1-s2.0-S0007850607602029-main

Manufacturing of Lightweight Components by Metal Forming M. Kleiner' (2), M. Geiger2 ( I ) , A. Klaus'

Chair of Forming Technology, Dortmund University, Germany Chair of Manufacturing Technology, University Erlangen-Nuremberg, Germany

1

2

Abstract Due to constantly increasing ecological concerns and demands for higher performance, lightweight construction is a key factor to success mainly in the transportation sector but also in general engineering, machine- tools, and architecture. This paper deals with current and future contributions of forming technology to the manufacture of lightweight components and structures. As design, materials, and manufacturing processes have to be considered integratively, it is pointed out which issues arise in the production of load adapted designs and using high strength materials. Frame and shell structure concepts as well as their related forming processes are presented. Finally, fields of further research are identified.

Keywords: Metal forming, material property, lightweight construction

1 INTRODUCTION In modern transportation engineering, the application of lightweight components is a central challenge. Due to economical and ecological reasons as well as to improve product properties, a mass reduction is necessary. This involves approaches from different engineering disci- plines. Therefore, lightweight construction can be defined as 'an integrative construction technique using all available means from the field of design, material science, and manufacturing in a combined way to reduce the mass of a whole structure and its single elements while at the same time the functional quality is increased'. Lightweight construction is crucial where mass is critical to enable the product function like in aeronautical applications. In case of masses subject to acceleration, lightweight components can increase the product performance e.g. allow higher revolutions with lighter crankshafts. Driving comfort and safety can be increased when unsprung masses are reduced like in a car chassis. At least, reducing masses improves the fuel consumption. (Figure 1) Much effort is being put into the development of light-

weight components and structures in automotive applications. Firstly, lightweight construction deals with the use of light materials. For example, the tailgate of the Volks- wagen Lupo consists of a magnesium cast inner part with an aluminum outer panel although severe corrosion issues have to be considered [ I ] . DaimlerChrysler uses a maintenance-free ceramic disc brake system in the sports car SLR thus eliminating 20kg of unsprung mass which significantly increases product costs [2]. Secondly, lightweight construction deals with different design strategies. For example, the ULSAS study examined chassis design possibilities providing different levels of suspension comfort, costs and weight [3] (Table 1). Concerning the body structure of trains or cars, frame and shell structures can be differentiated. Both design strategies are commonly linked to a specific material: aluminum in the case of frame structures [4], steel in the case of shell structures [5]. Therefore, different manufacturing demands arise using different design strategies [6]. Design, choice of material, and manufacturing technology are closely related as can be shown by wheel production, for example. Weight reduction at wheels is important due to its unsprung mass and the associated reduction of fuel consumption and the better ride-and-handling comfort. Especially in the front of the car, a weight reduction is necessary to ease the critical mass distribution at the front axle and therefore increase driving safety.

System type cost Mass saving [%] saving [%]

Figure 1: Purpose of lightweight components

Twistbeam 6 32 Strut & links 2 25 Double wishbone 0 17 Multi-link 30 3 (vs . al u m i n u m bench mark)

(vs. double wishbone) Lotus unique 22 34

Table 1: Cost and mass savings with different suspension designs [3].

Figure 2: Lightweight wheels made of aluminum, steel, and magnesium (from left to right) [7, 81.

Conventionally, two-piece steel wheels are manufactured by deep drawing of the disk, profile rolling of the rim, and a subsequent bolting or welding operation to join both parts. This wheel type covers less than half of the market today. Customers demand wheels with a larger diameter and tires with a smaller cross section. Therefore, wheels become heavier thus aggravating the above mentioned problems [9]. The further development of forming technology enables the optimized manufacture of lighter wheels made of aluminum, magnesium, or even steel (Figure 2). As a technology for large-scale production especially for automotive applications, metal forming provides eminent possibilities for the cost effective manufacture of lightweight components. Advantages like work hardening and load adjusted material orientation offer additional potential for lightweight constructions. This paper focuses on the interrelations between lightweight construction and metal forming. What are the related challenges to manufacturing processes and which contributions can be given? Looking at material specific issues of metal forming, common problems, and respective manufacturing processes in section 2, the use of steel and aluminum as well as the recent use of magnesium for lightweight components are discussed. Section 3 deals with lightweight structures and the manufacture of their particular workpieces. Frame and shell structures are discussed individually, followed by aspects of joining by forming. Finally, the field of further research is identified (section 4). 2 FORMING OF LIGHTWEIGHT MATERIALS In a material based approach to the manufacture of lightweight components, the use of light metals - keeping the same workpiece geometry - reduces the component‘s weight. Although the density of aluminum is a third that of steel, aluminum has only a third of the strength and tensile modulus. As the use of light metals must not decrease product properties, specific material properties should be taken into account (Table 2). The tensile modulus is metal dependent and cannot be changed by alloys or grades. An increase in specific stiffness as needed e.g. for structural automotive applications can therefore only be achieved by larger hollow cross sections. Relating the strength of a material to its density, high strength steel (HSS) and in particular stainless steel become lightweight construction materials compared to some aluminum alloys. Depending on the actual alloy and grade, steel and aluminum are likewise ‘light metals’ as well as magnesium and titanium. In addition, compound materials like metal matrix composites (MMC) provide means for ultra lightweight components. As for applications in shell structures with the lightweight construction criteria dent resistance and shell stiffness, aluminum and especially magnesium show much better properties than steel as shells with the same area weight have a higher wall thickness due to the lower density. In the case of the BMW M3, the front hood made of aluminum is 42% lighter than the standard steel front hood [ I 01.

Al Mg Steel Ti P 2.8 1.74 7.83 4.5 E 70 45 21 0 110

R m 150-680 100-380 300-1200 91 0-1 190

R,/p“’ 54-243 57-21 8 38-1 53 202-264

E/p”’ 25.0 25.9 26.8 24.4

K p 9.3 11.2 4.4 7.7

fi/~(~’ 14.7 20.4 7.6 10.6

p Density [kg dmq, €Tensile modulus [GPa], R, Tensile strength [N mm-2] (’) Specific strength [ l o6 N mm kg-’1 (2) Specific stiffness [ I O9 N mm kg-’1 (3) Dent resistance [ l o6 N112 mm2 kg-’1 (4) Shell stiffness [ l o7 N113 mm713 kg-’1

Table 2: Material properties. Unfortunately, a progress in alloy development in terms of higher strength always results in lower nominal strain at fracture thus limiting their formability [ I I ] :

steel: strength increases from 250 MPa up to 1000 MPa but strain decreases from 45% down to 12%; aluminum: strength from 150 MPa up to 530 MPa but strain from 30% down to 10%;

magnesium: strength from 200 MPa up to 380 MPa but strain from 20% down to 7%.

As a consequence, high strength alloys necessitate higher forces in forming operations as well as more rigid presses and more wear resistant tools. The latter can be achieved by ceramic inserts for forging [12, 131 or deep drawing operations [14], for example. At the same time, the low ductility restrains design possibilities. In order to obtain lightweight components, the material distribution is crucial. The material used should be distributed ideally according to the load applied to the component. Recent developments employ more and more topological optimization using bionic methods [ I 51. In an iterative design process, material is added to a component where required due to the load, and material is removed where it is obsolete. This process can be compared to the growth of a bone or a tree. Casting processes offer ideal prerequisites to manufacture complex components designed by conventional or bionic methods. Disadvantages can be found in the material structure like the existence of pores and in the limited choice of cast alloys with lower yield stresses compared to wrought alloys (Table 3). In contrast to casting

Material Cast alloy Wrought alloy Increase Aluminum AICu4Ti Mg AIZn5,5MgCu 26-66%

320-420 530 Magnesium AZ91 T6 AZ80AT5 27-44%

Table 3: Comparison of tensile yield stresses (in MPa) of

240-300 345-380

high strength cast and wrought alloys [I61

Casting Cutting Forming Figure 3: Schematic material orientation in

different manufacturing processes [29].

or cutting, forming processes enable a dense material structure orientated parallel to the load path (Figure 3). Furthermore, the already higher yield stresses of wrought alloys designated for forming processes are yet increased by the work hardening effect. Unfortunately, forming processes especially sheet metal forming processes do not allow as complex shapes as do casting or cutting processes. To overcome this restraint, one possibility is to use semi-finished products that already provide a suitable material distribution. With tailored sheet metal products, where different dimensions, materials, alloys, or grades are combined within a single workpiece, a more sophisticated product or process design and more complex shapes can be achieved. On the other hand, forming processes of such semi-finished products require increased process knowledge, the observation of different material behaviors, and the development of designated adaptive forming processes and tools [ 17, 18, 19, 201. High strength but low ductile materials used for lightweight components moreover aggravate the problem of limited material distribution options. In order to avoid this, some solutions are at hand including:

forming at elevated temperatures, incremental forming,

superplastic forming, and thixoforming.

Elevated temperatures Forming at elevated temperatures lowers forces and increases ductility as additional slip planes are activated, especially for magnesium [21]. Moreover, higher temperatures decrease spring back which is an important issue using high strength materials. But with the temperature being a sensitive factor in forming operations, process parameter windows have to be carefully observed in order to obtain reproducible results.

lncrem ent a1 forming Incremental forming processes are characterized by a successive local forming of the workpiece instead of forming the whole workpiece at one time. While spinning allows the manufacture of rotationally symmetric hollow products, the incremental sheet forming (ISF) process and its derivatives allow the manufacture of complex asymmetric shapes [22, 23, 241. With shear forming as well as ISF, very high strains compared to conventional stretching or deep drawing processes can be achieved (Figure 4) [25, 26, 27, 281. High achievable strains and a flexible manufacturing method make this process very promising for lightweight applications according to increased work hardening and larger design possibilities. Due to the successive and

Figure 4: Incremental forming: process principle (left and bottom) and high achievable strain (top right) [25].

local nature of incremental forming, the very complex process modeling and simulation is just at the beginning. Nevertheless, first results to predict forming limits by means of finite element methods are being researched [26, 301.

Superplastic forming The superplastic forming eminently increases the nominal strain at fracture provided that preconditions like a grain size below 10 vm, specific strain rates and forming temperatures are observed. For some applications, this process is mostly applied with aluminum and titanium. Also magnesium sheet metal forming experiments are being undertaken [31].

Thixoforming Finally, to overcome low ductility, the material can be formed in a semi-solid state. This so called thixoforming [32] allows for example the forging of more complex parts that can be produced net or near net shaped in one process step and therefore offers opportunities for the production of lightweight components [33]. Established for aluminum and magnesium, this process still lacks experience with steel due to specific die development challenges [34].

2.1 Steel With its high formability, steel is predestined for complex sheet metal parts and hence for difficult forming operations. It offers reasonable freedom to the designer as less manufacturing restrictions apply when choosing from alloys in ultra high strength grade to those with enormous ductility. Furthermore, sheets of different thickness, quality and surface coating can be welded together to achieve an optimized weight and material property distribution over the blank.

Stainless steel Though high strength and ductility are usually mutually exclusive properties, both are provided by stainless steel. Due to its price, about five times that of regular steel, this being even higher than the price of aluminum, stainless steel is scarcely used in automotive applications compared to the amount of carbon steel. Until recently, applications were limited to decorative use and products in the exhaust systems due to its good resistance to thermal fatigue, creep, and oxidation, here accounting for 300,000 tons/year in Europe (2002) [35, 361. Observing suitable process parameters, the forming of stainless steel grades is industrially applicable in large series. Differences to carbon steels primarily refer to spring back behavior which can be compensated by appropriate tool development [37]. In Japan, all-stainless-steel railway carriages today account for half of all carriage production [38]. Their easy recyclability and their considerable weight saving, even compared to aluminum carriages, are of great advantage. Stainless steel shows a strong tendency to work hardening so that even at low strains, a significant increase in yield strength can be achieved [39]. Combined with its high formability, stainless steel enables new, more lightweight product designs to be manufactured. The bumper of a passenger car has to be very stiff concerning crashworthiness reasons in order to lead primary impact forces into the main crash structure. High strength of the bumper is necessary to direct these forces on the non-impact side in case of an offset crash. On the other hand, a deformation zone between bumper beam and bumper cover is desired to increase pedestrian safety. It is assumed that up to 2,000 fatal casualties in the EU per year could be prevented by appropriate car redesign. In order to achieve this, the cross section dimensions of the

bumper have to be reduced without lowering the stiffness. A conventional bumper beam of a caravan vehicle is bent from a flat blank, welded into a closed shape, and bent slightly into the final form. A significant weight reduction is possible by the use of stainless steel. Here, a bumper beam design optimization introduced beads into the cross section that have to be deep drawn (Figure 5). This required for a material with higher formability but allowed a smaller sheet thickness to be used due to higher strength and a larger cross section area. At the same time, the high ductility can absorb additional crash energy. Simulated drop tower test evaluated the crash performance of the optimized design. In effect, to maintain the same performance as conventionally designed bumper, a weight reduction of 20% was achieved using stainless steel AlSl 301L in cold worked condition CIOOO.

For many years, fuel tanks for passenger cars have been made of plastic by the blow moulding process, accounting for about 70% of all tanks produced. It allows the manufacture of complex shapes required due to complex package limitations. But legislation demands zero emis- sion of hydrocarbon from tanks which plastics used at present do not meet. Besides diffusion tightness, stainless steel on the other hand provides high corrosion resistance, outstanding formability, and high strength compared to mild steel. Still, the manufacture of such a complex shape could only be achieved by the intense use of finite element simulation. In many optimization steps,

the best suiting forming processes using conventional

adequate parameters in the very small process window,

[40,411

and hydro mechanical deep drawing,

as well as

tool and workpiece design were achieved. As a result, the stainless steel tank is 20% lighter while providing 4% more capacity than the conventional plastic tank due to smaller wall thickness (Figure 6). [42]

Tailored blanks A load adapted material distribution is the key to successful lightweight components. In massive forming, a complex material distribution can easily be achieved with forging or extrusion processes. However, in sheet metal forming, this is not applicable. Here, the use of tailor- made semi-finished products enables the cost-efficient production of weight and load-optimized workpieces. Not only sheet metal of different thickness (tailored blanks) but also of different materials grades (tailored heat treated blanks), or even of different materials or alloys (hybrid blanks) are available. Tailored blanks are usually joined with a linear weld seam (tailor welded blanks), less often with non-linear weld seams (tailor engineered blanks, Figure 7). Furthermore, the varying wall thickness can be manufactured by rolling (tailor rolled blanks) leading to a continuous thickness transition. In 1983, Thyssen Steel were the first to manufacture tailored blanks today accounting for 50% of a typical car body, e.g. as doors, liftgates, floors and side beams. Although applicable to non-ferrous metal as well, tailored blanks are currently referred to as made from steel. [43, 441 Compared to conventional blanks with a uniform thickness, tailor welded and even more tailor engineered blanks provide the potential for a weight reduction of 20- 34% e.g. for a door inner panel [43]. When forming tailored blanks, new problems arise: [I71

Tribology: different surface textures and coatings lead to different friction in deep drawing processes thus in- fluencing the forming result. Varying friction coefficients require adaptive strategies;

Mechanical properties: the forming behavior cannot directly be estimated from both single materials. The

Figure 5: Stainless steel bumper beam - development from original to final design [41].

Figure 7: Wheel arch made from tailor engineered blank [18].

Figure 6: Comparison between plastic and steel tank [42].

Figure 8: Cross section and yield stress after flexible rolling [19].

higher strength and lower ductility of the weld seam cause much smaller yield strains in the tensile test of probes with a longitudinal seam, combined with a different failure mode; Material flow: Due to different wall thicknesses, the binder force is only applied to the thicker sheet area. This leads to wrinkling and cracks especially in the weld seam. Moreover, the material in the flange preferably flows in the milder part of the blank. Therefore, different tool concepts for tailored blanks are necessary.

The flexible rolling process on the other hand allows the production of blanks with almost arbitrary thickness dis- tributions in the rolling direction by varying the rolling gap. Hereby, multiple local sheet thicknesses can be ideally adapted to the load. Due to work hardening in the flexible rolling, the yield stress increases according to the cross- section reduction (Figure 8). Applying a dome height test using a hemispherical punch, tailor rolled blanks (TRB) with a thickness transition length above 40mm reached the same dome height as a regular blank. This is a great advantage compared to tailor welded blanks due to ductility reasons mentioned above [45]. Manufactured from such TRB, first prototype applications and even a mass-produced part are examined. A weight reduction of 25% compared to a regular sheet was reached for a Mercedes-Benz E-class cross member (Figure 9). This part has been stamped successfully from TRB with thicknesses of 0.8mm and 1.25mm in tools originally designed for TWB. In order to nevertheless match the specific tools, the shortest producible thickness transitions were chosen. An optimal lightweight design would have been achieved if longer, load-adapted transitions were used that then, however, would have required a new tool set. [19, 451 For the manufacture of a bumper made from stainless steel TRB, the air bending on a press brake and the profile bending on a three-roll-bending machine was investigated (Figure 10). lnhomogeneous springback due

to a continuously varying sheet thickness (1.02-1.22mm) and strength required an individually designed die. Using a regular punch, local variations of the die height preset a varying punch displacement in order to compensate the material behavior. Furthermore, for short transitions, a segmented rapid tooling die was manufactured by laser cutting v-shaped lamellas that were individually adapted. Conventional steel wheels are still the cheapest in the market (about 20 US$), compared to cast aluminum wheels (about 40 US$) or even forged aluminum wheels (about 70 US$). The disadvantage of heavier steel wheels can be compensated by a better material distribution in the rim and the use of high strength alloys. A varying thickness over the rim can either be achieved by flowforming or the use tailored strips. While spinning the rim, a defined seamless thickness distribution can be manufactured by a radial motion of the forming tool towards the mandrel thus reducing the wall thickness (Figure 11). In an application for a 15x6 base wheel of a mid size car, the rim was manufactured from a 2.29mm thick sheet, with a spun thinned area of 1.55mm thickness using a microalloy steel (Figure 12). This reduces the weight of the wheel by 20%. [7] By the use of tailored strips (narrow tailored welded blanks with multiple thicknesses) rolled into a tubular shape, a similar weight reduction can be achieved under economical conditions [43].

Forging With the improved material structure of forged workpieces, the forming technology provides advantages over competing cutting processes. Especially for the production of bevel gears, forged components offer higher strength and precision accuracy that lead to a power density improved by 25% compared to conventionally machined gears as

the grain flow in the formed teeth is parallel to the load

no fibres are open in areas of high load,

and ideal contact pattern can be achieved as all apexes of all gears of the differential are in the exact same point, and

direction (Figure 13),

Figure 9: Cross member made from tailor rolled blank [18].

Figure 11: Flowforming [46]

Figure 10: Bumper made from tailor rolled blank [20].

Figure 12: Microalloy wheel with non-uniform wall thickness in the rim [7].

the surface hardness is increased The increased power density can be used for smaller and lighter components. In addition, conventionally manufactured gears and gear box end pieces are designed with the cutting clearance for the hobbing of the splines. This run-out length is necessary for machining but not for forming. As a consequence, the cutting clearance can be avoided thus saving additional weight and space (Fig- ure 14). [47, 481 The technological, ecological, and economical benefits of forged gears have led to a broad market. But the gears require a precision forging process to meet the required tolerances. This is usually achieved by a hot forging and a subsequent cold coining operation. In order to apply this forming technology to helical gears, a wide knowledge of the essential process variables is necessary due to the more complex tooth geometry and the higher surface quality required. Therefore, the demands on the precision forging process increase. Especially slug mass, the slug temperature, and the energy of the forging press have an important influence on the gear quality and the tool stresses. By underfilling the tool edges not lowering the tool function, a significant reduction of the tool pressure, tool wear, and tool life can be achieved. [49]

2.2 Aluminum In aircraft applications, weight saving enables longer or faster flights while at the same time consuming less fuel. Commercial flying therefore becomes ecologically and economically more reasonable by allowing larger aircrafts. As aluminum alloys have been the most widely used structural material in aircrafts for several decades, new alloys and engineered materials are emerging. Low- density al u mi n u m-l ithi u m alloys, powder-metal I u rgy- processed 7000 series alloys, the aluminum based MMC, and metal-polymer hybrid composites have the potential to replace the conventional 2000, 6000, and 7000 series alloys. Demands for higher strength led to the development of several new EN-AW7075 derivative alloys. The

Figure 13: Comparison of a cut (left) and a formed (right) bevel gear [47].

Figure 14: Comparison of a cut (left) and a formed (right) gear box end piece [48].

alloy EN-AW7055-T77 e.g. shows a yield stress of 603 MPa. This progress resulted largely from tighter control over impurity levels and improvements in thermomechanical and heat-treatment practices. [50] With rapid solidification processes (RSP) like spray deposition or meltspinning, alloys with even higher yield stresses become possible. At spray deposition, the molten aluminum is sprayed on a rotating table thus growing a cylindrical billet. Hereby, materials that tend to segrega- tion in casting processes can be produced. It is even possible to add powders to the spray that would not blend with the molten matrix. During the meltspinning RSP, molten aluminum hits a fast rotating wheel and almost instantaneously releases a continuous metal ribbon at room temperature. This ribbon is converted into flakes and finally into an extrusion product. At RSP, the sudden temperature drop that takes place at a rate of more than 106"Cs-'. Due to this rapid quenching, a very small grains size of about 2vm in comparison to a conventional alloy with a grain size of about 100vm is produced. The yield strength (RSA-707) reaches 800MPa while the fracture elongation drops down to 2-5% compared to 10-14% of a conventional EN-AW7075 alloy. [51] Increased strength and lowered ductility requires developments in aluminum forming technology. The field of ongoing research can be divided into

improvements in massive forming, use of tailored blanks, forming at elevated temperatures, and superplastic forming.

Massive forming The application of cold formed aluminum parts is significantly increasing due to good formability and high strength in the final product. Machined steel products are replaced as e.g. in the case of a steering column (Fig- ure 15). Made from EN-AW6082, this part is backward can extruded followed by successive ironing operations reducing the wall thickness down to 1.5mm thus enabling a final forming of the bellow. This complex product design not only allows a very lightweight component. As added value beyond the reduction of weight, the design results into improved security of the driver as a side bending of the steering column is possible in case of an accident.

In fact, cold forging allows for net shape forming with no or very few machining and finishing operations. But high forging pressures considerably lower tool life and the lower formability at room temperature reduces the potential complexity of shape in cold forging. In case of a wheel suspension arm, aluminum hot forgings replace conventionally formed welded steel sheets. Over the past two decades, the product complexity more and more increases in order to save weight and space. In order to furthermore improve material characteristics and concurrently reduce process steps and production time, the

[521

Figure 15: Cold formed automotive steering column [52].

basic understanding of the material behavior with respect to time, temperature, and forming are essential for the development of new forging lines. Hot forging usually requires a separate batch heat treatment process step within the production line which causes the formation of a material microstructure resulting in low strength in the forgings. With an integration of the heat treatment into the production line and a shorter ageing time, a fibrous microstructure can be preserved in the material. As a consequence, the mechanical strength is 10% higher allowing for more lightweight components. [53] A further alternative to hot forging is the warm forging at intermediate temperatures. Sophisticated heat treatments can be avoided as the forming process uses solution treated and water quenched aluminum thus being in soft but indeed unstable condition. In warm forging, flow stresses are reduced and formability increased. Still a precision forming is possible since the lower temperatures compared to hot forging make it easier to adhere to close tolerances. With a proper flow control, better mechanical properties can be obtained by preserving the material orientation. [54] Although the forming of aluminum provides the possibility for the manufacture of rather elaborate components, forming cannot compete with casting processes regarding highest complexity of shape. Besides aspects of a lightweight construction due to load adapted material distribution, complex three-dimensional shaping of components also enables an aesthetic design. Especially in emotionally charged automotive applications, it can be assumed that the wide use of cast aluminum wheels does not necessarily derive from their quality being 40% lighter [55] than conventional steel wheels. Less design restrictions, personal distinction, and the sporting image of a lightweight component are probably of more concern for the consumer. Aluminum wheels made from sheet metal blanks in the same way as conventional steel wheels are lighter but unfortunately look the same which leads to serious marketing problems. Forged aluminum wheels on the other hand provide more freedom of design compared to non-cast wheels. Addi- tionally, forging saves 15% weight compared to cast aluminum wheels, due to improved material structure, high strength alloy, and work hardening. In case of coaches with eight or trucks with twelve wheels, several hundred kg can be easily saved. [55]

Tailored Blanks Although tailor welded blanks (TWB) offer both potential weight and cost benefits, the continuous weld-line and thickness difference in TWB can often result in difficulty in stamping. This problem is more severe in aluminum because of its limited formability as compared with typical drawing-quality steels. Additionally, welding of steel TWB tends to increase the strength of the weld material which helps prevent failure in the weld during forming. Alumi- num TWB do not experience this increase in strength and therefore may have a greater tendency to fail in the weld [56]. Here, the weld line geometry and the weld heat affected zone properties have to be taken into account when modelling TWB forming. Specifically, the weld line geometry was found to be more significant than weld material properties in predicting weld line shift by means of FE-simulation. [57, 581 A recently developed welding method uses two lasers simultaneously. Instead of using a C02-laser with the known restrictions in weld seam quality (Figure 16 left), a Nd:YAG-laser is used for known deep-penetration welding while a second laser smoothes the weld seam at the surface (Figure 16 right). This process is already used for the inner wheel arch in the Audi A4. [56, 591

Figure 16: Comparison of C02-laser (left) and dual-laser welded (right) aluminum sheets [59, 561.

Figure 17: Increase of limiting drawing ratio by locally heat-treating regular blanks (left) and tailor welded blanks (right) [61, 621.

Tailored heat treated aluminum blanks (see also 2.1) offer another possibility in designing product and process. Though sheet thickness remains the same all over the workpiece, material disadvantages in low ductility can be overcome by locally changing the mechanical properties by C02-laser or Nd:YAG-laser induced heating. As a result, this partial solution treatment induces a soft condition in the material and thereby reduces the flow stress in the affected areas. After the cold forming process, the material naturally ages back into the T4 condition within seven days for precipitation hardenable alloys. Conse- quently, the limiting drawing ratio (LDR) pmax significantly increases from 2.1 to 2.6 (Figure 17 left). [60, 61, 1371 This allows high strength aluminum alloys to be more extensively employed. Furthermore, the local heat- treating can also be applied to TWB thus easing the poor weld seam properties and raising the LDR from 2.0 to 2.36 (Figure 17 right) [62]. As a consequence, lightweight components with more complex shapes can be manufactured.

Elevated Temperatures At higher temperatures, the flow stress decreases hence lowering the required forming forces and at the same time increasing ductility. In warm forming processes like forging or bar extrusion, this is commonly done. As more and more sheet metal blanks of high strength alloys are used to decrease product weight, the formability limits can be extended by increasing forming temperatures. In order to be able to design process and product, simulation tools are employed that first and foremost rely on suitable material data. A sound acquisition of the flow curve prerequisites a suitable strain and temperature measurement. However, commercially available set-ups still lack to provide this. Recently, a new method to properly determine material data has been examined. Here, instead of assuring a highly constant temperature distribution over the specimen, a defined gradient was produced and calculated analytically. With a high resolution camera, the strain was recorded online. As only the center of the probe was

n

~

I I 50 100 150 200 250 300 € 4 1 0

Flange Temperature in "C DD =Deep-drawing HM =Hydromechanical deep-drawi ng 0 DD: EN-AW 5083 punch diameter: 100mm X DD: EN-AW 6016 die radius : 7mm 0 HM: EN-AW 5083 punch radius : 7mm 0 HM: EN-AW 6016 punch velocity : 5 m m k

Figure 18: Increase of limiting drawing ratio by deep drawing and hydro mechanical deep drawing

at elevated temperatures [63]. taken into account, reliable strain values could be com- puted [64]. Another method to determine flow curves at elevated temperatures is the hydraulic bulge test. In comparison to the tensile test, it allows higher true strains. Furthermore, it is in better accordance with deep drawing operation as two-dimensional stresses are applied. Besides flow stresses, also friction and heat trans- fer coefficients have to be considered in order to determine suitable tool and process parameters by means of FEM simulation. [65] Experiments show that the LDR in deep drawing is increased from 2.1 to 2.8 by using a heated flange area at a temperature of 300°C. The same increase in LDR can be achieved in hydro mechanical deep drawing already at a temperature of 200°C (Figure 18). The lower temperature is specifically of importance regarding problems with thermal lubricants and the tendency for pick-ups. [63]

Superplastic Forming At very low strain rates, extremely high deformation de- grees at low stresses are possible that exceed conventional forming processes by far. Using gas pressure, this so-called superplastic forming (SPF) is carried out at elevated temperatures where ductility is anyhow increased relative to room temperature. Although not common in ordinary metal alloys, most aluminum alloys exhibit a superplastic behavior depending on metallurgical structure (grain size about IOVm), temperature (35O0C-55O0C), and strain rate (about I O-5-10-2s-1). Due to the lack of work hardening, enhanced ductility, and significantly reduced springback, SPF allows for complex shapes to be manufactured. Especially the complex aerodynamic shapes are often difficult to produce using conventional forming methods. Using SPF, dimensionally accurate and high strength panels can be produced including three dimensional ribbed stiffening panels. This enables thinner and therefore lighter sheets to be used.

Unfortunately, the slow strain rates result in long cycle times. Therefore, SPF is currently used only for low volume production like aerospace applications, trains or niche vehicles (Figure 19). For the use in automotive mass production, a strain rate 100 times higher is required as such a rate can result in a production rate above 20 pieces per hour. Conventional deep drawing is even another 10 times higher but it often requires a series of progressive double die sets in comparison to SPF where only a single die is used. At quick plastic forming (QPF), the gas pressure increases at an appropriate temperature within 2-3 minutes instead of 20-30 minutes as at SPF. This comes with an increase of gas pressure

[661

Figure 19: Superplastic formed aluminum sheets [66]. of about 2-3 times. Furthermore, in a combined process, sheet metal blanks are first conventionally deep drawn to some extent. Then, the final forming step is carried out within the same die set with QPF. [31, 671 Another approach arranges the SPF in the first place with a subsequent hydro mechanical deep drawing. Hereby, SPF produces a preform without work hardening that can afterwards be formed into the final shape. Due to the cold forming in the second step, increased work hardening and superior accuracy of shape is expected. [68, 691 As the flow stress is a function of temperature besides strain and strain rate, an exact control of the temperature during the forming process is necessary to avoid local necking due to flow stress gradients. A variation should not exceed f5K. With special fibreboards used as thermal insulation between die and press, a temperature deviation below f0.5K was obtained. [70] By applying the SPF to tailor welded blanks, an even more detailed, more complex and lighter component can be produced. One significant complication that occurs is the behavior of the weld seam. While the process parameters can control the superplastic behavior of the uniform sheet parts, substantial grain growth in the seam due to the welding process prevents the local SPF. Hence, the deformation mechanisms during SPF will cause flow stress differences between the weld and the sheet material. [71] Uniaxial tensile tests carried out under SPF conditions examined differences in the flow curves between specimen without a weld seam, with a longitudinal seam, and with a transverse seam. The transverse-weld specimen exhibited strain in the sheet material uninfluenced by the seam, thus behaving like to two monolithic specimen undergoing series loading but separated by the weld. At higher tensile stresses, the longitudinal-weld specimen exhibited a significantly lower elongations of about 40- 60% compared to 220-360% of the parent material. Therefore, the flow stress and ductility incompatibilities have to be taken into account during component design.

2.3 Magnesium First applications of magnesium at Volkswagen started in the 1950s and reached its climax in 1972 with a 42,000 tons yearly consumption. Primarily, engine and gearbox casings from cast magnesium alloys AS41 and AZ81 were manufactured. Later on, less expensive and techni-

[711

Figure 20: Activation of additional sliding planes for magnesium at elevated temperatures [73].

cally more advanced aluminum alloys superseded the use of magnesium. [72] With a density of 1.74kg/dm3, magnesium is approximately 35% lighter than aluminum. But due to the closed packed hexagonal (cph) crystal lattice structure at room temperature, magnesium provides only low ductility for cold forming operations. At temperatures above 225"C, additional sliding planes are activated thus increasing ductility and lowering the yield stress, besides the conventional temperature effect on ductility and yield stress (Figure 20). [73] Furthermore, disadvantages of magnesium comprise poor creep resistance at temperatures above about 100°C as well as corrosion. Here,

chemical corrosion due to environmental influence e.g. salt,

electro-chemical corrosion due to a high electro nega- tivity compared to aluminum and steel leading to severe problems in joining, and stress cracks corrosion due to variations in stress

are of main concern. Nevertheless, the production of magnesium automotive parts is currently experiencing a rapid growth which results mostly from high pressure die castings accounting for 95% of the 120,000 tons magnesium worldwide yearly used (numbers by 2000). So far, only a limited number of different alloys is available compared to aluminum alloys. Therefore, much effort is being undertaken in alloy development improving material properties especially concerning higher creep resistance at elevated temperatures [74] or the increase of formability by the introduction of lithium as an alloying element [75]. Wrought alloys on the other hand did not experience this dramatic growth although they generally offer better mechanical properties that can even be enhanced by adequate heat treatment. Here, AZ80A and ZK6OA show excellent values regarding high tensile stresses and elongation [76]. Due to raised ductility above 225°C and the importance of formed magnesium parts for lightweight applications, many investigations are being undertaken to gain a deeper knowledge about the specific material behavior. Especially flow curves and their dependencies from temperature, strain, and strain rate have to be carefully observed prior to all forming experiments. Also specific effects like work softening have to be taken into account when theoretically describing the forming behavior for simulation purposes [77]. In massive forming, these influences can be examined under varying conditions by recording the flow curves with the uniaxial cylinder compression test [79]. At room temperature, magnesium is deformed with serious work hardening from 150 to 380 MPa until brittle fracture. With an increase of the homologous temperature (HT) up to 0.26, the compression test exhibits basically the same material behavior at lowered flow stresses and a slightly

400

2 300 H S .- ; 200

g 100 1 1 1 1 I I

g!

- Deformation rate =0.8 l / s

* v)

L

OO 0.2 0.4 0.6 0.8 1 Degree of deformation

Figure 21: Flow curves of AZ61 in compression test [78].

'0 50 100 150 200 250 300 3 Temperature in "C

j0

Figure 22: Dependency of elongation on temperature (pure magnesium, tensile test) [78].

larger degree of deformation up to 20%. But at a HT of 0.32, high deformations at very low flow stresses are achieved (Figure 21). This is due to the additional sliding planes and dynamic recrystallization. [78] As ductility is critical, the prediction of workpiece failure is essential. Under the simplified conditions of the compression test, the occurrence of cracking relates well to a calculated maximum tensile stress [21]. This makes it possible to design appropriate forming conditions. Be- sides temperatures and strain, frictional behavior is investigated. The oxides that magnesium quickly tends to develop at elevated temperatures enormously increase friction. Processing in non-oxidizing atmosphere therefore plays an important role besides tool coating, heating, and the use of appropriate lubricants [81]. In contrast to massive forming where semi-finished products are commercially available as cast slugs, rods, or bar extrusions, magnesium sheet blanks are not yet available in large quantities. But as workpieces with thin walled and large areas provide best lightweight potential, rolling of sheet metal blanks and their properties are subject of current intense research. As rolling involves high deformation ratios, special attention has to be paid to microstructure as well as intermitting heat treatment, and the resulting mechanical properties. Here, knowledge of the metallurgical and material physical characteristics, thermodynamic conditions, and the state of stress during forming is required. [78] In accordance to the compression test, the tensile test also shows a significant leap of strain at increasing temperature (Figure 22). Moreover, the grain size shows an influence on formability much larger than in body centred cubic materials as smallest grains allow for highest strains (Figure 23). By an adapted multistep forming and heat treatment process, the cast semi-finished product with large grains can be transformed into a sheet metal

10 100 1c Medium grain size in pm

Figure 26: Parts made from AZ 31B using an axisymmetric tool 01 50 mm, formed at

room temperature (left) and at 230 "C (right) [138]. in flow stress and formability necessary for the distinct

3

extension of the forming limits of magnesium (Figure 25). [771 Hydroforming at elevated temperatures also offers addi-

Figure 23: Dependency of elongation on grain size (pure magnesium, tensile test) [78].

Figure 24: Grain size before (right) and after (left) rolling of magnesium sheet [80].

Figure 25: Partially heated deep-drawing tool [73, 771 blank with fine grains and a stabilized structure (Fig- ure 24). [82, 801 As cold forming of magnesium is hardly realistic, deep drawing of magnesium sheet also has to be carried out at elevated temperatures. Due to the significant sensitivity of formability to temperature, partially heated blank hold- ers are able to control the forming process very accu- rately especially for complex geometries. In straight flange areas, mainly radial stretching with less true strain occurs. The corners are dominated by an overlapping of radial stretching and additionally a tangential compression with high true strains thus requiring higher formability. Here, a partially heated tool set provides a suitable temperature distribution and therefore a distribution

tional forming possibilities of magnesium sheet. At room temperature, the low ductility causes a failure of the component at the tool radius where tension and bending load can be detected at an early stage of forming (Figure 26 left). At elevated temperatures, a good form-filling can be reached using tools and hydroforming fluid heated up to 230°C (Figure 26 right). [I381 Despite the above mentioned limitations, cold forming of magnesium has been investigated. In cup drawing test, only small limiting drawing ratios between 1.1 and 1.5 were obtained. Nevertheless, in some applications for products with simple geometry, this ductility might be sufficient for cold forming operations like 3D-bending, coining, die pressing, and raising. In those cases, a production rate comparable to aluminum sheet becomes possible. [83] In applications like motorsport where the importance of low component weight excels all other issues like long-life reliability, first commercial formed magnesium products appear. In motorcycle racing e.g., forged magnesium wheels have to compete even with carbon composite wheels. For the 2000 championship, 70% of the wheels used were made of cast magnesium, 20% of carbon composite, and 10% are forged magnesium wheels. Still, the demand for forged wheels rises due to their weight

Figure 27: Application of forged magnesium wheels in motorcycle racing [84, 851.

Material Process Weight in kg Rel. Weight Steel Stamping, (1 4.2)' 100%

rim rolling Aluminum Casting 8.5 60% Aluminum Forging , (7.2)' 51 Yo

Magnesium Casting 3.9-4.2 -28.5% Magnesium Forging , 3.2-3.5 -23.6%

Carbon 2.9 20.4%

rim rolling

f lowfor mi n g

Composite

Table 4: Comparison of 6x1 7" motorcycle racing wheel weight [82, 83, 851.

' Weight estimation for steel and forged aluminum based on general weight reduction for wheels

improvement over cast versions (Table 4) Similar to the manufacture of forged aluminum wheels, the process chain of forged magnesium wheels consists of forging the wheel disc and flow-forming the rim from the flange of the disc. Both process steps have to be carried out at temperatures above 225°C. The flowforming itself runs in three steps: splitting up the flange, flow-forming the rim, and calibrating the rim contour. Depending on the required material properties, the wheel can afterwards be stabilized, aged, or heat treated. Af- terwards, the wheel is machined by turning the face sides and the rim, milling the spokes, and drilling the valve hole as well as coating and painting (Figure 27 right). As a consequence, the magnesium forged wheel at a weight of 3.2kg is 1 kg lighter than the cast version [82, 851. Only the carbon composite wheel is yet another 10% lighter

2.4 Titanium Apart from conventional and commonly used lightweight metals, various other new materials offer the potential for lightweight components that require forming operations and appropriate process knowledge. First of all, titanium is used in extreme applications that concern lightweight aspects. Furthermore, sandwich and foam materials become more and more available. Titanium offers supreme properties which amongst others include

a density near half that of steel, highest strength, corrosion and high temperature oxidation resistance,

a modulus of half that of steel Unfortunately, titanium is extremely expensive with about 30-120 US$ per kg compared to about 1 US$ per kg carbon steel (prices for sheet metal blanks). Therefore, titanium is only considered in lightweight applications where weight saving yields an outstanding economical benefit like in the aerospace industry, an edge in compe- tition like in motorsport, or a product value like for pros- thesis due to its tissue compatibility. In motorsport, titanium is ideal for products in the exhaust system, springs, connecting rods, pistons, valves, and many more. As a consequence, such items have become available commercially. In the aerospace industry, titanium products are widely used but not so much for lightweight reasons. In the main structure of the Airbus A330/340, titanium only accounts for 7% of the weight, in contrast to the engine where titanium is the main material in terms of volume (Fig- ure 28 left) [84]. This is first and foremost due to its high strength at high temperatures even compared to Ni- based superalloys. Furthermore, in the application of a helicopter rotor head, titanium is used for its highest durability under dynamically changing loads (Figure 28 right) [85]. The material properties and forming behavior of titanium alloys are well documented in many research studies

~ 3 1 .

and

Figure 28: Titanium applications in the aerospace industry [84, 851.

carried out to support the aerospace industry. This includes most of all the manufacture of spherical vessels where SPF is commonly used e.g. for satellite or rocket tanks [86, 871. In order to avoid a fusion welding process decreasing material properties, the SPF of a diffusion welded double sheet is being investigated. On the basis of numerical and experimental investigations, the manufacture of flangeless spherical vessels has been demon- strated [89]. As commercially pure (cp) titanium shows high anisotropy, finite-element models are being improved in order to optimize forming processes and workpieces [88]. As mentioned above, high material costs have excluded the use of titanium in high-volume automotive applications. Nevertheless, with the current trend in vehicle warranties climbing to a 10 year / 150,000 mile level in the US, vehicle manufacturers must consider the long- term cost associated with less durable materials. Having this in mind, titanium quickly becomes the lowest-cost option for some applications. [go] In order to provide appropriate forming processes for such applications, the deep drawing of cp-ti sheets is being analysed. In the exhaust system, titanium is able to gain a weight saving of 40-50% and a better corrosion prevention. To replace stainless steel with cp-ti, the manufacturing process demands new concepts and parameters. For such development, knowledge of the material behavior, an optimized tribological system, and improvements of the process limits are necessary. At room temperature, the examined titanium materials showed a higher limit drawing ratio in comparison even to stainless steel. [91, 92, 931 In order to reduce production cost, also the forming of titanium sheets using rubber as a flexible media was investigated. Different concepts with male and female rubber dies were verified by numerical models and experiments. It could be shown that the elastomer behavior is predictable and that the component could be manufactured. [94] 3 FORMING TECHNOLOGY FOR LIGHTWEIGHT

Lightweight constructions are optimal if material is used only in component areas where stresses appear and if the material used is charged near yield stress. Therefore, such a structure is primarily designed for strength i.e. the structure does not fail. This design principle is followed by in aerospace applications where materials with highest specific strength are used like the 680 MPa aluminum alloy EN-AW7449 in the wing up side of the Air- bus A380. In automotive applications, structures are additionally designed for stiffness i.e. the structure does not elastically bend too much. Whereas it is spec- tacular but irrelevant that an airplane wing tip bends several meters before failure, stiffness is a major comfort element in automotive applications. This additional requirement naturally increases the structural weight. Only crash relevant structures are solely designated for ab- sorbing crash energy by deformation and therefore are designed for strength. Depending on the purpose of a lightweight structure, two main construction principles are employed overlapping each other to a certain degree. As long as a structure only has to carry a given load, frameworks are used e.g. in cranes (Figure 29 left), scaffolds, bridges, or monu- ments like the Eiffel tower. A shell structure on the other hand is used if the structure has to seal against e.g. pressurized water, fuel, or air (Figure 29 right). Usually, lightweight structures combine both concepts, either by

increasing sheet stiffness by sheet or massive profiles (e.g. car body-in-white, fuselage), or geometrical elements within the sheet like beads (e.g. cans), or

STRUCTURES

covering a structural framework with sheet thus enhancing stiffness if, additionally, shear stresses are induced into the sheet.

While frameworks mainly involve the use of beams like tubes or profiles, shell structures deal with sheet metal blanks.

3.1 Frame structures

Concepts and semi-finished products In most cases of the above mentioned applications, straight semi-finished products are joined to complex structures. As steel is much cheaper than other metals with almost identical specific properties, those structures are nearly all made of steel. In contrast, in the case of transportation applications, curved profiles and tubes are necessary due to aerodynamics, structural properties, and design reasons. Here, a commercially driven compe- tition of materials has developed. Conventional steel shell structures are being replaced by aluminum frame structures. Achieved weight savings over existing structures are accredited to the employed lightweight material. At a closer look, however, each new design generation is lighter than the one before. Therefore, rather the improved design than the actual material is accountable.

In contrast to automotive shell structures, only simple geometries are used in frame structures. In many applications, most of the members are tubes with round or rectangular cross sections. Welded round tubes are very common in axle tubes, bicycle frames, garden chairs, or ski sticks. Extruded tubes are used in simple space frames like the BMW C1 (Figure 30). Unfortunately, they may show variations in wall thickness of up to 20%. Therefore, a subsequent cold drawing is applied to additionally yield closer tolerances and better mechanical properties. At least, seamless tubes offer best mechanical properties. Due to high cost, they only account for a small market segment like in helicopter landing vats, drive shafts, or hydraulic pipes. [95] In automotive applications, single hollow extrusions pre- vail. Especially in low volume productions like prototypes or niche cars, more and more space frame body-in- whites are made from aluminum extrusions (Figure 31) [4]. This is mainly due to the fact that extrusions offer excellent cross section design possibilities to include additional functions together with the mere structural property of high moment of area inertia [96]. Due to low tool costs, straight profiles are more economical in low batch sizes compared to conventionally deep drawn double half shell workpieces like e.g. roof rails or cross members that require more expensive tools [4]. On the other hand, deep drawn parts offer a better material distribution because complex 3D parts can be manufactured whereas extruded profiles are symmetric on the longitu-

Figure 29: Frame and shell structures.

dinal axis. This symmetry restricts design options. Fur- thermore, deep drawn parts can be directly manufactured in a curved shape while extruded profiles usually require a subsequent bending operation to obtain a curvature. Here, expensive tools raise the minimum economical batch size. Magnesium is also taken into account for the application in frame structures. Although specific stiffness of aluminum, magnesium, and steel are alike, magnesium offers a considerably higher specific strength compared to steel and regular aluminum alloys. In case of window frames, seats, or supporting structures where tensile strength and bending stress is relevant, the use of magnesium can contribute to weight saving. Although a car body is designed rather according to stiffness considerations than to strength, magnesium is also used in a first research demonstrator. In the Volkswagen one-litre car (one litre fuel consumption per 100km equals 16.4miles per gal- lon), 36kg of magnesium as thin-walled casting nodes, extrusions, and sheets were employed thus accounting for 13kg of weight saving compared to an aluminum space-frame. [97] In contrast to aluminum and magnesium, steel cannot be extruded into hollow profiles with walls thin enough to meet car body requirements. Therefore, space frames made of laser welded steel tubes are considered [98]. Because of low material cost and inexpensive tools, the use of straight tubes is very economical. However, a maximum weight saving is only possible if high strength steel tubes are hydroformed to appropriately vary the cross section achieving best load adaptation. In addition, hydroformed dents can trigger deformation and therefore direct crash energy. Again, this subsequent forming operation uses expensive tools and requires a sophisticated process design. In the full production chain, 2/3 of the total cost are determined by the hydroforming operation 50% of which are caused by the tooling [99]. Therefore, a weight saving of 35% to comparable cars is only expected at a cost increase of 15% for a production volume

Figure 30: Tubular frame structure of the BMW C1

Figure 31: Profile based frame structure of the Ferrari F360 Modena [4].

up to 100,000 per year [98, 1001. In steel tube making, continuous and discontinuous processes can be distinguished. The conventional first variant continuously feeds a sheet strip from a coil into a profile rolling tool set. These endless tubes then are laser or high-frequency welded in-process and cut into customer size workpieces. This process is limited to uniform cylindrical tubes that, however, can be joined to tailored tubes with different wall thicknesses, materials, or material properties. The discontinuous second variant uses sheet metal blanks being bent in three steps on a press brake into open tubes that are subsequently laser welded. Due to lower tooling and investment cost, this process offers an economical benefit at lower batch sizes. Additionally, for a better load adaptation, conical tubes can be manufactured. By the use of tailored blanks, this technology is also suitable for the production of tailored tubes. Fur- thermore, the process can be applied to materials like stainless and high strength steel, aluminum, or titanium.

Advances in forming technology In contrast to the sheet metal half shells of conventional car body manufacture, the aluminum profiles already show a high stiffness. Therefore, the handling equipment cannot assure defined gap geometries by applying forces to the workpiece. As a prerequisite for automated aluminum welding, a maximum gap of approximately a third of the wall thickness is required. In case of the aluminum extrusions of the Audi A8, taking into consideration handling tolerances and welding distortion, straight and curved profiles must meet the required contour tolerances of 20.3mm. As conventional profile manufacturers cannot fully reach this requirement due to limitations in the extrusion process and bending deviations caused by springback, curved and even straight profiles in some cases have to be calibrated expensively by hydroforming. Achievements in forming technology by increasing the accuracy of curved profiles contributes to lightweight forming because additional cost especially in low volume production prevent lightweight components to be economically manufactured and used throughout the market. By employing curved tubes, simple bending operations can be integrated into the hydroforming process. Al- though the tube might wrinkle in the curvature radius during pre-bending while closing the die, this effect is eliminated by the main hydroforming process step. [98,

Complex 3D bending of tubes and profiles requires new process technology. One approach uses a fixed tool and a moveable die. The die is positioned in six axes by a parallel kinematics that is determined by the required bending space and resulting bending forces. By variably adjusting the die position to the axial feed of the tube, the workpiece can be bent in a variable 3D shape (Fig- ure 32). [I021 Another kinematic approach in flexible 2D and 3D bend-

[99, 1011

1001

Figure 32: Complex 3D bending of tubes [I021

ing of structural profiles and tubes uses a polyurethane matrix. A vertically adjustable rigid roll presses the workpiece against the matrix casing. The elastic matrix de- forms and thus bends the workpiece. By a longitudinal movement of the casing, a curvature is manufactured over the length of the workpiece. A variation of the roll adjustment and of the forces applied on each side of the roll results in a variable 3D curvature. The maximum length of the curved workpiece is however limited by the length of the matrix casing. [I031 In conventional stretch bending, the accuracy of shape can be improved by an adaptive process control. Usually, the springback is taken into account in the tool design so that the profiles are over-bent. With the assumed springback, the desired shape is achieved. But variations in the bending behavior resulting from different wall thicknesses as well as quenching or heat treatment conditions may lead to varying springback behavior. By measuring the applied forces over the tool movement during the first bending phase, the material and springback behavior can be estimated. As the springback is also determined by the axial tensile stress, an adaptation of the tensile force to the estimated springback behavior improves the shape accuracy of the bent profile (Figure 33). [104, 1391 An innovative extrusion process variant [ I 051 produces curved profiles directly at the press. The strand exiting from the die is inserted into a guiding tool. By moving the tool to a numerically controlled lateral position, a resulting force is applied to the profile. As a consequence, the profile exits the die in a rounded shape (Figure 34 left). The forming mechanism consists of two effects that take place (Figure 34 right):

Applied on the strand over the distance of the guiding tool from the die, the lateral force leads to a resulting moment on the material flow inside the die. This moment leads to pressure stresses on the inner side, and tensile stresses on the outer side of the profile. The lateral force leads to a higher surface pressure on the bearing and thereby a higher friction force on the inner side of the profile. The lower surface pressure on

Figure 33: Adaptive stretch bending [104, 1391

by rounding during extrusion are better than those of bent profiles:

High accuracy of shape is possible: As the process shows basically no springback, curved profiles with highest contour accuracy can be manufactured. Minimal cross-section deformation: The curvature of the profile is formed inside the die where the cross-section is still guided through the bearing. Therefore, the cross- section deformation of rounded and straight profiles are almost equal. Reduced residual stresses: As there is basically no springback at rounding during extrusion, no forming induced residual stresses remain. Only very low thermally induced stresses remain in the profile. Unreduced formability: Rounding during extrusion is an integrated forming process leading to no reduction of formability. Subsequent forming operations like hydroforming can therefore make use of unchanged material properties. Furthermore, even highest strength alloys like sprayformed aluminum or magnesium can be produced into curved profiles. [106, 1071

Still, the exact positioning of the guiding tool is crucial to manufacture profiles with extremely narrow contour tolerances. In order to obtain a process control for the tool position, a contact-free online radii sensor that deter- mines the curvature by means of three laser sensors has been developed. By variably CNC-adjusting the guiding tool over the extruded length, a profile with a variable curvature can be extruded (Figure 35). Resulting forces in the strand due to the acceleration by the tool can be calculated and compensated to a certain degree. [I081 This process is currently in development to achieve an industrially safe process chain including high precision curved extrusion, automated cutting and handling operations, and an extrusion press design dedicated especially to rounding during extrusion. [I091 As magnesium provides low ductility at room temperature (ref. 2.3), conventional cold bending of magnesium profiles if limited. However, a very fine grain size of about 5pm achieved by drawing and annealing of magnesium tubes allows for the bending at a radius-diameter-ratio down to 2.8 without fracture [IIO]. As an alternative, a warm bending process can be applied. Due to the particular forming mechanism by changing the material flow inside the die instead of bending the profile afterwards, rounding during extrusion also allows for the production of curved magnesium profiles, providing a very high potential for lightweight applications. Sheet metal members of a frame structure are conventionally deep drawn using a rigid punch and a rigid die. For double half shell workpieces, two sheets have to be individually formed using at least four rigid tools. In con-

Figure 34: Process principle rounding during extrusion the outer side of the profile leads to a lower friction force.

Both effects result in a velocity profile of the material flow that is different from conventional straight extrusion. The material velocity on the inner side of the profile is lower than on the outer side. This causes the profile to exit the die in a rounded shape. As a consequence, rounding during extrusion is not a bending process. Referring to DIN 8586, bending is defined as a forming process where plasticity mainly results from a moment applied. The contour radius of the curved profile hereby is solely determined by the position of the guiding tool in relation to the die. Process or material related parameters like extrusion temperature, velocity, or alloy generally do not influence the profile curvature. The position of the guiding tool necessary for a desired radius therefore can be geometrically calculated. While rounding during extrusion, plasticity results from the extrusion process itself, not from the lateral force. Therefore, the properties of curved profiles manufactured

Figure 35: Rounding during extrusion of a variably curved bumper (top left and bottom).

Originally stretch bent bumper (top right). Figure 36: Hydroforming sheet metal pairs - process

principle (left) and applications (right) [ I 1 I ]

trast, for hydroforming of sheet metal pairs, only two tools (upper and lower die) are necessary (Figure 36 left). Furthermore, trimming and joining of the sheet metal pair can be integrated in a single hydroforming process step leading to a more robust and shorter process chain. [ I 1 I ] As this process makes use of sheet metal blanks, it is also predestined for the manufacture of hollow lightweight components as substructures (Figure 36 right) or larger shell structures.

3.2 Shell structures In contrast to frame structures used for small- and medium-lot production, shell structures for automotive car body applications are established for large-lot production. In contrast to casting processes, only forming technology is able to provide large thin walled hollow components with a surface quality suitable for outer skin panels. As the material price accounts for about 50% of the total vehicle cost at large-lot production [lo], steel is commonly used. With the need for weight reduction particu- larly in the front of the car, more expensive materials like aluminum and even magnesium are considered for sheet metal applications. Although providing the same specific strength and stiffness, their lower density results in a higher sheet thickness at the same weight per area thus considerably increasing specific dent resistance and shell stiffness. Due to this shell related material properties, weight savings of around 50% compared to steel and 20% compared to aluminum can be achieved using magnesium in applications without strength requirement like front hoods, trunk lids, and doors [97]. Whereas in crash relevant components like a B-pillar bottom reinforcement contributing to crashworthiness especially in pole crashes, ultra high strength steel grades like CP 800 in case of the DaimlerChrysler S-class coupe are employed

Different studies have been carried out to investigate the feasibility of ultra lightweight car bodies. While the ULSAB consortium propagates the mono-use of steel (Figure 37 left) [113], Ford developed the P2000 as an all-aluminum car body in a shell structure design (Fig- ure 37 right) [4, 1141. With the demand to decrease costs in lightweight structures, sheet metal parts have to become larger (Fig- ure 38) as

[ I 121.

Figure 37: Full steel and full aluminum car body concepts [ I 13, 41.

Audi A8

Audi A2

Figure 38: Multi part (top) and single part (bottom) side panel [ I 151.

joining processes and auxiliary joining parts decrease, logistics and finishing operations get easier, and the process chain becomes shorter. [ I 12, 1151

By this on the other hand, the workpieces and their respective forming processes are getting more complex and difficult not only due to the size but also because of the use of tailored blanks. Whereas before, parts of different thicknesses were joined in the assembly, now single parts consisting of different wall thicknesses are used as semi-finished products (Figure 39) [ I 131. From the material used, specific forming problems arise in deep drawing and related processes. In aluminum concepts, close attention has to be paid to the specific forming behavior of aluminum. Aspects like adapted drawing depth, larger radii, and a homogenous feed are to be taken into consideration. [ I 151 Furthermore, aluminum is extremely sensitive to surface defects caused mainly by its high adhesion tendency and the deposit of workpiece swarf. Once the lubrication film discontinues, aluminum instantly adheres to the tool surface. Subsequently, this leads to grooves and scratches on the workpiece as well as to an increase in tool wear. Additionally, due to the sensitivity of aluminum to slight changes in the cutting clearance, the cutting punch can generate swarf that is pressed into the workpiece surface. The use of modern tool coatings, tool adjustment, and lubricants helps to prevent high scrap rates. [ I 121 Also in the processing of steel sheets, the use of high strength grades leads to significant challenges as higher tool stresses result. In order to prevent wrinkling, the binder has to apply higher forces causing relevant tool wear and making premium tool material, tool coatings, or even the use of ceramic inserts necessary. The high strength of the material is also responsible for an increase in springback that has to compensated by a progress in the use of FEM-simulations [ I 161. Aggravatingly, slight batch changes in the material behavior cause the springback to vary significantly thus requiring the opera- tor to compensate this effect by changing the binder force distribution or the local lubrication. [ I 121 Due to the mentioned lightweight aspects, much effort is put into investigating the forming of magnesium sheets. First warm rolled sheet metal blanks with a homogenous material structure of 15pm, an anisotropy comparable to mild steel, and an elongation up to 16.8% are industrially available though at a price yet much too high. Here, the development of a continuous cast rolling process prom- ises a less expensive production of the blanks. [I171 Despite low formability at room temperature (ref. 2.3), first demonstrator parts have been deep drawn at elevated temperatures without cracks like a door inner and outer panel. But customary magnesium sheets are currently not capable of meeting the corrosion resistance

Figure 39: Use of tailored blanks for side panel [ I 131

Figure 41: Different multipoint blankholders [121, 1221

Figure 42: Process principle of HMD (left) and HBU (right).

stresses. Using sheet metal parts as deep drawing tools, they are welded or diffusion bonded with the foam serv- ing as cover plates for the whole structure (Figure 40 bottom). Subject of current research is the determination of appropriate process parameters such as forming force, velocity, and temperature in order not to damage the structure. [60] For materials with limited formability or which require higher forming forces, deep drawing and its related derivatives provide different approaches for the manufacture of lightweight components. In conventional deep drawing, the use of multipoint blankholders permits elaborate control of the material flow in the flange region (Figure 41) [121, 1221. By an appropriate determination of process parameters over the process time in the different segments, wrinkling and cracks can be avoided within extended forming limits. In the same way, the use of active drawbeads increases the limiting drawing ratio [ 1231. Forming by using working media and their relation to lightweight construction have been extensively discussed in a recent keynote paper [124]. In pneumo-mechanical deep drawing, the pneumatic preforming can be used to cause additional work hardening in the sheet metal or to pre-distribute material for the subsequent deep drawing operation [ 1251. In hydro mechanical deep drawing (HMD, Figure 42 left), the die is replaced by a fluid [126]. In high pressure sheet metal hydro forming (HBU, Figure 42 right), it is the punch that is replaced [127]. HBU allows for an increased work hardening in sheet metal by a distinctive stretching operation. Furthermore, if warm forming is desired, the use of warm working media is preferable to warm rigid tools in both processes (refer 2.2, elevated temperatures). As an advantage of HMD, a higher limiting drawing ratio can be achieved because of the higher surface pressure in a larger contact area between punch and workpiece which enables higher drawing forces to be transferred. On the other hand, HBU allows for an arbitrary distribution of stretching and deep drawing portions over the draw depth. Furthermore, a better shape accuracy is attained compared the conventional deep drawing

3.3 Joining Depending on the geometry of the lightweight structure and the material used, different joining processes can be

[121].

Figure 40: Deep drawing of unfoamed sandwich sheet (top and middle) [120], deep drawing of foam

using cover sheets (bottom) [60]. and surface finish requirements placed on the vehicle- body outer skin. While formability and corrosion protec- tion are issues that are dealt with currently, the future success of magnesium sheet applications solely depend on competitive price and surface quality. [72, 971 As an alternative for magnesium sheet, sandwich panels comprising two thin layers either of steel or aluminum with an plastic layer in between are being developed. The aluminum sandwich has a density and shell properties comparable to magnesium but without the corrosion and surface quality problems. It offers good ductility at room temperature while some process restrictions apply. [ I 00, 98, 1181 High shell stiffness is required in the application of convertible cars. Here, the loss of the roof structure heavily affects the body stiffness. A comparable stiffness near the non-convertible version can only be achieved by currently 100-150kg of additional weight [ I 191. In order to decrease this high amount, shell elements with highest specific stiffness are essential. Here, metal foam structures on aluminum base possess a large potential. They are characterised by a very small density, high rigidity, good energy absorption, and good recycling behavior. Deep drawing of aluminum foam sandwiches into 3D parts can be done in two ways: Firstly, the unfoamed double sheet is deep drawn using regular tools (Figure 40 top and middle). In this case, the forming behavior of the sandwich sheet with the com- pressed aluminum powder in between compares to a standard aluminum sheet of identical wall thickness. After forming, the workpiece is preheated for about 40s, foamed within a tool set for 45s, and solidified in another 30s. A post-processing is not necessary in most cases. But indeed, process time is still too long to be industrially used in large scale under economical circumstances.

Secondly, at increased temperature, the forming of the foam itself is possible without damage of the foam structure due to the increase of ductility and reduced flow

[ I 201

Figure 43: Friction stir welding process principle (left) and deep drawn FSW part (right) [TWI].

applied. Joining by forming is an alternative to established resistance or arc welding techniques especially in case of limited fusion weldability. Mechanical welding processes like stir and inertia friction welding have advantages as a solid state process, clinching and riveting are also applicable to hybrid structures [128], and electro-magnetic forming in addition provides a high velocity and contact free forming principle.

Friction stir welding Developed by TWI, friction stir welding (FSW) uses a wear resistant rotating tool which moves along the joint between two components. The tool shoulder being in close contact with the surface plastifies the material be- neath while the tool pin traverses through the joint line thus creating heat by friction (Figure 43 left). As a solid phase process, FSW operates below the melting point of the workpiece material. It can weld all aluminum and magnesium alloys, including joining dissimilar alloys and those materials that cannot be conventionally fusion welded such as aluminum-lithium alloys. No shielding gas or filler is required. Material properties of welded aluminum alloys show tensile strength similar to the parent material after heat treatment although full elongation is not restored. [129, 1301 As the weld seam still shows good formability and energy absorption for crashworthiness, FSW sheet metal blanks can be easily used as tailored blanks for deep drawing (Figure 43 right) or spinning [131].

Inertia friction welding Spindles as a chassis component serve as the main f’nterface between non-driven wheels and the suspension system. Traditionally, spindles are manufactured by

machining a single-piece steel forging or

joining a machined steel shaft to an iron spindle The spindle body, however, can be produced in aluminum with a 30% weight reduction while maintaining all structural requirements. Inertia friction welding was examined as an alternative joining method. In this process, one component is held stationary while the second is rotated at a controlled velocity. The faying surfaces contact each other under the applied pressure and create heat. The aluminum spindle body becomes plastic at the interface, filling the gap to the shaft. [132, 1331

Electro-magnetic forming In electro-magnetic forming, the energy of a pulsed magnetic field is used with a contact free tool to join metals with a good electrical conductivity, such as aluminum. The sudden discharge of a high voltage capacitor through a tool coil causes the generation of an intense magnetic field inside the coil. This magnetic field increases within a few microseconds up to its maximum so that, in turn, an eddy current in the workpiece is induced generating a second magnetic field reversely directed to the tool coil field. The forces acting between tool coil and

m- -- Jdnhp- L -- ton -1 m-1

Figure 44: Electro-magnetic forming - tool coil with workpiece (left) and

different joining principles (right) [135, 1361

Figure 45: Structural joining B-pillar to rocker (left), prototype vehicle (right) [136].

workpiece are determined by the current density and the magnetic flux density. [I341 Amongst others, tubular components can be narrowed or expanded by this forming process. This forming can be used to join two workpieces (Figure 44). As only one of the parts has to be primarily formed, the other workpiece to be joined may consist of an arbitrary material. There- fore, electro-magnetic joining offers advantages in the joining of hybrid structures. As electro-magnetic forming is a process where extreme strain rates of IO4s-’ and above can be achieved, FEM- simulations cannot make use of the generally employed material behavior data gained by e.g. tensile tests. In order to overcome this restriction, it is proposed to determine relationships between stress and strain and, even more important, between stress and strain rate by an iterative recursive calculation method matching calculated deformations to measured ones obtained through an electro-magnetic tube compression forming. [ I 351 As electro-magnetic forming offers advantages as a cold, fast, and clean process that supports a flexible assembly of modular car body structures, this joining technique has been investigated for feasibility in an a-class car at Ford (Figure 45). [I361 4 NEED FOR FURTHER RESEARCH With the effort of achieving more lightweight components, it can be observed and anticipated that materials with hcreasingly worse forming behavior will have to be dealt with, like

high strength steel and aluminum, magnesium, titanium, metal foams, and compound materials like e.g. sandwich panels or metal

Additionally, new alloys and material compositions will arise. As a common challenge, those materials exhibit decreasing ductility at increasing forming forces. The application of warm forming operations or superplastic forming therefore will have to continue as these techniques assure the extension of forming limits. Eventually, localization is a potential solution. Local forming like e.g.

matrix composites.

incremental forming or rotary forging, local heating, or local heat treating achieve higher strains. Meanwhile, new and yet uncommon materials for some products will allow new designs that cannot be manufactured with conventional material e.g. the growing use of stainless steel of titanium in applications where carbon steel is widely employed. At the same time, more complex shapes will have to be manufactured by forming processes as a consequence of an integrative lightweight construction. Only workpieces which are ideally adapted to the given load distribution and which use the best material available will succeed in lightweight construction. Forming processes here will have to ensure feasibility. Furthermore, in order to attain optimal load adaptation of producible workpieces, a combined product and process design by means of finite element simulation or the use of bionic methods is favor- able. This includes concurrently the use of more complex semi-finished products like tailored or hybrid parts. How- ever, those products require an increased process knowledge and the observation of different material behaviors. Here, adaptive processes and tools and the yet increasing use of simulation software is advantageous. Variations in manufacturing processes lead e.g. for extruded profiles to

variations of about 210% in wall thickness because of

variations of about 210% in material properties be-

Designing a lightweight workpiece, these variations have to be observed as a worst case assumption hence giving away potential 20% of weight saving. By achieving close tolerances in manufacturing processes and in the prediction of the workpiece behavior e.g. effects of differences in work hardening, additional lightweight potential can be activated. This also relates to increasing demands for close-tolerance semi-finished products accompanied by the necessary knowledge of the specific product history. The need for high accuracy forming faces a steady growth also due to other reasons and therefore becomes growingly relevant. However, it prerequisites appropriate modeling as well as the acquisition of more exact and better suiting material properties. Finally, the reduction of component weight alone will not be sufficient in the future as the benefit of less weight usually does not justify the often associated increase of cost. On the one hand, the reduction of cost e.g. by shorter or more flexible process chains is crucial. Mean- while, especially ultra lightweight components require flexible processes as they are generally manufactured in smallest batch size due to their limited applicability. On the other hand, added value beyond the mere reduction of weight will legitimize the use of more sophisticated processes, materials, and products. This potential added value can comprise issues such as:

safety, size, or durability.

The aluminum steering column provides driver safety as it bends sideways in a crash accident. Safety is furthermore increased if higher crash energies are absorbed as in case of the stainless steel bumper beams. The use of titanium springs, aluminum wheel suspensions, stainless steel tanks, or precision forged gears allow for a smaller package and a better product function i.e. larger volume and thus range in case of the tank. Using titanium for heat shields of a catalytic converter e.g., increased dura-

the extrusion process and

cause of the quenching conditions.

bility can decrease maintenance costs if prolonged warranties are considered. 5 CLOSING REMARKS Forming technology can substantially contribute to lightweight construction. This paper describes necessities and functional aspects if lightweight construction as well as the common problems in manufacturing lightweight materials, semi-finished products, components, and structures. It is pointed out how load adaptation is the central key to success. Therefore, a wide range of solutions are discussed in order to overcome limitations in forming. Here, innovative processes play a major role. Finally, fields of potential further research are identified and discussed. 6 The authors would like to give special thanks to the fol- lowing persons who have contributed to this paper (CIRP members denoted by *: N. Bay* E. Doege* H. Flegel F. Gabrielli* P. Groche* J. Jeswiet* M. Kiuchi* R. Kopp* R. Neugebauer* K. Osakada* K. Sieged* REFERENCES

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