Direct Chill Billet Casting of Al Alloys

SECTION 1

INDUSTRIAL PERSPECTIVE

Casting technology is developing rapidly, driven by a combination ofimprovements in understanding of underlying solidification theory,increased computer capacity for design simulation and on-line control, andenhanced industrial competition in increasingly globalized world markets.Wrought products are often dominated by the quality and yield of primarysolidification of ingot and billet feedstock.Moreover, many industrial sectorsare developing rapid prototyping and agile manufacturing processes toreduce time to market and speed up responsiveness to customer demands.Casting has much to offer as a near net shape technology, but importantissues of reproducibility and quality need to be improved. The overall indus-trial perspective on casting technology is discussed in this section, concentrat-ing on recent innovations and challenges for the future.

Chapters 1 and 2 discuss respectively semi-continuous and continuousmethods of casting aluminium alloys, and chapter 3 discusses continuouscasting of steel. Chapters 4 and 5 discuss different aspects of the importanceof casting in the important automotive sector.

© IOP Publishing Ltd 2003

Chapter 1

Direct chill billet casting of

aluminium alloys

Martin Jarrett, Bill Neilson andEstelle Manson-Whitton

Introduction

This chapter details the operational and technological developments of thedirect chill (DC) casting process for a high volume commercial extrusionbusiness. Other continuous casting processes are discussed in chapters 2and 3. Modelling of DC casting is discussed in chapter 6. Improved extrusionmanufacturing efficiency is driving the need for better and more consistentbillet quality. This has necessitated significant technological and processdevelopment of DC billet production, in order to produce extrusion ingotsof predictable performance. A thorough understanding is required of theinteraction of the equipment and process technologies that impact on themetallurgical macro- and microstructure of the DC cast ingot, and itssubsequent performance in the extrusion process.

The first commercial semi-continuous casting machine for aluminiumalloys was opened in Germany in 1936, following development of verticalcontinuous casting techniques for other metals, such as lead, since themid-nineteenth century [1]. Over the past 50 years, the DC casting processhas been developed to become the predominant process for extrusionbillet production [2, 3], with 6xxx alloy extrusion billet accounting for alarge percentage of the throughput of worldwide aluminium DC castingproduction.

Together with market driven advancement, environmental considera-tions are driving significant change in DC casting technology. The provisionof consistent high quality DC cast billets of predictable performance isof fundamental importance in operating extrusion presses at maximumefficiency, while meeting the stringent quality requirements of themarket place. Several key factors affecting billet quality, that impact onextrusion performance, have been previously described by Weaver [4] and


Langerweger [5], and more recently by Bryant and Fielding [6]. Theseemphasize the importance of characterization and control of the totalprocess. A schematic representation of a typical DC casting process isshown in figure 1.1. The key process stages form the basis of the subsequentsections of this chapter, which describe the critical aspects of both equipmentand process technology.

Direct chill billet production

The DC casting process frommelting, through melt in-line treatment, castingand homogenization is discussed in terms of the present best practiceand promising novel techniques. The impact of best practice on billetquality and ultimately the impact on extrusions are also then discussed,utilizing the extrusion limit diagram concept, the critical aspects of whichhave been summarized by Parson et al [7]. All discussion is in the context

Figure 1.1. Schematic representation of a typical direct chill casting process.

4 Direct chill billet casting of aluminium alloys


of a high throughput DC casting operation, concentrating on 6xxxseries alloys.

Melting

Current standard DC casting facilities use melting furnaces which arecharged with a mixture of primary and secondary scrap aluminium depend-ing on target billet specification. The molten metal is transferred to aholding furnace before casting. The use of a holding furnace maximizesefficiency by fully utilizing melting time. The ideal configuration balancesthe melting capacity of the furnaces with the casting capacity of the DCcasting machine. Alloying additions are made in the melting furnace, inthe holding furnace, or during laundering to the holding furnace. Bettermixing is achieved if alloying additions are made earlier, although there isa danger of significant loss if additions are made to the melting furnace,and, unless additions can be made with the furnace door closed, productiontime may be lost.

Melting and holding furnaces can be either induction or gas heated.Gas is preferred for efficiency, and for melt cleanliness as the churningresulting from induction heating can drag oxide particles from the drossback into the melt. More recently oxyfuel furnaces, burning gas withapproximately 10% oxygen, have been introduced, although a well-controlled gas furnace, incorporating either regenerative or recuperativeair heating systems to maximize fuel efficiency, remains the industrypreference.

Furnaces can be fixed hearth or tilting. Fixed hearth furnaces have lowercapital cost but tilting furnaces are preferred for metal cleanliness, processcontrol, and safety, as at any point the flow can be stopped by resettingthe furnace, whereas a fixed hearth furnace requires manual plugging.Older fixed hearth furnaces are generally being replaced with tilting furnaces.A further advantage of tilting furnaces is that they can be fully drained,allowing greater flexibility for alloy changes.

Together with other areas of the DC casting process, the environmentalimpact of furnaces is being minimized by the reduction of particulateemissions through more efficient furnace design (for example, the use ofregenerative burners) and, where necessary, the use of equipment for thecapture of both particulate and noxious gaseous emissions.

Temperature control is of paramount importance in casting, having adirect bearing on product quality and production efficiency. The optimummelt delivery temperature for 6xxx alloys is in the range 690–7508C, and isproduct and plant specific. Temperature measurement is predominantlythrough the use of thermocouples. Other methods of temperature measure-ment such as optical pyrometry are used, but encounter problems with

Melting 5


aluminium due to oxide skin formation. Immersed thermocouples, how-ever, remain the most accurate, robust and reliable technology. Heatingequipment is universally thermostatically controlled, to maintain thermalefficiency and process control.

Molten metal pre-treatment

Molten metal pre-treatments, carried out in the melting or holding furnace,can be distinguished from in-line treatments given during laundering of themelt to the casting machine. The main purpose of fluxing [8] is to clean themelt by degassing and to remove oxides and other inclusions. Other advan-tages of fluxing include the production of a dry dross, which minimizes metallosses during skimming. Fluxing, together with more recent developments indross reprocessing (pressing and recycling), has led to improved recoveries inthe industry, whilst maintaining cleaner and more efficient melting units.

Currently, two methods of fluxing are available, using chlorine gas orchlorine-based salt. Injection of gas (sometimes using a spinning nozzle)below the surface of the melt has until recently been the preferred method.However, environmental legislation is now necessitating the phasing out ofchlorine gas use at the melting stage. The use of fluxing salts, although amore mature technology than gas fluxing, is now being re-evaluated as areplacement for chlorine gas. However, there are still environmentalconcerns over the chlorine and fluorine reaction products produced whichremain in the dross, and the uncertain determination of potentially hazar-dous products of reaction emitted to the environment from the use ofthese fluxes. The need to reduce and eventually eliminate the use of chlorinein fluxes has led to a gap in the market for an alternative environmentallyfriendly fluxing method. Currently there are no processes yet capable ofcommercial operation.

In-line metal treatment

Certain melt treatments, namely grain refinement, degassing and filtration,must be given in-line during laundering to the casting machine to accruemaximum benefit. Grain refining inoculants have in the past been added tothe holding or even melting furnace. The disadvantages of this methodinclude fade (where the effectiveness of the grain refiner decreases withtime), and the formation of a boron-particle-rich sludge in the bottom ofthe furnace which contaminates the metal, and leads to through-lengthvariation in cleanliness and composition of the DC cast log. Degassing andfiltration are performed in the launder such that there is minimum turbulentflow, which can cause the reintroduction of oxides before the casting



machine. Recent technological developments of these techniques have beenreviewed by Fielding and Kavanaugh [9], who emphasize the criticality ofdegassing and filtration in the DC casting process.

Grain refinement

The purpose of grain refinement is to produce a refined, equiaxed grain struc-ture with modified second phase particle morphology through the thicknessof the DC cast log. Nucleation and grain refinement are discussed in detailin chapters 12 and 13. Current practice is to use approximately 10 ppm oftitanium to give the most desirable grain size of approximately 80–150 mm.There are a number of alternative grain refiners available, all based on Al,the most common being 6%Ti, 3%Ti, 5%Ti–0.1%B, 5%Ti–1%B and5%Ti–0.2%B. All these compositions are in commercial use for a varietyof applications and products. Best metal cleanliness is achieved if grainrefiner is added before de-gassing or filtration. Figure 1.2 shows a systemfor injection of grain refiner rod into the launder, which can be controlledautomatically to give the desired rate of addition.

Universally the 5%Ti–1%B grain refiner is the preferred inoculant [10].As a result of this it has been extensively researched, leading to a number ofpublications discussing in detail its capability as a grain refiner under variousconditions [11,12]. Its difficulty, however, is the high boron content. Boron isan insoluble element and boron particles are prone to flocculate, forminglarge clusters which can be deleterious to products in the form of pick-up,and as an abrasive to dies. To counter the problem of boron inclusions, itis advised to inoculate at a reduced rate that accounts for the boron contentof the recycled aluminium.

Figure 1.3 shows the effect of titanium content on grain size for a typical5 :1 grain refiner achieving the target grain size. In addition to grain sizemodification [13], experiments have shown that the morphology of theinsoluble iron-rich phase can be influenced, not only by the grain refinercomposition, but also by the manufacturing route used by different suppliers.(The presence of the iron-rich phase in the form of a-script has been shown toaffect adversely extrusion surface quality by increasing pick-up [14]). Thiswould indicate that the nucleation mechanism of the iron-rich particles isstrongly influenced by the grain refinement process. This effect is currentlybeing investigated to gain an understanding of the process. Figure 1.4 is aplot of a-script occurrence for different grain refiners, and shows that thechoice of grain refiner can have a significant effect on the amount of a-script in the microstructure.

The most promising new grain refiner currently available is titaniumcarbide [15–17], which, in addition to eliminating boron contamination,is reported to overcome problems of poisoning by zirconium- and

Grain refinement 7


Figure 1.2. System for automatic injection of grain refiner rod into the melt during

laundering to the casting machine.

Figure 1.3. Average grain size (determined at mid-radius) as a function of total melt

titanium content for a 180mm diameter 6063 billet following in-line inoculation with

5% Ti–1% B rod.



chromium-containing alloys, yet is as effective in reducing grain size as tita-nium boride [15]. Closer temperature control is, however, required in usingtitanium carbide and thus commercial implementation may require adjust-ment to processing control. Individual plants are currently evaluating theuse of titanium carbide. Other novel methods of grain refinement includeNb additions [18], and physical methods (which retard formation of andbreak up dendrites) such as ultrasonic vibration [19], sump displacementand electromagnetic stirring [20].

Degassing

Most high-quality cast houses now use in-line spinning nozzle degassingsystems for the removal of hydrogen. The most common systems are theAlpur (Pechinney) and SNIF (Foseco). However, some patents are nowending, and enterprising companies are designing their own systems basedon existing and new technology [9]. An example of an Alpur degasser isshown in figure 1.5, and a schematic of a SNIF degasser is shown in figure1.6. Efficiencies of spinning nozzle type degassers are very high, and theycan deliver hydrogen levels of less than approximately 0.1 cc/100 g, comparedto previous levels of approximately 0.3–0.4 cc/100 g. Figure 1.7, for example,shows the hydrogen content of a melt measured before and after passingthrough the Alpur degasser shown in figure 1.5.

Both Alpur and SNIF systems, when using chlorine gas, have also beenshown to reduce oxide inclusions by approximately 50%. They will reducethe overall volume fraction of inclusions, and through the stirring action

Figure 1.4. Number of a-script particles per mm2 for different grain refiners produced by

three different suppliers.

Degassing 9


Figure 1.5. The Alpur degasser in use at British Aluminium Extrusions, Banbury, UK.

Figure 1.6. Schematic of the SNIF R-140 degasser showing two chambers.



will break up coarse borides. However, as a result of this, the number densityof particles can increase. Improved process control using multi-chamberSNIF and Alpur systems, incorporating two-way metal flow and controlledargon and chlorine gas mixtures have, however, demonstrated significantlyimproved levels of particle removal. Figure 1.8 shows inclusion levels of

Figure 1.7. Hydrogen content of 6xxx alloys measured before and after passing through an

Alpur degasser.

Figure 1.8. Inclusion levels of 6063 before and after passing through an Alpur degasser.

Degassing 11


6063 casts before and after passing through the Alpur degasser shown infigure 1.5.

Filtration

There are a number of filtration methods in use throughout the industry [9].The most common methods are those that rely upon oxide entrapment by thetortuous route the aluminium must take as it travels though the filtrationmedium. These methods can be classified as ceramic foam filters (CFF),Alcan bed filter (ABF) and porous tube filter (PTF). Glass cloths are alsoused but remove only the coarsest particles, and their primary purpose isto give a uniform distribution of metal during the start of casts using thedip tube and float configuration.

The Alcan bed filter is viewed as being a particularly efficient system,and comprises a layered bed of graduated ceramic spheres through whichthe metal passes. Particles are trapped at the interstices between the spheres.Ceramic foam filters work in much the same way as the Alcan bed filter, butare less effective because of the shorter distance through which the metalflows. They allow flexibility as they are discarded after each cast, and arewidely used as secondary filters. The newest filtration development,shown in figure 1.9, is the porous tube filtration system [21], which filtersmore effectively than the Alcan bed filter or the ceramic foam filter systems.Porous tube filtration is currently used for quality-critical finished productssuch as photocopier tubes. Figure 1.10 shows a comparison of inclusioncontent following filtration with (a) a ceramic foam filter and (b) aporous tube filter, showing an order of magnitude difference. The Alcanbed filter and the porous tube filter will eventually block, and therefore acontinuous maintenance programme is required in order to sustainmaximum performance, and these are best used in conjunction with anin-line degasser.

Figure 1.9. Schematic of a porous tube filter. Metal flows into the box and is filtered

through the walls of a cartridge of porous tubes.



Casting configuration

The three main casting configurations in commercial use are dip tube andfloat, hot top, and airslip [3]. Within each of these general systems, manyvariations have been individually developed. Figure 1.11 shows an illustra-tion of the casting configuration for a typical dip tube and float system [22].Although this system has been in use for much longer than hot top andairslip and is gradually being replaced, thousands of tonnes of good qualityaluminium billet for a wide diversity of markets are still cast through thissystem annually. The dip tube and float system remains popular becauseof its low capital cost, ease of maintenance and fast turnaround in castingpreparation. Billet can consistently be produced with desirable macro- andmicrostructure. Good surface quality can be achieved using either grease orcontinuous lubrication. Mould type is critical, and aluminium moulds musthave a sustained maintenance programme to ensure consistent surfacequality, whereas graphite-lined moulds require minimal maintenance toachieve good surface quality, and improved shell. The drawbacks of thedip tube and float system are partly a result of the turbulent metal flow,leading to oxide entrainment, and occasional blocking of the dip tube.The other major disadvantage of the system is the unavoidable presenceof a shell zone, which must be minimized to reduce the impact on extrusionrecovery.

Figure 1.10. Inclusion content following filtration by (a) ceramic foam filter (average

inclusions �40K/kg), and (b) porous tube filter (average inclusions �0.1K/kg).

Casting configuration 13


The hot top system, shown in figure 1.12 [22], reduces turbulent flow butdoes not eliminate the shell zone and its associated problems. Moulds tend tobe either graphite or ceramic for ease of maintenance, and are again lubri-cated by either grease or continuous lubrication systems depending on castlength. In both dip tube and float and hot top configurations, the deleteriousshell zone can be minimized by casting with a low metal height in the mould

Figure 1.11. Schematic of the dip tube and float configuration for DC casting [22].



and using an appropriately high casting speed. Under optimum conditionsthe shell zone can be controlled to a thickness of less than approximately10mm, but cannot be eliminated.

The shell zone, shown in figure 1.13 for conventional dip tube and floatcasting, results from the region of slow cooling within the mould, between themeniscus and point of water impingement, where the air gap acts as effective

Figure 1.12. Schematic of the hot top casting configuration for DC casting [22].



insulation. Generally, the deeper the mould the longer the air gap and thedeeper the shell zone. The airslip system, shown in figure 1.14, was developedin response to this issue, and is so named because the annulus around whichthe billet is formed is a pressurized system, and the metal flowing into themould does not touch the mould but is suspended on an air/lubricant filmover which it slips. Solidification in an airslip mould then is not influencedby heat transfer through the mould but through the air/lubrication gapand, together with a short mould, the system effectively eliminates the shellzone, and minimizes surface inverse segregation. Figure 1.15 shows the

Figure 1.13. Microstructure of an as direct chill cast (dip tube and float) 6082 billet (a) at

the edge showing inverse segregation, (b)�5mm from the edge showing large grains in the

shell zone, and (c) in the centre of the billet.



shell zone in conventional dip tube and float DC cast billet, and eliminationof the shell zone using airslip technology. Figure 1.16 shows the shell zonethickness as a function of effective mould depth [23].

Increased billet quality, with resultant improved extrusion recoveriesfrom reduced discards, through the use of airslip is achieved at the expenseof a higher capital cost and maintenance. Balancing of the pressurizedsystem can, however, be problematic, and gas, whilst always shown as escap-ing downwards, can under some conditions escape upwards, leading to

Figure 1.14. Schematic of the airslip DC casting configuration.

Figure 1.15. Anodized sections through 180mm DC cast billet: (a) conventional dip tube

and float showing shell zone, and (b) airslip with no shell zone.



oxidation of the molten metal stream feeding the mould. The system is under-going continuing development by suppliers and will certainly play an increas-ingly major role in DC casting in future.

Casting speed

Casting speed is optimized for both productivity and quality. Cast speedsvary with ingot size and alloy type from 50 to 150mmmin�1 in the generalingot size range from 400 to 150mm diameter. Start speeds are nominally10% lower than the final cast speed and are controlled to lessen the effectof high-stress-induced centreline cracking at the start of the cast. This isalso controlled by careful design of stool shape. Ramping to the finalspeed is PLC controlled.

Cooling

Water is the universal coolant for all DC casting systems. It must be suppliedat a rate which continuously removes the film of boiling water adjacent to thebillet skin, promoting nucleate boiling to give a continuous, uniform coolingof the ingot [24]. Systems have been developed to slow cool at the start of the

Figure 1.16. Shell depth as a function of effective mould depth for conventional configura-

tions, and airslip which can eliminate shell entirely [23].



cast and reduce billet internal stresses [1]. Alcoa, Alcan and Reynolds havedevised their own methods (CO2, pulsed water, and air mixture respectively)[1, 3]. These systems rarely extend beyond the influence of their plants.Universally, the water flow rate for billet production is in the range 0.8 to1 gallon per minute per inch (1.42 to 1.77 litres per minute per cm) ofmould circumference. Temperature ranges of delivered water can varydepending on supply from ambient (0–258C depending on season) toconstant temperatures of approximately 258C where control or recycling isused. Water temperature is an important variable in achieving microstruc-tural control. Water composition is rarely viewed as a controlled factor,but the influence of composition is considered to be a potentially importantfactor. Contamination by lubricants is controlled for environmental reasonsonly, although it may have an influence on billet cooling rate, and thereforeon microstructural development.

Analytical systems and process control

All casting systems are now controlled by PLC systems. The list of factorswhich can be monitored and controlled is extensive. As computing powerincreases, so does the amount of information that can be collected andstored. This information can be used, not only for on-line monitoring, butas a valuable management tool for continuous improvement.

An example of what can be measured as metal flows though a castingplant is as follows: charge weight and make up; charging time; meltingtime and time at temperature; transfer temperature, time and weight;casting temperature; metal level; stool height in the mould (ram position);degassing parameters such as gas pressure, speed, and temperature; furnacetilt; casting speed including start speed and ramp time; cast time; laundertemperature; water temperature and flow rate; inoculation rate; log length,weight, and number; homogenizer time in, temperature, time out; and cool-ing rate following homogenization. Modern systems also provide conditionmonitoring of key equipment so as to provide early warning of potentialequipment failure and to aid maintenance and total productive maintenance(TPM) activities.

Melt composition and temperature are probably the two most crucialparameters in DC casting which must be measured and controlled. Compo-sitional control starts with charge composition, which varies with the amountand quality of scrap and primary metal used. For cost efficiency, the greatestquantity of scrap of a known quality is used which still maintains final billetquality. Primary aluminium (to purity specification based on a combinationof iron and silicon content, e.g. 99.7% Feþ Si ¼ 0.3% max.) and sometimescommercial scrap, make up the rest of the charge. Following melting, asample is taken from the furnace and analysed; compositional adjustment

Analytical systems and process control 19


is then made by introduction of primary aluminium, to dilute elements inconcentrations greater than alloy compositional limits impose, and/ormaster alloys which contain high concentrations of alloying elements, tobring the composition up to specification.

Master alloys are available as element-rich aluminium ingot, element-rich tablets, or ‘pure’ alloys (e.g. magnesium, silicon). The choice isdependent on furnace size, and method and position of addition. To increasemelting capacity, additions can be made in the launder or holding furnace,although this requires an initially dilute charge, as alloying element con-centrations can only be increased using this method. Control of elementswithin most alloys is nominally �0.03%. However, extrusion customersare increasingly specifying both tighter tolerances and capping of deleteriousimpurity elements to attain their own process consistency.

Homogenization

Homogenization of DC cast billet is necessary to achieve an optimizedmicrostructure for ease of extrusion. An ideal homogenization treatmenteliminates microsegregation, modifies the insoluble particle morphology,and on cooling precipitates a fine dispersion of Mg2Si which, by removingmagnesium and silicon from solution, lowers the flow stress of the billetbut allows full redissolution during extrusion. Homogenization is typicallyperformed at temperatures between 500 and 5958C for times of 1–4 h, anddepends upon the degree of microstructural refinement and homogeneity

Figure 1.17. Typical batch homogenizer for aluminium DC cast billets [27].



required. Following the high temperature soak, air cooling or water quench-ing is used, and a controlled cooling rate is critical in achieving the idealMg2Si size, morphology and distribution [25, 26].

Traditionally, batch homogenization is used, shown in figure 1.17, withthe disadvantage of temperature variations from log to log and along thesame log with position in the furnace, leading to variations in microstructureand mechanical properties from billet to billet. Recently, commercial fullyautomated continuous homogenization furnaces have been introduced,shown in figure 1.18 [27], which virtually eliminate many of the problemsassociated with batch homogenization. In the future, continuous homogeni-zation will allow delivery to the extruder of billet with microstructure andflow stress controlled to within very tight limits.

Summary

Extruders demand not only high quality DC cast billet, but also consistencyof quality and mechanical properties (most notably flow stress) from billetto billet and batch to batch. All technological development is aimed atachieving this, while keeping costs low and ensuring environmental targetsare met. Consistently high billet quality achieved through controlledproduction allows maximum recovery through elimination of quality basedrejections and minimization of top and tail discard and scalping (ifrequired).

Figure 1.18. An example of a continuous homogenizer, allowing greater control and

flexibility than a batch homogenizer [27].

Summary 21


Developments in equipment and process technology have provided thepotential for significant improvements in billet quality. The most significantof these developments are:

. airslip mould technology

. in-line multi-chamber degassing systems

. grain refinement, e.g. TiC, ultrasonics

. porous tube filtration

. continuous homogenization.

The optimization of DC cast billet requires a holistic approach to thecontinual development of the technology through

. the mechanistic quantification of the critical process stages throughlaboratory scale optimization and in plant trials

. an effective technology transfer process translating innovative technologyto robust industrial practice

. a proactive in-plant continuous improvement culture

. precise characterization of extrusion equipment and process capabilitieswith effective evaluation and performance measures.

Microstructural optimization

The ability to manipulate the as cast microstructure through controlledsolidification and grain refinement techniques, coupled with an ability toapply accurate heating and cooling cycles to individual logs during continu-ous homogenization, gives the capability to optimize billet microstructure forspecific products or processes. Ultimately the need for a homogenizationtreatment, separate from billet pre-heat, may be negated if sufficient micro-structural refinement is achieved during casting. This will only be achieved,however, through a fundamental understanding of the nucleation andgrowth mechanisms taking place during solidification under the influenceof (i) temperature and time, (ii) alloy chemistry, (iii) inoculant chemistryand (iv) physical turbulence, e.g. deliberate dendrite breakage. Moreover,the effective removal of the shell zone using airslip technology provides theextruder with an increased capability of achieving maximum recovery bydrastically reducing extrusion billet discard size.

In-line metal cleanliness

Removal of hydrogen, residual alkaline metals, and non-metallic inclusionsusing the next generation in-line degasser systems may remove the need forfinal filtration other than for the most critical applications. Control of



molten metal turbulence throughout the casting cycle provides furtheropportunity for cleanliness improvement, by reducing the formation andentrainment of oxide inclusions. Considerable benefits, for example instructural integrity, have been achieved for foundry casting by minimizingoxide formation through carefully designed runners and gating systems.Full characterization of the DC casting process is required to understandfully the potential improvement to be derived from better metal flow. Inorder to produce the cleanest metal, however, best available technologywould suggest the incorporation of the porous tube filter in conjunctionwith an in-line degassing system.

Impact on extrusion

With a supply of reliable high quality billet with tailored microstructure andconsistent flow stress, the extruder can operate presses at maximum efficiencyand minimum cost. Figure 1.19 shows a typical speed and surface qualitylimit diagram for extrusion of 6063. Such extrusion limit diagrams are avaluable aid in evaluating extrusion and maximizing productivity withinthe bounds of avoidance of surface defects, specific pressure requirements,and attainment of mechanical properties. Figure 1.19 shows that maximumproductivity is achieved by extruding under conditions at the apex of thetriangle bounded by insufficient pressure, and inadequate surface. Thus,high quality and consistent billet can increase extrusion productivity intwo ways. First, the position of the apex of the triangle in figure 1.19 canbe raised by improved billet microstructure leading to lower susceptibilityto tearing and pickup. Second, more consistent billet properties allow press

Figure 1.19. A typical extrusion limit diagram for 6063.

Impact on extrusion 23


operation much closer to the theoretical maximum speed without the risk ofdefects leading to expensive quality rejections.

In addition to maximizing productivity, consistent high quality billetallows the extruder to supply a more consistent product to their customers,while maximizing recovery due to improvements in planned and unplannedscrap. Assurance of quality also allows the extruder to design to tighterlimits, for example reducing wall thickness while maintaining mechanicalproperty requirements and improving through-batch consistency for theautomotive market. Mechanical property requirements of �5MPa proofstress variations within a one tonne batch of extrusions can be required forcertain critical applications. These stringent requirements are becomingincreasingly common, putting increased demands on the extruder andbillet suppliers’ ability to improve process control and to achieve predictableperformance.

Extensive programmes involving continuous improvement, total produc-tive maintenance, total quality and manufacturing excellence schemesunderpin many of today’s successful businesses. These must also be coupledwith a strong research and development capability in order to provide thefundamental science necessary for full exploitation of the technology.Indeed, without these processes, optimization of the technological advanceswill be stifled. It is therefore essential, in order to maximize the impact oftechnology within the business, that these processes be undertaken simul-taneously with effective communication and technology transfer processesin place.

References

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Direct Chill Billet Casting of Al Alloys

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