Assignment 3: Select materials and processing for a specified product. Tasks From the list products below, select one from trade and one other. Then answer the questions below:

High Flow Fan (vacuum cleaner)- (Manufacturing) Safe Pressure Vessels- (Mechanical) Turbine Blade for a Jet Engine- (Aerospace)

1) For your chosen products, carry out an analysis of what each has to do in terms of its function and identify the most important material property characteristics required. 2) Consider the design of each product and suggest the most appropriate processing method. Carry out a material selection exercise to identify the most suitable material. 3) Identify constraints on your material selection choices imposed by processing.

Your completed assignment should be: Technically and professionally presented, together with various calculations and

diagrams etc. Word processed Contain a bibliography Contain between 1800 and 2000 words.

Ray Gibbons April 2009

From the list products below, select one from trade and one other. Then answer the questions below:

High Flow Fan (vacuum cleaner)- (Manufacturing) Safe Pressure Vessels- (Mechanical) Turbine Blade for a Jet Engine- (Aerospace)

1) For your chosen products, carry out an analysis of what each has to do in terms of its function and identify the most important material property characteristics required.

2) Consider the design of each product and suggest the most appropriate processing method. Carry out a material selection exercise to identify the most suitable material.

3) Identify constraints on your material selection choices imposed by processing.

1. Safe Pressure Vessels.

A pressure vessel is a closed container designed to hold gases or liquids at a pressure substantially different from the ambient pressure. The vessel can be subjected to internal pressure contain liquids, gases or powder, such as fire extinguishers or simply an aircraft accumulator containing O2 or N2. Aircraft cockpit can also be classified as a pressure vessel.

PV’s are potentially hazardous if it exceeds its design limits through either high internal or external or excessive temperature ranges, leading to failure. This can be as little damage as an exploding coke can or an exploding oxygen bottle on board an aircraft. For these reasons there are various considerations to take on board during design, manufacture, test and inspection.

DesignThe pressure differential is dangerous and many fatal accidents have occurred in the history of their development and operation. Consequently, their design, manufacture, and operation are regulated by engineering authorities backed up by laws. For these reasons, the definition of a PV’s varies from country to country, but involves parameters such as maximum safe operating pressure and temperature. Depending on the requirements of the vessel would depend on the local countries in the UK this would be BS**** or ASME codes. The requirements of a safe pressure vessel are:

Function Pressure vessel = contain pressure, pObjective Maximize safetyConstraints Must yield before break or

Must leak before breakWall thickness reduced for mass and costs

Fabrication.

PV’s are fabricated in line with the design calculations and drawings, and the method of construction as defined in the approved code. Where welding is carried out the welding the standards in the code should be should be adhered to, this should also include that the necessary checks against the welders qualifications and experience for the type of welding and materials in use. During fabrication sage checks should be carried out by a competent inspector ensuring that the standards of the code are maintained.

Test and inspection.Testing and inspection are a very important part of the process of making sure that the PV manufactured will be able to withstand the pressure and temperature to which it will be subjected. Several methods of testing can be carried out these could be NDT such as:

Radiographic examination. Magnetic particle. Ultrasonic testing. Dye-penetrant examination. Pressure testing.

Markings.The manufacturer’s markings on a pressure vessel include information regarding the date of manufacture, the manufacturers name or registered mark, the serial number of the unit and the specification or exemption to which the container complies. For pressure vessels that have been re-qualified one or more times, additional markings indicate the date(s) of any previous hydrostatic retests and identification markings of the retest facility

As part of the production of PV’s a procedure for a safe work should be compiled by the manufacture company, this should cover fabrication and suitable testing of the PV. All workers involved in the fabrication and testing should be aware of the potential hazards that may occur during manufacture and the dangerous processes that are also involved.

Fail Safe.

In large PV’s safe design is achieved by ensuring that it “leaks before breaks” this means that the smallest crack propagates unstably, having a length greater than the thickness of PV wall, the leak will be detected and the pressure in the container will be reduced at a safely rate preventing any further damage. Small PV containers are generally designed to “yield before break” this means that the that the container wall will deform and will be detectable and the pressure can be release preventing any further hazard.

Stresses within a pressure vessel.In two dimensions, the state of stress at a point is conveniently illustrated by drawing four perpendicular lines that we can view as representing four adjacent planes of atoms taken from an arbitrary position within the material. The planes on this “stress square” shown in Figure below can be identified by the orientations of their normal; the upper horizontal plane is a +y plane, since its normal points in the +y direction. The vertical plane on the right is a +x plane. Similarly, the left vertical and lower horizontal planes are −y and −x, respectively.

Stress in two dimensions; the stress square.The sign convention in common use regards tensile stresses as positive and compressive stresses as negative. Besides, the stress square must be in equilibrium; therefore this arrow must be balanced by another force acting on the −x face and pointed in the –x direction. Of course, these are not two separate stresses, but simply indicate the stress state is one of uniaxial tension. It goes same with y direction for compression.

The sign convention for normal stresses.Consider now a simple spherical vessel of radius r and wall thickness b, such as a round balloon. An internal pressure p induces equal biaxial tangential tensile stresses in the walls, which can be denoted using spherical rθφ coordinates as σθ and σφ. The internal pressure generates a force towards the spherical wall, which is balanced by the wall stress.

Wall stresses in a spherical pressure vessel.

At the surfaces of the vessel wall, a radial stress must be present to balance the pressure there.

Free-body diagram for axial stress in a closed-end vessel.The stresses in the axial direction of a cylindrical pressure vessel with closed ends are found using this same approach as seen above and yielding the same thing (equilibrium state& equation).

Hoop stresses in a cylindrical pressure vessel.Note the hoop stresses are twice the axial stresses. This result — different stresses in different directions — occurs more often than not in engineering structures, and shows one of the 3 compelling advantages for engineered materials that can be made stronger in one direction than another (the property of anisotropy). If a pressure vessel constructed of conventional isotropic material is made thick enough to keep the hoop stresses below yield, it will be twice as strong as it needs to be in the axial direction.

2.2Material selectionThe selection of construction materials for Code pressure vessels has to be made from Code approved material specifications. It usually specifies the most economical materials of low first cost and/ or low future maintenance cost that will be satisfactory under operating conditions and will meet other requirements. There are many factors must be considered in selecting the most suitable materials including:

a. Corrosion resistance in the service corrosive environment b. Strength requirements for design temperature and pressure c. Cost d. Ready market availability e. Fabricability f. Quality of future maintenance

Selection of materialIt has been identified that the acceptable crack size is maximized by choosing a material with the largest value of:

M= K1c

σf Also, the maximum pressure is carried most safely by the material with the greatest value of

M2= K 2 1c

σf

Making the yield strength of wall very thin could make both M and M2 large. However, the material, lead for instance has high values of both, but would not be used in a PV because the vessel wall must also be as thin as possible, reducing costs and weight. Thus the thinnest wall is that with the largest yield strength σf.

Thus we wish also to maximise

M3= σf

This reduces the choice of material even further.

These selection requirements are applied by using the graph below with fracture toughness K1c plotted against strength σf. The three criteria appear as lines of slope 1,1/2 and as lines that are vertical. Take ‘yield before break’ as an example. A diagonal line corresponding to M= K1c = C links material with equal performance; σf those above the line are better. The line shown in the graph at M1=0.6m1/2 excludes everything but the strongest materials steels, copper and aluminium alloys, although some polymers come very close, some low pressure containers containing carbonated beverages are made from polymers. A second selection line at M3=100 Mpa eliminates aluminium alloys.

Large pressure vessels are always made of steel. Those for models, such as small scale model steam engine, are copper due to the higher resistance to corrosion.

The alternative criterion M2= K 2 1c favours more strongly, but does not change the σf conclusion.

Material M1= K1c (M1/2) M3 = 01(Mpa) σf

Comment

Tough steel >0.6 300 These are the PV steels, standard in their use.

Toughcopper alloys

>0.6 120 OFHC hard drawn copper

Tough A1 alloys >0.6 80 1000 and 3000 series A1 alloys.

Ti-alloys 0.2 700 High yield but low safety margin.High strength A1-alloys 0.1 500

Ashby fracture toughness - strength

Good for light pressure vessels.

GFRP/CFRP 0.1 500

The Ashby chart on the previous page identifies materials that are used in the fabrication of PV. Therefore, steel, copper alloys and aluminium alloys are ideal to best satisfy ‘yield before break’ properties. In addition, high yield strength gives a higher working pressure. The materials in the ‘search area’ triangle are the best choice. The ‘leak before break’ leads to essentially the same selection. There are fewer failures in the use pf PV’s as information from fracture mechanics improves engineering practice.

2.3 Processing method for a safe P.V.

With the increase in technology, materials available and improvement in techniques and skill levels ever increasing, the fabrication and manufacture of P.V.’s have allowed for a greater range in size, weight and pressure limits available when it comes to the fabrication in P.V.’s.

The pressure vessel material used for the shell of large P.V.’s commonly made of carbon steel and stainless steel.

Fabrication and component parts.

ShellThe steel plate rolling process will form the cylindrical shape. Because there may be limitations in forming and rolling machine width, these limits are taken into account during the design, meaning the PV shall be divided into several parts.

HeadsThere are several types of head:

Semi ellipsoidal Torispherical Hemispherical Flat Conical

The head is formed into shapes by two types of forming:

Hot formed – heads are formed at temperature suitable to the material. This method reduces residual stresses, to temper or soften the material itself. Typically carbon steel heads are hot formed at 899oC and stainless steels are hot formed at 1038oC.

Cold formed – heat treatment is usually required after cold forming to remove brittleness and stress relief.

NozzleThe nozzle existence will affect P.V. load calculations. The loads are strictly forces and moments imposed on the nozzle by the attached piping. Before applying these load numbers, check the piping layout connected to the nozzle to make assumption about the loading combination. Then, reference to supplier / fabricator range of dimension which suit with load.

Welding Welding plays a very important role during fabrication. There are several types of welds and applications used to assemble the plates and nozzle brining them together into on vessel.

Welding MethodWelding is a joining process that uses heat, pressure, and/or chemicals to fuse two materials together permanently.

The most widely used industrial welding is arc welding which is any of several of fusion welding processes wherein the heat of fusion is generated by an electric arc.

Type of weld

WELDING

WELDING METHOD

FORGE

FUSION

PRESSURE

TYPE OF WELD

GROOVE

FILLET

PLUG

WELDING PROCESS

Shielded Metal Arc Welding (SMAW)

Submerged arc welding (SAW)

Gas Metal Arc Welding (GMAW)

Gas Tungsten Arc Welding (GTAW)

Gas Welding

Resistance Welding

WELDED JOINT

Tee

Lap

Butt

Corner

Edge

i. Groove weldA type of weld that consists of an opening between two part surfaces, which provides space to contain weld metal. Groove welds are used on all joints except lap joints.

Groove weld can be subdivided according to the edge conditions.

Types of groove welds.ii. Fillet Weld

A fillet weld is a weld with an approximately triangular cross section, joining two surfaces at right angles.

iii. Plug weldA plug is a circular weld made either by arc or gas welding through one member of a lap or tee joint.

welding

Some form of welded joint in combination with different weld types.

Welding process.

Shielded Metal Arc Welding (SMAW)This form of welding is widely used. The heat for welding is produced by the resistance of the arc air gap to the flow of electric current. Also called stick electrode welding, SMAW is almost always done manually. As the electrode heats, the core wire which conducts the current to the arc melts and provides filler metal for the welded joint. The coating of the electrode breaks down to form a gaseous shield for the arc and weld puddle as well as small amount of slag, which protects the weld as it cools. Shielding is very important for the quality of weld, since it prevents the loss of alloying elements during the transfer of molten metal through the arc.

Submerged Arc Welding (SAW)This process, almost always fully automatic, is used in the fabrication of main vessel seams. It gives excellent welds at low cost. However, it can be used for horizontal positions only. A continuous consumable wire coil is used as electrode. Weld puddle and arc are protected by liquid slag, formed form granular mineral flux deposited ahead of the arc.

Gas Metal Arc Welding (GMAW)A consumable continuous wire is used as an electrode which melts and supplies the filler metal for the welded joint. A protective shield of inert gases (helium, argon, CO2, or a mixture of gases) is used. The process produces excellent welds at less cost than the GTAW process (see below) with higher weld deposition rate.

Gas Tungsten Arc Welding (GTAW)This process is used when the highest-quality welding with difficult to weld metals is required. An arc is formed by a non-consumable tungsten electrode, which carries the electric current; the filler metal, if required, is added separately form a rod or a continuous wire. Inert gas flows around the arc and the weld puddle to protect the hot metal.

i. Gas WeldingHeat of fusion is generated by burning a flame of gas with oxygen. Different gases are used, as described below:a. The oxyhydrogen process (OHW) uses hydrogen for

combustion.b. The oxyacetylene process (OAW) uses acetylene gas:

- Flame cutting- Flame machining

ii. Resistance WeldingThe heat of fusion is generated by the resistance at the interface to the flow of electric current. No shielding is required. Pressure must be applied for good metal joining. Resistance spot welding (RSW) or resistance seam welding (RSEW) are used to fix corrosion-resistant linings to the wall of a vessel shell.

Heat TreatmentHeat treatment intended to be done on individual parts of a vessel after they have been shaped for assembly. Post hot/cold work heat treatment (heating about 2/3 of material melting temperature) followed by natural cooling to remove the residual stress.

Clad Construction Integrally clad materials are used for fabricating equipment of reactive metals economically, robustly and durably. Bonding these materials onto lower cost, stronger backers extends the economical service of these materials into higher pressures and temperatures and more challenging service environments than are feasible with solid alloy construction.

Tolerances.The entire components have their percentage of permissible dimensional difference which provided in the code.

1. Inspection, examination and testing.

The inspection must be run during the fabrication. This because to ensure the vessel is in the best quality and satisfy with the required operating requirement.

Inspection and Test Plan (ITP)

It is basically the test planning before the fabrication of the pressure vessel. Refer Table 1 for an example of ITP

Material Identification and Certification

i. MaterialMake sure the materials that supplied by the suppliers are as stated in the datasheet.

ii. DimensionalThe inspectors have to ensure that the dimension of all fabricated plate, shell, nozzle and head are same as stated in the datasheet.

iii. Sizing

This matter also same with above. Running of size is the biggest problem during fabrication. It will lead to other problems i.e. leakage and deforming.

Inspection and Examination NDT and visual & optical testing must be performed in suitable conditions, however, fine cracks may remain invisible.

Radiographic examination.It is used in cases of volume defects revealed by ultrasonic as an additional method for the characterization the defects. In cases of weld repair, it is always recommended to use RT.

Magnetic particle. It is the best method for the detection of fine surface cracks. This method is the most sensitive and it reveals many cracks that are not detected by other methods with lower defect sensitivity. AC yoke WMFT is preferred over DC or prod methods. DC methods are not as sensitive and prod methods may leave arc strikes that, if not ground out, can serve as crack initiators.

Ultrasonic testing. With straight and angle-beam probes is the main method for the detection of subsurface and deeper surface defects of sufficient size. UT shear waves, longitudinal waves, and crack-tip diffraction ultrasonic inspections

are used. UT may also be used for the examination of a vessel under pressure without discharging of the liquid gas from the vessel.

Dye-penetrant examination.It is used only occasionally, is less sensitive than MT, it is however, easier to perform. In case, some complicated areas that MT, UT and RT are not accessible, the PT is used

Pressure testing.Pressure testing of the PV using coloured water is known as hydrostatic test, during a hydrostatic test, a pressure vessel is placed inside a closed system, usually a test jacket filled with water, and a specified internal water pressure is applied to the container inside this closed system. The applied internal pressure causes an expansion of the container being tested, and the total and permanent expansion that the container undergoes is measured. This volumetric expansion measurement, in conjunction with an internal and external visual inspection of the container, are used to determine if a pressure vessel is safe for continued use, or has suffered from a degradation in its structural integrity and then must be condemned.

Marking.The manufacturer’s markings on a pressure vessel include information regarding the date of manufacture, the manufacturers name or registered mark, the serial number of the unit and the specification or exemption to which the container complies. For pressure vessels that have been re-qualified one or more times, additional markings indicate the date(s) of any previous hydrostatic retests and identification markings of the retest facility.

Preservation and protection for shipment.This preservation and protection basically to protect the pressure vessel from accidental deformation, leakage and rust initiator during transportation to the site. The vessel may be large heavy thus very dangerous to residential areas if something were to happen during transportation.

The action below is taken to overcome these issues:

Rust prevention. Protection and safety during shipment. Transportation saddles.

Turbine blade for a jet engine.

1) For your chosen products, carry out an analysis of what each has to do in terms of its function and identify the most important material property characteristics required.

Jet turbines are made up of one or more stages of stators and rotors, these are aerofoil in design. The turbine rotors are assembled on disc which are attached to the shaft of the compressor. The stators, nozzle guide vanes (NGV’s) are located on the turbine casing. The turbine uses the energy from the hot air gases to turn the compressor. In the early Rolls Royce engines the turbine needed to be air-cooled as it operated in a gas stream temperature of about 15000c which is higher than the melting point of the blade material. The total power produced by these engines are 250.000hp and exhaust gases exit at 1000mph.

NGV and turbine blade consideration.

NVG’s and turbine blades are aerofoil in shape and form a convergent space between adjacent blades to convert pressure energy of the gas flow to kinetic energy. The gas flow needs to be turned through an extremely large angle of deflection fron the axial direction and, at the same time undergo expansion from low velocities to sonic or near supersonic speeds. NGV’s operate ‘choked’ under design conditions in order to effect the maximum energy conversion. NVG’s must be able to withstand higher gas temperature than those which the turbine blades are subjected to and they are normally hollow to allow HP air passing through to cool them. NVG’s are also subjected to temperature and stress fluctuations caused by either uneven flow distribution from the compressor, or by flame pulsating from within the combustion chamber. NGV’s are normally manufactured with shrouds fitted to both top and bottom of the vane.

Turbine blade and NVG coolingTo identify the intricacies, both internally and externally of the turbine blade it should be noted that they are cooled with air trapped from the compressor, which is at a usable temperature and pressure for cooling. The volume of air required is only a small percentage of the total compressor output and is taken into account during the design stage. Air-cooling of blades currently uses either convection or film cooling.

Convection cooling.Convection cooling is achieved by passing cooling air through longtiduinal holes or hollow blade sections.

Turbine blade with cooling holes for film coolingFilm Cooled Blades.Film cooling of the blade successfully overcomes the effective limits of cooling by convection by passing a film of cold air over the surface, thus protecting it from the hot gas flow. Although it is expensive to develop and produce so designers have identified where to provide cooling to the portions of the blade which have the greatest need, the leading and trailing edges.

Turbine blade design considerations.Durability of the blade is the most significant attribute required by a gas turbine. The nature of the tasks required dictates the impact of premature wear or failure. In military and commercial aviation sectors both are concerned with safety and military engines must meet all tasking required of them.

Loss of tip clearance.The service life of a gas turbine is generally limited by the condition of high temperature components. The designed tip clearance between the turbine blades and the shroud ring can reduces due to the blade creep and bearing wear. In military engines this damage is inspected as part of the boroscope visual inspections while in service.

Buckling, cracking and distortion.Hot spots from misaligned, damaged combustion chamber systems or faulty blade cooling may give rise to blade troubles, particularly distortion and cracking of the NVG trailing edges. If cracks are not speedily identified pieces of material could break off and cause further damage to the turbine or impose eccentric dynamic loading on the assembly. The turbine can be visually inspected using boroscope techniques utilising inspection ports included in the design of the engine.

FODOn occasion damage can result from foreign objects, although the compressor damage will occur as well. Turbofan engines are not so prone to FOD on turbines as, after passing through the LP compressor, most of the debris is centrifuged into the bypass duct. The majority of FOD damage to the turbine comes from the failure of the combustion liner, or by carbon breaking away and passing through the turbine. Small scratches and chips are allowed, but these limits are very small because of the extreme operating conditions of the turbine.

Turbine blade detachment and containment. If a turbine blade becomes detached from the disc during use the destructive force is so large that secondary damage caused by the blade

can result in the loss of the aircraft. Various methods of containment are used such as increasing the strength of the turbine casing and having reinforced bands located around the aircrafts engine bay, protecting the vital areas of the aircraft structure and systems such as fuel and flying controls.

Turbine blade overtemp.The over temping of a turbine blade can lead to premature failure of the blade with catastrophic results for the engine.

Blade damage due to over temp.

Engine temperature and pressure.

Creep and fatigue.The cyclic loading of a turbine blade associated with duty cycles such as take-off/ cruise or combat mission throttle excursions is a principal source of damage in turbine blades. Even manufacturing processes can cause fatigue damage. However fatigue degradation is not confined to metals. Most engineering materials such as ceramics and composites are also susceptible to fatigue damage.

Blade material creep.Creep is the gradual increase in length of the blades material because due to the constant loads applied. This must be considered as the blades materials are required to work continuously at high temperature. Creep can be seen during 3 phases.

Primary creep. This starts at a rapid rate but slows down as work hardening sets in and the strain rate decreases.

Secondary creep.During this phase the creep increases in strain is approximately proportional to time, that is, the strain rate is constant and at its lowest value.

Tertiary creep.Through this phase, creep the strain rate increases rapidly, necking occurs and the blade may fail. Therefore, the initial stress, which was within the elastic range and did not produce early failure, did eventually end in failure after a period of time.

Fatigue.There are also three commonly identified forms of fatigue, high life cycle fatigue, low cycle fatigue and thermal mechanical fatigue. The principal distinction between HCF and LCF is the of the stress strain curve where

repetitive application of load is taking place, resulting in deformation or strain.

High cycle fatigue (HCF).HCF is characterized by low amplitude high frequency elastic strains. An example would be an airfoil subjected to repeated bending. One reason this bending occurs is when the turbine blade passes behind the stator vane. When the blade emerges into the gas path it is bent by high velocity gas pressure. Changes in rotor speed change the frequency of the blade loading. The excitation will at some point match the blade’s resonant frequency causing the amplitude of vibration to increase significantly.

Low cycle fatigue (LCF).LCF is the mode of material degradation when plastic strains are induced in an engine component due to the in use environment. LCF is characterized by high amplitude low frequency plastic strains. The act of permanently bending a turbine blades means that the elastic limit point on the stress strain curve has been exceeded, thus crossing over into the plastic region. Forcing the blade back into the original position will require it to be bent or yielded, thereby completing one LCF cycle. Each blade can endure only a very few of these cycles before it will fail due to LCF. It is worth noting that in a turbine blade these large strains occur in areas of stress concentration.

Most turbine blades have a verity of features like holes, internal passages, curves and notches, it is these features raise the local stress level to the point where plastic strain can occurs. As already mentioned the turbine blades and vanes have a configuration at the base known as a dovetail or fir-tree and this feature is used to attach the blade to the turbine disk. As engine rotational speed increases centrifugal forces result in local plastic strains at the attachment surfaces and LCF damage can occur.

Thermal mechanical fatigue (TMF)When it comes to HCF and LCF I have looked at strain and the result in the material when it is stressed. In the case of TMF, present in turbine blades, vanes and other hot section components, large temperature changes result in significant thermal expansion and contraction and therefore significant strain excursions. These strains are reinforced or countered by mechanical strains associated with centrifugal loads as engine speed changes. The combination of these events causes material degradation due to TMF.

Fatigue data.Engineering personnel can use fatigue data from test machines similar to that used for tensile tests and similar specimens are used. The chief difference lies in the application of load. In an HCF specimen test, the load is applied to the specimen at 30 to 60 cycles per second and often at a lot higher frequencies. In the engine components where HCF is a

concern, designers observe what is referred to as a material’s fatigue strength. This is determined by running multiple specimen tests at a number of different stresses with the objective to identify the highest stress that will produce a fatigue life beyond ten million cycles. This stress is also known as the material’s endurance limit. Gas turbines are designed so that the stresses in engine components do not exceed this value including an additional safety factor.LCF testing is carried out in a similar method, the chief difference being the need for higher (plastic) loading and lower frequencies. The graphic results appear similar but the lives are much lower and there is no fatigue strength. However, components subjected to LCF loading are designed such that stresses remain well below the average lives determined in the LCF tests.The fatigue life graphs give information in a number of ways. As well as determining the maximum stress allowed for a component life to meet ten million cycles, they can also be used to compare the durability of different alloys and show temperature and other environmental effects. Fracture mechanisms.Metals are crystalline in nature with metal crystals known as grains. They are composed of uniformed layers of atoms stacked one on another. The atoms occupy regular positions in what is called a lattice structure and atomic forces maintain the atoms in place. When the grain deforms, these layers attempt to slide past each other in a sheering process known as slip. If the slip displacement is small, within the limits of the elastic section of the stress/strain curve, the deformation will be reversible. If the slip exceeds the plastic limits plastic deformation will occur along the slip plane and the slip will be irreversible. As fatigue cycles accumulate, the number and intensity of bands increase and the microstructural damage is concentrated onto a few intense slip bands. The process continues until a persistent slip band forms.

Conclusion.If we look at our stress strain and fatigue curves it would appear that higher alloy strength equates to superior fatigue life. But alloy microstructure can be more important than strength. LCF and HCF cracks frequently initiate at the intrinsic defects previously mentioned. This is not to say that the material is defective. Defects are a normal feature of an alloy's microstructure; however, alloys can be produced with fewer

defects. In the alloy comparison shown in figure below Alloy A exhibits clearly superior durability of 70ksi vs <40ksi. What is not shown, but is especially interesting, is that Alloy A is significantly lower in strength than Alloy B. The improvement stems from a low defect content obtained in Alloy A by special heat treatments.

Graph plotting HCF test data.

2 Consider the design of each product and suggest the most appropriate processing method. Carry out a material selection exercise to identify the most suitable material.

Material Selection.Turbine blade materials need to have some definitive requirements, these are; high fatigue strength, good creep resistance qualities, resistance to thermal shock and corrosion, also they need to be economical to manufacture and resistant to wear. With these requirements and the selection of materials available to industry we can create a material selection matrix to help with the selection making process identifying materials against the requirements needed, listed below in order of preference.

1, Corrosion Resistance2, Wear Resistance3, Ease of Manufacture 4, Toughness5, Heat Resistance6, Strength and Resistance to Creep

Materials looked at are; Mild Steel Stainless Steel Titanium GRP Aluminium

These materials are rated in terms of their properties with a rating of 1 – 5 with 1 being poor and 5 being good. The mark is then multiplied by the requirement score giving a total score and the combined totals in a tabulated form,

RequirementsMaterial Strength

6

Heat Resistan

ce5

Toughness

4

Manufacture

3

Wear Resistan

ce2

Corrosion

resistance1

Totals

Mild Steel

4(24) 4(20) 4(16) 5(15) 4(8) 3(3) 86

Stainless Steel

5(30) 4(20) 4(16) 4(12) 4(8) 3(3) 92

Titanium 5(30) 5(25) 5(20) 3(9) 5(10) 5(5) 99GRP 3(18) 2(10) 3(12) 3(9) 4(8) 5(5) 62Aluminium

3(18) 3(15) 3(12) 3(9) 3(6) 3(3) 63

Turbine Blade Material Selection Matrix.

Using this matrix above we can identify Titanium as the most suitable material to use. A metallic element, titanium is recognized for its high strength-to-weight ratio. It is a strong metal with low density that is quite ductile, especially in an oxygen-free environment, lustrous, and metallic-white in color. Titanium’s relatively high melting point, more than 1,650 °C or 3,000 °F, makes it useful as a refractory metal. It is paramagnetic and has fairly low electrical and thermal conductivity.

Commercial (99.2% pure) grades of titanium have ultimate tensile strength of about 63,000 psi (434 MPa), equal to that of common, low-grade steel alloys, but are 45% lighter. Titanium is 60% more dense than aluminum, but more than twice as strong as the most commonly used 6061-T6 aluminum alloy. Certain titanium alloys (e.g., Beta C) achieve tensile strengths of over 200,000 psi (1,400 MPa). However, titanium loses strength when heated above 430 °C (806 °F).

It is fairly hard although not as hard as some grades of heat-treated steel, non-magnetic and a poor conductor of heat and electricity. Machining requires precautions, as the material will soften and gall if sharp tools and proper cooling methods are not used. Like those made from steel, titanium structures have a fatigue limit, which guarantees longevity in some applications. Titanium alloys specific stiffness’s are also usually not as good as other materials such as aluminum alloys and carbon fiber, so it is used less for structures which require high rigidity.

Properties of Titanium High resistance to corrosion.

Resistant to sea water as platinum. Bio compatible material. Non-magnetic property.

Magnetic permeability is 1.0001 making it nearly perfectly non-magnetic.

Low specific gravity.Ti is 4.5 around 50% of Ni or Cu and 60% of steel.

High specific strength.About 3 times as Al and higher than stainless steel and resists temperature up to 400oc.

Ashby chart above identifies nickel alloy has a greater density to strength characteristics than titanium alloys. This is why nickel alloys are now used in the manufacture of turbine blades which are located in the hottest sections of a jet engine. Although titanium has won the material selection, the material of choice is now the super alloy and a super alloy is a metallic alloy, which can be used at high temperatures, often in excess 0.7 of the absolute melting temperature. Creep and oxidation resistance are prime design criteria can be based on iron, cobalt or nickel, the latter being best suited for aero engine applications.

Turbine blade manufacture is now a major user of nickel based super alloys. A single blade is free from grain boundaries as boundaries are easy diffusion paths and therefore reduce the resistance of the material to creep deformation. The directionally solidified columnar grain structure has many grains, but the boundaries are mostly parallel to the major stress axis; the performance of such blades is not as good as the single-crystal alloys over conventionally cast polycrystalline super alloys is that many of the grain boundary strengthening solutes are removed. This results in an increase in the incipient melting temperature. The single-crystal alloy can therefore be heat treated to temperatures in the range between 1240-1330oc, with titanium blades in the colder regions. This is because there is a danger of titanium igniting in special circumstances if its temperature exceeds 400oc.

The Ashby chart above shows that nickel alloys have a grater fracture toughness than titanium alloys and the Ashby chart below shows that titanium is and expensive metal to use, even higher than nickel.

Use of ceramics.To understand why ceramics are not used in aero-engines even though they have the best high temperature properties we need to look at what their other properties they have consider which ones are important.

Property Metals CeramicsToughness( Foreign object damage)

Good Very Poor

Oxidation / CorrosionResistance

Fair Good

Forming Good(forging)

Fair(sintering)

Joining Good DifficultCreep Resistance Fair GoodCost High High

Poor toughness is the main reason why ceramics have not been successfully introduced into the majority of engines. While ceramics are not used on major components, Zirconica (ZrO) coatings are being used to increase the operating temperature. These coatings can operate at a much higher temperatures and protect the metal from chemical attack. Oxides e.g. ZrO, Al2O2 are often used for high temperature coatings. Combined, the effects of air cooling and ceramic coatings mean that the

Titanium

Nickel Alloy

combusting gases can reach temperatures over 1600oc – higher than any metal can cope with.

Use of Coatings.

Jet engines and land based power generation turbines provide different environments. In general a jet engines high pressure turbine blade, HTP blades, is expected to survive for around 30,000hrs. When it comes to land based power generation this is expected to reach 50,000 and 75,000hrs. HTP blades in jet engines will normally be refurbished, strip coating and re-coat, during its life, for power generation this can be up to two depending on its target life. HTP blades in jet engines mainly suffer from oxidation; Pt-aluminide coatings are preferred in these conditions and are commonly used to coat the main body of the blade.

A jet engine HPT blade. Aluminised jet engine HPT blade.

Other areas of the HTP blade in engines may receive a different coating, the base and tips can be coated with MCrAlY coating as corrosion can have more of an effect in these areas. Blades are simply aluminised; they are not cooled and do not receive a coating. In the coating may contain additions to enhance corrosion resistance.

To conclude, further research and development of titanium and nickel alloys might be limited to the coatings applied, with ceramics taking the lead.

3 Identify constraints on your material selection choices imposed by processing.

Sand casting the most widely used casting process, utilizes expendable sand molds to form complex metal parts that can be made of nearly any alloy. Because the sand mold must be destroyed in order to remove the part, called the casting, sand castings typically has a low production rate. The sand casting process involves the use of a furnace, metal, pattern, and sand mold. The metal is melted in the furnace and then ladled and poured into the cavity of the sand mold, which is formed by the pattern.

The sand mold separates along a parting line and the solidified casting can be removed. The steps in this process are described in greater detail in the next section. In sand casting, the primary piece of equipment is the mold, which contains several components. The mold is divided into two halves - the cope (upper half) and the drag (bottom half), which meet along a parting line. Both mold halves are contained inside a box, called a flask, which itself is divided along this parting line. The mold cavity is formed by packing sand around the pattern in each half of the flask. The sand can be packed by hand, but machines that use pressure or impact ensure even packing of the sand and require far less time, thus increasing the production rate. After the sand has been packed and the pattern is removed, a cavity will remain that forms the external shape of the casting. Some internal surfaces of the casting may be formed by cores. Sand casting is used to produce a wide variety of metal components with complex geometries. These parts can vary greatly in size and weight, ranging from a couple ounces to several tons. Some smaller sand cast parts include components as gears, pulleys, crankshafts, connecting rods, and propellers. Larger applications include housings for large equipment and heavy machine bases. Sand casting is also common in producing automobile components, such as engine blocks, engine manifolds, cylinder heads, and transmission cases. Sand casting is able to make use of almost any alloy. An advantage of sand casting is the ability to cast materials with high melting temperatures, including steel, nickel, and titanium. The four most common materials that are used in sand casting are shown below, along with their melting temperatures. Materials Melting temperature Aluminium alloys 1220 °F (660 °C) Brass alloys 1980 °F (1082 °C) Cast iron 1990-2300 °F (1088-1260 °C) Cast steel 2500 °F (1371 °C) The material cost for sand casting includes the cost of the metal, melting the metal, the mold sand, and the core sand. The cost of the metal is determined by the weight of the part, calculated from part volume and material density, as well the unit price of the material. The melting cost will also be greater for a larger part weight and is influenced by the material, as some materials are more costly to melt. However, the melting cost in typically insignificant compared to the metal cost. The amount of mold sand that is used, and hence the cost, is also proportional to the weight of the part. Lastly, the cost of the core sand is determined by the quantity and size of the cores used to cast the part.

Advantages: Can produce very large parts can form complex shapes, many material options, Low tooling and equipment cost, Scrap can be recycled, Short lead time possible.

Investment casting can make use of most metals, most commonly using bronze alloys, stainless steel, and tool steel. This process is beneficial for casting metals with high melting temperatures that can not be moulded in plaster or metal. Parts that are typically made by investment castings include those with complex geometry such as turbine blades or firearm components. High temperature applications are also common, which includes parts for the automotive, aircraft, and military industries. The process is generally used for small castings, but has produced complete aircraft door frames, steel castings of up to 300 kg and aluminium castings of up to 30 kg. It is generally more expensive per unit than die casting or sand casting but with lower equipment cost. It can produce complicated shapes that would be difficult or impossible with die casting, yet like that process, it requires little surface finishing and only minor machining. Investment casting is used in the aerospace and power generation industries to produce turbine blades with complex shapes or cooling systems. Blades produced by investment casting can include single-crystal (SX), directionally solidified (DS), or conventional equiaxed blades. It is also widely used by firearms manufacturers to fabricate firearm receivers, triggers, hammers, and other precision parts at low cost. Other industries that use standard investment-cast parts include military, medical, commercial and automotive.

Investment casting offers high production rates, particularly for small or highly complex components, and extremely good surface finish (CT4-CT6 class accuracy and Ra1.6-6.3 surface roughness) with very little machining.

The drawbacks include the specialized equipment, costly refractories and binders, many operations to make a mould, and occasional minute defects.

Advantage; Can form complex shapes and fine details Many material options, High strength parts, Very good surface finish and accuracy, Little need for secondary machining

Applications: Turbine blades, armament parts, pipe fittings, lock parts, hand tools, agriculture parts, marine parts, medical parts, hardware, automobile parts, ect.

Die casting is a manufacturing process that can produce geometrically complex metal parts through the use of reusable molds, called dies. The die casting process involves the use of a furnace, metal, die casting machine, and die. The metal, typically a non-ferrous alloy such as aluminium or zinc, is melted in the furnace and then injected into the dies in the die casting machine. There are two main types of die casting machines - hot chamber machines (used for alloys with low melting temperatures, such as zinc) and cold chamber machines (used for alloys

with high melting temperatures, such as aluminium). The differences between these machines will be detailed in the sections on equipment and tooling. However, in both machines, after the molten metal is injected into the dies, it rapidly cools and solidifies into the final part, called the casting. The steps in this process are described in greater detail in the next section. The castings that are created in this process can vary greatly in size and weight, ranging from a couple ounces to 100 pounds. One common application of die cast parts are housings - thin-walled enclosures, often requiring many ribs and bosses on the interior. Metal housings for a variety of appliances and equipment are often die cast. Several automobile components are also manufactured using die casting, including pistons, cylinder heads, and engine blocks. Other common die cast parts include propellers, gears, bushings, pumps, and valves. Die cast parts can vary greatly in size and therefore require these measures to cover a very large range. As a result, die casting machines are designed to each accommodate a small range of this larger spectrum of values. Sample specifications for several different hot chamber and cold chamber die casting machines are given below.

Additive Manufacturing (AM) is the term given to a group of technologies that are capable of creating physical objects from computer aided design (CAD) files by incrementally adding material such that the objects “grow” from nothing to completion. Most of today’s AM technologies start with a CAD solid model which is sliced into thin layers. Each layer comprises a 2D cross section profile of the part which is then made layer by layer. Different technologies use different methods and materials to create objects, ranging from lasers melting metal powder through to inkjet heads printing photo-curable resins. 

AM is a nascent technology area with the first recognised commercial machines being sold in the late 1980’s. However the concept of making objects in an additive approach is hardly new – consider how the pyramids in Egypt were produced. The automated approach to creating objects from CAD files in the additive manner that AM allows is beginning to have a profound effect on how objects are made. Almost all manufactured objects that we see today are made with some form of tooling at some point – parts may be machined using a cutting tool or parts may be formed from a moulding tool. The use of tooling imposes numerous restrictions such as design limitations, restrictive costs to prepare for manufacture and the need to create products at a single location prior to being shipped globally. AM technologies invariably require no tooling to create parts and this profoundly affects how products can be designed, tailored, made, distributed etc. Figure below shows an example of the design of an automotive housing manufactured by traditional casting and machining on the top with a version design for AM on the bottom. The design on the bottom is able to use 40% less material than the design on the top and AM allows the design to employ curved channels rather than straight line drilled channels that are needed by traditional processes

Benefits such as design freedom and speed of one-off part manufacture add value to products that can be made by AM processes. This addition of value, coupled with high costs of technology development have meant that equipment, material and maintenance costs have been high. These high costs have often restricted ownership of AM equipment to large organisations or organisations dedicated to supplying parts made by AM.

As most of today’s processes create parts layer by layer, each layer can be thought of in many respects as similar to printing. Instead of printing text on a page, the processes “print” the 2D profile of a part under construction so an analogy with printing is clear. The fact that the process repeats with subsequent “prints” to create 3D parts instead of 2D profiles has led to the term 3D Printing being used to describe the processes. Today 3D Printing is considered mainly as a reference to low cost machines or those that employ print heads usually for prototyping purposes but the user friendly nature of the expression might mean that this becomes the standard for all the process currently referred to as AM. 

In conclusionAM technologies have come a long way in a short time and their future looks bright, especially for use as production machines rather than just for prototypes. This is likely to lead to the generation of new types of products that were economically unviable or geometrically impossible to make in the past. In industry the shift will be away from visual aids to functional products.

Refernces

http://files.asme.org/IGTI/Knowledge/Articles/13048.pdfhttp://www.scribd.com/doc/20064635/Coatings-for-Turbine-Bladeshttp://www.thomas-sourmail.org/coatings/single-page.htmlhttp://emergingtechnologies.becta.org.uk/index.php?section=etr&rid=15280&filter=ArtTec_001http://www.casting-manufactory.com/prolist.htm