Alloys

We are going to be focussing on mechanical properties in this module, but can you think of a common metallic engineering component (apart from medical implants) where mechanical properties like strength or toughness are not the most important property ?

Steels for electric motor applications are one example. Good magnetic and electrical properties are needed, and strength is not the main priority. Magnets are another example where strength is not top of the list of priorities.

In the figure below we can see 5 different materials that you would typically find in a jet engine. Think about the property requirements for engine components and also the environmental conditions of different parts of this engine and try and answer the following (i) What is it about the properties of titanium that makes it suitable for components at the front of the engine ?

Titanium is light, strong and corrosion resistant. It has good specific strength for front-of-engine components, where temperatures are lower than the turbine section. Its use is limited at higher temperatures, roughly above about 500°C.

In the figure below we can see 5 different materials that you would typically find in a jet engine. Think about the property requirements for engine components and also the environmental conditions of different parts of this engine and try and answer the following (ii) Why are nickel components located at the back of the engine ?

Nickel-based alloys are used at the back of the engine because they retain high-temperature strength in the hot combustor and turbine sections. Coatings can improve corrosion and oxidation resistance.

In the figure below we can see 5 different materials that you would typically find in a jet engine. Think about the property requirements for engine components and also the environmental conditions of different parts of this engine and try and answer the following (iii) Why could aluminium be used for fuselage material?

Aluminium is lightweight and cheaper than titanium. It is suitable for cooler fuselage structures where high-temperature capability is not the controlling requirement.

In the figure below we can see 5 different materials that you would typically find in a jet engine. Think about the property requirements for engine components and also the environmental conditions of different parts of this engine and try and answer the following (iv) Why are carbon fiber reinforced polymers (CFRP) used ?

CFRPs are used because they can provide high specific strength and stiffness with low density. This gives weight saving compared with many metallic structures when the service conditions are suitable.

Powder metallurgy routes are becoming more widespread for producing aerospace components. We will discuss these in more detail in future lectures, but can you think of some disadvantages of using this type of process route ?

Disadvantages include cost of powder, energy required to produce the powder, porosity in the finished product, limitations on component size, and unsuitability of some metals and alloys.

Why might porosity be a problem in cast products ?

Porosity reduces mechanical properties such as UTS, ductility, fracture strength and fatigue strength.

Which casting process might be most suitable for producing 2 one-off, large cast -iron castings for a forging press housing ?

Sand casting.

Which casting process might be most suitable for producing 1 million small aluminium smartphone components ?

Permanent mould die casting.

Define w and χ in the rolling load equation

w = width of the material being rolled, and χ is the friction factor.

When preparing to roll a piece of plate steel, if the calculated rolling load exceeds the capacity of the rolling mill, suggest 2 possible solutions to the problem.

Increase temperature to decrease flow stress, reduce the reduction (h0 - hf), or use a bigger capacity mill.

One of the solutions to the problems of incorporating impurities in the finished extruded product is to leave a discard. Think of one disadvantage to this solution.

It is not very efficient in terms of material usage.

6000 series alloys like 6063 are ideal for extruded products. We looked at the example of a ladder lock in the lecture but find out 3 more possible applications for 6000 series alloys.

Examples include bike frames, yacht masts, aircraft wing skin, transport applications such as rail carriages and buses, and window frames.

Which of the forging techniques that are discussed would be the most suitable for producing a titanium fan blade for an aeroengine ?

Isothermal forging, because it gives great control over microstructure and therefore mechanical properties.

How might you eliminate the problem of anisotropy in forging and rolling ?

Rotate the component by 45 or 90 degrees between deformation steps.

Q1. Choosing the most suitable process route is a vital part of producing any metallic component. Traditional routes such as forging, rolling and extrusion are in competition with newer powder metallurgy processes such as additive layer manufacturing (ALM)and metal injection moulding (MIM). Each of these processes will have associated advantages and disadvantages a) Discuss the main features of the metal injection moulding process. Use diagrams if necessary and explain in your answer why it might be advantageous to choose this process route for the production of small aerospace components (6 marks)

Main features: powder plus binder are mixed in the correct ratio, injected into a mould, debound and then sintered. Fine spherical pre-alloyed powder is ideal but expensive. Binder composition may be complex and closely guarded. Draft angles are needed for ejection. During sintering the part must be supported and the correct atmosphere chosen. MIM is advantageous for small aerospace components because it can make complex high precision parts with less waste material and a lower energy footprint than some alternatives.

Q1. Choosing the most suitable process route is a vital part of producing any metallic component. Traditional routes such as forging, rolling and extrusion are in competition with newer powder metallurgy processes such as additive layer manufacturing (ALM)and metal injection moulding (MIM). Each of these processes will have associated advantages and disadvantages b) List 4 potential disadvantages of powder metallurgical processes such as MIM (4 marks)

Possible disadvantages: powders can be costly, potential defects such as porosity, size limitations, significant energy requirements, surface finish problems, and not all current alloy compositions are suitable for ALM.

Q2. Some process techniques are more suited than others for producing metallic aerospace components. Casting, rolling and forging remain the most widely used of these processes. i) Which of the three processes mentioned above would be most suited to producing a) an aircraft wing panel

Rolling.

Q2. Some process techniques are more suited than others for producing metallic aerospace components. Casting, rolling and forging remain the most widely used of these processes. i) Which of the three processes mentioned above would be most suited to producing b) a nickel superalloy turbine blade

Casting.

Q2. Some process techniques are more suited than others for producing metallic aerospace components. Casting, rolling and forging remain the most widely used of these processes. i) Which of the three processes mentioned above would be most suited to producing c) a titanium aeroengine fan blade

Forging.

Q2. Some process techniques are more suited than others for producing metallic aerospace components. Casting, rolling and forging remain the most widely used of these processes. ii) Describe the main features of the investment casting process which can be used to produce certain aeroengine components (4 marks)

A pattern is made of wax or plastic, coated in a refractory slurry, and multiple moulds may be combined into one larger mould called a tree. It is costly, but gives good surface finish and tolerances.

Q2. Some process techniques are more suited than others for producing metallic aerospace components. Casting, rolling and forging remain the most widely used of these processes. iii) Explain why forging is generally considered the most suitable of the different processes for producing a component with excellent fatigue properties (3 marks)

Sophisticated forging processes allow close control over temperature and strain rate, giving good control over microstructure and mechanical properties.

Q1. What is the maximum temperature at which we can use most aluminium alloys ?

About 125°C.

Q2. What could be done to increase the maximum operating temperature of aluminium alloys ?

Add a reinforcement to create a metal matrix composite.

Q3. Which of the light metals exhibits allotrophy ?

Titanium.

Q4. What do the terms FCC, BCC and HCP mean and what do they refer to ?

FCC = face centered cubic, BCC = body centered cubic, HCP (or CPH) = hexagonal close packed. These refer to crystallographic arrangements.

Q5. Which series of aluminium alloy is based on the Al-Cu system ?

The 2000 series.

Q6. Which series of aluminium alloy is based on the Al-Mg system ?

The 5000 series.

Q7. Why are 2000 series alloys suitable for lower wingskin applications ?

The lower wingskin is in tension and, with the correct heat treatment, 2000 series alloys can be damage tolerant, meaning resistant to crack propagation.

Q8. Name the 4 main ways in which we can increase the strength of aluminium ?

Grain refinement, work hardening, solid solution strengthening and precipitation (age) hardening.

Toughness is a key property of metals and is affected by the crystal structure of the metal and the availability of slip systems. The crystal structure will also affect the magnitude of the critical resolved shear stress, which in turn will affect which slip systems are activated first. (a) Define the term toughness for a metal. [2 marks]

Resistance to crack propagation, and the ability to absorb energy before fracture.

Toughness is a key property of metals and is affected by the crystal structure of the metal and the availability of slip systems. The crystal structure will also affect the magnitude of the critical resolved shear stress, which in turn will affect which slip systems are activated first. (b) Toughness can be measured by carrying out impact testing such as a Charpy test. Draw a schematic impact energy vs temperature graph obtained for copper and steel. [2 marks]

Expected sketch: impact energy on the y-axis and temperature on the x-axis. Steel shows a low-energy brittle region at low temperature, then a steep ductile-to-brittle transition into a higher-energy ductile plateau. Copper, as an FCC metal, should stay relatively ductile and should not show the same sharp DBTT.

Toughness is a key property of metals and is affected by the crystal structure of the metal and the availability of slip systems. The crystal structure will also affect the magnitude of the critical resolved shear stress, which in turn will affect which slip systems are activated first. (c) Briefly explain why FCC metals such as aluminium do not exhibit a ductile to brittle transition. [4 marks]

FCC metals always have at least 5 independent slip systems, satisfying the Von Mises criterion. The slip systems are not strongly affected by temperature as in BCC and HCP metals, and lattice friction stress is low. Dislocation motion remains relatively easy at all temperatures, so the metal remains ductile across a large temperature range.

Q1 Briefly explain the reasoning behind the choice of aluminium, titanium and nickel in a modern aeroengine

Titanium is light, strong and corrosion resistant, but it cannot be used over about 500°C, so nickel is used at the hotter back end of the engine. Nickel has good high-temperature strength and can be coated to improve corrosion resistance. Aluminium is used for fuselage material because it is lightweight and cheaper than titanium.

Q2 Titanium alloys must be melted in a vacuum before can be rolled or forged into a final component. Explain why the vacuum melt is necessary and what mechanical properties would be affected if the vacuum treatment was not used

Vacuum melting prevents impurity contamination of titanium, especially oxygen and nitrogen. Many mechanical properties would be affected by high impurity levels, especially fatigue strength.

Q3 Why is specialist equipment such as a Sendzimir Mill (Z-mill) required in the rolling of high strength metal sheet, such as stainless steel ?

Stainless steel is very strong when cold rolled. To deform it and achieve the required thickness, a very rigid mill such as a Z-mill is needed to prevent deflections. Lower strength materials such as aluminium generally do not need such specialist equipment.

Q4 Define the terms used in the extrusion pressure equation, P=σ a+bLn(R)

P = extrusion pressure or load. σ = material flow stress. R = extrusion ratio. a and b are extrusion constants.

Q5 One of the advantages of additive layer manufacturing techniques is that the buy to fly ratio is reduced. What do we mean by the buy to fly ratio ?

The ratio of the raw material used for a component to the weight of the component itself. Ideally the ratio should be 1.

Q6 Explain what you understand by the term near-net shape

Using a technique to initially produce a component very close to the dimensions of the final product. This reduces the number of production steps, particularly finishing or surface treatment steps.

Q7 Briefly explain the importance of the sintering step in metal injection moulding

Before sintering, the product is not fully dense and contains significant porosity, so its mechanical properties are poor and it has limited use. Sintering densifies the component.

Q8 What is the fundamental reason that aluminium is more ductile at low temperatures than the other important light metals, titanium, magnesium and beryllium ?

Aluminium has a face centered cubic structure with many available slip systems that operate at low temperatures. Dislocation motion is relatively easy and ductility remains good. Be, Ti and Mg have CPH/HCP structures with limited available slip systems at room temperature.

Q9 How can we increase the strength of non heat treatable alloys such as AA5083 ?

Use cold working such as cold rolling to increase strength by work hardening, use solid solution strengthening through alloying, or use grain refinement.

Q10 The homogenization heat treatment process is designed to eliminate segregation in cast materials. What is segregation ?

Segregation is a difference in chemical composition over short distances, such as microns, or larger distances, such as millimetres, within the material.

Q11 Which equation can be used to relate the yield strength of a material to the average grain size ?

The Hall-Petch equation: σy = σi + k d^-1/2, where σy is yield strength, σi and k are constants, and d is the average grain diameter.

Q12 In metals processing what is anisotropy and why is it a problem ?

Anisotropy is orientation of grains within the metal caused by the production process. In extrusion or rolling, grains may be elongated in the extrusion or rolling direction. This means mechanical properties, or how the material deforms, may differ depending on the direction of applied stresses.

Give another example of a metallic engineering material where magnetic or electrical properties may matter more than strength.

Electrical steels in motors or transformers are designed mainly for magnetic and electrical performance rather than maximum strength.

Why are titanium alloys suitable for fan and compressor regions but not the hottest turbine stages?

They have high specific strength, low density and corrosion resistance, but they are limited at high temperature, roughly above about 500°C.

What properties make nickel-based superalloys suitable for combustor and turbine sections?

They maintain high-temperature strength and resist mechanical and chemical degradation in severe operating environments, often helped by coatings or cooling.

Why is aluminium suitable for airframe structures but not hot engine sections?

It is low density and relatively cheap, but most aluminium alloys have limited high-temperature capability.

Why might CFRP be chosen instead of a metal in aircraft structures?

CFRP can give high specific strength and stiffness with low density, giving useful weight saving where the service conditions are suitable.

Why can powder-based aerospace manufacturing routes be expensive?

Powders can be costly to produce, powder manufacture can have a high energy footprint, and specialist equipment or post-processing may be required.

How does porosity reduce fatigue strength in a casting?

Pores act as stress concentrators and crack initiation sites, reducing fatigue life.

Which casting process would suit a very large one-off component with low dimensional accuracy requirements?

Sand casting.

Why is permanent mould or die casting suited to high-volume small aluminium components?

The reusable mould gives high production rates and good finish or tolerance once the tooling cost is justified.

In the rolling force equation F = χ w R(h0 - hf) σflow, what happens if the strip width increases?

The rolling force increases because width is one of the multiplying terms in the equation.

How does increasing the rolling temperature help when the mill load is too high?

It lowers the material flow stress, which reduces the rolling load.

Why is leaving a discard in extrusion a trade-off?

It helps avoid impurity or piping defects in the product, but it wastes part of the billet.

Why are 6xxx aluminium alloys often used for extruded products such as window frames?

They are Al-Mg-Si heat-treatable alloys with good extrudability and useful strength for complex profiles.

What is the key benefit of heating the dies in isothermal forging?

Cooling of the workpiece is reduced, low flow stress is maintained, and microstructure and properties can be controlled more closely.

Why might rolling in one direction produce direction-dependent properties?

Grains can elongate or become oriented along the rolling direction, so properties may differ parallel and perpendicular to that direction.

List the basic processing sequence for metal injection moulding.

Mix powder and binder, injection mould the feedstock, remove the binder, then sinter to densify the part.

Name two defects or limitations that may affect powder metallurgy routes such as MIM or ALM.

Examples include porosity, surface finish problems, size limits, costly powders and unsuitable alloy systems.

Which process is normally used to make flat sheet or plate for aircraft panels?

Rolling.

Why can investment casting be suitable for nickel superalloy turbine blades?

It can produce complex shapes with good surface finish and tolerances, and it is suitable for components such as turbine blades.

Why is forging preferred for titanium aeroengine fan blades?

Forging gives good control of microstructure and mechanical properties, which is important for fatigue performance.

What is meant by a casting tree in investment casting?

Several wax or plastic patterns are joined into one larger mould arrangement so multiple components can be cast together.

How does control of strain rate and temperature affect a forged part?

It helps control the microstructure, which affects the final mechanical properties.

Why do most aluminium alloys have a limited maximum service temperature?

Their mechanical properties are not retained well at high temperature compared with materials such as titanium or nickel alloys.

What are the two main constituents of a metal matrix composite?

A metal matrix and a reinforcement, such as an oxide, carbide, nitride, particle, fibre or whisker.

What does allotropy mean for titanium?

Titanium can change crystal structure with temperature, from alpha CPH/HCP to beta BCC.

Which important light metal in the slides has an FCC crystal structure?

Aluminium.

What main alloying element defines 2xxx aluminium alloys?

Copper.

What strengthening mechanisms are used in non-heat-treatable 5xxx aluminium alloys?

Solid solution strengthening from magnesium, cold work and grain refinement.

Why is damage tolerance important for lower wing skin?

The lower wing skin is in tension, so resistance to crack propagation is important.

Which strengthening mechanism relies on forming useful fine particles in the grains?

Precipitation hardening, also called age hardening.

Why is toughness important when considering DBTT?

Toughness is resistance to crack propagation and ability to absorb energy before fracture, which is critical at low temperature.

What does low Charpy impact energy at low temperature indicate?

Brittle behaviour with low energy absorption before fracture.

Which crystal structures tend to exhibit a ductile-to-brittle transition according to the slides?

BCC and HCP/CPH materials can exhibit a DBTT because slip behaviour is temperature dependent.

Why are different materials used at different positions in a modern aeroengine?

Temperature, loading, density requirements and environmental conditions vary along the engine, so the best material changes by location.

Which impurities are especially problematic during titanium melting?

Oxygen and nitrogen are especially problematic because they contaminate titanium and damage mechanical properties.

What is the purpose of a highly rigid cluster mill or Z-mill?

It prevents deflection while cold rolling thin sheet of high-strength material such as stainless steel.

In the extrusion pressure equation P/σ = a + b ln(R), what does R represent?

R is the extrusion ratio, usually initial billet area divided by final extrudate area.

Why can additive layer manufacturing reduce the buy-to-fly ratio?

It builds a near-net-shape part, so less material is removed as machining waste.

Give one advantage of near-net-shape processing.

It reduces machining, finishing steps, waste, cost or lead time by making the initial part close to final dimensions.

What happens to porosity during sintering in metal injection moulding?

Porosity is reduced as the component densifies, improving mechanical properties.

Why does limited slip in HCP or CPH metals reduce ductility?

Fewer available slip systems make dislocation motion harder, especially at low temperatures.

Why does cold rolling strengthen AA5083?

Cold rolling increases dislocation density, so dislocations obstruct each other and the alloy work hardens.

Why should cast ingots be homogenised before hot working?

Homogenisation reduces microsegregation and macrosegregation, otherwise defects and processing problems can occur during hot working.

According to the Hall-Petch relationship, what happens to strength as grain size decreases?

Yield strength increases as average grain size decreases.

How can anisotropy affect a rolled sheet during forming?

The sheet may deform differently in different directions, causing direction-dependent properties and forming behaviour.