Exam Questions

Question 1

The yield strength of heat-treatable aluminium alloys after extrusion and ageing depends on the shape of the component, the processing, and the alloy. Summarise the key parameters and characteristics of the geometry, the process history and the alloy composition and microstructure, and explain briefly how they influence the outcome.

Answer 1

• The question relates to wrought precipitation hardened aluminium alloys, e.g. Al- 4%Cu. These are typically shaped by extrusion, as here, immediately followed by ageing to produce the required distribution of precipitates and increase the strength.
• Key parameters and characteristics:
• section geometry: governs the cooling rate, particularly important for the quench stage. i.e. distance from surface to point of slowest cooling
• process history: billet pre-heat temperature, uniformity of deformation heating and temperature rise, whether fully solutionised, cooling process after extrusion, ageing temperature and time. Recrystallisation occurs during extrusion; grain size is affected by thermomechanical history and presence of dispersoids (particle stimulated nucleation of recrystallisation).
• alloy composition and microstructure: composition of heat-treatable alloy determines artificial ageing response, presence of dispersoids from earlier process steps may lead to quench sensitivity (coarse non-hardening precipitation during cooling, depending on imposed cooling rate)
• Overall effect: cooling rate imposed across whole profile needs to avoid C-curves for coarse precipitation, to retain 100% supersaturated solid solution and achieve full potential peak hardening during artificial ageing.

Question 2

Write brief notes to explain, in each case, one beneficial and one detrimental aspect of the following alloying elements in the alloys indicated:

• either Mn or Fe in heat-treatable aluminium alloys;
• Pb in carbon steels;
• Ni in low alloy steels.

Answer 2

• Mn and Fe in heat-treatable aluminium alloys Beneficial: both form second-phase dispersoids during casting and homogenisation, which can be used to control grain size on recrysallisation (via PSN, particle-stimulated nucleation).
  
  • Detrimental: during quenching after solution heat treatment, Mn and Fe containing dispersoids can nucleate non-hardening phases, using up solute that would otherwise remain in supersaturated solid solution for subsequent age hardening. Peak aged strength is then reduced (the effect known as quench sensitivity).
• Pb in carbon steels Beneficial: addition of Pb used in free-machining steels, with Pb forming inclusions on grain boundaries promoting chip formation, enabling self-lubricating machining with lower cutting forces.
  
  • Detrimental: various aspects – Pb is expensive and environmentally damaging; also it lowers toughness and fatigue resistance of the steel.
• Ni in low alloy steels
  
  • Beneficial: increases hardenability, enabling quenching of larger components prior to tempering for optimum strength and toughness. (May also contribute directly to strength via solid solution).
  • Detrimental: higher hardenability leads to poor weldability, with the risk of forming martensite in the heat-affected zone, with consequent embrittlement.

Question 3

Why is it important to be able to control the size and shape of grains in cast components?

Describe three ways in which grain size can be reduced during casting.

Explain how a single-crystal gas turbine blade can be made from a nickel-base superalloy.

Answer 3

Grain size: Small grains improve mechanical properties (yield strength and toughness). Reduce effects of impurity segregation to grain boundaries by increasing grain boundary area so reducing impurity concentrations. Chemical inhomogenity can more easily be reduced by heat treatment alone in fine grained materials.

Large grain size – particularly eliminating boundaries normal to the stress axis - used to improve creep properties. Single crystals avoid the problem altogether.

Grain shape: Columnar grains may lead to interconnected porosity.

Control during casting: Inoculants to control nucleation; reduce pouring temperature to increase nucleation; vibrate mould to increase turbulence and displace dendrite arms which act as nuclei. Casting into a chilled mould increases nucleation so reduces grain size.
Post-casting control: Deformation and heat (or hot deform) to recrystallise.

Single crystal turbine blade: investment casting with a ‘pigtail’: a helical cavity which selects one crystal from the chill zone by allowing only the most favourably oriented crystal to grow.

Question 4

State the assumptions made in the upper bound method for predicting forces in metalworking processes.

Answer 4

The upper bound method is based on the upper bound theorem which states that the plastic collapse load estimated by equating the rate of working of external forces to the internal rate of energy dissipation, for any compatible mechanism of deformation, will form an upper bound to the true load. We usually assume plane strain deformation and a rigid plastic material, with shear on internal shear planes.

Question 5

Mild steel can be protected against corrosion under damp conditions by using coatings made from a range of different materials.

Explain why galvanised mild steel does not corrode even if the coating is incomplete, whilst painted steel may start to corrode as soon as the paint layer is scratched.

Where will corrosion then take place?

Answer 5

Galvanised iron is coated with a thin layer of zinc. The zinc dissolves to form Zn2+ ions, releasing electrons. The electrons flow into the iron, which becomes the cathode. On the iron surface, oxygen dissolved in the water reacts with water to form hydroxyl ions OH-. This uses up electrons, so the corrosion of the zinc continues. If the zinc (with the lower SEP) were not present, the anodic reaction would be oxidation of the iron, which would therefore corrode. The presence of the zinc protects the iron.

Corrosion when paint films have been disturbed is an example of differential aeration or concentration corrosion. (It is the essentially the same reaction as the one above but instead Iron is being oxidised)

The oxygen levels are highest directly under the crack in the paint film, and the lowest oxygen levels are under the paint film next to the crack. Corrosion (the anodic process) will be concentrated here where the oxygen concentration is lowest, so cracks spread under the paint film.

Question 6

Welding of steels is associated with a number of possible defects. For each of the following, explain briefly the nature of the phenomenon and how it arises, and suggest ways in which it can be reduced or eliminated:

• residual stress
• stress concentrations
• weld decay

Answer 6

• Thermal residual stresses result when only part of a component is heated, as usually occurs in welding. The material close to the weld (HAZ) is heated and expands. The hot region is constrained by the cooler surrounding material away from the weld pool, and so is put into compression. However, being hot, its yield stress is reduced, so it can plastically deform relatively
  
  • easily (and it may also creep). Where the material can flow to depends on the geometry. When the region cools, there is contraction, and the HAZ and weld pool are put into tension. Stresses also arise because of solidification of the molten metal in the weld pool: this will increase tensile stresses in the HAZ.
  • Tensile stresses are generally of the order of the material yield stress, and are balanced by compressive stresses elsewhere.
  • Reducing Stresses: Pre-heat the whole component; post-weld heat-treat to allow stresses to relax.
• Stress concentrations arise at any section change or change in profile, such as are usually found at weld beads. They can be reduced by grinding the beads flat, or grinding our convex profiles to make them concave thus reducing the contact angle. Surface grinding marks can themselves act as stress concentrators.
• Occurs in the HAZ in unstabilised stainless steels. Chromium reacts with carbon to form carbide particles which precipitate on grain boundaries. This leaves a chromium-depleted region around grain boundaries, which means that chromium oxide (which normally acts as a protective layer on the steel surface) cannot form, leaving the grain boundary regions vulnerable to corrosion. These regions will form localised anodes, and deep cracks can form quickly.
  
  • Problem can be avoided by using steel of a different composition: stabilised by the inclusion of small amounts of niobium or titanium, which form carbides preferentially, so leaving the chromium in solid solution and able to form the oxide surface layer.

Question 7

State the assumptions made when doing equilibrium analysis calculations.

Answer 7

Assumptions​

• Homogeneous deformation, constant yield stress (i.e. neglect deformation heating)
• principal stresses assumed to be in 1 and 2 directions: correct on the centre-line, and also throughout the element when frictionless. But when sticking friction occurs, the principal axes will deviate from the 1,2 directions as the die wall is approached since the shear stresses become significant compared to the direct stresses. Hence it is an approximation to treat σ1 and σ2 as principal stresses
• von Mises yield criterion is itself a model for the real material behaviour

Question 8

In a failure investigation, samples of a high-strength stainless steel were loaded in tension at different strain rates in a bath of hot salt solution.

At low and at
high strain rates the steel exhibited ductile failure, but at an intermediate strain rate it showed brittle fracture at a lower stress.

Explain these observations.

Answer 8

This is an example of strain-rate sensitivity in stress corrosion cracking. At low strain-rates, the metal surface can re-passivate. At high strain-rates, the rate of crack growth is limited by the rate at which the reactive agent (in this case, chloride ions) can be transported to the crack tip. There is therefore a critical strain-rate range for SCC as shown in the sketch below.

Question 9

Explain the following observations and identify the underlying physical principles in each case:

(i) the grain size in an Al alloy casting is reduced by adding TiB2 powder to the melt before casting;
(ii) the grain size in an Al alloy component which has been hot-forged after prolonged homogenization depends on its Mn and Cr content;

Answer 9

• TiB2 powder acts as an inoculant in Al alloy castings, stimulating extensive heterogeneous nucleation of the initial solid on solidification. Each nucleus becomes a grain, reducing the grain size. The important physical behaviour is the competition between the surface energies ofliquid-inoculant, solid-inoculant, and solid-liquid.
• This is captured by the wetting angle at the boundary of the solid-liquid interface on the inoculant surface. A low angle is most effective solid rapidly spreads over the particle, so that the stable nucleus radius is exceeded with a lower volume of solidified liquid.
• Mn and Cr are elements that form dispersoids in Al alloys (second phase particles). The average size and volume fraction of dispersoids may be changed by varying the homogenization treatment after initial ingot casting. The physical effect ofthe dispersoids is that in hot worked Al alloys, they act as nuclei for recrystallisation (in the solid state, after forming) so-called particle-stimulated nucleation (PSN). Each nucleus becomes a recrystallised grain, giving a grain size that is dependent on the Mn and Cr content and homogenization treatment (even ifthe deformation and annealing conditions are exactly the same).

Question 10

Explain the following observations and identify the underlying physical principles in each case:

Answer 10

• Magnesium poisons the growth ofbrittle flake graphite, leading to spherodised graphite nodules and higher strength and ductility (as premature fracture is avoided in tension). The physical effect of poisoning is that the trace alloy addition blocks ledge growth of the non-metal phase, leading to a finer and more rounded morphology of the brittle phase.
• The metal blocks in the wall ofthe mould are chills, designed to accelerate the cooling rate in this part of the casting. By solidifying this region first, feeding of liquid metal is improved, avoiding shrinkage problems such as macroporosity, which are associated with trapped volumes of liquid solidifying with no route for the dissolved gas to escape.