Metals- Novel Processing Flashcards
What is friction stir welding?
A cylindrical tool is placed on the joint line between two materials. The tool rotates at high speeds to generate frictional heat. The workpiece starts to soften and as the tool moves along it leaves behind a solid phase bond between the two pieces. No need for filler materials
What is friction stir welding used for (FSW)?
Joining of plates or sheets up to about 50mm thick. Particularly suitable for joining of dissimilar metals.
Advantages of FSW
Excellent mechanical properties of join (often stronger than the parent material). No porosity or distortion. Non-consumable too which can be used for up to 1000m. Possible to perform on the spot repair work.
Disadvantages of FSW
Costly equipment. Some materials difficult to FSW. Al and Mg easier than steel or Ti.
Applications of FSW
Shipbuilding and marine structures. Aerospace e.g wings, fuel tanks. Train bodies, railway tankers and container bodies. Automotive and military.
Features of alloys powder metallurgy is used for
High cost and high performance. Difficult to machine. High scrap rate if conventionally processed via e.g casting. Multiple steps required to achieve final geometry
How does additive layer manufacturing work?
Layer upon layer addition of material. The feedstock may vary (powder, foil, wire). Near-net shape or net-shape achieved.
Buy-to-fly ratio
Amount of metal bought to amount of component made
Advantages of additive layer manufacturing
Way of reducing buy-to-fly ratio for aerospace. Low amount of scrap once optimised. Design iteration friendly (don’t need new tooling for new designs). Customisable. Low lead-times. Can make parts deemed impossible with conventional techniques.
Laser systems for ALM
They apply heat only to the part slice. Surrounding powder very cold (means residual stresses). Laser scanning changes by 90° on each layer. Layer thickness 20-100μm. Thin wall parts almost impossible
Electron beam for ALM
Arcam the most promising. Predominantly for Ti-6Al-4V and CoCr and other aerospace alloys. Preheating step occurs on every layer. Means part is annealed with no residual stresses. Fatigue properties much better than cast and comparable to forged. UTS and elongation % greater than cast.
Why is there excess material used during AM?
To support parts of the component while it is being made and then is removed
Disadvantages of ALM
Hardware and powders can be expensive. Need highly skilled workers
How does metal injection moulding work?
Direct evolution of polymer injection moulding. Mix metals with binders and grind into granulated feedstock. Put into injection moulder via hopper and is heated (not melted) and forced into the mould. Moulded part put in oven and debound part (brittle) removed. This is sintered in vacuum furnace. Then hot isostatic pressing, heat treatments, reworking into final product nearly 100% dense
Metal injection moulding considerations
Fine and spherical powder is the best but more expensive. Prealloyed powder generally used. 35/65 ratio binder to metal. For sintering need correct atmosphere, temperature and time to account for movement and shrinkage of parts
Advantages of MIM
Great for very small parts. Many high precision aerospace components made. Large flexibility on chemistry and geometry. Is solid state processing route so much lower energy footprint per part than casting. High production rates for high complexity parts
Disadvantages of MIM
Carbon pickup is a problem of high end applications. Powders very expensive for specialty alloys.
Advantages of hot isostatic pressing
Consolidation of materials in solid state. Minimes segregation of heavy atoms. Only way to make some metal matrix composites. Shorter cycle times at elevated temperatures than sintering. Smaller grain size than sintering. Heals internal porosity in castings. Used for cladding for functionally graded materials
HIP considerations
The capsule (vessel) needs to be joinable and low cost. Design of which important to transfer pressure efficiently. Finite element tools used for simulating process. Cycle is 1000°C and 100Mpa for several hours. Mould removal uses strong acids and is lengthy process
Disadvantages of HIP
Costly, complex, low volume.
But shale complexity and surface quality unparalleled
3 other powder processes
Sintering, field assisted sintering, continuous consolidation
How does spray deposition work?
Not strictly powder metallurgy process. Molten metal atomised and spray directed to fall onto a solid former where a layer of dense metal of a pre-determined shale is formed. Structure similar to from powder material (fine grains and no segregation).
Advantages of spray deposition
Properties can be superior to those of powder based wrought materials. Inclusion content much lower e.g superalloys. Unconventional alloy compositions possible. Composite formation possible by adding reinforcements into jet stream
