Materials Part C - Powder metallurgy

How is friction stir welding done?

Rotating tool with flat face is pressed ontop of mating edge and draw across seam while spinning so soften and join the metals.

What are the advantages behind friction stir welding?

• Excellent mechanical properties

• No porosity or distortion

• Non consumable tool can be used for many operations

• Can be used for on the spot repair work

What are the disadvantages behind friction stir welding?

• Expensive equipment

• Very difficult to apply to some materials.

What are some applications for friction stir welding?

• Shipbuilding and marine structures

• Wings, fuel tanks

• train bodies, railway tankers, container bodies

What are the main manufacturing processes in powder metallurgy?

• Additive layer manufacturing

• Metal Injection moulding

• Hot isostatic pressing

How is the metal powder created?

Through a repaid solidification process where a liquid stream of metal is blasted with an inert gas or water.

What are the 2 types of powder additive layer manufacture?

• Blown powder

  • C02 laser is used to melt layers of powder in place are they blown in by several nozzles

• Powder bed

  • Powder is inserted layer by layer and a laser passes over each layer solidifying the powder in area where it is needed.

What are some drawbacks behind this method of manufacturing (laser systems specifically)?

• Since heating is very localised, surrounding powder is cold leading to residual stress in the part

• Thin walls are almost impossible

What are other limitation with additive style manufacture?

• Creation of overhangs is difficult leading to wasted support material.

• Cant have enclosed space for powder bed manufacture since void will be full of loose powder.

What are the advantages of using electron beam instead?

• Preheating occurs on every layer

• This leaves the part annealed with no residual stress

• Fatigue properties are therefore much better than cast parts and comparable to forged

What are the overall advantages to additive manufacture processes?

• Shorter lead time therefore designs can be iterated faster

• Less wasted material

• Allows for more creative freedom when making parts

• Can use difficult to machine alloys

What are 2 other powder metal processing routes?

• Metal injection moulding

• Hot Isostatic Pressing

What are the main stages involved in metal injection moulding?

• Metal powder and binder is mixed

• Binder metal mix is heated and forced into mould

• Part is then placed in oven and heated further to remove binder

• Part is placed in even hotter oven and fed with highly pressurised noble gas to compress and sinter final part

• This is done to remove porosity left from the binder.

What are some considerations when looking at metal injection moulding of a part?

• Powder need to be of high quality (spherical and fine)

• Binder - Secret formula

• Careful handling of debinded part (high porosity)

• Sintering - Correct atmosphere and temperature are required

• Part shrinkage

What are advantages to metal injection moulding?

• Offers high flexibility for chemistry and geometry

• Much lower energy footprint per part than casting

What are some limitations for metal injection moulding?

• Carbon pickup

• Powders for specialty alloys are very expensive

What are key advantages of hot isostatic pressing?

• Consolidation of materials in solid state

• Minimises segregation of heavy atoms (happens in melt routes)

• Smaller grain size than sintering

• Heals porosity of castings

What are some key considerations of hot isostatic pressing?

• Capsule needs to be manufactured

  • Efficient at transferring pressure

  • Joinable

  • Low cost

• Design of part needs to be carefully considered and process needs to be simulated

• Removal of mould requires strong acids and is a lengthy process

Give key conclusions to HIP?

• Costly

• Complex

• Low volume

• Shape and surface quality is excellent