Powder Metallurgy Flashcards
What are the key factors used to describe the geometry features of engineering powders ?
- Particle size and distribution ( how well powders pack and flow )
- Particle size
- Internal structure
- Surface area
How does particle shape impact the flow ability and interparticle friction of powders ?
- Spherical particles : low inter particle friction & better flow ability
- Irregular/ flake-like particles: higher friction & poor flow leading to difficulty in packing.
What is the primary difference between elemental powders and pre-alloyed powders ?
Elemental powders consist of pure metal ( iron, copper, aluminum).
Pre-alloyed powders particles are already from an alloy ( stainless steel, high-speed steel) offering more precise chemical composition control.
What are the advantages of using the gas atomization method for producing metallic powders ?
~ Produce spherical particles with excellent flow ability
~ Can use inert gases to prevent oxidation during powder production
~ Offers a narrow particle size distribution, improving packing efficiency
Why are powders compacted before sintering ?
To give the powder a specific shape and mechanical integrity
What is meant by “green compact” ?
It refers to the pressed powder part that has not yet been sintered
Explain how sintering contributes to the bonding and densification of the powder ?
- Powders are heated 70-90% of their melting point
- The particles bond at contact points through diffusion, reducing pore size and increasing strength
- Shrinkage occurs as pores close, enhancing density and mechanical properties of the part.
What secondary operations can be applied after sintering and why are they necessary?
I- Repressing : increases density and mechanical properties
II- sizing: Ensures dimensional accuracy
III- Coining : Adds surface details
IV- Machining: Creates features like threads or holes that cannot be formed by pressing.
Which metals are most commonly used in powder metallurgy and why ?
Iron/ steel : Strength and wear resistance
Copper : High conductivity
Aluminum : Lightweight
Tungsten Carbide : Hardness and Wear resistance
How does the porosity of PM parts enhance their functionality in certain applications?
Porous structure allows absorption of lubricants during impregnation enabling self-lubricating in gears and bearings
Porosity can also reduce weight while maintaining part integrity
What are some limitations and disadvantages associated with powder metallurgy processes?
- High tooling & Equipment cost
- Raw materials are expensive
- Storage challenges
- Design limitations
How do part geometry and powder flow ability influence the design of PM?
• Parts must have simple shapes to facilitate ejection from the die
• Holes and undercuts are difficult to form requiring seconds operations
• Poor flow ability can lead to uneven density affecting strength and performance
What are the advantages of isostatic pressing instead of conventional pressing in PM ?
Isostatic applies pressure uniformly from all directions leading to more consistent density throughout the part.
How does powder injection molding differ from conventional PM method?
In PIM a mixture of powder and polymer binder is injected into the mold.
The polymer is later removed and the part is sintered.
PIM is used for highly complex geometry.
Why is powder metallurgy economically viable for high production volumes ?
High initial cost of equipment & tooling
Cost per part decreases with higher volume due to economies scale.
