Powder Metallurgy

As particle size increases, interparticle friction (a) decreases, (b) increases, or (c) remains the same?

a) increases

Which of the following design features would be difficult or impossible to achieve by conventional pressing and sintering (three best answers): (a) outside rounded corners, (b) side holes, (c) threaded holes, (d) vertical stepped holes, and (e) vertical wall thickness of 1/8 inch (3 mm)?

a) outside rounded corners b) side holes c) threaded holes

Which of the following statements is correct in the context of metallic powders (three correct answers): (a) porosity + packing factor = 1.0, (b) packing factor = 1/porosity, (c) packing factor = 1.0 - porosity, (d) packing factor = - porosity, (e) packing factor = bulk density/true density?

a) porosity+packing factor=1 c) packing factor=1-porosity e) packing factor=(bulk density)/(true density)

In cold isostatic pressing, the mold is most typically made of which one of the following: (a) rubber, (b) sheetmetal, (c) textile, (d) thermosetting polymer, or (e) tool steel?

a) rubber

Impregnation refers to which of the following (two best answers): (a) filling the pores of the PM part with a molten metal, (b) putting polymers into the pores of a PM part, (c) soaking oil by capillary action into the pores of a PM part, and (d) something that should not happen in a factory?

b) putting polymers into the pores of a PM part c) soaking oil by capillary action into the pores of a PM part

What are the principal methods used to produce metallic powders?

(1) atomization - the conversion of molten metal into droplets which solidify into powders (2) chemical reduction - reducing metallic oxides by use of reducing agents which combine with the oxygen to free the metals in the form of powders (3) electrolysis - use of an electrolytic cell to deposit particles of the metal onto the cathode in the cell.

The particle size that can pass through a screen is obtained by taking the reciprocal of the mesh count of the screen: (a) true or (b) false?

(b) false The given description neglects consideration of the screen wire thickness

PM is especially suitable for what metals?

- Metals with low ductility - Metals with high melting temp

Name some of the reasons for the commercial importance of powder metallurgy technology.

- PM parts can be mass produced to net shape or near net shape, eliminating or reducing the need for subsequent processing. - Certain metal alloy combinations and cermets can be formed by PM that cannot be produced by other methods. - The PM process itself involves very little waste of material. - Certain metals that are difficult to fabricate by other methods can be shaped by powder metallurgy. - PM production methods can be automated for economical production

What are some of the disadvantages of PM methods?

- Tooling and equipment costs are high - metallic powders are expensive -There are difficulties with storing and handling metal powders (such as degradation of the metal over time, and fire hazards with particular metals). - There are limitations on part geometry - Variations in material density throughout the part may be a problem in PM, especially for complex part geometries.

What are some of the ingredients usually added to the metallic powders during blending and/or mixing?

- lubricants: reduce friction between particles - binders: required in some cases to achieve adequate strength in the pressed but unsintered part - deflocculants: inhibit a clump of powders for better flow characteristics during subsequent processing.

What are the advantages of infiltration in PM?

Advantages of infiltration are (1) toughness and strength are improved (2) the resulting structure is nonporous structure

What is the technical difference between mixing and blending in powder metallurgy?

Blending means combining particles of the same chemistry but different sizes, while mixing refers to the combining of metal powders of different chemistries.

List and describe the two forms of isostatic pressing

Cold isostatic pressing Involves compaction performed at room temperature. Advantages include more uniform density, less expensive tooling, and greater applicability to shorter production runs. Hot isostatic pressing Carried out at high temperatures and pressures.

How would one measure the angle of repose for a given amount of metallic powder?

One measure would be to let the powders flow through a small funnel and measure the angle taken by the resulting pile of powders relative to the horizontal.

What is the difference between open pores and closed pores in a metallic powders?

Open pores: spaces into which a fluid can penetrate. They are external to the individual particles. Closed pores: internal voids in the structure of an individual particle. Influence is minor but can effect the density measurement.

What are some of the reasons why a controlled atmosphere furnace is desirable in sintering?

Some of the purposes of a controlled atmosphere furnace are (1) to protect against oxidation (2) to provide a reducing atmosphere to remove existing oxides (3) to provide a carburizing atmosphere (4) to remove lubricants and binders used in pressing.

What is meant by the term green compact?

The green compact is the pressed but not yet sintered PM part.

In the screening of powders for sizing, what is meant by the term mesh count?

The mesh count of the screen is the number of openings per linear inch.

What is meant by the term aspect ratio for a metallic particle?

The ratio of maximum dimension to minimum dimension for a given particle.

What are the three basic steps in the conventional powder metallurgy shaping process?

The steps are (1) blending and/or mixing (2) pressing (3) sintering.

What are the three steps in the sintering cycle in PM?

The three steps in the cycle are (1) preheat, in which lubricants and binders are burned off (2) sintering (3) cool down.

Define bulk density and true density for metallic powders.

True density: density of the true volume of the material (powder melted to solid). Bulk density: the density of the powders in the loose state after pouring (has pores)

Which of the following powder shapes would tend to have the lowest interparticle friction: (a) acicular, (b) cubic, (c) flakey, (d) spherical, and (e) rounded?

d) spherical