ISE FINAL: CHAPTER 15

What is the technical difference between blending and mixing 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.

Define bulk density and true density for metallic powders.

Bulk density refers to the weight per volume of the powders in the loose state, while true density is the weight per volume of the true volume of metal in the powders (the volume that would result if the powders were melted).

What are some of the disadvantages of PM methods?

Disadvantages include (1) high tooling costs, (2) metal powders are expensive, (3) difficulties in storing and handling metallic powders, (4) certain limitations on part geometry imposed by the uniaxial press methods, and (5) variations in density in a PM component can be troublesome.

What is the difference between impregnation and infiltration in powder metallurgy?

Impregnation is when oil or other fluid is permeated into the pores of a sintered PM part. Infiltration is when a molten metal (other than the PM metal) is permeated into the pores of a sintered part.

How is isostatic pressing distinguished from conventional pressing and sintering in PM?

Isostatic pressing applies hydrostatic pressure to all sides of the mold, whereas conventional pressing is uniaxial.

What is the difference between powder injection molding and metal injection molding?

Metal injection molding is a subset of powder injection molding, in which the powders are metallic. The more general term includes powders of ceramic.

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 are air spaces between particles, while closed pores are voids internal to a particle.

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

PM is important because (1) parts can be made to net or near net shape, (2) parts can be made with a controlled level of porosity, (3) certain metals difficult to process by other methods can be processed by PM, and (4) PM allows the formulation of unusual alloys not easily obtained by traditional alloying methods.

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, and (4) to remove lubricants and binders from pressing.

Describe what happens to the individual particles during compaction.

Starting with the initial powder arrangement, the particles are first repacked into a more efficient arrangement, followed by deformation of the particles as pressure is increased.

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

The additives include (1) lubricants, (2) binders, and (3) deflocculants.

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

The aspect ratio of a particle is the ratio of the maximum dimension to the minimum dimension of the given particle.

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 are the principal methods used to produce metallic powders?

The powder production methods are (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; and (3) electrolysis - use of an electrolytic cell to deposit particles of the metal onto the cathode in the cell.

Why is PM technology so well suited to the production of gears and bearings?

The reasons are (1) the geometries of these parts lend themselves to conventional PM pressing, which consists of pressing in one direction, and (2) the porosity allows impregnation of the PM parts with lubricants.

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

The steps are (1) blending and/or mixing, (2) pressing, and (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, and (3) cool down.