16.11 What are some of the ingredie

16.11 What are some of the ingredients usually added to the metallic powders during blending and/or
mixing?
Answer. The additives include (1) lubricants, (2) binders, and (3) deflocculants.
16.12 What is meant by the term green compact?
Answer. The green compact is the pressed but not yet sintered PM part.
16.13 Describe what happens to the individual particles during compaction.
Answer. 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.
16.14 What are the three steps in the sintering cycle in PM?
Answer. The three steps in the cycle are (1) preheat, in which lubricants and binders are burned off,
(2) sintering, and (3) cool down.
16.15 What are some of the reasons why a controlled atmosphere furnace is desirable in sintering?
Answer. 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.
16.16 What are the advantages of infiltration in PM?
Answer. Advantages of infiltration are (1) the resulting structure is nonporous structure and (2)
toughness and strength are improved.
16.17 What is the difference between powder injection molding and metal injection molding?
Answer. Metal injection molding is a subset of powder injection molding, in which the powders are
metallic. The more general term includes powders of ceramic.
16.18 How is isostatic pressing distinguished from conventional pressing and sintering in PM?
Answer. Isostatic pressing applies hydrostatic pressure to all sides of the mold, whereas
conventional pressing is uniaxial.
16.19 Describe liquid phase sintering.
Answer. Liquid phase sintering occurs when two metals of different melting temperatures are
sintered at a temperature between their melting points. Accordingly, one metal melts, thoroughly
wetting the solid particles and creating a strong bonding between the metals upon solidification.
16.20 What are the two basic classes of metal powders as far as chemistry is concerned?
Answer. The two classes are (1) elemental powders - powders of pure metal such as iron or copper,
and (2) pre-alloyed powders - powders of alloys such as stainless steel or brass.
16.21 Why is PM technology so well suited to the production of gears and bearings?
Answer. 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.