Powder Metallurgy
What are some of the disadvantages of PM methods?
high tooling costs, (2) metal powders are expensive due to the time and technology involved in powder production, (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 technical difference between blending and mixing in powder metallurgy?
refers to combining particles of the same chemistry but different sizes, whereas mixing means combining metal powders of different chemistries.
. What is meant by the term "green" compact?
the pressed but not yet sintered PM part.
What are the different atomization techniques in powder production?
(1) Gas atomization, where the molten metal is drawn through a siphon and sprayed with a gas jet; (2) Gas atomization, where the molten metal flows via gravity into a gas jet stream; (3) Water (or oil, other fluid) atomization, where the molten metal flows via gravity into a water jet stream; (4) Centrifugal atomization, where the molten metal falls onto a rotating disc and is sprayed into powder.
What are the three 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; and (3) electrolysis - use of an electrolytic cell to deposit particles of the metal onto the cathode in the cell.
What are the three basic steps in the conventional powder metallurgy shaping process?
(1) blending and/or mixing, (2) pressing (compaction), and (3) sintering.
What are the two basic classes of metal powders as far as chemistry is concerned?
(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.
Name some of the reasons for the commercial importance of powder metallurgy technology.
(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 the three steps in the sintering cycle in PM?
(1) preheat, in which lubricants and binders are burned off, (2) sintering, and (3) cool down.
Why is PM technology so well suited to the production of gears and bearings?
(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.
Why might porosity in the final part be an advantage or disadvantage?
. Adv: Can be used for self-lubricating parts such as bearings and gears, and parts where porosity is a design function such as filters. Disadv: Material anisotropy, lower strength than 100% dense part, water permeation could cause oxidation.
What is the difference between impregnation and infiltration in PM?
Impregnation is the term used when oil or other fluid (e.g., polymer melt) is permeated into the pores of a sintered PM part, whereas infiltration is an operation in which the pores of the PM part are filled with a molten metal whose melting point is below that of the PM part.
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.
Which is the most common atomization technique? What are its advantages/disadvantages?
Water atomization. Adv: Rapid cooling, water velocity is related to powder particle size and can be controlled. Disadv: Causes oxidation in ferrous powders, so water may be replaced with synthetic oils