Powder Metallurgy Flashcards
Powder Production - Describe the comminution process
A mechanical process that involves crushing, milling or grinding.
Small particles are produced using a ball mill - rotating hollow cylinder partly filled with steel or white cast iron balls
(Most widely used)
What are the methods of compaction?
Cold Isostatic Pressing Hot Isostatic Pressing Powder Rolling Extrusion Injection Moulding
What is the main disadvantage of PM processing?
Initial cost for punches, dies, and equipment is high so a high production volume is required to justify usage. Break-even point is usually around 10,000+ units for the process to be economically viable
Blending - Describe the mechanical alloying process
Powders of 2 or more pure metals are mixed in a ball mill
The powders fracture and intermingle with one another (diffusion), forming alloy powders
Blending - How can a homogeneous mix of powders be obtained?
Selecting good particle size distribution
Using suitable lubricant
What is powder metallurgy also known as?
Sintering
Sinter metal
What is the volume of a sintered compact equal to?
(Volume of green compact)x(1 - linear shrinkage)^3
Density other way round
Finishing - Describe the forging process
A workpiece is shaped by compressive forces applied through various dies and tools.
It is used to shape sintered compacts to the desired shapes
Compaction - Describe the Powder Rolling process
Powders are fed through the gap between rollers in a two-high rolling mill and compacted into a continuous strip
Compaction - What are the advantages of using Hot Isostatic Pressing
Produces compacts with 100% density
Produces good metallurgical bonding among particles, and good mechanical properties
Compaction - Describe the Extrusion process
Powders are encased in a metal die and extruded
Compaction - What is the core rod used for?
Central features
Compaction - Describe the Cold Isotatic Pressing method
Metal powders are placed in a flexible rubber mould and then pressurised hydrostatically in a chamber
Why is blending required?
Powders may have different sizes and shapes (they must be mixed to obtain uniformity)
Powders of different metals and other materials may be mixed (to impart special physical and mechanical properties and characteristics to the PM products)
What is the formula for linear shrinkage?
change in length of a compact after sintered / original length of compact