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
What are the methods of powder production?
Atomisation
Reduction
Electrolytic Deposition (Electroplating)
Comminution
What are the factors that determine the powder production method to be used?
Economic, purity issue (also including particle size), desired physical or chemical characteristics of a compact
What is the main advantage of PM processing?
Eliminates many secondary manufacturing and assembly operations, also eliminates many secondary machining costs
Powder Production - Describe the reduction process
Gases such as hydrogen and carbon monoxide are used as reducing agents for metal oxides
Very fine metallic oxides are reduced to the metallic state
Powders produced are spongy and porous, with uniformly sized spherical or angular shapes
Finishing - Describe the infiltration process
A process whereby a slug of lower MP metal is placed against the sintered part and the assembly is heated to a temperature sufficient to melt the slug
Molten metal infiltrates the pores by capillary action, resulting in a pore-free part with good density and shape
Used to improve the hardness and tensile strength of PM products
Pores within products are filled to prevent moisture penetration that could cause corrosion
Compaction - Describe the Injection Moulding process
Very fine metal powders are blended with polymer or wax-based binder and then pressed
Moulded greens are placed in a low temperature oven to remove the binder and then sintered in a furnace
Powder Production - Describe the electrolytic deposition (electroplating) process
Uses either aqueous solutions or fused salts
Powders produced are very pure
Electric circuit, deposit the metal we want onto a cathode and then remove the powder from the cathode
Compaction - Describe the Hot Isostatic Pressing method
A die is made of a high MP metal
Inert gas is used as the pressurisation medium
Compaction - What is the pressed powder called and what does the density depend on?
Green compact
Density depends on the pressure applied and size distribution of particles
What does finishing do?
Further improves properties of sintered PM products
Why is powder metallurgy widely used?
No material is wasted
Only semi-skilled labourers are required
Final products require little or no finishing
What are the methods of finishing?
Forging (Including open die forging)
Coining
Infiltration
Plating
What properties does sintering increase?
Strength, density, durability and thermal and electrical conductivities of the compacts
However, compact shrinks after sintering so allowances should be made. Levels of shrinkage depend on material composition, density/porosity etc
Describe the basic process for powder metallurgy
Powder Production Blending Compaction Sintering Finishing operations
Compaction - What is the main advantage of the Injection Moulding process
Complex shapes can be produced and removed easily from dies
What are the applications of powder metallurgy?
Gears, cams, bushes, cutting tools, porous products (e.g. filters, oil-impregnated bearings) and piston rings
Powder Production - Describe the atomisation process
A liquid-metal stream is ejected through a small orifice
The stream is broken up by jets of inert gas, air, or water to create small particles
The size of the particles depends on the temperature of the metal, rate of flow, nozzle size, and jet characteristics
Finishing - Describe the plating process
A coating process that imparts resistance to wear and corrosion, high electrical conductivity, better appearance and reflectivity, and other desirable properties
Why is compaction needed?
Obtain the required shape, density, and particle-to-particle contact
Strengthen a part for further processes
Finishing - Describe the coining process
A type of forging process where the slug is coined in a completely closed die cavity
Used to improve dimension, strength, and surface finish of PM products
Describe Sintering
A process whereby compressed metal powders are heated in controlled atmosphere to a temperature below their MP, but high enough to allow bonding of individual particles
Protective atmosphere is employed to prevent oxidation, and reduce the probability of future surface oxidation
Outline the properties of PM products
Produced at near net shape to eliminate scraps
Can be produced with a wide range or properties from low density, high porosity with tensile strength as low as 70MPa to a high density with tensile strength of 1250MPa
Most mechanical properties are dependent on density
Lower tensile strength than those wrought of similar composition
Describe the compaction process
The process of pressing the blended powders into a shape using either hydraulically or mechanically activated presses
Normally performed at room temperature
