Powder metallurgy technique is used to mix the material which does not form an alloy. Some of the major applications of the process are:
- Filament of a bulb, tube.
- Cutting tool and grinding wheel.
- Nozzles for abrasive jet machining.
- Porous bearing or self lubricating bearing.
- Filters used in casting process
- Friction material in anti-lock braking system (ABS).
Sequence of Processes in powder metallurgy
- Methods of Making Metal Powders:Metal Crushing and pulverizing: Brittle materials can be converted into powder by crushing. If the material is not sufficiently brittle, its temperature is decreased.Atomization: Ductile materials or the materials having low melting point, can be converted Into powder by this method. Liquified material is poured over a high speed rotating disk Due to the centrifugal action, liquid metal comes down in the form of very fine droplets The material can also be sprayed by using a plasma torch.Corrosion: When stainless steel is kept in the environment of sulphuric acid and copper sulphate, it dissolves and settles down at the bottom of tank. But this method takes time.
- Blending: By mixing the lubricant with powders, a layer of lubricant will be deposited over the particles. This will increase the interaction between them and as a result of that powder can be given some shape called green compact. This is called as green because it is freshly prepared.
- Compaction: It is also called an ISO-static pressing. To give initial strength to the green compact, powder is pressed on a press. Variation in the properties of compact will be more when the compaction is done on a single or a double action press. But on a double action press, properties will be more uniform. Smaller is the size of particle, better will be under diffusion and the compact will be stronger, if the particle size is smaller the strength will be better because more area will be under diffusion.
- (a) Pre-sintering: When heating is done along with compaction it is called Hot Iso-static pressing (HIP) Compact is heated and due to that lubricant on the surface of particle will bur. Characteristic of lubricant should be such that after burning, it should not leave any residue. This provides the localized heating which increases the extent of diffusion between the particles: When Pre-sintering is clubbed with compaction, it is called Hot Iso-static pressing. Rough particles give better strength as interlocking among the particles is better (b) Sintering: In the sintering stage, compact heated 60-70% of the melting point of the base metal. This increases the extent of diffusion between the particles. Those powder which could not form interlocking and whose melting point are below this temperature. they will be liquefied and will fill up the voids. When the compact is a mixture of large number of powders having large difference in their melting point two or three stage melting process is performed. After sintering, product appears to be very hard and brittle, so normally no machining is advisable but to give some simple shapes to the part sizing is performed. Powder metallurgy can only be applied for mass production because of expensive tooling arrangement.
Limitations of powder metallurgy:
- The cost of dies and equipment is very high and also rapid wear of dies occurs during compaction. Hence it is only suitable for mass production and not for products less than 10000 in quantity.
- This process is not used in some cases, due to the problem of storing the expensive powders without deterioration.
- Intricate shapes can not be manufactured easily with this process.
- Parts with theoretical density can not be manufactured
- Powders like magnesium, titanium etc, sometimes causes explosions and fire hazards.