Powder metallurgy – basics & applications
Powder metallurgy –
science of producing metal powders and making finished/semi finished objects from mixed or alloyed powders with or without the addition of nonmetallic constituents.
Steps in powder metallurgy:
Powder production, Compaction, Sintering, & Secondary operations
Powder production:
Raw materials =>
Powder; Powders can be pure elements, pre-alloyed powders
Methods for making powders –
Atomization: Produces powders of both ferrous and non-ferrous powders like stainless steel, super alloys, Ti alloy powders; Reduction of compounds: Production of iron, Cu, tungsten, molybdenum; Electrolysis: for making Cu, iron, silver powders.
Powders along with additives are mixed using mixers
Lubricants are added prior to mixing to facilitate easy ejection of compact and to minimize wear of tools; Waxes, metallic stearates, graphite etc. Powder characterization – size, flow, density, compressibility tests.
Compaction:
Compaction is performed using dies machined to close tolerances
Dies are made of cemented carbide, die/tool steel; pressed using hydraulic or mechanical presses.
The basic purpose of compaction is to obtain a green compact with sufficient strength to withstand further handling operations.
The green compact is then taken for sintering Hot extrusion, hot pressing, hot isostatic pressing => consolidation at high temperatures.
Sintering:
Performed at controlled atmosphere to bond atoms metallurgically; Bonding occurs by diffusion of atoms; done at 70% of abs. melting point of materials.
It serves to consolidate the mechanically bonded powders into a coherent body having desired on service behavior.
Densification occurs during the process and improvement in physical and mechanical properties are seen.
Furnaces – mesh belt furnaces (up to 1200C), walking beam, pusher type furnace,batch type furnaces are also used Protective atmosphere: Nitrogen (widely used).
Secondary operations:
Operations include repressing, grinding, plating can be done; They are used to ensure close dimensional tolerances, good surface finish, increase density, corrosion resistance etc.
