Friday, 7 October 2016

Powder metallurgy - post 1

Powder metallurgy – science of producing metal powders and making finished /semifinished objects from mixed or alloyed powders with or without the addition of nonmetallic constituents . The Process of blending fine powdered materials , compacting the same into a desired shape & form inside a mould followed by heating of compacted powder in controlled atmosphere , referred to as sintering to facilitate the formation of bonding of powder particles to form the final part. 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, superalloys, 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.

Advantages & limitations

• Efficient material utilization
• Enables close dimensional tolerances – near net shape possible
• Good surface finish
• Manufacture of complex shapes possible
• Hard materials used to make components that are difficult to machine can be
readily made – tungsten wires for incandescent lamps
• Environment friendly, energy efficient
• Suited for moderate to high volume component production
• Powders of uniform chemical composition => reflected in the finished part
• wide variety of materials => miscible, immiscible systems; refractory metals
• Parts with controlled porosity can be made
• High cost of powder material & tooling
• Less strong parts than wrought ones
• Less well known process

Dies & Equipments cost are high
Material cost in powder form is high.
Metal powders are difficult to store without some deterioration.
Part & weight are restricted .
Some metal powders in a finely divided state present chance of explosion , fire hazard eg. Al, Mg , Zr & Ti .

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