53. POWDERED METALLURGY
Basic Process - a die cavity is made in the form of a metal part. When the part is to be made, a metal powder is placed in the die, and then compacted. When the die is opened, the part is stiff. It goes through a sintering operation that partially melts the powder and gives the part its strength.
Powders can be manufactured from most metals inexpensively using techniques such as,
- atomization/metal spraying - low melting point metals are sprayed to form irregular particles
- granulations - as metals are cooled they are stirred rapidly
- electrolytic deposition - often used for iron, copper, silver
- machining - coarse powders such as magnesium
- milling - crushers and rollers to break down metals
- shotting - drops of molten metal are dropped in water
- reduction - metal oxides are turned to powder when exposed to below melting point gases
Powders often come in elemental forms and must be blended in correct ratios for metallurgical purposes. Lubricants may also be added to increase powder flow, and to reduce mold adhesion during and after compaction.
During sintering the metal parts are put in ovens with temperatures just below the melting point. (These ovens also have controlled atmospheres). As the parts are heated the compacted particles melt slightly and bond. There is a reduction in part size.
Features:
- For high tolerance parts, a sintering part is put back into a die and repressed. In general this makes the part more accurate with a better surface finish.
- A part has many voids that can be impregnated. One method is to use an oil bath. Another method uses vacuum first, then impregnation.
- A part surface can be infiltrated with a low melting point metal to increase density, strength, hardness, ductility and impact resistance.
- Plating, heat treating and machining operations can also be used.
Production of magnets:
- 50:50 Fe-Al alloy is used for magnetic parts
- Al-Ni-Fe is used for permanent magnets
- Sintering is done in a wire coil to align the magnetic poles of the material
- H2 is used to rapidly cool the part (to maintain magnetic alignment)
- Total shrinkage is approximately 3-7% (for accurate parts an extra sintering step may be added before magnetic alignment)
- The sintering temperature is 600°C in H2
Conducting contacts can also be made,
Other applications include,
Advantages,
- good tolerances and surface finish
- highly complex shapes made quickly
- can produce porous parts and hard to manufacture materials (e.g. cemented oxides)
- pores in the metal can be filled with other materials/metals
- surfaces can have high wear resistance
- porosity can be controlled
- physical properties can be controlled
- variation from part to part is low
- hard to machine metals can be used easily
- no need for many/any finishing operations
- permits high volume production of complex shapes
- allows non-traditional alloy combinations
- good control of final density
Disadvantages,
- metal powders deteriorate quickly when stored improperly
- fixed and setup costs are high
- part size is limited by the press, and compression of the powder used.
- sharp corners and varying thickness can be hard to produce
- non-moldable features are impossible to produce
