27. 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.
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.
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,
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
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
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
Problem 27.1 TRUE / FALSE: Heating is involved in powdered metallurgy.
Problem 27.2 TRUE / FALSE: Refractory materials are used as conductors.
Problem 27.3 Describe the sintering process.
Problem 27.4 What advantages does powdered metal have over other processes?
Problem 27.5 What limitations exist in the powdered metal process.
Problem 27.6 How can the porosity in a powder metal part be reduced? How can it be used to introduce advantageous materials?