Q. What are the limitations of the powder metallurgy process ? How are some of these overcome ?
Ans. Limitations : 1. Dies and equipment costs are high. So, the process is economically feasible only for mass production. Minimum production run of 10,000 parts is needed to offset the tooling costs. Dies are made for long production runs, of the order of 1,50,000 to 2,00,000 parts.
2. The material cost in powder form is high. However, high powder material costs are usually offset by an almost total absence of scrap. Part costs are also reduced by eliminating machining. Labour costs are also comparatively low.
3. Metal powders are sometimes difficult to store without some deterioration.
4. There is little flow of metal during compaction, so intricate designs in the products are difficult to attain.
5. A completely dense product is not possible by this process. Also, a uniformly high-density product is very difficult to produce.
6. Part size and weight are restricted. These are regulated by the capacity of available press and compressibility characteristics of the various powder combinations. P/M is principally restricted to the manufacture of comparatively small parts. Part weight can be under 5 Newtons. However, parts weighting upto 900 Newtons have been successfully mass produced by P/M. Typical part
100 mm Φ x 150 mm long on 900 kN capacity press
250 mm Φ x 200 mm long on 900 to 2700 kN capacity press.
7. Some metal powders in a finely divided state, for example, of Al, Mg, Zr and Ti, present explosion and fire hazards.
8. Product Design limitations, which will be discussed ahead.
9. The residual porosity, makes sintered compacts rougher than the compacting die.
10. Low mechanical properties compared to other methods for similar parts. Low impact and fatigue properties.
11. Some thermal difficulties are experienced during sintering operation. This is particularly true for low melting point metals such as Sn, Pb, Zn and Cd. During the sintering operations, the atmosphere in the furnace should be reducing to avoid the formation of oxides which will have adverse affects on sintering process and result in an inferior product. The oxides of the above mentioned metals can not be reduced at temperatures below the melting point of the metal, whereas the sintering temperature is always less than the melting point of the metal.