Different types of metal powder in specific amount are mixed in a Y type mixer for specific time. The Uniform mixing and specific amounts of each metal are very important in preparation of parts with right physical and mechanical properties.
Very Complex forms can be compacted in special and complicated dies and presses. The mixed powder is filled in die cavity in right proportion. Compaction proceed by double action of upper and lower punches to a proper green density. At this stage, by relative movement of lower punch, the green part is ejected from die. It is necessary to achieve a right density in different sections of green part with no microcracking.
The green parts then are transferred to a tunnel furnace under a reducing H2 atmosphere for sintering. The reducing atmosphere produced by ammonium cracking in an electric furnace at 830C. The sintering is done at an electric furnace at 1120C for 45min. Sintering temperature Ts>0.7Tm and sintering time are extremely important in producing parts with right physical and mechanical properties. The metal particles during sintering start to surface fusion and inter-diffusion which lead to pore rounding and porosity elimination.
The presence of remaining porosity in PM parts are inevitable. That is why almost all of PM parts are immersed in oil to prevent subsequent atmospheric inter corrosion. The inter porosity channels are oil filled by capillary forces. The self lubrication property in these parts observed as the result of service heating or friction condition.
Deburring and Polishing
To remove sharp Edges and burrs, PM parts along with grinding and polishing stone are charged in vibrational deburring machine for 0.5h. For better cleaning and removing the burrs, specific amounts of water and detergents are added to each charge.
PM part sections, holes or inserts need to be sized precisely to very close tolerances. In these cases, a sizing press and very accurate punch and die set are used.