lec 8

2.3 Mechanical Methods
These processes are not much used as primary methods for the production of metal powders. Mechanical comminution is possible by methods such as impact, attrition, shear and compression. The formation of metal powders by mechanical methods relies on various combinations of these four basic mechanisms. Such methods have been used as the primary process for the following cases:
– materials which are relatively easy to fracture such as pure antimony and bismuth, relatively hard and brittle metal alloys and ceramics.
– reactive materials such as beryllium and metal hydrides.
– common metals such as aluminium and iron which are required sometimes in the form of flake powder. A common method is the use of a ball mill consisting of a rotating drum with hard wear resistant balls. The critical factor is the speed of the drum’s rotation. A very high speed will cause the material and the ball to be pressed against the walls of the drum, because of the centrifugal forces and prevent relative motion between the material and the balls. Too low a speed will result in an insignificant amount of movement in the lower part of the drum. The optimum speed corresponds to a situation in which some amount of ball and material is lifted up to the top of the drum and falls down on the remaining material.