Friction measurement and lubrication in unloading and ejection stages in powder pressing cycle

The development of a floating die shear plate apparatus and its use to explore lubrication mechanisms (both admixed and die wall) and to assess the effectiveness of lubrication under a wide range of both compaction and normal loading conditions are described. The equipment allows the independent exploration of the effect of density and normal stress on the friction between compact and tool set surface. Tests were carried out using DistaloyAE, a zirconia target surface and Kenolube as lubricant (added or applied). A friction correlation that includes both density and normal stress effects and that reduces in response to both of these parameters was found. The results also suggest that the friction coefficient during ejection will be lower than that during compaction; at very high contact stresses, there is a likelihood that the lubrication regime will break down leading to galling. With regard to the friction mechanism, surface analysis using EDX did not detect the presence of lubricant on the compact surface; however, it did reveal two distinct pore types, a ‘dark’ one having a higher than average level of carbon and a ‘light’ one having a higher than average level of nickel. Image and topographical measurement of compacts subjected to sliding against the target surface revealed significant differences. Both analyses suggested significant closure of open pores due to sliding with topographical analysis revealing remnants of pores under the deformed particles that make up the exterior of the compact surface. Results confirmed there is no noticeable benefit in increasing admixed lubricant content above 0˙5%. Die wall lubricant was demonstrated to be very effective, achieving a reduction in friction of up to 58% for unlubricated powder and up to 27% for powder containing 0˙5% admixed lubricant. This was achieved for a lubricant film of typical thickness 5–20 μm.