Internal or Die Wall Lubrication for Compaction of Al Powders?

Abstract

The influence of varying lubricant admixtures on the dimensional stability and on the green and sintered strength of Al–Cu compacts has been studied and compared to the properties of compacts made using only die wall lubrication, Admixed lubricant decreases the true green density and also the green strength of the compacts. The lubricant used here also contributes to the dimensional instability of the compacts through the volume effect of a solid state transformation of the wax, and through effects connected with the spreading of liquefied wax and the concomitant enclosure of gas in the compact during heating. Compacts containing lubricant are weakened during dewaxing whereas in lubricant free compacts the corresponding heat treatment produces a slight increase in strength. The sintered strength and the swelling-shrinking behaviour which is typical of the sintering of Al–Cu compacts is also influenced by the lubricant; the effects are compatible with the assumption that remnants of the wax impede the sintering process. For small parts or simple geometry, and for parts the specifications of which require sizing or coining in any case, pressing with die-wall lubrication would seem both to afford a simpler process and give improved properties. PM/0233