Improving Conductivity of Copper PM Parts by Pretreatment of Green Compact

Abstract

There is a maximum green density to which pure copper can be compacted and still exhibit no growth upon sintering. This limits the maximum density and thereby the maximum conductivity and strength that can be achieved with sintered copper bodies. The swelling of a part pressed above this critical compacting pressure is caused by premature closure of the outer pores of the compact, thus sealing off the egress of internally generated gases. In this study a number of carefully selected chemical compounds were added to the as-pressed compacts using two methods in an effort to find a way of keeping these outer pores temporarily open until all internal gases are eliminated. These pores are then sealed off in the later stages of sintering, resulting in a higher density body with improved electrical conductivity throughout. Two compounds proved to be most effective in accomplishing this without leaving a residue deleterious to conductivity. Two commercial copper powders were die pressed into thick discs and then impregnated with an aqueous salt solution, the preferred treatment. After a special sintering procedure, the best conductivity obtained in an 18 mm diameter × 10 mm thick disc using the most responsive powder and treatment was 93·9% IACS at the centre and 94·2% IACS at the surface. Untreated, the same powder showed a maximum centre conductivity of 89% IACS.