Effect of heat treatment temperature on tensile strength and hardness of isostatically compacted aluminum alloy blend

Abstract

The strength and density of powder metallurgy (P/M) parts are influenced by powder characteristics, consolidation technique and sintering conditions. In the case of an alloy the properties are further improved by heat treatment. However, the properties of such P/M parts, which are not subsequently subjected t o a forging operation, are always lower than those obtained by conventional techniques for wrought metals and alloys. The objective of this study is t o investigate the effect of one of the processing variables, heat treatment, on the tensile strength and hardness of an isostatically compacted aluminum alloy corresponding t o 6061 (1.0 pct Mg, 0.6 pct Si, 0.25 pct Cu and 0.25 pct Cr). Although aluminum powder metallurgy is now practiced quite extensively, relatively few r e s e a r c h papers on this subject have been published.1"4 Aluminum-copper alloys (2xxxs e r i e s and A1-Mg-Si alloys (6xxx s e r i e s ) are usually solution heat t r e a t e d for dissolving maximum amounts of solute elements such as copper, magnesium and silicon in the aluminum solid solution at the highest possible temperature below the eutectic melting point . Homogenization of the solid solution thereby achieved increases the strength and hardness properties of the component with increasing solution heat treating temperature. For heat treatable alloys in the 2xxx and 6xxx s e r i e s , this treatment is conventionally done at temperatures in the range of 940 t o 970°F (777 to 794 K) which is close t o the eutectic melting temperature for most alloys in the s e r i e s except the alloy 6061. This aluminum-magnesium-silicon alloy contains 1.0 pct magnesium and 0.6 pct silicon, whichforms 1.5 pct magnesium silicide. Since some of the magnesium silicide remains undissolved when heat treating the alloy at the conventional temperature of 970°F (794 K) which is below the complete dissolution temperature of 1025°F (824 K), the strength of the alloy should be improved further by heat treating at higher temperatures.%° Therefore, the purpose of this study was to investigate the effect of solution heat treatment temperature on the tensile strength and hardness of 6061 components made from isostatically compacted powder of this blend. The commercial grade powderblend, 601AC, containing 1.0 pct magnesium, 0.6 pct silicon and 0.25 pet copper and which is similar in composition to commercial 6061 alloy, was processed in the following manner. The blend was isostatically compacted at a pressure of 40,000 ps i (276 MN/m2) in a