Strengthening of tunsten heavy alloys by a second strain aging treatment : Z. Baosheng (General Research Inst. for Non-Ferrous Metals, China)

Abstract

A method to disperse oxide particles (primarily alumina) in the matrix of aluminium alloys, involving additions of particles to alloy melts prior to the solidification of castings, is described. Over 4 wt.% of 80 μm size α-alumina particles were successfully introduced by either hand or mechanical stirring, in pure aluminium or in AlSiCu alloy melts, to which 4.5 wt.% magnesium was freshly added just prior to the addition of α-alumina particles. Under similar conditions, it was not possible to introduce more than 0.05 wt.% alumina particles in the melts of the same alloys in the absence of the magnesium in the aluminium alloy melts. The minimum critical levels of the magnesium required in the melt for successful introduction of alumina particles is higher, if it is present simply as a previously added alloying element (that is, not added as a fresh addition immediately prior to the introduction of alumina particles). When magnesium is present as a previously added alloying element, the minimum amount of magnesium needed for successful introduction of the alumina particles is still higher if silicon is present in the melt. Switching to mechanical (from hand) stirring, involving higher speeds, increases the recovery of the alumina particles in the castings, other conditions remaining the same. The alumina particles in the cast alloys contain various amounts of magnesia as a result of their reaction with the magnesium present in the melt; in extreme cases the entire alumina particles changed to magnesium oxide. The particles are uniformly distributed in the matrix on a macroscopic scale in permanent mold castings of these composite alloys; however, on a microscopic scale there is frequent clustering of the oxide particles. Occasionally the alumina particles show cracking probably due to volume expansion in the outer layers due to the formation of MgO and to a lesser extent due to the thermal shock when the particles are introduced in the melt. Owing to the solidification of aluminium alloys in the presence of suspended oxide particles, the cast structure is refined and the extent of central piping in permanent mold castings is considerably reduced. The room temperature and high temperature hardness and yield strengths of oxide containing alloys were much higher than those of the base alloys without the oxides; the ductilities of the oxide containing alloys were of course lower. This technique of dispersing oxide particles in alloys through foundry technique could be extended to other systems to produce cast composite alloys for a variety of applications such as bearings, cutting tools, high temperature components and pistons. They represent a new family of future foundry products to produce aluminium-graphite, aluminium-mica and aluminium-silicon carbide composites. The foundry technique to produce these composites represents a much simpler energy consuming alternative to the conventionally used powder metallurgy techniques to produce these alloys.