Consolidation of Aerospace Grade Aluminum 7055 Powder Via PM Technologies

Abstract

The objective of this research was to assess the response of aerospace grade gas atomized aluminum 7055 (Al-8Zn-2.1Mg-2.3Cu-0.2Zr) powder to sinter-forge and SPS-forge styles of powder metallurgy processing. In meeting this objective the powder was processed through a three-stage sequence of cold isostatic pressing, liquid phase sintering, and rotary forging or through a two stage sequence of spark plasma sintering and rotary forging. In the prior, core variables included sintering temperature, particle size, the effects of admixed sintering activators (Mg, Sn), and forging temperature. Here it was found that the removal of fines (x<45μm) was required to ensure a crack free product while forging. It was also observed that while tin helped to produce a sintered product with higher density, the addition was detrimental to the post-forged mechanical properties. Overall it was found that a sintering temperature of 580°C and a forging temperature of 470°C, produced a fully dense product with wrought like tensile properties and hardness. In SPS-based research, core variables included sintering temperature, sintering time, and the timing of the application of pressure. Here it was found that the bend properties of sintered products were substantially higher when pressure was applied in a Type II mode of pressure application (pressure applied once the compact was at full temperature). Overall a sintering temperature of 500°C was found to produce products with the highest strength along with a sintering time of 2400 seconds. Once the sintered billets were forged and heat treated, products were found to be fully dense and have wrought like tensile properties and hardness. In both cases forging of the CIP-Sinter and SPS’ed products greatly increased the tensile properties of the product and was believed to help break up and refine the oxide shell present on the aluminum particles to a point where this constituent was dispersed within the material and thereby acted as a strengthening agent within the product.