The Effect of Powder Characteristics and Processing Conditions on the Microstructure and Mechanical Properties of Titanium Alloys Made by Powder Forging

摘要

Powder compact forging was used to produce Ti and Ti-6Al-4V rocker arms using pre-alloyed and blended elemental powders. Green powder compacts with high relative density were manufactured by warm compaction. Due to the characteristics of raw powders, interlocking and cold welding are the main mechanisms for HDH powder compaction, while warm welding is the main mechanism for GA powder compaction. During induction heating of the powder compact, it was found that necks formed extensively in as-sintered HDH Ti powder compact, leading to an average elongation to fracture of 7.5%, whereas necks did not form so extensively in as-sintered HDH Ti-6Al-4V and GA Ti-6Al-4V powder compacts, which make them brittle due to their low relative density.As a rapid consolidation process, the densification rate was enhanced by powder compact forging due to pore collapsing caused by material flow driven by a large amount of localized plastic deformation. The degree of powder consolidation of powder compact forging using HDH Ti, HDH and GA Ti-6Al-4V powders were studied by characterising their porosity distributions, microstructure, mechanical properties and fracture behaviour. Due to the positive effect of shear deformation on the powder consolidation of powder compact forging, the powder compact in the centre of forged parts were consolidated completely with full density and had better mechanical properties than those from ingot metallurgy.The effects of heat treatments on microstructure and mechanical properties of as-forged HDH Ti part, HDH and GA Ti-6Al-4V parts were investigated, and the ductility of forged HDH Ti part was improved significantly by annealing treatment. Recrystallization annealing was regarded as one of the best heat treatments to achieve the high ductility of as-forged HDH and GA Ti-6Al-4V parts, which can open the applications of forged HDH Ti-6Al-4V part with high oxygen content (~0.5%). The mechanical properties of forged HDH and GA Ti-6Al-4V parts after solution and aging treatment and recrystallization annealing were much better than those from both the ones from reported pre-alloyed approaches and wrought parts, which were caused by the enhancement of powder consolidation during recrystallization process.Blended elemental approach and mechanical alloying method were applied to produce Ti-6Al-4V rocker arm by powder compact forging at 1350 oC. The effect of holding time at forging temperature on the samples produced by forging the compact of HDH Ti/Al-V master alloy powder mixture was studied, and it was found that a holding time of 5-10 minutes at forging temperature is required to get the as-forged part with good mechanical properties and homogeneous microstructure of free undissolved master alloy powders. Also, in order to reduce the holding time for achieving composition homogeneity, the powder mixture of Ti and Al-V master alloy powders was milled to produce Ti/Al-40wt%V composite powder. With such composite powder, the master alloy layers/particles were dissolved rapidly into Ti matrix, but the oxygen pick up during milling and powder passivation make the mechanical properties of the forged samples inferior to those of the parts made by powder compact forging of the powder mixture or pre-alloyed powder.