Hot Isostatic Pressing Of Titanium Alloy Powders

Titanium alloys have been widely used in aerospace industry due to their high strength-to-weight ratio,excellent fracture and fatigue properties and outstanding corrosion resistance.Nowadays,in order to improve the thrust-to-weight ratio and fuel efficiency of aero-engines and lightweighting of airframes,it is necessary to establish an integral method to fabricate the titanium alloy components with high complexity and precision.Powder metallurgy(PM)through hot isostatic pressing(HIP),being a near-net-shape forming techinique,is very suitable to make componnets from refractory metals and hard-to-machine materials.It is believed that the mechanical properties of PM components produced through HIP route are comparable to those of wrought materials,and upto 90%of the raw materials can be used.Therefore,the PM-HIP route is also considered as a technology upgrade of precision casting.Recently,the relationships between the microstructure and mechanical properties of PM titanium alloys using HIP route have been studied by various researchers.However,the effects of processing parameters on the microstructure and mechanical properties of PM titanium alloys are still far from incomplete.Besides,the PM-HIP technique which is a complex process,involves the powder production and tooling design,followed by HIP compaction to full density,but there is not a clear understanding of the importance of process control of PM-HIP technique.In the present study,Ti-5Al-2.5Sn(α alloy),Ti55(near a alloy)and Ti-6Al-4V(α+β alloy)prealloyed powders which were produced by electrode induction melting gas atomization(EIGA)technique were used for HIP compaction.The effects of processing parameters on the microstructure and mechanical properties of PM titanium alloys were systematically investigated,which is helpful to obtaining a better understanding of PM-HIP route for producing titanium alloy powder components.The main findings can be summarized as follows:The characterization of titanium alloy powders was conducted.The results showed that Ti-5Al-2.5Sn,Ti55 and Ti-6A1-4V prealloyed powders show similar particle size distribution,powder.surface considtion,phase composition and frequency of hollow powder with gas bubble.Meanwhile,it is neccessary to conduct the degassing treatment of powders for fabricating the high performance titanium alloy powder component,and the suggested degassing temperature should be below the maximum use temperature of titanium alloys in this study.The particle size has no obvious effects on tensile and stress rupture properties of Ti-6Al-4V powder compacts,but the Ti-6A1-4V alloys HIPed from the powder with different size fractions have different fatigue strength.The Ti-6Al-4V powder compacts HIPed from the powders with full size range have the best mechanical properties.For Ti-6A1-4V powders,the effects of HIP conditions and heat treatment on the microstructure and mechanical properties of HIPed powder compacts were investigated.The results showed that the optimized HIP parameters are:HIP temperature from 920 to 940℃,HIP pressure over 120 MPa.The mechanical properties of Ti-6Al-4V powder compacts are close to those of wrought materials.There is no obvious difference in tensile properties of Ti-6Al-4V powder compacts after different heat treatments,which is is good for the fabrication of PM components.It is noticed that the globularization of lath-like a phase of powder compacts occurs after the duplex annealing treatment,and the globularized percentage increases with increasing annealing time.Meanwile,temperature induced porosity(TIP)in powder compact occurs after annealing at 930℃for 1 h plus aging.The TIP has an adverse effect on high cycle and very high cycle fatigue properties.For Ti55 alloy powders,the effects of HIP temperature,heat treatment and powder surface state on the microstructure and tensile properties of HIPed powder compacts were investigated.The results showed that the optimized HIP temperature of Ti55 alloy powder is 940℃.The tensile properties of heat-treated Ti55 powder compcacts are comparable to those of wrought materials.There is a metallic oxide layer on the surface of Ti55 alloy powder,and the thickness of the oxide layer remains to a constant in the initial stage and then increases gradually during the further powder storage.The thick oxide layer on the Ti55 alloy powder surface may not be broken sufficiently during the HIP process,which increases the microstructural inhomogeneity and reduces the elongation of HIPed powder compacts.Based on the densification of metal powder,a novel HIP procedure with two steps was developed.The thick oxide layer was broken sufficiently by using the novel HIP procedure,and the Ti5 5 powder compacts with homogeneous microstructure and excellent elongation were obtained.The effects of minor porosity on mechanical properties of Ti-5Al-2.5Sn and Ti-6Al-4V powder compacts were investigated.The results showed that when the relative density is above 99%,the tensile properties of HIPed Ti-5Al-2.5Sn and Ti-6Al-4V alloys are close to those of fully dense powder compacts.When the relative density is above 99.5%,the impact toughness of HIPed Ti-5Al-2.5Sn alloy is close to that of folly dense powder compacts.With the increase of the relative density from 99%to 100%,the high cycle fatigue strength of HIPed Ti-5Al-2.5Sn alloy increases gradually,but the high cycle fatigue strength of HIPed Ti-6Al-4V alloy first increases and then decreases.This can be attributed to the different microstructures of as-HIPed titanium alloys.In addition shielding effect on the encapsulated powders may occur during the HIP precess,which would hinder the powder densification.The shielding effect was found to be equivalent to a departure from the optimized HIP conditions,namely the reduction of HIP pressure and/or HIP time.It was found that the reduction of HIP pressure is much worse for the powder densification.