On the improvement in fatigue resistance of Ti-6Al-4V castings

Abstract

The fatigue resistance of two-phase titanium alloys like Ti -6AI-4V is known to critically depend upon microstructurat parameters such as Widmanst~tten colony sizes, their relative orientation and the size of individual a/~ platelets [t ]. This improvement in fatigue fracture resistance with fine microstructure is established in the forged products [2]. Recently widespread interest has been evinced in the use of net-shaped titanium alloy castings for aerospace and other applications. The castings under normal freezing conditions have a coarse microstructure with targe a/~ p/atelets which can lead to poor fatigue properties. One of the viable routes to refine this structure without any mechanical working is by faster cooling [2-4]. However, the use of ceramic moulds impose an upper limit on the achievable cooling rates. Hydrogen dissolved in titanium brings down the /3-transus temperature. This beneficial effect has been used to improve the superplastic formability of titanium alloys [5], In this paper, we report on the use of h}~drogen to ref'me the as-cast microstructure and thereby improve the fatigue life of near net-shaped castings made of these alloys. Ti-5.9A1-3.8V pancakes were melted and cast by a consumable arc melting process, The alloy was melted four times to obtain homogeneity. Oxygen content in the alloy was 80 to 100 ppm. These alloys were made to simulate the cast T i -6AI-4V alloy. However, on account of solidification in contact with the water-cooled copper mould, the alloy displayed a much finer mdcrostructure than if it were to be cast in investment ceramic moulds, The hydrogen treatment described betow was carried out to examine if this microstructure could be refined further. Cut pieces of the pancake were hydrogenated in a tubular furnace fitted with on inconel tube at 850°C for 24 h by maintaining a steady flow of hydrogen over the hot samples. At the end of the run, the samples were cooled to room temperature in a fast flowing argon atmosphere, It was noted that coolhag in the hydrogen gas itself led to shattering of the samples possibly due to the formation of hydride while cooling. The samples were then degassed at 650 ° C at a 10 -4 torr vacuum level for 24h and then quenched by a jet of helium gas. The weight gain and subsequent loss of hydrogen was 0.6wt%. After the hydrogen treatment, the hydrogen content of the samples was measured to be about 20 ppm. Some of cast samples were directly subjected to the vacuum