Linear friction welding of Ti–6Al–4V: Modelling and validation

Abstract

The linear friction welding (LFW) process – of the type required for the production of bladed discs for the next generation of civil aero-engines – is modelled using numerical and analytical methods. For model validation and testing, experimental work is carried out on the Ti–6Al–4V alloy using pilot-scale apparatus. Welds were instrumented with thermocouples to deduce the heat transfer effects prevalent in the process. The sensitivity of the measured rates of upset to the critical process variables – amplitude, frequency and the applied pressure – is shown to be consistent with the predictions of the modelling. The flash produced is dependent upon the ratio of oscillation amplitude to applied load; when this is large, a rippled morphology is produced. An analytical model of the process is proposed, in which the rate of mechanical working is balanced against the enthalpy associated with flash formation; at steady state, the temperature is predicted to decrease exponentially with distance in the heat-affected zone (HAZ), and the temperature gradient in the HAZ to increase as the upset rate increases, consistent with observation. By consideration of the form of the analytical model and the processes occurring during LFW it is suggested that, for a given upset rate, the weld temperature decreases as the pressure increases. Analysis of the experimental data indicates that the efficiency of adiabatic heating is close to 100%.

Introduction

Linear friction welding (LFW) is a process which – if successfully implemented in the gas turbine industry – would enable the weight of modern aero-engines to be reduced significantly, with concomitant benefits in terms of performance, fuel economy and CO₂ emissions [1]. This is due to its use for the production of integrally bladed discs, which are known colloquially as blisks [2], [3]. Blisks offer a significant advantage over conventional disc/blade arrangements which are reliant on mechanical fixturing and dovetail joints (see Fig. 1). Currently, the LFW process is the most attractive one for blisk production. Whilst machining a blisk from a single forging is more practical and more cost-efficient for smaller gas turbines, for the larger ones required for large civil aero-engines – which are typically manufactured from a high-strength titanium alloy such as Ti–6Al–4V [4], [5], [6] – the blades are large so that machining of blisks from solid forgings produces considerable material waste. In these circumstances, it is cost-efficient to fabricate the blade and disc separately, and then join them using the LFW process.

In this paper, modelling of the LFW of Ti–6Al–4V is carried out. Since LFW is a relatively new process which has not yet been adopted as widely as it might be in the future [1], validated models will prove useful for the purposes of analysis but also to aid in the identification of optimal values of the process variables. Moreover, since many aspects of LFW are not amenable to direct measurement during the process – partly on account of the significant strain rates incurred and its speed – modelling represents the only pragmatic way to make estimates of the behaviour of the material during joining. So far, very little work has been reported on the modelling of the LFW process. The modelling described here is used to predict factors such as thermal profiles, flash formation, and strains and strain rates experienced within the joint and the surrounding parent material. Experimental data gained from welding trials are used for the purposes of testing the predictions.