The microstructures of Ti-6Al-4V and Ti-6Al-2Sn-4Zr-6Mo and their relationship to processing and properties

Sammanfattning: The research described in the thesis concerns phase transformations and mechanical properties of the titanium alloys Ti-6Al-4V and Ti-6Al-2Sn-4Zr-6Mo. The need to weld the alloys for certain engine components can expose the alloy locally to non-optimum thermal cycles and it is therefore of importance to gain an understanding of the kinetics involved in the phase transformations during heating and cooling. Moreover, for the purpose of modelling and computer simulations of heat treatments and welding processes, quantitative descriptions of the transformation are necessary. One focus in this work has been to examine the feasibility of using high temperature X-ray diffraction (HT-XRD) to study the phase transformation kinetics. Another aim in this work was to develop a weld simulation model leading to a prediction of the joint structure and microstructure of an electron beam welded component of the alloy Ti-6Al-4V. Electron beam welding (EBW) and friction welding (FRW) of Ti-6Al-4V and Ti-6Al-2Sn-4Zr- 6Mo have also been investigated, including microstructural weld characterization and the effect of post weld heat treatments on selected mechanical properties. In addition, two components of Ti-6Al-4V produced by ring-rolling and closed die forging, respectively, showed unexpected differences in low cycle fatigue (LCF) under certain loading conditions. Quantitative metallographic studies and texture examinations were conducted in order to find possibly reasons for the observed difference in fatigue properties. The HT-XRD technique was successfully used to monitor the alpha-to-beta transition during heating and the beta-to-alpha transition during cooling and including the transformation kinetics during isothermal hold. From the recorded spectra the thermal expansion properties for the alpha and beta phases could be extracted up to a temperature of 1050C, enabling the determination of the overall thermal expansion for the alloy by using rule of mixture (ROM) calculations. The weld simulation model here presented proved to predict the joint structure and microstructure of an electron beam welded Ti64 component with reasonable good accuracy, both in terms of thermal history during welding and also in terms of martensitic fraction formed during cooling and the distribution of martensite through the cross section of the FZ and HAZ. All mechanically tested Ti64/Ti6246 EBWs and FRWs, independent of PWHT temperature, fractured in the Ti64 base material and not in the weld region, thus, both types of welding processes seem to be promising candidate methods for successful joining of the Ti64 alloy to the Ti6246 alloy. Quantitative metallographic studies revealed that the closed die forged material exhibited a finer primary alpha grain size and a finer Widmanstätten platelet colony size which would be expected to provide a superior resistance to fatigue crack initiation. Observed differences in the texture of the two materials as determined by electron back-scattered diffraction might also have contributed to the difference in fatigue properties.