On The Friction Stir Welding of Titanium Alloys: Experimental Measurements and FEM Model Fine Tuning

Abstract

Friction Stir Welding (FSW) is a solid state welding process patented in 1991 by TWI; initially adopted to weld aluminum alloys, is now being successfully used also for magnesium alloys, copper and steels. Recently, research is focusing on titanium alloys thanks to the high interest that such materials are getting from the industry as welding of titanium alloys by traditional fusion welding techniques presents several difficulties due to high material reactivity resulting in bonding with oxygen, hydrogen, and nitrogen with consequent embrittlement of the joint. In this way FSW represents a cost effective and high quality solution. The study of the temperatures reached at the varying of the main process parameters allows a deeper knowledge of the process enabling the prediction of the microstructural evolutions occurring during the process and dramatically influencing the mechanical properties of the obtained joints. In the paper a 3D FEM model of the FSW welding process, based on a thermo-mechanical fully coupled analysis, is presented. In particular the model is tuned following an inverse identification approach starting from welding temperatures on the tool acquired during FSW experiments on the widely commercially diffused Ti-6Al-4V alloy. The inverse approach led to the identification of the most correct friction factor to be introduced in the numerical model. The obtained results permit to asses that the tuned FEM model of the FSW process can be utilized as an effective design tool.