A comprehensive numerical stress-strain analysis of laser beam butt-welded titanium compared with austenitic steel joints

Experimental tests on laser beam butt-welded joints made of commercially pure titanium and Ti-6Al-4V alloy showed a fatigue behaviour quite different from that usually observed in the case of AISI 304 joints connected with the same welding technique. In fact, fatigue failures of titanium joints were observed at the level of the base material but not in the notch region, as have been found for austenitic steel. In order to explain this experimental evidence, very detailed finite element models are developed in this paper. The models reproduce the real cord profiles of Ti-6Al-4V and AISI 304 steel welded joints detected by a high-precision coordinate measuring machine. Furthermore, mechanical properties given as input to the finite element models correspond to the real joint microstructure revealed by micro-hardness tests carried out in the different regions of the specimen. Finally, finite element models reproduce the distribution of porosity detected via microscopic inspection of the fractured surfaces. Interactions of weld geometry, material properties, presence/absence of porosity, and level of applied stress are considered in this study, which includes nine different finite element models of titanium and austenitic steel joints. A relevant complication in the analysis of titanium joints is represented by the small thickness and stress concentration effects. Numerical results show that the stress field in titanium joints is sensitive to the combined effect of weld seam geometry and presence of welding defects (pores). The latter produces stress concentration much more severe than that caused by the weld cord. Microstructural support length evaluated near the pores of titanium joints is about the same as that computed near the notch of austenitic steel joints (i.e. near 0). Furthermore, the Topper parameter evaluated for the titanium joints in the presence of pores is much smaller than for the stainless steel joints. Microstructure modifications must be accounted for in order to assess correctly the fatigue behaviour of austenitic steel joints.