Influence of laser heat input on weld zone width and fatigue performance of Ti-6Al-4V sheet

During this study, the fatigue life of laser welded Ti6Al4V sheet was evaluated as a function of process heat input. Heat input was varied by manipulating laser power and welding travel speed and was categorised into three heat input ranges (Low = 40–60 J/mm; Medium = 65–120 J/mm and High = 160–230 J/mm). Fatigue data was acquired from as-welded and polished specimens in order to study the effect of weld geometry resulting from a change in process parameters. Results showed that there was an increase in fatigue life for low heat input welds, mainly derived from the associated higher traverse speeds, demonstrating that laser power variation was not the sole determinant in fatigue life. A higher fatigue life and lower heat input relationship is related to the occurrence of a narrower fusion zone and increased weld zone hardness that corresponds with a lower heat input obtained from higher traverse speeds. Two predominant crack initiation mechanisms were observed; internal initiation from discontinuities that is related to inadequate optimisation of welding process parameters for the polished samples, while surface initiation occurred in the welded specimens, due to the stress concentration effect of the weld bead geometry. An increase in welding speed or decrease in laser power both led to a reduction in weld undercut or a lower stress concentration at the weld toe. As expected, the fatigue data for the polished samples showed a marked improvement in life and a reduction in scatter compared to the as-welded data.