The Effect of Fusion Zone Oxygen Content and Weld Geometry on Hardness and Porosity of Laser-Welded CP Titanium Sheets

The current study aims to understand the governing mechanisms behind weld hardness and porosity in laser-welded titanium sheets. Hardness measurements indicate increasing hardness with increased heat input and grain size, which is inconsistent with the general knowledge on hardness behavior. Since hardness is influenced by the weld microstructure, this study examines morphology and grain size. A transformation of Serrated structure to Widmanstätten structure is observed with increased heat input. To understand this phenomenon, oxygen is analyzed in the fusion zone using EDS and XRD analyzes. A greater amount of oxygen (Ti2O3 and TiO2 phases) is detected in the fusion zone for samples with higher heat input. Results show that while increased heat input increases the grain size, it also causes a greater chance of oxygen absorption from the environment in the fusion zone by increasing the melt pool width. Consequently, it is revealed that the fusion zone oxygen content has a more significant effect on titanium weld hardness compared to the grain size. Quantitative measurements indicate that porosity in titanium welding is controlled by the weld depth. The results of the present research can be applied to reduce hardness and porosity formation in laser welding of titanium sheets.