Influence of the mode of laser welding parameters on microstructural morphology in thin sheet Ti6Al4V alloy

Abstract

The transients and gradients generated due to localized heating and cooling by a laser source governs the thermal-metallurgical-mechanical performance of the welded structure. An attempt is made to investigate the significance of the pulse parameter over continuous mode in Yb-fiber laser welding of 800 μm thin Ti6Al4V alloy. The full-depth of penetration with minimum weld width is achieved at the lowest heat input of 12 J/mm for the pulse laser. The behaviour pattern of the thermal history is critically assessed with the aid of FE based heat transfer model and corresponding relation with evolved microstructural morphologies are systematically investigated. Several stages of transformation occurred in the weld zone such as α-phase dissolution, β-transus in the heating cycle, and diffusionless β→α'/α martensitic transformation during the cooling cycle are well explained by thermal history. A relatively high amount of α'-martensite is observed in the pulse laser welding whereas the transformed β-phase fraction gradually reduces further away from the weld line. Blocky plate-shaped α'-martensite within the coarseβ-grain boundary is apparent at higher heat input (26–80 J/mm), whereas acicular morphology within the fineβ-grain boundary is observed at a low heat input of 12–19 J/mm. The dimensional variation of α'-lath at the fusion zone has a significant influence on strength. Weld metal containing very fine α'-lath have comparable strength to that of the base metal. The images of the fractured surface shows the dimples as well as microspores in continuous mode, whereas combinations of large and small dimples are observed for pulse mode of welding. The level of contamination is examined by surface discoloration technique and found to be the highest for the heat input of 80 J/mm.