Int. J. Mechatronics and Manufacturing Systems, Vol. 11, Nos. 2/3, 2018 203 Copyright © 2018 Inderscience Enterprises Ltd. Microstructure evolution in thin sheet laser welding of titanium alloy Bikash Kumar, Daniel Kebede and Swarup Bag* Department of Mechanical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India Email: bikash.kumar@iitg.ernet.in Email: kebededaniel333@gmail.com Email: swarupbag@iitg.ernet.in *Corresponding author Abstract: The effect of cooling rate on microstructural morphology and mechanical properties of laser welded thin sheet Ti6Al4V alloy is studied. The numerical investigation has been performed to predict the weld pool geometry at different heat input by pulse Nd:YAG laser. The cooling rate is estimated from simulated time-temperature history. The solidified structure is complex and may acquire various microstructural transformations with different morphology of mainly α and β phases depending upon the particular cooling rate followed. Diffusional, α′-martensitic and mixed structures are found in the welded joint. Massive diffusion-controlled α lamellae has found in the range of 52–325 K/s. Volume fraction of α′-martensitic phase in the fusion zone increases with cooling rate. It shows that the dimensional variation of α lamellae plays an important role on mechanical properties. Substantial improvement of the mechanical properties with increase in cooling rate is characterized by the volume fraction of primary α-phase and the α + β lamellae spacing. Keywords: Ti-6Al-4V alloy; α and β phases; laser welding; Nd:YAG; thin plate; thermal analysis; cooling rate; microstructure; mechanical properties; grain size. Reference to this paper should be made as follows: Kumar, B., Kebede, D. and Bag, S. (2018) ‘Microstructure evolution in thin sheet laser welding of titanium alloy’, Int. J. Mechatronics and Manufacturing Systems, Vol. 11, Nos. 2/3, pp.203–229. Biographical notes: Bikash Kumar is currently a Research Scholar in the Department of Mechanical Engineering of Indian Institute of Technology Guwahati, Guwahati, Assam, India. He completed his graduation in Mechanical Engineering from the West Bengal University of Technology, Kolkata, 2014. His research interest includes phase transformation effect on thermomechanical analysis of thin sheet welding process. Daniel Kebede received his Master’s degree in the Department of Mechanical Engineering of Indian Institute of Technology Guwahati, Guwahati, Assam, India. He completed his graduation in Mechanical Engineering from Ethiopia. His research interest includes thermomechanical analysis of fusion welding process.