Molten pool modeling, microstructure and grain refinement in Ti-alloys

Abstract

<p>It is known that titanium alloys are widely used in a variety of industries such as aerospace, automotive and biomedical. Good corrosion resistance and the high strength/weight ratio are among the properties that have made these alloys attractive for abovementioned industries. Although these alloys have been extensively investigated for the past decades, the growing demand, the high cost and the recent development of additive manufacturing techniques require better understanding of these alloys as well as the existing relationship between composition, processing and properties. The approach here is to model the molten pool dynamics, created during additive manufacturing processing of titanium alloys, via Comsol Multiphysics to assess key aspects such as grain growth direction and cooling rates. The Laser Engineered Net Shaping (LENS™), was used to produce a set of specimens to evaluate microstructure and grain refinement in the Ti-W system.</p> <p>The computational results indicate the importance of are the fluid dynamics variables (e.g. Marangoni and buoyance effects) and the preferential grain growth in the <001>direction. A compositionally graded titanium binary system (Ti-xW specimen (0≤x≤30 wt%) was used to evaluate the influence of composition on grain refinement by applying the Easton & St. John model that shows how the grain refinement is mainly governed by the nucleant particles mechanism. In addition, a set of nine Ti-6wt%W specimens were deposited using LENS™ with different laser energy densities and the results shows how the energy density is proportional to the grain size due to two effects. The first effect is that as the energy density increases, the potential availability of nucleant particles can be reduced. Second the energy density (represented by power in the model) is inversely proportional to the cooling rate which confirms the proportionality between grain size and energy density.</p>