Evolution of Microstructure During Shaped Metal Deposition

Abstract

In a companion paper, the correlation between the temperature field and the microstructure has been studied in a series of specimens made by Shaped Metal Deposition (SMD). The aim of the present paper is to improve the microstructure evolution model by taking into account not only the temperature and its rate but also the kinetics of the different solid phase transformations occurring in the Ti-6Al-4V alloy along the multiple heating and cooling cycles. At high temperature (above the beta-transus ~1000°C), Ti-6Al-4V is in the beta phase field. During cooling, the beta phase may transform to martensitic or massive alpha, Widmanstätten alpha or grain-boundary or colony alpha, depending on the cooling rate. Transformation to martensitic/massive alpha takes place under rapid cooling and is assumed to be diffusionless or instantaneous. At lower cooling rates, Widmanstätten and grain boundary/colony alpha growth is diffusion-controlled. Further, the diffusive transformation of martensitic/massive alpha into Widmanstätten alpha is considered during cooling based on observations. In addition, during heating, diffusion-controlled transformation of martensitic/massive alpha into Widmanstätten alpha and instantaneous growth of the beta phase fraction at the expenses of the fractions of martensitic/massive alpha, Widmanstätten alpha, and grain-boundary/colony alpha is assumed.
For diffusive transformation, the temperature history of a point (computed as described in the companion paper) is approximated as a series of isothermal steps where the transformation is governed by the Johnson-Mehl-Avrami law. Massive/martensitic transformation is described using the Koistinen-Marburger law, while transformation of martensitic/massive alpha, Widmanstätten alpha, and grain-boundary/colony alpha into beta during heating is assumed to follow the beta equilibrium curve.
Numerical results are compared to the experimentally observed microstructure, which helps to understand the complex transformation modes taking place during the repeated heating and cooling sequences of a SMD run.