Microstructural Evolution in Laser-Deposited Multilayer Ti-6AI-4V Builds: Part II. Thermal Modeling

The thermal history developed in laser metal deposition (LMD) processes has been shown to be quite complex and results in the evolution of an equally complex microstructure. A companion article (Part I. Microstructural Characterization) discussed the LMD of Ti-6Al-4V, where the resultant microstructure consists of a periodic, scale-graded layer of basketweave Widmanstatten alpha and a banding that consists of colony Widmanstatten alpha. In order to understand the microstructural evolution in Ti-6Al-4V, a numerical thermal model based on the implicit finite-difference technique was developed to model LMD processes. The effect of different laser-scan velocities on the characteristics of the thermal history was investigated using an eight-layer single-line build. As the laser-scan speed decreases and the position within a layer increases, the peak temperature increases. The heating rate and the peak thermal gradient within a deposited layer were shown to follow the same trend as the peak temperature after two layers were deposited on top of the substrate. In general, the laser-scan speed or z-position within a layer did not have a significant effect on the cooling rate. The cooling rate in a newly deposited layer decreases as the number of layer additions increases. Given the predicted temperature vs time profile from the thermal model, the evolution of phase transformations occurring in the deposit is mapped as each layer is deposited. As a result of the thermal cycling imposed by the periodic deposition of material, a characteristic layer, consisting of two regions heated above and below the beta transus, forms in layer n due to the deposition of layer n + l.