Microstructural evolution in laser-deposited multilayer Ti-6Al-4V builds: Part II. Thermal modeling
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5/5/04
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Microstructural Evolution in Laser-Deposited Multilayer Ti-6Al-4V Builds: Part II. Thermal Modeling S.M. KELLY and S.L. KAMPE 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 Widmanstätten alpha and a banding that consists of colony Widmanstätten 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 1.
I.
INTRODUCTION
MICROSTRUCTURAL evolution in laser metal deposition (LMD) builds has been demonstrated (Part I of this article[1]) to be quite complex. Without knowledge of the thermal history developed during the build process, analysis of the resultant microstructure becomes difficult. Recent research on understanding the microstructural evolution in LMD builds from a thermal standpoint has been performed on builds of H13 tool steel,[2,3] P20 tool steel,[4] and 316 stainless steel.[5,6] The investigation of H13 builds used experimental thermocouple data to obtain a thermal history at a given location within the build and utilized equilibriumphase diagrams to reconcile the microstructural evolution in the deposit. The work performed on stainless steel examined the shape and thermal profile of the laser-induced melt pool and the resulting solidification behavior. Vasinota et al.[7] have also investigated the effect of process parameters on the solidification behavior in laser-engineered net-shaping (LENS*) builds. *LENS is a trademark of Sandia National Laboratory and the United States Department of Energy, Albuquerque, NM.
Limited work has been performed to understand the microstructural evolution in LMD Ti
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