Characterization and Thermal Modeling of Laser Formed Ti-6Al-4V

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Virginia Tech

The current work focuses on three aspects of laser formed Ti-6Al-4V: an evaluation of the as-deposited and heat treated macro and microstructures and preliminary results obtained from a model developed to calculate the temperature profile resultant of the laser forming process. A "solution treat and age" heat treatment with a variable cooling rate was performed on the Laser Formed Ti-6Al-4V single line builds. Increasing the cooling rate decreases the acicular alpha grain size in the basketweave Widmanstätten alpha plus untransformed beta microstructure. Distinct features of the as-deposited macrostructure include: large columnar prior-beta grains that have grown epitaxially through multiple deposited layers; a well defined heat affected zone in the substrate; and the presence of "layer bands," a macroscopic banding present at the top of every layer except for the last three layers to be deposited. The nominal microstructure between the layer bands consists of acicular basketweave Widmanstätten alpha outlined in untransformed beta. The alpha grain width is smaller just above a layer band and larger just below a layer band. The microstructure of the layer band consists of larger colonies of acicular alpha outlined in untransformed beta. The gradient in the alpha grain size and presence of the layer band is due to thermal cycling as opposed to segregation effects which were ruled out using quantitative compositional analyses. Through analysis of the microstructural results the gradient in the nominal microstructure and formation of the layer band in layer n was caused by the deposition of layer n+2, and n+3, respectively.

A thermal model has been developed to assist in the prediction and interpretation of the as-processed microstructure. The model is used to explain that the microstructural evolution of the layer bands and gradient microstructure in layer n is due to the deposition of layer n+2. The difference in the two analyses of microstructural evolution based on microstructural observations and thermal model results are due to differences in the parameter sets used to build and model the deposit.

rapid manufacturing, laser metal deposition, thermal modeling, Ti-6Al-4V