IPM04: Material Model for Microstructure Evolution During Laser Additive Manufacturing

Laser Additive Manufacturing (LAM) is a promising near net-shape additive manufacturing technology for performing repairs on Ti-6Al-4V aerospace components due to its potential for significant cost savings in comparison to part replacement. The reliability required in service dictates the need for achieving desired microstructure and an absence of physical defects. Currently, expensive trial and error experimentation is required to obtain the robust processing parameters to achieve sound mechanical properties in the repaired region. Applied Optimization has developed the SAMP software capable of performing heat transfer and residual stress analyses for guiding the LAM process optimization. Such existing software provides the framework for quantitatively studying the microstructure evolution during LAM.

The primary objective of this work is to develop and validate a microstructure model for understanding the microstructure evolution in Ti-6Al-4V LAM build. The microstructure model, based on simultaneous transformation kinetics (STK) theory, is calibrated by quantitative microstructure characterization of Ti-6Al-4V LAM builds. The calculated thermal history from SAMP is input to the microstructure model to predict the a-Ti to b-Ti transformation during heating and the decomposition of b-Ti during cooling. The STK model is further enhanced with a b-Ti grain growth model that predicts the b-Ti grain size prior to decomposition. Such prior b-Ti grain size is essential to improve the accuracy of predicting important microstructure such as basketweave-α which has been known to benefit mechanical properties of Ti-6Al-4V. As shown in the above figure, the STK prediction is fairly consistent with the experimental data. 

Parts produced using LAM are prone to porosity from lack-of-fusion and gas entrapment, which can have a detriment effect on the mechanical properties. The second objective of this work is to minimize the porosity defect through process control. In particular, a design of experiments method has been carried out to study the effect of deposition parameters such as powder flow rates, laser scanning speed, and hatch spacing on the porosity. Molten pool heat transfer and fluid flow based simulation is used to aid in optimizing hatch width. Based on these experiments, lack-of-fusion porosity has been reliably eliminated. As-build parts have shown sound fatigue strength comparable to the forged alloy.

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