Investigation of the influence of process parameters on crack formation and mechanisms in Ti-48Al-2Cr-2Nb alloy via laser directed energy deposition

Abstract

The Ti-48Al-2Cr-2Nb alloy was fabricated using laser directed energy deposition(LDED), and the impact of various process parameters on the macroscopic crack morphology was analyzed. The mechanism of crack formation was investigated through the analysis of crack microstructure, phase composition, crystal orientation, and elemental composition. The process parameters were optimized by response surface methodology(RSM) and the laser remelting method was used to suppress the crack formation. The results showed that the cracks were mainly caused by lack of fusion, residual stress during LDED and stress between different phases of TiAl alloy. The mismatch of process parameters results in insufficient energy for powder melting, ultimately leading to lack of fusion occurrence. To minimize crack formation, the response surface method was employed to optimize process parameters and reduce crack generation. The higher temperature gradient led to the existence of residual stress in the sample, and the higher stress between α 2 phase and B2 phase formed in the deposition process due to the difference of thermal expansion coefficients. The region where the two phases converge was the region with the highest crack sensitivity, and cracks occured in the region where α 2 phase and B2 phase converge in the form of excellent transgranular fracture. The samples prepared by using the optimized parameters can effectively restrain the cracks caused by lack of fusion, but can not restrain the cracks caused by the stress between phases. Laser remelting after LDED can not only reduce the temperature gradient and residual stress, but also remelt the unmelted powder on the surface of the as-deposited samples, effectively inhibiting the generation of cracks, and preparing crack-free samples.