Optimization of directional energy deposition forming process of high-strength aluminum alloy based on laser mode control

Abstract

Due to the low boiling points of the main constituents of 7075 aluminum alloy and the high surface tension gradient of the molten alloy， combined with the uneven energy distribution of the circular Gaussian （CG） laser beam spot commonly used in directed energy deposition （DED） processes， the forming quality of 7075 aluminum alloy DED specimens is generally poor. This significantly restricts the further application of DED in 7075 aluminum alloy. By regulating the laser mode， it is possible to effectively modify the temperature and flow fields of the melt pool， as well as the corresponding solidification conditions. Thus， the forming quality of laser additive manufacturing specimens and the active control of microstructure are improved. An off-axis parabolic integrating mirror is employed to homogenize the CG laser mode into a circular flat-top （CF） laser mode， which is further tilted to achieve a transversely elliptical flat-top （TE） laser mode. Using these three different laser modes， single-track cladding， single-wall structures， and 7075 aluminum alloy in DED block specimens are prepared. Combined with numerical simulations， the influence laws and underlying mechanisms of the laser modes on the forming quality and solidification structure are revealed. The simulation results indicate that by modulating the laser beam spot from the conventional CG laser mode to the CF and TE laser modes， the uniformity of the heat flux on the surface of the cladding layer is significantly improved， subsequently reducing the temperature gradient of the melt， and increasing the solidification rate. Compared to the CG laser mode， the surface forming quality of single-wall structures and block specimens under the CF and TE laser modes is significantly enhanced. The width variation of the single-wall structures is greatly reduced， and the dimensional accuracy in the horizontal direction of the block specimens is improved. Additionally， the density of the block specimens increases from 95.8% to 97.2% and 97.7%， respectively. In terms of solidification structure， the texture is significantly weakened， the grain size is reduced by approximately 50%， and the number of nano-precipitated η-phase within the grains is also increased.

Keywords

• directional energy deposition
• high-strength aluminum alloy
• laser mode control
• forming quality
• microstructure