Evaluation of powder mixing homogeneity for laser-directed energy deposition (L-DED) of functionally graded materials

Improving the mixing homogeneity of heterogeneous powder in real time plays an essential role in ensuring the forming quality of functionally graded materials (FGM) produced by laser-directed energy deposition (L-DED). In order to achieve a high performance of material composition and proportion control as forming FGM parts, this paper investigates the mixing mechanism of several typical powders including 316L and NiCrCoAlY used for the L-DED process driven by compressed gas. An innovative pneumatic powder mixer is designed and optimized by the CFD-DEM simulation method to analyze the mixing characteristics of heterogeneous particles. To improve the real-time mixing homogeneity of different mixing ratios in the conveying process, mixing experiments based on differences in powder properties were conducted by the additive-subtractive composite machining center, and a coefficient of variation method was proposed to describe the mixing homogeneity of non-uniform powders. The results of micro-morphology and EDS indicate that the mixing quality of the two-phase flow is significantly enhanced with the four 45 degrees compression inlets and 2 m/s inlet velocity structure. The results show that the numerical analysis method proposed in this paper can effectively describe the mixing law of heterogeneous particles, and the compressed air intake velocity should not exceed the initial two-phase flow velocity. The horizontal and vertical components generated by the compressed air intake at 45 degrees are more conducive to the formation of favorable perturbations to the two-phase flow, and the number of four and eight compressed air intakes that act uniformly on the flow field is more appropriate. After a certain period of time, the two-phase flow with low error of mixed powder quality can be produced continuously. The mixed powder quality is obviously improved after the disturbance of compressed air intake. The mixing uniformity of powder with low spherical particle proportion can be effectively improved by increasing the preset mixing proportion of non-spherical particles.