Abstract Scope |
Additive manufacturing technology is a "bottom-up" material accumulation manufacturing method. It has the advantages of three-dimensional molding and significantly saving unnecessary materials during the manufacturing process. Directed Energy Deposition (DED) technology is a laser additive manufacturing technology developed by coaxial powder feeding and laser cladding technology. The material source is metal powder, and the energy source is a high-energy laser beam. Blue laser with a wavelength of 450 nm, as a new type of semiconductor light source, can increase the energy absorption rate of highly reflective metal materials (e.g., Al, Cu, Au, and so on) compared to the infrared laser, having broad application prospects. However, the feasibility and performance of applying blue laser in additive manufacturing of aluminum matrix composite material remain unclear. Here, based on FLOW-3D v11.2, we simulate the single-pass process of DED and investigate the influence of different process parameters (laser power, spot diameter, powder feeding method, particle initial temperature, powder feeding rate) on the molten pool dynamics and the shape of the single pass. When particles of different sizes enter the molten pool in different ways, the interaction among the particles, laser and molten pool is studied, and the possible causes of pores in the solidified molten pool are analyzed from the perspective of fluid movement.
The simulation is based on the Finite Volume Method(FVM). The physical models involved in the simulation include mass transfer model, viscous flow model, heat transfer model, evaporation model, solidification and surface tension model, Lagrangian particle model. In addition, according to the customization function of FLOW-3D, the design of the surface heat source with beam tracking and the recoil pressure model are also carried out, The material in simulation is Al-Si/TiB2 composite material prepared in the laboratory, and the relevant material parameters are obtained from the related literature and theoretical calculations in JMatPro. In order to describe the DED process well, two models are built in the software: the initial time model of single particle impacting on the molten pool and the stable process model of particles deposition. The computing device is a 64-core Dell Precision T7820 series workstation with the Windows 10 Professional Edition system. The experimental equipment for DED uses the Zhongkeyuuchen LDM-4030 system with a 1000 W Laserline blue laser. To verify the simulation model, A high-speed camera (Qianyanlang® Revealer X113) is used to in-situ observe the surface of the molten pool, and a metallographic photo of the vertical section of the printed single-pass sample is obtained by optical microscope.
According to the initial time model of single particle impacting on the molten pool, it is concluded that the particle will be heated under the irradiation of the blue laser. The melting degree is related to the position of the particle relative to the laser center, particle size, and the irradiation time. A temperature gradient pointing to the center of the laser is generated inside the particle, and partial or complete melting occurs. The interaction between particles and molten pool can be roughly divided into three stages: particles impact on the molten pool, molten pool surface oscillates, and the surface recovers. For particles with different particle sizes and different incident modes, these three stages always exist, but their effect and duration are related to the state of the particles.
According to the simulation results, it can be concluded that in the stable process model of particles deposition, the blue laser power density will significantly affect the structure and size of the molten pool, and affect the melting behavior of the particles. A smaller power density will cause the molten pool to be narrow and shallow, which cannot fully cover the depth of the melt channel or form a fewer defects melt channel. To determine the best initial temperature for simulating laser-heated particles, we compare the simulation in different initial particles temperature conditions and experimental results. By changing the simulation of the powder feeding rate, it can be concluded that a lower powder feeding rate will result in a lower melt channel height, which can increase the resolution of additive manufacturing in the vertical height direction. A higher powder feeding rate will cause the molten pool to move up, causing the laser to fail to melt the substrate well, which could cause that the mechanical bond between the deposited layer and the substrate is not tight. From the comparison between blue laser and infrared laser under the same process parameters, we can learn that due to a greater absorption rate for the blue laser in the aluminum alloy, the size of the molten pool is larger and it can ensure that the powder and the surface of the substrate are fully melted.
In this manuscript, we simulate the interaction among the particle, molten pool and laser with high time resolution and spatial resolution, and the models are validated by both in-situ and off-line experiments. The reasons for porosity formation in the DED process with blue laser are proposed. The influence of process parameters on the deposition process is investigated, and the optimal process parameters window is found. The process parameters will significantly affect the characteristics of the molten pool and finally affect the porosity and mechanical properties of molded parts. To sum up, we prove the feasibility of applying blue laser in powder deposition additive manufacturing of aluminum composite material and the single-pass weld bead without obvious defects could be achieved at the optimized parameters window.
Keywords: Additive Manufacturing, Directed Energy Deposition, Multi-physics Simulation, Blue Laser |