Tungsten alloy is a composite material with tungsten as the hard phase and transition metals such as copper, nickel and iron as the binder phase. Tungsten alloy is widely used in aerospace, electronics, nuclear industry and defense industry due to its high strength and hardness, low thermal expansion coefficient, corrosion resistance and good radiation shielding. However, the high hardness, high melting point and room temperature brittleness of tungsten make it difficult to form tungsten alloy by traditional machining process. At present, it is mainly prepared by sintering process. However, the process has the disadvantages of long production time, high cost and inability to form parts with complex shapes, which limits the application of tungsten alloy. Laser direct deposition, as an additive manufacturing technology, has the advantages of short forming time, low cost, no need for molds and the ability to form parts with complex shapes. It is expected to replace some traditional sintering processes in the future and become a new technology for preparing tungsten alloy parts.
(1) The optimal process parameters were obtained by studying the influence of laser deposition process parameters on the microstructure, density and tensile properties of 60W40Cu alloy thin-walled parts. On this basis, the microstructure defects were reduced by increasing the W content and adding the activation element Ni. The results show that the W particles in the 60W40Cu alloy deposition layer are unevenly distributed, and more W particles are gathered on the outer surface of the deposition layer. At a scanning speed of 2mm/s, many residual pores caused by the aggregation of W particles are easily formed in the interlayer bonding area of the thin-walled parts, resulting in a lower density than that at 3mm/s, and also reducing the tensile strength. Increasing the W content or adding the activation element Ni can promote the uniform distribution of W particles and improve the density of the structure, but the promotion effect of Ni is more significant, and it can also significantly improve the tensile strength. (2) Based on the geometric profile of the W-Cu powder beam and the flight speed of the powder particles measured by high-speed photography, combined with the nozzle structure, a numerical model of the interaction between the laser and the powder beam was established. The calculation results show that due to the large difference in the absorption rate of W and Cu to the laser, there is a huge temperature difference between the two. When the laser power is increased from 100W to 900W, the temperature of the Cu particle at the center of the spot on the substrate surface only increases from 317K to 535K, and the temperature rise is small and far below its melting point. In contrast, the temperature of W particles can rise from 654K to 3538K, with a larger temperature rise.
(3) Based on the morphological change process of the W-Cu melt pool observed by high-speed photography, a two-phase flow (W-Cu deposition layer and air)-fluid dynamics model is established. The simulation results show that when the surface temperature of the substrate rises to its melting point under laser irradiation, W and Cu powder particles begin to deposit on the substrate surface. However, since the W-Cu composite has a lower laser absorption rate than the substrate and a larger thermal diffusion coefficient than the substrate, the temperature drops. Subsequently, under the continuous irradiation of the laser, the temperature of the deposition layer gradually rises to above the melting point of Cu and forms a melt pool. Finally, when the laser irradiation ends, the molten Cu cools and solidifies rapidly at a cooling rate of about 104K/s.
(4) A three-phase flow (W, Cu and air)-fluid dynamics model is established for the transient motion process of particles at a more microscopic level involved in laser direct deposition. The simulation results show that the surface tension σCu-W between W and Cu is balanced with the inertial force of the W particles falling, thus preventing the falling W particles from completely sinking into the molten pool, and finally making the W particles float on the surface of the molten pool. When Cu particles fall on W particles on the surface of the molten pool, they absorb heat from W particles and melt under the action of laser irradiation, and finally flow into the molten pool. With the accumulation of time, the number of W particles floating on the surface of the molten pool will gradually increase, which explains why W particles in the 60W40Cu deposition layer are aggregated on the outer surface of the deposition layer.
(5) The optimal process parameters were obtained by studying the influence of laser process parameters on the microstructure and properties of 85W15Ni high-density tungsten alloy thin-walled parts. The results show that the increase of laser volume energy density is conducive to improving the density of the sample and reducing residual pores. When the laser volume energy density reaches 380J/mm3 or above, the densification curve tends to be stable, and the density is basically stable in the range of 95%~97%. When the laser volume energy density is 395J/mm3, the tensile strength and elongation of the sample reach the highest. Too high laser energy density will promote the mutual adhesion of W particles or W dendrites, forming more fragile W-W interfaces, reducing the tensile strength and elongation. The laser direct deposition process of tungsten alloy reveals the mechanism of action of laser and powder beam, optimizes the organization and performance, and lays the foundation for the future application of laser direct deposition technology in the preparation of tungsten alloy.