Effects of Laser Scan Strategies on Densification, Residual Stress, and Mechanical Properties of W-Ti Heavy Alloy Fabricated by Laser Powder Bed Fusion

Objective Owing to the high melting point, high thermal conductivity, high creep resistance, high physical sputtering rate, and low hydrogen retention of tungsten (W) and its alloys, W has been widely used in the nuclear industry as well as rocket nozzles, medical protection, and other industrial fields. However, W is difficult to process, with a high ductile-brittle transition temperature ( DBTT, 200 400.). Traditional processing methods, such as powder metallurgy, plasma sintering, and hot isostatic pressing, are unable to realize the formation of complex components from W, limiting its engineering application. Fortunately, the development of laser powder bed fusion (LPBF) additive manufacturing provides a feasible method for fabricating W. In this study, we design a W-Ti heavy alloy and successfully fabricate it using LPBF. We investigate the effects of laser scan strategies on the densification, residual stress, and mechanical properties of LPBF-printed W-Ti heavy alloys, and further optimize the laser scan strategy. We hope that these findings can promote the optimization of laser additive manufacturing of difficult- to-process W- Ti heavy alloys by elaborating the relationship between the laser scan strategy and the properties of the LPBF-fabricated W- Ti alloy. Methods Pure W and Ti spherical powders were used in this study. First, W and Ti powders were mixed uniformly by mechanical milling under an argon atmosphere. Then, the mixed powder was processed using self-developed LPBF equipment according to a CAD model. After printing, the samples were cut from the substrate and subjected to ultrasonic cleaning. The relative density of W-Ti was measured using the Archimedes method. The microstructure and densification behavior were characterized using an optical metallographic microscope (PMG3). The phase composition and residual stress were analyzed using an X-ray diffractometer (Bruker D8 Advance). An FEI Quanta 200 scan electron microscope equipped with an energy-scattering spectrometer was used to observe the surface morphology. To characterize the mechanical properties, a CMT5205 testing machine (MTS Industrial System, China) was used at room temperature, and subsequently, an S-4800 field emission scan electron microscope was used to observe the fracture morphology. The nanohardness was determined using a nano- indenter ( DUH-W201S, Japan). Results and Discussions The LPBF-printed W-Ti sample obtained using the island scan strategy has a smooth surface morphology with a clear scan track (Fig. 3). Meanwhile, microcracks and balling phenomena are reduced ( Fig. 3). Highly dense W- Ti (99. 4%) is obtained using the island strategy because of its sound wettability in the molten pool (Fig. 4). When the island scan strategy is applied, the island feature can reduce the residual stress during the LPBF process, thereby inhibiting crack growth. The maximum surface residual stress along the x- and y- directions is approximately 450 MPa, which is the smallest among the applied scan strategies (Fig. 7). The LPBF-processed W-Ti sample using the island strategy has the highest nanohardness, which is due to the uniformly distributed residual stress ( Fig. 8). Finally, an ultimate compressive strength of 1906 MPa and fracture strain of 20. 4% are obtained using the island strategy (Fig. 9). The enhancement of the mechanical properties results from reduced defects and residual stress. Conclusions In the present study, a W-Ti heavy alloy is fabricated by LPBF under three scan strategies: island, zigzag, and remelting. When island and zigzag scan strategies are used to prepare the W-Ti alloy, the microstructures of the W-Ti alloy are almost free of metallurgical defects such as pores and microcracks, resulting in densities of 99. 4% and 99. 3%, respectively. The W- Ti alloy using the remelting strategy has some metallurgical defects, such as pores and cracks, which leads to a reduction in density to 98. 1%. The unique multizone scan mechanism of the island strategy can shorten the laser scan path, reducing the stress concentration level by making the melting and solidification rates of metal powders at different positions more uniform. Owing to the high density and relatively high solid-solution degree of the island sample, the distribution of residual stress is more uniform, resulting in the highest nanohardness (8. 44 GPa). The ultimate compressive strength and fracture strain of the sample formed using the island strategy are the highest, reaching 1906 MPa and 20. 4%, respectively. The ultimate compressive strength and fracture strain of the zigzag specimens are 1856 MPa and 16. 6%, respectively. However, the ultimate compressive strength of the sample is the lowest at 1785 MPa, and the fracture strain is 17. 8% when the remelting scan strategy is adopted. This is due to the large and uneven distribution of residual stress in the remelted sample, which leads to a low resistance to crack propagation and the resultant propagation along grain boundaries.