Objective Spherical powders with particle sizes of 15-53 mu m are usually employed in selective laser melting (SLM) process. The difficulty in producing SLMed parts results in high costs. Large particle size powders with particle sizes of over 100 mu m are comparatively easier to produce and inexpensive. Hence, the application of large- size powders in the SLM process reduces the cost of the SLMed parts because high- power lasers are available. Additionally, the use of large- size powders for high layer- thickness SLM can significantly reduce the manufacturing costs and enhance efficiency. However, the process is challenging, owing to the potential defects associated with the SLM of high layer- thickness large- size powders, which ultimately results in lower density and inferior mechanical properties. Research on the large- size powder SLM is still in its early stages, and the process parameters are not optimized. In this study, large- size Ti6Al4V powder with a particle size of 100-200 mu m is used in a SLM process to fabricate samples with a layer thickness of 120 mu m. The effects of the laser power, scanning speed, and hatch spacing on the SLM process, defects, microstructures, and properties of the fabricated samples are investigated based on the results of numerical simulations and experiments. The process parameters are optimized, and high- density samples are further analyzed to explore their microstructure and mechanical properties. Hence, this study provides guidance for the application of large- size Ti6Al4V powder in the SLM process. Methods The ANSYS software is used to simulate and track the temperature field of a single layer and calculate the size of the molten pool. Ti6Al4V powder with a particle size of 100-200 mu m, produced via the gas atomization method, is employed to fabricate the SLMed parts. The process parameters are: a layer thickness of 120 mu m, laser power of 340 W to 370 W, and scanning speed of 800 mm/s to 1100 mm/s. Samples and tensile blocks are fabricated via laser scanning with an interlayer rotation of 67 degrees , and the parameters are optimized based on the results of the numerical simulations and single- pass experiments. After wire- cutting the substrate, the samples and blocks are cleaned, the relative density is measured via the Archimedean method, and the samples are polished for metallographic observation. The hardness of the samples is measured using an automatic micro- Vickers hardness tester, mechanical properties are tested using a tensile testing system and tensile fractures are characterized by a scanning electron microscope. Results and Discussions The width and depth of the molten pool scanned by a 370 W laser exceed 200 mu m at all scanning speeds, which makes obtaining dense samples possible. The calculated results of the established finite element model are in good agreement with the experimental results, with a difference of less than 6 degrees o, which confirms the reliability of the model. The hatch spacing has a significant effect on the defects in the fabricated samples. It is difficult to obtain a high- density sample when the hatch spacing is greater than 0.14 mm (Fig. 10). The largest- size samples with a relative density of 99.64 degrees o (Fig. 10), are achieved at a laser power of 370 W, scanning speed of 1050 mm/s, and hatch spacing of 0.10 mm without noticeable defects, such as porosity and cracks (Fig. 13). The tensile and yield strengths (sigma(0.2)) of the optimized samples are 1197 MPa and 1112 MPa, respectively, and the elongation is 8. 2 degrees o (Fig. 14). The tensile samples show mixed tough- brittle fracture characteristics with disintegration features and several tough dimples with 2 -5 mu m size (Fig. 15). The mechanical properties of the SLMed samples with large- size powders under high layer thickness are comparable to those of the SLMed samples with small- size powders under low layer- thickness, whereas the building rate is 3-5 times that of the conventional process (Fig. 16). The processing cost significantly reduces because of the low price of the large size Ti6Al4V powder and high building rate. Conclusions The SLM process is optimized for obtaining large- size Ti6Al4V powder with a particle diameter of 100-200 mu m. The relative density of the fabricated samples reaches 99.64 degrees o at a layer thickness of 120 mu m, laser power of 370 W, hatch spacing of 0.1 mm, and scanning speed of 1050 mm/s. The tensile strength, yield strength, and elongation of the sample prepared under the optimized process are 1197 MPa, 1112 MPa, and 8.2 degrees o, respectively, which are comparable to the performance of the samples fabricated using small size powders under low layer- thickness. The building rate of the sample is 12.6 mm(3)/s, which is 3-5 times that with the small- size powders under lower layer- thickness.
