| Selective laser melting (SLM) is one of the greatest potential technologies in the field of Rapid prototyping & manufacturing (RP&M), which can make metallic parts with complex shapes. SLM technology has been a research highlight due to its irreplaceable advantages compared with other conventional forming technology by the melting of metal powders with the principle of layered manufacturing. However, the SLM technology is just in its beginning, often accompanied by universal engineering roadblocks such as balling, pore and crack etc. Moreover, the chemicophysics metallurgical theory and material universality also need intensive study.According to above background, firstly, the universal theories such as balling phenomenon, pore formation and control, physical metallurgy were deeply and systematically investigated with AISI316L as representative material, in order to improve the SLM forming qualities and promote the technological advance. In addition, the SLM of rare metals, and SLM combined with hot isostatic pressing (HIP) technology were studied to develop new research directions. The main works are as follows:(1) The formation mechanisms and inhabiting methods of balling phenomenon were investigated. The balling phenomenon was characterized and analyzed. There exist two typical balling characteristics:big-sized ball (ellipsoid, about 500μm) and micro ball (sphere, about 10μm). The first kind of balling can lead to defect very easily due to worsened wetting characteristic; the second kind of balling is usually caused by energy transformation from laser impact energy to surface energy of metal ball and the technical defect can be neglected due to its subsize. The influence factors of balling effect were disclosed by regulating the processing techniques and powder characteristics. The inhibiting methods of balling were also proposed as follows:using gas-atomized metal powder, lessening the oxygen content of powder and forming atmosphere, improving the energy density of scan line, shortening the slice thickness. The laser melting line of pure nickel was researched and the balling principle was found to be similar with that of AISI316L.(2) The kinds, formation mechanisms, technical factors and controlling methods of pores in SLM process have been researched. The water-atomized powder possesses higher oxygen content and lower packing density compared with gas-atomized powder, which is very easy to give rise to balling effect. Lower energy density of scan line and higher slice thickness can also lead to balling easily. The above balling factors can also yield pore formation. It should be noted that the unfavorable overlapping condition of scan line can also lead to pore formation even if the scan line is free of balling. Thus the pore formation mechanisms were summarized:balling (irregular shape), unfavorable overlapping condition of scan line (regular shape), crack and gas induced pore (sphere shape). Aiming to build dense metal parts, the optimized parameters were obtained (laser power 100～200W, scan speed 90-100mm/s, scan interval 0.1-0.15mm, layer thickness 20-60μm) and 96% relative density can be obtained using above parameters. Moreover, the porous or gradient pore metal parts can also be made sucessfuly by regulating processing conditions.(3) The microstructure formation and property of SLMed materials were discussed. In the single line scan process, the oxygen, silicon, carbon contents are rich in scale line while the contents of metal elements were poor in that. The growth direction of columnar grain is perpendicular to the molten interface due to the heat dissipation way. With the columnar grain extenting to the inner of molten pool, it turns into the cellular grain gradually. The multi-layered sample exhibits ultra fine and multi-directional grain. The former is due to the special metallurgical mode with rapid scanning and solidification. The latter is because of the complex laser scanning process based on line, surface and entity. The microstructure is also influenced by techniques:higher molten pool temperature, prolonged solidification time (higher laser energy, thicker layer thickness, cantilever construction) can result in a coarsened microstructure. The mechanical property of SLMed material is mainly controlled by its densification, such as the 96% dense part with the tensile strength of 654MPa while the 78% dense part with 135MPa in that.(3) In light of the above universal SLM theories, the SLM material universality (W-Ni-Fe hyperbar alloy, Ni-Cr-Fe-Al multi-components alloy) was investigated and the corresponding metallurgical mechanisms were also disscussed. The forming mechanisms of 90W-7Ni-3Fe are melting and liquid phase sintering, on account of the partially melting of tungsten and full melting of nickel and iron. Above all the SLM of tungsten alloys shows favorable forming ability. In the SLM of Ni-based composite powders, the partial Cr particles were un-melted and the other elements can be dissolved into Ni crystal lattice. The microstructure possesses triangle or hexagon shapes with grain size about 2μm.(5) The SLM/HIP technology was probed into to improve the quality of SLMed part. HIP can close most of the cracks and pores, leading to a higher relative density approaching to 100%, while the HIP can hardly improve the densification of porous part. After the HIP process, the fine grains (1～2μm) could merge into relative jumbo size (about 100μm) grains.According to above researches, the measures for improving SLM qualities were proposed:reducing the oxygen contents in powder material and forming atmosphere, using the pre-heating system. Thus the molten pool with better wetting abilities can be obtained and accordingly the forming quality can be improved.
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