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In this review, the analysis of melting and solidification phenomena and the mechanism of the occurrence of defects as well as the analysis of melting and solidification using the numerical simulation in laser powder bed fusion (LPBF) process were introduced. In addition, the strategies of suppression of defects were described. The melting and solidification phenomena during LPBF process are relatively similar to those during welding.Since the plume brings about the strong recoil pressure on the melt pool, the keyhole takes place. And when the depth of keyhole becomes more than a threshold, the keyhole pore remains at the bottom of melt pool. Since the plume also brings about spattering and blows out powder, the gas pores are prone to occur easily. The micro-simulation of melting and solidification enables to reproduce the real phenomena. The macro-simulation of melting and solidification phenomena is one of the effective tools to predict the optimum fabrication condition. In order to prevent the occurrence of defects, it is significant not only to obtain the optimum fabrication condition using the process map but also to develop the simulation software. In addition, the use of the monitoring and feedback control system is greatly effective. Therefore the development of the cyber-physical system is needed.

Selective laser melting (SLM) process has advantages in building free shape and simplification of manufacturing process. Since Ni-base superalloys have lower ductility at lower temperature, it is difficult to produce the parts by means of other process like forging. Therefore, SLM process has already applied to produce Ni-base superalloy parts. However, SLM process needs a long process time comparing to casting and machining. One of the means to solve this problem is an application of the high scanning speed condition under high power laser output. In this research, the optimum fabrication condition of Inconel 718 superalloy by SLM process under high power and high scanning speed condition was investigated. As a result, the optimum fabrication condition was obtained using the process map. However, the relative density of the as-built specimen fabricated under high power and high scanning speed condition is lower than that of the as-built specimen fabricated under the condition of 300 W and 600 mm/s. This may be mainly due to the occurrence of gas-pores by key-hole like phenomenon in melt pool and the increase of spattering at high power and high scanning speed condition.

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Brazing of a stainless steel to various aluminum alloys was carried out using an Al-Si filler metal and a fluoride-active flux in air. The brazeability was remarkably different by the aluminum alloys and the brazing conditions. It was considered that the differences were originated with the compositions of base metals and the filler metal, the solidus temperature and the partially melting behavior of the aluminum alloys, and the behavior of the surface oxide film layers of both base metals. On the other hand, the obstruction of brazeability was identified as the rapid reaction between the aluminum alloys and the brazing filler metal, which makes the molten brazing filler metal disappear at the joining interface before the wetting occurs to the stainless steel. Taking this phenomena into consideration, it was attempted to make previous wetting of the brazing filler to the stainless steel before brazing to the aluminum alloys. This method provided the successful brazed joints for the most combinations of the stainless steel and the aluminum alloys.

Pure copper is utilized as a material for products with complicated shape and high thermal conductivity such as heat exchangers. However, it is difficult to fabricate pure copper parts with high density by the selective laser melting (SLM) process. One of the reasons is considered to be its high thermal conductivity by which the heat in the melt pool rapidly diffuses away. Additionally, the lower rate of energy absorption of fiber laser power for pure copper makes the size of melt pool smaller. In this research, the optimum fabrication condition of highpurity 99.9% copper fabricated by SLM process was investigated by evaluating the density and microstructure. As a result, it was found that the optimum condition of laser power and scan speed are 800~900W and 300 mm/s, respectively, and the optimum energy density is around 1000 J/mm 3 , which is much higher than that of other materials due to high reflectivity and high thermal conductivity of pure copper. And also, it was found that the hatch pitch is important factor to achieve the densification of the as-built specimen and the optimum hatch pitch was 0.01 mm. The high density parts were successfully fabricated by the optimum fabrication condition. The maximum density of the as-built specimen was 96.6 % and was much higher than that of the as-built part already reported.

Recently, high-entropy alloys (HEAs) have attracted much attention because of their superior properties, such as high strength and corrosion resistance. This study aimed to investigate the influences of process parameters on the microstructure and mechanical properties of CoCrFe NiTiMo HEAs using a laser-based powder bed fusion (LPBF) process. In terms of laser power and scan speed, a process map was constructed by evaluating the density and surface roughness of the as-built specimen to optimize the process parameters of the products. The mechanical properties of the as-built specimens fabricated at the optimum fabrication condition derived from the process map were evaluated. Consequently, the optimum laser power and scan speed could be obtained using the process map evaluated by density and surface roughness. The as-built specimen fabricated at the optimum fabrication condition presented a relative density of more than 99.8%. The microstructure of the as-built specimen exhibited anisotropy along the build direction. The tensile strength and elongation of the as-built specimen were around 1150 MPa and more than 20%, respectively.

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