Additive manufacturing (AM) has been widely used in aerospace, military, medical, automotive, nuclear power, and other fields due to its great potential in producing lightweight parts with complex structures and high personalization. As one of the most widely used AM technologies, laser powder bed fusion (LPBF), also known as selective laser melting, is characterized by huge temperature gradients, drastic phase changes, and extremely unstable molten pools. Because of its particularity in the manufacturing process, there easily exist internal defects of parts and harmful influence on forming quality and mechanical properties. Some of the defects cause the reduction of density, and further reduce other mechanical properties. For example, micropores and lack of fusion lead to the reduction of density, resulting in the decrease of its strength, hardness, and fatigue strength. The balling and spattering caused by lack of fusion also influence the surface morphology and phase composition. In addition, the residual stress generated in the machining process also causes cracks and warping deformation. The cracks influence the performance, while warping deformation influence the dimensional accuracy. Therefore, it is of great significance to understand the characteristics, formation mechanism, and influencing factors of defects, so as to explore the control mechanism of defects and control the quality and performance of parts.
This paper introduces the characteristics, formation mechanism, and influence of common defects on density, including unmelting caused by lack of fusion (Fig. 2), balling (Fig. 3), spatter (Fig. 5) and micropores (Fig. 6) , and the crack (Fig. 7) and warping deformation (Fig. 8) caused by residual stress. The effects of control methods of forming process and composite manufacturing on defects are discussed. The control methods of forming process can be classified into five categories: process gas supply, powder bed, laser beam, processing parameters, and scanning strategy (Fig. 9). A suitable processing environment and powder bed are the basis to prevent defects. As one of the most important factors in LPBF, a laser has many controllable aspects (Fig. 10). Proper focus shift, spot size, and intensity distribution can not only enhance the stability of the molten pool, but also improve the morphology of the molten pool and the microstructures of the parts. In addition, choosing an appropriate wavelength of a laser according to the absorptivity of materials can both improve the energy utilization of the laser and effectively inhibit the generation of lack of fusion defects. Process parameters are the most flexible means to control defects. Scanning strategy can change the overlap ratio and stress distribution in the process to inhibit the generation of defects, effectively inhibit the continuation of defects, and even eliminate the defects ever generated (Fig. 12). The composite manufacturing control methods are divided into additive-subtractive hybrid manufacturing and the multi-energy assisted process. Additive-subtractive hybrid manufacturing has the flexibility of AM and the ability of milling to eliminate the internal defects and improve the internal quality of parts (Fig. 13). However, it is inevitable that the alternate processing of adding and reducing materials greatly reduces the processing efficiency of parts, so a more efficient way of additive-subtractive hybrid manufacturing needs to be explored urgently. Magnetic field assisted LPBF has the effects of homogenizing microstructure, refining grains, inhibiting segregation, and reducing density. Ultrasonic-assisted LPBF has a positive effect on reducing residual stress, and improving anisotropy and performance of parts.
Understanding the typical features, formation mechanisms, and influence of defects can discover the relationship between various factors in the forming process and defects or internal quality of parts more effectively, which is helpful for researchers to explore various control methods of the forming process. With the development of the LPBF technology, the adjustment of laser parameters in forming process and the new composite manufacturing control methods have also been investigated, providing some multi-dimensional and more advanced control methods of defects. For example, the problem that LPBF is difficult to process pure copper and other infrared high reflection metal materials is solved by adjusting laser wavelength. Meanwhile, these control methods have a great potential and play a great role in eliminating defects, improving product quality and processing efficiency, and regulating microstructure. Adjusting spot morphology can improve the heat distribution in the process, which has the potential to change the morphology and improve the stability of molten pools, and further improve the stress distribution of parts. The magnetic field assisted LPBF forming process has the function of refining grains and homogenizing structures and compositions. The ability of ultrasound to refine grains and reduce defects has been demonstrated in directional energy deposition. In addition, the on-line inspection technology assisted LPBF is also a trend of defect control technologies. Finding the defects by on-line monitoring and combining with defect characteristics, formation mechanism, and control methods are able to achieve the closed-loop control of defects, which can greatly improve the stability and reliability of part forming quality and performance.
