Metallic components have been widely used in aviation, aerospace, marine and other industrial departments for their excellent properties. Over the past decades, metallic components are developing toward to high-performance and multi-function but with low production cost, which pushes new manufacturing techniques to be used. Laser additive manufacture (LAM) is one of the new additive manufacturing techniques, which is widely used to manufacture near-net-shaped metal parts layer-by-layer by melting metal powders using a laser beam. Now, it is also widely used to manufacture large and critical high performance metallic components with advantages in reducing material waste, production time and cost. In the LAM process, the components undergo periodic and unstable thermal cycling, which influences the microstructure and internal stress. More importantly, the inappropriate selection of process parameters leads to the appearance of defects with degradation of mechanical property. For example, if there exists porosity in the material powder, the pores occur during LAM and the liquid condensation speed is faster than the gas escape speed. If the selected laser power is too low, the powder is hardly fully molten and the unfused pores and inclusions are generated.
Therefore, the extensive researches worldwide focus on studying the generation mechanism of defects in the LAM process. In Deng’s research, the 15%(volume fraction) SiC ceramic reinforced steel (MS) metal matrix composites were prepared by selective laser melting (SLM). Facing the compatibility and cracking problems raised between SiC and metal matrix, great efforts on the suppression of defects during the SLM process are taken from various aspects, including laser melting, substrate preheating, and design of support and build directions. Substrate preheating can be used for the significant suppression of cracks. Tillmann et al. treated the IN718 component with the HIP method and the density of components was increased to 99.985%-99.989% within a certain range. However, most of the researches focus on just one kind of defect as well as its corresponding control methods. There are a few summaries of formation mechanisms and control methods of typical defects in LAM of metallic components.
We also summarize and analyze the formation mechanisms and control methods of three types of defects (cracks, inclusions and pores) in LAM of metallic components. The thermal cycle during the LAM process usually causes the generation of large internal stress, which causes the micro-crack formation during or after deposition. According to the formation temperature, cracks can be divided into cold cracks and thermal cracks (Figs. 1 and 2). Pores are always formed due to the insufficient energy input during the deposition process (Fig. 3) or the trapping of the residual gas inside the molten pool, which is related to the flowing behavior of molten metal fluid in the pool (Figs. 4 and 5). There are two main types of inclusion defects: one is oxide inclusions mostly caused by the mixing of oxygen in the production atmosphere, and the other is high-melting metal inclusions caused by the mixing of powders with high-melting metal powders (Fig. 6).
Different control methods are required for different types of alloys and defects. For cold cracks, the currently commonly used control methods include optimizing the compositions of metal powder and adding post-heating treatment. For hot cracks, the currently commonly used control methods include optimizing the compositions of metal powder and the process including selecting appropriate parameters (Fig. 7) and preheating substrates (Fig. 8). The pore defects can be minimized by either improving powder quality (Fig. 9) or adopting post-processing. For high melting-point metallic inclusions, the effective control methods include the selection of reasonable process parameters and scanning schedule. For oxide inclusion defects, the main control method is to control the oxygen content in environment.
The laser melting deposition additive manufacturing technology for metallic components is a high-performance, low-cost, and designable manufacturing technology. However, the degradation of material properties due to the defects is still one of the problems that must be solved during component production. It is still necessary to conduct a more in-depth research in the following aspects. For crack defects, it is necessary to discuss the stress evolution during the deposition as well as the relationship between mechanical behaviors and internal stress distributions. For pore defects, the quantification of the influence of each process parameter on the formation mechanism is required. For inclusion defects, it needs to research the thermodynamic mechanism of the protective gas composition and their flowing behaviors.
