Original Title: Types, Causes and Adverse Effects of Defects in SLM Process Defects in SLM process can be divided into defects in powder spreading process and defects in printing process. Defects in powder spreading process include irregular powder layer, and defects in printing process include splash, spheroidization, pores, poor surface quality, cracks, geometric deformation, etc. Dr. Cao Longchao from Huazhong University of Science and Technology summarized the common defects and their formation mechanism in SLM manufacturing reported in the literature. The different defects and their formation process are introduced below, and the impact of different defects on the application of printing components is summarized. Defect is one of the biggest bottlenecks for the wide application of SLM technology in industry. 1. Splash Splash is one of the most common defects in SLM process, which can directly affect the interaction between laser and material and lead to other defects. The study shows that the spatter is mainly caused by the lateral shielding gas flow, the fluctuation of the molten pool and the recoil pressure. The splashing on the powder will form large metal particles, and then produce underfusion and porosity defects, which are harmful to the tensile strength and fatigue performance of the component. In addition, the splashing on the surface of the solidified layer will affect the next layer of powder, resulting in the next layer of powder being uneven and uneven, and even damaging the scraper. Physical process of interaction between laser molten pool and laser beam In order to keep the metal powder bed from being contaminated by spatter, a high-speed protective gas flow can be used to remove the spatter. However,
nickel titanium wire, excessive airflow will affect the surface quality of the powder layer, and how to optimize the airflow technology has become the direction of manufacturers'continuous efforts. 2. Spheroidization Expand the full text Spheroidization is a unique metallurgical defect in the manufacturing process of metal-based powder bed. Spheroidization occurs when the liquid metal solidifies into a spherical shape under the action of surface tension. If the laser beam energy density is too high or too low, it will lead to this phenomenon. If the energy is too low, the metal powder will not be completely melted, which will lead to spheroidization. If the energy is too high, the liquid metal will splash on the unmelted metal powder, which will also lead to spheroidization. Spheroidization will affect the quality of the next layer of powder,
titanium sheet grade 5, affect the surface quality of components, and also lead to defects such as poor fusion and slag inclusion. Further, spheroidization reduces the tensile strength and fatigue resistance of the member. To reduce and avoid spheroidization, it is necessary to optimize the printing process and find the optimal parameter combination. Defects in 3D printed Ti6Al4V alloy and examples of their existence as fatigue cracks 3. Air Holes Porosity is the main defect type of SLM components, and it is one of the defects that have the greatest impact on the mechanical properties of SLM components, and it is also one of the focuses of industry and academia. In the process of SLM, the rapid melting and solidification of the material and the violent fluctuation of the molten pool will lead to the formation of pores. The size, number, morphology and location of the pores have an important impact on the mechanical properties of the component. The higher porosity will shorten the fatigue life of the molded parts, and the pores near the surface have a greater impact on the fatigue properties of the molded parts than those at other locations. According to the formation mechanism of stomata, it can be divided into raw material-related stomata and laser-induced stomata. The process of pore formation is very complex and closely related to the process parameters. Reasonable printing process and uninterrupted process optimization for material use times are still important conditions to avoid pores. 4. Surface quality With the development of SLM technology, the types of printable materials are more and more abundant,
ti6al4v ,
titanium sheet grade 5, and the mechanical properties of components have been greatly improved, but the relatively poor surface quality is still one of the main obstacles limiting the development and industrial application of SLM. The surface roughness of the intermediate layer will affect the quality of the next layer, leading to internal defects; the surface roughness of the forming surface will seriously affect the fatigue performance of components. SEM and reconstructed images of the upper surface at different laser powers The surface quality is closely related to the flow behavior of the molten pool.The surface quality can be effectively improved by adjusting the process parameters such as laser power, scanning speed and powder layer thickness.The surface quality can also be improved by post-treatment, but it will increase the cost and reduce the efficiency.Therefore, it is necessary to explore new methods to improve the surface quality of components. 5. Crack The formation of cracks in SLM process is related to temperature distribution, residual stress and poor fusion. The cracks caused by residual stress can be divided into solidification cracks and liquefaction cracks, which are related to materials. Solidification cracks are caused by the large temperature gradient between the molten pool and the solidified metal, resulting in large deformation of the molten pool. However, the insufficient fluidity of the liquid can not compensate for the deformation of the molten pool; Liquefaction cracking occurs in the partially melted zone and is related to the extent of liquefaction, grain structure, thermal elongation, shrinkage and confinement of the metal. Cracks inside a 3D printed metal turbine blade In addition, underfusion caused by poor fusion is also a common type of crack in SLM components, which has a fatal impact on the mechanical behavior and fatigue life of SLM components. Underfusion cracks often occur between adjacent scanning paths or between deposition layers, which are mainly caused by incomplete melting of metal powder, and may lead to delamination defects when the cracks are serious. An example of a layere defect Cracks have a fatal impact on SLM components, and it is an important challenge for academia and industry to reduce crack defects of SLM components. Alloying and adjusting process parameters are two possible ways to improve microcracks, but the success rate is still limited. At present, there is little research on the cracks of SLM components, and it is urgent to further study. 6. Geometric deformation Due to the geometric characteristics of the component, heat accumulation, stress concentration and other reasons, different degrees of geometric defects will be formed in the SLM process. The lighter degree may cause deformation, resulting in dimensional errors, and the serious degree may lead to incomplete structure, or even failure of the processing process. The geometric accuracy of SLM components will affect the assembly and use performance of the components. It has been found that different scanning methods will have an important impact on the temperature field, and the temperature field will affect the distribution of residual stress and the geometric deformation of the component; the laser spot, power and scanning speed will affect the dimensional accuracy and surface quality, and improving the geometric accuracy of the component will greatly promote the industrial application of SLM technology. 7. The powder layer is irregular In the SLM process, the filling quality of the powder will affect the thermal conductivity of the powder bed and its absorptivity to the laser. The filling density of powder layer will affect the thermal conductivity of powder bed and the flow of molten pool, the thickness of powder layer will affect the stability of molten pool, the melting state and the internal defects of components, and the irregular surface of power layer will also cause the quality problems of components. At the same time, SLM is formed layer by layer, and a component has to be manufactured through multiple layers, so each layer of powder is very important to the quality of the component. Powder spreading is the key step of SLM manufacturing, the quality of the powder layer is directly related to the absorption of the material to the laser, the thickness of the powder layer will affect the stability of the molten pool, too thick powder layer will cause defects such as lack of fusion, pores and so on,
ti6al4v, too thin powder layer will reduce the manufacturing efficiency, therefore, strictly controlling the quality of powder spreading is of great significance to improve the quality of SLM components. Return to Sohu to see more Responsible Editor:.
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