Objective As one of the most promising additive manufacturing technologies, selective laser melting (SLM) is commonly used in metal mold forming. However, there are few types of materials used for SLM forming of the metal mold. Most die steels are prone to crack and porosity because of the effect of carbon content, limiting the application of SLM in metal mold manufacturing. A new type of maraging stainless steel, SS-CX (corrax stainless steel, referred to as CX stainless steel), can exhibit excellent mechanical strength and good corrosion resistance through the intermetallic compound precipitation and has a lower carbon content, which is considered to be an ideal candidate material for manufacturing metal mold. Because of the novelty of CX stainless steel, its SLM forming has not been systematically studied. The process parameters of SLM forming have been widely studied. Among them, defocus distance as one of the important parameters is rarely reported. The spot size and energy density can be adjusted, and the molten pool shape can be effectively controlled by changing the defocus distance, which is helpful to improve the production efficiency and obtains high-density parts. This study reports the CX stainless steel samples formed through SLM based on defocus parameters, combined with microstructure observation, phase analysis and experimental research, and the sample’s printing quality and forming performance. We believe that the research results obtained will provide a valuable reference for the SLM forming of CX stainless steel and help expand SLM’s range of materials used for metal mold manufacturing.
Methods First, the SLM forming process of CX stainless steel is optimized and a reasonable process window is established by conducting the single weld channel test combined with the cross-section observation. Then, the square and tensile specimens are formed through SLM based on different defocus distances. The effects of defocus distance on the sample’s density, hardness, and surface roughness are analyzed through optical microscopy and scanning electron microscopy. Then, the microstructure and phase composition of the sample are studied using metallurgical microscope and X-ray diffraction. The effect of the mechanical properties of the sample is studied before and after heat treatment. The samples’ microstructure evolution and strengthening mechanism after solution, aging, and solution aging heat treatment are then investigated using metallographic observation, scanning electron microscopy, X-ray diffraction, energy dispersive spectroscopy, and hardness testing. Furthermore, the variation of mechanical properties of the sample before and after heat treatment is investigated in combination with the tensile test.
Results and Discussions In the SLM forming process window, the welding channel in the stable melting region is continuous and straight and the cross section shows a fine wetting effect (Fig.6). The density and hardness of the sample are first increased and then decreased with the change of defocus distance, whereas the variation of surface roughness is opposite (Fig.12). The main composition of the sample is martensite and austenite. The grain refinement is visible as the defocus distance increases, which is beneficial in promoting martensitic transformation. Simultaneously, the tensile fracture transitions from quasi-cleavage to ductile fracture (Fig.18), the number of dimples increases, and the mechanical properties considerably improve. However, excessive defocus distance leads to incomplete powder melting and reduces the sample’s mechanical properties (Table 4). In addition, some differences are present in the microstructure and tensile fracture morphology of different heat-treated samples. After solution aging heat treatment, the boundary of the welding channel disappears; a large number of lath martensite exist in the structure. Meanwhile, the hard second phase particles of NiAl are precipitated to produce a precipitation strengthening effect. Consequently, the hardness and tensile properties of the sample are considerably improved, the tensile fracture appears as river-like morphology with a few shallow deformation dimples, exhibiting quasi-cleavage fracture characteristics (Fig.27).
Conclusions The single weld channel test is used in this study to determine the SLM process window of CX stainless steel, which includes severe melting, stable melting, and incomplete melting regions. The molten liquid phase, for example, exhibits a good melt-wetting effect in the stable melting region. The shorter defocus distance causes an excessively high laser energy density, molten pool instability, and increased spheroidization. The results show that the density and hardness of the sample are reduced and the surface roughness is increased. The tensile characteristic shows quasi-cleavage fracture. With the increase in the defocus distance, the suitable energy density and spot size are conducive to forming a good metallurgical bond between the adjacent weld channels and layers and the sample’s mechanical properties are improved. Under the condition of 3.5 mm defocus distance, the sample’s maximum cross-section and longitudinal-section hardness are 35.94 HRC and 36.82 HRC, respectively, and the surface roughness is 7.315 μm. The tensile fracture mechanism is transformed into ductile fracture characteristics, and the maximum tensile strength is 1218 MPa. Simultaneously, the sample’s mechanical properties are considerably improved after the solution aging heat treatment due to the precipitation and precipitation strengthening effect of the hard second phase NiAl. The maximum hardness of the cross section and longitudinal section is 43.17 HRC and 44.52 HRC, respectively, and the tensile strength is 1746 MPa, which is 43.35% higher than that of the printed sample. When the defocus distance increases excessively, the laser energy density and penetration depth decrease and the liquid melt’s diffusion and infiltration effects become poor. Unmelted metal powder is present between the layers, resulting in the decrease of the density and mechanical properties of the sample.
