GH3536 is a solid-solution-strengthened nickel-based superalloy with excellent corrosion resistance, oxidation resistance, thermal strength, and microstructural stability. 304 austenitic stainless steel exhibits excellent mechanical properties, strong corrosion resistance, and high strength, as well as incurs low cost. Both materials have been widely used in the aerospace field. Currently, selective laser melting (SLM) of these two materials is close to maturity. Laser welding technology has the advantages of accurate energy control, low heat input, and small welding deformation. It can be implemented in an atmospheric environment and involves flexible beam regulation. This technology is well established in the aerospace field and has been increasingly employed. Therefore, this study proposes the use of laser welding to connect SLMed GH3536 and SLMed 304, two dissimilar materials. Further, the weld structure and performance may be examined to confirm whether this approach combines the technical advantages of the two advanced laser manufacturing methods while reducing the manufacturing cost and process risk of 3D printing large-sized parts. This approach is expected to enable low-cost, high-quality manufacturing of large size components in the aerospace sector.

The welding equipment uses a fiber laser with a wavelength of 1060?1070 nm and a maximum output power of 6 kW. The core diameter of the transmission fiber is 200 μm, focal length of the fiber-coupled collimator is 200 mm, focal length of the focusing lens is 300 mm, and diameter of the focusing spot is 0.3 mm. The test materials are SLMed GH3536 and SLMed 304, and the butt-welding sample sizes of both materials are 50.0 mm × 50.0 mm × 3.8 mm. During the welding process, the laser beam is perpendicular to the surface of the plate, and the focus is located on the upper surface of the plate. Ar is used as a protective gas at a flow rate of 15 L/min. The angle between the axis of the protective gas nozzle and upper surface of the plate is 45°, and a welding test is performed using drag welding. The welding process uses a special fixture to maintain the sample plate butt without a gap, and the welding direction is perpendicular to the forming direction of the SLM process. The SLM stacking forming direction is defined as the Z-axis; the XOY plane is perpendicular to the forming direction, and the XOZ plane is parallel to the forming direction. The SLMed GH3536/SLMed 304 butt joint is obtained under a laser power of 2500 W and welding speed of 2 m/min. The weld microstructure is observed using a large depth-of-field microscope. The microstructures of the SLMed GH3536 and SLMed 304 substrates perpendicular to the forming direction and parallel to the forming direction are examined via electron back scattering diffraction (EBSD). The phase compositions and elemental distributions of the weld and base metal are measured and analyzed using X-ray diffraction (XRD) and energy spectrum analysis (EDS). A hardness tester is used to test the microhardness distribution of the weld. The indenter load of the hardness tester is 200 g, and the loading time is 15 s. The weld and two base materials are tested using a tensile test machine.

The weld microstructure of SLMed GH3536/SLMed 304 is mainly composed of dendrites and equiaxed crystals. Near the fusion line of the 304 base metal, there exists a mixed-crystal region composed of fine crystal, equiaxed crystal, and dendrite. An unmixed zone with a width of 5?20 μm appeares near the fusion line of the 304 base metal. Some unmixed areas, resembling islands or peninsulas, are observed in the weld near the fusion line of the 304 base metal. The microstructure near the fusion boundary of GH3536 is composed of short dendrites and equal crystals, and characteristics of layer-by-layer solidification are observed. The central structure of the weld is mostly dendrite (Fig. 4). The elemental content of the weld is significantly different from those of the two substrates, and the elemental distribution curve near the weld fusion line changes sharply; however, the change inside the weld is small (Fig. 8). The weld is in the complete austenitic phase (Fig. 9). The average microhardness of the upper part of the weld is 195.6 HV, that of the middle part of the weld is 216.18 HV, and that of the lower part of the weld is 196.06 HV (Fig. 10). The tensile strength of the weld is 587.75 MPa, elongation reaches 55.5%, and the fracture mode is ductile (Fig. 11).

A 3.8-mm-thick SLMed GH3536/SLMed 304 dissimilar material butt joint is successfully prepared using laser welding technology. The welded joints resemble nail heads in shape. The microstructure of the weld is dominated by dendrites and equiaxed crystals. The tensile test results show that the sample breaks at the SLMed 304 base metal, and the tensile strength reaches 587.75 MPa. The fracture mode is ductile.