Stainless steel 316 (STS316)/Fe functionally graded materials were fabricated by direct energy deposition (DED) method using laser as a heat source. The feeding amount of the mixed powder was evaluated and the powder feeding condition was optimized through the section evaluation. The reliability of the powder feed was evaluated by regression analysis, and it was confirmed through the energy dispersive spectrometer (EDS) analysis and X-ray diffractometer (XRD) that the graded functional material of the designed composition was manufactured. Defects and microstructures were analyzed by scanning electron microscope (SEM). and cladding speed rate were the main parameters for controlling the clad layer width. Most of the deformation was at the clad height, but the clad bead side angle also depended on the powder feed rate and cladding speed. Saqib et al. [5] presented the result of laser cladding using P420 steel cladding powder deposited on low carbon structural steel plates. Process parameters such as laser power, scanning speed, and powder flow rate, among others, affected the bead height, width, penetration, area, dilution area, and bead shape. to process parameter relationships, the bead shape was analyzed using an artificial neural network (ANN). Calleja et al. [7,8] optimized the parameter value based on quantitative analysis and deviation of each parameters such as bead height, width, deposition rate, and wetting angle and assigned different weight factor to each parameters. In addition, the feed rate for uniform supply was controlled via the developed algorithm [8].In the metallurgical point of view, another solution for issues of dissimilar joints is functionally graded materials (FGM) that gradually change the composition or properties. Including mild steel-STS FGM [9], various FGM such as Inconel-STS FGM [10,11], Ti6Al4V-STS FGM [12][13][14][15], Invar-STS FGM [16], and compositional gradient STS [17,18] have been manufactured. Especially mild steel and STS are the most widely used materials in structural parts, requiring their dissimilar welds for pipes, valves, and pressure vessels [19]. However, the direct dissimilar joints cause defects at the interface, which originates from brittle phase formation, thermal deformation, solidification cracking, hydrogen cracking, porosity, and so on. Thus, it can lead to earlier failure than the expected life [20][21][22]. used V, Cr, and Fe elemental powder to make an FGM between Ti6Al4V and STS, because directly additive manufacturing results in fracture by intermetallic phases, FeTi and Fe 2 Ti. The Fe powder is employed in the transition zone between Cr and STS. Fe could be used for austenite-ferrite transition joints, but the brittle sigma phase at these interfaces should be controlled by adjusting the Cr content and cooling rate [13].

The fatigue life of the resistance spot weld of 980 MPa grade transformation induced plasticity (TRIP) steel was investigated and failure modes and fracture surfaces according to the fatigue load were analyzed. The fatigue life according to the nugget size was observed by using two electrodes with face diameters of 8 mm and 10 mm. When an electrode face diameter with 10 mm was used, the nugget size was large, and the fatigue life was further increased. After the fatigue test, three types of failure modes were observed, namely pull-out, plug, and heat affected zone (HAZ) failure, depending on the fatigue load. The fracture surfaces in each failure mode were analyzed. In all failure modes, a crack was initiated in the HAZ region, which is the interface between the two materials in all failure modes. In the case of pull-out failure, the crack propagates as if it surrounds the nugget at the outer edge of the nugget. In the case of HAZ failure, the crack propagates in the thickness direction of the material and outward in the nugget shell. Plug failure occurs with pull-out failure and HAZ failure mixed. The propagation patterns of cracks were different for each failure mode. The reason why the failure mode and the fracture surface are different according to the fatigue load is that the propagation speed of the fatigue crack is fast when the fatigue load is relatively large and is slow when the fatigue load is low.