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High entropy alloys (HEAs) have superior mechanical properties that have enabled them to be used as structural materials in nuclear and aerospace applications. As a dissimilar joint design is required for these applications, we created a dissimilar joint between CoCrFeMnNi-HEA and duplex stainless steel (DSS) through laser beam welding; a technique capable of producing a sound joint between the two materials. Microstructure examination using SEM/EBSD/XRD analysis revealed that the weld metal (WM) exhibits an FCC phase regardless of the postweld heat treatment (PWHT) temperature (800 and 1000 °C) without forming detrimental intermetallic compounds or microsegregation. The heat-affected zone of the CoCrFeMnNi-HEA showed CrMn oxide inclusions while that of the DSS showed no inclusions. Moreover, a lower hardness was recorded by the WM compared to the base metal after welding. After PWHT, the hardness of the WM, CoCrFeMnNi-HEA, and DSS decreased with an increase in the PWHT temperature. However, the decrease in the hardness of the HEA was more significant than in the WM and DSS. The cause for this reduction in hardness was attributed to recrystallization and grain growth. In addition, a strength of 584 MPa with low ductility was recorded after welding. The obtained strength was lower than that of the BMs, but comparable to that of the welded CoCrFeMnNi-HEA. The application of PWHT resulted in over a 20% increment in ductility, with only a marginal reduction in strength. The deformation mechanism in the as-weld joint was mainly dominated by dislocation while that for the PWHT joint was twinning. We propose laser beam offset welding as a technique to improve the mechanical properties of the dissimilar joint, which will be the subject of future studies.

In this work, the effects of post weld heat treatment (PWHT) on the microstructure and mechanical properties of functionally gradient materials (FGM) was investigated. The FGM consisted of five different layers which were mixtures of austenitic stainless steel (type 316L) and ferritic steel (LAS). The ratio of type 316L and LAS powder in each deposition layer was 100:0, 75:25, 50:50, 25:75, and 0:100. FGM samples were successfully fabricated without cracks or delamination by a direct energy deposition process. The sensitization phenomenon of the FGM samples was investigated after PWHT. The PWHTs were conducted at 700oC, 900oC, and 1100oC for 4 hours and the samples were then air-cooled. After PWHT, the annealed specimens were observed by optical and scanning electron microscopy to analyze their microstructure. The occurrence of sensitization was found in the specimen annealed at 700oC. The contents of Cr and C increased substantially along grain boundaries. However, the sensitization did not occur in other samples annealed at 900oC and 1100oC. In the C and D layers of the 1100oC annealed sample, micro-hardness was measured to be very high due to the formation of bainitic ferrite and a lath martensite structure. In addition, a reduction of the austenite fraction was confirmed by Electron Back Scatter Diffraction.

This study investigated the effects of laser power on the microstructure and mechanical properties of functionally gradient materials (FGM) produced by direct energy deposition. The FGM consisted of five different layers, which were a mixture of austenitic stainless steel (Type 316L) and ferritic steel (HSLA). During the direct energy deposition, two different laser power conditions (450W and 380W) were used. The ratio of Type 316L and HSAL at each deposition layers were 100:0, 65:35, 50:50, 25:75, and 0:100. After the direct energy deposition process, no cracks or delamination were seen between layers of the FGM. The effects of laser power on chemical composition and microstructure were not significant. However, as the laser power decreased, tensile strength and elongation changed with a small change in grain size.

This paper mainly demonstrates an advanced type of the vaporizing foil actuator welding (VFAW) process between GPa-grade steel (TRIP1180) and aluminum alloy (AA5052-H32) without applying standoff. To secure a flying distance during the VFAW process, the preformed target sheet shaped like a circular indentation has been utilized. It is necessary to optimize process parameters integrated with geometrical design of the preform since the welding strength can be decreased beyond the optimum input energy in the standoff-free VFAW process. The welded surface was evaluated by SEM-EDS, XRD, EBDS, and TEM to analyze the welding mechanism and composition at the welding interface. The diffusion zone including the AlFe3 phase was observed at the welded interface which has high grain density due to the high-speed impact by increasing the welding strength, which leads to the perfect welding between the dissimilar materials.

Pack rolling is an effective process for manufacturing Ti-6Al-4V products, especially thin sheets; however, it tends to cause surface defects. Surface ridging is the most significant surface defect produced during the pack rolling of thin sheets. Hence, in this study, the effects of pack-rolling conditions on the origin of surface ridging on Ti–6Al–4V alloy surfaces were investigated using experimental and numerical techniques. Pack rolling was carried out at temperatures of 830, 860, 930, and 980 °C until a thickness reduction of up to 30 % was achieved. The surface ridging mechanism, microstructural evolution, and mechanical properties after pack rolling were studied using optical microscopy, scanning electron microscopy in the backscattered electron mode, microhardness tests, and the finite element method to identify the influence of pack-rolling parameters and the surface quality at various temperatures. The stress, strain, and temperature distributions after pack rolling at different temperatures revealed that the difference in the plastic flow between the core and cover resulting from the temperature rise was the mechanism controlling surface ridging on the surface of the Ti–6Al–4V plate. These results can be used for optimizing the pack-rolling processes of Ti-6Al-4V alloys.

In the current study, fundamental deep drawing characteristics of Ti-6Al-4V alloy sheets were investigated to establish the effect of processing conditions on large size square deep drawn cups . To accomplish this study, FE-simulations (Abaqus) wereperformed to determine optimum blank size, friction coefficient, the gap between punch and die, etc. The simulated processing parameters were verified experimentally. Based on the FE-simulation results, deep drawing was performed with various blank holding loads and sample sizes. In order to improve the formability of Ti-6Al-4V sheet, various lubricant methods were evaluated. Tensile tests and thickness measurements were conducted on the formed sheets. Processing parameters including blank holding force, lubricants, and optimum blank size, were selected to achieve improved drawing quality. With ×the optimum processing condition, a 200mm 200mm cup was deep drawn successfully. Key Words : Titanium, Ti-6Al-4V Sheet, Deep Drawing, FEM-simulation, Abaqus Code, Lubricant