Friction Stir Spot Welding (FSSW) has been applied to a dissimilar metal lap joint of an aluminum alloy and steel by stirring only the upper aluminum alloy sheet. Therefore, FSSW cannot be used to weld a lap joint composed of three or more sheets and a lap joint with an adhesive interlayer. In the present work, we propose a novel spot welding process for dissimilar metal lap joints using a new tool with the tip made of spherical ceramics. When this process is applied to the lap joint of the aluminum alloy and steel, the tool can be plunged into the lower steel sheet, then a steel projection is formed in the aluminum alloy sheet. The height of this steel projection increases with the plunge depth, and accordingly, the weld strength increases; the tensile shear strength and the cross tensile strength reached about 3.6 kN/point and 2.3 kN/point, respectively.
The radial plates (RPs), which is used in Toroidal field (TF) coil in ITER, are quite large, such as 13 m tall and 9 m wide, but thin, such as 10 cm thick, and are made of stainless steel. Even though they are very large structures, they require very high manufacturing tolerances and high mechanical strength at 4 K. The similar requirements will be required in the next generation fusion reactor. Therefore, the authors intend to develop efficient manufacturing methods in parallel with ITER TF coil RP manufacture. The authors therefore performed trial manufacture of the RP segments using a diffusion bonding method, namely Hot Isostatic Pressing (HIP). As a result of trials, it was clarified that even when HIPping is applied, the mechanical characteristic of base metal is not deteriorated. The machining period can be reduced by about 1/3 compared with the traditional manufacturing method. On the other hand, mechanical strength at 4 K is degraded due to weak bonding, that is no grain growth through joint, by HIPping. However, additional test indicates promising possibility of much better joint by higher temperature and joint surface treated HIPpings. These results justified that RP segment manufacturing is not only possible, but it is a technically valid manufacturing method that satisfies all requirements.
This research concerns a dissimilar metals joining of steel and aluminum (Al) alloy by means of zinc (Zn) insertion. The authors proposed a joining concept for achieving strong bonded joints between Zn coated steel and Al alloys. The ultimate aim of this research is to apply this joining concept in the resistance spot welding process for manufacturing vehicle bodies. This paper presents the results of fundamental investigations concerning the effect of Zn insertion on the properties of diffusion bonded joints. Bonded joints of a combination of galvanized (GI) steel and Al alloy were subjected to joint interface observations; joint strength tests were followed by fracture surface observations of the test specimens. The results confirmed that the Al-Zn eutectic reaction effectively removed the oxide film on the Al alloy surface at low temperature in air, enabling the formation of a uniform Al-Fe intermetallic compound (IMC) layer. The formation of this Al-Fe IMC layer facilitated metallurgical bonding that lead to strong joints. In addition, considering an actual application for car body production, bonded joints of not only a combination of GI steel and Al alloy but also of a combination of galvannealed (GA) steel, a steel grade widely distributed in Japan, and the Al alloy were prepared by diffusion bonding and rapid cooling. Detailed observations were performed, and the influence of different types of Zn coating on the bonding process has been discussed. During the bonding process of GI steel and Al alloy, the oxide film on Al alloy surface and the Zn coating on steel are then removed together with the Al-Zn eutectic liquid phase, as bonding pressure is applied, resulting in the formation of an Al-Fe (Zn) IMC layer. On the other hand, during the bonding process of GA steel and Al alloy, aluminum and the Fe-Zn alloy coating layer react to form an Al-Fe (Zn) IMC and a mixed layer consisting of Al (Zn) and a Zn liquid phase. Therefore it is more difficult to push them out around the periphery of the joint. As a result, a thick Al-Fe (Zn) IMC layer is more easily formed, and the bondability is lower in comparison with GI steel and the Al alloy.
Defect formation mechanism during friction stir welding (FSW) was investigated by three-dimensional visualization of material flow around a tool. The three-dimensional flow patterns in various FSW conditions were obtained using two pairs of x-ray transmission real-time imaging systems. It revealed that the tilt of the horizontal material flow around the tool and the stagnation of the material flow at retreating side of the tool were tightly connected with the defect formation mechanism. Additionally, the material flow rates during FSW were directly calculated by the results of the three-dimensional visualization. The material flow rate at advancing side was obviously decreased in the defect formation process conditions.
In many industries, there are applications that require the joining of stainless steel and copper components; therefore, the welding of dissimilar stainless steel/copper joints is a common process. For this investigation, the optimal brazing conditions and suitable filler metals for laser brazing of stainless steel/copper lap joints were studied. Tensile shear force increases with increases in the laser spot diameter or in the laser irradiation angle, which is associated with increased bonding width; however, as bonding width approaches 2 mm, tensile shear force reaches a saturated value due to fracturing at the HAZ of the Cu base plate. In order to obtain joints with high tensile shear strength, laser brazing was optimized by using Cu-Si-based filler metal under the following conditions: laser power: 4 kW; spot diameter: 3mm; laser irradiation angle: 80 degrees; irradiation position shift: 0.6 mm; brazing speed: 0.30 m/min; and filler metal feed speed: 0.30 min. Concerning filler metals, it was found that the Ni-Cu type showed relatively large tensile shear force even at high welding speeds in comparison with those of the Cu-Si, Cu, Cu-Ni, Ni-Cu and Ni types, respectively.