Seiji Furusako

Strengthening spot weld joint by autotempering acceleration at heat affected zone

Abstract High tensile strength steel sheets in automotive body are composed of high weight fraction carbon and other additional elements. Therefore, securing the joint strength is difficult in spot welding. This study proposes a new technique for improving the spot weld joint strength. Typically, high joint strength can be obtained by prolonging the post-heating duration. This post-heating is expected to prevent propagation of crack into the nugget and improve the toughness of the heat affected zone. Compared to the conventional spot welding and other methods, the heat input is higher, thereby increasing the highest reaching temperature of heat affected zone. Hence, the austenite grain size becomes larger and martensite start temperature increases. Consequently, autotempering is promoted, resulting in stronger joint strength.

Establishment of a model predicting tensile shear strength and fracture portion of laser-welded lap joints

This study was aimed at establishment of a model that can predict tensile shear strength and fracture portion laser-welded lap joints in the tensile test. To clear influence of the bead length and width on them, the joints employed steel sheets with a thickness in the range of 0.8 mm to 1.2 mm were evaluated. It was found that the tensile shear strength increased with the bead size, and the fracture occurred at base metal (BM), weld metal (WM) or portion between them with a curvature (referred to as portion R). Also to clarify rotational deformation process around WM during the tensile test, joint cross-sections were observed at some applied load levels in the test. This observation derived the relationship between the radius, Ri, at the inner plane of portion R and the rotational angle, θ, of the center of sheet thickness, and the relationship between Ri and applied load. A plastic analysis based on these functions and assumptions that the joint consists of BM, WM and R, which are under simplified stress mode respectively, could estimate the tensile shear strength and the fracture portion of the joints. This estimation made good accord with experimental results.

Dependence of strength and fracture behaviour on chemical compositions in spot-welded L-type joints

Abstract In order to achieve lighter and stronger car bodies by applying high strength steel sheets, one of the key technologies is enhancement of joint strength. In this study, we investigated dependence of strength and fracture behaviour on chemical compositions of steels in spot-welded L-type joints in detail. Consequently, the following experimental results were obtained. (1) Maximum load of the joint decreased with increase of carbon (C) and phosphorous (P). The maximum load was decreased by 0.4–0.7 kN with increase of 0.1% in C, with C content ranging from 0.03 to 0.5%, and 0.5 kN with increase of 0.01% in P, with P content ranging from 0 to 0.03%. (2) Fracture portion changed from the outside to the inside of weld metal with increase of C and P. (3) The fracture path was estimated to accord with the solidification segregation site in the weld metal in the case of a steel of 0.2% C, 0.03% P. (4) By implementation of an appropriate post heat during spot welding process for the steel of 0.2% C, 0.03% P, the degree of solidification segregation was clearly reduced and the maximum load of the joints was improved by 70%.

Tensile shear strength of laser lap joints

This study was aimed at establishment of a model that can predict tensile shear strength and fracture portion of laser-welded lap joints in the tensile test. To clear the influence of bead length and bead width on them, the joints that used steel sheets with a thickness in the range of 0.8–1.2 mm were evaluated. It was found that the tensile shear strength increases with the bead size, and the fracture occurs at base metal (BM), weld metal (WM) or a portion between them with a curvature heat-affected zone (HAZ), in the tensile test. Also to clarify the rotational deformation process around WM during the tensile test, cross-sections of joints were observed, which were applied to several loads in the tensile test. This observation derived the relationship between the radius, Ri, at the inner plane of the HAZ and the rotational angle at the centre of the sheet thickness. Furthermore, the relationship between Ri and the applied load was obtained by linear regression. A plastic analysis for deformation of the joints was carried out based on these functions and some assumptions. These assumptions consider that the joint consists of BM, WM and HAZ, which are under a simplified stress mode. Finally, estimation of the tensile shear strength and the fracture portion of the joints was achieved. This estimation made good accordance with the experimental results.

Porosity formation in CO2 laser welding of steel sheets

In laser welding there are weld metal internal defects, such as porosity and cracking, and shape inferiorities like undercut and droop. The mechanism of porosity formation alone is thought to vary with the plate thickness, material and the groove conditions. For CO2 laser welding of steel sheet, porosity formation is empirically probable with thinner plate and inferior butt conditions, such as a widened gap. The objective of this study was to clarify the mechanism of porosity formation in the CO2 laser welding of steel sheet. Ultra-low-carbon cold rolled steel sheet (thickness 0.7 mm) was mainly employed as the specimen material; in addition, in order to avoid externally disturbing factors, such as contamination at the groove face and the variation in the butt condition, melt-run welding was carried out. Under these circumstances Ar coaxial shielding gas was employed; welding experiments were carried out where Ar + N2 mixed gas with varied nitrogen content was used as either coaxial shielding gas or for the backside atmosphere, in addition to normal welding with a steel sheet backside air atmosphere; the mechanism for porosity formation was also studied.