Yasunobu Miyazaki

Optimization of an electron beam remelting of HVOF sprayed alloys and carbides

Due to the lamella structure of the coating and the lack of metallurgical bonding, thermal sprayed coatings may have disadvantages such as low coating cohesive strength and low interface strength between the substrate and coating. It was considered that the electron beam remelting process is one of the most convenient processes to reduce or remove these disadvantages. The optimization of electron beam remelting process parameters and the evaluation of this process were investigated. Not only electron beam conditions, but also the thickness of copper substrate and electroplated Ni affect the penetration depth and the remelted width at the interface. Also, in order to reduce the number of pores and the unevenness of the surface morphology, it was found that relatively low fusing speed and homogeneous heating are preferable. Moreover, because the metallurgical bonding could dominate the interfacial strength, the interfacial shear strength is independent of the coating hardness. Thus, it is successful to deposit coatings, Stellite 6, Ni based self-fused alloy 4 and WC-12Co with an adhesion strength value of approximately 350 MPa.

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.

Effect of applying electrical potential to a CO2 laser welding of different thickness plates

Abstract This study was aimed at developing laser welding with an applied voltage potential to increase the bead root width in laser welding. Also, in order to enhance the welding speed and the butt joint gap tolerance, the influences of the experimental conditions: supplied voltage between plate and backside electrode, welding speed, plasma operate gaseous species, and the butt joint gap, on the bead root width were investigated. Although it is necessary to avoid over heating and melting the plates, it is applicable for higher speed and wider gap butt joint welding than a conventional laser welding. In the case of butt joint welding with a thickness of 2.0 and 0.8 mm steel sheets by using 5 kW CO2 laser system, it is concluded that this method is effective for increasing of the welding speed from 5 to 8 m/min. Knowledge of optimum conditions and configurations has guided to extend this process to more challenging structural materials such as a tailored blank steel sheet.

Minimization of Heat-Affected Zone Size in Welded Ultra-Fine Grained Steel under Cooling by Liquid Nitrogen during Laser Welding

Ultra-fine grained steel (UFGS) with an average grain size of less than 1μm has been developed and is expected to demonstrate superior properties. However, its welded heat-affected zone, HAZ, substantially affecting the strength of a welded joint, will be easily softened after welding. Therefore, minimization of UFGS’s HAZ size during laser welding was carried out using the cooling conductor liquid nitrogen. It was found that a shielding gas with adequate flow rate for the liquid nitrogen depth was used to displace nitrogen on the area of laser beam irradiation to stabilize the weld bead. Also, because YAG laser system was mainly used because it has a lower laser induced plasma or plume temperature, which results in a decreased occurrence of pit and blowhole. HAZ size minimization strongly depends on the initial plate temperature. Reduced initial plate temperature shrinks the specific heated temperature range in which softening occurs. However, due possibly to decreasing thermal conductivity under room temperature, which prevents heat removal, the benefit of reducing the initial plate temperature is limited. The optimal initial temperature to minimize the HAZ size, in the present work, was found to be 123K.

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.

Experimental study of laser welding with applied electrical potential

Abstract The objective of the present study is to develop laser welding with an applied voltage potential, to increase the weld bead root size in laser welding and thereby to improve the welding speed and the butt joint gap tolerance. The influences of the experimental conditions, namely, input laser power, applied voltage between plate and backside electrode, welding speed, plasma operating gaseous species (air, argon, or argon–helium), and the butt joint gap, on the plasma stability and the weld bead were investigated. The weld beads are evaluated from the point of view of the bead appearance, the penetration depth, the ratio of the widths of the weld bead root and weld bead face, and the smoothness of the bead surface. It is found that to stabilise the plasma, it is preferable to set the plate polarity as the cathode (electrode positive), and to use argon or helium gas as a plasma operating gas. Also, it is concluded that this novel method is effective in increasing the bead root/face width ratio, and the melting area. Although it is necessary to optimise the experimental conditions to avoid overheating and melting of the plates, the present method is applicable for higher speed and wider gap butt joint welding than in conventional laser welding.

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.

Influence of laser-induced plume on penetration in laser welding

The influence of a laser-induced plume on penetration is discussed in remote laser welding. Melt-runs accompanying plumes of various lengths were carried out in order to investigate the influence of plume on penetration using a 4.5 kW YAG laser and air knife. Penetration depth obviously reduced in conditions with a long plume. It is considered that the decrease of penetration depth comes from attenuation of laser power in the plume, through the decrease in weld metal cross-sectional area. Attenuation of laser power was estimated at about 7% and focus shift was estimated to be about 0.7 mm in the experimental conditions with a plume of 90-mm length, compared with the conditions with a plume of 11-mm length.

Joint strength and behaviour of coated aluminium in laser lap welding of steel sheets

When galvanized steel sheets are closely overlapped and welded by laser lap welding, a large amount of molten metal spatters, resulting in a poor surface appearance of the weld and weakened strength of the welded joint, as compared with that of cold-rolled steel sheets. Whereas in the case of aluminium-coated steel sheets, even when they are closely overlapped and welded by laser lap welding, no spattering occurs. Thus, a good surface appearance of the weld is obtained, but the welded joint has lower strength. In both the mentioned cases, it is known that if a clearance of about 0.1 mm is provided between the steel sheets, laser lap welding produces a good surface appearance of the weld and the welded joint strength equal to that of the cold-rolled steel sheets. This report discusses specifically how, in laser lap welding of overlapped Al-coated steel sheets, Al of the coated layer comes to enter the weld metal, also specifically how to reduce the joint strength, as well as what behaviours of Al are present when a clearance is provided between the steel sheets. When the steel sheets are closely overlapped and welded, Al becoming molten on the base metal side of the bond of the overlapped face becomes swallowed up by the bath streams of the molten pool, flowing into the molten pool, then forming the Fe–Al intermetallic compound, while not being sufficiently stirred. It is considered that when subjected to the tensile shear test, the Fe–Al intermetallic compound starts to fracture, thereby causing a partial loss of the weld metal and a reduction in the joint strength. On the other hand, when a clearance is provided between the steel sheets, it may be inferred that the fusion Al on the base metal side of the bond stays in place without flowing into the molten pool, consequently not forming the Fe–Al intermetallic compounds within the weld metal.