Papers by Author: Masao Toyoda

Abstract: Different techniques are being studied to make the automotive electrical parts more compact and lightweight; along with improving their functionality for saving energy and protecting the environment. In the wake of this trend, the joints in these parts are required to have little distortion and a high degree of accuracy. Although laser welding, which yields low-distortion joints, was developed in response to this demand for high accuracy, it has been difficult to meet the needs of modern, high-accuracy automobile parts with conventional laser welding. In this research, in order to realize that degree of accuracy, a phenomenal and theoretical analysis of the melting solidification process was conducted and high-accuracy laser welding was investigated for controlling welding deformation.

Abstract: In order to reduce the number of fabricating processes, it is preferable to control welding distortion during welding; instead of using restraints before the process, or correcting the distortion after the process. The benefits of eliminating extra processes include the reduction of the time and manufacturing cost. This paper presents a method of back heating and welding in tandem, with the back heating occurring at a constant distance from the welding torch during welding. Traditionally, the back heating method has reduced angular distortion in two different ways; One is to bend the welded material in the opposite direction, and the other is homogenization of temperature distribution along the thickness. But, it has recently become known that the angular distortion produced by multiple heat sources, in tandem placement, is not always predicted by the total heat input to the welded joint; and it is possible for the distortion to differ greatly due to factors such as the distance between two heat sources, heat input parameters, and the heat input ratio. Based on these findings, angular distortion is expected to be reduced more effectively by choosing the proper condition for the heat source arrangement in back heating. In this paper, reduction of angular distortion by in-process control welding, using a back heating source, is numerically analyzed by the three-dimensional thermal elastic-plastic analysis, considering moving heat source with weld metal deposition. It was confirmed that the back heating method is effective in reducing angular distortion without restraint or correction. Proper condition concerning the heat source arrangement can be chosen and angular distortion can be perfectly controlled by back heating with ten percent of the welding heat input.

Abstract: The welding residual stress of a butt-welded pipe joint is evaluated, using inherent strain analysis. The residual stress distribution is obtained in detail along the thickness direction. The residual stresses are similar to values obtained by direct measurement on the specimen surface; as if though direct measurement is not used for the inherent strain analysis. These results indicate that inherent strain analysis is effective in evaluating through-thickness residual stress in primary piping of girth welded joint.

Abstract: Recently, stress corrosion cracking (SCC) of primary piping of stainless steel has been observed. SCC is considered to initiate and progress at near the welding zone in butt-welded pipes, because of the tensile residual stress introduced by welding. In present work, three-dimensional and axisymmetric thermo-elastic-plastic finite element analyses have been carried out, in order to clarify the effect of geometrical and welding conditions on through-thickness residual stress. In particular, butt-welding joints of SUS316L pipes have been examined. The residual stress was simulated by three-dimensional and axisymmetric models and the results were compared and discussed in detail.

Abstract: Development of a non-contact method for measuring the degree of corrosion is highly desired by those who are involved in the maintenance and control of steel structures. Increasingly, it is expected that application of an electromagnetic acoustic technology that does not require a coupling medium will meet this demand. Various problems have been reported regarding measuring thickness of a corroded material using the resonance method. In order to clarify these problems, it is important to make evaluations based on the resonance spectrum that corresponds to the thickness of each plate. In this paper, experiments using grooved specimens for a basic investigation were performed and a new method for wall-thinning measurement based on the resonance spectrum amplitude was proposed. In addition, examination was performed by using a numerical simulation about this method’s detection principle.

Abstract: This study investigated a method for estimating hardness distribution in welds, considering the effect of phase transformation and weld thermal cycles. Hardness distribution in welds was estimated from fractions and hardness of each microstructure by using rule of mixture. Finite element heat conduction analysis was performed to calculate weld thermal cycles. Microstructures formed corresponding to the thermal cycle were also calculated based on the continuous cooling transformation (CCT) diagram. The method mentioned above was applied to welds of Ultra-Narrow Gap welding process, which was developed for welding of ultrafine-grained steels. The calculated thermal cycles in the welds corresponded with measured results. Moreover, the estimated hardness distribution in the welds, which were estimated by using calculated thermal cycles and the phase fraction of each microstructure, was also in good agreement with measured values.

Abstract: Joint performances such as tensile strength and hardness in multi-pass welds are induced from both metallurgical and mechanical heterogeneity due to the difference of welding conditions. Hardness distribution in multi-pass weld metal is evaluated with a numerical simulation considering multiple heat cycles and phase transformation. Hardness of multipass weld metal is calculated with the rule of mixture by using fraction and hardness of each microstructure. In order to calculate fraction of each microstructure, CCT diagram was used. Conventional CCT diagrams of weld metal is not available even for single pass weld metal, thus new diagrams for multi-pass weld metals are created in this study. Modified diagrams for multi-pass weld metals with reheating effect were more dependent on the maximum temperature in reheating than the welding conditions. Hardness distribution is precisely predicted when the created CCT diagram for the multipass weld metal was used and the detailed calculation of weld thermal cycle is done.

Abstract: It has been well known that ductile fractures of steels are accelerated by triaxial stresses. The characteristics of ductile crack initiation in steels are evaluated quantitatively using two-parameter criterion based on equivalent plastic strain and stress triaxiality. It has been demonstrated by authors using round-bar specimens with circumferential notch in single tension that the critical strain to initiate ductile crack from specimen center depends considerably on stress triaxiality, but surface cracking of notch root is in accordance with constant strain condition. This study fundamentally clarifies the effect of strength mismatch, which can elevate plastic constraint due to heterogeneous plastic straining under static loading, on critical conditions for ductile cracking from the pre-notch root. In order to evaluate the stress/strain state in the pre-notch root of specimens, a thermal elastic-plastic finite element (FE) analysis has been carried out.