In this study, the effects of resistance spot welding conditions on the nugget diameter, which was one of the major influencing factors of resistance spot weld joint strength, was modeled by a machine learning method which had been proposed by the authors in recent years. Then, the applicability of constructed model and the effect of resistance spot welding conditions on the nugget diameters were discussed. The feature of the machine learning method used in this study was that the relationship between input and output could be derived as an easy-to-understand mathematical expression. A resistance spot welding condition-nugget diameter database was created through experiments using 590MPa class steel plates, and a nugget diameter prediction model was constructed to reproduce the database appropriately. As a result, it was indicated that the nugget diameter prediction model can predict the nugget diameter under welding conditions used for model construction and those not used precisely. Furthermore, it was found that the nugget diameter prediction model was composed of two terms that were presumed to reflect the spread of material melting due to heat input and the phenomenon at the beginning of energization.
Laser-arc hybrid welding is divided into laser-based and arc-based welding. In the arc-based welding, the laser beam irradiation provides the guiding effect of the arc plasma. Some studies have reported that an unstable arc plasma becomes stable due to this guiding effect. This is considered as the influence of metal vapor generation by laser beam irradiation, however, there are few reports on the measurement of metal vapor and arc plasma state during the guiding process. Therefore, the mechanism has been still not fully understood. In this study, the fundamental study of the guiding arc phenomenon was conducted in TIG arc welding (60A) with laser beam irradiation (170W) under relatively low power condition. When the base metal was stainless steel, the guiding arc by the laser beam irradiation was observed unlike the mild steel. Spectroscopic measurement for the guiding arc phenomenon in stainless steel showed that the metal vapor was composed of manganese and chromium, and was widely distributed near the irradiation point, but not concentrated. We also observed the decrement of plasma temperature, which is considered to be caused by distributed metal vapor and the resultant decrement of the current density.
Solidification cracking can be explained by the intersection between the high temperature ductility curve and thermal strain curves; thus, to predict the location and length of solidification cracking during arc welding, these two curves were calculated. The critical strain rate (εCSR) of the high temperature ductility curve was measured by an in-situ observation technique. The solidification initiation and completion temperature were calculated by the Kurz-Giovanola-Trivedi (KGT) and solidification segregation models respectively. The thermal strain curve was calculated using a finite element simulation model. The solidification cracking occurrence in the actual U-type hot cracking test during the arc welding corresponded with the prediction results estimated from the metallurgical and thermal elastic-plastic analysis models.
Solidification cracking susceptibility in arc & laser welding of DSSs (duplex stainless steels) was quantitatively evaluated and the numerical simulation of solidification cracking susceptibility has clarified the affecting factors. The BTRs (Brittle temperature range) of standard, lean and super DSSs in GTAW were 58K, 60K and 76K, respectively. The solidification cracking susceptibilities of DSSs were lower than those of austenitic SSs with A mode solidification modes. The BTRs of standard, lean and super DSSs in LBW were 40K, 45K and 56K in LBW, respectively. The BTRs in LBW were reduced by 15-20K compared to those in GTAW. These results suggested that DSSs had a significantly low risk of solidification cracking in LBW as well as GTAW. In order to clarify the affecting factors of solidification cracking, numerical simulation of solidification cracking susceptibility was carried out. The segregated concentrations of P, S and C in LBW were slightly lower than those in GTAW, suggesting that the solidification cracking susceptibility in LBW was reduced to GTAW attributed to the inhibition of solidification segregation because of the rapid solidification in LBW. In addition, we discussed that there is a difference in the hot cracking susceptibility compared with the austenitic stainless steel having the A mode. To explain this phenomenon, the segregation amounts of S and P by arc, laser and solidification modes were investigated. The segregation of Standard DSS was lower than that of Type310S when using same amount of P and S. This clearly indicates that F mode solidification was less segregated than A mode solidification. This is because the equilibrium partition coefficient and the diffusion coefficient of the standard DSS are larger than that of Type 310S.
The aim of this study is to propose the prediction approach of ductile crack growth resistance of cracked component with steels, based on ductile crack growth simulation using only mechanical properties of material obtained from tensile test for a round-bar specimen. Ductile crack growth was simulated in accordance with nonlinear damage accumulation model with stress triaxiality dependency of critical strain derived from void growth analyses of unit cell. In terms of simplicity, the prediction formula of the stress triaxiality dependency of critical strain using mechanical properties of material was proposed in this paper. In order to confirm the applicability of proposed prediction approach, ductile crack growth simulations were performed with elasto-plastic finite element analyses implementing nonlinear damage accumulation model using tensile test result for a round-bar specimen. As a result, it was found that ductile crack growth resistance could be predicted with high accuracy by the proposed approach. In addition, it was confirmed that the proposed approach was also applicable to steels with different mechanical properties and could reproduce the effect of plastic constraint related to specimen dimension to ductile crack growth resistance.
The optimum weld line shape for the remote laser welding were investigated to improve shear strength and peel strength of the lap joints of 980 and 1180MPa grade automotive high tensile strength steel sheets. Lap joints were prepared with two kinds of weld line shapes that were linear line and circle. These lap joints were exposed to three different tensile tests of tensile shear test, cross tension test and L-form tension test. In tensile shear test, weld line shapes didn’t affect on tensile shear strength (TSS), and TSS was proportional to weld line length. In cross tension test, weld line shapes much affected on cross tension strength (CTS). CTS of the circle weld line shape joints was much higher than that of the linear weld line shape joints. In L-form tension test, the weld line shapes also much affected on L-form tension strength (LTS). LTS of continuous linear line weld joints was much higher than that of circle shape weld joints. The factors which controlled TSS, CTS and LTS were discussed by conducting FEA of the tensile tests from a viewpoint of local stress concentration considering the loading types of shear stress and tensile stress.