Koutarou Inose

Characteristics of welding distortion/residual stress generated by fillet welding with laser beam and verification of generality of its highly accurate prediction

It was found that the characteristics of welding distortion and residual stress generated by laser beam welding (LBW) had the same tendency as those by arc welding but that the absolute value was small. Moreover, the process of LBW is quite different from that of arc welding, the validity and generality of the dominant factors to predict high accuracy welding distortion and residual stress generated by welding could be verified by the high accurate prediction of welding distortion and residual stress which had been proposed based on arc welding. Inherent force was proposed as a parameter which could indicate the magnitude of compressive residual stress, which might influence the load-carrying capacity. Although inherent forces with LBW were 40% that of arc welding, it was found that control of welding heat input was important so as to improve the load-carrying capacity.

Effects of heat input ratio of laser–arc hybrid welding on welding distortion and residual stress

For predicting welding distortion and residual stress generated by laser–arc hybrid welding, a series of experiments and analyses were carried out. A bead-on-plate welding was performed on SM490 steel by using a fibre laser and CO2 arc welding by changing their heat input ratio. The experiment was simulated by thermal elastic–plastic analysis with the proposed simulation model considering the penetration shape by laser and arc separately. By using this model, the experimental results could be simulated with high accuracy. Therefore, the validity and generality of the numerical simulation model could be verified. The tendency and magnitude of angular distortion varied with the heat input ratio of laser and arc. The results indicated the possibility of the ideal heat input ratio of laser and arc for controlling angular distortion generated by hybrid welding. On the other hand, it was confirmed that the heat input ratio of laser and arc did not affect residual stress generated by hybrid welding.

Verification of Validity and Generality of Dominant Factors in High Accuracy Prediction of Welding Distortion

The two dominant factors, satisfying the yield condition (especially at high temperature above 700 °C) and regarding the weld metal shape as the deposited metal, in the high accuracy prediction of welding distortion had been previously determined using a bead-on-plate specimen with a groove. To verify the validity of the dominant factors, the experiment and thermal elastic-plastic analysis by FEM were conducted using a specimen for butt welding and fillet welding. The validity of the dominant factors in the high accuracy prediction of welding distortion using the bead-on-plate specimen with the groove was verified by the results of butt welding and fillet welding, and its generality was confirmed. It was confirmed that residual stress was also accurately predicted because welding distortion was accurately predicted.

Determination of Dominant Factors in High Accuracy Prediction of Welding Distortion

The experiment was carried out using a bead-on-plate specimen with the groove placed to accurately predict welding distortion. The experiment was simulated with thermal elastic-plastic analysis based on FEM. Comparing and investigating both results, the dominant factors for high accuracy prediction of welding distortion were determined. In the thermal elastic-plastic analysis, the yield condition should be satisfied regardless of the magnitude of yield stress of the material. It was not known if the predicted results of welding distortion obtained without satisfying the yield condition in the whole temperature range were overestimated or underestimated. The accuracy of prediction of welding distortion was improved by applying heat input to the weld metal bead shape, not just the groove profile. It was determined that satisfying the yield condition in the entire temperature range and considering the weld metal shape as the deposited metal were the dominant factors for high accuracy prediction of welding distortion.

Design and welding method for high-strength steel structure using laser-arc hybrid welding

The study intended for rational usage of high-strength steel in structure by applying laser-arc hybrid welding has been conducted. Key design requirements and improvement target have been clarified considering their key performances as structural members. The evaluation methods for risk of brittle fracture and cold cracking have been established for quality control. Safety of welded joint has been confirmed and their fatigue strength has been improved by optimizing welding process.

Safety of Mega Container Ship Focusing on Brittle Crack Initiation and Arrest Behavior of Heavy Thickness Plate

The enlargement of containership requires the higher tensile strength steel and thicker plate around the hatch coaming more than the prescribed classification rules. These call the concerns about the safety of brittle crack initiation and arrestability of hull structure. It is known that increasing of stress and thickness of plates will reduce the toughness of base metal and welding joints. Therefore, the authors conducted to assure the toughness of base metal and welding joint for new higher tensile strength steel of 460MPa yield strength (YP460) and 60mm thickness. It has been considered that weld assembled steel plate structure has some arrestability at intersections of plates, though there are no prescription on the rules. And it was reported that very thicker mother plate has not enough toughness to arrest brittle crack recently. The authors investigated the structural arrestability by model testing and FE analysis and established it. Furthermore, structural details for intersections were also established. Both preventing the brittle crack initiation and arresting the brittle crack of weld structure were described, focusing on developing higher yield strength and heavier thickness of YP460 for containership in this paper.© 2008 ASME

Distortion and residual stress generated by laser beam welding of high strength steel

In order to elucidate distortion and residual stress generated by welding of high strength steel (HT780) by laser beam, a series of experiments and analyses were carried out. The angular distortion generated by bead-on-plate welding was V-shaped and its magnitude was about 2 mm. The longitudinal bending distortion was extremely small. On the surface of the plate, the residual stress component in the welding direction was tensile and it was smaller than the yield stress in the weld metal. The residual stress was almost zero outside the weld metal. The phase transformation range in the cooling stage and the temperature dependency of mechanical properties were obtained. The mechanical properties in the phase transformation range in the cooling stage could not be specified due to transformation expansion. Therefore, they were idealized by considering transformation expansion and transformation superplasticity. The validity of the idealized mechanical properties was verified by simulating the experiment by the thermal elastic–plastic analysis. It was elucidated that the welding out-of-plane distortion and tensile residual stress were largely controlled by phase transformation in the cooling stage, although the bead width of laser beam welding was extremely narrow.

Functional Assessment of Laser Arc Hybrid Welded Joints and Their Application for Bridge Construction

It has been made clear that the tensile strength of fillet joints using laser arc hybrid welding satisfies the requirements retaining base metal strength. The fatigue strength of laser arc hybrid welded fillet joints can be considered at least equal to that of conventional arc welding in the preparation of the structural design specification. By applying laser arc hybrid welding to the pedestrian walkway mount-up panel, the welding distortion was too small to require corrective work. The number of welding passes was decreased by single-sided fillet weld. The welding speed of laser arc hybrid welding is about twice that of arc welding.

Evaluation of Charpy impact toughness using side-grooved specimen for hybrid laser-arc welds of ultra-high-strength steel

This paper discusses the significance of a narrow hard weld bead, produced by hybrid laser-arc welding, on the Charpy absorbed energy. A parametric finite element analysis with various strength mismatching and bead widths has been conducted on the strain field of side-grooved and normal Charpy specimens. The narrow weld bead promotes the accumulation of plastic strain energy on the base metal side. This feature is more significant in the normal specimen than in the side-grooved specimen. The results show that the method based on the Weibull stress criterion for converting the absorbed energy of the side-grooved specimen to that of the normal specimen can be applied to hybrid laser-arc welds of ultra-high-strength steel. The converted energy is consistent with the absorbed energy measured in the normal specimen. This study indicated that the absorbed energy of side-grooved specimen corresponding to the absorbed energy of the normal V-notch specimen increases with increasing hard zone width. On the other hand, the influence of strength mismatch ratio on the difference of the absorbed energy is rather weak.

Dominant factor of HT780 non-load-carrying T-joint fatigue strength using laser arc hybrid welding and quality control

ABSTRACT The study intended for clarify that dominant factors of fatigue strength of T-joint which welded by laser arc hybrid welding. Fatigue test has been conducted using five types of HT780 T-joints fabricated under different welding conditions respectively. Dominant factors for fatigue strength of T-joint are flank angle and curvature at toe of T-joint fillet. The fatigue strength is increasing inverse proportional to flank angle and proportional to curvature at toe. Heat-filler feeding ratio affects flank angle and curvature at toe. The former would be decreased and later should be increased by heat-filler feeding ratio increasing. In the study, fatigue strength of the one type of joint shall be recognized as JSSC fatigue strength class D. All types of joint in the study can be dealt as at least JSSC fatigue strength class E. Flank angle of these joints are smaller than 47 degree and curvature of toe are larger than 1.3mm. These quantitative results of the study are very useful for quality control of practical fabrication. That secures the fatigue strength of non-load-carrying T-joint welded by laser arc hybrid welding.