It has been broadly accepted that root gap during welding continues to open in case of butt-welding without restraints. So, the restraints, such as strong back and tack weld, are essential countermeasures to prevent root gap from opening. In order to improve manufacturing sufficiency, active control of root gap behavior during welding even becomes significant. In this study, the root gap behavior during in-process control welding by additional cooling is discussed based on measurement experiments and numerical analyses. As the results, the root gap behavior during butt-welding is possible to be considerably affected by temperature distribution during in-process control welding. The cooling position and cooling intensity of controllable factors in additional cooling method have a significant influence on the root gap behavior during welding. In addition, the root gap can be closed due to rapidly cooled temperature distribution by additional cooling with appropriate cooling position. The appropriate cooling position is dependent on the mechanical melting length behind welding torch. It is significant to consider the effect of weld shrinkage during rapid cooling for closing of the root gap. It is concluded that root gap behavior during butt-welding can be controlled by the optimization of temperature distribution due to additional cooling.
It has become much more necessary to control weld distortion, which has negative influences on structural integrity, without loss of manufacturing efficiency. Commonly, angular distortion, which is controlled by temperature distribution along direction of plate thickness, is controlled by heat input parameter (Qnet / h2). However, it is also known the conventional heat input parameter is not always applicable in any welding process conditions. In order to clarify the effect of welding conditions on weld distortion more obviously, a better understanding and considering of the characteristics of temperature distribution during welding is required. In this study, the more accurate numerical analysis of weld distortion using arc physics based heat source modeling has been developed, as one approach for a better considering of the characteristics of temperature distribution during welding. Computational simulation of tungsten inert gas arc plasma based on mathematical modeling of the heat transfer from arc plasma to a welded plate is performed to obtain a more precise temperature distribution during welding. The temperature distribution obtained is used for a large-deformation thermal elastic-plastic analysis of weld distortion. In addition, the effects of welding process conditions on angular distortion are examined considering weld penetrations with the arc plasma process, and verified experimentally. Finally, a better understanding of the relationship between heat input parameter and angular distortion is discussed.
High-strength steel sheets of HT980-class are beginning to be used for automobile bodies in order to reduce body-weight and improve fuel-efficiency. Resistance spot welding is an important method in joining of high-strength steel sheets. The microstructure of welds becomes martenstic structure and high tensile residual stress is generated. It is important to ensure the performance of resistance spot welds in order to use high-strength steels more widely in automobiles. Therefore, the evaluation of characteristics of resistance spot welds is quite important. In this study, residual stress of resistance spot welds of HT980 sheets were investigated by numerical simulation and X-ray stress measurement. First, resistance spot welded test pieces were fabricated and nugget formation process was observed. Residual stress distribution in the welds of the test pieces was measured by X-ray diffraction method with a two-dimensional detector. High tensile residual stress was measured in the spot welds. The measurement results for the three test pieces were comparable. Second, residual stress distribution in the resistance spot welds was calculated by current-temperature-microstructure-mechanics coupled simulation. Comparison between the experiment and the numerical simulation proved that nugget formation process was reproduced by the simulation. Residual stress distribution obtained by the simulation was in good accordance with the measurement results when initial residual stress in HT980 sheet before welding was taken into consideration.
When steel structures are constructed by welding, welding distortion and residual stress (hereafter welding imperfection) are inevitably generated. It was verified in our studies that welding imperfection was predicted with high accuracy by thermal elastic-plastic analysis based on the infinitesimal deformation theory. On the other way, in the case that the ultimate strength of thin plate with the welding imperfection is predicted, elastic-plastic (hereafter El-Pl) analysis based on the large deformation theory with up-date Lagrangian method is generally carried out. However, it was known that the welding imperfection could not be reproduced by the unbalanced forces caused by the differences with both analysis theories when the welding imperfection was introduced into El-Pl large deformation analysis. Therefore, thermal El-Pl large deformation analysis program was developed. From the analysis results, it was elucidated that prediction accuracy of welding distortion was very low by thermal El-Pl large deformation analysis and that it took very long CPU time to predict welding residual stress by it.
There is considerable interest in joining of aluminum to iron because of various advantages such as weight reduction of vehicles. However, the intermetallic compound (IMC) formed at the welded interfaces causes a brittle fracture of the dissimilar joint. Therefore, it is important to prevent the formation of IMC. It is well-known that friction stir welding (FSW) is suitable for the inhibition of IMC formation due to the low heat input. This study investigated the formation mechanism of IMC during FSW by comparing the results from previous studies, in which dissimilar welds were prepared by the diffusion bonding and roll bonding. The time compensated IMC thickness in FSW was larger than that in the other joining methods. This is because three factors of fresh surface, stored strain and plastic flow were strongly affected the formation mechanism of IMC. However, the amount of IMC was significantly small because the welding time was very short. It is therefore considered that a dissimilar joint of aluminum to iron can be achieved by FSW.
A new Thermal Cycle Tempering Parameter (TCTP) to deal with the tempering effect during multi-pass thermal cycles has been proposed by extending Larson-Miller parameter (LMP). Experimental result revealed that the hardness in synthetic HAZ of the low alloy steel subjected to multi tempering thermal cycles has a good linear relationship with TCTP. By using this relationship, the hardness of the low-alloy steel reheated with tempering thermal cycles can be predicted when the original hardness is known.
Recently, ultra high strength steel plate (in this study, over 780N/mm2 class steel is called 'ultra') such as 1000N/mm2 class strength is focused as material of various type of steel structure. 1000N/mm2 class steel has already applied in hydropower plant in order to enjoy its effect on cost reduction. However, in building construction field, high strength steel remains unfamiliar material. There are only a few example of application of 780N/mm2 class strength steel in building field. On the other hand, safety issue in a huge earthquake is more focused recently. Very large extent of elastic region of ultra high strength steel come to the front. Authors started the investigation of application of 1000N/mm2 class steel plate for buildings. When ultra high strength steel is investigated, welding always become an issue. However, authors think welding is absolutely necessary in assembling steel material for buildings because welding is the most common and economic way of joining the materials at the moment.In this report, welding material, which can match 1000N/mm2 class steel plate and can optimize the properties of welded joint, has been developed. Especially wide range of inter-pass temperature and heat input is tried to be available. By controlling carbon equivalent and oxygen content of deposit metal, finally new welding material of GMAW and SAW for building has been developed.
Recently, ultra high strength steel plate such as 1000N (950N/mm2 Class) class strength is focused as material of various type of steel structure. 1000N class steel has already applied in hydropower plant in order to enjoy its effect on weight and cost reduction. However, in building construction field, high strength steel remains unfamiliar material. There are only a few example of application of 780N/mm2 class strength steel in building field. Authors have already developed 1000N class steel plate and welding consumables for building. However, from viewpoint of actual application, structural performance under massive earthquake must be evaluated. Local area of column-to-beam connection seems be subject to large amount of strain when seismic event. So, in this report, low cycle fatigue properties of 1000N class steel and its weld was evaluated and structural experiments were conducted. As a result, it is revealed that Low fatigue property of 1000N class steel is equivalent to that of 780N class and can maintain the loading capacity under 26∼51 times of strain amplitude at cδp. Furthermore, it is also shown that structural performance can be estimated by test results of small specimen with strain calculation by FE analysis and finishing of edge of beam is effective for extension of lifetime through decrease of strain.
In order to propose and verify a method of non-destructive inspection for cracks in existing steel structures by FSM supported with an electric field analysis, a series of experiments and analyses were carried out. It was confirmed that the electric field pattern of the actual-sized steel plate deck in which cracks did not existed could be simulated with high accuracy by the electric field analysis. The electric field pattern was largely changed when a crack was initiated and propagated in the deck. By comparing the electric field pattern actually obtained by FSM with that obtained by the analysis simulating the virgin situation, whether cracks exist or not could be judged and evaluated if there was a difference between the both electric field patterns.