Shigetaka Okano

Arc physics based heat source modelling for numerical simulation of weld residual stress and distortion

Abstract Heat source modelling based on weld arc physics has been developed for the more accurate numerical simulation of weld residual stress and distortion. Computational simulation of tungsten inert gas arc plasma based on mathematical modelling 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 stress analysis of weld residual stress and distortion. In addition, the effects of welding process conditions on weld residual stress and distortion are examined considering weld penetration with the arc plasma process and verified experimentally. Finally, the effectiveness of arc physics based heat source modelling for numerical simulation of the weld residual stress and distortion is evaluated.

Distortion behaviour of Fillet T-Joint during In-Process Control Welding by Additional Cooling

It is desirable to control welding distortion and to apply this procedure during welding, rather than before or after welding in order to reduce the cost. The method suggested in this paper is to cool the hot section of weld metal during the welding of a fillet T-joint, in order to control rotational distortion in the web. Using this method, increasing the gap distance between web and flange is avoided so that it is possible to continue the welding procedure. Distortion behaviour of a fillet T-joint during in-process control welding by additional cooling is numerically analysed by the three-dimensional thermal elastic-plastic analysis considering a moving heat source with weld metal deposition. It is confirmed that the additional cooling method is effective to reduce the rotational distortion, which makes welding process impossible. Under the proper condition of cooling, it is possible to weld a T-joint without any tacking or restraint.

Experimental study on relationship between heat parameter and angular distortion

It is well known that weld residual stress and distortion should be controlled appropriately for structural integrity. Recently, it has become much more necessary to control weld distortion to highly improve manufacturing efficiency. Various studies on control of weld distortion had been conducted based on clarification of influential dominant factors for that. The influential dominant factors had been studied from a viewpoint of temperature distribution in plate thickness section. Without considering moving the weld heat source, the temperature distribution is controlled by weld heat input (Qnet) per weld length. Angular distortion, which is controlled by temperature distribution along the direction of plate thickness (h), is controlled by heat input parameter (Qnet/h2). However, it has recently become known that the conventional results cannot be applied to all welding processes because such processes are becoming more diversified. It is significant for more accurate control of angular distortion to investigate once again the relationship between the heat input parameter and angular distortion. In this study, a series of experiments on the relationship between heat input parameter and angular distortion are carried out. The effects of welding current and welding speed are investigated individually in both TIG and MAG welding. The difference between these welding methods is also investigated. Based on the result, the effects of them are discussed in relation to temperature distribution during welding. It is considered that angular distortion is affected by temperature distribution not only in plate thickness section but also along welding direction. So, angular distortion is not always controlled by only the conventional heat input parameter because the heat input parameter is proposed as the influential dominant factor for temperature distribution in plate thickness section. It is concluded that generation characteristics of inherent strain should be considered in relation to three-dimensional temperature distributions during welding for more accurate control of angular distortion.

Temperature distribution effect on relation between welding heat input and angular distortion

Conventional and activated tungsten inert gas (TIG) welding were conducted under the same welding conditions in an experiment to compare the weld penetration, temperature distribution and angular distortion. The results showed that the quantities of heat input per unit welding length were almost the same, but the shapes of the weld penetration and heat-affected zones changed drastically. The difference in angular distortion between conventional and activated TIG welding was strongly dependent on the thickness of the welded plate. A distortion analysis was developed by considering convective heat transfer in the weld pool to obtain a more detailed understanding of the temperature distribution effect on the relation between welding heat input and angular distortion. The resulting distinguishable curves for conventional and activated TIG welding, which can lead to the thickness dependence of differences in angular distortion, showed the traditional relation between the heat input parameter and the angular distortion.

Method of X-ray residual stress measurement for phase transformed welds

In this study, we attempted to evaluate the residual stress distribution in welds accompanied by phase transformation using X-ray stress measurement. First, relationship between the phase transformation behavior and the residual stress of welded specimens with different phase transformation was discussed. In the specimen with martensite in the weld, reduction in tensile stress due to martensite transformation shown in residual stress distribution followed conventional behavior. However, in the specimen with bainitic ferrite in the weld, the residual stress in the transverse direction was almost the same as the residual stress in the longitudinal direction, and the residual stress did not follow conventional behavior. Next, X-ray elastic constant in the weld was measured, and then the residual stress was reevaluated. In the specimen with bainitic ferrite in the weld, X-ray elastic constant had anisotropy, and the reevaluated residual stress followed conventional behavior. In conclusion, it was shown that depending on the phase transformation behavior, it is difficult to use X-ray elastic constant estimated by Kröner model; however, if we use the measured value, we are able to evaluate the residual stress more accurately.

Effect of welding conditions on reduction in angular distortion by welding with trailing heat sink

Abstract The effectiveness of welding with a trailing heat sink in reducing the angular distortion of a weld has been experimentally investigated with focus on the cooling position. A numerical model of welding with a trailing heat sink is constructed through the comparison of measured values of weld penetration, thermal cycles and angular distortion with those calculated. On the basis of this model, the effect of welding heat input conditions on the reduction in angular distortion is discussed to evaluate the versatility of welding with a trailing heat sink. The results indicate that the choice of an appropriate cooling position behind the welding heat source is essential for the effective reduction in angular distortion. The reduction in angular distortion by the heat sink at the appropriate cooling position increases with the heat input parameter Qnet/h2, where Qnet is the weld heat input and h is the thickness of the plate.

Angular Distortion Reduction by In-Process Control Welding Using Back Heating Source

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.

An attempt to enhance NUMERICAL MODELS OF ANGULAR DISTORTION by considering the physics of the welding arc

Recently, it has become necessary to control or reduce weld distortion, which has a negative influence on structural integrity, without loss of manufacturing efficiency. Some studies on the fundamental characteristics of weld distortion and its control or reduction have been conducted. However, the results cannot be applied to all welding processes because such processes are becoming more diversified. For this reason, to understand the fundamental characteristics of weld distortion based on the welding conditions, the heat transport phenomenon in arc physics must be investigated. In this study, an experiment and numerical simulations are conducted to investigate the link between arc physics and weld distortion. As a result, the relation between weld distortion and the heat transport phenomenon is further clarified.

A discussion of the relationship between heat input parameter and angular distortion by considering moving heat source effect

In recent years, it has become much more necessary to control or reduce weld distortion, which has negative influences on structural integrity, without loss of manufacturing efficiency. Higher accurate prediction of weld distortion brings in one of the contributions for improvement of manufacturing efficiency. Commonly, angular distortion, which is controlled by temperature distribution along the direction of plate thickness, is controlled by heat input parameter (Qnet/h2). However, it is also known that there are several concerns against applicability of conventional heat input parameter for prediction of angular distortion. One of the concerns is the effect of welding speed. This is because the conventional heat input parameter is derived from welding thermal conduction theory without considering moving of the weld heat source. Actually, a pre-study by experiment about the effect of welding conditions, including welding speed, on angular distortion has shown that angular distortion is not always controlled by heat input parameter in the case of different welding speed. Then, in order to clarify the effect of welding speed on that, more detailed study is further required. In this paper, a numerical study on the effect of welding speed on the relationship between heat input parameter and angular distortion is performed based on the past experimental results. In addition, the reason why the relationship between heat input parameter and angular distortion is affected by welding speed is discussed from a viewpoint of distribution of the maximum temperature rise and residual plastic strain. It is concluded that the effect of welding speed on the relationship between heat input parameter and angular distortion is understood by the difference in distribution of mechanical melting temperature, which identifies the distribution of residual plastic strain for angular distortion, in plate thickness section.

Weld residual distortion produced due to locally cooled temperature distribution and its reduction effect

It is well known that weld distortion, which has a negative influence on material properties, structural fabricability and structural integrity, should be controlled appropriately. There are many methods to control or reduce weld distortion, but most of them involve some costly process in addition to welding. In-process control of weld distortion becomes more preferable than post-welding process or other methods, when manufacturing efficiency is considered. In recent years, in-process control welding by additional cooling has been proposed as one of techniques for reducing weld distortion and partially applied for thin-plate structure in industries. However, the effectiveness of additional cooling method has not yet been fully clarified. In this study, the effectiveness of additional cooling method and appropriate cooling condition for effective reduction in weld distortion are investigated by three-dimensional thermal elastic–plastic analysis. In addition, the effect of locally cooled temperature distribution on generation behaviour of plastic strain is discussed. As a result, it is concluded that the effectiveness of additional cooling and appropriate cooling condition for reduction in weld distortion are dependent on weld distortion under consideration and welding conditions. Especially, it is necessary for reduction in weld distortion to set the cooling torch at the appropriate position. For example, in order to reduce angular distortion effectively, the appropriate cooling position is dependent on the mechanical melting length during welding.