An incompressible SPH (Smoothed Particle Hydrodynamics) method was applied to numerical simulation of the thermofluid behavior of an anode metal in a TIG (Tungsten Inert Gas) welding process, taking account of the phase change of the anode material, free surface deformation of the liquid, and four dominant flow-driving forces, namely, gradient of surface tension (Marangoni effect), gas drag on the liquid surface, buoyancy, and electromagnetic force (Lorentz force). The present method successfully simulated that the direction of the temperature gradient of surface tension causes a significant difference of weld penetration. The penetration geometries of the present results agreed with those of actual welding processes. It is shown that the particle method used in this study is applicable to arc welding simulations.
In order to establish the method to analyze the welding deformation and residual stress of the whole structure, the authors have developed a new analysis method called Idealized Explicit FEM (IEFEM). In this paper, Iterative Substructure Method (ISM) introduced IEFEM is proposed to achieve larger scale computation. The proposed method was applied to simple girth weld problem of a pipe and analysis results were compared to those by the existing method. As a result, it was found that the proposed method has the almost same analysis accuracy as the existing method. Computing time and memory consumption were also investigated and it was shown that the proposed method can analyze large scale problem in dramatically shorter computing time and less memory consumption compared to those of the existing method. In addition, the proposed method was applied to the large scale problem of the welding process of the canister of high level radioactive waste. Analysis results were compared to measured experimental results and it was found that the results obtained by the proposed method and measured were in good agreement. And it was also found that the proposed method can analyze the large scale problem having more than 3 million degrees of freedom in realistic time.
Direct dissimilar joining of various Al alloys (A1050, A3004, A5052 and A5083) plates and a polyamide 6 plate was performed using friction lap joining (FLJ) with the Al alloy plate as a top and the polyamide 6 plate as a bottom. The effect of Mg content in Al alloys on the joining strength was investigated. TEM analysis made clear that the polyamide 6 and Al alloys were joined via oxide layer consisted of Al2O3 and MgO. The results of XPS analysis indicated that MgO was formed by the heating during FLJ, and the quantity of MgO was increased with increasing Mg content in Al alloys. The tensile shear strengths of A3004, A5052 and A5083 joints were saturated to about 2 kN because of the tensile fracture at the polyamide 6 plate outside the tool passed zone, which were higher than that of the A1050 joint fractured at the joint interface. The peel strength of the joint was increased with increasing Mg content in Al alloys, and the fracture occurred both at the joint interface and at the polyamide 6 plate on the joint area. The fraction of polyamide 6 plate fracture was also increased with increasing Mg content in Al alloys.
Maximum cause to make mechanical toughness of the weld metal reduce in the process management is known to be a mixture of nitrogen including in the atmosphere by breaking the shield condition. Mixture of the atmosphere is prevented by blowing the shielding gas such as carbon dioxide, argon, and this mixture to the arc and the molten pool in the gas metal arc welding, but it is easily affected by the wind. Therefore, it has been recommended conventionally that wind velocity should be controlled less than 2.0m/sec. But it is thought that this recommendation value is unsuitable to produce multi-pass weld metal with high mechanical and porosity toughness property. Because this was provided from examination results by only consideration porosity toughness of single-pass weld metal but non-consideration mechanical toughness. In this paper, shielding condition is evaluated not only chemical analysis and mechanical properties of multi-pass weld metal in some velocity wind environment but also visualizing varied shielding gas behavior by the Schlieren method. As a result, it is necessary to be controlled the wind velocity less than 0.5m/sec to produce multi-pass weld metal with good properties. And the calculated velocity of shielding gas should be controlled more than twice the wind velocity.
In the post weld heat treatment (PWHT) process, the reheat cracking which might occur in the weldments of low alloy steels has been a serious problem. So, it is considered to be important to predict the possibility of occurrence of the reheat cracking in these steels. It is however recognized as time consuming procedure to evaluate quantitatively the susceptibility to this type of cracking. In the present study, a new quantitative evaluation method of reheat cracking susceptibility by in-situ observation and measurement using a laser confocal microscope has been proposed. Through this new method, the reheat cracking susceptibility of any kind of steels can been evaluated with the same standard. Moreover, because the position of initial crack can be focused and the critical ductility to initiate crack is measured by in-situ observation, the reheat cracking susceptibility can be evaluate by using only one specimen. So the newly developed method can provide efficient quantitative assessment of the reheat cracking sensitivity with high accuracy.
The oscillation laser beam is considered to be effective as a heat source of the narrow gap multi-layer welding, because the oscillation laser welding can control the penetration shape and prevent the lack of fusion. In this study, in order to establish a narrow gap welding process by oscillation laser beam, butt welding experiments of 50mm thickness carbon steel plate were performed. By the appropriate control of the heat input area using the in-process sensor for recognizing the groove shape, narrow gap welding of thick plate with groove which was cut by gas cutting was achieved. Properties of welded joint had been confirmed by nondestructive testing, tensile test, and side bend test. A two-dimensional numerical calculation model for welding deformation was developed. This calculation model was used for investigation of the optimal groove angle. The results of calculations were in quantitative agreement with the experimental results. Microstructure of the weld zone had multiple thermal histories. According to the hardness test results, maximum hardness of the heat affected zone of the upper layer has been lowered than that of the lower layer.
Recently, stress corrosion cracking (SCC) has been observed near the welded zone of the primary loop recirculation pipes made of low-carbon austenitic stainless steel type 316L in boilling water reactors. SCC is initiated by superposition effect of three factors. They are material, environmental and mechanical factors. For the non-sensitized material such as type 316L, residual stress as a mechanical factor of SCC is comparatively important. In the joining processes of pipes, butt-welding is conducted after surface machining. Surface machining is performed in order to match the inside diameter and smooth surface finish of pipes. Residual stress is generated by both processes. Moreover, residual stress distribution generated by surface machining is varied by subsequent welding process, and it has the maximum residual stress around 900 MPa near the weld metal. The variation of metallographic structure, such as recovery and recrystallization, in surface machined layer due to welding thermal cycle is an important factor for this residual stress distribution. In this study, thermal aging tests were performed in order to evaluate hardness variation due to thermal cycle in surface machined layer. The results of thermal aging tests were applied to finite element method (FEM) as the additivity rule of the hardness variation. Varied hardness was converted into equivalent plastic strain. Then, thermo-elastic-plastic analysis was performed under residual stress fields generated by surface machining. As a result, analytical results of surface residual stress distribution generated by bead-on-plate welding after surface machining show good agreement with measured results by X-ray diffraction method. The maximum residual stress near the weld metal is generated by same mechanism as both-ends-fixed bar model in surface machined layer that have high yield stress.
In this study, a new integrated simulation methodology has been proposed for the more accurate numerical simulation of weld distortion generated by gas metal arc welding. The integrated simulation model of gas metal arc welding was constructed by coupling between welding process and mechanics. The welding process models consist of arc plasma (heat source) model and bead formation (process) model. In the arc plasma (heat source) model, computational simulation based on mathematical modeling of the heat transfer from arc plasma to a welded plate is performed to obtain a more precise properties of heat source from welding heat input conditions. In the bead formation (process) model, computational simulation based on a coupling analysis between weld bead balance and thermal conduction is performed to obtain a more precise temperature distribution and weld bead configuration during welding from the properties of heat source obtained. In the stress and distortion (mechanics) model, computational simulation based on a large deformation thermal elastic-plastic analysis is performed to obtain a more precise weld distortion from the temperature distribution and weld bead configuration obtained. Through the developed simulation technique, weld distortion becomes to be available almost exclusively from welding heat input and process conditions. A bead-on-plate welding of high strength steel was performed under the same welding heat input and process conditions to compare the simulation results with the experimental results. It was concluded that both results of angular distortion were in extremely good agreement and thus the developed simulation technique has the potential to become useful for a highly accurate predictive simulation of weld distortion.