Among various additive manufacturing technologies, wire arc additive manufacturing (WAAM) is one of the most suitable methods for producing large-scale aluminum components, owing to its high deposition rate. However, achieving high-quality components by WAAM remains challenging, due to the heterogeneous microstructures and defects formed during WAAM process. To improve the performance of components, understanding the formation of the microstructure and defects is essential. In this study, the objective was to clarify the effect of heat input on grain morphology and the relationship between microstructure and mechanical properties of Al-Mg component manufactured by WAAM. Wall specimens were fabricated using Al-Mg wire (ER5356) with three different heat inputs, and their microstructures and mechanical properties were examined. The dominant grain morphology transitioned from feathery grains to columnar grains and finally to equiaxed grains. The number of porosity decreased with decreasing heat input. The component fabricated at the medium heat input exhibited superior tensile properties, i.e. ultimate tensile strength and elongation, by simultaneously suppressing the formation of detrimental feathery grains and porosity. These findings demonstrated that appropriate control of heat input can change the grain morphology and suppress the formation of porosity, thereby improving the tensile properties of Al-Mg components manufactured by WAAM.
Toward the realization of a sustainable society, the realization of multi-material structures requires the development of low-cost, versatile aluminum dissimilar material joining technology and the development of structures suitable for a circular economy. We have established a high-speed eutectic bonding method that generates and expels eutectic at the interface of dissimilar metals in a short time, aiming to reduce costs and improve versatility, and have developed a bonding technology with a bonding strength that can fracture the base material. In this study, we developed a thermal analysis method to gain insight into a bonding judgment system for dissimilar metal bonding method using high-speed eutectic reaction. This thermal analysis method is characterized by using energization heat balance experiment model to calculate interface heat including energizing environmental factors and then perform thermal analysis. The accuracy of thermal analysis was compared based on temperature distribution of experimental values and analyzed values, using bonding conditions of multi-material structure that obtains base metal fracture strength through high-speed eutectic reaction. Error rate was about 5% within 4 mm from the interface, which is a reasonable accuracy for determining conditions for bonding by eutectic reaction, and prospects for developing bonding judgment system were obtained.
Linear Friction Welding (LFW) was applied to medium-high carbon steel S55C, which had difficulty for arc welding process, and fatigue strength of welded joints were evaluated. In LFW process, peak temperature can be easily controlled by applied pressure and fatigue strength of welded joints for two levels of peak temperature are compared. As a fatigue test result, the welding defects are not detected in the joints. Sound welded joints can be fabricated by LFW. Because, the fatigue crack initiated at the surface of interface between joint area and base material, not in the joint area itself. A remarkable difference was recognized for the hardness distribution between two levels of peak temperature, and low peak temperature joint has low and flat distribution. Low peak temperature joints had almost equivalent fatigue strength to that of a base material. The reason why the fatigue strength of LWF joints is maintained at the same level as the base material is because compressive residual stress is generated on the surface, which is the initiation site of fatigue crack. This residual stress is due to the tensile plastic deformation due to uneven temperature distribution within the cross section during the cooling process after welding.
Hot cracking is one of the most serious welding defects that can significantly reduce the strength of structures, and it is important to prevent hot cracking. In the Japanese shipbuilding industry, one-sided submerged arc welding with multiple electrodes is used to improve the production efficiency of welding large steel plate joints, and it has been reported that hot cracking may occur at the end of the weld. In this study, a new "parallel heating method" is proposed to prevent hot cracking by using the thermal expansion caused by additional heating applied in parallel with the welding torch. In the analysis of bead-on-plate welding, the proposed method reduced high-temperature strain not only at the steady state but also at the beginning and end of the weld. It was confirmed that the effect of reducing high-temperature strain varies depending on the position of the additional torch, and that there is an appropriate heating position. In the analysis of multi-electrode single-sided submerged arc welding, the proposed method has little effect on the weld penetration shape. It was also confirmed that the proposed method has a significant reduction effect on high-temperature strain generated by localized opening deformation, demonstrating the usefulness of the proposed method.
Fatigue damage in welded steel structures such as bridges has become a serious social problem due to cyclic loading and especially has become severe situation at the joints of horizontal members and horizontal stiffeners which joint type is out-of-plane gusset welded joint. The fatigue fracture initiation of out-of-plane gusset welded joint often occurs weld toe where stress concentration is high. As a method to increase the fatigue strength of the weld toe, there is a grinder treatment which smooths the weld toe to reduce the stress concentration and a hammer peening treatment which applies plastic deformation to the weld toe to introduce compressive residual stress. However, these treatments lead to a decrease in construction efficiency because additional treatments are necessary after welding. In this study, a new welding technique as a "split boxing "is developed to improve fatigue strength without decrease of the construction efficiency.The mechanism of improving fatigue strength is also discussed in the developed new welding technique," split boxing ". The procedure of the developed welding is that the welding line is divided into short and long side of the gusset. The weld line at the long side of the gusset is extended. The effect of improving fatigue strength of out-of-plane gusset welded joints by applying the developed welding method is described from the viewpoints of the suppressing the initiation of fatigue cracks and the delay of fatigue crack propagation rate.
