Objective Aluminum alloy has the advantages of low density and great strength; therefore, it is commonly used in several industries. However, aluminum alloy is difficult to be welded via fusion welding, which limits its utilization in the aerospace field. Friction stir welding (FSW) can conveniently solve this problem. Nevertheless, tensile residual stress is introduced by FSW during the welding process, affecting the mechanical properties of the joint; therefore, strengthening the joint is essential. Laser peening (LP) is a technology that uses instantaneous shock pressure to strengthen the impacted parts, and the mechanical properties of the impacted parts will be improved by introducing compressive residual stress on the surface and subsurface of the materials. Currently, LP is commonly utilized in the modification of metal materials. Simultaneously, research concerning the utilization of LP to modify metal welds is advancing. However, there are few studies regarding LP strengthening for friction stir welding zone, particularly the finite element simulation for the composite process. In this study, a finite element model of the FSW and LP composite processes was developed to investigate the effect of LP on the residual stress in the friction stir welding zone of aluminum-lithium alloy and examine the cause of stress wave attenuation under various processes.
Methods In this study, a composite process model was developed using Hypermesh and ABAQUS software. The simulation analysis was divided into two processes: FSW and LP. First, the sequential coupling approach was adopted during the simulation process of FSW. The Dflux subroutine was called as the heat source program to calculate the welding temperature field, and the stress analysis was carried out based on the temperature field. Then, the LP simulation was performed with welding stress filed as the initial condition, and the Vdload subroutine was called as the shock pressure program for LP simulation. Finally, a stable residual field was achieved. Next, the residual stress values along the X, Y, and Z axes were extracted for comparative investigation, taking the center of the welding as the original node. Moreover, the attenuation law of stress wave was summarized by comparing the propagation phenomenon of stress wave after LP and composite process, and the reason for this phenomenon was explained.
Results and Discussions By studying the residual stress distribution diagram of the sample under various treatment processes, we discovered that the tensile residual stress was caused by FSW, the residual compressive stress was caused by LP, and the tensile residual stress in the welding zone treated by the composite process substantially decreased (Fig.10). In addition, by studying the residual stress distribution diagram of the sample after the composite process, we discovered that the residual stress field generated by the composite process was not equal to the linear superposition of the residual stress field generated via LP and the residual stress field generated by FSW but was determined by the coupling effect of the two processes (Fig.11). The reduction of residual stress caused by LP and the value of residual stress caused by FSW were further studied, and we discovered that the reduction of residual stress was positively correlated with the residual stress caused by FSW (Fig.12). Based on the stress wave propagation law, we discovered that the attenuation of the internal stress wave of the sample treated by LP and the sample treated by the composite process exhibited an exponential changing trend. As the plastic wave transformed into an elastic wave, the stress wave decayed faster in the initial stage compared with the later stage (Fig.13). The stress wave generated by the composite process decayed slightly faster compared with the LP process. The possible reason for this phenomenon was that tensile residual stress caused by FSW. The introduced tensile residual stress increased the conversion rate of stress fluctuation energy into plastic deformation energy to offset the tensile residual stress. Consequently, the stress wave attenuation rate of the composite process rapidly decreased (Fig.14).
Conclusions In this study, a finite element simulation model for the composite process of FSW and LP was developed. The accuracy of the model was proven by comparing it with the experimental results in a related study. The model was used to simulate the FSW, LP, and composite processes. The differences among the residual stress distribution of the samples treated via various processes were compared. The study discovered that FSW introduced tensile residual stress on the surface and subsurface of aluminum-lithium alloy, whereas the LP introduced compressive residual stress. In addition, the tensile residual stress in the welding zone treated by the composite process was effectively reduced. Furthermore, the residual stress caused by FSW was found to be positively correlated with the reduction of residual stress, whereas it did not affect the affected area of LP. The propagation law of the stress wave in the material was investigated, and we discovered that the composite process attenuated the stress wave faster than the LP process. Therefore, under the same laser parameters, the reduction of residual stress caused by the composite process was greater than that caused by LP. Thus, the strengthening impact of LP on the aluminum-lithium alloy’s friction stir welding zone was superior to that of aluminum-lithium alloy.