materials Article Effect of Shot Peening on Redistribution of Residual Stress Field in Friction Stir Welding of 2219 Aluminum Alloy Lin Nie 1,2 , Yunxin Wu 1,2,3,*, Hai Gong 1,2,3 , Dan Chen 1 and Xudong Guo 3 1 School of Mechanical and Electronic Engineering, Central South University, Changsha 410083, China; [email protected] (L.N.); [email protected] (H.G.); [email protected] (D.C.) 2 State Key Laboratory of High-Performance Complex Manufacturing, Central South University, Changsha 410083, China 3 Light Alloy Research Institute, Central South University, Changsha 410083, China; [email protected] * Correspondence: [email protected]; Tel.: +86-130-8731-9063 Received: 23 June 2020; Accepted: 14 July 2020; Published: 16 July 2020 Abstract: Welding is one of the essential stages in the manufacturing process of mechanical structures. Friction stir welding structure of aluminum alloy has been used as a primary supporting member in aerospace equipment. However, friction stir welding inevitably generates residual stress that promotes the initiation and propagation of cracks, threatening the performance of the welded structure. Shot peening can effectively change the distribution of residual stress and improve the fatigue properties of materials. In this paper, friction stir welding and shot peening are performed on 2219 aluminum alloy plates. The residual stress fields induced by friction stir welding and shot peening are measured by using the X-ray diffraction method and incremental center hole drilling method, and the distribution characteristics of residual stress fields are analyzed. The effect of the pellet diameters and pellet materials used in shot peening on the redistribution of welding residual stress field are investigated. The pellet diameter used in the experiment is in the range of 0.6–1.2 mm, and the pellet material includes glass, steel, and corundum. This study provides guidance for the application of shot peening in friction stir welding structure of 2219 aluminum alloy. Keywords: residual stress; shot peening; friction stir welding; 2219 aluminum alloy 1. Introduction The application of high strength aluminum alloy can effectively reduce the structural weight of spacecraft. Basically, 2219 aluminum alloy has been selected as the main structural material for the fuel and oxidant tank of the new-generation launch vehicle due to its excellent high/low-temperature mechanical properties [1]. However, the tank is difficult to be manufactured at one time because of its complex structures, which requires the use of welding technology to complete the assembly and connection. In fact, the longitudinal seams of the cylinder parts are connected by friction stir welding. Friction stir welding, as a new solid-state joining technology, has been widely used in the aerospace manufacturing [2]. In the welding process, the tool pin is inserted into the joint of the workpiece, and then the high-speed friction between the tool and the workpiece generates heat, which softens the material and complete the welding. Although the main problem in welded structures is that the harmful residual stresses will inevitably be generated during the welding process [3–5]. Lemos et al. [6] performed friction stir welding with different process parameters to analyze the characteristics of residual stresses and observed higher difference of the residual stresses state between the advancing side and the retreating side. Huang et al. [1] measured the residual stresses in variable polarity plasma arc welded joint and friction stir welded joint on propellant tank of aluminum alloy by using the Materials 2020, 13, 3169; doi:10.3390/ma13143169 www.mdpi.com/journal/materials Materials 2020, 13, 3169 2 of 13 indentation strain-gauge method. The results showed that there are large residual tensile stresses in the center and inner areas of the friction stir welded joint, and the weak spot of the tank appears in the region of maximum tensile stress. Yu et al. [7] developed an analysis model for friction stir welding to obtain the distribution of residual stresses. The results indicated that the tensile stresses are concentrated in the welded zone. Welding residual stresses promote the initiation and failure of fatigue cracks and even cause instability and fracture of components in complex service environments [8–11], which greatly affects the performance of components. Ferro et al. [12] observed that the fatigue strength of welded joints is sensitive to the initial residual stresses caused by welding under high cyclic load. Lee et al. [13] established a fatigue damage model to calculate the fatigue crack growth of welded joints. Compared with the test results, it was found that the welding residual stresses cannot be ignored in the estimation of the fatigue life of the weld. Fratini et al. [14] investigated the influence of the residual stresses on the fatigue crack propagation in welded joints of 2024 aluminum alloy and found that the fatigue crack propagation behavior is related to the welding residual stresses. Welding is the only option for joining different parts in some industrial applications [15], but the residual stresses profile in the friction stir welded joints affects the quality of the joints. Therefore, it is of practical significance to study the real state of welding residual stresses for eliminating the residual stresses and evaluating the service performance of welded structure. To improve the fatigue life of industrial parts and components, various surface treatment technologies have been developed in recent decades [16–19]. Shot peening is a well-known surface strengthening technology, which can significantly enhance the fatigue strength and effectively prevent the failure of components. This method is flexible and can be applied to almost any shape of components and has been widely used in industry [20,21]. The application of shot peening has great attraction to the welded joints, because of the low fatigue resistance of welded joints due to the high stress concentration. The mechanism of shot peening is the process of impacting the material surface with high-speed pellets to plastically deform the impacted surface. During the process, residual compressive stresses beneficial to improve the fatigue performance are introduced into the components [22,23]. Therefore, for improving the shot peening technology and component performance, it is of great significant to analyze the relationship between the shot peening process and the residual stress field in the components and grasp the distribution of the residual stress field introduced by shot peening. Chen et al. [24] shot peened the surface of 2507 stainless steel, and the measurement results indicated that high residual compressive stresses are induced on the surface layer after shot peening, which is helpful to improve the fatigue performance of components. Wu et al. [25] studied the effect of shot peening parameters on the residual stress of aluminum alloy. Zhang et al. [26] presented shot peening experiments with different process parameters and established a finite element model to study the residual stress distribution. The results indicated that the residual compressive stresses are proportional to the shot peening speed, and the compressive stress depth increases with the increase of coverage. Zhou et al. [27] proposed a numerical method to analyze the residual stresses introduced by shot peening, and the experiment verified the validity of the model. Liu et al. [28] used SPH method to simulate shot peening to eliminate welding residual stresses. The evolution of residual stresses was proposed, and the change of welding residual stresses before and after shot peening was analyzed. In addition, the effect of shot peening velocity on residual stresses is investigated. Although many scholars have studied a lot of shot peening [29–31], the experimental research on shot peening of aluminum alloy-welded structures is rarely involved. In addition, operators mainly depend on experience to select shot peening parameters in the manufacturing practice [32]. There is a lack of experimental basic research on the variation of residual stress field under various shot peening parameters. In this paper, the experiments of friction stir welding and shot peening on 2219 aluminum alloy plates are carried out at the State Key Laboratory of High-Performance Complex Manufacturing, Central South University. The distribution characteristics of residual stress field under different Materials 2020, 13, 3169 3 of 13 process parameters are measured, and the effect of shot peening on the redistribution of residual stress field is analyzed, which provide experimental data for determining the optimal shot peening process parameters. 2. Materials and Methods 2.1. Materials The 2219Materials aluminum 2020, 13, x FOR alloy PEER used REVIEW in the experiment is high-strength aluminum alloy3 of that14 belongs to Al-Cu-Mn series, which can be enhanced by heat treatment. However, small precipitates of Cu at high stress field is analyzed, which provide experimental data for determining the optimal shot peening temperatureprocess may be parameters. the cause of cracking and the limitation of weldability. The chemical composition of 2219 aluminum alloy as listed in Table1. The yield strength at room temperature was 352 MPa, the tensile strength2. Materials was and 455 Methods MPa, and the elongation was 10% [33]. 2.1. Materials The 2219 aluminumTable alloy 1. Compositionused in the experiment of the 2219is high-strength aluminum aluminum alloy. alloy that belongs to Al-Cu-Mn series, which can be enhanced by heat treatment. However, small precipitates of Cu at highElement temperature Cumay be the Mn cause of cracking Fe an Sid the limitation Ti of Mg weldability. Zr The chemical Al compositionwt.% of 22195.8~6.8 aluminum0.2~0.4 alloy as listed 0.3 in Tabl 0.2e 1. The 0.02~0.1yield strength0.02 at room0.1~0.25 temperatureBal. was 352 MPa, the tensile strength was 455 MPa, and the elongation was 10% [33]. 2.2. Friction Stir Welding Table 1. Composition of the 2219 aluminum alloy.