1. Introduction
In spacecraft tanks, 2219 Aluminum Alloy is widely used for its excellent high and low temperature mechanical properties, and strong stress corrosion cracking resistance [
1]. Due to the shape requirements, many 2219 Aluminum Alloy components need to be connected by means of welding. Significant and uneven distribution residual stress is inevitably induced inside the welded components for its local non-uniform heating and cooling process [
2,
3]. The residual stress and the working stress caused by the load are superimposed on each other during the service of the weldment, causing secondary deformation and redistribution of residual stress, which not only reduces the rigidity and dimensional stability of the welded structure, but also the fatigue strength, brittle fracture resistance, resistance to stress corrosion cracking, and high temperature creep cracking of the structure will be seriously affected [
4,
5,
6]. These effects are very unfavorable for structural performance, safety, and reliability of welded components. Thus, stress relief is a necessary technology before welding components are used. The most commonly used residual stress relief methods are thermal stress relief (TSR) and vibration stress relief (VSR).
TSR refers to the process of relieving residual stress under temperature by heating, holding, and cooling the parts for a certain time. Bruno et al. [
7] studied the residual stress distribution and microscopic changes after isothermal annealing treatment of metal matrix composites by means of neutron diffraction. In order to adjust the residual stress state of the satellite box and reduce the processing deformation, Younger et al. [
8] studied the quenching process, mechanical deformation treatment, and thermal aging treatment of the two aluminum alloys, thus effectively solving the problem. Tanner et al. [
9] performed a thermal aging treatment at 170 °C for 12 h on the forging of aluminum alloy 2014 after solution treatment, and the results show that the residual stress amplitude decreased. Edwards et al. [
10] evaluated the residual stress state of the weld seam after 45 min of post-weld thermal stress relief of the weld at 760 °C. The results show that the post-weld thermal stress relief can effectively reduce the residual stress caused by welding.
VSR works by applying a cyclic load for a certain time. Hebel et al. [
11] studied the effect of VSR at sub-resonance frequencies on the residual stress relief of welded structures. The article considers that the sub-resonance frequency is the preferred excitation frequency for the residual stress release of the weldment. Rao et al. [
12] established a mathematical model of residual stress relaxation under dynamic stress, proving that the evaluation of the effect of vibration aging by dynamic stress is feasible. Kwofier [
13] pointed out that the residual stress is self-balancing and the release of local stress causes the stress of the part to be redistributed, and a plastic model was constructed to simulate the VSR phenomenon. Djuric et al. [
14] studied VSR treatment on the residual stresses for welded specimens of ultra-high strength steel. The results show that VSR treatment has a limited effect on the macroscopic residual stress state but significantly increases the damage of the microstructure. Li et al. [
15] studied the effect of VSR on the welded components of DH36 high strength low alloy steel, with the results showing that the residual stress of the weld toe is reduced by 29–72% after VSR treatment; the metallographic structure is not changed much, but the dislocations increase. Sonsino et al. [
16] pointed out that the fatigue life of the parts after VSR is equivalent to that after high temperature thermal aging, but the residual stress elimination effect is not as good as the thermal aging.
The Thermal and Vibratory Stress Relief (TVSR) is a combined method of TSR and VSR. Under a certain temperature (the organization and mechanical behavior of the workpiece should not be affected), a cyclic dynamic load is applied to a workpiece to achieve the relief and homogenization of the residual stress. Lv et al. [
17] compared the residual stress release effects of TVSR and VSR treatment and the experimental results showed that the maximum stress in the high stress region after TVSR treatment was reduced by 55.9%, which is greater than the result after VSR treatment. Li et al. [
18] applied the waste heat to VSR during a multi-pass welding procedure, with the result indicating that the method seems more effective in decreasing the residual stress.
As can be seen from the above studies, TSR, VSR, and TVSR treatment have all been proven to be effective for residual stress relief. So far, no comparison has been made between the effects on residual stress relief of these three stress release methods. In addition, due to the rapid heat input and cooling during the welding process, the residual stress has a distribution unevenness in the direction perpendicular to the weld seam and along the depth direction of the weld component. However, most of the current research focuses on the evaluation of the effect on residual stress relief and homogenization on the surface of the weld parts. In this study, twelve 2219 aluminum alloy welding samples were divided into four groups. One of the groups is used as a reference without any stress relief treatment. The other three groups were processed by TSR, VSR, and TVSR, respectively. The residual stresses of depths of 0–1.2 mm are measured to compare the relief and homogenization in stress.