1. Introduction
Metal additive manufacturing technology has been widely used since it has been introduced to the market, particularly in aerospace and biomedical applications. However, the residual stresses generated in continuous transient melting and solidifying process are still a significant barrier for additive manufacturing of high-performance large-size metal parts, which can lead to defects such as excessive deformation and cracking [
1,
2,
3,
4,
5]. Therefore, the stress-relieving operations during or after the part construction process is necessary in the metal additive manufacturing. The Ultrasonic peening treatment (UPT) technology is used to modify the residual stress by applying high-frequency mechanical peening or vibration on the part surface, which can lead to plastic deformation near the surface [
6]. In the early 1970s, the ultrasonic peening method was proposed by Statnikov [
7] and it has been widely used in industrial fields such as aerospace, ship and ocean engineering, bridge structures, etc. [
8,
9,
10,
11] The International Welding Association has been showing great enthusiasm for the application of UPT to post-weld treatments to modify residual stresses [
12]. UPT of the weld toe can improve the microstructure and optimize the geometry of the welding joint, so that tensile stresses in the surface areas can be transformed into compressive stresses after UPT, thereby the mechanical properties of parts such as hardness, fatigue life and strength can be improve greatly [
13,
14,
15]. On account of the similarity of welding process and metal additive manufacturing technology, UPT is also very promising in the field of metal parts additive manufacturing [
16,
17].
Common methods for residual stress measurement include X-ray diffraction, contour method, incremental deep-hole drilling and ring-core technology [
18,
19,
20,
21]. By means of these measuring methods, a large number of experimental studies have been conducted on the effects of UPT on the residual stress field of the welded parts. Liu et al. [
22] measured the interior and surface residual stress distribution of the welded specimen after UPT by means of contour method and X-ray diffraction, respectively. The results show that the compressive stresses generated by UPT in the welded area has the same effect in the lateral and longitudinal directions of the specimen, and the residual stress distribution is more uniform after the peening. Ficquet et al. [
23] used the incremental deep-hole drilling method and the ring-core techniques to measure the residual stress distribution of the 50D steel specimens after the ultrasonic peening, respectively, at a depth of 0.5 mm, 1 mm and 2 mm. The experimental results indicate that the UPT technology can generate a zone near the surface and the maximum depth up to 2 mm, where the tensile stresses are transformed to the compressive stresses. Yu et al. [
24] performed UPT and corrosion tests on welded joints of 6005A-T6 aluminum alloy material. Ganiev et al. [
25] used metallography methods to investigate the surface hardening and internal residual stress of specimens treated by ultrasonic peening. It was found that the formation of favorable compressive stress was accompanied by the fine crushing of crystalline grains in the narrow surface layer of the treated metal.
Because the residual stress measurement experiments are time consuming, costly and highly dependent on the operator’s skills, some researchers have carried out numerical simulation studies on the UPT process and residual stresses prediction. Yin et al. [
26] used the finite element software ABAQUS to build the impact-rebound-impact model to predict the residual stress distribution of the specimen after ultrasonic peening. An elastoplastic model was used to describe the constitutive relationship of the material during UPT. The simulation results are basically consistent with the residual stress X-ray diffraction measurements in the experiment. Yuan and Sumi [
27] used SYSWELD and LS-DYNA to implement the thermomechanical welding and elastoplastic ultrasonic peening simulation successively, and the welding residual stress field was treated as prestress field in the peening simulation. Khurshid et al. [
28] studied the effects of UPT on residual stresses in S355, S700MC, and S960 grades steel experimentally and numerically based on different material models.
The aim of this article is to investigate residual stress modification for laser additively manufactured parts by UPT process. A numerical simulation method was developed to analyze the influence of UPT on the residual stress of specimens fabricated by SLM, including three-dimensional thermomechanical coupling SLM process simulation and transient dynamic UPT process of the as-fabricated specimens. The Johnson–Cook constitutive model was used in the ultrasonic peening process. The model can consider various complex factors such as ultrasonic peening process parameters and mechanical properties changes. In addition, a series of experimental studies were carried out to determine the parameters of the material model and measure the residual stress in order to validate and increase the reliability of the numerical simulation analysis strategy.
6. Conclusions
In this paper, the SLM building process of AlSi10Mg specimens, the UPT process of the specimen and the effect of UPT on the residual stress distribution were studied by means of experiment and FEM simulation. The UPT process can significantly modify the residual stresses distribution in the laser additively manufactured specimens, the tensile stresses (the maximum value 50 MPa) can be changed into compressive stresses (the minimum value −150 MPa) after UPT. An experimental study strategy and setup was presented first. A FEM method for simulating the UPT process of the specimens was proposed. Simufact Additive was used to simulate the SLM building process and predict the residual stress field. The specimen’s model file containing residual stress was processed using Simufact Forming, and then it was applied as the initial FEA model for UPT simulation in MSC Marc. Based on the initial prestressed specimen model, a dynamic finite element model of UPT established in MSC Marc was used to analyze the influence of UPT on residual stress. The material mechanical property data and constitutive model parameters required in the simulation analysis were obtained through a series of experiments. The Young’s modulus and Poisson’s ratio of the specimens were measured by ultrasonic detector measurement method. The parameters of Johnson–Cook constitutive model were determined by SHPB experiments, and the model was used to define the material properties of the specimens in the MSC Marc software. The residual stress obtained by the simulation agrees well with the experimental measurements, and the simulation model and simulation strategy are proved to be reliable. On this basis, the influence of parameters such as time, amplitude and frequency of ultrasonic peening on residual stress was discussed by numerical simulation method. Increasing the peening time, amplitude and frequency can increase the effect of ultrasonic shock on residual stress. The influence of time and peening amplitude on the residual stress is greater, and the maximum depth of action could reach 2 mm.