Effect of high frequency impacting and rolling on fatigue crack growth of 2A12 aluminum alloy welded joint
Introduction
With the attention of resources and environment, energy conservation and emission reduction are an inexorable trend for green development [1], aluminum alloy has become the preferred material to realize this target because of its light weight. As the representative of 2000 series aluminum alloy, 2A12 aluminum alloy has the advantages of good ductility, low density and good corrosion resistance, and is widely used as structural materials in the transportation industry such as automobiles and rail vehicles [2], [3], [4]. Welding technology is one of the important and common method in structural parts assembly [4], [5]. In general, welded structures are easily fracture at cyclic loading conditions. Therefore, the research of FCG behaviors of welded structures is important to ensure the security of structural applications and reduce the accident incidence [6], [7].
Micro-cracks usually occur on the metal materials surface under cyclic loading [8]. In order to extend the service life of weldment parts, some advanced surface severe plastic deformation (S2PD) technologies have been developed such as ultrasonic peening treatment (UPT) [9], [10], shot peening (SP) [11], [12], laser peening (LP) [11], [12] and laser shock processing (LSP) [13] et al. It is well known that the weldment parts with fine-grain microstructure produced by S2PD technology exhibit better fatigue property. For examples, Yin D et al. verified that FCG rate of Q235 steel welded joints and Q345 steel welded joints subjected to UPT was significantly decreased, which was attributed to the formation of CRS zones [10]. Hatamleh O et al. research the effects of SP treatment and LP treatment on the FCG performance of 2195 aluminum alloy welded joint and 7075-T7351 aluminum alloy welded joint, respectively [11], [12]. A systematic research of SP treatment and LP treatment effects showed that SP-treated welded joint samples had an significant reduction in FCG. Yun W et al. reported that fatigue life of LSP-treated EH36 weldment could be increased by about 270% compared to that of LSP-untreated EH36 weldment [13]. Hence, S2PD technology can effectively reduce the driving FCG force and prolong the FCG life of metal materials by introducing CRS [14].
HFIR treatment is an effective and newly developed S2PD technology for its own advantages of small noise, high efficiency, low cost and energy conservation. Especially a gradient nano-structure and effective CRS can be obtained on the metal materials surface by HFIR treatment. The metal materials with nanograins have some excellent mechanical properties, such as high microhardness [15], [16], better resistance to fatigue [17], [18] and good friction and wear [19], [20], [21] performance. In view of the significant advantages of HFIR treatment in improving the properties of metal materials, we innovatively proposed that the surface of welded joint was strengthened and processed by HFIR treatment and the effect of HFIR treatment on the FCG life of BM and WS was researched, respectively.
Hence, microstructure, nanomicroharness, elastic modulus and RS of BM and WS of 2A12 aluminum alloy welded joints before and after HFIR treatment were studied by optical microscope (OM), Carl Zeiss EVO-18 scanning electron microscopy (SEM), JEM-2010 transmission electron microscopy (TEM), nano-Indenter and micor-AIS(F) indentation instrument. Then, the effect of HFIR treatment on FCG behavior of 2A12 aluminum welded joint at BM and WS were studied. In order to reveal the influence of HFIR treatment on improvement of FCG life, the statistical analysis of FCG life of BM and WS were calculated. In addition, a comparison of fatigue fracture characteristics is carried out between HFIR-untreated samples and HFIR-treated samples at different FCG stages, which will provide the fatigue strengthening mechanism of HFIR treatment on BM and WS of 2A12 aluminum alloy welded joint.
Section snippets
Experiment procedures
The base material of 2A12 aluminum alloy with a thickness of 6 mm was welded by variable polarity plasma arc (VPPA) welding (Fig. 1a). Its nominal chemical composition and mechanichal properties are shown in Table 1 and Table 2. Welding wire was ER2319 with diameter of 1.6 mm. The chemical composition of welding wire is shown in Table 3. The welding parameters is described in the Ref.16. Before HFIR treatment, weld reinforcement was removed by grinding machine. Next, BM and WS of 2A12 aluminum
Microstructure
Fig. 3a shows the macroscopic cross-section morphology of welded joint before HFIR treatment. The microstructure of BM are shown in Fig. 3b and Fig. 3c. As shown, the microstructure of BM is characterized by elongated rolling microstructure, and the grain size of BM is about 70 μm. The microstructure of WC consists of aluminum matrix and equiaxed dendritic structure (Fig. 3d). The grain size of WC is about 20 ~ 50 μm (Fig. 3e).
Fig. 4 shows microstructure of BM and WS after HFIR treatment.
Conclusions
In this paper, microstructure, nanomicrohardness, elasticity modulus, residual stress and FCG behavior of 2A12 aluminum alloy welded joint before and after HFIR treatment were studied. The key conclusions can be summarized as follows:
After HFIR treatment, the cross-section of BM and WS produced about 120 μm and 300 μm severe plastic deformation layer, respectively. Grain size of the top surface of BM and WS was refined to about 400 nm and 13 nm. Nanomicrohardness and elastic modulus of BM and
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This research was funded by the National Nature Science Foundation of China, grant number 51875246, the Natural Science Foundation of Jilin province through Grant No. 20180101323JC.
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