Chen Furong, Liu Chenghao
School of Materials Science and Engineering, Inner Mongolia University of Technology, Hohhot 010051, China.
Materials (Basel). 2021 May 22;14(11):2742. doi: 10.3390/ma14112742.
To improve the loose structure and serious porosity of (Al-Zn-Mg-Cu) 7075 aluminum alloy laser-welded joints, aging treatment, double-sided ultrasonic impact treatment (DSUIT), and a combination of aging and DSUIT (A-DSUIT) were used to treat joints. In this experiment, the mechanism of A-DSUIT on the microstructure and properties of welded joints was analyzed. The microstructure of the welded joints was observed using optical microscopy, scanning electron microscopy, and electron backscatter diffraction (EBSD). The hardness and tensile properties of the welded components under the different processes were examined via Vickers hardness test and a universal tensile testing machine. The results showed that, after the aging treatment, the dendritic structure of the welded joints transformed into an equiaxed crystal structure. Moreover, the residual tensile stress generated in the welding process was weakened, and the hardness and tensile strength were significantly improved. After DSUIT, a plastic deformation layer of a certain thickness was generated from the surface downward, and the residual compressive stress was introduced to a certain depth of the joint. However, the weld zone unaffected by DSUIT still exhibited residual tensile stress. The inner microhardness of the joint surface improved; the impact surface hardness was the largest and gradually decreased inward to the weld zone base metal hardness, with a small improvement in the tensile strength. Compared with the single treatment process, the microstructural and mechanical properties of the welded joint after A-DSUIT were comprehensively improved. The microhardness and tensile strength of the welded joint reached 200 HV and 615 MPa, respectively, for an increase of 45.8% and 61.8%, respectively. Observation of the fractures of the tensile specimens under the different treatment processes showed that the fractures before the aging treatment were mainly ductile fractures while those after were mainly brittle fractures. After DSUIT of the welded joints, a clear and dense plastic deformation layer was observed in the fracture of the tensile specimens and effectively improved the tensile properties of the welded joints. Under the EBSD characterization, the larger the residual compressive stress near the ultrasonic impact surface, the smaller the grain diameter and misorientation angle, and the lower the texture strength. Finally, after A-DSUIT, the hardness and tensile properties improved the most.
为改善（Al-Zn-Mg-Cu）7075铝合金激光焊接接头组织疏松和气孔严重的问题，采用时效处理、双面超声冲击处理（DSUIT）以及时效与DSUIT相结合的方法（A-DSUIT）对接头进行处理。本实验分析了A-DSUIT对焊接接头组织和性能的作用机制。采用光学显微镜、扫描电子显微镜和电子背散射衍射（EBSD）观察焊接接头的微观组织。通过维氏硬度试验和万能拉伸试验机检测不同工艺下焊接构件的硬度和拉伸性能。结果表明，时效处理后，焊接接头的树枝晶组织转变为等轴晶组织。此外，焊接过程中产生的残余拉应力得到减弱，硬度和抗拉强度显著提高。DSUIT处理后，从表面向下产生了一定厚度的塑性变形层，并在接头一定深度引入了残余压应力。然而，未受DSUIT影响的焊缝区仍表现出残余拉应力。接头表面的内部显微硬度提高；冲击表面硬度最大，向内逐渐降低至焊缝区母材硬度，抗拉强度有小幅提高。与单一处理工艺相比，A-DSUIT处理后焊接接头的微观组织和力学性能得到综合改善。焊接接头的显微硬度和抗拉强度分别达到200 HV和615 MPa，分别提高了45.8%和61.8%。观察不同处理工艺下拉伸试样的断口发现，时效处理前的断口主要为韧性断裂，时效处理后的断口主要为脆性断裂。焊接接头DSUIT处理后，拉伸试样断口中观察到清晰致密的塑性变形层，有效提高了焊接接头的拉伸性能。在EBSD表征下，超声冲击表面附近的残余压应力越大，晶粒直径和取向差角越小，织构强度越低。最后，A-DSUIT处理后，硬度和拉伸性能改善最为明显。
