With the development of the aerospace industry, increasing attention has been paid to large-scale integral structural parts that contribute to aircraft's lightweights. Aviation aluminum alloy parts face the urgent need for larger sizes and more complex structures. A single additive manufacturing method cannot meet the needs of aviation parts. The structure and size requirements can be realized by combining the high-precision advantages of selective laser melting (SLM) technology with the size advantages of wire arc additive manufacturing (WAAM) through connection technology to realize an additive manufacturing scheme of large aviation aluminum alloy parts. A new type of connection technology is required for large and complex structural components to meet the high-quality connection requirements of large-scale integral components. The laser additive jointing (LAJ) technology can adjust the process parameters during the layer-by-layer joining to realize reasonable control of the heat input, thereby reducing the workpiece deformation. LAJ is used in this study to join SLMed AlSi10Mg aluminum and WAAMed 2024 aluminum alloys. The microstructure and mechanical properties of the jointing areas are compared and analyzed. We hope that our basic strategies and research results can help advance efficient, high-quality intelligent manufacturing of large aluminum alloy components for aerospace.
In this study, SLMed AlSi10Mg aluminum alloy and WAAMed 2024 aluminum alloy are joined by adding the AlSi10Mg alloy powder and 2024 alloy powder, respectively. The joining process is realized using LAJ. During the LAJ, specimens using different filling powders are annealed at 240 ℃ for 2 h to prevent deformation due to stress concentration, and the microstructure is systematically investigated using an optical microscope (OM) and energy dispersive spectrometer (EDS). The microhardness of SLMed AlSi10Mg substrate-jointing area-WAAMed 2024 substrate is measured. The mechanism of hardness change in the jointing area is revealed. In addition, tensile tests at room temperature are performed to evaluate the mechanical properties of the specimens using two different powders.
The results show that the microstructure of the jointing area for the AlSi10Mg joining specimen is mainly composed of columnar crystals, and its growth direction is roughly parallel to the deposition direction (Fig. 4), whereas the grain size of the jointing area of 2024 jointing specimen is substantially refined (Fig. 5). The microhardness of the substrate on both sides of the AlSi10Mg jointing specimen is higher than that of the jointing area (Fig. 7), and the average microhardness of the 2024 jointing area is higher than the average microhardness of two types of heterogeneous aluminum alloy substrates. The room temperature tensile properties of the specimens using different powders and the two heterogeneous aluminum alloy substrates are tested (Fig. 8). The results show that the fracture positions of the two specimens are in the jointing area (Fig. 9). The tensile strength in the jointing areas of the two specimens is lower than that of the substrate (Fig. 10). In addition, the fracture mechanisms of both jointing specimens are quasi-cleavage fracture (Fig. 11).
According to the above analysis, the results of the morphology analysis show that the microstructure of the AlSi10Mg jointing area is columnar crystals with obvious growth direction characteristics. The microstructure of the grain in the jointing area is finer when the 2024 aluminum alloy powder is used as filling powder, and bright white strengthening phase is precipitated at the grain boundary. The microhardness and tensile strength of the jointing specimen using 2024 aluminum alloy powder are better than those using AlSi10Mg aluminum alloy powder under the same annealing heat treatment system. The generation of the second strengthening phase and fine grain strengthening can improve the mechanical properties. The fracture positions of the jointing specimens using two powders are all in the jointing area, indicating that the tensile strength of the jointing specimen does not reach the tensile strength of the substrate. The tearing edges forming around the pores in the jointing area are observed in the tensile fracture. All fracture mechanisms are quasi-cleavage fractures.
