As structural materials, aluminum alloys are widely employed in aerospace, especially in the 5 series and 7 series aluminum alloys. Currently, most of these aluminum alloy materials are prepared by traditional forging processes. Additive manufacturing technology, especially selective laser melting (SLM) forming technology, has gradually demonstrated enormous technological advantages under numerous demanding requirements such as weight reduction and functional upgrading of aerospace structures. However, currently, SLM forming of aluminum alloy structural components mainly relies on low-strength aluminum alloys, and these aluminum alloys' strength and other indicators cannot meet the performance requirements of 5 series and 7 series aluminum alloys. Additionally, the size of structural aluminum alloy components formed by SLM often has certain limitations. The development of high-strength Al-Mg-Sc-Zr forming and joining processes is significant for the large-scale and integrated development of aerospace equipment. Currently, there is relatively little research on the joining technology of SLM-formed Al-Mg-Sc-Zr alloys both domestically and internationally. Therefore, we hope to find a method to improve the joining performance of high-strength aluminum alloys.

Due to the difficulty in forming large-scale high-strength aluminum components by SLM directly, we investigate the directed energy deposition (DED) joining process of Al-Mg-Sc-Zr fabricated by SLM. The distribution and morphology of defects and their influence on the mechanical properties are analyzed. Moreover, the microstructure, element distribution, and properties of specimens joining with different DED process parameters and the addition of ultrasonic external field assistance are compared, and mechanical properties are improved by hot isostatic pressing.

The results indicate that the defects are mainly distributed in the fusion zone, which is the interface between the base and the joining zone (Fig. 4). The aggregation of dense pores at the fusion zone leads to a lower hardness than that of the joining zone and the base and then affects the mechanical properties of the whole specimens. With the laser energy density of 75-150 J/mm², the higher energy density leads to higher density and tensile strength (Fig. 6). The highest fusion zone hardness, joining zone efficiency of space filling, and tensile strength of 90 HV, 90.83%, and 203.38 MPa respectively are obtained using 3000 W laser power, 5 mm/s scanning rate, and 3.7 g/min powder feeding rate. Ultrasonic vibration promotes the precipitation of the Al₃(Sc,Zr)-enhanced phases and refines the grains, and ultrasonic vibration reduces the pore number and size. With ultrasonic vibration, the comprehensive mechanical properties of the specimens are significantly improved (Fig. 7). Hot isostatic pressing after ultrasound can further enhance the comprehensive mechanical properties.

We employ the DED process to join SLM forming Al-Mg-Sc-Zr and explore the influence of different process parameters and ultrasonic external field assistance conditions on the microstructure and tensile mechanical properties of the joining samples. We also elucidate that the suppression of pore defects is a key factor in improving the microhardness and tensile mechanical properties of the connecting sample. Between 75 J/mm² and 150 J/mm² laser energy densities, the larger energy density brings fewer pores and higher tensile strength. The highest hardness, efficiency of space filling, and tensile strength of the fusion zone are obtained using 3000 W laser power, 5 mm/s scanning rate, and 3.7 g/min powder feeding rate, with values of 90 HV, 90.83%, and 203.38 MPa respectively. Ultrasonic vibration promotes the formation and precipitation of Al₃(Sc,Zr)-enhanced phases, refines the grains, and solves the defects, causing the pores to tend to escape outward and disperse into the joining zone. With ultrasonic vibration at a frequency of 19.66 kHz and a 1.6 A current, the Al-Mg-Sc-Zr joining is carried out by DED. Ultrasonic vibration generates a stirring effect in the melt pool, providing sufficient escape speed for the upward movement of pores in the melt pool. Compared with the alloy samples without ultrasonic vibration, the pore defects in the sample are significantly reduced and distributed more evenly, with notably improved mechanical properties such as strength and hardness. The hardness at the fusion zone is 95 HV, the efficiency of space filling is 93.06%, and the tensile strength is 292 MPa, all of which are 5%, 2.4%, and 44% higher than those without ultrasonic vibration respectively. The post-treatment method using hot isostatic pressing after ultrasonic vibration can further improve the comprehensive mechanical properties. The hardness of the fusion zone is 160 HV, the efficiency of space filling is 99.99%, and the tensile strength is 405.71 MPa, which are 68.4%, 7.4%, and 38.9% higher than those without hot isostatic pressing respectively.