Results and Discussions The results reveal that the external cooling field can significantly improve the efficiency of cumulative heat transfer from the substrate to the deposited samples, so as to reduce the porosity and improve the density of the deposited samples (Fig. 4). Ultrasonic vibration assistance by its unique acoustic flow effect and acoustic cavitation effect promotes the accumulation and floating of small bubbles in the molten pool, and the number and size of pores are significantly reduced, with the eutectic phase at the grain boundary refined. Under optimized external field-assisted conditions, the optimal mechanical properties of the deposited samples are obtained. Specifically, the yield strength, tensile strength, specific elongation, and micro-hardness are 234 MPa, 385 MPa, 17.1%, and 125.84 HV, respectively (Fig. 6 and Fig. 7).1) Compared with the air cooling condition, the water cooling cycle assisted to alleviate the heat accumulation problem of the LMD formed Al-Mg-Sc-Zr samples, and improved the density and mechanical properties of the deposited samples. This shows that higher solidification cooling rate plays an optimization role in LMD forming of Al-Mg-Sc-Zr alloy. The tensile mechanical properties are improved from 339 MPa tensile strength, 159 MPa yield strength, and 14.1% elongation under air cooling condition to 385 MPa tensile strength, 234 MPa yield strength, and 17.1% elongation under ultrasonic vibration assistance condition.2) Under the assistance of ultrasonic vibration with a frequency of 20 kHz, the porosity defects of the LMD formed Al-Mg-Sc-Zr alloy samples were significantly reduced, and the mechanical properties such as strength and hardness were significantly improved. The acoustic cavitation, acoustic flow effect, and molten pool stirring effect generated by ultrasonic vibration provide sufficient escape velocity for the floating of pores in the molten pool, reduce the size of α-Al grains and eutectic phases at grain boundaries, and finally improve the mechanical properties of Al-Mg-Sc-Zr alloy samples. The tensile properties of ultrasonic assisted forming specimens are 385 MPa tensile strength, 234 MPa yield strength, and 17.1% elongation.

Al-Mg alloy modified by Sc/Zr microalloying is used to realize the generation of high-performance aluminum alloy materials. It has many advantages, such as high specific strength, strong corrosion resistance, low hot crack sensitivity, high thermal stability, and good creep resistance, and is widely used in aerospace, rail transit, chemical engineering, and other fields. The additive manufacturing process can adjust and control the deposition process window. Although it can inhibit the generation of solidification defects to a certain extent and improve the metallurgical structure, it is still unable to achieve the preparation of high-strength, high-density, and high-performance aluminum alloy bulk deposition samples. Multi physical field assisted metal solidification has always been an important means to achieve traditional casting products to break dendrites, refine grains, and reduce workpiece defects, which has important reference significance for improving the microstructure and mechanical properties of additive manufacturing components. To achieve the preparation of high-strength and high-density Al-Mg-Sc-Zr alloy, we use the laser melting deposition (LMD) technology to prepare the bulk samples of this alloy under different process conditions.

The purpose of this study is to clarify the influence of different external field assisted processing conditions on the pore defect derivation behavior, tensile stress, and other mechanical properties during LMD. The heat transfer conditions in the deposition layer are controlled by air cooling (AC) and water cooling (WC) substrates. At the same time, the influence of ultrasonic vibration on the pore defect inhibition behavior and mechanical properties such as micro-hardness and strength of the samples is studied. In order to provide some process reference and data support for the preparation of large size, high-performance,and high-density aluminum alloy parts by LMD technology, and to break through the limitation of SLM technology forming cavity size, the effects of water-cooling conditions and ultrasonic vibration assistance on the microstructure, pore defect evolution, and mechanical properties of deposited samples are studied by means of the metallographic microscope, scanning electron microscope, micro-hardness test, and room-temperature tensile test.

Al-Mg-Sc-Zr alloy bulk samples were prepared by LMD technology under different external field conditions. The influence of air cooling, water cooling, and ultrasonic vibration assistance conditions on the microstructure, tensile, and other mechanical properties of the deposited samples was studied. It was clarified that grain refinement and the inhibition of pore defects were the key factors to improve the micro-hardness and tensile mechanical properties of the deposited samples. The conclusions are as follows: