Ultrasonic vibration-assisted laser-directed energy deposition of high-strength AlMgScZr alloy: Microstructural transformation and strength enhancement

AlMgScZr is an alloy designed for laser additive manufacturing, which has obtained an ultimate tensile strength of more than 500 MPa under the laser powder bed fusion (LPBF). However, satisfactory mechanical properties have not yet been achieved under the laser-directed energy deposition (LDED) process, which is more efficient and flexible than LPBF. In this study, to improve the microstructure and reduce defects of the LDED-fabricated AlMgScZr, ultrasonic vibration was applied to influence the thermal field and improve the flow field of the molten pool. The porosity, microstructure, and tensile properties of the as-deposited AlMgScZr samples fabricated by LDED and ultrasonic vibration-assisted LDED (UV-LDED) were characterized. Results indicated that ultrasonic vibration during the LDED process not only reduced the porosity and micro-shrinkage of the as-deposited AlMgScZr but also promoted the microstructure of the nano-scale Al3(Sc, Zr) precipitates transforming from L12 to D022. Consequently, favorable mechanical properties with a yield tensile strength of 215 MPa ultimate, a tensile strength of 392 MPa, and an elongation of 22.1 %, were obtained, which are at a leading level among current studies. The thermal history of the UV-LDED and LDED processes was investigated. The effect of ultrasonic vibration in reducing defects and the microstructure transformation of the nano-scale Al3(Sc, Zr) are discussed. The contributions of three kinds of strengthening mechanisms on yield strength were theoretically calculated and used to understand the effects of ultrasonic vibration. Understanding the mechanism of ultrasonic vibration in LDED helps achieve ideal mechanical properties in LDED-fabricated AlMgScZr in the future.