Metal melting and forming involves heating one or more types of metal to their melting point or above, allowing for alloying or metallurgical bonding. This process is utilized in methods such as welding, surface cladding, and additive manufacturing. Metal melting and forming technology has always advanced alongside human progress. At present, this technology is integral to aerospace, marine, and high-end equipment manufacturing, facilitating the formation, joining, and repair of metal structural components. However, the large temperature gradients that occur during metal melting and forming can have varying effects on structural metal parts.

Welding, surface cladding, and additive manufacturing technologies involve the use of high-energy-density heat sources to melt metal materials in a confined area. The rapid cooling rate in these processes results in significant temperature gradients within the melt pool. This can lead to the predominant formation of columnar or dendritic crystal grains, which are prone to cracking and porosity under thermal stress, significantly affecting the performance of the workpiece. Regulating the internal structure of metal melting and forming parts and suppressing internal defects are critical issues in these processes. To address these challenges, researchers have proposed applying external physical fields?including electric, magnetic, and ultrasonic fields?during the metal solidification process. These fields interact with the molten metal to regulate its solidification structure. Initial studies have shown that ultrasonic vibration can refine grains, inhibit segregation, and reduce temperature gradients, thus positively impacting grain growth and solidification processes. Currently, the integration of ultrasonic vibration as an external energy field into metal melting and forming technology has become a research hotspot, offering potential solutions to enhance the quality and performance of metal parts.

This study presents a discussion and summary of the recent progress and challenges in ultrasonic vibration-assisted metal melting and forming technology, focusing on its combined application with welding, surface cladding, and additive manufacturing. First, we briefly introduce the mechanism of ultrasonic vibration in metal melting. Then, we discuss the effects of ultrasonic vibration on metal melting and forming technology, considering different additive modes based on the ultrasonic transmission characteristics. Next, we explore the role of ultrasonic vibration in regulating the organization and properties of molded parts for casting, welding, surface cladding, and additive manufacturing processes. Finally, we identify the key scientific issues and technical challenges associated with ultrasonic vibration-assisted metal melting and forming technology.

In metal melting and forming technology, the excessive temperature gradients and rapid cooling rates often lead to significant non-equilibrium solidification structures. This results in issues such as grain size variation, solute segregation, and stress concentration. Ultrasonic vibration, a high-frequency mechanical vibration, induces several effects in molten metal, including cavitation, acoustic flow, mechanical, and thermal effects. Finite element simulation and experimental observations have revealed that these effects can effectively break down coarse grains, enhance melt flow, and reduce temperature gradients (Figs. 1?3).

However, the high temperatures during metal solidification can significantly impact the working life of ultrasonic components. This study categorizes the various methods of incorporating ultrasonic vibration into metal melting and forming technology into contact and non-contact methods. Contact methods are further subdivided into below, side, and above, on the basis of the direction of ultrasonic vibration propagation in the metal melt. In the repair process of a small area, the time span is short; hence, the effect of ultrasonic vibration in different directions on the molding area is minimal. However, in the metal manufacturing process of a large area, the prolonged solidification process can intensify the impact of ultrasonic vibration, leading to significant differences in the molded parts based on the direction of ultrasonic vibration. This study compares the advantages and shortcomings of these methods in the application process (Table 1).

Welding, surface cladding, and additive manufacturing are three typical metal melting and forming technologies. This study presents a literature review to explore the effects of ultrasonic vibration on these technologies from two perspectives: the microstructure and physical properties of the molded parts. The effects include gas exclusion, grain refinement, stress reduction, and enhancement of mechanical properties and corrosion resistance (Table 2).

In this study, we review the research progress of ultrasonic vibration-assisted metal melting and forming technology. We discuss the mechanism of ultrasonic vibration in metal melts, various methods of adding ultrasonic vibration, and effects of ultrasonic vibration on the microstructure and physical properties of molded parts in welding, surface cladding, and additive manufacturing processes. This study summarizes the impact of ultrasonic vibration on the forming quality of various metal melting and forming technologies, seeking to aid the realization of high-quality metal melting and forming. We also aim to promote further research and application of ultrasonic vibration-assisted metal melting and forming technologies.