Effect of welding energy on microstructure and mechanical properties of Mg/Ti Ultrasonic Welded Joint

Magnesium alloy is widely used in aviation, aerospace, automobile and other industrial fields due to its high specific strength, high specific stiffness, good shock absorption performance, abundant resources, easy processing and manufacturing, and low cost. However, the corrosion resistance and mechanical and thermodynamic stability of magnesium alloy at high temperature are inadequate. Titanium alloy has high specific strength, good heat and corrosion resistance, strong fracture toughness and so on. Therefore, it is of great practical significance to combine titanium alloy with magnesium alloy to obtain high-strength welded joint based on its excellent performance and apply it to L6 in the field of high and new technology. Both titanium and magnesium have a dense hexagonal structure, the solubility of titanium in magnesium is less than 0.1%, there is no metallurgical reaction, no intermetallic compounds are formed, and the melting point of Ti and its alloy (1477~1682 ℃) is higher than the boiling point of Mg (1363℃).Therefore, it is difficult to connect the two directly by fusion welding, usually using the fusion welding method with additional alloy elements or solid phase welding method to connect them. At present, the common welding methods of Mg/Ti dissimilar metal connection are laser welding, diffusion welding, resistance spot welding, friction stir welding and ultrasonic welding. Ultrasonic welding, as a new type of green welding technology, has the advantages of solid forming, low pressure, low energy consumption and high production efficiency, so that the technology is not only used for the connection of the same metal sheet, but also for the connection between different metals. However, at present, the formation mechanism of Mg/Ti dissimilar metal ultrasonic welding joint is not well understood. Therefore, it is necessary to study its interface forming mechanism in depth.

Conclusion:
1) The effective connection between AZ31B magnesium alloy and TC4 titanium alloy can be achieved by using ultrasonic welding technology. The overall interface is relatively flat, with minor fluctuations in local interface. No cracks, no fusion and other defects are found, and no obvious reaction layer is seen.

2) With the increase of welding energy, the thickness of the interface diffusion layer gradually increases. When the welding energy is 1700 J, the thickness of the interface diffusion layer reaches the maximum, which is 4.7um. No obvious intermetallic compounds were found at the interface of Mg/Ti ultrasonic welded joint.

3) The shear performance of Mg/Ti joint increases first and then decreases with the increase of welding energy. When the welding energy is 1700 J, the maximum shear force of the joint is 1832n.

4) When the welding energy is lower than 2000 J, the joint fracture mode is interface fracture; when the welding energy reaches 2000 J, the joint fracture mode is button fracture. When the interface fracture occurs, the joint fracture occurs in the magnesium side diffusion zone and the magnesium side non-diffusion zone.