The laser welding technique of magnesium-on-aluminium lap joint and magnesium/aluminium interlayer with the addition of Ti foil is proposed to weld AZ31 magnesium alloy and 6061 aluminium alloy, and the organisation and properties of magnesium/aluminium joints are investigated under the effect of Ti foil-laser. The results show that the addition of Ti-foil can achieve an effective Mg/Al joint under the laser welding process, and the shear strength (line strength) of the Mg/Al joint reaches 58 N/mm, and the molten pool morphology changes from the "V" shape without Ti-foil to the "wine glass shape" with Ti-foil. The shape of the melt pool changes from "V" shape without Ti foil to "cup shape" with Ti foil. With the increase of the thickness of Ti foil, the depth of the melt pool of Mg/Al joint increases, the Ti foil near the aluminium side of the substrate partially melts, and the Ti elements are distributed in the melt pool to form Ti3Al compounds; the addition of Ti foil inhibits the direct contact between magnesium and aluminium, and avoids the reaction of Mg and Al to form brittle Mg/Al compounds, so that the addition of Ti foil has a certain degree of barrier effect, but the Ti foil has a lower thermal conductivity, and is far away from the laser heat source. However, the thermal conductivity of Ti foil is low, it is far away from the laser heat source, the melting of Ti foil is incomplete, and the bonding of Ti foil with the base material needs to be improved.
With the rapid development of material science, light metals (aluminium, magnesium, titanium), lightweight and high strength ceramics and ceramic composites, C/C composites, etc. are becoming more and more popular in the field of aerospace engineering technology, and there is an increasingly urgent need to weld aluminium alloys with titanium alloys or magnesium alloys to form composite structural parts. Laser welding is a rapidly developing joining technology in recent years, which uses a high-energy beam to rapidly melt the material and solidify it at the same time. Due to its small heat input, small post-weld deformation, and welding time is short, high efficiency, very suitable for dissimilar materials connection. However, the use of laser welding method of magnesium / aluminium dissimilar metal welding, due to the magnesium / aluminium melting point difference is small, the two liquid metal contact is unavoidable, the reaction speed between the liquid state is much faster than the reaction speed between the solid metal, the Mg / Al reaction is prone to brittle Mg / Al compound, these compounds are not only in the formation of the interface between magnesium / aluminium, will also be entered into the interface of the molten pool near the area, leading to welded joints These compounds not only form at the Mg/Al interface, but also enter the molten pool area near the interface, leading to brittle fracture of the welded joint and reducing the performance of the Mg/Al joint. The formation of brittle Mg/Al compounds is a challenge for the efficient and economical joining of magnesium/aluminium by fusion welding.
Domestic and foreign researchers have carried out exploratory studies on Mg/Al welding, among which representative works such as Zhao Limin et al. found that zinc interlayer can prevent Mg/Al elements from diffusing with each other; Chang et al. investigated the organisation of welded joints in Mg/Al laser friction stir welding with Ni foil; Chen Ying investigated the microstructure and properties of Mg/Al friction stir lap welded joints and found that the formation of Chen Ying studied the microstructure and properties of Mg/Al friction stir lap welded joints and found that the formation of Mg17Al12 compounds improved the joint properties; Qi et al. carried out Mg/Al lap welding with the addition of a soft steel foil interlayer and found that the joint shear strength varied with the thickness of the interlayer, and the shear strength increased and then decreased with the increase of foil thickness; Liu and Gao et al. carried out Mg/Al laser composite welding with the addition of a middle interlayer, and found that Zn, Ce, and Ti had the effect of inhibiting the formation of brittle Mg/Al compounds. The role of Zn, Ce, Ti all have the effect of inhibiting the formation of brittle Mg/Al compounds, of which Ti with high melting and boiling points has a better effect of inhibiting the formation of brittle Mg/Al compounds, but the mechanism is not clear; Ma Yucheng et al. explored the feasibility of aluminium/steel lap welding with the addition of Ni intermediate layer; Wang et al. carried out the laser-arc composite fusion welding of magnesium/aluminium with the addition of Ni interlayer, and found that the phase composition of intermetallic compounds in fusion zone Khodabakhshi et al. performed oscillating laser welding of AZ31 magnesium alloy plate and 6022 aluminium alloy with Ni foil and found that the oscillation of the laser beam reduces joint crack formation and enhances the mixing inside the molten pool, and the addition of Ni foil better inhibits the reaction of Mg and Al to form a brittle Mg/Al compound; Li Mingfeng et al. performed ultrasonic welding of AZ31B magnesium alloy plate and aluminium alloy with metal foil. Li Mingfeng et al. carried out ultrasonic welding of AZ31B magnesium alloy plate and aluminium alloy with the addition of metal foil, and found that the addition of Cu foil prevented the formation of Mg/Al compounds, and most of the surfaces of the Mg/Cu breaks were smooth; Zhang Shumai et al. carried out laser welding of magnesium alloy/aluminium alloy with the addition of Ni foil, and found that the addition of Ni foil restricted the diffusion of Al from aluminium into the magnesium bath, and decreased the reaction of Al and Mg to produce brittle Mg/Al compounds, and the reaction of Ni and Al in the area of joint pool produced AlNi and AlNi. In addition, Ni in the joint pool area reacts with Al to produce Al-Ni compounds such as AlNi and Al3Ni. Peng et al. carried out ultrasonic welding of magnesium alloys and aluminium alloys with the addition of Ag foil and found that Ag foil interacted with Mg and Al to inhibit the generation of brittle Mg/Al compounds, and the Mg/Al interface was replaced by the Mg/Ag and Al/Ag interfaces. From the existing research results on Mg/Al welding, the key factor affecting the performance of Mg/Al joints is the brittle Mg/Al compounds. In order to improve the properties of Mg/Al joints, the formation of Mg/Al compounds is mainly influenced by optimising the welding process parameters and by the use of additive materials. In view of the problem that liquid magnesium/aluminium is prone to react to form brittle Mg/Al compounds and reduce the joint properties, this paper proposes the laser welding technology of magnesium-on-aluminium overlap and the addition of Ti foil between the magnesium/aluminium layers to weld AZ31 magnesium alloy and 6061 aluminium alloy, and to study the organisation and properties of magnesium/aluminium welded joints under the action of the titanium foil-laser, the results of the study are expected to be useful for the advancement of the development of composite structural components made of dissimilar materials in aerospace manufacturing. The results of this study are expected to provide a new idea to promote the application of heterogeneous composite structural components in aerospace manufacturing.
AZ31 magnesium alloy and 6061 aluminium alloy were welded by using the laser welding technique of magnesium-on-aluminium interface and adding Ti foil between magnesium/aluminium layers to study the structure and properties of magnesium/aluminium fusion welded joints under the action of Ti foil-laser, and the main conclusions are as follows.
(1) Under the process conditions of laser power of 1600-1700 W, welding speed of 1200 mm/min, laser head deflection of 20°, zero defocus, and the use of Ar protective gas for side-blowing and back-protecting, the addition of Ti foil can achieve an effective connection of magnesium/aluminium, and the magnesium/aluminium joints can reach a shear strength (line strength) of 58 N/mm.
(2) With the addition of Ti foil, the melt pool morphology changes from a "V" shape without Ti foil to a "wine glass" shape with Ti foil. As the thickness of Ti foil increases, the depth of the melt pool of the magnesium/aluminium joints increases, and the foil near the aluminium side of the substrate partially melts, and the Ti elements are distributed inside the melt pool. The Ti elements are distributed inside the melt pool and Ti3Al compounds are formed.
(3) The addition of Ti foil inhibits the direct contact between magnesium and aluminium and prevents the reaction of Mg and Al to form brittle Mg/Al compounds. The addition of Ti foil has the effect of barrier, but the thermal conductivity of Ti foil is relatively low, and it is far away from the laser heat source, so the melting of Ti foil is incomplete, and the bonding of Ti foil with the base material needs to be improved.
