Kehong Wang

Effect of joining temperature on microstructure and properties of diffusion bonded Mg/Al joints

The joining of AZ31B Mg alloy to 6061 Al alloy was investigated at different joining temperatures by vacuum diffusion bonding method. The microstructures of Mg/Al dissimilar joints were studied by means of optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX). The results show that the thickness of each layer in the diffusion zone increases with the increase of joining temperature, and the microstructure changes obviously. At joining temperature of 440 °C, the diffusion zone is composed of Mg2Al3 layer and Mg17Al12 layer. At joining temperatures of 460 and 480 °C, the diffusion zone is composed of Mg2Al3 layer, Mg17Al12 layer, eutectic layer of Mg17Al12 and Mg-based solid solution. The width of high-hardness zone in the joint increases with increasing joining temperature, and the micro-hardnesses at different locations in the diffusion zone are significantly different. The joining temperature of 440 °C offers the highest tensile strength of 37 MPa, and the corresponding joint exhibits brittle fracture at the intermetallic compound layer of Mg17Al12.

Light-Weight Mg/Al Dissimilar Structures Welded by CW Laser for Weight Saving Applications

With the increasing demand of light-weight alloys, such as magnesium (Mg) and aluminum (Al), the need for joining these two alloys is unavoidable. In this study, AZ31B Mg and 1060 Al alloys were joined by continuous wave laser micro-welding using a 0.05 mm thick Cu/Zn interlayer. The microstructure and phases constituent of the weld seam were examined by optical microscope, SEM and EDS. The formation and distribution of the intermetallic compounds (IMCs) and the relationship between these structures and the micro-hardness of the weld were discussed in detail. The effect of Cu/Zn interlayer on the performance of Mg/Al joint was also analyzed. The results showed that Mg/Al IMCs were formed in the weld, which indicates that the Cu/Zn foil could not prevent the reaction between Mg and Al. However, the addition of Cu and Zn into the weld pool refined the microstructure by improving the number of eutectic structures. The micro-hardness of Mg/Al IMCs in the middle of the weld was very high which can be detrimental to the toughness of the Mg/Al joints.

Effect of strontium and SmO on the microstructure and fracture mode of AlSi-Mg2Si brazing filler metal

An AlSi-Mg2Si self-fluxing brazing filler metal was obtained by activating AlSi brazing filler metal with the Mg2Si phase and applying a metamorphism treatment of Mg2Si and Si crystals with Sr and SmO. A good wetting performance between the modified AlSi-Mg2Si and 3003 Al alloys was achieved in the vacuum brazing without Mg vapour as the activator. The melting point of the brazing filler was measured by STA409Pc differential scanning calorimetry. A scanning electron microscope was used to analyse the microstructure and the component distribution of the brazing filler. The results indicated that the change in morphology of the Mg2Si phase was remarkable after metamorphism. The analyses of the microstructure indicated that the Mg2Si and Si phases presented small needle-type and granular morphologies. The follows were found to occur: intergranular penetration of Mg and Si on the base metal, a large amount of granular eutectic structure, and a significant grain boundary effect. The shear fracture of the lap joint presented cleavage fracture; in addition, with the decrease of the Mg2Si phases, the fracture morphology transformed from a rock candy shape to a steam shape, and cleavage steps appeared. The rock candy-shaped fracture areas enlarged after metamorphism treatment.

Study on Lightweight Design and Connection of Dissimilar Metals of Titanium Alloy TC4/T2 Copper/304 Stainless Steel

Under the background of lightweight design, manufacturing and improvement of comprehensive performance, dissimilar metals connection has been becoming a research focus recently and will have a broad application prospect. Titanium alloy TC4 and 304 stainless steel have many excellent properties, achievement of effective connection between these two materials has a significant promoting effect on Industrial Technology. However, the bonding connection of TC4/304 is very poor, so it is necessary to redesign the joint and further study the strengthening mechanism. In the paper, connection experiment of TC4/304 was carried out using two methods: Electron Beam Welding and Friction Stir Welding. Optical microscopy, SEM, EDS were applied for the analysis of microstructure and phase structure. The results state that EBW and FSW are effective and the maximum strength are 196 Mpa and 178 Mpa respectively. Both failure mode are brittle fracture.