Microstructure and Properties of Diffusion Bonded Mg/Al Joints

In this study, magnesium alloy AZ31 was successfully welded with aluminum alloy 6061 by diffusion bonding method. In addition, annealing process was applied to refine micro-structure and improve mechanical property. Microstructure and elemental distribution of interface were investigated with Scanning Electron Microscope (SEM) and Electron Probe Micro Analyzer (EPMA). Furthermore, experiments on diffusion bonded specimens with the usage of Transmission Electron Microscope (TEM) were carried out. At last, tensile strength was measured. It can be obtained that the width of diffusion layers increase with the increasing annealing temperatures. Elemental distribution of specimens with annealing were more uniform than that without annealing. The intermetallic compounds in diffusion layers are Al3Mg2 and Al12Mg17, their crystal structure are respectively face-centered cubic (fcc) and body-centered cubic (bcc). What’s more, tensile strength turns to be strongest after annealing at 250°C.


Introduction
As an extremely light metal, magnesium and its alloys have received growing attention due to their low densities, high specific strengths, excellent casting ability, outstanding hot formability, good electromagnetic interference shielding and recycle ability, but it is easily corroded [1][2][3][4][5]. While aluminum and its alloys have good corrosion resistance and they are also widely used for their attractive mechanical and metallurgical properties, such as high specific strength, good workability, high thermal and electrical conductivity [6][7][8][9]. With the growing emphasis on energy economy and environmental concerns, Mg alloys have become a favored choice in the automobile field. If aluminum alloys and magnesium alloy can be bonded and form a kind of composite material, not only would the flexibility and availability of the material be substantially improved, but also the weight and cost would be reduced. Presently, there are many studies on this topic. Many welding methods have been applied to join Mg alloys and Al alloys. However, no matter which technique is used, brittle and hard intermetallic compounds form in the joints. This weakens the tensile strength of the joints. As was previously reported elsewhere, the tensile strength of bonded specimens was only 37 MPa [10]. Annealing can soften and restore the ductility and formability of materials [11]. What's more, it can transform the structure of crystals and eliminate defects in microstructures, thereby reducing the brittleness and improving the mechanical characteristics of materials. In this study, in order to investigate these effects, annealing treatments were applied at different temperatures. In addition, the microstructures and their properties were evaluated. Based on this work, applications of the composite material formed by diffusion bonding of aluminum alloy and magnesium alloy could be extensive. The composite materials will lead to light-weight components, which will in turn lead to decreased depletion of resources and reduced energy usage, which can help mitigate environmental pollution.

Experimental
The first step in the experimental procedure was to cut AZ31B magnesium alloy sheets and 6061 aluminum alloy sheets according to the dimensions shown in Fig. 1. The oxide layers on the surface of the substrate were then polished with abrasive papers and the ground samples were wiped with acetone before joining. According to the Mg−Al phase diagram, the joining temperature was chosen as 440℃. Specimens were successfully joined with a method called vacuum diffusion bonding under the protection of argon gas. In order to refine microstructures and improve the properties of the bonding layers, annealing treatment was carried out. Samples were annealed at the temperatures of 200, 250 and 300℃, holding time is 1 h, which is based on previous research [12], then cooled down to room temperature in electric furnace [13].
For the purpose of studying microstructures and properties of interfaces, a series of specimens annealed at different conditions were cut across the diffusion zone. The cut sections were ground and polished. The microstructures and elemental distribution of the joints were then studied using SEM, TEM and EPMA. The tensile strength was also measured.

Results and Discussion
The microstructure was observed with SEM and shown in Fig.2 to Fig.5.The diffusion layers are clearly distinguishable in SEM micrographs and their width increases with increasing annealing temperature. This is because the diffusion rate increases with increasing temperatures. What's more, it can be depicted as that the microstructure after annealing at 250℃ is the most uniform than others.  Fig.6 and Fig.7. According to the scale in Fig.6, and the measurement of the distance to the center, the actual interplanar spacing was calculated, after compared with the database, the conclusion was obtained that d1 is 0.283nm, d2 is 0.187nm. The results indicated the (771) and (1511) planes of Al3Mg2, whose crystal structure is face-centered cubic (fcc). Based on the scale in Fig.7 and the distance from the diffraction spots to the center, interplanar spacing can be calculated. After referring to the database, the planes index can be obtained [14]. The interplanar spacing d1, d2, d3 are respectively 0.2480nm, 0.2640nm, 0.1600nm. So the corresponding plane index are (330), (400) and (622) of Al12Mg17. And the crystal structure is body-centered cubic (bcc).
The results of elemental analysis with EPMA are shown in Fig.8. Elemental distribution of specimens annealed at 200℃, 250℃, 300℃, are shown in (b), (c) and (d) respectively. While (a) corresponds to the condition without annealing. Clearly, the diffusion layers become wide when the heat treatment temperature is increased. The reason for this variation in the width of diffusion layers and element abundances is the diversity of annealing temperature. Elemental diffusion rates increase with increasing temperature, and diffusion layer is therefore the widest when the samples are annealed at 300℃. However, elemental distribution is the most uniform in the condition of 250℃. The results of tensile strength are shown in Fig.9. The tensile strength is the highest when the annealing temperature is 250℃, of about 45 MPa. When the samples are annealed at 200 and 300℃, the values of tensile strength are approximately 30 and 28 MPa respectively. Untreated specimens exhibit a tensile strength of 29 MPa. In this work, the application of annealing treatment results in a higher tensile strength.