Effect of Prior Rolling on Microstructures and Property of Diffusion-Bonded Mg/Al Alloy

In this study, magnesium alloy AZ91, which was cast by double roll casting system, was rolled by a rolling mill. Then, rolled magnesium alloy and magnesium alloy without being rolled were, respectively, welded with aluminum alloy 6061 by diﬀusion bonding method. Furthermore, annealing process was applied to reﬁne the microstructure and improve mechanical property. The microstructure and elemental distribution of diﬀusion zone were investigated with a scanning electron microscope (SEM), an electron probe micro analyzer (EPMA), and a transmission electron microscope (TEM). In addition, hardness and tensile strength were measured. When cast magnesium alloy was used, the width of diﬀusion layers was wider than that with rolled magnesium alloy. And the width increased with the increasing annealing temperatures. Element distribution of specimens with annealing was more uniform than that did not undergo annealing process. Furthermore, tensile strength turns to be strongest after annealing at 250 ° C. And the strength of the specimens with rolled magnesium alloy was stronger than that with cast magnesium alloy which was not rolled.


Introduction
With the rapid development of the transportation, aerospace, national defense, and military industry, magnesium alloys and aluminum alloys are both widely used in aerospace, automotive, machinery, and electrical and chemical industry [1][2][3][4]. In addition, with growing energy, economy, and environmental needs, Mg alloys have become the favorite choices in automotive industry [5]. If aluminum alloy can be bonded with magnesium alloy and form a kind of composite material, not only would the flexibility and availability be improved substantially but also the weight and cost would be reduced obviously. So it is significant to achieve reliable connection of Mg/Al dissimilar metals. Presently, there are many researchers studying in this field.
Fusion welding is one of the most widely used methods for the joining of metals [6]. And there are many other welding methods which have been used to join Mg alloys and Al alloys such as soldering, electron beam welding [7], resistance spot welding, explosive welding [8,9], laser welding, and vacuum diffusion bonding applied in this study [10,11]. However, no matter which technique is used, the brittle and hard intermetallic compounds such as Al 3 Mg 2 and Mg 17 Al 12 were formed in the joints as annealing can transform the form of crystal and improve the defect of microstructures and rolling process can refine the microstructures. erefore, brittleness is reduced and mechanical characters turn to be better. In this study, in order to improve the microstructures and mechanical properties, rolling process on magnesium alloy before diffusion bonding was carried out, and the welded specimens were annealed at different temperatures. In addition, microstructures and properties were investigated. In diffusion process of this paper, specimens used for diffusion bonding were put into the device justly and cannot move along the longitudinal direction. No pressure is added to the specimen before heating. But pressure will generate due to thermal expansion. And it is nearly the first time the different effect of cast magnesium alloy       Advances in Materials Science and Engineering and rolled magnesium alloy on di usion bonding between magnesium alloy and aluminum alloy using this process was studied. Based on the results of this study, the application of di usion bonding and composite material of magnesium alloy and aluminum alloy will be used widely.

Materials and Methods
In this study, the AZ91 magnesium alloy and 6061 aluminum alloy were used for di usion bonding and annealing process. Experiments were also carried out via using SEM, XRD,     Advances in Materials Science and Engineering TEM, Vickers hardness tester, and tensile machine to evaluate the microstructure, crystal structure, and mechanical behavior.

Experimental.
As the rst step in the experimental procedure, a 6-high rolling mill with the rolls diameter of 80 mm was used to roll AZ91 magnesium alloy sheets from 2 mm to 1 mm. e parameters of rolling process are shown in Table 1. e second step in the experimental procedure was to cut AZ91 magnesium alloy sheets and 6061 aluminum alloy sheets to the dimensions shown in Figure 1.
en, the oxide layers on the surface of substrate were polished with abrasive papers and the ground samples were wiped with acetone and then put into the device justly. Samples cannot move along longitudinal direction without pressure to the ends. e device was put into the electric furnace, joining temperature was set at 440°C according to the Mg-Al phase diagram, and holding time was 60 min; after cooled down to room temperature in electric furnace, specimens were successfully joined in the equipment with the atmosphere of argon. e ow chart of di usion bonding is shown in Figure 2.
Besides, in order to improve the microstructures and mechanical properties, the welded samples were used for the annealing treatment experiments. According to the Mg-Al phase graph and previous annealing experience, the samples were annealed using heat treatment temperatures of 200, 250, and 300°C, and the holding time was 60 min. After heat treatment, samples were cooled to room temperature in an electric furnace.
For the purpose of studying the e ect of rolling process and annealing temperatures on microstructures and the properties of the interfaces, a series of specimens annealed at di erent conditions were cut across the di usion zone. e cut sections were then inlaid into resin to facilitate the investigation of microstructures. Using a grinder and abrasive papers (GRIT 240, 600, 800, and 1200), the samples were ground and polished with a polishing compound. e microstructures and elemental distribution of the joints were studied, respectively, by SEM, XRD, TEM, and EPMA. en, tensile strength and Vickers hardness were investigated.  Figure 6 shows the microstructures of joints that did not undergo annealing.

Results and Discussions
Based on the SEM micrographs above, the conclusions can be obtained that as the annealing temperature rises, the width of the di usion layers increases. is is because the di usion rate increases with increasing temperature. Moreover, interface of the specimens with rolled magnesium alloy is thinner than that with cast magnesium alloy. e reason is that rolling process makes the atomic spacing of magnesium alloy decrease, and then it is di cult for atoms to di use between magnesium alloy and aluminum alloy, so the width of di usion layers becomes thinner. e results of surface scanning for elemental distribution are shown from Figures 7-10, and the same conclusions with SEM micrographs can be obtained. In addition, when rolling process was applied, the element concentration of joints turns to be more than without rolling because the atom spacing decreases, so the atomic density increases. 0.48 mm, and 0.51 mm. However, in case of rolled magnesium alloy, they exhibited thicknesses of 0.065 mm, 0.07 mm, 0.075 mm, and 0.19 mm. So, the same conclusions with SEM micrographs and surface scanning for elemental distribution can be obtained.
For the purpose of analyzing and identifying the crystal structure, investigation of welded specimens using XRD was carried out. At the same time, AZ91Mg alloy and 6061Al alloy were also investigated. e results are shown in the following gures. Figure 13 shows the di raction diagram of di usion layers. It can be depicted that the di raction peaks of Mg side, Al side, and di usion layers were di erent from each other. e di raction peaks of di usion layer on Mg side occurred at the place of 2θ 67.3°and 72.5°. In case of Al side, the locations of peaks were 37.9°, 44.2°, and 81°. e For the purpose of identifying crystal structure of intermetallic compounds in di usion zone further, experiments with the usage of TEM were carried out. And the results are shown in Figures 16 and 17.
According to the scale in Figure 16 and the measurement of the distance to the center, the actual interplanar spacing was calculated; after compared with the database, the   Based on the scale in Figure 17 and the distance to the center, the interplanar spacing can be calculated. erefore, the plane index can be con rmed by referring to the database. e interplanar spacings d 1 , d 2 , and d 3 are, respectively, 0.2480 nm, 0.2640 nm, 0.1600 nm. So, the corresponding plane index is (330), (400), and (622) of Al 12 Mg 17 whose crystal structure is body-centered cubic.
For studying the e ect of prior rolled magnesium alloy and annealing on mechanical property of di usion-bonded Mg/Al alloy, microhardness of di usion layers annealed at di erent temperatures was investigated, with the load set to be 1 kg. e distributions of hardness in the annealed samples are shown in Figures 18(a) and 18(b). e trends in the hardness distributions are broadly similar in that the hardness of the Mg side is higher than that of the Al side and signi cantly increases in the di usion zone. When cast magnesium alloy was used, in the condition of no annealing and annealing at 200°C, 250°C, and 300°C, the highest hardness in the di usion bonding region was, respectively, 214 HV, 201 HV, 187 HV, and 212 HV, which is shown in Figure 18(a). However, in the case of rolled magnesium alloy, the samples exhibited the highest hardness in di usion zone of 115 HV, 110 HV, 101 HV, and 104 HV in Figure 18(b). e shape and dimensions of the samples using for the test of tensile strength are shown in Figure 19.  e conclusions can be obtained that tensile strength increases after annealing treatment; when with cast magnesium alloy, the maximum of strength is 44 MPa, which was obtained after being annealed at 250°C. As what has been reported in other study, the strength is about 37 MPa when without annealing [12]. However, when with rolled magnesium alloy, the maximum of strength increases to 53 MPa, which was also achieved after being annealed at 250°C. So, it can be thought that the most appropriate annealing temperature is 250°C [13]. Furthermore, rolling and annealing process can refine microstructure, and the mechanical character can be improved.
It is said that a fine-grained material is stronger than one that is coarsely grained because it has a greater total grain boundary area to impede dislocation motion. For many materials, the yield strength, σ s varies with grain size according to the following equation: In this expression, d is the average grain diameter and σ 0 and k are constants for a specific material [14]. According to this equation, it can be known that for one metallic material, the smaller the average grain diameter is, the stronger the yield strength will be. So, it is significant to use any processing to refine grain structure of metallic materials, in order to enhance its strength. In this study, as annealing and rolling can refine microstructure and make the grain diameter decrease, tensile strength increases.

Conclusions
Based on the results above, it can be thought that rolling process on magnesium alloy before welding and annealing after welding can refine the microstructure and have a good effect on tensile strength. e conclusions can be summarized as follows: (1) For the specimens with rolled magnesium alloy, the width of diffusion zone is thinner than the specimens which were made with the magnesium alloy without rolling. Rolling process can refine microstructure, so the microstructures and elements distribution are more uniform. (2) Intermetallic compound in diffusion zone near Al alloy is Al 3 Mg 2 whose crystal structure is facecentered cubic, while the one near Mg side is Al 12 Mg 17 whose crystal structure is body-centered cubic. (3) Annealing can refine microstructure and improve mechanical property. After being annealed at 250°C, the strength turns to be the strongest. Furthermore, the strength of the specimen which was with rolled magnesium alloy is stronger than the one with cast magnesium alloy that was not rolled. (4) e width of diffusion layers turn to be wider with the increasing annealing temperatures, and 250°C is the most appropriate annealing temperature.

Data Availability
No data were used to support this study.

Conflicts of Interest
e authors declare that they have no conflicts of interest.