Study On Contact Reactive Brazing (CRB) Of Magnesium Alloy To Aluminum Alloy

Mg alloy and A1 alloy are used in the fields of aerospace, electronic industries and energy sources for structural applications due to their unique performances. The application of Mg alloy is extending into civil products of high additional value such as automobile, computer and communication equipment. As different materials are used in the same products, it is necessary to bond Mg and A1 to form complex structure.This paper is about the research on the joint between Mg (AZ31) and A1 (6061) which formed by Contact Reactive Brazing (CRB). Al-Si alloy, Sn and Zn were selected as the braze-metal on consideration of the phase diagram and literature. The joints were analyzed making use of the methods Micro-Photographs, X-ray, strength and energy spectrum. The relationships between the thicknesses of the braze-metal, brazing time, brazing temperature and the property of the joint were researched. A good interface status was obtained by the optimization of the technological parameter. This will do well to the property of the Mg/Al joint.The results show that Mg alloy and A1 alloy can joined successfully by CRB in traditional furnace, but the shear strengths of the Mg/Al joints are very low when the joint formed by CRB directly or using AI-Si braze-metal. The average value of this kind of joints is lower than 10MPa. The shear strength of Mg/A1 joint can be improved a lot when Sn was used as the braze-metal, but another problem was occurred. A lot of solution phenomena occurring on the Al base metal for the large dissolution into the Sn braze metal during hot dipping. Zn is considered as the best braze metal for CRB between Mg and Al because a good shear strength and confection joint can obtain by using Zn brazes metal. The shear strength of the joint is as high as the braze metal. The reason for the above phenomena was studied by analyzing the microstructure, elements behavior and the phase composition of different Mg/Al joint. The addition of the Al-Si braze metal can improve the microstrucmre of the joint for the fir-tree crystal and pores existed in the joint formed by Mg/Al CRB directly, But whether Mg alloy was joined to Al alloy directly or with Al-Si braze metal a lot of Mg-Al intermetallic will formation in the joint and this will decrease the shear strength of the joint obviously. When the joint between Mg/Al was formed by using Zn as the braze metal the intermetallic compound formation in the joint was replaced by Mg-Zn intermetallic. The diffusion between Mg and Al was impeded by the addition of the Zn filler metal, so the Mg-Al intermetallic disappeared in the joint and Mg-Zn intermetallic was formed. According the Mg-Zn binary phase diagram we can find that little Mg-Zn intermetallic compound will form in the joint for the ratio of the Zn elements in the eutectic is as low as 3%wat. Moreover the distribution of the Mg-Zn intermetallic is homogeneous for fast diffusion speed of the elements in the liquid phase. These lead to a good mechanical performance of the joint by CRB using Zn braze metal.