| Recently, Magnesium and its alloys have increasingly applied in automotive, aerospace, photographic equipment and other industries because of its advantages such as low density, high damping vibration and easy to recycle. The joining between magnesium and steel(especially galvanized steel) can make the car body lightweight, so as to meet the requirements of energy saving and environmental protection. The combination of titanium with light quality and Mg not only play their respective advantages, but make up for their shortcomings. Mg-Cu dissimilar metal structure can not only reduce the cost of materials, but reduce the structure weight. In this paper, the research on wetting behavior and interfacial structures of molten Mg on the surface of steel, titanium, copper could provides certain theoretical basis for performance improvement of dissimilar metal weld.The wetting behavior and interfacial structures of AZ61 Mg alloys on surfaces of different base sheets(Q235 steel、galvanized steel、TA2 Ti and T2 Cu) under the condition of CMT(cold metal transfer) were mainly studied in this paper, and the optimized process parameters of Mg/steel、Mg/Ti、Mg/Cu were respectively obtained. The results showed that when wire feeding speed varied from 3.5m/min to 9.5 m/min, Zn coating made droplet transition unstable and caused to splash. Only when wire feeding speed was higher than 10.5m/min, the final wettability of Mg/ galvanized steel was better than that of Mg/bare steel. Aluminum enrichment layer were forming in the interfaces of Mg/ galvanized steel and Mg/bare steel. ie Al element in Mg alloys and Fe element in steel react,and form Al-Fe intermetallic compounds,which plays a important role in bonding of Mg-Steel. Considering the affinity of Al-steel is greater than that of Zn-steel, so Al is greatly tend to absorb in Mg-Fe interface and Zn is squeezed to triple lines. Mg-Zn eutectic layer produced by reaction of Zn and Mg exist over the Al-Fe IMC layer. In addition, the role of addition of alloy powder to bare steel sheets on wettability were also investigated. The results showed that Mg2 Si intermetallic compounds were formed at interface of Mg/steel with addition of silicon powder, while it had not obvious effect on wettability. With addition of copper powder, the Mg-Cu intermetallic compounds which were formed by reaction between Mg and Cu promoted the wettability of Mg-steel. The addition of nickel powder could make the wetting better, but poor interface bonding. For the addition of zinc powder, because oxide film existed between zinc powder and steel sheet, zinc evaporation did not have the effect of removal of oxide films and hindered the wettability of Mg-Steel through reduction of heat put. For wetting process of Mg-Ti, the wire feeding speed with more than 2.5m/min could be all meet the requirements of the wettability. The bonding of Mg and Ti were realized through the Ti3 Al intermetallic compounds produced by reaction of Al elementin Mg alloys and Ti element in Ti sheet because of no reaction between them. The enrichment of aluminum on interface between magnesium and titanium conformed to the formation of thermodynamics. At the same time, the wettability became better when wire feeding speeds varied from 2.5m/min to 9.5m/min, which was attributed to heat in the interface increased and interfacial reactions was more fully with wire feeding speeds increased. The oxide films of titanium sheet’s surface were broken through Al-Ti intermetallic compounds precipitated from the interface, which made the system intrinsic wettability become more obvious. Two layers of intermetallic compounds were formed at brazing interface between Mg and Cu: the first layer was consist of Mg17Al12 and Mg2 Cu eutectic phases, and the second layer was consist of Mg2 Cu and Mg Cu2 intermetallic compounds. But Mg Cu2 with white bulks was very brittle, which indicated the poor mechanical properties.Finally, the wetting behaviors of Mg/steel、Mg/Ti and Mg/Cu were compared and we discovered that the change rule of the contact angle with increasing time could all approximately expressed by following formula: The wettability of Mg/Ti was better than that of Mg/steel and Mg/Cu, which was related to heat in the interface except for the better intrinsic wettability of Mg/Ti. The heat dissipation capacity of titanium sheets were worst than that of steel sheets and copper sheets. Therefore, molten Mg wet better on the titanium surface because of the largest heat input at the interface between Mg and Ti under the same temperature. |
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