Investigation Of Interfacial Structure And Performance Of AM60/6061Al Liquid-solid Diffusion-bonding

Magnesium/aluminum bi-metallic castings offer significant mass saving opportunities inautomotive structural applications. In this thesis, the bi-metallic experiments using aself-designed high-vacuum test apparatus provides valuable understanding of the interfacialphenomena of Mg (AM60)/Al (6061) bi-metallic samples. The focus lies on surfacetreatment of the solid-state6061Al、the liquid-solid diffusion-bonding process and theinvestigation of the interfacial structure and performance of AM60/6061Al bimetal.1. A “zincate+galvanizing” surface treatment has been developed for aluminumalloys to provide improved wetting and metallurgical bond with molten magnesiumin bi-metallic casting, due to the removal of Al2O3layer and the formation of with avery thin metallic zinc film on the aluminum surfaces. This procedure comprises acombined pickling/redox-reaction and galvanization procedure: degrease、alkalietching、pickling、zincate immersion and zinc galvanizing. The results show thatthe zinc coating thickness is about50μm with the appearance of gray、uniformcolor and bright surface. Between zinc layer and the substrate of6061Al, there isno obvious defects.2. There are technical challenges in achieving strong metallurgical and mechanicalbond in these bi-metallic castings due to the fact that both Mg and Al have highthermal conductivities, prone to form brittle intermetallic phases, and a largedifference in thermal expansion coefficient. The results show that pre-heating timeof6061Al is hoped to be under80s so that we can control the pre-heatingtemperature from500℃to590℃.The thickness of Zinc layer will decline as thepre-heating temperature rises, and the zinc layer will disappear when thepre-heating temperature rises to590℃It is recommended to adjust the pre-heatingtemperature and time of the aluminum substrates to control the interfacial reactionsfor optimum interfacial properties in the actual bi-metallic casting processes.3. In this paper, The optical micrograph shows a clear Mg/Al metallic boundarywith no evidence of oxide in the interface. Further investigation using SEM coupledwith EDS scans show evidence of metallurgical reactions in the interfacial regions,some dendritic branches of α-Al exhibit in the interface. The intermetallic particlesaway from the Mg/Al interface were identified as Mg17Al12per EDS analysis. From the microhardness profile across the interfacial region of a Mg/Al bi-metallicsample (furnace temperature750°C, and substrate pre-heating temperature560°C),It is obvious that the interfacial region (about200mm wide) shows significantlyhigher hardness values, due to increased concentrations of alloying elements andMg17Al12intermetallic particles.The shear strengths of the samples with aluminumtreatment are30-90%higher than that of untreated sample. Also, the shearstrength of the treated samples increased with the increasing pre-heatingtemperature. When the pre-heating temperature (590°C) is slightly above thesolidus temperature of the6061alloy, the shear strength increased significantly dueto the enhanced metallurgical bond between magnesium and the substrate. Fromthe fracture surfaces of AM60/6061Al bimetal, it shows that the fractures from the6061Al side are mostly brittle and fractures from the AM60side are partly ductilefractures.