| Owing to their high strength-to-weight ratio,high stiffness and high corrosion resistance,Al/Mg composite materials are a promising structural material in wide applications.In this work,an Al coating was applied on the magnesium alloy surface by means of a multi-pass friction stir processing（MP-FSP）to abtain the good interface bonding and excellent corrosion resistance of Al/Mg composite materials.The effect of process parameters（travle speed,welding speed and pin penetration）on the formability of the interface was first studied.According to the feasibility study results on the formability of magnesium alloy friction stir processing directly decided the forming quality of Al/Mg composite materials,it is confirmed that the preparation technology of firction stir processing should hold with next parameters:a tool rotation speed of 1900 r/min,a pin penetration depth of 0.3 mm,and a tool travel speed ranging from 4 to 14 mm/min.On this basis,the effcet of travel speed and overlapping ratio on the interfacial microstructure and properties were studied.Without the regard to overlapping ratio,the travel speed had a great impact on the interfacial macro-features.For travel speeds of 4 mm/min and 14 mm/min,the interfacial macro-features showed defects such as microcracks and microvoids.However,a structurally sound weld bond in the interfacial macro-features without any microcrack and microvoid was formed for a travel speed of 10 mm/min.At a 50%overlapping ratio,the intermetallic compounds（IMC）were formed in the conversion zone at different travel speeds.An Al12Mg17 was formed close to the bottom Mg alloy and an Al3Mg2 was formed close to the top Al alloy.The grain size of Al12Mg17 and Al3Mg2 in the previous pass zone was larger than that of the latter pass zone.It was observed that submicron Al3Mg2 had a diffuse distribution both in the previous pass zone and the latter pass zone.It was also observed that an annealing twin formed in the Al3Mg2 phase.In the overlapping area of the interface transition zone near the side of magnesium,the Al12Mg17 phase possessed different microstructure morphologies for different travel speeds.The shear force of the interface of the single pass increases with the decrease of the travel speed,however,the shear force of the interface of the overlapping area first increases and then decreases.In addition,with the increase of the overlapping ratio,the shear force of the interface of the overlapping area also first increases and then decreases.The better interface shear performance was obtained for a travel speed of 10 mm/min and an overlapping ratio of 50%.The interfacial failure location of the single pass is in the interface between the Al alloy and Mg alloy.When the the travel speed of 14 mm/min,the failure mode is brittle fracture.However,when the travel speed of 4 and 10 mm/min,the failure mode presented a mixture of plastic and brittle fractures.For a travel speed of 10 mm/min,rotation speed of1900 r/min and pin penetration depth of 0.3 mm,the interfacial failure location of the interface of overlapping area were all in the interface between the Al3Mg2 phase and the Al matrix in different overlapping ratios.All the failure mode were brittle fractures.The formation of the Al3Mg2 phase has a great effect on the shear force of the interface of the overlapping area.The distribution of the microhardness for the Al/Mg composite materials using the overlapping ratio of 50%exhibited low microhardness in the Al alloy and Mg alloy parent materials and high microhardness in the interface between these parent materials.A l060 Al coating was applied on the AZ31B Mg alloy surface by means of a mult-pass friction stir processing greatly enhancing the corrosion resistance of AZ31B Mg alloy by shifting the corrosion potential from-1.52VSCE to-1.09VSCE,decreasing the corrosion current density from 2.75×10-6 A/cm2 to 5.20×10-8 A/cm2 and increasing the transfer resistance from 86?·cm2 to 29860?·cm2.In this multi-pass friction stir processed Al/Mg composite material,crystallographic orientation relationships were observed between the IMCs and the matrix.Two new orientation relationships between the IMC and the matrix were observed in the form of[101]Al12Mg17//[0112]Mg,（101）Al12Mg17//（0111）Mg;[531]Al12Mg17//[1210]Mg,（0001）Mg 1.7°from 21（）1Al12Mg17;（2）02Al3Mg2//（2）02Al,（02）2Al3Mg2//（02）2Al,（20）2Al3Mg2//（20）2Al, [111]Al3Mg2//[111]Al;（111）Al3Mg2//（111）Al,[110]Al3Mg2//[211]Al.The Al12Mg17 had a coherent interface with the Mg matrix in order to minimize the activation energy for nucleation. |
