| Magnesium alloys can reduce the weight of their structural design due to their low density,specific strength,and high specific stiffness.However,poor corrosion resistance and poor processing plasticity at room temperature are the main reasons limiting their widespread application.Stagger spinning is a powerful spinning process with local loading and local deformation,which is beneficial to the deformation of magnesium alloys and its large spinning pressure is beneficial to the interface bonding of composite tubes.In this study,6061 aluminum alloy was coated on the outer surface of AZ31B magnesium alloy,and the metal composite tubes with light weight,high strength and excellent corrosion resistance was prepared by stagger spinning.Then the investigation of the optimal preparation conditions of the Al/Mg composite tubes,the rheological behavior during metal deformation,the evolution of the magnesium alloy in the microstructure,and the interface bonding mechanism of the composite tubes have certain theoretical and application values.In this paper,the ABAQUS finite element analysis software is used to first simulate the spinning process of single-layer 6061 aluminum alloy and AZ31B magnesium alloy,and the effects of the main parameters on the forming quality of the two are studied in this process.Then in this window,the numerical simulation of the spinning process of the aluminum/magnesium composite tubes are carried out,and the stress and strain field distribution of the composite tubes after deformation are analyzed to obtain the deformation characteristics of the material during the spinning process.Establish the optimal process range of composite tubes through simulation and conduct subsequent tests.The coordinated deformation behavior of composite tube is represented by deformation coordination index.The microstructure of AZ31B magnesium alloy under different spinning conditions is observed and analyzed by metallographic microscope,and tested by the microhardness tester.The residual stress of the composite tube was eliminated by annealing treatment,and the effects of different annealing conditions on the interface morphology and structure were studied.X-ray diffraction（XRD）was used to test the phase composition at the interface after annealing,test and analysis of interfacial bonding strength of composite tubes under different annealing conditions using tensile-shear tests,analysis of mechanical properties of micro-zones at the interface of composite tubes,establish the structure-phase-performance relationship and explore the interface binding mechanism.According to the simulation of the stagger spinning of single-layer tubes,the unevenness of the deformation between the inner and outer metals of tubes obtained by forward spinning is much smaller than that of backward spinning,which is conducive to the coordinated deformation between the inner and outer tubes.In addition,when the spinning temperature is 300～400°C,the feed ratio was 1 mm/r,and the spindle speed is within the range of 100～150 r/min,the forming effect of single-layer aluminum alloy and magnesium alloy is good.Then,at 350°C,the feed ratio is 1 mm/r,and the spindle speed is 150 r/min.A single-pass spinning with 30%thickness reduction of both materials are performed.The tubes a well formed,which verifies the reliability of the simulation results.In the obtained parameter window,the numerical simulation of spinning of the composite tubes are carried out.The results show that when the temperature is 350°C,the feed ratio is selected to be 1 mm/r,and the thickness reduction is30%～40%,the inner tube and outer tube have less axial offset.By studying the stress and strain distribution of the inner and outer tubes,it is found that the equivalent plastic strain of the inner and outer tubes is discontinuity at the interface.At the same time,during the spinning process,the equivalent stress and three stress components generated by the local loading of the roller along the radial direction present a gradient distribution,which is the cause of the plastic discontinuous plastic strain of the inner and outer tubes.The aluminum/magnesium composite tubes with good quality are prepared according to the parameters obtained by simulation.When the single-pass reduction increase and the number of spinning passes increase with 60%total thickness reduction,the difference in the reduction of the inner and outer tubes is reduced,meaning the deformation coordination between the two is improved.The microstructure in the AD-RD plane of AZ31B magnesium alloy gradually refines with increasing deformation during single-pass spinning,showing that the original coarse and uneven squeezed structure is gradually transformed into fine and uniformly recrystallized grains,at this time,the recrystallization mechanism is mainly dynamic recrystallization induced by twins.As the number of spinning passes increases,the recrystallized grain size decreases and the distribution is more uniform.The OM and SEM were used to observe the interface of the composite pipe after spinning.The results showed that the critical value of the thinning amount of a good joint at the interface of the aluminum/magnesium composite pipe was about 40%.After annealing under different conditions,a continuously distributed diffusion layer appears at the interface.The thickness of the diffusion layer is significantly affected by the annealing temperature and holding time.The diffusion layer is divided into two layers with different thicknesses.XRD results indicate that the aluminum layer is near the aluminum side with Al3Mg2 phase,and the average hardness is 3.32 GPa,and the magnesium side is with Al12Mg17 phase,with average hardness value of 2.89GPa.Combined with the EDS results,the interface structure is Al-Al3Mg2,Al3Mg2-Al12Mg17 and Al12Mg17-Mg. |
