Effect Of Accumulative Roll Bonding On Microstructure And Mechanical Properties Of AZ31 Mg Alloy

As the lightest metal structure material,magnesium（Mg）alloy has been paid attention to by scientific research workers,which has good specific strength,specific stiffness,corrosion resistance,damped vibration and machinability.It is the best potential material to replace steel and aluminum in industry at present.It is a solution for the shortage of traditional metal resources and decrease of energy consumption at present.However,the hexagonal close-packed structure（hcp）crystal structure of Mg alloy,result in the poor plastic deformation and plastic processing ability of magnesium alloy at room temperature,which greatly limits the application of magnesium alloys.Grain refinement is a way to improve strength and obtain superplasticity of materials at the same time.Strength and plasticity of Mg alloy can be improved by refining grain size.In this paper,AZ31 alloy sheet was prepared by Accumulative Roll Bonding（ARB）.Microstructure was observed by metallography（OM）,X-ray diffraction（XRD）and scanning electron microscopy（SEM）.Strength and elongation was characterized by tensile test and microhardness was characterized by microhardness test at room temperature.Interface bond strength is evaluated by a single-lap tensile test.The effects of different rolling temperatures,reduction,and pass on the microstructure,mechanical properties,and interface bonding quality of AZ31 alloy with ARB were investigated.The evolution of microstructure,formation mechanism of the edge cracking and interface bond mechanism for AZ31 with different process parameters were explored.Because the ARB temperature of AZ31 alloy is above recrystallization which different with the theory of cold rolling welding.This paper with a highlight that explored the bonding mechanism of interface for AZ31 alloy with ARB.The following conclusions are obtained:1.Dynamic recrystallization occurs in AZ31 alloy after ARB,forming a shear band by fine dynamic recrystallized grains in the direction of shear stress.ARB temperature influences the width of shear band.This shows that the microcracks initiation in the shear band and connect with each other to form a large crack and expand along the shear band.Microstructure of interface was inhomogeneous than the surface after ARB.With the increase of ARB temperature from 350°C to 450°C,fine dynamic recrystallized grains are formed in the grain boundaries and no shear bands are formed at first,then fine dynamic recrystallized grains form the line-like shear band region.Finally,shear band gradually widened and gradually narrowed from surface to inside.With increase reduction of ARB from 32%to 46%,width of shear band was increase from 11.3μm to 51.6μm.Shape of shear band was changed from line to lath and extended deeper.With increase pass of ARB from one to two,angle of shear bands with the rolling direction was increased from 10°to 30°.The increasing of pass with angle becomes smaller.Cracks are generated because the ARB process introduces a large amount of strain energy,strain energy cannot be completely absorbed,and then released in the form of cracks.2.Yield strength（YS）,ultimate tensile strength（UTS）and microhardness of AZ31alloy was enhancement after ARB,but the decrease of elongation is obvious which result by crack.Increasing the rolling temperature,reduction and rolling passes can improve the strength and elongation of AZ31 alloy.YS,UTS and elongation of the sample were increased by 3.7%,9.8%and 9.1%with ARB temperature increase from400°C to 450°C,respectively.Yield strength and ultimate tensile strength of the sample were increased by 8.2%,11.1%,and 14.5%,with enhance ARB reduction from 32%to46%,respectively.YS,UTS and elongation of the sample with were increased by 5.2%,3.1%and 25.5%with increase ARB pass from one to two,respectively.Microhardness of AZ31 alloy can be increased by increasing the reduction and rolling pass.However,increasing the rolling temperature will increase recrystallization grain size and decrease the microhardness of the alloy,but it is still higher than that of the as-received one.The significantly of elongation of AZ31 alloy after ARB is related to the microcracks in the matrix.Microcracks growth seriously affects the toughness of the alloy during the tensile deformation.Because of the shear band in the outer plate,microhardness distribution of the outer plate is not uniform,and the value of microhardness of the outer plate is lower than that of the inner plate because of the softening of the shear band.3.Stress concentration was generated on the surface by the wire-brushing before ARB,result in dynamic recrystallization occurs more easily at the interface during the ARB process.A large number of recrystallized grains nucleate and grow at the interface.The fine recrystallized grains gradually swallowed original coarse grains.Increase of rolling temperature,reduction and pass all improve the interface bonding quality of AZ31 alloy with ARB.With the increase of ARB temperature from 400°C to 450°C,the bonding strength of the interface was increased by 15.8%.With increase reduction of ARB from 32%to 46%,interface bonding strength was increased by 38.3%,.Interface bonding strength of sample with ARB₂ is improved by 244.5%,compared with the sample with ARB₁.