Rheo-rolling Of Mg-3Sn-1Mn Alloy Under Coupling Effect Of Shearing/Vibrating

Heat-resistant magnesium alloys are ideal materials for automotive transmission and engine parts applications.With the rapid development of automobile industry,the heat-resistant magnesium alloy strips used in automobile industry have broad market prospect.However,the application of commercially available AZ and AM series alloys are limited at elevated temperature.This is due to the main strengthening phase?-Mg17Al12 parallel to slip planes of Mg-matrix,and the P-Mg17Al12 exhibits low thermal stability owing to the low melting temperature of 462?,it is difficult to pin the dislocation and grain boundary.The addition of Sn into Mg alloys leads to the formation of thermally stable(melt temperature 778 ?)and high-hardness(119 HV)Mg2Sn phase.Because the price of Sn is low,Mg-Sn system alloys are thought as the potential corrosion and creep resistant-magnesium alloys.Most magnesium alloy strips are prepared by hot-rolling,the process is long,and the cost is high.In order to prepare magnesium alloy strip with good heat resistant by high efficiency and short process,our group developed rheo-rolling of semi-solid alloy under coupling effect of shearing/vibrating,which is supported by National Natural Science Foundation of China(No.51034002).The semisolid slurry consisting of fine non-dendrites and remnant liquids can be obtained by the shearing/vibrating sloping plate.The semisolid slurry flow directly into the roll gap between the convex and the concave rollers.The technology integrates and combines casting,heating and rolling,so it is high efficiency and energy saving.In this paper,Mg-3Sn-1Mn(wt.%)alloy strips processed by rheo-rolling under coupling effect of shearing/vibrating were studied.The following conclusions were drawn:(1)The microstructure evolution of Mg-3Sn-1Mn(wt.%)alloy strip processed by rheo-rolling under coupling effect of shearing/vibrating was obtained.The cooling rate and shearing stress of melt on the slope plate increase with the increase of vibration frequency and amplitude of slope plate.The nuclei on the plate surface would escape from the plate surface and enter into the melt under the shearing stress,which refines the microstructure of alloy.From the entrance to the exit of the slope plate,the primary grain size decreases,the solid fraction increases,and the grain shape becomes rounder and rounder.Secondary crystallization occurs in the roll gap.The change of primary grain shape is not obvious in the roll gap,and the microstructures of the alloys are mainly composed of fine spherical primary grain and eutectic structure.(2)The mathematical model of flow shear constitutive relation during rheo-rolling process has been established:(m = e89.8-0.142t,n=0.039t-24.27,(634<t<640).In roll gap,the nearer the roll is,the higher shearing stress is.The shearing stress increases firstly and then decreases sharply from the entrance to the exit of the roll gap,and the peak value of shearing stress appears near the exit of the roll gap.The shearing stress increases with the increment of the roll speed and the roll radius,but deceases with the increase of the thickness of the strip.The shearing stress increases with the decrease of the casting temperature.(3)The distribution of heat-flow coupling fields in Mg-3Sn-1Mn alloy during rheo-rolling was obtained.The alloy temperature decreases gradually from the entrance to the exit of the slope plate,and the alloy velocity increases gradually.The nearer the slope plate surface is,the lower velocity is.The alloy temperature decreases gradually from the entrance to the exit of the roll gap.In roll gap,the nearer the roll is,the lower the temperature is.The proper casting temperature is 670??730?,and the proper roll speed is 0.052m/s?0.087 m/s.(4)The effects of process parameters on the microstructure and mechanical properties of Mg-3Sn-1Mn alloy strip processed by rheo-rolling were obtained.The average grain size of Mg-3Sn-1Mn alloy strip increases with the increase of casting temperature,and the tensile strength and elongation of the alloy strip decreases correspondingly.The average grain size of Mg-3Sn-1Mn alloy strip decreases with the decrease of roll speed.The primary grain size decreases firstly and then increases with the increase of vibration frequency,and the tensile strength and elongation of alloy strip increase firstly and then decreases with the increase of vibration frequency.When the casting temperature is 670 ?,the roll speed is 0.052 m/s and the vibration frequency is 60 Hz,Mg-3Sn-1Mn alloy strip with a cross-section size of 4 mmx 160 mm is produced by the proposed process.The ultimate tensile strength and elongation of Mg-3Sn-1Mn alloy are 175 MPa and 5.6%at room temperature,respectively.The ultimate tensile strength and elongation of Mg-3Sn-1Mn alloy at 150 ? are 159 MPa and 61.4%.Mechanical properties of the present product at room temperature or 150 ?are higher than that of Mg-3Sn-1Mn-0.87Ce.(5)The effects of Lanthanum(La)on the microstructure and mechanical properties of Mg-3Sn-1Mn(wt.%)alloy strip manufactured by rheo-rolling were obtained.When La content is 0.2 wt.%,La is completely dissolved in the a-Mg matrix to form solid solution,Mg2Sn is the main phase of the Mg-3Sn-1Mn(wt.%)alloy.When La content increases to 0.6 wt.%,a new plate-shaped compound with an average length of 380 nm is formed,which is composed of La5Sn3,Mg2Sn and Mg17La2 phases.As La content increases to 1.0 wt.%,the length of the compound increases to 565 nm.This plate-shaped compound is coherent with a-Mg matrix along the[1 0-1-2]Mg crystal axis,and grows towards the inside of a-Mg grain perpendicularly,thus having pinning effects on the a-Mg grain boundary and dislocation.As La content increases to 1.4 wt.%,the length of this compound is 2300 nm,however,the strengthening effects of this plate-shaped compound is weakened.The Mg-3Sn-1Mn-1.0La(wt.%)alloy plate show tensile strength of 226 MPa and elongation rate of 7.4%at room temperature,and show tensile strength of 194 MPa and elongation rate of 65%at 150?.The mechanical properties of Mg-3Sn-1Mn-1.0La alloy strip at room or elevated temperature are better than that of Mg-3Sn-IMn-0.87 Ce alloy.