Effect Of Pulse Current On Microstructure And Properties Of Cast-rolled AZ31 Magnesium Alloys

With advantages of short flow,high efficiency,high comprehensive yield and low cost.Magnesium alloy casting and rolling process has attracted the attention of many researchers.However,the defects of cast-rolled magnesium alloys such as uneven microstructure,segregation and excessiveβ-Mg₁₇Al₁₂ phase result in inferior mechanical properties,which will confine the extension and adhibition of the process.Pulse current can improve mechanical properties of deformed metal by accelerating microstructure transformation,promoting dislocation movement and improving microstructure.At present,the effect of low frequency（0～300 Hz）high intensity（0～600 A）pulse current on the microstructure and mechanical properties of magnesium alloys and its mechanism are rarely studied.Therefore,the microstructure and mechanical properties of Cast-rolled AZ31 magnesium alloy sheet under pulsed current were studied in this paper;The quantitative relationship between the pulse current parameters and the microstructure and mechanical properties of the plate was clarified;The mechanism of tissue homogenization and mechanical property strengthening under pulsed current treatment was revealed;Based on the aforementioned content,combined with the rolling deformation process,the microstructure evolution mechanism and mechanical property strengthening mechanism of magnesium alloy under the process of"rolling+low frequency and high strength pulse current"were further explored.（1）The experimental results of pulse current treatment on cast-rolled AZ31magnesium alloy show that with the increasingly pulse current sections such as current density,frequency,duty cycle and action time,the average grain size and Mg₁₇Al₁₂ phase quantites decline,while the elongation enhances significantly.When the current density,frequency,action time and duty cycle are 30 A/mm²,200Hz,300 s,30%respectively,the cast-rolled samples have the most uniform and fine average grain size of 27.52μm,the highest elongation of 14.26%,the tensile strength of 194.14 MPa and the microhardness of 54.01 HV.This result can be explained by the theory that the pulse current promotes the static recrystallization of the alloy and accelerates the dissolution ofβ-Mg₁₇Al₁₂ phase,thus making the grain finer and the microstructure uniform.（2）Pulse current treatment process can induce recrystallization in the alloy at relatively low temperature.The principle is that as an energy pul se current,it adds an extra amount of energy to the recrystallization of th e alloy.This extra energy makes up the driving force of recrystallization,reduces the recrystallization barrier,improves the nucleation rate and reduc es the grain size.（3）Under the rolling temperature of 380℃,the rolling deformation experiments of cast-rolled AZ31 magnesium alloy with different reduction rates（30%,40%,50%）were conducted.Although the samples whoes reduction rate in50%are at the best level in microstructure and mechanicial properties in this experiment,dense cracks have been produced on the surface of the samples.The microstructure and mechanical properties of 40%reduction rate sample are slightly lower than that of 50%sample,but no cracks appear on its surface.Therefore,the optimum rolling parameters were selected as 380℃rolling temperature and 40%reduction rate.（4）With the increasingly current density and frequency,the mean of grain size andβ-Mg₁₇Al₁₂ phase content reduced,and the elongation increased significantly.For AZ31 magnesium alloy cast-rolled sheet with 40%reduction rate,after pulsed current treatment with current density of 37.5 A/mm²,frequency of 200 Hz,action time of 300 s and duty ratio of 30%,the rolled cast-rolled AZ31magnesium alloy has the most uniform and fine microstructure,with an average grain size of 18.65μm.At this time,the tensile strength is 203.84 MPa and the microhardness is 59.88 HV.This result is attributed to the relief of continuous recrystallization and theβ-Mg₁₇Al₁₂ phase during pulsed current processing and the work hardening generated during rolling.