Effect Of Dual-frequency Ultrasonic On Microstructures And Mechanical Properties Of Magnesium Alloys

Magnesium alloy has the characteristics of low density,high specific strength and high specific stiffness,but low absolute strength and plastic deformation ability,which seriously restricts range of its use.Refinement of solidified structure can not only improve the strength of magnesium alloy,but also effectively improve its plasticity.A large number of studies have shown that ultrasonic treatment can significantly refine the solidification structure and improve the mechanical properties of magnesium alloys.However,most of the current research focuses on single-frequency ultrasonic.Due to the serious attenuation of ultrasonic in metal melts,it is difficult to effectively handle large-volume melts.The fixed-frequency characteristics of singlefrequency ultrasonic are difficult to achieve real-time efficient resonance,which seriously affects ultrasonic non-linear effect,so how to improve the range and effect of ultrasonic treatment of magnesium alloy melt is the key issue to realize the industrial application of ultrasonic solidification of magnesium alloy.In view of the above problems,this paper takes AZ80 magnesium alloy melt as the object,numerically studies the attenuation and cavitation behavior of dual-frequency ultrasonic in magnesium alloy melt,and experimentally studies the effects of dual-frequency ultrasonic process parameters on the solidification structure and mechanical properties,and the process parameters are optimized.On this basis,the effects of dual-frequency ultrasonic melt treatment on the as-cast solidification structure and mechanical properties of rare earth magnesium alloys such as Mg-5Sm-xAl and Mg-xZn-Y were carried out.Finally,the effects of ingots obtained by dual-frequency ultrasonic treatment on the deformation structure and mechanical properties were compared.The full text draws the following main conclusions:(1)Numerical simulations show that at the same total power,the attenuation coefficient of(15+20)kHz dual-frequency ultrasonic in the melt from 0.09 dB/cm(20 kHz)or 0.05 dB cm(15 kHz)is reduced to 0.038 dB/cm.The cavitation area of dual-frequency ultrasonic is greater than single-frequency ultrasonic(15 kHz and 20 kHz)at different initial cavitation bubble radius.(2)Experiments show that dual-frequency ultrasounic has a more significant refinement and strengthening effect than single-frequency ultrasonic.Under the same conditions,the refinement rate and tensile strength increase rate of AZ80 solidified structure from respectively 29%and 7%(20 kHz),43%and 20%(15 kHz),increase to 54%and 33%of dual-frequency ultrasonic.Meanwhile,it shows that the refinement effect is better when the power ratio is P15:P20=1:2 and the processing time is 5min.(3)The research results on Mg-5Sm-xAl alloy show that both the increase of Al content and the application of dual-frequency ultrasonic can significantly refine the solidification structure of the alloy.However,when the Al content is high,the effect of dual-frequency ultrasonic weakens because of the heterogeneous nucleation of Al2Sm.The tensile strength of the alloy increases with the degree of grain refinement,but the contribution to strength during dual-frequency ultrasonic treatment is closely related to dispersion of the second phases and its grain refinement.(4)The research on Mg-xZn-Y alloys shows that the refine rate of applying dual-frequency ultrasonic reach 84%(Mg-6Zn-Y)and 80%(Mg-10Zn-Y),and the phase I changes from lamellar to dot and strip.The tensile and yield strengths of Mg-6Zn-Y alloy increased by 56 MPa and 37 MPa,respectively,Mg-l0Zn-Y alloy increased by 25 MPa and 23 MPa,respectively,and the elongation increased from 6%and 3%to 12%and 5%.(5)The AZ80 ingot obtained by dual-frequency ultrasonic treatment has a finer grain size after extrusion at 250?.The strip-like Mg17Al12 phase decreases and the dotteed Mg17Al12 phase increases.In addition,the recrystallization fraction is reduced,more cone slip is easily activated,and the dislocation density is increased.The tensile strength and yield strength of the extruded AZ80 are increased to 341 MPa and 236 MPa,it increased by 19 MPa and 20 MPa,respectively,compared with not application of ultrasonic.