Study On Solidification Behavior Of Magnesium Alloy Melt By Variable-Frequency/Dual-Frequency Ultrasonic Treatment

Magnesium alloy,the lightest structural metallic material until now,has extensive application prospect in many fields,while low strength,deformation capacity and corrosion resistance limit its larger application.Grain refinement is one of the most important method to improve alloy's strength,properties and deformation capacity during subsequent deformation process.Nowadays,large studies have proved that ultrasonic treatment has outstanding abilities on grain refinement,purification and degasification.However,fixed frequency of current ultrasonic equipment cannot satisfy melt's resonant frequency moment by moment,and heavy attenuation can lead to difficult application in large volume melt,which restrict its industrialized application heavily.In this thesis,the influence of AZ80 and ZK60 alloys' microstructure and mechanical properties were investigated,and the ultrasonic threshold and attenuation coefficient in magnesium melt were investigate as well.Besides,numerical simulation method was used to investigate the acoustic pressure distribution in melt and cavitation bubble's dynamic behavior under dual-frequency ultrasonic field.The refinement mechanism of new acoustic field was also investigated with the combination of results of simulation and experiments.The main conclusions are following(1)According to the calculated results,the ultrasonic threshold of AZ80 magnesium melt is about 0.987MPa at 660? and the attenuation coefficient is about 0.081942 dB/cm.The variable-frequency ultrasonic field has higher average and maximum acoustic pressure and larger valid cavitation area.(2)Introducing the ultrasonic field into semi-continuous casting process can refine the grain,change the morphology and distribution of second phase reduce the segregation rate of main elements,and improve billet's mechanical properties,while variable-frequency ultrasonic field has higher efficiency than fixed-frequency ultrasonic field.The grain size of AZ80?255mm billet are decreased to 732?942?m from 679?1454?m treated by fixed-frequency ultrasonic field,and further reduced to 150?241 ?m after variable-frequency ultrasonic field treatment.The grain size of ZK60 0255mm billet are decreased to 149?186?m from 168?201 ?m treated by fixed-frequency ultrasonic field,and further decrease to 70?101 ?m after variable-frequency ultrasonic field.The second phase become smaller and dispersed after variable-frequency ultrasonic field treatment.The mechanical properties of refined billet are greatly promoted,for example,the yield strength and ultimate tensile strength of AZ80 are 92MPa and 169MPa,which is 23%and 20%higher than untreated billet.Refinement efficiency is improved slightly in edge with higher insert depth,while heavier segregation rate can be brought in this way.(3)The dual-frequency ultrasonic field can refine the grain of ZK60 alloy with larger extent than single-frequency ultrasonic field.The refinement efficiency is improved by the increase of ultrasonic power appropriately,while it will be decreased with further increment of power.The grain size is decreased to 125?m from 183 ?m treated by single-frequency ultrasonic field,and further decrease to 72?m after dual-frequency ultrasonic field,which is 30.4%higher than traditional technology weith same power.The ? phase of ZK60 alloy can be seen as dotlike and dispersed distribution after dual-frequency ultrasonic field treatment.Ultrasonic treatment can also decrease the microporosity of alloy,and the billet has less microporosities after dual-frequency ultrasonic field treatment.The mechanical properties are improved dramtically because of grain refinement,? phase's morphology and distribution and decrease of microporosities.(4)Both variable-frequency and dual-frequency ultrasonic fields can improve refinement effect by enlarging the cavitation bubble's radius and increasing the number of bubble,while they have different mechanism.Variable-frequency ultrasonic field can achieve this goal by producing higher acoustic pressure and bring larger action area.Dual-frequency ultrasonic field can enlarge the cavitation bubble's radius and increase the number of bubble under same acoustic pressure.