Study On Microstructures And Mechanical Properties Of Two-stage Pretreatment Mg-9Al-1Si Heat Resistant Magnesium Alloys By Equal Channel Angular Pressing

Developing heat-resistant magnesium alloys with low cost and excellent mechanical properties on Mg-Al-Si alloy can overcome the limitations of rare earth magnesium alloy with high cost and is of great significance to broaden the commercial application of heat resistant magnesium alloys.Mg-9Al-1Si alloy with ultrasonic vibration and ultrasonic vibration + heat treatment were designed and fabricated,and the alloy with ultrasonic vibration + solution heat treatment were subjected to equal channel angular pressing(ECAP)deformation.The effect of different pretreatment process on the microstructure of as-cast and ECAPed Mg-9Al-1Si alloys were investigate by OM,SEM and TEM.The mechanical properties of alloys were researched by Electro-mechanical Universal Testing Machines,and the effect of pretreatment on the creep resistance properties of ECAPed alloys were discussed.The research results are showed below:After ultrasonic vibration,the Mg2 Si phase of Mg-9Al-1Si alloy distributed very uniform,changed from coarse Chinese script shapes to fine Chinese script shapes,the β-Mg17Al12 phase changed from coarse bulk to lamellar shape.After ultrasonic vibration + solution heat treatment,with the increase of solid solution temperature and time,the β-Mg17Al12 phase was gradually reduced.The coarse Chinese script shaped Mg2 Si phase was refined,and part of the Mg2 Si tended to dissolve,break and spheroidize.After ultrasonic vibration + aging treatment,with the increase of aging time and temperature,non-continuous lamellar Mg17Al12 precipitated phase along the grain boundary increased.The optimized pretreatment process were: ultrasonic vibration(20 min),ultrasonic vibration(20 min)+ solution heat treatment(420℃/24 h),ultrasonic vibration(20 min)+ aging treatment(420℃/24 h +180℃/2 h).The Mg-9Al-1Si alloys with ultrasonic vibration + solution heat treatment were subjected ECAP at 350℃.After 1-2 passes ECAP,the matrix grains were significantly refined.However,after 4 passes,the matrix grain grew slightly due to higher extrusion temperature.With the increase of ECAP passes,Chinese script shaped Mg2 Si phase was gradually broken and distributed relatively uniform.The β-Mg17Al12 precipitated phase increased first and then decreased,distributing in the grainl boundary and interior.Part of β-Mg17Al12 phase was re-dissolved to α-Mg matrix.The mechanical properties of Mg-9Al-1Si alloy are the best after 2 passes ECAP.With the decrease of ECAP temperature,the average grain size of Mg-9Al-1Si alloys gradually decreased,the precipitation of Mg17Al12 phase increased and became larger,the fragmentation of Mg2 Si phase was more obvious.But the fragmented Mg2 Si was agglomerated after 2 passes at 310℃.The mechanical properties of ECAPed Mg-9Al-1Si alloys with ultrasonic vibration + solution heat treatment were the best after 2 passes ECAP at 330℃.The morphology and distribution of second phases in ECAPed Mg-9Al-1Si alloys were different due to the difference of second phases in as-cast Mg-9Al-1Si alloy after pretreatment.The yield strength,tensile strength and elongation of ECAPed Mg-9Al-1Si alloys with ultrasonic vibration + solution heat treatment were reached 290 MPa,350 MPa and 14.77% respectively.With the increase of temperature,the creep resistance properties of ECAPed Mg-9Al-1Si alloy decreased.At the same temperature and stress,the creep resistance properties of ECAPed Mg-9Al-1Si alloys with ultrasonic vibration and ultrasonic vibration + solution heat treatment were improved significantly.The creep life and steady tensile creep rates(175℃,50 MPa)of ECAPed Mg-9Al-1Si alloys with ultrasonic vibration were 92.37 hours and 4.17×10-7 s-1 respectively.Compared with the direct ECAP without pretreatment,the steady tensile creep rates was reduced by nearly an order of magnitude.