Effect Of Alloying And Physical Field On Solidified Structure And Mechanical Properties Of AZ91 Magnesium Alloy

Magnesium alloy is a new structural material, which is low density and high strength ratio, known as the 21 st century green engineering material. AZ91 magnesium alloy is the most widely used cast magnesium alloy at present, which has good casting property and high yield strength. Alloying and physical field can refine solidification structure of magnesium alloy effectively, and improve comprehensive mechanical properties. This paper studies the influence of trace elements Sr and Ca on microstructure and mechanical properties of AZ91 magnesium alloy, and on this basis,effects of pulsed magnetic field, ultrasonic treatment and mechanical vibration on microstructure and mechanical properties of AZ91-0.4Ca magnesium were studied to further enhance overall mechanical properties of AZ91 magnesium alloy, and the related mechanism was also discussed. The major conclusions are as follows.1) Solidification structure of the alloy is change and the primary phase is coarse,when trace amounts of Sr and Ca are added in AZ91 alloy. Second phase changes from refinement to coarsening and then refines with the increase of Sr addition. Refining effect is the best when Sr addition is 0.05%. With the increase of Ca addition, second phase refines at first and then coarses. Refinement effect is the finest when the amount of Ca is 0.4%. The new phase Al4 Sr and Al2 Ca are found in the alloys when the amount of Sr is 0.1% and 0.2% and the amount of Ca addition is 0.5%. When Ca addition is0.4%, comprehensive mechanical properties of the alloy is the best, tensile strength and elongation of the alloy are increased by up to 51% and 691%, respectively, compared with the alloy without Ca addition.2) The solidification microstructure of AZ91-0.4Ca magnesium alloy treated by pulsed magnetic field, ultrasonic and mechanical vibration is obviously refined and the grain size and the volume fraction of the second phase is decreased, and the mechanical properties is improved. The primary phase of AZ91-0.4Ca magnesium alloy is changed from developed dendrites to rose-shaped crystal, and the change law of compressive strength is consistent with the microstructure.3) Second phase consists of strip or block into small punctate and globular under the action of pulsed magnetic field. Solidified microstructure of AZ91-0.4Ca magnesium alloy is refined gradually with the increase of pulse voltage, pouringtemperature or the decrease of mold temperature, when pulse voltage is at 0~300V,pouring temperature is at 640~730? and mold temperature is at 200~600?. The solidified structure of the alloy refines at first and then coarses with the increase of pulse frequency, when pulse frequency is at 1~10Hz. Solidified structure of the alloy is the best when the pulse frequency is 5Hz. The compressive strength of AZ91-0.4Ca magnesium alloy treated by pulsed magnetic field is up to 365 MPa, which is increased by up to 11.7%, compared with the untreated alloy. Orthogonal experiment shows that the pulse voltage is 300 V, the pulse frequency is 5Hz, the pouring temperature is 720?,the mold temperature is 200?, are the ideal process parameters of pulsed magnetic field treatment.4) Solidified microstructure of AZ91-0.4Ca magnesium alloy is gradually refined with the increase of ultrasonic power. The prolongation of action time or the decrease of pouring temperature when action time is at 0~120s, pouring temperature is at640~720?, microstructure refines gradually. The compressive strength of AZ91-0.4Ca magnesium alloy treated by ultrasonic treatment is up to 360 MPa, which is increased by up to 8.5% compared with the untreated alloy. The best optimization scheme of ultrasonic treatment is as follows. Ultrasonic power is 800 W, ultrasonic action time is120 s, and the pouring temperature is 640?.5) Solidified microstructure of AZ91-0.4Ca magnesium alloy is gradually refined with the increase of the amplitude voltage, the vibration frequency, the pouring temperature or the mold temperature when the amplitude voltage is at 0~300V, the frequency of vibration is at 0~75Hz, pouring temperature is at 640~730?, and the mold temperature is at 200~600?. The compressive strength of the alloy under the action of mechanical vibration is up to 365.67 MPa, which is increased by up to 68.8%, compared with untreated sample. The optimal scheme of mechanical vibration is that the amplitude voltage is 300 V, the vibration frequency is 75 Hz, the pouring temperature is730?, and the mold temperature is 600?.