Microstructure Formation and Mechanical Properties of AZ31 Magnesium Alloy Solidified with a Novel Mechanical Vibration

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ED wrought alloys are of special interest for applications in lightweight structural components. However, their applications are still limited due to relatively low strength, poor formability, and limited ductility at room temperature. It has been found through investigation that AZ31 can only be extruded at an elevated temperature of about 623 K (350 °C) at a relative low rate of 10 to 20 m min1.[1] Microstructure refinement is an effective way of increasing strength, favoring uniform deformation and allowing deformation at room temperature.[2–4] Mg-Al-based wrought alloys are usually cast into billets to be used for rolling, forging, or extrusion. Therefore, a billet with a refined and uniform microstructure after solidification is desirable for subsequent thermo-mechanical processing. Chemical inoculation treatments are often preferred to refine solidified microstructure. Although, many grain refiners for Mg-Al alloys have been reported, such as C, Sr, Mn, RE, Ca, AlN, SiC, and Al4C3,[4–7] grain refining of commercial Mg-Al alloys has proven to be a scientific and technological challenge by using chemical inoculation. H.M. GUO, A.S. ZHANG, and X.J. YANG, Professors, M.M. YAN, Senior Lab Master, and Y. DING, Lab Master, are with the School of Materials Science and Engineering, Nanchang University, 999 Xuefu Road, Nanchang 330031, Jiangxi, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted March 11, 2013. Article published online September 7, 2013 438—VOLUME 45A, JANUARY 2014

Apart from the inoculation, grain refinement can also be obtained by introducing an external field into a molten alloy during solidification. Electromagnetic vibration is an attractive technique because it is a noncontact method and can effectively refine solidified microstructure of Mg-Al alloys.[8,9] The force produced in a molten alloy is proportional to the cross product of the current density and the magnetic field in it. So, electromagnetic vibration is costly and requires tremendous amount of current to be effective. Ultrasonic vibration is an energetically more efficient method to refine solidified microstructure of Mg-Al alloys because it allows direct propagation of ultrasonic waves within the molten alloy during solidification.[10,11] The basic problem is that the refinement is always limited near the area of the vibration source due to the fact that the vibration intensity attenuates quickly in the molten alloy. Recently, melt conditioning by advanced intensive shearing using a twin screw during solidification has proven to be the effective means to refine grain structure in Mg-Al alloys.[12,13] However, the molten alloy is always aggressive to the twin screw, which may contaminate the molten alloy and increase the cost of treatment. Mechanical vibration is another technique to refine solidified microstructure by introducing an external field into a molten alloy during solidification. We consider using mechanical vibration because it has the following advantages: (1) relatively high refining effectiveness; (2) easy operations;