Study On Refining Technology And Theory In AZ Magnesium Alloys

Magnesium alloys have been hailed as the21st century’s green engineering materials, and attracted wide attention. However, the production, especially, quality of magnesium alloy wrought products with high added value in China are quite lower comparing with that in developed countries because of low metallurgical quality of ingots. It is well known that grain refining is the main method to improve mechanical proprties of Mg alloys, and because of the low cost and high efficiency, carbon-containing refiners have become the most important refining method for magnesium alloys. This study is the part work of the foundational resurech in liquid forming of Mg alloys(2007CB613702), a project of the National Basic Research Program of China(973). The refining effects of carbon-containing salts in AZ31Mg alloy at various conditions were studied systematically. The work is forced on the processing of Al-Al4C3refiner, effect of technic parameters on microstrucutres, refining mechanism, and effects of Al-Al4C3on the as-cast microstructures AZ system alloys when ultrasound was applied in solidification and the electromagnetic field was applied in DC casting, which will provides new economic and efficient grain refiner to produce high performance ingots of Mg alloys.The effects of several carbonates or carbon-containing refiner including MgCO3, CaCO3, SrCO3and C2Cl6on the as-cast microstructure of AZ31magnesium alloy were investigated first. The results showed that all refiners used in this study have refining effect, which is a complicated result of many factors, such as content of additives, inoculation time, temperature, and the adding method, etc. Considering the energy consumption, product quality, and environmental problem, for MgCO3the best refining effect is obtained at0.6wt%MgCO3at680℃holding for40min, in which the average grain size of the alloy is148μm; tensile strength is210MPa, elongation is5.9%, compared with unaddition of refiner, tensile strength is increased by57.9%and the elongation increases to11.2%. A refining method of repeated adding in small amount of MgCO3each time was developed, which can obtain higher refining effect than adding same amount of MgCO3one time.For C2Cl6the best refining effect is obtained at adding0.5%C2Cl6at680℃and holding for60min, in which the average grain size is190μm, tensile strength is190MPa, elongation is11.1%, compared with unaddition of refiner, the strength is increase by42.9%and elongation is increased by109.4%. For CaCO3, the best refining effect is obtained at adding0.5%CaCO3at760℃and holding for lOmin, in which the average grain size is148μm, the tensile strength and elongation are increased to138MPa and11.1%respectively. For SrCO3the best refining effect is obtained at adding0.6%SrCO3at760℃and holding for lOmin, in which the average grain size is110μm; the tensile strength and elongation are increased to189MPa and5.5%respectively.Al, C and MgCO3podwers were milled, then dried3hours. Subsequently, the liquid Al with low overheat was put on the milled podwers wrapped by Al foil, which were heated in an intermediate frequency induction furnace to950℃for5min and cast in a mold, which can be extruded into wires. This processing method can be used in industry because of high income of C, low processing temperature and low coast. The results show that the higher the amount of carbon in Al-Al4C3refiner, the better the refining effect will be. As Al4C3particles, which service as heterogenous nucleation sites, is unformly distributed throughout AZ31magnesium alloy melt, therefore creating more grain with samller grain size during solidification. Al-Al4C3refiner has better refining effect in AZ magnesium alloys. The experimental results show that the average grain size of AZ31magnesium alloy decreases to86μm by the addition of0.6wt%Al-Al4C3at680℃and hoding20min, which is15%of grain size of the alloy without refiner only. The refiner mechanism of Al-Al4C3refiner is that part of Al4C3particles increases heterogenous nucleation sites and other part of Al4C3particles restraining grain from growing by dragging the boundariy movement, which will results in a microstrucutre with fine grains. Because samller grain size and more grain boundaries Al put to grain boundaries forming β-Mg17Al12phases which can prevent the grians from growing.The grain size of pure Mg can be effectively reduced to106μm by the addition of1wt%Al-Al4C3refiner, in which activation ratio of Al-Al4C3is1.09%. The power ultrasonic waves also can refine the pure Mg. As the vibrating power increased, the heating effect caused by ultrasonic waves will make grain size becomes bigger. When pure Mg solidified, the ultrasonic wave was applied and Al4C3refiner was added, the obvious refining effect is obtained, which is attributed to the activation of Al4C3particles by ultrasonic waves, the average grain size of the pure Mg is77μm. A lot of Al4C3particles were activated and subsequently form heterogeneous nucleation sites, increasing the number of nucleation, at which the activation ratio of Al-Al4C3particles is2.11%which is two times of that without ultrasonic waves. When the Al-Al4C3refiner was added to AZ31magnesium alloy in DC casting the average grain size of the α-Mg in center of the ingot was decreased to about430μm, at which the activation rate of the Al4C3is0.51%. The average grain size of the α-Mg grain in center of the ingot was decreased to about570μm when a low frenquency elenctromagnetic field was applied in DC casting(LFEC). When the0.8%Al-Al4C3refiner was added to the AZ31Mg alloy melt in LFEC at680℃and holding20min, the grain size of α-Mg can be further refined and its average size is180μm. The electromagnetic field with low frequency can activate more Al4C3particles and refine microstrucutres, at which the activation rate of the Al4C3is1.65%.