Study On Heavy Reduction Rolling Of AZ31Magnesium Alloy

Magnesium alloy has been used rapidly in aerospace, electro communication, automobile and many other fields for its high modulus, high specific strength, excellent damping vibration attenuation and electromagnetic shielding characteristic. But due to its own characteristics of magnesium alloys, magnesium alloy cast ingot by traditional casting way is serious composition segregation, large organization and poor surface quality etc. And in the wrought magnesium alloy, especially in the rolling process of AZ31magnesium alloy, because of the low magnesium alloy plastic, it is rolled difficultly and after rolling the tissue of slab is inhomogeneity.Low frequency electromagnetic half the continuous casting (LFEC) technology which is developed by Northeastern University can thin the tissue of ingot and blank, inhibit the composition segregation, reduce the surface crack, improve the surface quality significantly. In the dissertation, different preparation technology and alloy composition have been investigated to the effect of the surface quality, mechanical property, composition uniformity and as-cast structure etc. of magnesium alloy flat blank with changing the casting speed, changing the electromagnetic field conditions and adding rare earth elements in the self-designed200x60mm square stainless steel mould. To study the effect of heavy reduction rolling of the microstructure and surface quality of AZ31magnesium alloy slab, heavy reduction rolling experiments of single pass or two-pass were carried out in250℃-450℃temperature. We want to get the best preparation technology of magnesium alloys flat blank and best rolling technology of AZ31magnesium alloy. Through the experiment and research, the results are as follow:(1) Appling electromagnetic field can refine the solidifying structure of AZ31alloy flat and reduce organization difference. Improving casting speed can refine the crystallite dimension of the centre of flat significantly, decrease the organization difference of center and the edges at the same time, and improving the surface finish quality of flat blank and surface quality. But if the casting speed is too high(180mm/min),it will get the opposite effect. The best casting speed is150mm/min when electromagnetic field is I=30A,f=30Hz.The effect of casting speed to macrosegregation had something with the effect of electromagnetic, the macrosegregation of Al and Zn will be more serious in the condition of electromagnetic.(2) When magnesium alloy of AZ31is being rolled, if the deformation is10%, and the second pressure rate is less than30%, there is no edge crack basically. when rolling reduction is increaseing, the depth and width of the crack along with the direction of the board face will be linear increase with total rolling reduction. Decreasing rolling temperature, the depth of the crack will reduce mildly, while the width of the crack increases obviously. When the deformation is10%, and the second pressure rate is about50%, it will have the greatest influence on rolling temperature.(3) Improving the reduction of every pass and decreasing the rolling temperature both can enhance degree of uniformity of rolling deformation of AZ31alloy, and promote dynamic recrystallization behavior, thin the organization of slab, that is to say, low temperature and heavy reduction rolling technology realized AZ31slab hot-rolling blooming affine deformation and refining microstructure.(4) As far as slab deformation uniformity be concerned, good rolling blooming system is300℃rolled, and the deformation is10%, the second pressure rate is50%. For the time being the tissue of the slab is perfect recrystallization, as far as controlling the changed crack be concerned, the total reduction should not more than40%.