Effect Of Fine-grained Master Alloy On Microstructure And Mechanical Properties Of AZ91D

In this paper,fine-grained master alloys and AZ91D were studied.On this basis,the fine-grained Al-Sr,fine-grained Mg-Ce and fine-grained Mg-Al-Ti-C master alloys were prepared,and their microstructure was analyzed.Three kinds of fine-grained master alloy were used on AZ91D respectively,to study the relationship between amount of master alloy addition with microstructure and properties of AZ91D and obtain the most suitable amount of addition.Meanwhile,the mechanism of action of fine-grained master alloy on AZ91D were discussed in depth.Based on that,On this basis,the effect of fine-grained Mg-Ce and fine-grained Mg-Al-Ti-C synergistic treatment on the microstructure and properties of AZ91D was studied.The results indicate that:The fine-grained Al-Sr master alloys were prepared by copper mold inject casting,and the average size of the second phase（Al₄Sr）of it decreased significantly.Meanwhile,the distribution of Al₄Sr was more disperse.The melting point of the fine-grained Al-Sr is low,and the melting rate of it is fast.The modification effect of coarse and fine grained Al-Sr master alloy on AZ91D increases with the increase of the addition amount and then decreases.When the addition amount is 0.7%,the modification effect was the best.In contrast,the properties of AZ91D using fine-grained Al-Sr to modification treatment were better than that of using coarse-grained Al-Sr alloy to modification treatment.In addition,compared 0.7%fine-grained Al-Sr modification of AZ91D with 0.7%coarse-grained Al-Sr modification of AZ91D,the tensile strength increased by 8.4%,the elongation increased by11.3%,Vivtorinox hardness increased 9.8%,the corrosion rate decreased by 16.25%.After copper mold casting,the fine-grained Mg-Ce alloys were obtained,of which the average size of Mg₁₂Ce phase decreased significantly,and the microstructure was refined.The microstructure and mechanical properties of AZ91D could be improved effectively by using coarse or fine-grained Mg-Ce master alloy.But compared with AZ91D adding0.9%Ce coarse-grained Mg-Ce master alloy,the properties of AZ91D with 0.9%Ce fine-grained Mg-Ce alloy were better.The tensile strength increased by 5.23%,elongation15.14%,Vickers hardness increased by 6.92%,the corrosion rate decreased by 44.62%.In adition,the modification effect of fine-grained Mg-Ce was better than that of the fine-grained Al-Sr.The master alloy of Mg-Al-Ti-C can be prepared respectively by in-situ synthesis and two-step method.However,much TiC of master alloy prepared by in-situ synthesis method combined together and can’t be dispersed.But master alloy prepared by two-step method didn’t exist this problem,and then on the basic of the two-step methord,fine-grained Mg-Al-Ti-C master alloy containing uniformly dispersed TiC were prepared by copper mold inject casting.The tensile strength and Vickers hardness of AZ91D adding fine-grained Mg-Al-Ti-C firstly increased and then decreased.Among them,2vol%TiC-AZ91D was the best,and the elongation rate decreased with the increase of the content of master alloy.After synergistic treatment,elongation of AZ91D treated by 2vol%TiC and 0.9%Ce fine-grained master alloy was between 2vol%TiC-AZ91D and AZ91D modificated by0.9%Ce fine-grained Mg-Ce,tensile strength and Vickers hardness were higher than both of them,the corrosion rate was lower than that of AZ91D and higher than that of AZ91D modificated by 0.9%Ce fine-grained Mg-Ce.