Technical Study On Preparation Of Semi-solid Billet Of Mg-Al-Zn And Mg-Zn-Y-Zr Magnesium Alloys

As a near-net shape forming technology, semisolid metal processing (SSP) technology is catching many researchers’attention in various fields. Comparing with traditional casting processes, SSP has special technical advantages and wide application prospect. Combine with the advantages of Magnesium (Mg) alloy for properties and resource, the semi-solid forming of Mg alloy has become one of the hotspots in the field of metal processing.In this paper, AZ80Mg alloy as the main object of study, and Mg-Zn-Y-Zr alloy as supplemented. Semi-solid billet prepared by use of semi-solid isothermal heat treatment (SSIT). This paper deals with the evolution law of microstructure of two alloys after being treated through SSIT and the quantitative relationship between the a-Mg grain size as well as Shape Factor (SF) and process parameters of extruded AZ80Mg alloy. Meanwhile, the changes in morphology and size of two alloys under the multi-stage isothermal pre-heat process were studied.Based on the study results, the main conclusion are as follows:1) If the isothermal temperature is lower than350℃and isothermal treatment time within12h, a-Mg grains grow of extruded AZ80Mg alloy as a stable normal grain growth mode and the variation of a-Mg grain size is as substantially linear. Average increment rate of the grain size of a-Mg temperature under different temperatures are:0.03μm/h (250℃),0.19μm/h (300℃),0.49μm/h (350℃), respectively.2) When heat-treated at450℃, and treated after lh and2h, the a-Mg grain as unitary isometric crystal and its distribution is more uniform; Meanwhile, the a-Mg grains grow is as a stable normal grain growth mode. With the isothermal time increase to3h, the growth mode of a-Mg grain tends to be abnormally.3) With the same parameters, after the extruded AZ80magnesium alloy is through SST, the a-Mg grain size is more spherical and its distribution is more uniform; And the liquid phase inside the a-Mg grain is caused with the main mechanism of intracrystalline partial melting priority due to solution elements enrichment; while, during the semi-solid isothermal process of the cast AZ80magnesium alloy, the liquid phase inside the a-Mg grain is caused with the main mechanism of liquid phase entering inside the grain due to the dendrite arms coalescing.4) For extruded AZ80Mg alloy, with the semi-solid isothermal process, merger of grain coarsening occurs firstly, and then liquid phase appears and expansion; with the increase of isothermal holding time, the a-Mg grains separated by liquid phase and tend to spheroid constantly, Ostwald ripening and grain coalescing will become the main mechanism with continue increase holding time. After through SST with different processing parameters, the a-Mg grain sizes conforms to Gauss distribution.5) When isothermal temperature is450℃, and isothermal time within4h, a-Mg grains grow of extruded Mg-Zn-Y-Zr alloy as a stable normal grain growth mode. With the increase of isothermal time, the growth tendency of a-Mg grain is decreased. For different isothermal stage, the growth rate of a-Mg grain are:1.45μm/h (1h-2h),0.68μm/h (2h-3h)、0.38μm/h (3h-4h), respectively.6) As cast Mg-Zn-Y-Zr alloy is composed of a-Mg and I-phase (Mg3Zn6Y), and the a-Mg grain exists as isometric crystal, and the average grain size is about40μm. While, extruded Mg-Zn-Y-Zr alloy is only composed of a-Mg, and the a-Mg grain is smaller.7) In addition, the semi-solid billet’s morphology of both cast and extruded Mg-Zn-Y-Zr alloy is as well-distributed of solid/liquid and higher roundness of solid particle can be obtained by SSIT. Combine with solid fraction, the uniformity of solid/liquid’s distribution, a-Mg grain size of the solid phase in semi-solid billet and particle roundness of a-Mg of these four evaluation, comparing with cast alloy, extruded Mg-Zn-Y-Zr alloy as raw material can obtain a higher efficiency due to the change of liquid phase is rapid. Combine with the stability of liquid fraction and a-Mg grain size, cast Mg-Zn-Y-Zr alloy as raw material to prepare semi-solid billet by SSIT has obvious advantage.