| In the1970s with the worldwide oil crisis intensified, magnesium alloy due to the light weight, high strength characteristics than more and more attention, and the Mg-Zn-Cu alloy is a heat-resistant magnesium alloy based on studies of developing in the1980s, and some research organizations mainly focused on aging microstructure and alloys phases, and the coverage is limited, ZC alloy grades are now mature application ZC62, ZC63and ZC71.The topic to Mg-Zn binary alloy series is based on the preparation of metal casting method using Mg-xZn-yCu(x=3,6,9; y=0,1,3,5) alloys, analysis of different content of Zn、Cu elements for Mg-xZn-yCu (x=3,6,9; y=0,1,3,5) changes of microstructure and mechanical properties of the alloys as-cast and after heat treatment, as well as to explore the best ratio of Zn/Cu under non-dendritic structure evolution of semi-solid, in order to provide a theoretical basis for the further development and utilization of Mg-Zn-Cu alloys.The experimental results show that, the ratio of Zn/Cu haved impacted on the Mg-xZn-yCu eutectic phase particle morphology, number, size and distribution, and Mg-xZn binary alloys microstructure are mainly composed of a-Mg matrix and MgZn eutectic phase composition, with increasing Zn content, the primary eutectic phase shifted gradually from granulation to white strip. After adding the appropriate amount of Cu content, the granular microstructure was gradually transformed into intermittent, continuous network. After solution treatment, the eutectic Mg-9Zn-yCu alloys changed significantly, mostly presented semi-continuous, and the second phase eutectic most dissolved. After aging treatment, the precipitation of grain boundary precipitated obviously, along with a large number of precipitated phase. With the increase of aging temperature, the grain boundary precipitates particle size and spacing changed more larger, which showed the grain boundary precipitated. High Zn (9wt.%) or high Cu (5wt.%) can be refined grains, while the grains of the high Zn (9wt.%) and high Cu (5wt.%) were more diffuse small.The mechanical properties of alloys as-cast and heat treatment, which studied showed that the as-cast, without adding Cu, Mg-6Zn alloy tensile strength and elongation were the highest for221MPa/19.6%, and after adding Cu, the tensile strength and elongation of Mg-9Zn-Cu and Mg-9Zn-3Cu alloys reached for213MPa/10.7%and225MPa/11.1%respectively. After solution treatment and aging, with increasing Zn content, Mg-xZn alloys showed an increasing trend in the tensile strength, elongation rate was decreased. When the Zn content of9%, the tensile strength reached254MPa, the elongation rate was only7.25%. After added different amounts (1%,3%,5%) of the Cu, the tensile strength gradually decreased after aging160℃for20h, the elongation rate gradually increased, respectively299MPa/12.1%.288MPa/16.5%and262MPa/16.6%. And changed the tensile strength and elongation of Mg-xZn-yCu alloys before and after heat treatment, which were influenced by the different degrees of cleavage fracture and intergranular fracture.Combined with the analysis of the mechanical properties, and selected research the semi-solid micro structure of the Mg-9Zn-yCu(y=1%,3%) alloys, which gradually shifted from cast dendrite structure to spherical particles at different isothermal temperature and isothermal time. While the liquid tissue content of the alloy was mainly affected by isothermal temperature, isothermal time had a great influence on the solid particle morphology and particle size. Determined that the optimum semi-solid process parameters of Mg-9Zn-Cu alloy was610℃+insulation15min, which the average particle size, shape factor and solid fraction were37.53μm,1.65, and48.44%; and which the average particle size, shape factor and solid fraction were41.87μm,1.41and42.86%for Mg-9Zn-3Cu alloy was600℃+insulation15min. Respectively, which can provide suitable for subsequent thixotropy of semi-solid slurry for semi-solid processing. |
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