Effects of composition, mold temperature, rotating rate and modification on microstructure of centrifugally cast Zn–27Al–xMg–ySi alloys have been investigated. In situ composites of Zn–27Al–6.3Mg–3.7Si and Zn–27Al–9.8Mg–5.2Si alloys were fabricated by centrifugal casting using heated permanent mold. These composites consist of three layers: inner layer segregates lots of blocky primary Mg2Si and a litter blocky primary Si, middle layer contains without primary Mg2Si and primary Si, outer layer contains primary Mg2Si and primary Si. The position, quantity and distribution of primary Mg2Si and primary Si in the composites are determined jointly by alloy composition, solidification velocity under the effect of centrifugal force and their floating velocity inward. Na salt modifier can refine grain and primary Mg2Si and make primary Mg2Si distribute more evenly and make primary Si nodular. For centrifugally cast Zn–27Al–3.2Mg–1.8Si alloy, the microstructures of inner layer, middle layer and outer layer are almost similar, single layer materials without primary Mg2Si and primary Si are obtained, and their grain sizes increased with the mold temperature increasing.

采用热模金属型工艺,离心铸造Zn-27Al-9.8Mg-5.2Si和Zn-27Al-6.3Mg-3.7Si合金,获得了内层聚集大量块状初生Mg2Si、少量初生Si,中层不含初生Mg2Si和初生Si,外层含有初生Mg2Si和初生Si的自生锌基复合材料.离心铸造Zn-27Al-3.2Mg-1. 8Si合金,获得了不含初生Mg2Si和初生Si的单层材料.考察了复合材料的组织形貌,检测了复合材料的硬度和耐磨性,分析了复合材料的断裂模式.结果表明:复合材料的内层因聚集大量的初生Mg2Si和初生Si具有较高的硬度和较优的耐磨性.复合材料的断裂方式为脆性断裂,含共晶Mg2Si和共晶Si的中层在断裂中比含块状初生Mg2Si和初生Si的内层经历了更多的塑性变形.

In situ three-layer surface composites of Zn-27Al-9.8Mg-5.2Si and Zn-27Al-6.3Mg-3.7Si alloys were obtained by centrifugal casting with preheated metal mould, which possesses the microstructures of accumulated lots of blocky primary Mg_2Si and a litter blocky primary Si in inner layer, accumulated primary Mg_2Si and primary Si in outer layer, and without primary Mg_2Si and primary Si in middle layer. The effects of composition, mold temperature, rotating rate and modification on microstructures of centrifugally cast Zn-27Al-Mg-Si alloys were investigated. The results show that the inner and outer layers of Zn-27Al-9.8Mg-5.2Si alloy are respectively thicker than those of Zn-27Al-6.3Mg-3.7Si alloy and have more primary Mg_2Si. During the centrifugal casting of Zn-27Al-6.3Mg-3.7Si alloy, with rotating rate increasing, the thickness of the inner layer increases and the outer layer becomes thinner and the addition of Na-salt can refine grain and primary Mg_2Si, and make primary Mg_2Si distribute more evenly and make primary Si nodular. Moreover, the forming process of composites was also analyzed.

In situ composites of Zn–27Al–6.3Mg–3.7Si alloy have been fabricated by centrifugal casting using the heated permanent mold. This kind of composites is consisted of three layers: inner layer segregates lots of blocky primary Mg2Si and a little blocky primary Si, middle layer does not contain primary Mg2Si and primary Si, outer layer contains a little primary Mg2Si and primary Si. The position, quantity and distribution of primary Mg2Si and primary Si are determined by solidification velocity under the effect of centrifugal force and their floating velocity inward. The inner and outer layer contained primary Mg2Si and primary Si exhibit much higher hardness and wear resistance than the middle layer.

In-situ composite of Zn-27Al-6.3Mg-3.7Si alloy was fabricated by centrifugal casting using the heated permanent mold. This kind of composite is consisted of three layers, inner layer segregates lots of blocky primary Mg_2Si and a little blocky primary Si, middle layer contains no primary Mg_2Si and primary Si, outer layer contains a little. Zn-27Al-6.3Mg-3.7Si alloy solidifies according to following steps: firstly, the precipitation of primary Si and primary Mg_2Si, secondly, the precipitation of Zn-phase enriched by Al, thirdly, the precipitation of Mg-Zn compound, and lastly, the occurrence of ternary or quaternary eutectic reaction. The position, quantity and distribution of primary Mg_2Si and primary Si are determined by solidification velocity under the effect of centrifugal force and their floating velocity inward. The inner and outer layer contained primary Mg_2Si and primary Si exhibit much higher hardness and wear resistance than the middle layer.

The combination of magnesium alloys with the expendable pattern casting (EPC) process will bring a bright future for the application of magnesium alloys. Vacuum is a pre-requisite parameter in the EPC process of magnesium alloys, because without vacuum, the fluidity of the magnesium alloy in the EPC process is too poor to fill the mold completely, especially for the thin-section castings. In this investigation, the effect of vacuum on the fluidity of AZ91 magnesium alloy has been explored. A modified model has been presented to explain the effect of vacuum on mold filling, which was verified by optical microscopy.The results obtained indicate that vacuum is the most effective parameter in improving the fluidity, the effect of vacuum on the fluidity interacting strongly with the pouring temperature and coating, Vacuum greatly changes the mass and heat transfer in the EPC process. Vacuum may not only control the profile of the metal-foam interface, which will influence the mass transfer process, but may also greatly speed up the removal rate of pattern decomposition products at the metal-coating interface. It also changes the primary heat-transfer mode to heat convention, which has a great influence on the distribution of the casting temperature field and solidification process. The microstructures of castings cast with vacuum exhibit a fine grain size and a small amount of precipitated Mg17Al12, but vary insignificantly with the location in the castings. (C) 2002 Elsevier Science B.V. Ail rights reserved.

A new in-situ Zn-Al-Si surface composites was fabricated by using centrifugal casting. A high-volume fraction of primary Si particles in the inner layer was achieved. The formation mechanism of the gradient was discussed. The effect of technological parameters on the composites was investigated. Testing results show that the hardness and wear resistance of the composites depend mainly on the distribution of the primary silicon particles along radial direction. The inner layer of the composite shows the highest hardness and best wear properties. The fracture mode of the composites is primarily brittle. The inner layer of the composite exhibits more deformation than the inner layer during the fracture.

Effects of Sb addition on the microstructure, mechanical properties, and fracture behaviors of AZ91 magnesium alloy, as well as the sensitivity to section thickness of the structure and mechanical properties, have been studied. The results show that when Sb is added into the AZ91 alloy, the grain is refined, the Mg17Al12 phase is refined and granulated, and a new Mg3Sb2 phase is formed and becomes coarse needle-shaped as Sb content increases. The room-temperature tensile strength, elongation, and impact toughness increase first, and then decrease with increasing Sb content. The study on sensitivity to section thickness shows that, when composition is constant, the room-temperature tensile strength and elongation increase with the reduction of section thickness; when section thickness is constant, the room-temperature tensile strength and elongation increase first, and then decrease with increasing Sb content. Additionally, the Sb addition improves the tensile strength of the AZ91 alloy at 100 °C and 150 °C. The room-temperature tensile and impact fractographs of the AZ91 alloy show intergranular fracture. With increasing Sb content, the tearing deformation zones on the both fractographs enlarge at first, and then diminish, which is consistent with the change of tensile strength, elongation, and impact toughness increasing first, and then reducing with increasing Sb content.

The effects of Ca addition on the microstructure and mechanical properties of AZ91 magnesium alloy have been studied. The results show that the Ca addition can refine the microstructure, reduce the quantity of Mg17Al12 phase, and form new Al2Ca phase in AZ91 magnesium alloy. With the Ca addition, the tensile strength and elongation of AZ91magnesium alloy at ambient temperature are reduced, whereas Ca addition confers elevated temperature strengthening on AZ91 magnesium alloy. The tensile strength at 150°C increases with increasing Ca content. The impact toughness of AZ91magnesium alloy increases, and then declines as the Ca content increases. The tensile and impact fractographs exhibit intergranular fracture features, Ca addition changes the pattern and quantity of tearing ridge, with radial or parallel tearing ridge increasing, tensile strength, elongation and impact toughness reduce.