**2. Study on fabrication methods and various properties for magnesium matrix composites**

### **2.1 Fabrication methods**

The AZ61 alloy was used as the matrix material. The chemical composition of AZ61 was 5.8%~7.2%Al, 0.15% Mn, 0.40%~1.5%Zn, 0.10% Si, 0.05%Cu, 0.05%Ni, 0.005%Fe, and the rest is Mg. Its solidus temperature was 525℃, and the liquidus temperature was 625℃. The reinforcement was green -SiC particle whose average diameter was 10*<sup>m</sup>* . A selfmanufactured electric resistance furnace was used for melting Mg alloy (shown in Fig.1). The liquid metal was stirred with the mechanical stirrer driven by the timing electrical machine in the melting process. In order to improve the accuracy of controlling temperature, the thermocouple was inserted directly into the liquid metal, and combined with the artificial aptitude modulator BT608 that adopted the industrial micro-processor, whose precision was only 士10℃. The MMCs specimens were sampled by the pipette connecting to a vacuum pump in this experiment.

The fabrication processes of SiCp/AZ61 composites were described as follow. AZ61 alloy matrix was heated to melt, gas was gotten rid of and slag was removed. Then SiC reinforcement was added into the molten. There were the three addition processes. In the fully-liquid stirring casting process (about 680℃), SiC particles were introduced into the fully-liquid molten, and then sampled after stirred to reach predetermined time. (2) In the stirring-melt casting process (590℃), SiC particles were introduced at the semi-solid state, then sampled after reached to a fully-liquid temperature of 680℃. (3) In semi-solid stirring casting process, SiC particles were introduced at the semi-solid state, then sampled after stirred to reach predetermined time. In above experiments, their volume fractions of SiC particles were 3%, 6%, and 9% respectively, whose preheated temperature was 500℃ with holding time 2h. The stirring rate was 500r/min with holding time 10min. Then the specimen was made to the metallurgical phase sample and corrupted with 0.5℅ ammonium HF liquor, and its microstructural changes were observed under the optical microscope. Finally, the Vickers hardness was measured in a micro-sclerometer HXS-1000AK.

technology of magnesium matrix composites is a new method. The semi-solid material forming technology has advantages such as lower deformation resistance, good material mobility and so on [Flemings 1991, Yan et al., 2005]. It was composed of three processes such as: semi-solid billet fabrication [Yan et al., 2005], partial remelting [Yan et al., 2006] and thixoforming [Yan et al., 2008]. For this reason, the research on the basic theory of semi-solid stirring melting fabrication method and thixoforming process for the advanced MMCs is studied in this item. The works include the study of semi-solid stirring melting fabrication method [Yan & Fu et al., 2007; Yan & Lin et al., 2008] and reheating process [Yan & Zhang et al., 2008; Zhang et al., 2011] for the particle-reinforced MMCs. The material constitutive relation will be proposed [Yan & Wang et al., 2011]. Then the finite element model coupled with multi-physical fields will be built. The simulation will be gone based on the developed analytical program. The forming performances and deformed laws in the thixoforming for the particle-reinforced MMCs will be studied by the way of combining theoretical analysis with experimental method [Yan & Huang, 2011]. The results will play an important function to bulid the theoretical and technological fundament for the thixoforming process of the

**2. Study on fabrication methods and various properties for magnesium** 

The AZ61 alloy was used as the matrix material. The chemical composition of AZ61 was 5.8%~7.2%Al, 0.15% Mn, 0.40%~1.5%Zn, 0.10% Si, 0.05%Cu, 0.05%Ni, 0.005%Fe, and the rest is Mg. Its solidus temperature was 525℃, and the liquidus temperature was 625℃.

manufactured electric resistance furnace was used for melting Mg alloy (shown in Fig.1). The liquid metal was stirred with the mechanical stirrer driven by the timing electrical machine in the melting process. In order to improve the accuracy of controlling temperature, the thermocouple was inserted directly into the liquid metal, and combined with the artificial aptitude modulator BT608 that adopted the industrial micro-processor, whose precision was only 士10℃. The MMCs specimens were sampled by the pipette connecting to

The fabrication processes of SiCp/AZ61 composites were described as follow. AZ61 alloy matrix was heated to melt, gas was gotten rid of and slag was removed. Then SiC reinforcement was added into the molten. There were the three addition processes. In the fully-liquid stirring casting process (about 680℃), SiC particles were introduced into the fully-liquid molten, and then sampled after stirred to reach predetermined time. (2) In the stirring-melt casting process (590℃), SiC particles were introduced at the semi-solid state, then sampled after reached to a fully-liquid temperature of 680℃. (3) In semi-solid stirring casting process, SiC particles were introduced at the semi-solid state, then sampled after stirred to reach predetermined time. In above experiments, their volume fractions of SiC particles were 3%, 6%, and 9% respectively, whose preheated temperature was 500℃ with holding time 2h. The stirring rate was 500r/min with holding time 10min. Then the specimen was made to the metallurgical phase sample and corrupted with 0.5℅ ammonium HF liquor, and its microstructural changes were observed under the optical microscope.

Finally, the Vickers hardness was measured in a micro-sclerometer HXS-1000AK.


*<sup>m</sup>* . A self-

particle-reinforced MMCs applied the industry area.

**matrix composites 2.1 Fabrication methods** 

The reinforcement was green

a vacuum pump in this experiment.

Fig. 1. Schematic diagram of SiCp/AZ61 composites fabricated in stirring casting process 1. thermocouple 2. resistance thread 3. crucible 4. BT608 (artificial aptitude modulator) 5. resistance furnace 6. strring lamina 7. vacuum jar 8. pressure meter 9. pipette 10. vacuum pump 11. guiding windpipe 12. timing electrical machine 13. stirring bar

The microstructures of SiCp/AZ61 composites in three casting processes were shown in Fig.2. The variations of influence of three casting processes on the microstructures of SiCp/AZ61 composites were shown in Fig.2- Fig.4. The distribution of SiC particles was a little uniform in the fully-liquid casting process where a lot of gas cavities and slacks were presented, and SiC particles were easy to sink and float. There were a few gas cavities in the semi-solid casting process where the distribution of SiC particles was inhomogeneous. SiCp/AZ61 composites fabricated by the stirring-melt casting method possessed not only few gas cavities but fairly uniform distribution.

The existence of gas cavities and slacks was attributed to the following factors: (1) Gas was involved in the molten during the mechanical stirring process. (2) Their non-uniform volume shrinkages presented in the composites solidification process due to the differences of their thermal expansion coefficient and heat conduction between matrix and reinforcement. (3) Hydrogen produced in the chemical reactions between Mg and H2O was dissolved in the molten and formed gas cavities during solidification. (4) The formation of gas cavities was resulted from the particles clustering.

The main problem in the stirring casting process was the inhomogeneous distribution of reinforcement phase. The major reasons were followed as. (1) Due to having the different densities between matrix and reinforcement, SiC particles were settled down. (2) The higher surface tension and poor wettability between SiC and matrix presented, a few SiC particles were floated on the surface of the molten.

SiC particles were introduced at the semi-solid state during the semi-solid stirring casting process where the high viscosity semi-solid alloy can help withstand SiC particles from sinking and floating, but the uniform distribution can not be solved (shown in Fig.3). During the stirring-melt casting process the reinforcement were added at the semi-solid state, and the composites were poured immediately after reached 690℃ (liquidus). The quite uniform SiCp/AZ61 composites can be obtained in this method (shown in Fig.4).

Study on Thixotropic Plastic Forming of Magnesium Matrix Composites 257

In this study, the composites were fabricated by a stirring melt casting method. The effects of volume fraction of SiC particles, stirring temperature and stirring time on the mechanical properties and microstructure of SiCp/AZ61 composites were investigated. The main technological parameters of preparing SiCp/AZ61 composites were optimized, which was

The effects of volume fraction of SiC particles, stirring temperature and stirring time on the mechanical properties of SiCp/AZ61 composites were investigated by an orthogonal experimental method, in which average particle size and stirring speed were maintained the same. The orthogonal test table with three factors and three levers is shown in Table 1. According to design of the primary experiment, volume fractions of SiC particles were 3%, 6% and 9%, stirring temperatures were 580℃, 587℃ and 595℃, and stirring times were 3min, 5min and 7min. Three factors were volume fraction of SiC particles, stirring temperature

The effects of volume fraction of SiC particles, stirring temperature and stirring time on the tensile strength and elongation at room temperature of SiCp/AZ61 composites are shown in Table 2. (1) Tensile Strength Analysis. The level two (the volume fraction of SiC 6%) was

Table 2. The table of three factors and three levels in the orthogonal experiment

**2.2 Optimization on stirring melt casting process** 

and stirring time. Two targets were tensile strength and elongation.

helpful for obtaining its good properties.

Table 1. Factors and levels of test

(a) fully-liquid casting process (b) semi-solid casting process (c) stirring-melt casting process Fig. 2. Microstructures of SiCp/AZ61 composites in three casting processes

(a) 500r/min, 10min, 3Vol.% (b) 500r/min, 10min, 6Vol.% (c) 500r/min, 10min, 9Vol.%

Fig. 3. Microstructures of SiCp/AZ61 composites with various volume fractions of SiC particles in semi-solid stirring casting process

(a) 500r/min, 10min, 3Vol.% (b) 500r/min, 10min, 6Vol.% (c) 500r/min, 10min, 9Vol.% Fig. 4. Microstructures of SiCp/AZ61 composites with various volume fractions of SiC particles in stirring-melt casting process
