**2.2 Optimization on stirring melt casting process**

256 Mechanical Engineering

(a) fully-liquid casting process (b) semi-solid casting process (c) stirring-melt casting process

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

(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

Fig. 3. Microstructures of SiCp/AZ61 composites with various volume fractions of SiC

particles in semi-solid stirring casting process

particles in stirring-melt casting process

Fig. 2. Microstructures of SiCp/AZ61 composites in three casting processes

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 helpful for obtaining its good properties.

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


Table 1. Factors and levels of test

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

Study on Thixotropic Plastic Forming of Magnesium Matrix Composites 259

The test results showed that the tensile strength of SiCp/AZ61 composites, that increased as the increasing of volume fraction of SiC particles increasing, and were higher than that of AZ61 (about 165MPa). The tensile strength was up to the maximum 189MPa when the volume fraction of SiC particles was 6%. However, the tensile strength decreased as the volume fraction of SiC particles increased continuously. Comparison with the volume fractions of SiC particles, the change trend of elongation decreased gradually with addition of SiC particles. The reason was a mass of rigid second phase existence in the matrix of SiCp/AZ61 composites, which could improve its rigidity and tensile strength. With the increasing volume fraction of SiC particles, problems of particle packing, agglomerating and clustering were presented in the matrix (Fig. 5), which caused tensile strength to decrease. Decreasing elongation was due to the non-uniform distribution of SiC particles and weak

The fracture morphology of AZ61 matrix at the ambient temperature is shown in Fig.6a. Ductility dimples existed, and cleavage cracks were present in a part. Fig.6b showed the fracture morphology of SiCp/AZ61 composites at the ambient temperature by a better processing plan (A2B3C2). Compared with the fracture morphology of the matrix, the fracture morphology of SiCp/AZ61 composites at the ambient temperature were brittle

(a) AZ61 matrix (b) SiCp/AZ61 composites

(a) Fractographs of SiC particle (b) EDS analysis

cracks in boundaries between the reinforcement and matrix.

where the fractured SiCp particles were found (Fig. 7a).

Fig. 6. SEM of tensile fracture surfaces

Fig. 7. Fractographs of SiC particle and EDS analysis

the best among the levels of factor A. The level three (595℃) was the best among the levels of factor B. The level 2 (5 min) was the best among the levels of factor C. Thus the optimum combination was A2B3C2. (2) Elongation analysis. The level two (the volume fraction of SiC 3%) was the best among the levels of factor A. The level 3 (595℃) was the best among the levels of factor B. The level three (7 min) was the best among the levels of factor C. Thus the optimum combination was A1B3C3. (3) Range Comprehensive Analysis. The greater the Range (R), the greater the effect of the lever change of the factor on the test target. This factor was more important. From Table 2, the sequence of tensile strength was RA>RC>RB. So the sequence of primary and secondary in factors of A, B, C was the volume fraction of SiC particles, stirring time and stirring temperature. The sequence of elongation was RA>RC>RB. So the sequence of primary and secondary in factor of A, B, C was also the volume fraction of SiC particles, stirring time and stirring temperature. Besides, from the ranges(R) in table 1, factor A has a more notable impact for tensile strength and elongation, and other factors do not have great impact. After comprehensive analysis, a better combination of factors was A2B3C2, namely, the optimum processing plan of SiCp/AZ61 composites in the experimental condition was volume fraction of SiC particles 6%, stirring temperature 595℃ and stirring time 5 min.

Fig. 5. Microstructures of SiCp/AZ61 composites with various volume fractions of SiC particles

the best among the levels of factor A. The level three (595℃) was the best among the levels of factor B. The level 2 (5 min) was the best among the levels of factor C. Thus the optimum combination was A2B3C2. (2) Elongation analysis. The level two (the volume fraction of SiC 3%) was the best among the levels of factor A. The level 3 (595℃) was the best among the levels of factor B. The level three (7 min) was the best among the levels of factor C. Thus the optimum combination was A1B3C3. (3) Range Comprehensive Analysis. The greater the Range (R), the greater the effect of the lever change of the factor on the test target. This factor was more important. From Table 2, the sequence of tensile strength was RA>RC>RB. So the sequence of primary and secondary in factors of A, B, C was the volume fraction of SiC particles, stirring time and stirring temperature. The sequence of elongation was RA>RC>RB. So the sequence of primary and secondary in factor of A, B, C was also the volume fraction of SiC particles, stirring time and stirring temperature. Besides, from the ranges(R) in table 1, factor A has a more notable impact for tensile strength and elongation, and other factors do not have great impact. After comprehensive analysis, a better combination of factors was A2B3C2, namely, the optimum processing plan of SiCp/AZ61 composites in the experimental condition was volume fraction of SiC particles 6%, stirring temperature 595℃

(SiC)=0% (b)

(SiC)=6% (d)

Fig. 5. Microstructures of SiCp/AZ61 composites with various volume fractions of SiC

(SiC)=9%

(SiC)=3%

and stirring time 5 min.

(a) 

(c) 

particles

The test results showed that the tensile strength of SiCp/AZ61 composites, that increased as the increasing of volume fraction of SiC particles increasing, and were higher than that of AZ61 (about 165MPa). The tensile strength was up to the maximum 189MPa when the volume fraction of SiC particles was 6%. However, the tensile strength decreased as the volume fraction of SiC particles increased continuously. Comparison with the volume fractions of SiC particles, the change trend of elongation decreased gradually with addition of SiC particles. The reason was a mass of rigid second phase existence in the matrix of SiCp/AZ61 composites, which could improve its rigidity and tensile strength. With the increasing volume fraction of SiC particles, problems of particle packing, agglomerating and clustering were presented in the matrix (Fig. 5), which caused tensile strength to decrease. Decreasing elongation was due to the non-uniform distribution of SiC particles and weak cracks in boundaries between the reinforcement and matrix.

The fracture morphology of AZ61 matrix at the ambient temperature is shown in Fig.6a. Ductility dimples existed, and cleavage cracks were present in a part. Fig.6b showed the fracture morphology of SiCp/AZ61 composites at the ambient temperature by a better processing plan (A2B3C2). Compared with the fracture morphology of the matrix, the fracture morphology of SiCp/AZ61 composites at the ambient temperature were brittle where the fractured SiCp particles were found (Fig. 7a).

Fig. 6. SEM of tensile fracture surfaces

(a) AZ61 matrix (b) SiCp/AZ61 composites

(a) Fractographs of SiC particle (b) EDS analysis Fig. 7. Fractographs of SiC particle and EDS analysis

Study on Thixotropic Plastic Forming of Magnesium Matrix Composites 261

Fig. 9. shows the microstructural evolution of SiCp/AZ61 composites during partial remelting. When the heating temperature reached 590℃ with isothermal holding time of 15min, the grain boundaries had almostly been merged and could not be seen clearly. At the same time, SiC particles were inside the grains away from the grain boundaries (Fig. 9a). A separating tendency in the grains of coalescence emerged with the prolongation of isothermal holding time (Fig. 9b). While the holding time reached 60 min, a few grain boundaries became clear. A few globular grains appeared with SiC particles presented in the grain boundaries, but the liquid volume fraction was lower (Fig.9c). When the reheating temperature increased to about 595℃ with holding time of 15 min, the grain microstructure evolved quickly, and a globular microstructure appeared, then the eutectic structure began to melt (Fig.9d). The grain boundaries appeared completely with holding time 30 min, and fine globular grains emerged. The effective liquid fraction of SiCp/AZ61 composites was about 31%, and the mean diameter of grains was approximately 60µm (Fig.9e). When the isothermal holding time was further increased to 60min, the grain microstructure was entirely spheroidized, which became more clear and round, and SiC particulate returned to the grain boundaries from interior of grains (Fig.9a,b). At the same time, the mean diameter of grains was about 85µm (Fig.9f). As the reheating temperature increased to 600℃ with holding time of 15 min, the microstructural evolution of the sample during remelting was rapid. Some of grains began to spheroidize (Fig.9g). When the holding time reached 30 min, all grains had been spheroidized, whose sizes became relatively fine (Fig.9h). With the prolongation of holding time to 60 min, the grain microstructure tended to spheroidize and increase in size, and the effective liquid fraction was about 37% (Fig.9i). When the reheating temperature was above 610℃, the semi-solid microstructure began to dissolve and disappear. The specimens were susceptible to serious deformation, the liquid flow emerged from the sample, which would prevent semi-solid microstructure from partial remelting (Fig.9j). Therefore the optimal technological parameters of SiCp/AZ61 composites were the reheating temperature of 595℃~600℃ and isothermal holding time of 30min~60min.This temperature interval was suitable for semi-solid thixoforming of SiCp/AZ61 magnesium

The microstructures of SiCp/AZ61 composites during partial remelting (Fig.9e, f) were compared with that of AZ61 alloy (Fig.10). It was observed that the microstructures of SiCp/AZ61 composites coalescenced basically before isothermal holding time at the predetermined temperature for 15min, and a separating tendency in the grains didn't appear obviously. After isothermal holding at the predetermined temperature for 25min, the grain microstructure began separating and spheroidizing. However the rate of separation and spheroidization for AZ61 alloy increased. When the reheating temperature reached 595℃ with holding time of 0min,the grain microstructure was separated completely, and a few globular grains had appeared. With the prolongation of holding time from 20min to 40min, the mean diameter of the globules was 85µm and 110µm respectively. In addition, compared with AZ61 alloy, the microstructures of SiCp/AZ61 composites were finer during partial remelting due to addition of SiC particulates. Coalescence was restricted since the globules were isolated one with respect to the other by the presence of SiC particulates. At the same time the effective diffusion coefficient of the liquid phase was also reduced because of the presence of reinforced particulates, and during the subsequent isothermal

phase was hindered, and Ostwald ripening was also

matrix composites.

holding process coalescence of

restricted.
