**3. Semi-solid isothermal heat treatment technology for the partial remelting of composites**

In this study, semi-solid isothermal heat treatment technology was used for the partial remelting of composites. The round semi-solid microstructure had been obtained by controlling the reheating processing parameters such as heating temperature and isothermal holding time. The law of microstructural evolution in the remelting process of SiCp/AZ61 composites was investigated, which was expected to offer some theoretical references for the design of thixoforming technology.

The reheating temperatures were taken as 590℃, 595℃, 600℃ and 610℃ respectively with isothermal temperature heat treatment times of 15min, 30min and 60min. When the scheduled time and temperature were reached, the specimen was taken out and water quenched. Then the specimens were made and etched with 4% nitic acid liquor, and its microstructure change was observed under the optical microscope. The Image-pro Plus software was used to measure the diameter of equal-area of microstructure. The average radius of grain microstructure was then calculated.

According to the Sheil equation (1) and equation (2), the liquid phase volume fraction of the partial remelting structure was calculated (shown in Fig.8).

$$f\_L = \left(\frac{T\_m - T}{T\_m - T\_L}\right)^{-1\mathfrak{f} - \mathbb{K}\_0} \tag{1}$$

$$f\_E = f\_S(1 - f\_P) - f\_P \tag{2}$$

Where *Lf* , *Ef* and *Pf* represent the liquid phase volume fraction of matrix alloy, effective liquid phase volume fractions of composites and enforcing particles. *Tm* and *TL* are melting point of the pure metal and the liquidus temperature of the alloy. *K*0 is represented for the coefficient of distribution.

Fig. 8. Relationship between liquidphase volume fraction and temperature

**3. Semi-solid isothermal heat treatment technology for the partial remelting** 

In this study, semi-solid isothermal heat treatment technology was used for the partial remelting of composites. The round semi-solid microstructure had been obtained by controlling the reheating processing parameters such as heating temperature and isothermal holding time. The law of microstructural evolution in the remelting process of SiCp/AZ61 composites was investigated, which was expected to offer some theoretical references for the

The reheating temperatures were taken as 590℃, 595℃, 600℃ and 610℃ respectively with isothermal temperature heat treatment times of 15min, 30min and 60min. When the scheduled time and temperature were reached, the specimen was taken out and water quenched. Then the specimens were made and etched with 4% nitic acid liquor, and its microstructure change was observed under the optical microscope. The Image-pro Plus software was used to measure the diameter of equal-area of microstructure. The average

According to the Sheil equation (1) and equation (2), the liquid phase volume fraction of the

*m L*

Where *Lf* , *Ef* and *Pf* represent the liquid phase volume fraction of matrix alloy, effective liquid phase volume fractions of composites and enforcing particles. *Tm* and *TL* are melting point of the pure metal and the liquidus temperature of the alloy. *K*0 is represented for the

*T T*

*T T*

*K*

(1)

( ) 1 *ES P P ff f f* (2)

<sup>0</sup> 1 1

**of composites** 

design of thixoforming technology.

coefficient of distribution.

radius of grain microstructure was then calculated.

partial remelting structure was calculated (shown in Fig.8).

*L m f*

Fig. 8. Relationship between liquidphase volume fraction and temperature

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 matrix composites.

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 holding process coalescence of phase was hindered, and Ostwald ripening was also restricted.

Study on Thixotropic Plastic Forming of Magnesium Matrix Composites 263

(a) 595 0C, 0min (b) 595 0C, 20 min (c) 595 0C, 40 min Fig. 10. Microstructures of semi-solid AZ61 magnesium alloy billet during partial remelting

The characteristics of semi-solid composites deformed mechanism can be understood well only when the relationships between stress and strain are described. So the semi-solid compression tests for SiCp/AZ61 composites were conducted, whose mechanical properties

The experiments were conducted in a Thermecmastor-Z dynamic material testing machine, whose set-up was shown in Fig.11. The specimen was heated by electromagnetic wave, whose temperature was monitored by thermocouples. The graphite slices were placed between the specimen and the compression heads for reducing the influence of friction on experiment. In order to study and master the characteristic mechanics of semi-solid magnesium matrix composites at high solid volume fractions, the deformation temperatures were taken as 530℃, 545℃, 560℃ and 570℃ respectively. According to the heating procedure shown in Fig.12, the initial heating rate was 10℃/s; when the specimen temperature reached 500℃, the temperature rate was down to 1℃/s. Then the semi-solid compression experiments were done under the strain rates of 0.1s-1, 0.5 s-1, 5.0 s-1and 10 s-1

The stress-strain curves of semi-solid SiCp/AZ61 composites with various volume fractions of SiC particles are shown in Fig.13. The tendency of curves implies that the deformation temperature has a significant effect on the flow stress. It is observed that for a constant strain

**4. Thixotropic compression deformation behavior of composites** 

and destruction model were investigated.

respectively, in which the total strain was 0.6.

Fig. 11. Schematic diagram for the compressive tests

Fig. 9. The microstructural evolution of SiCp/AZ61 composites during partial remelting

(a) 590 0C, 15min (b) 590 0C, 30min (c) 590 0C, 60min

(d) 590 0C, 15min (e) 590 0C, 30min (f) 590 0C, 60min

(g) 600 0C,15min (h) 600 0C, 30min

(i) 600 0C, 60min (j) 610 0C, 60min

Fig. 9. The microstructural evolution of SiCp/AZ61 composites during partial remelting

Fig. 10. Microstructures of semi-solid AZ61 magnesium alloy billet during partial remelting
