【Citation Format】
LU Caixuan, WU Hao, MA Heng, et al. Study on Spheroidization of Primary Si in Pereutectic Al-Si Alloys by Heat Treatment[J]. Special Casting & Nonferrous Alloys,2024,44(3):372-376.
Citation:LU C X，WU H，MA H，et al. Spheroidization of primary silicon in hypereutectic Al-Si alloy by heat treatment［J］. Special Casting & Nonferrous Alloys，2024，44（3）：372-376.
Aluminum alloy is widely used as a structural material in the power industry due to its light weight, good corrosion resistance and weather resistance. Power fittings such as overhang wire clamps and tension clamps are mainly made of cast Al-Si alloy, which is easy to heat up due to direct contact with wires in the actual use process, and often has problems such as size changes and reduced anti-occlusion load capacity. Therefore, the properties of subeutectic and eutectic Al-Si alloys are difficult to meet the requirements of use, and it is necessary to develop supereutectic Al-Si alloys with smaller thermal expansion coefficients and better heat resistance and wear resistance than subeutectic and eutectic Al-Si alloys. Due to the increase of Si content, the appearance of sharp edges and corners and coarse size of primary Si in the solidification structure makes the alloy prone to cracks, splits the matrix, reduces the performance, and seriously limits the application of supereutectic Al-Si alloys.
From the perspective of reducing the density and thermal expansion coefficient of the material, and improving the dimensional stability and wear resistance of the material, it is very advantageous to increase the Si content in the supereutectic Al-Si alloy, and the key to solving this problem is to control the morphology of the Si phase, and the metamorphic treatment and appropriate heat treatment process can effectively control the morphology of the second phase elements in the alloy, which is an important means to improve the performance of Al-Si alloy.
NARI Group Co., Ltd., State Grid Electric Power Research Institute, Wuhan NARI Co., Ltd., Zhejiang Huadian Equipment Testing and Research Institute Co., Ltd. and other joint research teams published an article entitled "Research on Heat Treatment Spheroidization of Primary Si of Pereutectic Al-Si Alloys" in the journal "Special Casting and Nonferrous Alloys", Volume 44, No. 3, 2024. Semi-solid heat treatment promoted the homogeneity and spheroidization of primary Si, and the optimized treatment process was determined to improve the alloy properties. The results showed that the optimal parameters of solid-state diffusion heating were 540 °C for 12 h, and the primary Si gradually grew and passivated, while the optimal parameters of semi-solid heating were 615 °C for 15 min, and the morphology of primary Si was significantly improved and spheroidized, and the shape factor could reach more than 0.80.
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【Methodology】
The test material is a supereutectic Al-24Si alloy, and its chemical composition is shown in Table 1.
The process of metamorphic treatment of Al-24Si alloy mainly includes: preheating the dry alloy ingot at 200 °C, heating to 800 °C, after the alloy is completely melted, adding ZnS detergent powder to press it into the bottom of the alloy liquid and stirring evenly, holding it for 30 min and then cooling to 700 °C when pouring the sample, see Figure 1. Finally, the specimen is cut into uniform cylindrical short pieces, which are polished with metallographic water frosting paper and mechanically polished by a polishing machine to obtain the specimen to be used.
Fig.1 Schematic diagram of the deterioration process
In order to further improve the phenomenon of uneven microstructure distribution in the metamorphic Al-24Si alloy, the metamorphic Al-24Si alloy samples were heated to promote the uniform distribution and spheroidization of primary Si in the alloy.
In order to improve the microstructure of the Si phase, especially the primary Si microstructure, the sample was subjected to solid-state diffusion heating treatment, so that the Si phase was more dissolved into the aluminum matrix, and the morphology of the primary Si could be improved through the diffusion of Si atoms. The temperature of solid-state diffusion heating should be controlled below the eutectic temperature, and it is advisable to avoid the growth of primary Si caused by too high heating temperature, and the temperature of solid-state diffusion heating treatment in this experiment is 540 °C. In order to observe the evolution of primary Si at different times, the holding time was 6, 12 and 24 h, respectively. The cooling method after solid-state diffusion heating is water cooling, which fixes the tissue obtained by high-temperature diffusion. The test scheme of solid-state diffusion heat treatment is shown in Table 2.
(2) Semi-solid state heat treatment The primary Si structure in the supereutectic Al-Si alloy after solid diffusion heat treatment will be improved, but not obviously. In order to further improve the morphology of primary Si and spheroidize primary Si particles, a semi-solid heat treatment method was used to spheroidize primary Si. Secondary heating involves heating the specimen in a temperature range within the semi-solid temperature range. The equilibrium phase diagram method can be used to determine the temperature range of the liquid-solid interval, i.e., the liquidus temperature is 730 °C and the solidus temperature is 577 °C. Therefore, the secondary heating temperature was 600, 615, 630 and 645 °C, and the holding time was 5, 15 and 30 min, respectively, and water cooling was used. The semi-solid heat treatment test scheme is shown in Table 3.
After conventional grinding, the microstructure was observed and analyzed using an optical microscope, and the average grain size of the alloy was calculated using Image Pro Plus 6.0.
【Research Results】
The initial Al-24Si alloy structure is mainly dendrite structure, and the slat-like primary Si phase and needle-like eutectic Si are unevenly distributed, and the coarse primary Si directly affects the properties of the material. Solid diffusion heating treatment at 540 °C showed that the primary Si phase was uniformly distributed after 6 h of insulation, more uniformly distributed after 12 h of insulation, and the corners were passivated, and the size of the primary Si phase increased after 24 h, and the homogenization became worse.
Fig.2 Microstructure of Al-24Si alloy at 540 °C with different holding times
With the extension of holding time, the average diameter and shape factor of primary Si increased. According to the comprehensive analysis, 12 h is the best holding time for solid-state diffusion heating of Al-24Si alloy.
Fig.3. Effect of different holding time on the average diameter and shape factor of primary Si
Secondary heating, also known as partial remelting, is a heating process in which the prepared secondary blank is heated to a certain temperature in the semi-solid temperature range to carry out thixomorphing. The supereutectic Al-Si alloy was heated to the liquid-solid two-phase region, and the changes of primary Si particles in the secondary heating were studied, so as to understand the effect of secondary heating on the primary Si morphology. At 600 °C, the sharp corners of the primary Si have been passivated, but a large number of sharp corners remain, and the eutectic Si is in the form of discontinuous elongated strips. After holding at 615 °C for 15 min, the sharp corners of the primary Si were passivated obviously, and only a small number of sharp corners existed, and most of the eutectic Si was rounded and short rod-shaped. After holding at 630 °C for 15 min, the passivation of primary Si was very obvious, and the sharp corners were basically absent, and the unmelted α phase and less liquid phase appeared.
Fig.4 Microstructure and morphology of Al-24Si alloy after holding for 15 min at different secondary heating temperatures
After incubation at 645 °C for 15 min, the sharp corners of the boundary of primary Si in the tissue could not be seen, and the unmelted α phase decreased and the liquid phase increased.
For the spheroidization of primary Si, the higher the temperature of the secondary heating, the better, if the temperature is too high, the morphology of the primary Si will deteriorate. After holding at 660 °C for 15 min, the primary Si grew new sharp corners at the boundary, and the spheroidization effect deteriorated, indicating that in addition to the dissolution of the primary Si phase in the secondary heating, there was also a process of growth, and in addition, the α phase became dendritic.
For the secondary heating of Al-24Si alloy in semi-solid state, the higher heating temperature can provide energy for the dissolution and growth of the primary Si phase, so the morphology of the primary Si is different at different heating temperatures. The average diameter of primary Si increases with the heating temperature, and its growth rate is proportional to the heating temperature. Before 630 °C, the size of primary Si was mostly below 42 μm, and when the heating temperature was 660 °C, the average diameter of primary Si was 49.74 μm. The shape factor of primary Si showed a parabolic trend, reaching a maximum of 0.82 at 645 °C. Compared with 0.65 before solid-state diffusion heating, it is an increase of 26.2%. At temperatures above 615 °C, the α phase occurs because the α phase is softer and more massively distributed, which theoretically reduces the wear resistance of the material. Based on the analysis of the average diameter, shape factor and microstructure, the optimal secondary heating temperature of Al-24Si alloy was 615 °C.
Fig.5 Effect of different heating temperatures on the average diameter and shape factor of primary Si
When incubated for 5 min, the primary Si in the tissue had a certain spheroidization, and the eutectic Si was long and short rod-shaped. After holding for 15 min, the spheroidization effect of primary Si was better, the sharp corners of primary Si were few, and the eutectic Si was mostly short rod-shaped, and some spherical Si were still present. When the temperature was kept for 30 min, the spheroidization effect of primary Si was very good, but there was a phenomenon of overlapping of primary Si particles, and the tissue contained more liquid phase and α phase.
Fig.6 Microstructure and morphology of Al-24Si alloy at 615 °C under different holding times
With the extension of holding time, the size of primary Si gradually increases. The average diameter of primary Si was 39.47 μm, 40.89 μm for 15 min, and 42.72 μm for 30 min. With the extension of holding time, the morphology of primary Si was improved, and the spheroidization effect was the best when holding for 30 min, but at this time, α phase appeared in the alloy microstructure and the microstructure deteriorated. According to the comprehensive analysis, the optimal holding time of Al-24Si alloy for secondary heating is 15 min.
Fig.7. Effect of different holding time on the average diameter and shape factor of primary Si
The principle of solid-state diffusion heating is based on the diffusion motion of atoms. In the casting process, the solidification and cooling rate of the alloy is fast, the alloying elements have no time to diffuse, the alloy is a non-equilibrium supersaturated solid solution, and there is a non-equilibrium metastable phase between the grain boundary and the dendrite. By holding the alloy for a long time at a solid diffusion heating temperature, atomic diffusion intensifies, and a series of microstructure changes occur in the alloy. In the early stage of heat preservation, most of the primary Si existed in irregular block and plate shape, and the distribution was uneven, with obvious aggregation state, forming a serious segregation zone. With the extension of holding time, the solute atoms began to dissolve and diffuse, and the size of the second phase became smaller, showing a spheroidal trend, which manifested as the decomposition of lumpy and flaky primary Si into small pieces, and some large particles were not fully dissolved, and there were also undissolved tissues. As the diffusion motion progresses, the second phase between the grains is more fully dissolved, but the size of the particles formed in the alloy is not the same. There is a solute concentration gradient from high to low between the matrix around the small particles and the large particles, and the solutes around the small particles have a tendency to diffuse around the large particles, and the system is in a metastable state. With the intensification of atomic diffusion, the concentration of solute around the small particles decreases to less than the concentration in the metastable equilibrium state, and the size of the small particles decreases and dissolves, while the concentration around the large particles is higher than their own solubility, which promotes the growth of the large particles, and in order to minimize the interface energy, the large particles grow in multiple dimensions and form a spherical shape.
Analyzing the evolution mechanism of secondary heating, it is found that at the lower heating temperature and the initial stage of heat preservation, the liquid phase is less, and the low melting point phase that melts first is limited to some α-Al grain boundaries. In order to reduce surface energy, the grains tend to aggregate with each other, and some grains will come into contact and grow together. The merger and growth of grains is a spontaneous process of decreasing the interfacial area and reducing the energy of the system. As the liquid phase increases, the liquid phase fills almost all the grain boundaries, which reduces the possibility of merging and growing at the low-angle grain boundaries, and when the solute atom diffusion between the solid and liquid phases reaches equilibrium, the solid phase will gradually grow driven by interfacial energy. At this point, the Ostward growth mechanism is dominant, and the difference in grain radius leads to different equilibrium concentrations in its vicinity, thus creating a concentration gradient. With the extension of holding time and the increase of heating temperature, the large size grains continue to grow, and the small size grains gradually decrease or even disappear. Based on this, in the secondary heating process, with the increase of heating temperature and the extension of holding time, the grains merged and grew, Ostward grew and rounded.
Fig.8. Schematic diagram of the spheroidization principle
【Conclusions】
(1) In the process of solid-state diffusion heating of Al-24Si alloy, the primary Si gradually grows and passivates with the extension of holding time, but after a long period of heat preservation, the ideal spheroidization effect is not obtained. The optimal solid-state diffusion heating temperature of Al-24Si alloy was 540 °C and the holding time was 12 h.
(2) During the secondary heating process, the structure of Al-24Si alloy underwent eutectic Si fuse, a small amount of liquid phase formation in the gap between the unmelted α phase, and the volume expansion of the liquid phase surrounded α phase, and the primary Si spheroidization occurred in the secondary heating.
(3) With the increase of secondary heating temperature or the extension of holding time, the primary Si showed a trend of spheroidization growth, which could be attributed to grain merging growth, Ostward growth and rounding. The optimal secondary heating parameters of Al-24Si alloy were 615 °C and 15 min of insulation.