The Effect of CRITSIMA Process Parameters on the Microstructure Evolution and Element Segregation of Semi-Solid CuSn10P1 Alloy Billet

Abstract

In this paper, based on the as-cast CuSn10P1 alloy. Semi-solid CuSn10P1 alloy billet was prepared by cold-rolled isothermal treatment strain-induced melting activation (CRITSIMA). The effects of cold-rolling reduction, isothermal temperature, and isothermal time on the microstructure of semi-solid copper alloy billet were studied by metallographic microscope and Image-Pro Plus software. The changes of primary elements in as-cast and semi-solid microstructure were analyzed briefly by a scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS). The results show that with the increase of cold rolling reduction, the average grain diameter of semi-solid microstructure decreases gradually, the average grain roundness increases first and then decreases, and the liquid fraction of the microstructure remains unchanged. During semi-solid isothermal treatment, with the increase of isothermal temperature and the extension of isothermal time, the average grain diameter increases gradually, the average grain roundness increases first and then decreases, and the liquid fraction increases gradually. When cold rolling reduction is 30%, isothermal temperature is 900 ℃, and isothermal time is 20 min, a better microstructure can be obtained. The average grain diameter, average grain roundness, and liquid fraction of semi-solid alloy billet are 66.45 μm, 0.71, and 12.78%, respectively. Sn and P diffuse from the intergranular liquid to the grain inside during the isothermal treatment from as-cast to semi-solid.

1. Introduction

Semi-solid metal forming technology is a forming technology that relies on the coexistence state of the liquid phase and solid phase [1,2]. This forming method has the advantages of casting and forging, which only require one forming, and has good mechanical properties. At the same time, it has the characteristics of a smooth filling process, good formability of parts, high density, and a short forming cycle. It is considered one of the near-net forming technologies with development prospects [3,4,5]. In semi-solid metal forming technology, it is necessary to prepare semi-solid billet with uniform distribution of solid and liquid phases and fine microstructure, which affects the quality of subsequent processing and forming. Therefore, it is necessary to prepare semi-solid metal billets of good quality.

Thixoforming is a semi-solid metal processing type that has significant advantages such as near-net forming, low forming temperature, and prolonged die life [6,7]. The thixoforming process is generally divided into semi-solid billet preparation, secondary remelting, and forming [8]. In this paper, the preparation process of semi-solid billet was studied. The semi-solid billet was obtained by accumulative reduction of cold rolled as-cast copper alloy and isothermal treatment in the semi-solid temperature range. The non-dendritic microstructure largely determines the thixotropic properties of semi-solid billets, which can be obtained by several preparation methods summarized by Atkinson [9]. Among them, the strain-induced melting activation method (SIMA) does not require a liquid metal stirring process, so it has higher production efficiency and lower production equipment cost. Semi-solid billet prepared by the SIMA method has high density, no pollution, and a wide application range. It has a unique and large commercial advantage for preparing high melting point and non-dendrite alloys. At present, researchers have carried out extensive research on semi-solid billet preparation by the SIMA method. The influence of different pre-deformation processes and isothermal treatment parameters on the microstructure and properties of semi-solid billets were studied, and the semi-solid billets with better performance were prepared. At the same time, the evolution law of semi-solid microstructure was also studied, which provides a reference for the preparation of semi-solid billets by the SIMA method [10,11,12,13]. Generally speaking, the coarsening rate of the semi-solid microstructure and the melting rate jointly determine the average grain diameter and average grain roundness during isothermal treatment [14]. The semi-solid microstructure’s average grain diameter and average grain roundness will affect the final mechanical properties. Therefore, it is necessary to study grain coarsening behavior and understand its coarsening law to prepare semi-solid parts with good mechanical properties. At present, most of the research work on semi-solid billet preparation is aimed at low melting point alloys such as aluminum alloy and magnesium alloy, and relatively little research on semi-solid processing of high melting point alloys such as copper alloy. The earliest research on semi-solid copper alloy processing was the semi-solid slurry of C905 copper alloy prepared by Young et al. [15], and the parts were made by rheological forming. Cao et al. [16,17] proposed and used rotary swaging strain-induced melt activation (RSSIMA) to prepare C5191 and C3771 semi-solid copper alloys. This RSSIMA process is improved based on the SIMA method, and higher mechanical properties of semi-solid copper alloy slurry can be prepared. Youn et al. [18] studied the semi-solid billet forming of Cu–Ag alloy and made parts of a small induction motor. The study found that lower filling speed is conducive to the filling of semi-solid slurry. Shao et al. [19] prepared QSn7-0.2 copper alloy parts using a semi-solid extrusion process. It was found that the distribution of solid and liquid phases in different sections of the parts was significantly different. However, the overall microstructure was dense, and the spheroidization effect was good. Chen et al. [20] studied the effect of Cu content on the mechanical properties of Ti–Cu alloy in semi-solid forming. The results showed that the increase of Cu content would improve the plasticity and toughness of the alloy.

In this paper, the as-cast CuSnl0P1 copper alloy is taken as the research object. Due to the severe reverse segregation of cast CuSnl0P1 copper alloy, it is easy to crack during hot working by using the traditional SIMA method. Based on the SIMA method, a new strain-induced melting activation method, CRITSIMA, was proposed to prepare semi-solid billets. The effects of cold rolling reduction, isothermal temperature and isothermal time on the microstructure of semi-solid billet were studied. Coarsening behavior of grains in the semi-solid temperature range were studied. The distribution of elements in tissue before and after isothermal treatment was analyzed. It provides a theoretical basis for preparing semi-solid copper alloy billet and a theoretical reference for applying semi-solid copper alloy forming technology.

2. Materials and Methods

2.1. Materials

As-cast CuSnl0P1 copper alloy was used in the experiment. The chemical composi-tion of the material is shown in

Table 1. Chemical composition of as-cast CuSn10P1 copper alloy material (wt.%).

Sn	P	Fe	Pb	Cu
10.2	0.9	0.02	0.05	Bal.

.

STA449F3 synchronous thermal analyzer was used for differential scanning calorimetry (DSC). The solidus temperature was 876.1 °C and the liquidus temperature was 1024.2 °C, as shown in Figure 1. The semi-solid microstructure can be obtained by isothermal treatment of the alloy in this temperature range.

2.2. SIMA, RAP and CRITSIMA Processes

As shown in Figure 2, the traditional SIMA method was first proposed by Young [21] in 1983. The cast alloy is deformed by hot working processing at the recrystallization temperature and then cooled to room temperature for cold deformation processing. Finally, the deformed billet is heated to the semi-solid temperature range for isothermal processing to obtain the semi-solid billet. Hot working processing above recrystallization temperature improves grain anisotropy in as-cast microstructure, and cold deformation processing at room temperature stores deformation energy and provides energy for grain spheroidization during isothermal treatment. Compared with the liquid phase method, the traditional SIMA method is short in process, simple in operation, and does not require large complex equipment, so scholars have widely studied it in China and abroad. However, the SIMA method requires multi-pass pre-deformation treatment at different temperatures, which includes many processes and is not suitable for preparing large-size billets, so it needs further improvement. In 1992, the RAP method (Recrystallization and Remelting) was put forward demand by Kirkwood [22], the technique is improved based on the SIMA method. As shown in Figure 2, in the RAP method, “warm working” under the recrystallization temperature instead of “hot working + cold deformation processing” considerably shortened the preparation process of semi-solid billet, thus improving production efficiency.

Because the ternary eutectoid phase (α + δ + Cu₃P) in as-cast CuSnl0P1 copper alloy makes the alloy have hot cracking properties and severe cracking will occur when it is deformed by hot working, the semi-solid CuSnl0P1 copper alloy cannot be prepared by SIMA and RAP methods mentioned above. In the research group, Wang et al. [23] compared the microstructure of semi-solid billets prepared by isothermal treatment after various cold deformation methods. The results showed that CRITSIMA is not only easy to operate and control, but also that its semi-solid grain microstructure is fine and uniform, and its spheroidization effect is good. On this basis, the process of “cold rolling + isothermal treatment” is proposed to prepare a semi-solid billet. The as-cast copper alloy was subjected to severe plastic deformation by multi-pass cold rolling at room temperature, which achieved the purpose of thermal deformation and grain breakage in the traditional SIMA process and shortened the pulping process.

In the CRITSIMA process, CuSn10P1 copper alloy is cold-rolled in multiple directions by a two-high rolling mill, and the final cumulative reductions are 10%, 20%, 30%, and 40%. The rolling method was that one surface was first-pass rolled, and after being turned 90° around its long axis, it was second-pass rolled. The rolling reduction was calculated by adding two passes of rolling reduction. After rolling, the copper billet was cut into small block samples of 15 mm × 20 mm × 25 mm and put into the induction furnace for isothermal treatment. Isothermal treatment was carried out at 880 °C, 900 °C, and 920 °C for 10 min, 15 min, 20 min, and 25 min. After isothermal treatment, the samples were quickly water quenched to obtain semi-solid billets.

2.3. Observation of Microstructure and Detection of Element Distribution

The microstructure of the semi-solid billets was analyzed by metallographic microscope and scanning electron microscope. The distribution of elements in the samples was determined and analyzed by scanning electron microscope and energy dispersive spectroscopy. The semi-solid microstructure was quantitatively analyzed by Image-Pro Plus 6.0 software, and the average grain diameter, average grain roundness, and liquid fraction of semi-solid samples obtained under different processing conditions were statistically analyzed. The average values measured by three metallographic microstructure diagrams were used as the final average grain diameter, average grain roundness, and liquid fraction of the process. The effect of cold rolling reduction, isothermal temperature, and isothermal time on the semi-solid microstructure was studied. Grain diameter (D) and roundness (R) are calculated from Equations (1) and (2) [24,25,26].

$$\mathrm{D}=\frac{{{\displaystyle \sum }}_{\mathrm{i}=1}^{\mathrm{N}}\sqrt{4{\mathrm{A}}_{\mathrm{i}}/\mathsf{\pi }}}{\mathrm{N}}$$

(1)

$$\mathrm{R}=\frac{{{\displaystyle \sum }}_{\mathrm{i}=1}^{\mathrm{N}}4{\mathsf{\pi }\mathrm{A}}_{\mathrm{i}}/{\mathrm{P}}_{\mathrm{i}}^2}{\mathrm{N}}$$

(2)

In the formula, N is the total number of grains, A_i is the area of particle i, P_i is the perimeter of particle i.

3. Results and Discussion

3.1. Effect of Cold Rolling Reduction and Isothermal Time on the Microstructure of Semi-Solid Billet

Figures 3–6 show the metallographic microstructure of the semi-solid CuSn10P1 billet obtained at 900 °C for 10, 15, 20, and 25 min under different cold rolling reductions. In the microstructure of each phase diagram, the near-spherical grain is the α-Cu phase, and (α + δ + Cu₃P) eutectoid is the gray-black grain between the near-spherical grain. The isothermal temperature of each group was 900 °C, and four different cold rolling reductions were set in each group to compare the influence of different cold rolling reductions on the microstructure of the semi-solid billet. Four different isothermal times were used between each group to compare the influence of different isothermal times on the microstructure of semi-solid billets.

Figure 3 shows the microstructure evolution of semi-solid billet at 900 °C for 10 min under different cold rolling reductions. As shown in the figure, at 900 °C for 10 min, due to the short isothermal time, the low melting point eutectoid (α + δ + Cu₃P) in the microstructure melted to form a small amount of liquid. The liquid could not aggregate and separate grains, so grain spheroidization is not obvious. Most grains in the microstructure bonded to each other, resulting in large grain diameter and poor roundness. The stored energy in the alloy increases with the increase of cold rolling reduction. In the same isothermal treatment process, the stored energy is released. Thus, the liquid in the microstructure increases. As shown in grain A in Figure 3d, the increase and aggregation of the liquid phase separated the grains, the grain was gradually refined, and its roundness improved. Figure 3a–3d observed liquid droplets in the solid phase grains. The liquid droplets slowly grew with the increase of cold rolling reduction and aggregated with the intergranular liquid to cut the grains, forming “C” shaped grains (grains B in Figure 3d).

Figure 4 shows the microstructure evolution of semi-solid billet at 900 °C for 15 min under different cold rolling reductions. As shown in the figure, after 15 min isothermal at 900 °C, the effect of grain spheroidization was significantly improved, and most grains were spherical. However, the grains were still bonded to each other and were not evenly separated. The evolution behavior of grains can be roughly observed from Figure 4a. First, in the isothermal process, many small liquid droplets melted from the solid phase, then small liquid droplets aggregated to form large droplets. The intergranular liquid penetrated the grain, and the large liquid droplets migrated to the grain interface. They converged to reduce interfacial energy, thus forming C-shaped grains (grains B in the figure). Finally, the liquid formed by aggregation gradually cut the original grain and separated the fine, rounded spherical grain (grain A in the figure). The liquid phase segregation was observed with the increase of cold rolling reduction. A large amount of stored energy could not be easily released at high temperatures for short periods. Part of the stored energy was used for melting low melting point eutectoid into intergranular liquid, and part of the stored energy was used for the consolidation and growth of grains (grains C and D in the figure). The inhomogeneous distribution of solid and liquid phases was formed by merging and squeezing the intergranular liquid phase. Very fine burr grains were observed at the liquid phase aggregation and grain boundary. With the increase of cold rolling reduction, the shape of the burr grains gradually changed from sawtooth to dendritic.

Figure 5 shows the microstructure evolution of the semi-solid billet at 900 °C for 20 min under different cold rolling reductions. As shown in the figure, after isothermal at 900 °C for 20 min, the spheroidization effect is good, and the liquid phase distribution is uniform. The intergranular liquid phase forms a liquid film to evenly separate grains. With the increase of cold rolling reduction, the stored energy can accelerate grain consolidation and growth. When the cold rolling reduction increased from 10% to 20%, the grains C and D grew together, the intergranular liquid film decreased obviously, and the intergranular liquid phase extruded to form a large aggregation liquid phase. The grain melting and coarsening mechanism can be observed simultaneously with the increase of cold rolling reduction. There are merging and growth of grains, diffusion of the liquid, and melting small grains in the microstructure.

Figure 6 shows the microstructure evolution of semi-solid billet at 900 °C for 25 min under different cold rolling reductions. As shown in the figure, after 25 min isothermal time at 900 °C, the liquid phase in the microstructure increased significantly, and a large number of serrated burr grains appeared on the grain boundary. Excessive isothermal time makes the grain melting mechanism dominant, and the liquid phase inhibits grain growth. Compared with 20 min, the liquid fraction increased significantly at 25 min. It can be seen that longer isothermal time is not conducive to the preparation of semi-solid billets. Figure 6d shows that many burr grains were generated in the liquid. They are either distributed along grain boundaries in jagged shapes or distributed in the liquid phase in rose shapes and aggregate and grow into secondary dendrite arms. This phenomenon is consistent with the burr grain phenomenon observed in the study of semi-solid AlSi7Mg alloy billet by Jiang et al. [27], which may be caused by the exfoliation of high-energy sub-grains near the grain boundary by liquid infiltration.

In conclusion, in the process of cold rolling, the broken dendrites in the alloy increase with the increase of cold rolling reduction, and the semi-solid microstructure is formed in the isothermal process. With the increase of cold rolling reduction, the average grain diameter in semi-solid microstructure gradually decreases due to the accumulation of more stored energy for grain refinement. The average grain roundness change in microstructure is complex. When the cold rolling reduction is 40%, the average grain diameter is small. However, the growth rate is too fast, and the grains bond to each other, resulting in low average grain roundness. It is similar that semi-solid billet microstructure evolution behavior with different cold rolling reductions (10–30%). However, when the cold rolling reduction is 40%, the semi-solid billet microstructure grows excessively and requires longer spheroidization time, and more liquid is precipitated.

The microstructure of semi-solid CuSn10P1 copper alloy can be obtained under different cold rolling reduction conditions after 900 °C, 10 min and more isothermal time. In the isothermal process, with the increase of isothermal time, the liquid in the microstructure increases, and the liquid dissolves the connected grains to form independent grains. At the same time, the grain gradually grows into a spheroid. Longer isothermal time will cause severe coarsening of the grain and decrease the uniformity of the grain. Ostwald ripening and grain growth mechanisms can explain grain coarsening behavior during the isothermal treatment. When the isothermal time is 10 min, the microstructure shows extremely irregular shape and non-uniform particle size, and there are many microstructure bonding phenomena. When the isothermal time increased to 15 min, the liquid phase gradually increased and separated the grains, and the grains gradually tended to spherical grains. When the isothermal time increased to 20 min, the liquid phase separated each grain and presented a nearly spherical shape, while smaller droplets appeared in the grain. When the isotherm reached 25 min, the microstructure continued to coarsen and began to show irregular shape. The liquid aggregation phenomenon was serious and many burr grains appeared.

The average grain diameter, average grain roundness, and liquid fraction of the microstructure under different isothermal time and cold rolling reduction were quantitatively calculated, and the results are shown in Figure 7. As shown in Figure 7a, when the isothermal time is short (10–20 min), the increase of reduction can reduce the average grain diameter and achieve the effect of grain refinement. When the isothermal time is too long (25 min) and the amount of deformation is large (40%), the stored energy is fully released. The grain growth rate is faster in the isothermal process, and the average grain diameter increases. During the isothermal process, the average grain diameter increases gradually with the extension of isothermal time. As shown from Figure 7b, when the isothermal time is 10 min, the average grain roundness is less than the reference value (less than 0.5). When the isothermal time is 15–25 min, average grain roundness is relatively high when the cold rolling reduction is 10% or 40%, and relatively small when the cold rolling reduction is 20% or 30%. With the increase of isothermal time, average grain roundness increases first and then decreases. Excessive isothermal time will lead to severe grain coarsening, resulting in irregular grain shape and reduced average grain roundness. Figure 7c shows that the liquid fraction has little difference under different cold rolling reductions. With the prolongation of isothermal time, the liquid fraction increases gradually. At the same isothermal temperature (900 °C), the average grain diameter, average grain roundness, and liquid fraction of semi-solid billets were compared under four kinds of cold rolling reduction. It can be seen that when the reduction is 30% and isothermal time is 20 min, the average grain diameter of the semi-solid billet is lower, the average grain roundness is the largest, and the liquid fractions are suitable, which are 66.45 μm, 0.71, and 12.78%, respectively. Therefore, under the conditions of 30% cold rolling reduction and 20 min isothermal time, the semi-solid CuSn10P1 copper alloy billet prepared by the CRITSIMA method can produce a better semi-solid billet.

3.2. Effect of Isothermal Temperature and Isothermal Time on the Microstructure of Semi-Solid Billet

Semi-solid isothermal experiments were carried out on copper alloy billets with a cold rolling reduction of 30%. Figures 8–10 show the metallographic microstructure of semi-solid CuSn10P1 billets obtained at different isothermal temperatures and times. Four different isothermal times (10, 15, 20, and 25 min) were set for each group of figures to compare the influence of different isothermal times on the microstructure of semi-solid billets. Three different isothermal temperatures (880, 900, and 920 °C) were set for each figure to compare the effects of different isothermal temperatures on the microstructure of semi-solid billets.

Figure 8 shows the microstructure evolution of semi-solid billets obtained at different isothermal times at 880 °C after 30% cold rolling. As shown in the figure, the low melting point eutectoid (α + δ + Cu₃P) between the broken dendrites is melted and precipitated when the isothermal time is 10 min, and the liquid aggregates. At this time, many broken dendrite microstructures (grain E in the figure) were not segmented. When the isothermal time was extended to 15 min, the liquid droplets grew and aggregated with the intercrystalline liquid phase to cut the grains, forming the “C” shaped grains (grain B in the figure). When the isothermal time was extended to 20 min, the grains merged and grew, and the intergranular liquid film extruded, resulting in liquid phase aggregation. When the isothermal time was extended to 25 min, the grains combined and grew too much, showing an irregular shape.

Figure 9 shows the microstructure evolution of semi-solid billets obtained at different isothermal times at 900 °C after 30% cold rolling. As shown in the figure, when isothermal time is 10 min, the liquid phase is evenly distributed between the grains. However, due to the short isothermal time, the grains are still in the shape of broken dendrite. When the isothermal time was extended to 15 min, the grains began to grow together (grains C and D in the figure), the intergranular liquid film extruded, and the liquid aggregated. When the isothermal time was 20 min, the grain melting mechanism gradually dominated, and the liquid’s path was formed by increasing the amount of liquid, making the liquid’s distribution more uniform and average grain roundness better. When the isothermal time was 25 min, the isothermal time was too long and many burr grains appeared in the liquid.

Figure 10 shows the microstructure evolution of semi-solid billets obtained at different isothermal times at 920 °C after 30% cold rolling. Figure 10a shows that long strip grains with relatively uniform orientation (white rectangles in the figure) can be observed at an isothermal time of 10 min. It is a backbone of broken dendrites. Compared with 880 °C and 900 °C, dendrite fragmentation is more significant at the same isothermal time. The results show that the spheroidization rate of grain microstructure accelerates obviously with increasing isothermal temperature. With the increase of isothermal time, the grain evolution mechanism is similar to that at other isothermal temperatures. The grains broke, spheroidized, merged and grew, and were dissolved by the liquid. If the isothermal time is too long, many burr grains will appear in the liquid and grain boundary.

Figure 11 shows the quantitative calculation of grain diameter, roundness, and liquid fraction of the microstructure at different isothermal temperatures and times in the isothermal process. The isothermal temperature and isothermal time greatly influence the microstructure of the semi-solid billet. With the increase of isothermal time, the average grain diameter and liquid fraction increased, and the average grain roundness increased rapidly at the isothermal time of 10–15 min, and slowly increased and then decreased at the isothermal time of 15–25 min. The average grain roundness is low when the time is 10 min, and the grain is very irregular. This is because with the short isothermal time, the broken dendrites have not had time to spheroidize. The average grain roundness was higher at 15 min, 20 min and 25 min, and the grains were nearly spherical. Due to the irregular grains during the growth and melting process at 25 min isothermal time, the average grain roundness increases. The average grain diameter increases with increasing isothermal time. However, when isothermal time is 25 min, the average grain diameter decreases at 900 °C and 920 °C. Because of the higher temperature and longer time, the microstructure has a high liquid fraction. At this time, the melting mechanism of grains is dominant, and the grains melt under the high-temperature liquid, resulting in the reduction of the average grain diameter. On the whole, during the isothermal process, when the temperature is 900 °C for 20 min, the semi-solid billet of copper alloy has a relatively rounded microstructure and moderate size and liquid fraction, which is suitable for semi-solid forming of copper alloy.

3.3. Coarsening Rate

The influence of cold rolling reduction and isothermal temperature on the coarsening rate of average grain diameter was fitted by LSW (Lifshitz, Slyozov and Wanger) empirical formula, as shown in Equation (3):

$${\mathrm{D}}_{\mathrm{t}}^{\mathrm{n}}-{\mathrm{D}}_0^{\mathrm{n}}=\mathrm{KT}$$

(3)

In the formula, the exponent n is the empirical constant of coarsening, and the value is 3; D_t is the average grain diameter at isothermal time T; D₀ is the average initial grain diameter; K is the grain coarsening rate coefficient.

Grains begin to form semi-solid spherical grains at isothermal temperature for 10 min. The average grain diameter at 10 min of each cold rolling reduction was taken as the initial grain diameter D₀. Origin software was used to calculate and linearly fit the cubic value of grain diameter at each deformation amount and isothermal time, and the grain coarsening rate coefficient at different isothermal temperatures was obtained, as shown in Figure 12. The adjusted goodness of fit R² is 0.89–0.99, indicating that the fitting curve can reflect the coarsening rule of average grain diameter with isothermal time.

As shown in Figure 12a, the increase of cold rolling reduction increases the stored energy, accelerates the volume diffusion efficiency, and thus accelerates the coarsening rate of semi-solid billets. So, when cold colling reduction increases, the coarsening rate coefficient K increases. With the increase of isothermal temperature, the melting rate of grains is higher than the combined growth rate, and the liquid inhibits the combined growth of grains, so the coarsening rate coefficient K decreases gradually.

3.4. Distribution Changes of Elements during Isothermal Treatment

Figure 13 shows SEM and EDS surface scanning images of as-cast CuSn10P1 copper alloy without isothermal treatment with a cold rolling reduction of 30%. Dendrite fracture can be seen in Figure 13a. The distribution of elements can be seen by EDS surface scanning images in Figure 13b–d. Cu is the matrix distributed in the whole region, but there is a higher content in the dendrite (the position of point 1). Sn is less distributed in the dendrite, concentrated in the bright field region of the secondary phase (the position of point 2), and P is concentrated in the dark field region of the secondary phase (the position of point 4). There is severe element segregation in the secondary phase of as-cast microstructure.

Figure 14 shows SEM and EDS surface scanning images of semi-solid CuSn10P1 copper alloy after the cold rolling reduction is 30% and isothermal treatment for 20 min at 900 °C. The distribution of major elements in the microstructure can be clearly observed through the EDS surface scanning. As the main element, the Cu is still evenly distributed in the microstructure, but it still shows a higher content inside the grain boundary. There is a large amount of Sn and P in the semi-solid intergranular.

The elemental content distribution of the seven points in Figure 13a and Figure 14a can be further obtained by EDS point scanning quantitatively characterized in the original as-cast and semi-solid microstructure element. As shown in

Table 2. Element content before and after isothermal treatment (wt.%).

Element	As-Cast				Semi-Solid
	1	2	3	4	1	2	3
Cu	96.86	68.68	88.87	85.78	96.13	91.98	75.48
Sn	2.99	31.32	10.68	-	3.55	7.71	19.23
P	0.15	-	0.45	14.22	0.32	3.17	5.29

, the Sn, P element concentration distribution in the secondary phase and intergranular organization of initial as-cast microstructure. Dendritic structure distribution is less. There is serious intergranular segregation. Compared with the as-cast microstructure, the Sn and P elements of the semi-solid intergranular microstructure diffuse into the inside grain, resulting in the decrease of Sn and P in the intergranular microstructure and the increase of Sn and P in the inside grain. The results show that the isothermal treatment can also improve element segregation to a certain extent. As can be seen from the distribution of elements in semi-solid microstructure, the content of Cu gradually decreases from inside grain to intergranular microstructure (point 1 to point 3), while the content of Sn and P gradually increases. Although element segregation in semi-solid microstructure has been improved, it is still serious on the whole and needs to be further improved by the subsequent heat-treatment process.

4. Conclusions

In this study, CuSn10P1 copper alloy was treated by the CRITSIMA method to obtain near-spherical microstructure. The effects of cold rolling reduction, isothermal time, and temperature on microstructure evolution of semi-solid billet were studied. The following conclusions are drawn:

(1)

The dendrite network microstructure in the as-cast CuSn10P1 copper alloy is destroyed after multi-pass cold rolling, and the dendrite and dendrite walls in the microstructure are broken and fractured. The dendrite direction of fracture is shifted in the same direction by multi-direction rolling. With the increase of cold rolling reduction, the degree of dendrite fragmentation in the microstructure is intensified, and the more stored energy is stored in the microstructure, which provides more energy for the subsequent isothermal treatment process.

(2)

The increase of cold rolling reduction can refine the average grain diameter of semi-solid grain. However, when it reaches 40%, the bonding phenomenon of grain is more severe, and the average grain diameter increases. In the isothermal process, the increase of isothermal temperature and the extension of isothermal time will make the grains grow up. When the deformation is 30% and isothermal temperature is 900 °C for 20 min, the average grain diameter, average grain roundness, and liquid fraction are 66.45 μm, 0.7, and 12.78%, respectively.

(3)

With the increase of cold rolling reduction, stored energy increases, and grain consolidation and growth rate are accelerated. Therefore, the grain coarsening rate coefficient K increases gradually. With the increase of isothermal temperature, the increase of the liquid phase makes the grain melting rate more than the coarsening rate. Therefore, the grain coarsening rate coefficient K decreases gradually.

(4)

Element segregation exists in both as-cast and semi-solid microstructure of CuSn10P1 alloy. Sn and P accumulate in the secondary phase of the as-cast microstructure. Sn and P accumulate in the intergranular microstructure of the semi-solid microstructure. Isothermal treatment can improve the segregation of Sn and P to a certain extent, but cannot eliminate the segregation of elements.