Study on the Bonding Mechanism of Copper-Low Carbon Steel for Casting Compounding Process

Abstract

The casting compounding process for copper-steel composite material has broad prospects of application, but due to the lack of supporting theories (especially the bonding mechanism of copper-steel at high temperatures), it is developing slowly. In this research, copper-steel composite materials for different casting temperatures have been prepared by the casting compound process. The results show that, for the casting compound process, the stable copper-steel transition layer can be formed in a short time, and the bonding of copper and low carbon steel is the result of both the diffusion of Cu in low carbon steel and the dissolution of Fe in molten copper. The diffusion coefficient of Cu in the low carbon steel is mainly concentrated in the range of 4.0 × 10⁻¹⁵–8.0 × 10⁻¹⁴ m²/s. However, for casting compound process of copper-steel, as the temperature rises the thickness of the copper-steel transition layer gradually decreases, while the Fe content in the copper layer gradually increases. At the same time, the analysis of the glow discharge results shows that, during the solid-liquid composite process of copper-steel, the element C in steel has a great influence. As the temperature rises, the segregation of C intensifies seriously; the peak of the C content moves toward the copper side and its value is gradually increases. The segregation of C would reduce the melting point of the steel and cause irregular fluctuations of the diffusion of Cu in low carbon steel. Therefore, a relatively lower molten copper temperature is more conducive to the preparation of copper-steel composite materials.

1. Introduction

Copper is the earliest metal used by mankind. Due to its excellent electrical conductivity, thermal conductivity, and corrosion resistance [1], copper is now widely used in electrical appliances such as motors and transformers. However, at the same time, the enlargement of copper devices has been limited by its lower strength and higher ductility. To solve this problem, metallurgical researchers have conducted a lot of work, and their main solutions to this problem could be divided into two categories. One is to develop copper alloys with high-strength and high conductivity [2,3,4]. However, the higher cost of this solution makes it impossible to attempt a large scale of application. The other is to develop steel-based composite Cu-Fe materials; since the conductivity of copper is a typical skin effect behavior, the conduction could be accomplished by the extremely thin copper layer on the surface of the composite material. A composite material made of copper and high-strength metal could not only ensure the electrical conductivity of the material, but also effectively improve the overall structural strength of the material [5,6,7]. Thus, the copper-steel composite material has wide prospects of application.

Common manufacturing processes for copper-steel composite materials include composite rolling, electrodeposition, explosive forming, diffusion welding, powder metallurgy, hot-dip plating, and casting compound [8,9,10,11,12,13,14,15,16,17,18,19,20]. Compared with other processes, the casting compound of copper-steel could form a more stable Cu/Fe transition layer, which would determine the subsequent processing performance of the composite material. The cost of the casting compound is also lower, and it is safer and more stable than explosive forming. The environmental pollution problems involved are also fewer. However, the casting compound process of copper-steel also has some big shortcomings. The melting point of copper is 1083 °C, and the melting point of steel is in the range of 1350–1520 °C; they are both high and close. The core technology of the casting composite process is the designing of the temperature of molten copper and the preheating temperature of the steel. If the temperature of molten copper or the preheating temperature of the steel is too low, then when the molten copper contacts with the steel, it will quickly solidify, shrink, and form a strong solidified shell. The solidification and shrinkage of molten copper will cause the solidified shell to separate from the steel, and the bonding between the copper and steel cannot be formed. Therefore, higher molten copper temperatures and higher preheating temperature of steel are conducive to the recombination of the two metals. However, high temperatures would also accelerate the dissolution of Fe in the molten copper. The increase of Fe content in copper would affect the copper conductivity seriously [21,22], and this is why the casting compound process of copper-steel cannot continue to develop.

Cu-Fe alloys also need to face the problem which facedd Fe content. In recent years, researchers have carried out a lot of results on the control of the existence of Fe in Cu-Fe alloys. Cui Wenfang et al., found that the enrichment of Fe in Cu-Fe alloy could be effectively achieved through rolling and aging diffusion [23] and, at the same time, Dawei Yuan et al., found that adding Mg to the Cu-Fe alloy could effectively promote the enrichment and precipitation of Fe [24], thereby reducing the effect of higher Fe content in copper on the conductivity of Cu-Fe alloy. They also found that the lower the Fe content in copper, the easier the subsequent treatment. In comparison, for the casting compound, the Fe content in the copper region of the copper-steel composite material is much lower than that of Cu-Fe alloy. Therefore, the above results could also be used for casting compound process and will provide a new opportunity for the development of the copper-steel casting compound.

This is different from other composite processes; the copper-steel casting compound involves both the diffusion of Cu in steel and the fusion of Fe in molten copper, and the diffusion behavior of Cu in steel is the key to the forming of stable transition layer between the copper and steel. Although researchers have conducted a lot of work on the diffusion coefficient of Cu in Fe or steel [25], the temperature ranges are all below the melting point of copper; they are not fit for the casting compound. The lack of relevant data is a huge constraint on the study of solid-liquid compounding behavior and the casting compounding process for copper and steel.

In this study, the casting compound experiments of copper and steel at different temperatures had been designed. For the copper-steel composite metals, the corresponding metallographic analysis, scanning electron microscope analysis, and glow discharge detection were carried out to analyze the diffusion behavior of the elements involved in the casting compound process. The results will be the theoretical basis for the casting compound process of copper and steel.

2. Experimental

The key of the copper-steel casting compound experiment is the control of the preheating temperature of steel and the temperature of molten copper.

For the preheating temperature of steel, the increase in the temperature of the steel will promote the bonding between copper and steel. However, when the temperature reaches its austenitizing temperature, the surface of the steel will be easy to oxidize. The forming of FeO is not good for the bonding between copper and steel, so the preheating temperature of steel was set to 600 °C.

The temperature of the molten copper is the key to the formation of a stable bonding between copper and steel. Considering that the melting point of copper is 1083 °C, and if its temperature is too high it will increase the solidification defects of copper, the temperature of molten copper was sat between 1100 and 1200 °C.

In this study, the mold and the steel were preheated together, and then both were immersed in the molten copper. When the required holding time was reached, the mold, the steel, and the molten copper would be taken out. When the surface of the molten copper solidified, they all would be immersed in water to cool.

2.1. Experimental Materials

The low carbon steel used in this study is ISO: HR2(σs195) steel, whose chemical composition is shown in

Table 1. The composition of ISO: HR2(σs195) steel.

Composition	C	Si	Mn	P	S	Fe
Content %	0.07	0.20	0.30	0.020	0.020	others

, and the pure copper ingot is selected for the preparation of molten copper, its composition is shown in

Table 2. The composition of the copper.

Composition	Cu	Bi	Sb	As	Pb	S
Content %	99.90	0.001	0.002	0.002	0.005	0.005

.

2.2. Experimental Procedure

In this study, the tube furnace which was designed by ourselves was used to heat the copper ingot to molten copper. The heating range of the tub furnace is from room temperature to 1450 °C, and its temperature error is less than 2 °C. In order to prevent the oxidation of the molten copper, graphite crucible was used as the container, and the whole experimental device was ventilated with Ar gas throughout the experiments.

The schematic diagram of the experimental device is shown in Figure 1a. The mold used for casting compound of copper and steel is shown in Figure 1b, and its material is high-purity graphite. The size of the composite zone of the casting compound sample is 80 mm × 55 mm × 12 mm.

Under high temperature conditions, when the molten copper contacts with the air that brought by the casting mold, it is easy to oxidate. To prevent that oxidation, the graphite flakes was placed on the surface of the molten copper.

In order to analyze the diffusion coefficient of Cu in low carbon steel under different temperature conditions, the casting temperature was sat to 1100 °C, 1125 °C, 1150 °C, 1175 °C, and 1200 °C, respectively. The holding time at high temperature was 15 min.

For the casting compound experiment, the steel should be washing by the acid and alkali at first, and then be placed in the graphite mold. When the temperature of the molten copper reached the target temperature, the sealing plug of the tube furnace would be opened, and the steel with mold should be placed in the tube where the temperature was preheated to 600 °C. After 3 min, the mold would be immersed in the molten copper. When holding time reached 15 min, the mold would be taken out and be cooled in water. Then the samples would be analyzed by OM, SEM and glow discharge.

3. Results

The profiles of casting compound samples at different casting temperatures are shown in Figure 2, the surfaces of the samples are shown in Figure 2a, and the cross-section of samples are shown in Figure 2b.

As shown in Figure 2a, when the holding time is 15 min, whether the casting temperature (molten copper) is 1100 °C or 1200 °C, the mold could be filled by molten copper effectively. Due to the large solidification shrinkage coefficient of copper, especially under water cooling conditions, more or less shrinkage cavities had been formed on the surface of the samples. However, considering that the main purpose in this study is to analyze the bonding mechanism of copper-steel and the diffusion coefficient of Cu in low-carbon steel for casting compound process, the surface defects of the cast copper could be ignored.

As shown in Figure 2b, from a macro point of view the bonding between copper and steel is very good, and there is no obvious crack in the transition layer.

In order to further analyze the bonding between copper and steel, the microstructures of copper-steel interface at different casting temperatures had been analyzed. The results are shown in Figure 3.

As shown in Figure 3, it can be seen that when the casting temperature is below 1150 °C, the bonding area between copper and low carbon steel is a relatively regular straight line. At this time, the formation of the bounding would be mainly by the diffusion of elements. It should be noted that the grains on the side of the low carbon steel have been significantly refined and the grain boundaries corresponding to the refined grains are relatively coarse. The above results show that the segregation of non-Fe elements occurs in the grain refinement area and results in the formation of new fine phases.

When the casting temperature continues to rise, the grain refinements start to disappear gradually. The coarse grain boundaries are also difficult to find, and the copper-steel bonding area is no longer a relatively regular straight line; there are some humps on the side facing the steel.

When the casting temperature rises to 1200 °C, the width of the bonding area between the copper and steel has increased significantly, it is most likely that the formation of the bonding has been transitioned from diffusion to fusion. At this time, the width of the bounding area is close to 3 μm.

For the casting compound materials with different casting temperatures, the microstructures of the copper-steel bounding area have clearly changed. In order to further analyze the elements composition in bounding areas, the line scan of EDS was carried out. The results are shown in Figure 4.

As shown in Figure 4a–e, it is different from Figure 4b–e, the components line scan result in Figure 4a shows an obvious truncation. As shown in Figure 4a, there is a lot of element O at the edge of the truncated area, which means that the truncated area is most likely that one inclusion attached to the surface of steel.

Except for Figure 4a, as shown in Figure 4b–e, there is a significant Cu and Fe diffusion area at the Cu/Fe interface. From the micro point of view, the thicknesses of the Cu/Fe transition layers for different casting temperature are between 3 and 7 μm, and as the temperature rises, the thickness of the Cu/Fe transition layer gradually decreases.

For the elements O and C in Figure 4a–e, their diffusion tendency is significantly different from that of elements Cu and Fe. As shown in Figure 4, during the casting compound process of copper and steel, there is an obvious segregation of element C at the Cu/Fe interface. For the distribution of element O, the O content in the low-carbon steel side is relatively higher than that in the copper side, which may be related to the fact that the O content in the low-carbon steel is higher than that in the copper.

The results shown in Figure 4 could reflect the change trend of the content of Fe, Cu and other elements in the bonding area, but the elements content has large fluctuations, especially the element Cu which is easy to segregate at grain boundaries [26,27]. Thus, the elements distribution in a local area could not reflect the real elements distribution in the whole transition layer. In order to verify this, the copper layer of the sample that casted at 1100 °C had been removed, and the area scan of the transition layer of steel side had been carried out. The results are shown in Figure 5.

As shown in Figure 5, the element Cu is mainly distributed at the grain boundary, so the single results of line scan could not truly reflect the distribution of Cu in the whole transition layer. In order to further analyze the diffusion behavior of Cu, and to calculate the diffusion coefficient of Cu in low-carbon steel for casting compound process, the glow discharge instrument had been used to detect the content of different elements in the whole transition layer.

For copper-steel composite samples, the glow discharge instrument could only detect within 200 μm once, so it has extremely high requirements for the thickness of the metal that on the surface the sample. In this study, due to the metal luster difference between the copper and steel, the low-carbon steel layer had been grinded to an extremely thin thickness. The sample morphology after grinding is shown in Figure 6a, and the sample morphology after the detection of glow discharge is shown in Figure 6b. Also, as shown in Figure 6b, the area of the glow discharge detection is a circle with a diameter of Φ5 mm.

The distribution of elements Cu and Fe of the transition layers for the samples at different casting temperature had been tested by the glow discharge instrument. The results are shown in Figure 7a–e, wherein the content of Cu and Fe is the mass percentage.

As shown in Figure 7a–e, for all the casting temperature involved in this study, the stable transition layer could be formed between copper and low carbon steel. The change trends of elements Cu and Fe for different casting temperature are all smoother, and the thicknesses of different transition layers are all above 70 μm, which has a huge difference with the line scan results. This means that the distributions of elements Cu and Fe at the microscopic level are extremely uneven.

Tong Xi and Dieter Isheim et al. [26,27] found that the element Cu would be easily segregated at grain boundary, while Osman Yilmaz and Zhang Haichao et al. [28,29] found that the microscopic grain boundary of the steel was the diffusion channel of element Cu. Although Ting Dong Xu and Shigeru Suzuki et al. [30,31,32,33] found that the element Cu in steel would achieve uniform distribution under certain high temperature aging conditions, but for the 15 min holding time in this study, it is far from reaching the critical time requirement. All of the above results show that the line scan results could not be used to determine the diffusion behavior of Cu or Fe for the copper-steel composite material.

For the diffusion behaviors of elements Cu and Fe at different casting temperatures, as the casting temperature rises (except for 1125 °C), the changing rate of the mass percentage of Cu and Fe in the early stage clearly increases, which is shown in the curves that the slope of the concentration curve of Cu and Fe near the intersection point increases significantly. As far as the glow discharge detection results are concerned, when the casting temperature rises, the diffusion rate does not increase but shows a decreasing trend, and the corresponding transition layer width gradually decreases too. This result is not according to the Arrhenius equation. At the same time, it should be noted that as the temperature rises, the content of Fe in copper has a tendency to increase gradually. Considering that rising temperature would increase the dissolution rate of Fe in molten copper, the influence of the temperature for the diffusion behavior of Cu in low carbon steel should be further analyzed and discussed.

4. Discussion

The above analysis results show that, the results of glow discharge instrument could reflect the diffusion behavior of Cu in the low carbon steel for casting compound process better, so the following discussion would be mainly on the analysis of the glow discharge test results.

4.1. Analysis on the Diffusion Coefficient of Cu in Low Carbon Steel

In this study, the diffusion coefficient of Cu in low carbon steel had been calculated by the optimized Boltzmanm-Matano model proposed by Den-Broeder [34], the calculation formula is as shown in Equation (1)

$$D=\frac{V\prime m}{2t{(\partial C/\partial x)}_{x_0}}[\frac{C_1-C_0}{C_1-C_2}{\int }_{-\infty }^{x0}\frac{C-C_2}{Vm}dx+\frac{C_0-C_2}{C_1-C_2}{\int }_{x_0}^{+\infty }\frac{C_1-C}{Vm}dx]$$

(1)

where:

• V’m—molar volume of one location, where the concentration is C₀, mol/m³;
• t—diffusion time, s;
• C₁, C₂—maximum and minimum concentration, mol/m⁻³;
• x₀—the location where the concentration is C₀, m;
• Vm—molar volume of the system, when temperature is T (°C), mol/m³.

The thermal expansion coefficients of copper and low carbon steel are both relatively small, so their molar volumes can be seen as the same. Therefore, the Vm can be seen as a constant, and V’m = Vm.

In this study, the mass concentration of Cu and Fe in Figure 7 has been converted into the corresponding molar concentration, then the corresponding diffusion coefficient of Cu in low carbon steel with different Cu mass concentrations at different casting temperatures had been calculated by Equation (1). The results are shown in Figure 8.

Since the amount of data tested by the glow discharge instrument is extremely large, and there are also some fluctuations of the data at some positions, the diffusion coefficient values calculated also have certain fluctuations. However, the calculation results could still reflect the Cu diffusion coefficient changing trend with its mass concentration at different casting temperatures, and the results are mainly concentrated in the range of 4.0 × 10⁻¹⁵–8.0 × 10⁻¹⁴ m²/s.

For the calculation results of the diffusion coefficient of Cu, it is much larger than its corresponding value at high-temperature for solid-solid phase diffusion. These results show that, for casting compound process, the diffusion bonding between copper and steel could be completed in a shorter time. For the formation of stable diffusion bonding, the efficiency of casting compound process is much higher than the high-temperature annealing diffusion between solid steel and solid copper.

In order to further analyze the change of Cu diffusion coefficient in low carbon steel at different casting temperatures, the linear fitting for the calculation result of the diffusion coefficient in Figure 8 has been carried out, and the fitted lines are shown in Figure 8. The slopes and intercepts of the corresponding fitted straight lines in Figure 8 are shown in

Table 3. Slopes and intercepts of the fitted straight lines at different casting temperatures.

Temperature °C	Intercept	Slope
1100	1.77 × 10−10	1.62 × 10−10
1125	1.94 × 10−10	9.25 × 10−11
1150	1.42 × 10−10	9.92 × 10−11
1175	1.35 × 10−10	5.21 × 10−11
1200	7.69 × 10−11	1.49 × 10−10

.

As shown in Figure 8 and Table 3, it can be seen that the Cu mass concentrations has a certain effect on the its diffusion coefficient in low carbon steel. At different casting temperatures, the Cu diffusion coefficients all show an increasing trend as Cu content rises. In comparison, when the temperature is 1100 °C or 1200 °C, the diffusion coefficient of Cu rises the fastest with the increase of Cu content, and the slope value of the corresponding fitting line is also the largest. However, when the temperature is 1175 °C, the increase rate of Cu diffusion coefficient is the slowest, and when the Cu content is close to 100%, its corresponding diffusion coefficient is the smallest among all temperatures.

Theoretically, in addition to the influence of Cu content, the temperature is another important influence factor for the diffusion coefficient. But as shown in Figure 8, during the casting compound process in this study (except for temperature 1200 °C), as the temperature rises the Cu diffusion coefficient in low carbon steel does not increase. On the contrary, it shows a certain decreasing trend, especially when the Cu content is bigger than 25%. This is obviously different from the changing law of Cu diffusion coefficient at high temperature for solid-solid composite process. For the temperature 1200 °C, when the Cu content is less than 58%, its diffusion coefficient is smaller than the corresponding value of temperature 1175 °C. However, due to the faster growth rate of the diffusion coefficient, when the Cu content is higher than 58%, its diffusion coefficient has exceeded the corresponding value for temperature 1175 °C.

From the above results, it is not difficult to see that for casting compound process, the higher temperature is not conducive to the diffusion of Cu in low carbon steel. The casting compound is a typical solid-liquid composite process, and the melting points of copper and steel are relatively close. For solid-liquid composite of copper-steel, there must be some Fe dissolving in the molten copper, and the dissolution of Fe would increase significantly as the casting temperature rises. This is a great influence factor for the diffusion behavior of Cu in low carbon steel.

For the solid-solid composite process, the diffusion of Cu and Fe is a typical inter-diffusion behavior. Therefore, the formation of the copper-steel transition layer is mainly attributed to the combined effect of the diffusion of Cu in low-carbon steel and the diffusion of Fe in copper. However, for solid-liquid composite process, since copper is liquid, the interface of copper and steel renews extremely fast and the diffusion of Fe in copper is essentially the dissolution of Fe in molten copper. At the same time, for the diffusion of Cu in low carbon steel, as the Cu content increases the melting point of the Cu-Fe compound will be lower; when the Cu content increases to a certain amount, and under certain experimental conditions, the Cu-Fe compound will be molten and will accelerate the dissolution of Fe in the molten copper. Therefore, for solid-liquid composite process, the formation of a stable copper-steel transition layer is the result of both the diffusion of Cu in low carbon steel and the dissolution of Fe in molten copper. To a certain extent, the dissolution of Fe in molten copper would also affect the diffusion of Cu in low carbon steel.

The diffusion coefficient of Cu in low carbon steel above could not represent the absolute diffusion coefficient of Cu in low carbon steel, but for the casting compound process of copper and steel it could reflect the real diffusion behavior of Cu in low carbon steel. Thus, the result has a greater significance for real production.

4.2. Analysis on the Dissolution Behavior of Fe in Molten Copper

With the above analysis, it can be found that the dissolution of Fe in the molten copper is also the key to the formation of a stable transition layer between copper and steel. In order to further analyze the dissolution behavior of Fe in molten copper, the saturation solubility of Fe in the liquid Cu at different temperatures had been calculated by FaceSage software, and the test results of glow discharge instrument had also been compared in this study. The results are shown in Figure 9.

As shown in Figure 9, whether the theoretical calculation result or the actual test result, the Fe content in molten copper shows a gradually increasing trend as temperature rises. Due to the relatively short holding time, the dissolution of Fe in the molten copper has not yet reached its maximum saturation. However, as temperature rises the dissolution rate of Fe in the molten copper gradually increases. When the temperature increased from 1100 °C to 1200 °C, the Fe content in the molten copper rises rapidly from 1.84% to 3.14% and the increase reaches 70.1%.

The increase of Fe content in copper would greatly reduce the copper’s conductivity. Although the rolling or aging treatments could make the Fe to exist in the form of aggregation in copper, the lower the Fe content in copper, the easier the control of the segregation later. The manufacturing cost will be reduced too. This also means that on the premise that a stable transition layer could be formed, the lower the casting temperature and the shorter the holding time, the more advantageous it is to control the production cost of the casting compound of copper and steel.

The increase of casting temperature would accelerate the dissolution of Fe in the molten copper liquid and, at the same time, it would also affect the transition layer thickness of copper-steel composite material, especially when the diffusion rate of Cu in low carbon steel is lower than the dissolution rate of Fe in the molten copper.

The transition layer thicknesses of the corresponding samples at different casting temperatures are shown in Figure 10. It can be seen from Figure 10, as the temperature rises, the transition layer thickness shows a clear trend of thinning. When the temperature rose from 1100 °C to 1200 °C, the thickness of the copper-steel transition layer decreased from 82.1 μm to 70.0 μm. The thinning of the transition layer thickness indicates that, as the temperature rises, the dissolution rate of Fe in the molten copper would gradually exceed the diffusion rate of Cu in the low carbon steel. This also shows that too high temperature is not conducive to the casting compound of copper and steel.

With the above analysis, it is not difficult to conclude that a relatively low casting temperature is good for the casting compound of copper-steel.

4.3. The Influence of Other Elements on the Formation of Transition Layer

Low carbon steel contains elements C, Si, Mn, P, S, O, and others. In order to further analyze the influence of the above elements on the formation of the copper-steel transition layer, the distributions of elements C, Si, Mn, P, S, and O in transition layers of corresponding samples at different casting temperatures had been analyzed in this research. The results are shown in Figure 11.

As shown in Figure 11, for the element P, it mainly exists in low carbon steel. However, the P content near the copper/steel interface now is significantly higher than it in the steel before. This means that, as an easy segregation element, P has obviously diffused and segregated to the high temperature area. As the temperature rises, the diffusion amount of P into the copper begins to increase and then dissolves in the molten copper.

For elements Mn, Si, and O, their contents are relatively high in steel and relatively low in copper, and their change trends are consistent with the element Fe. This also means that the elements Mn, Si, and O have little effect on the diffusion of Cu.

For element S, both low carbon steel and copper contain S, and the S content in steel is relatively high. When the casting temperature rises to 1125 °C, part of S appears to enrich at the copper-steel interface; when the temperature continues to rise to 1150 °C, lots of S in the steel begins to diffuse to the copper/steel interface and segregate. However, its start temperature of diffusion and segregation is higher than that of element P. As the temperature continues to rise, the distribution of S begins to be even, which means that, at this time, for the copper-steel interface and nearby areas, the requirement of the critical time for the segregation of element S has been reached and its segregation starts to ease.

For element C, it mainly exists in steel. And for casting compound process, as the temperature rises, the tendency of C to diffuse to the copper/steel interface gradually increases, the peak of C content is moving toward the copper side gradually and its value is increasing too. The maximum C content reaches 4.5% when the temperature is 1200 °C. It could be seen from the Fe-C phase diagram that, when the C content is 4.3%, the melting point of the Fe-C alloy will be lowered to 1148 °C, which is the lower than the casting temperature. Therefore, the enrichment of element C in the transition layer would greatly reduce the melting point of the original steel, and promote the dissolution of Fe in molten copper. This is also the reason for the rapid thinning of the transition layer of copper-steel composite material as the temperature rises.

From a microscopic point of view, to a certain extent the decrease in the melting point of the steel in the transition layer would accelerate the transition of the atoms in the corresponding area from the short-range order to the long-range order. The increase of the atomic distance in a local area would inevitably affect the diffusion behavior of Cu in this area, which is also an important reason why the diffusion coefficient of Cu in low carbon steel fluctuates as the temperature rises. Therefore, a relatively low casting temperature is conducive to the formation of a stable copper-steel transition layer.

5. Conclusions

The stable copper-steel transition layer could be formed quickly by casting compound process, and as the temperature rises, its fusion area gradually widens. As the temperature rises, the thickness of the copper-steel transition layer gradually decreases. When the temperature rises from 1100 °C to 1200 °C, the thickness of the transition layer decreases from 82.1 μm to 70.0 μm; the Fe content in the copper layer increases gradually as the temperature rises. When the temperature rises from 1100 °C to 1200 °C, the Fe content in the molten copper increases from 1.84% to 3.14%. For solid-liquid composite process, the bonding of copper and low carbon steel is the result of both the diffusion of Cu in low carbon steel and the dissolution of Fe in molten copper. The diffusion coefficient of Cu in the low carbon steel is mainly concentrated in the range of 4.0 × 10⁻¹⁵−8.0 × 10⁻¹⁴ m²/s. For the casting compound process of copper and steel, the elements in the steel also diffuse. Among them, element C has the greatest influence. As the temperature rises, the peak of C content is moving toward the copper side gradually, and its value is increasing too. When the temperature reaches 1200 °C, the peak C content reaches 4.5%, which would greatly reduce the melting point of steel and cause the diffusion of Cu in low carbon steel to show an obvious irregular fluctuation. In addition, elements Mn, Si, and O have little effect on the formation of the transition layer, while elements P and S would segregate at the copper-steel interface to a certain extent. For casting compound process, the relatively lower casting temperature is more conducive to the preparation of copper-steel composite materials.