Study on Softening Resistance of Cu-Sn-Fe-P alloy for Advanced Electrical Connectors

The effect of adding iron element in tin bronze on the microstructure and properties of Cu-Sn-Fe-P alloy is studied by means of phase diagram calculation and microstructure observation. The work hardening curve of Cu-Sn-Fe-P alloy is plotted by cold deformation of the alloy with different reduction rates. The test and microstructure observation are conducted for tensile strength and ductility of the alloy sample annealed at different temperatures, so as to determine the recrystallization temperature of the alloy. The results show that the addition of iron element plays a significant role in grain refinement and improves the subsequent working performance. After the cold rolling deformation, an obvious work hardening effect is imposed on the alloy. With the increase of the deformation, the alloy presents a trend of increasing first and then becoming stable in the tensile strength, while a reverse trend in ductility. Cu-Sn-Fe-P alloy has a softening resistance equivalent to that of QSn6.5-0.1 high-tin bronze, both of which will be softened at 400ºC; therefore, Cu-Sn-Fe-P alloy can replace QSn6.5-0.1 alloy at high temperature.The present study proposes how to maintain the original properties of the tin-phosphorous bronze alloy while reducing the content of the tin, and achieve the more effective industrialized production.


Introduction
With the rapid development of 5G communications, new energy vehicles, smartphones, and electronics industries, the demand for tin-phosphorous bronze strips for electrical connectors is increasing day by day.The high-speed electrical connectors are developing towards the direction of high current, high frequency, high speed and high density, resulting in the higher technical requirements for the comprehensive performance of high-performance tin-phosphorous bronze, so that the tin-phosphorous bronze should have higher strength, elastic performance, stress relaxation resistance, and high precision dimension.Therefore, the development of high-performance tin-phosphorous bronze strip products is constantly expanding from a single QSn6.5-0.1 at the beginning to a number of designations such as low tin-phosphorous bronze QSn4-0.3,high tin-phosphorous bronze QSn8-0.3, and QSn10-0.3 [1][2][3][4][5] .
At present, the improvement of the properties of tin-phosphorous bronze alloy remains a problem on how to increase the content of tin and how to suppress the segregation of tin in the process of horizontal continuous casting.Generally, it is required to add high content of Sn to obtain the tin-phosphorous bronze with good elastic properties.In the regular tin-phosphorous bronze produced in China such as QSn8-0.3,QSn6.5-0.1 and QSn4-0.3, the minimum content of Sn is approximately 4%.At present, due to the higher price of metal Sn, the increase in the content of Sn leads to the rise in material manufacturing cost.Moreover, in case of the higher content of Sn in the tin-phosphorous bronze alloy, Sn is easy to form segregation in the process of casting, so only the process of horizontal continuous casting + homogenizing annealing can be used in the production generally, resulting in low production efficiency and yield.Therefore, the present study proposes how to maintain the original properties of the tin-phosphorous bronze alloy while reducing the content of the tin, and achieving the more effective industrialized production [6][7][8][9][10] .

Test material and method
The material under test is a 1.0mm thick alloy strip composed of Cu-Sn-Fe-P, which has undergone hot rolling and annealing.The specific chemical components of the test material are shown in Table 1.The large ingot is cast semi-continuously into Cu-Sn-Fe-P alloy flat ingots.After the heat preservation for 3~5 hours at 900±50º C, the ingots are hot rolled and broken down into slabs with a thickness of 15 mm, which are then sprayed and cooled down.A study was conducted on the properties of Cu-Sn-Fe-P alloy strips by subjecting them to cold rolling deformation at various reduction rates in the annealed state.The aim was to investigate the impact of different cold rolling reduction rates.After heat preservation for 4~6 hours, Cu-Sn-Fe-P alloy strips are subsequently annealed at various temperatures, ranging from 400 to 600º C.This annealing process aims to determine the temperature range for recrystallization of the Cu-Sn-Fe-P alloy.
For the test samples subject to cold rolling deformation, the metallographic samples are cut off in the rolling direction, and then their microstructures are observed with DSX-500 optical microscope after grinding, polishing and etching.Furthermore, the tensile strength and ductility of the samples in various conditions are evaluated using the SANS CMT-5105 universal electronic testing machine.According to the thermodynamic analytical results of Cu-Sn, Cu-Fe and Cu-P binary systems and kinds of literature, a copper-base alloy thermodynamic database is constructed, and the optimization is conducted on the basis of the thermodynamic parameters in the copper-base alloy thermodynamic database.According to the optimized thermodynamic parameters, the phase equilibrium of Cu-Sn-P and Cu-Sn-Fe as ternary alloys is calculated.For the ternary phase diagram of Cu-6.5Sn-0.3P, the phase transformation of Cu-6.5Sn, and the phase transformation of Cu-2Sn-0.1Feat normal temperature, please see the calculation results of the isothermal section phase diagram in the copper-rich region as shown in Fig. 1. Figure 1 is the ternary phase diagram of Cu-6.5Sn-0.3Pat normal temperature calculated by Panda phase diagram calculation software.It can be concluded from the phase diagram that there are mainly Cu3P phase, Cu3Sn phase and Cu6Sn5 phase at Cu Angle.

Effect of Fe content on cast structure
Alloys can be strengthened through various mechanisms, including grain refinement, solid solution strengthening, precipitation strengthening, and work hardening.By fine grain strengthening, it is possible to improve both strength and plasticity, so it is an ideal strengthening method.By adding a trace of the iron element to copper alloy, it is possible to refine the grain properly.At the melting temperature of 1050 º C, the solubility of iron in copper will reach 3.5%; at the melting temperature of 635 º C, the solubility will decrease to 0.15%.In Figure 4, the macro structure of Cu-Sn-Fe-P alloy ingot with different contents of an iron element is shown.As depicted in Figure 4, the incorporation of iron element plays a crucial role in refining the grain structure.Consequently, the abundance of columnar crystals is considerably diminished, leading to a reduction in the resultant organizational stress. .In Figure 5, the photos of the microstructure of alloy with different contents of Fe after heat preservation for 0.5h at different temperatures are shown.
As seen from Figure 5, the addition of iron element hinders the growth of the grains.According to the Hall-Petch empirical formula(1): The smaller the metal grain is, the higher the yield strength of the material is.Due to the low solubility of iron element in copper, it is precipitated during the process of cooling, usually forming a dispersed second phase with the phosphorus element in the copper alloy.The second phase precipitated from the matrix raises the recrystallization temperature, while realizing the strengthening effect by dislocating pinning and impeding grain boundary migration [13][14][15] .

Cu-Sn-Fe-P Alloy: Impact of Reduction Rate and Annealing Temperature on Properties
Figure 6 demonstrates the relationship between the tensile strength, ductility, and relative reduction rate of the Cu-Sn-Fe-P alloy.The significant work hardening effect on the alloy after cold rolling deformation can be observed from Figure 6.The strength of the alloy improves significantly as the reduction rate increases, leading to a noticeable decrease in the ductility of the alloy.Based on the experimental data, it is evident that when the reduction rate of the alloy exceeds 40%, there is a tendency for the tensile strength and ductility of the alloy to stabilize without significant changes.
The analysis shows that during the cold rolling process of the strips, the cold deformation dislocation density continues to increase, the mutual dislocation intersection in movement is intensified, producing solid obstacles such as jog and dislocation tangling.[18] .Figure 7 illustrates the impact of annealing temperature on the tensile strength and ductility of QSn6.5-0.1 alloy and Cu-Sn-Fe-P alloy.rom Figure 7, it is evident that the alloys undergo a softening process around 400 º C. Interestingly, the Cu-Sn-Fe-P alloy, despite having a considerably lower tin content, exhibits consistent softening properties while maintaining high-temperature strength.This suggests that the high-tin content tin-phosphorous bronze alloy traditionally used at high temperatures could potentially be substituted with the Cu-Sn-Fe-P alloy.After cold working, the copper alloy strips will show a phenomenon of work hardening; when the working rate reaches a certain value, the strength and hardness are enhanced rapidly, the plasticity is reduced rapidly, and it is impossible to perform the cold working; therefore, it is necessary to perform intermediate annealing in the process of cold working, in order to reduce the strength and hardness, enhance the plasticity, and make preparation for the subsequent working.
Figure 8 illustrates the metallographic microstructure of Cu-Sn-Fe-P alloy after undergoing heat preservation for 4 hours at various annealing temperatures.From Fig. 8, it is evident that the grain size of the Cu-Sn-Fe-P alloy sample becomes coarse when annealed at 600 º C for 4 hours.Conversely, when the alloy is annealed at 500 º C for the same duration, the grain size appears to be relatively finer.The sample of Cu-Sn-Fe-P alloy annealed at 600 º C has a coarse grain size,a tensile strength of 304 MPa, and ductility of 47%.After annealing at 500 º C, the sample has a relatively fine grain size, a tensile strength of 343 MPa, and a ductility of 45%.According to the study and testing conducted, the performance curves of a Cu-Sn-Fe-P alloy strip with a thickness of 1.0 mm after undergoing 50% deformation at various annealing temperatures have been obtained.These curves are depicted in Figure 9.

Conclusion
(1) According to the results of the calculation with the phase diagram, Cu3Sn at the brittle phase in Cu-6.5Sn tin bronze is fully re-dissolved in solid solution at approximately 250 º C, while Cu-2Sn-0.1Felow-tin bronze at the brittle phase after Fe is added is fully re-dissolved in Cu matrix at approximately 60 º C, and the intermediate phase between Fe-Cu and P suppresses the precipitation of Cu3Sn from the matrix.
(2) The inclusion of iron element plays a crucial role in the refining of grains, leading to a substantial reduction in the formation of columnar crystals.Consequently, the organizational stress caused by columnar crystals is significantly diminished, resulting in improved working properties to a certain degree.
(3) Based on the softening resistance test on QSn6.5-0.1 and low-tin ferrous bronze (Cu-Sn-Fe-P alloy), it is found that the low-tin ferrous bronze (Cu-Sn-Fe-P alloy) is softened at the temperature equivalent to that of QSn6.5-0.1 alloy, both of which will be softened at the annealing temperature of 400º C; therefore, it can replace QSn6.5-0.1 alloy at high temperature.
(4) In the field of advanced electrical connectors, low-tin ferrous bronze (Cu-Sn-Fe-P alloy) has better comprehensive performance and lower cost, and can replace traditional tin bronze.

Figure 2 (Figure 2 .
Figure 2 (a) shows the phase transformation rule of Cu-6.5Sn alloy calculated by Panda phase diagram calculation software at different contents of phosphorous and different temperatures.The low temperature section is magnified as shown in Figure 2 (b), and it can be seen that Cu3Sn at the brittle phase is fully re-dissolved in the solid solution at approximately 250 º C.

Figure 3 (
Figure 3 (a) shows the phase transformation rule of Cu-2Sn-0.1Fealloy calculated by Panda phase diagram calculation software at different contents of phosphorous and different temperatures.The low temperature section is magnified as shown in Figure 3(b), and it can be seen that Cu-2Sn-0.1Feat the brittle phases is fully re-dissolved in the Cu matrix at approximately 60 º C, and the intermediate phase of Fe-Cu and P suppresses the precipitation of Cu3Sn from the matrix.

Figure 5
Figure 5 Metallurgical microstructure analysis was conducted on the Cu-Sn-Fe-P alloy after heat preservation for 0.5h at various annealing temperatures(The thickness of the sample used for analysis was 200μm).

Figure 6 .
Figure 6.The impact of various reduction rates on the properties of the Cu-Sn-Fe-P alloy.

Figure 9 .
Figure 9.Effect Curves of Different Annealing Temperatures on Properties of Cu-Sn-Fe-P Alloy (Reduction Rate of 50%).

Table 1．
Cu-Sn-Fe-P Alloy Composition (Mass Fraction, wt%) 3 Test results and analysis3.1 Ally phase diagram calculationBoth Pandat and Thermo-Calc phase diagram calculation software is used to optimize the alloy parameters.