ABSTRACT: High Lead Bronze CUPB15SN7/45 STEEL BI-METALLIC LAMELLAR composites were prepared by melt-casting method. The effects of heating conditions and cooling rate on the interface and microstructure of copper-steel composites were investigated. The results show that when the charging temperature is 900 °c, the holding temperature is 1015 °c, the holding time is 5 min and the nitrogen is used to cool the interface, the interface has excellent metallurgical bonding property and tensile fracture strength reaches 200 MPA The results show that the distribution of lead particles in the copper alloy zone is uniform, the loss of lead element is less than 1% , the microstructure and grain size of the steel matrix are reasonable, and the composite effect of melting and casting is excellent. With the increasing demand for engineering materials, it is more and more difficult for parts made of single metal materials to meet all-round performance requirements. Due to the scarcity of precious metal resources, bimetallic lamellar composites have been widely used in industry. On the basis of maintaining the original properties of each metal layer, the overall performance of layered metal composites has been significantly improved, and its preparation process has been paid more and more attention. High lead bronze has good thermal conductivity, wear resistance, impact resistance and bite resistance, widely used in the manufacture of hydraulic components such as Piston pump cylinder. At the same time, because the lattice type, lattice constant and the number of outer electron atoms of copper and iron are very close, they have good metallurgical compatibility. COPPER-STEEL BIMETALLIC layered composites with steel as Matrix layer and high lead bronze as composite layer have excellent properties of both materials. At present, the commonly used methods for preparing copper-steel bimetal laminated composites include explosive cladding, rolling cladding, diffusion cladding, centrifugal casting, melting casting and powder sintering. Among them, explosive compounding and rolling compounding methods are mainly used to prepare bimetallic laminates, diffusion compounding method has long preparation period and long holding time, which is easy to cause serious lead oxidation and burning loss, and the cost is also high The Centrifugal casting method is easy to lead macrosegregation, the Powder metallurgy method involves many processes, the preparation period is long, the cost is high. And the method of melting-casting composite with two materials heated at the same time has obvious technical and cost advantages in preparing copper-steel bimetallic layered composite materials, in this paper, the effect of process parameters on the microstructure and properties of copper-steel bimetallic materials is studied by melting-casting composite test.

1 test plan 1.1 the materials used in the test include high lead bronze Cupb15sn7 and 45 steel. The chemical compositions of the two materials are shown in Table 1. Before the test, 45 steel was processed into a cylinder with a diameter of 42 mm × 45 mm, and a groove with a diameter of 32 mm × 7 mm was made at one end. The copper alloy was processed into a circular sheet with a diameter of 30 mm × 6 mm. After machining, surface treatment processes such as sanding, Alkali cleaning, acid cleaning and soaking in 80 °c saturated borax water are needed.

1.2 The high lead bronze/45 steel bimetal composite was prepared by co-heating fusion casting process. The melting and casting test was carried out in the resistance heating furnace of model SG-XQL1400, during which high purity nitrogen gas was injected into the furnace as a protective gas. When the temperature in the furnace rises to 900 °C, the assembled sample is put into the furnace, heated to the holding temperature and kept warm for a period of time. In the melting-casting test, three process parameters, i. e. holding temperature, holding time and cooling mode, are mainly controlled to study the effect of each process parameter on the microstructure and properties of high lead bronze/45 steel, especially the effect of interface recombination and the distribution of lead in copper alloys. The heat preservation temperature is set to 1000 °C, 1015 °C and 1030 °C, and the heat preservation time is set to 0,5 Min and 10 min respectively. 1.3 The copper-steel bimetallic samples obtained from microstructure observation and property test were cut out by machining, and the 45 steel matrix was corroded by 3% nitric acid alcohol solution, the microstructure and the particle size and distribution of PB in each region of the composite were observed by optical microscope with Axio Imager M2M. The change of lead content in copper alloy before and after the test was analyzed by ICAP7600 Spectrometer. The mechanical properties were tested by UNIAXIAL tensile test to evaluate the interfacial bonding effect of copper-steel composite specimens. According to the prepared copper-steel melt-cast Specimen and the National Standard Gb/t 228.1 as 2010, the drawing specimen size and drawing fixture are designed as shown in Fig. 2. The tensile specimens were fixed on a fixture and tensile tests were performed on a universal testing Machine Model Sans Cmt5305(MTS) to test the interfacial bonding strength of the composite. The tensile fracture strength was calculated according to the fracture diameter and the load after the test.

2 Test Results and analysis 2.1 microstructure analysis when the charging temperature is 900 °c, the holding temperature is 1000 °C and the holding time is 10 min, the cast sample of copper steel is prepared, the microstructure of the bonding interface and the copper alloy region is shown in Fig. 3. As can be seen from Fig. 3A, compared with the diffusion process, there is no transition zone at the interface of copper and steel due to the shorter holding time. In the process of melting and casting, the gap and pit on the surface of 45 steel are completely filled with molten copper alloy liquid, and good metallurgical bonding effect is formed after the sample is cooled. The results show that the bonding morphology can increase the bonding area at the interface, decrease the relative slip of the interface and increase the bonding strength of the composite.

As can be seen from Fig. 3B, the distribution of PB in the cast copper alloy is uniform, and the PB particle size ranges from 20 μm to 100 μm. During the solidification and cooling stage of the melt-cast composite copper alloy, the CU element is crystallized out as Dendrite, and the PB element fills the DENDRITE gap due to the low melting point, thus forming a strip-like distribution characteristic. 2.2 the effect of holding temperature on microstructure and properties when the charging temperature is 900 °c, holding time is 5 min and cooling mode is air cooling, the interface bonding morphology of cast copper steel samples prepared under different holding temperature conditions is shown in Fig. 4. As can be seen from Fig. 4, at different holding temperatures, after a certain period of time, the copper alloys can be completely melted and form a good metallurgical bond with 45 steel at the interface. As the liquidus temperature of the high lead bronze theory is 986 °C, when the copper alloy temperature exceeds the liquidus temperature, the good thermal conductivity promotes the rapid melting of the copper alloy, the molten copper alloy penetrates into tiny gaps in the surface of 45 steel to achieve metallurgical bonding. The results show that the excellent metallurgical bonding state of the steel-copper interface can be achieved by the combined heating and casting process.

It is important to note that the metallurgical bonding effects of the three interfacial bonding morphologies shown in Fig. 4 are slightly different, as can be seen from the tensile test results. The tensile fracture strength of the three groups of specimens is shown in Table 2. The test results in Table 2 show that the tensile fracture strength of the interface decreases to some extent when the holding temperature is 1000 °C and 1030 °C compared with 1015 °C. By analyzing the interface bonding state and morphology of steel and copper, it can be found that the formation of metallurgical bonding first needs the complete melting of copper alloy, when the holding temperature is on the low side, the short holding time can not guarantee the complete melting of copper alloy, because of the low superheat, the metallurgical bond formed at the interface is not strong. With the increase of holding temperature, the superheat increases and the copper alloy melts completely, which is helpful to promote the diffusion of alloy elements and strengthen the metallurgical bonding effect of the interface. But when the holding temperature is too high, due to the rapid accumulation and growth of lead elements, and the Brittle and hard nature of lead itself, the large-sized lead particles tend to separate at the interface during the tensile deformation and produce micro-voids, in the process of tensile fracture, the lead particles will break off preferentially, and then the tensile fracture strength will decrease.

In order to study the effect of holding temperature and holding time on the microstructure and morphology of copper alloy, composite specimens were prepared at holding temperature of 1000 °C and 1015 °c, holding time of 0, the interface morphology of the two groups of specimens is shown in Fig. 5. Among them, the copper alloy in Fig. 5A is not completely melted, and there is obvious void appearance at the interface, while the copper alloy in Fig. 5B is completely melted and the interface is completely metallurgical bonded. It can be concluded that the process of holding at 1000 °C for 0 ~ 5 min is just the process of gradual melting of copper alloy. However, lower holding temperature and shorter holding time mean lower superheat and shorter heat transfer time, so the copper alloy can not melt completely, which leads to the loss of the melted alloy at the interface and the formation of small holes. At this point, the interface of copper and steel can still form metallurgical bond in macro-size, but the tensile fracture strength is low. When the holding temperature is raised to 1015 °c, the metallurgical bond will be formed in different holding time, but the short holding time will cause the similar bond strength to be lower. Therefore, only under the combined effect of proper holding temperature and holding time, good metallurgical bonding effect can be achieved.

When the holding temperature is 1000 °C, holding time is 0 and 5 min respectively and air-cooled, the microstructure of the copper-steel composite sample is as shown in Fig. 6. It can be found that there are more large size lead particles in the 6B sample than in the 6A sample with small and uniform lead particle size. The above results have shown that the process of holding at 1000 °C for 0 ~ 5 min is a gradual melting process of copper alloy. Therefore, in the melting process of copper alloy, lead will be melted earlier because of the low melting point, and then quickly aggregate and grow, and the length of holding time directly affects the size and distribution of lead. The accumulation and growth of lead is a key technical problem in the process of composite casting, which is one of the factors to judge whether the parameters of the process are reasonable or not.

The length of holding time also affects the size and distribution of pb after the copper alloy is completely melted. Three groups of specimens were prepared by air cooling at 1015 °C for different holding time. Among them, 7A sample is not completely melted, there is a longer and wider void space at the interface, and the dimension of PB element is smaller. 7b sample and 7C sample are completely melted and metallurgical bond is formed at the interface. However, there are more large size lead particles in the copper alloy region of 7B sample, while the lead particle size in 7C sample is relatively small, mainly showing as strip shape. The results of ICP test of three samples show that the lead content of 7A and 7B samples is about 14.8% , while that of 7C samples is only 9% , and the oxidation loss is about 40% .

Based on the above experimental results, the Change Law of copper alloy structure and lead element in the process of melting and casting can be summarized. In the first stage, before heating the copper alloy to melt, because the lead element does not dissolve in the Copper Alloy Matrix and does not form compounds with other elements, the low melting point lead element melts earlier, under the restraint of the solid copper alloy and the continuous input of external heat, the molten lead is in a more active state. In the second stage, in the process of gradual melting of copper alloy, the molten lead accumulates rapidly and tends to form larger lead particles, but there is no lead burning loss in this stage. In the third stage, with the extension of holding time, the large-size lead particles begin to burn and change into strip-like and point-like distribution, and the content of lead particles decreases rapidly. 2.3 The effect of cooling method on microstructure and properties is similar to that of steel heat treatment process. Cooling rate has a significant effect on the microstructure and properties of the composite prepared by melting-casting process. When the holding temperature is 1015 °C and the holding time is 5 min, the microstructure of copper-steel composite prepared by different cooling methods is shown in Fig. 8. It can be seen that the distribution of lead in the copper alloy region of the three groups of samples is uniform. Compared with 8B and 8C samples, the lead particle size of 8A samples prepared by Air Cooling is smaller and more fine point-like distribution. The results showed that the lead content of 8A sample was 12% , which was about 20% lower than that before the test, and the lead content of 8B and 8C samples was over 14% . It can be inferred that the cooling rate is relatively low by air cooling method, which means a longer residence time in high temperature stage, which results in a certain degree of lead burning loss, by using Nitrogen Cooling and water cooling, the cooling rate is high, and the original burning loss of lead can be effectively avoided.

The Matrix morphology of 45 steel in three groups of specimens in Fig. 8 is shown in Fig. 9. The microstructure of the samples prepared by Air Cooling is mainly pearlite and Ferrite, and contains a small amount of widmanstatten microstructure. The samples cooled by nitrogen are mainly pearlite, ferrite and a small amount of austenite. The grain size is similar to that before the experiment. The specimens prepared by water-cooling method were obtained at 1000 °c, the water-cooling temperature was in the single-phase zone of Austenite, and the AUSTENITE and martensite with larger grain size were obtained. Therefore, the cooling rate should be controlled at a moderate level for the BIMETALLIC layered composite material and its parts made of 45 steel as matrix and high lead bronze alloy as composite layer, on the premise of ensuring the content and size of lead in the copper alloy, the ratio of pearlite and ferrite in the cooling steel matrix was increased to ensure the grain size of the cooling steel 45 and the mechanical properties of the composite.

2.4 reasonable melting and casting parameters were obtained by observing the microstructure of the copper-steel composite and testing the mechanical properties of the interface, the effect of the heat preservation condition and cooling rate on the interface bonding of the composite and the microstructure of the copper alloy is analyzed, the reasonable technological parameters for preparing copper-steel bimetal composite by co-heating melting-casting process are as follows: When the furnace temperature is 900 °c, the sample is installed in the furnace, the holding temperature is 1015 °c, the holding time is 5 min, and the sample is cooled by nitrogen gas. The copper-steel composite was prepared with reasonable melting-casting process parameters. The lead particles in the copper alloy matrix were uniformly distributed, the lead content was 14.6% , and the burning loss was less than 1% . The tensile fracture strength of copper-steel interface is over 200 MPA, which can ensure excellent metallurgical bonding effect. 3. Conclusion (1) when the size of copper-steel sample is determined, the furnace temperature, holding temperature, holding time and cooling mode are the key process parameters, which directly affect the microstructure and properties of copper-steel composite and the metallurgical bonding quality of interface. For the copper steel sample, the reasonable casting process parameters are: charging temperature 900 °c, holding temperature 1015 °c, holding time 5 min, cooling mode is nitrogen cooling. (2) the cu-steel composite prepared by reasonable melting-casting process parameters has a uniform lead particle distribution, 14.6% lead content and less than 1% burning loss, and the microstructure and grain size distribution of the Steel Matrix are reasonable; The interface of copper and steel has excellent metallurgical bonding property, and the tensile fracture strength of the interface reaches 200MPa, which shows good melting-casting composite effect. 

Source: Foundry magazine