Fabrication of phosphor bronze/Al two-phase material by recycling phosphor bronze chips using hot extrusion process and investigation of their microstructural and mechanical properties

1.   Introduction

Currently, recycling is one the world’s most popular topics. The lower cost of recycled materials makes them attractive, as there are so many waste materials produced by many industries, especially when alloys or metals are used in machining processes that generate a large amount of chips. Accordingly, much attention has been paid recently to the recycling of chips [1–5].

Bronze alloys are used in a wide range of industries. Since they are generally shaped by machining, a large volume of bronze chips is produced, which is mostly waste. The surface area of bronze chips is relatively large and covered with oil, so they must be recycled by re-melting, which is energy-intensive. The conventional recycling process is characterized by high energy consumption, high operating costs, and many operations. However, if the chips have sufficient formability or are mixed with a metallic powder as a binder, recycling can be accomplished through a hot extrusion process, thus eliminating the need for melting [6]. Using an extrusion process, the recycling costs can be reduced and non-oxidized production along with fewer environmental impacts can be expected [7–8].

In fact, by extruding the chips–metallic-powder mixture, a metal–metal composite material can be produced in which the chips reinforce the metal matrix. By choosing the appropriate proportion of chips in the mixture and performing hot extrusion at the optimum temperature, desirable mechanical properties can be obtained in the fabricated composite. In this process, a billet is produced by cold-pressing. After being preheated to a specific temperature, it is extruded to create mechanical and diffusional bonds between the chips and metallic particles that achieve near-theoretical density and desirable mechanical properties [9]. To ensure a good distribution of chips in the mixture, a mixing (milling) process is also required prior to shaping the initial billet by cold-pressing. Although Fogagnolo et al. [10] employed a new technique that eliminates the milling step and results in a more economic recycling process. Shirvani moghaddam and most other investigators [11–12] believe that poor dispersion affects the final mechanical properties of the produced composite and that mixing the composite’s components by milling can ensure a better distribution of fibers in the metallic matrix.

As a metallic powder with low specific gravity, optimal mechanical and physical properties, and corrosion resistance, aluminum has been widely used for the fabrication of metal-matrix composites and as a metallic binder in the recycling of metallic chips by hot extrusion [3,6,13]. Alternatively, phosphor bronze alloy has excellent mechanical properties, including corrosion resistance, high strength, and plastic fracture in tensile tests [14]. Regarding the good mechanical properties of phosphor bronze and the high volume of chips fabricated during machining processes, their recycling in combination with Al powder could be used to fabricate valuable composite materials.

However, the final properties of a metal-matrix composite depend not only on the properties of the matrix and reinforcement materials but also on the quality of the bonds at the interface of the matrix and fibers [11]. In the case of metal-matrix composites and metallic fibers, it has been shown that a good metallic bond between two metallic phases leads to better mechanical properties, excellent fatigue characteristics along with high load-bearing strength and impact resistance [15–16].

In the recycling of chips, Castro et al. [17] used high-pressure torsion at room temperature to achieve good consolidation in the production of a magnesium–alumina composite, which exhibited higher hardness and improved creep resistance than those reported in previous works. Tekkaya et al. [18] extruded AA6060 aluminum chips and observed no dependency between the properties of the final product and the chip sizes. In their study, the authors demonstrated that any kind of chips can achieve good consolidation if the appropriate pressure, temperature, and strain are applied. Even so, Peng et al. [19] observed that coarser chips achieve interfacial bonding more easily during the recycling of Mg when using the solid-state method and hot extrusion. Hu et al. [20] also showed that appropriate mechanical properties could be achieved in the recycling of Mg alloy AZ91D chips through a hot extrusion process. Further studies were conducted by Fogagnolo et al. [10], in which pre-compacted AA6061 including Al₂O₃ as reinforcement fibers was extruded and notable mechanical properties were achieved, although the surface quality of the produced composite was not good. Sherafat et al. [6] also recycled aluminum alloy Al7075 chips using the same method, with Al powder being the metallic binder and matrix. In another study, Guluzade et al. [21] innovatively recycled two different types of chips together, i.e., AISI 1040 alloy steel and aluminum alloy chips.

In this work, we recycled phosphor bronze chips by fabricating aluminum matrix composites in a hot extrusion process, in which the chips served as metallic reinforcements. We then studied the effect of extrusion temperature and the proportion of phosphor bronze chips used on the quality, microstructure, density, and mechanical properties of the two-metallic-phase fabricated composites.

2.   Materials and methods

In machining processes, phosphor bronze chips are produced that have an average length of 1 mm. The chips required for this study were produced using a cutting process and a cutter machine to reduce the chip size to an average length of 0.4 mm and cross section diameter of 0.1 mm, which were collected using an appropriate sieve. These chips were then used as reinforcement fibers in the composite. As the matrix of the composite, we used air-atomized commercially pure Al powder with an average particle diameter larger than 45 μm.

After obtaining the required materials, a jar milling process was used to prepare mixtures containing different proportions of chips in the range of 20vol%–60vol%, which took 90 min for each mixture. To consolidate the mixtures, first they were cold-pressed at 175 MPa of pressure in a tool steel die with the internal diameter of 27 mm. Using this process, billets were produced with a diameter of 27 mm and average length of 35 mm. To reach final densification, the billets were preheated using a laboratory furnace and then hot extruded at different temperatures of 400, 430, 450, 465, 480, and 500°C, with an extrusion ratio and extrusion punch speed of 7.29 and 0.2 mm/s, respectively.

Using an optical microscope (Olympus BX JIM), the microstructures of the fabricated composites were studied to determine the degree of porosity remaining and the quality of the bonding between the chips and Al matrix. The densities of the fabricated samples were also measured based on Archimedes’ law to determine the degree of densification for comparison with the observed microstructure results. Furthermore, to calculate the relative densities of the different extruded samples, the theoretical density of each sample was calculated based on the rule of mixtures, and the densities of pure Al and phosphor bronze alloy were determined to be 2.71 and 8.22 g/cm³, respectively.

Micro hardness and tensile tests were also conducted to investigate the mechanical properties of the produced composite. Micro hardness was determined based on the E384-89 standard using a Vickers indenter on polished and chemically etched surfaces. The test was performed by applying about 3 N load for 15 min and the average values obtained in at least five experiments were considered to be the micro hardness.

The tensile tests were conducted at a strain rate of 0.02 mm/s based on the ATSM–E8 standard. The stress–strain curves for fabricated commercially pure Al (not including chips) and composites including different volume fractions of chips and those extruded at different temperatures were compared to investigate the resulting improvement in the mechanical behavior of the fabricated composites. Scanning electron microscopy (SEM) images were also obtained to study the fracture surfaces of the produced composites.

3.   Results and discussion

3.1   Materials investigation

Table 1 shows the chemical composition of the phosphor bronze chips after sieving and cutting. Fig. 1 shows the cut and sieved phosphor bronze chips. Fig. 2 shows a micrograph of the Al particles used. Table 2 lists the different mixtures prepared in this study. The samples are categorized based on the volume fraction of chips in the mixture.

Table  1.  Chemical composition of the phosphor bronze chips wt%

Sn	P	Fe	Pb	Zn	Cu
6.0	0.12	0.02	0.05	0.2	Bal.

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Fig. 1.  Cut and sieved phosphor bronze chips.

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Fig. 2.  Micrograph of the Al particles used.

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Table  2.  Volume fractions of the chips in the different mixtures vol%

Mixture A	Mixture B	Mixture C	Mixture D	Mixture E
20	30	40	50	60

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3.2   Extrusion process

We attempted to extrude a mixture containing 20vol% of chips at 400, 430, 450, 465, 480, and 500°C, but at 400 and 430°C, the samples could not be extruded due to their poor workability at those temperatures and the maximum applied load. The samples extruded at 480 and 500°C were also unsuccessful, because the phosphor bronze alloy melted at those temperatures. The melting at these temperatures, which are lower than the melting point of the phosphor bronze alloy used, was considered to be due to the cold working during the machining, cutting of the chips, compaction process, and the increased temperature due to friction and redundant work during the extrusion process. Therefore, we used the two temperatures of 450 and 465°C for the extrusion process. Fig. 3 shows the surface quality of the samples containing 20vol% chips extruded at 450 and 465°C, in which we can see that the surface quality is not good for the sample extruded at the lower temperature. However, the consolidation was perfect in both samples and an appropriate product was achieved after machining the surfaces.

Fig. 3.  Mixture A (including 20vol% of chips) extruded at (a) 450°C and (b) 465°C.

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Other mixtures containing 30vol%, 40vol%, 50vol%, and 60vol% of chips were also tested and extruded at 465°C. Except for the mixture containing 60vol% of chips, all the others were successfully extruded. The powder mixture containing 60vol% of chips could not be extruded due to the high percentage of chips and the high degree of friction between the chips and Al powder, which increased the strength but reduced the workability of the prepared mixture. This mixture must be extruded at a higher temperature, which would then likely cause it to be molten, and is therefore not applicable.

3.3   Density and microstructure

Table 3 shows the relative density measurement results for the extruded samples containing 20vol% of bronze chips at 450 and 465°C, from which we can observe that the samples were consolidated perfectly at both extrusion temperatures. Also, for the extruded samples containing different volume fractions of chips extruded at a constant temperature of 465°C, the relative density decreased significantly with an increasing amount of chips (Fig. 4) due to the decreased workability of the powder mixture with increases in the proportion of chips. It can be observed that by increasing the proportion of chips in the mixture, the intensity and size of the pores and defects primarily located at the interface of the chips and matrix also increased, which led to a decrease in density. These results also indicate that the rate of the decrease in density increased by increasing the proportion of chips in the samples.

Table  3.  Physical and mechanical properties of mixture A extruded at two different temperatures

Extrusion temperature / °C	Relative density / %	Vickers micro hardness, HV	Ultimate tensile strength / MPa	Elongation / %
450	99.79	108	135.7	9.9
465	99.82	93	154.0	14.1

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Fig. 4.  Variation in relative density as a function of the volume fraction of chips used in samples extruded at 465°C.

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Fig. 5 shows SEM images of the cross sections of the samples containing 20vol% of chips extruded at 450 and 465°C, wherein pores are completely visible in the sample extruded at the lower temperature. For composites containing different volume fractions of chips extruded at 465°C, as shown in Fig. 6, it can be seen that increasing the proportion of chips clearly increased the amount of remaining pores. The darker points indicated by the white arrows in Figs. 5 and 6 are pores that can be clearly observed at the interface of the chips and Al matrix. It is obvious that the pores became even bigger in the sample containing 50vol% chips. These results agree completely with the measured relative densities and are considered to be due to the lower consolidation efficiency achieved due to poorer formability of the Al and bronze materials at lower temperature and their greater hardness [6].

Fig. 5.  Cross section of samples containing 20vol% of chips extruded at (a) 450°C and (b) 465°C.

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Fig. 6.  Cross section of samples containing (a) 30vol% of chips, (b) 40vol% of chips, and (c) 50vol% of chips extruded at a constant extrusion temperature of 465°C.

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3.4   Mechanical properties

3.4.1   Vickers micro hardness

Table 3 shows the micro hardness of the samples extruded at 450 and 465°C that contained 20vol% of chips. It shows a decrease in hardness with an increase in the temperature of the extrusion, which may be caused by the release of additional stress due to the higher processing temperature. This additional stress may be due to the cold-work strain that accumulated during the machining and cutting of the chips, the milling process, and the production of the initial billets. The increasing grain size at higher temperature can be expected and might also have caused a reduction in the hardness of both the chips and powder regions [6].

Fig. 7 shows the average micro hardness values of the samples containing different proportions of chips extruded at 465°C. It can be seen that the micro hardness of the fabricated composites improved by increasing the proportion of chips in the mixture, but not significantly. The same results were also observed in a study of a composite of Al7075/Al fabricated using Al7075 chips as reinforcements [6].

Fig. 7.  Micro hardness values of mixtures A, B, C, and D extruded at 465°C.

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3.4.2   Tensile properties

Fig. 8 shows the tensile stress–strain curves for samples that contained 20vol% of chips and were extruded at 450 and 465°C. In Table 3, it can be seen that by increasing the extrusion temperature, both the ductility and ultimate tensile strength (UTS) increased, which may be due to the improvement in the interfacial bonding between the chips and powders and the increases in the density and size of the grains [6]. It is obvious that the surfaces of the chips have different degrees of roughness and coarseness, some having micro sharpness at the corners and micro pores on their surfaces due to the machining process, as is clearly observable in Figs. 5 and 6. Micro extrusion occurred in these regions and these pores were filled with the matrix due to the notable flowability of the aluminum during the hot extrusion process, thus perfectly bonding the chips and matrix in the fabricated composites. This strong interfacial bonding achieves better UTS due to the greater force needed to extract the chip fibers from the Al matrix. The extraction of chips can be anticipated based on the strong connection between the fibers and Al matrix, which causes the fabricated composite to exhibit ductile fracture behavior in tensile tests [12].

In contrast, the stress–strain curves for the extruded samples that contained different volume fractions of chips (Fig. 9) show that all the composites that contained chips had a higher UTS, but less elongation-to-failure in comparison with the extruded pure aluminum. This result is completely expected. Since phosphor bronze chips have greater strength and lower strain-to-fracture than pure Al, their presence enhances the strength and reduces the elongation-to-fracture of fabricated composites [22]. As a result, more brittle behavior is observed when chips are added to pure aluminum and also when their volume fraction is increased in the composites. Thus, either by comparing the tensile test results of the extruded pure aluminum and fabricated composites or by increasing the proportion of chips in the extruded composites at a constant temperature of 465°C, the ductility is found to decrease whereas the UTS experiences a notable increase. Moreover, this result definitely reveals that chips serve as ideal fibers for effectively reinforcing the Al matrix [6].

Fig. 8.  Tensile stress–strain curves for samples containing 20vol% of chips extruded at 450 and 465°C.

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Fig. 9.  Tensile stress–strain curves for samples containing different proportions of chips extruded at 465°C.

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Fig. 10 shows the UTS and ductility of these samples in which it can be seen that by increasing the volume fraction of chips to more than 20vol%, both the strength and ductility of the composites decrease. In comparison with the sample containing no chips, the chips in the composite containing 20vol% of chips served as reinforcements and caused a significant increase in the UTS and not much decrease in the ductility. However, in the samples containing a greater proportion of chips, weak bonds resulted between the chips and Al powder and more pores formed around the chips, which caused a significant reduction in the strength and ductility of the fabricated composites.

With an increase in the volume fraction of chips, the distance between the chips in the composite decreases, which might cause difficulty in the movement of dislocations, whereby a pile up of dislocations can occur and lead to a reduction in elongation [21]. Based on the obtained results, it can be concluded that reducing the proportion of chips resulted in improved toughness, and 20vol% of chips may be the optimum volume fraction to be used in the applied extrusion conditions.

Fig. 10.  UTS and elongation values for mixtures A, B, C, and D extruded at 465°C.

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3.4.3   Fractography

Fig. 11 shows the fracture surface of the phosphor bronze/aluminum composite. It can be seen that the ductility of the aluminum matrix at the fracture surface is crystal clear in Figs. 11(a) and 11(b) whereas the phosphor bronze chips exhibited a brittle fracture, which contributed to the expected increase in the composite’s UTS. Figs. 11(a) and 11(b) show the fracture surface of the specimen containing 20vol% chips extruded at 465°C. Figs. 11(c) and 11(d) show the fracture surface of composite that contain the same proportion of chips but that were extruded at the lower temperature of 450°C. In Figs. 11(c) and 11(d), we can see some visible voids at the interface region of the chips and aluminum (as indicated by the white arrows) in the fracture surface of the composite. Some slight extraction can also be observed in both composites, but more occurred in the specimen extruded at higher temperature (Figs. 11(a) and 11(b)). As noted above, the coarseness of the chip surfaces results in perfect interfacial bonding between the chips and matrix. The observed voids at the interface of the chips and matrix in Figs. 11(c) and 11(d) clearly show that de-bonding of the chips and matrix has occurred to a certain extent during the tensile test [22]. However, the interfacial bonding was strong enough to keep the fibers in the matrix, and the fracture of the fibers occurred prior to the extraction process. On the other hand, as argued earlier, the mixture extruded at temperatures higher than 465°C experienced melting of the chips. Composites extruded at 465°C may have experienced local melting on the surface of some chips during hot extrusion, which may have caused a reduction in the coarseness of their surfaces. This phenomenon would lead to weaker interfacial bonding between some of the chips and matrix, emerging a wider range of extraction in the fracture of those composites. Some cavities also emerged in the composites extruded at 465°C, as indicated by the white arrows in Fig. 11(b), which was due to the extraction of fibers from those areas. These fibers would likely be found on the mirror side of the fracture surface.

Fig. 11.  SEM images of the fracture surfaces of composites: (a, b) sample containing 20vol% chips extruded at 465°C; (c, d) sample containing 20vol% chips extruded at 450°C; (d, f) sample containing 30vol% chips extruded at 465°C.

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Figs. 11(e) and 11(f) show the fracture surface of the composite containing 30vol% chips extruded at 465°C. It can be seen that voids appeared around chips along with some micro cracks and pores in the aluminum region (as indicated by the white arrows in Figs. 11(e) and 11(f)). The increased volume fraction of the reinforcement chips poses an obstacle to the formability of the Al matrix during plastic deformation of the composite in fracture. This stress constraint leads to the emergence of cracks in the Al matrix, which reduce the hardness and strength of the composites [22]. The fracture also seems to be less ductile than the composite containing 20vol% of chips extruded at the same temperature. This result could also be predicted based on the tensile tests results and microstructure observations. Moreover, the orientation of the fibers appears to be in the direction of extrusion in all the figures, which is a desirable result. This result can be predicted because the hot extrusion process aligns the secondary phase in a direction parallel to the extrusion direction when the amount of the secondary phase is large [23].

4.   Conclusions

The successful recycling of phosphor bronze chips was accomplished using a hot extrusion process with the use of chips as reinforcement in an aluminum matrix to fabricate a metal–metal composite. Based on the results obtained in this study, the following conclusions can be drawn.

(1) The number and size of the pores in the fabricated composite increased with a decrease in the temperature of the extrusion and an increase in the volume fraction of the chips in the mixture, which led to a reduction in density.

(2) Composites containing 20vol% chips extruded at both 450 and 465°C showed ductile fracture behavior in tensile tests. However, increasing the temperature of the extrusion caused an improvement in both the ductility and UTS of the fabricated composite.

(3) It was proved that the fracture in the aluminum regions was ductile in nature and that of phosphor bronze chips area was brittle. In addition, a micro extrusion occurred in the interface of the chips and powder which caused by coarseness and micro sharpness of the chips surface and resulted in a better bonding between the chips and matrix. This phenomenon led to the fabrication of a composite with relatively high strength and moderate ductility properties, especially for the composite containing 20vol% chips.

(4) Phosphor bronze chips can be recycled directly into a high-density composite using aluminum powder as a matrix. The optimum amount of chips in the composite and extrusion temperature were determined to be 20vol% and 465°C, respectively, which achieved the best physical, mechanical, and microstructural properties. The results proved that 20vol% chips effectively reinforced the Al matrix, whereas addition of more chips was accompanied by emergence of cracks and pores, which led to a decrease in strength and ductility.

Acknowledgement

The authors would like to acknowledge the financial support provided by Shiraz University through grant number 97-GR-ENG-16.