1 Introduction

In the construction of machinery and plants as well as in the manufacture of automobiles, components made of copper and copper alloys are utilized in a variety of applications [1,2,3,4,5]. On the one hand, copper-based components are essential to the operation of electrical and electronic components in machines and motor vehicles due to their high electrical conductivity. On the other hand, they can be used in a variety of ways as sliding components and bushings. In machines, plants, and internal combustion engines, bearings, bushings, and sliding components guarantee the transmission or conversion of the drive energy [5]. One of the situations that imposes significant expenses on related businesses is a lack of stability under load. As copper wrought alloys used in the automotive or maritime industries reach their breaking point for increased mechanical, tribological, and thermal loads, these mechanisms exhibit very obvious wear, and copper surface composites are given particular attention [6]. The FSP could be used to gradually push copper base metals' stability under load limits further [4,5,6,7,8,9,10,11]. Cu/Gr composites with high strength and good electrical conductivity were explored by Naik et al. [7] using friction stir processing. They claimed that following FSP, the hardness of the Cu/Gr composite increased by 40%. Using FSP, Kaushal Kumar et al. [8] created several copper matrix surface composites that were strengthened by four different ceramic particles, including TiC, SiC, B4C, and Al2O3. They showed that B4C-reinforced surface composites had the lowest wear rate.

Friction stir processing is a good solution to the aforementioned issues because it is carried out at temperatures lower than the melting point of base alloys [12,13,14,15,16,17,18,19,20,21]. The physics of friction stir welding (FSW) serves as the foundation for this relatively recent technology [21, 22]. In contrast to FSW, friction stir processing locally modifies the alloy microstructure to obtain the required special characteristics. FSW is intended to connect several solid materials. It has distinct benefits over other surface treatment methods for metallic materials, including the following: (1) FSP is a solid-state, one-stage processing technique that provides grain refinement, strengthening, and structural homogeneity without changing the shape and size of the processed metallic material [23,24,25,26,27,28,29,30,31,32]; (2) the microstructure and mechanical properties of the processed items may be easily adjusted by changing the process parameters [33, 34]; (3) the approach is both economically and ecologically friendly. FSP has seen substantial development over the past few decades and has been extensively used in both science and industry [32]. The microstructure and mechanical properties of metal alloys can also be altered by prolonging the tool pass time, changing the tool rotation direction between passes, or using multi-pass FSP, although the results differ depending on the material [35,36,37,38]. Multi-pass FSP is frequently used in composite fabrication and creates materials with more homogeneous phase distributions and more efficient in situ processing reactions than single-pass FSPed materials [32, 33, 35, 39]. Multi-pass FSP can also be used to create materials with a large processing area by using neighboring tracks that are closely spaced apart; the overlap zone between two successive tracks has a complex structure [40]. According to studies on the effect of pin shape on heat generation during the plunge stage, the effective pin area directly affects friction-induced deformation and heat production. The effect of tool size and geometry on the microstructure and properties of materials is carefully explored in references [31, 32, 35, 39, 41]. According to Selvam et al.'s macrostructure research [42], the formation of a defect-free FSP zone is dependent on the tool profile and rotating speed. Joints formed using a triangular tool profile are less likely to have faults than joints made with other rotational speeds (1200 and 1400 RPM) when compared to other pin geometries.

Particle-reinforced metal matrix composites increase some mechanical properties of metals, particularly the elastic modulus, without the production challenges and cost penalties associated with continuous fiber-reinforced composites. In some systems, there are sometimes slight improvements in the frictional and wear behavior as well as a decrease in the coefficient of thermal expansion. Their main drawback, like with other composites, is that they are less ductile and robust than unreinforced metals. However, because of the physical enhancements they offer, particle-reinforced metal matrix composites are already locating commercial niche markets [1]. They have primarily been made using molten metal processing or the powder metallurgy (P/M) route [1, 12]. The FSP is utilized because it can provide the following functions in one step: (a) severe plastic deformation to promote mixing and refining of constituent phases in the material; (b) hot consolidation to form a fully dense solid.

Thanks to its high melting point, high density, and resistance to oxidation, tungsten (W) could be an excellent additive to produce metal matrix composites, which opens up a wide range of applications in high-temperature structures [13,14,15,16,17,18]. Naik et al. [7] showed that 10% of W reinforcement particles were required to achieve the best wear resistance in copper base metal at room temperature. The findings of the variance analysis showed that the quantity of reinforcement particles had a substantial impact on the characteristics of the other parameters. On the other hand, the solid solution strengthening feature may be a well-known method of strengthening, notably within the field of copper-based alloys [43,44,45]. When pure copper is alloyed with a small amount of alloying elements, it forms a homogeneous structure. Its exact amount depends on the solubility of particles within the copper [42]. Once the solid solubility limit is exceeded, two separate structures with totally different compositions and hardness are shaped. It is interesting to note that copper-nickel has been proposed as a surface coating for situations where fretting occurs, such as mechanisms where the dominating contact conditions in them appear to be high-frequency, large stress waves, as Surendrababu et al. demonstrated [46].

Most studies on the use of secondary particles, such as nickel particles, in the friction stirring process have focused on the base metal aluminum. As a result, less attention has been paid to studying the wear behaviors of copper base metal at high temperatures and the effects of introducing secondary nickel particles during the friction stirring process. Jaswinder et al. [47] produced an A413/Ni surface metal matrix composite via friction stir processing. They assert that increasing the number of sliding wear test passes caused the Al3Ni intermetallic particles to be distributed uniformly, considerably improving wear resistance at both low and high temperatures. Several efforts have used FSP to prepare Cu-based surface composites. In these investigations, SiC [4,5,6,7,8,9], a pure copper surface particle, was used to enhance B4C [6] via FSP. These studies have focused on enhancing the process variables (rotation, tool traverse speed, and depth of penetration) in order to increase the resistance to indentation and wear. The tool pin profile has also been altered to test its effect on equally distributing reinforcements in the treated zone. Only three of the aforementioned variables—tool rotation speed, tool traversing speed, and amount of reinforcement—were shown to have a substantial impact on the mechanical and wear characteristics of the surface composites. However, there hasn't been much research done on how these composite materials wear at high temperatures. In order to study the impact of the modified surface's microstructure, mechanical properties, and high-temperature friction behavior, which has not previously received much attention, the current work aims to produce two simultaneous strengthening mechanisms, including (1) the formation of a copper and nickel solution and (2) the addition of a second rigid phase of tungsten to the substrate of pure copper using the FSP method.

2 Materials and Methods

On the surface of pure copper sheets, a groove with dimensions of 2 mm * 2 mm * 100 mm was machined along the length of the sheet in the rolling direction. A combination of nickel and tungsten powders with volume percentages of 70% Ni + 30% W and 40% Ni + 60% W was milled. Reinforcing particles of nickel and tungsten with a particle size of fewer than 15 microns were used. The S1 specimen groove was filled with nickel powder. The S2 and S3 specimen grooves were filled with a blend powder of 30 vol% W and a blend powder of 60 vol% W, respectively.

The atomic size difference between the principal element and a solute is known to increase the strengthening effect of solid solutions. The expansion of the fcc lattice caused by the addition of the substitutional alloying elements leads to strengthening [20, 21]. Table 1 lists the solubility information for these elements in Ni at 1000 °C as well as the approximate atomic diameter changes for Ni, Cu, and W elements. W clearly provides the ideal atomic radii mismatch and significant solubility required for solid solution strengthening. Due to its great solubility in Ni, Cu can also be employed to reinforce solid solutions [41].

Table 1 Approximate atomic size variations and solubility limits of Cu and W in Ni
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The triangular pin profile was used to create the FSP tool. The pin profile and mixed nickel and tungsten powder reinforcement are shown in Fig. 1. Three such plates were friction stir processed by an axial force of 8 kN, a tool transverse speed of 70 mm/min, and a tool rotational speed of 1200 rpm. This group of process parameters was chosen after trial experiments. For better mixing of reinforcing powders and substrate, the FSP tools were held with a slope of 2° toward the surface of the specimens, and the operation was done in two passes with the same starting point. It was determined from the FSP literature that multi-pass FSP up to two passes enhances grain refinement. The counterclockwise rotation of the FSP was chosen to increase the concentration of reinforcing powder dispersion.

Fig. 1
figure 1

a Friction stir processing pin profile, b Nickel-Tungsten powder; milled for 3 h

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For metallographic evaluation of the FSPed sections, the etching solution used to reveal the structure of the specimens was 4 g FeCl3, 2 ml HCl, and 75 cc of C2H5OH. Through energy dispersive spectroscopy (EDS) with the map and line scan methods, the formed phases were examined. Moreover, the micro-hardness of specimens was also measured with a 50-g load.

In addition, due to the rotational nature and temperature rise at the junction site for some mechanisms, pin-on-disk tribometer tests were utilized to evaluate the wear behavior and friction coefficient of the FSPed specimens. Since the actual parts have a reasonably constant relative linear velocity, this technique has been proposed for abrasion problems and testing industrial policies. The pins used in the wear test were made of the material in the center of the processed region of all specimens. The pin-on-disk test was performed at 250 °C with a counterpart made of WC. The nominal load and sliding speed in the wear test were 65 N and 0.2 ms-1, respectively. The friction coefficient was evaluated at a sliding distance of 500 m. For a more accurate evaluation of the wear phenomenon on each specimen, SEM images were prepared of the pin surfaces of specimens and counterparts, and the weight loss in specimens and disks was measured before and after the wear test at high temperatures.

3 Results and Discussion

3.1 Microstructure of Surface Composites

Dynamic recovery and recrystallization have been identified as the primary mechanisms for grain refinement during FSP by researchers [22,23,24]. The prospect of dynamic recovery is brought about by the heat produced during FSP, consistent with Sakai et al.’s theory [25]. According to this theory, during dynamic recovery, dislocations organize themselves at low-angle grain boundaries (LABs). As processing goes on, dynamic recrystallization takes place, misorientation between the boundaries grows, and LABs change into high angle grain boundaries as a result (HABs). Accordingly, the primary mechanisms for grain refining are dynamic recovery and recrystallization [26, 27].

Some of the images considered for calculating the average grain sizes of base material and friction stir processed specimens are shown in Fig. 2. It is observed that the grain size has become finer in the FSP specimens in comparison to the base metal (21 µm). This decrease in grain size was observed in other specimens, including W. An increase in W in the S3 specimen caused further reduced grain size. The grain size of the recrystallized nugget zone is determined by the linear intercept method [28] and is presented in Table 2.

Fig. 2
figure 2

Microstructures of processed specimens and base material a base material, b S1 specimen, c S2 specimen, d S3 specimen

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Table 2 Average grain size of processed specimens and pure copper
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The authors of this research believe that the presence of W promotes material straining during the FSP process based on the evidence provided above. In fact, it is generally known that dislocation density rises as strain does. This phenomenon will be easier to see when the W value of the processed specimens rises. In specimens that have undergone FSP processing in comparison to the base material, greater dynamic recovery and recrystallization occur, and also finer grains will form [29,30,31,32,33,34,35,36]. This explains why samples processed with FSP containing W would have smaller grains than samples processed with Ni.

EDS and mapping intercept images from a specimen with W have been shown in Fig. 3 to examine the phases in the composite Cu/Ni/W produced. As can be seen, after performing two passes of FSP on the specimens, some of the reinforcing nickel powder was detected as residual nickel in the composites. Sharma et al. also reported the agglomeration of reinforcing nickel particles after doing a pass of FSP [37]. The zones rich in nickel contained tungsten particles, and it was observed that W was dispersed in the Cu zones, which could be used to increase uniform hardness in the FSPed areas.

Fig. 3
figure 3

EDS and Map-scanning images on a processed specimen including W particles

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3.2 Micro-hardness Analysis

Figure 4 depicts the micro-hardness measurement of the stir zones of processed specimens. The micro-hardness of Cu/Ni composite (specimen S1) increased in comparison to pure copper (57 Vickers) by adding reinforcing nickel powder; this increase in micro-hardness was attained in the range of 81–103 Vickers (Fig. 4c). In the outside area of the stir zone (specimen S2), the minimum value of micro-hardness was determined to be 145HV (Fig. 4b); in the same area of specimen S3, micro-hardness was discovered at around 166HV (Fig. 4d). Specimen S3 had a maximum micro-hardness of 236HV (Fig. 4a). In other words, the use of two mechanisms simultaneously increased the micro-hardness of these composites by 50% when compared to composite Cu/Ni. These mechanisms included the use of solid solutions with nickel as a process participant and the addition of secondary hard reinforcement particles of tungsten in a pure copper matrix.

Fig. 4
figure 4

Micro-hardness measurement a, d S3 specimens, b S2 specimens, c S1 specimens

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3.3 Wear Mechanisms

Figure 5 illustrates the average of wear degrees of surface hybrid metal matrix composite and its counterparts under load. Moreover, it shows a steady increase in material elimination from the surface hybrid metal matrix composite pins with a decrease in the volume percentage of tungsten in specimens at a constant descending speed.

Fig. 5
figure 5

The average of wear degrees of surface hybrid metal matrix composites and their counterparts

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As can be seen, specimen S1's pins have worn out severely compared to specimens S2 and S3, as seen by the disc's weight increasing from specimen S1 to specimen S3. When compared to other specimens, specimen S3 shows increased resistance to wear due to an increase in the volume percentage of reinforcing tungsten particles. This is indicated by an increase of 0.5 mg in the weight of the disk used in the wear test of specimen S3.

Figure 6 displays the chart of the S1, S2, and S3 specimens' friction coefficients. The stick–slip behavior of the pin surface with the counterpart in specimen S1 during a wear test at 250 °C is clearly seen in Fig. 6a. In comparison to specimen S1, which contains 30% wt. of reinforcing tungsten particles, specimen S2 in Fig. 6b has better conditions than Fig. 6a.

Fig. 6
figure 6

Variation of friction coefficient with sliding distance for a S1 specimen, b S2 specimen, and c S3 specimen

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The specimen S3's friction factor graph (Fig. 6c) demonstrated that a high wear resistance was attained as a result of the specimen's pin surface's minimal adhesion rate to its steel counterpart. The existence of abrasion between the three items is one of the factors contributing to stick–slip friction. Three-object abrasion may result from the separation of tungsten particles in these composites.

However, as seen in Fig. 6b, c, the addition of tungsten has resulted in a further decrease in variations of the specimen's friction coefficient, which demonstrates the existence of stick–slip caused by the three-body abrasion brought on by copper's inherent features. The results of this study's friction coefficients showed that specimens S1, S2, and S3 had increased abrasion resistance. Figure 6a demonstrates an increase in the pin's adhesion in specimen S1 at a distance of 300 m, which indicates significant pin wear and the removal of the copper-nickel solid solution layer in this specimen.

Moreover, a 300-m distance observation revealed an increase in the specimen S2's coefficient of friction. The composite Cu/Ni used in this work for high-temperature applications had an optimum percentage of tungsten particles based on the friction factor graph of specimen S3 (Fig. 6c), which had a consistent slope. The wear test results for specimens S1, S2, and S3 indicated friction coefficients of 0.48, 0.39, and 0.36, respectively. Specimen S3's lower coefficient of friction compared to the other specimens showed that nickel was continuing to dissolve in copper and that the residual nickel and tungsten particles were distributed more uniformly on the surface of this specimen.

Figure 7 displays the SEM images of the wear surfaces between pins and their specimen counterparts. As demonstrated in specimen S1 (Fig. 7 a, b), the layer produced by wear particles had only partially separated. It might be a sign of relatively good copper and nickel dissolution in this specimen, which would explain its relative hardness. The incomplete separation of layers produced by wear particles in specimen S1 (see Fig. 4) may be caused by an increase in the specimen's micro-hardness.

Fig. 7
figure 7

Wear surfaces morphology between pins and counterpart, a, b S1 specimen, c, d S2 specimen, e, f S3 specimen and g, h counterpart surface

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On the surface of the pin of specimen S1, which was typically associated with a certain load of 65N, a series of tiny cracks were formed that were perpendicular to the direction of the wear test motion. Deep plows were produced on the pin surface of the S1 hybrid metal-matrix composite at higher rates of penetration through the hard asperities of the counterpart steel surface.

With an increase in testing temperature, the strength of the copper matrix decreased gradually, which led to heavy plastic deformation in the subsurface region. According to the shear instability model of Zykova [38], the void density of the shear layer increased with decreasing distance to the surface. When the void density was high enough to reach a certain critical value, a so-called "crack-like" region, in which the strength is lower than the nearby materials, was formed. In the friction process, shear instability occurred in some of these regions, resulting in an accelerated increase in shear displacement and then turbulent plastic deformation in the subsurface region. When the longitudinal cross-section of Cu/Ni composite tested at 250 °C was examined, long cracks propagating (preferentially along the Ni–copper interfaces) almost parallel to the sliding direction could be clearly seen (Fig. 7). When adhesion occurred between the contacting surfaces, massive materials could be easily broken as a result of this. Therefore, over a particular threshold temperature, there was extreme wear (even seizure). This phenomenon may be clearly seen in Fig. 6a.

Figure 7c, d depict the surface wear on the pin of the S2 specimen. According to Ravinder [39], the worn hybrid composite pin surface under load employed in this investigation was covered with grooves that were parallel to and vertical to the sliding direction, which were indicative of abrasive wear. Wear was caused by the removal of tiny pieces of the pin material due to hard asperities on the surface of the steel counterpart and/or hard particles between the pin and the disk.

The S2 hybrid metal matrix composite pin wear surface demonstrated an adhesive wear mechanism with rows of furrows, evidences of plastic deformation, and a distinct transferred materials-sheltered wear track on the counter steel disk.

The threshold softening of the hybrid composite required to initiate the adhesion between the pin and disk appears to be caused by frictional heating at the loading state. Despite this, Singh et al. [40] revealed adhesive wear as the primary mode of wear in copper composite materials.

The decreased volume percentage of tungsten particles utilized in specimen S2 compared to specimen S3 is responsible for the development of an adhesive wear mechanism. Similar to how specimen S2's lower micro-hardness compared to specimen S3 could be a factor in the development of this mechanism.

The S3 hybrid metal matrix composite pin's extruded layers at the trailing edge are depicted in Fig. 7. With barely protruding layers of materials at the trailing edge, thermal softening and local melting of the corroded hybrid composite S3 pin face became evident. Additionally, the counter steel disk showed more material transfer than the worn surface, which was similarly found to be relatively unremarkable. On the counterpart surface, delamination had worn away small craters (Fig. 7g, h). The results on the impact of delamination on the copper hybrid metal matrix composite versus its hard-working tool steel counterpart were consistent with previously published information, as reported by Selvam et al. [42].

However, the major wear mechanism in the S3 specimen is the adhesive wear mechanism. According to SEM, the evaluated surface hybrid metal matrix composite's worn faces exhibit four distinct wear mechanisms functioning singly or in combination. Worn faces exhibit four distinct wear mechanisms that function singly or in combination. Abrasion, delamination, adhesion, thermal softening, and melting are the recognized wear mechanisms. As Sharma et al. [43] indicated, statements about the dominant roles of abrasives, abrasion, and adhesion as wear mechanisms in this analysis are consistent with those of past investigations.

Hybrid metal matrix composites (MMCs) offer a greater potential for usage in structural engineering and functional device applications compared to their conventional counterparts because they overall perform better in terms of mechanical and functional response. Copper metal matrix composites may be utilized in a variety of complex or numerous processes in heavy industries that operate at high temperatures.

4 Conclusions

  1. 1.

    When Ni + W powder was utilized as the reinforcement phase, the grain size of the treated specimens fell from 8 to 3 m. The grain size was reduced by 85% as compared to the base metal (21 m).

  2. 2.

    Instead of the Cu/Ni specimen, the stick–slip phenomenon was observed to be less pronounced when Ni + W particles were added to the copper matrix.

  3. 3.

    When compared to base metal (57 HV), a progressive rise in micro-hardness values from 103 to 236 HV was seen, along with a reduction in grain size.

  4. 4.

    The worn-out morphology of the FSPed specimen demonstrated that abrasive and adhesive wear were both involved in the wear processes.