Electric Resistance Sintering of Al-TiO 2 -Gr Hybrid Composites and Its Characterization

: In the present work, Al-TiO 2 -Gr hybrid composites were fabricated through a sustainable manufacturing approach, i.e., ERS (Electric Resistance Sintering) technique. In this experimental work, sintering is performed in a high-density graphite die, which also works as a heating element. The green compacts kept in the graphite die are sintered in two ways simultaneously (conduction and resistance heating). This facilitated the accomplishment of the sintering at a lower current (300–500 A). The aluminum (Al) was reinforced with 9 wt. % TiO 2 (rutile) nanoparticles and 3 wt. % graphite microparticles to synthesize a self-lubricated high wear resistance material. Mechanical properties such as density, hardness, and wear loss of the Al-TiO 2 -Gr hybrid composite were investigated. Scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) were performed for microstructural investigation. The experiments were performed according to the Taguchi design of the experiment, where three input process parameters (temperature, holding time, and sintering load) were taken to fabricate the Al-TiO 2 -Gr composite. The sintering temperature of 550 ◦ C resulted in the maximum value of mean sintered density (approx. 2.45 gm/cm 3 ). The holding time of 10 min for the sintering resulted in the maximum value of mean sintered density and mean hardness (HRB 53.5). The mean value of wear loss was found to be minimum for the composites sintered at 600 ◦ C for 10 min. The maximum value of the sintering load (800 N) revealed better density and hardness. Worn surfaces and wear debris were also analyzed with the help of SEM images. The sintering temperature of 600 ◦ C resulted in imparting more wear resistance which was proved by smooth surfaces, micro-cutting, and fewer crates, grooves, and smaller pits.


Introduction
Aluminium metal matrix composites (AMCs) have always been in high demand in the automotive, aerospace, and mineral processing industries due to their superior strengthto-weight ratio [1], high strength and modulus, good ductility, excellent wear, corrosion resistance [2], high temperature creep resistance, low coefficient of thermal expansion, and better fatigue strength, etc. [3]. The mechanical properties of composites are mainly dependent upon the type of reinforcement, particle size and fraction and processing method [4,5]. Various manufacturing techniques, such as stir casting, mechanical alloying, and powder metallurgy, are adopted to fabricate AMCs [6]. Each manufacturing approach has a unique set of benefits and drawbacks [7]. For instance, the casting process offers a clear advantage in terms of low-cost manufacturing, but non-uniform dispersion of reinforcement and undesired interfacial reactions result in a loss of mechanical properties [2,7]. On the other hand, powder metallurgy (PM) has gained popularity in the production of AMCs because of its potential advantages, such as uniform dispersion of reinforcements in matrix material, and low possibility of developing any intermetallic phase due to processing at a lower temperature than casting. Further, the fabrication of near-net-shaped products and the ability to produce parts of complex shapes to close tolerance are the unique feature of the powder metallurgy process [8][9][10]. The final properties of the product are significantly influenced by the sintering procedure as well as its processing variables like temperature, holding time, pressure, etc. [7,11]. Therefore, powder metallurgists, in collaboration with material scientists, are continuously putting effort into developing efficient and improved sintering techniques to achieve superior microstructures and mechanical characteristics [7]. In the conventional sintering method, slow heating rates and prolonged holding times typically cause excessive grain coarsening. In addition, the non-uniform distribution of heat creates a thermal gradient in the specimen that results in internal stresses in the product [9,11]. In recent years, some advanced sintering techniques such as microwave (MW), laser, electric resistance, and spark plasma sintering (SPS) have been utilized to sinter AMCs [11,12]. These non-conventional sintering techniques offer many advantages over conventional heating methods, including improved diffusion processes, high heating rates, shorter processing times, and better control over microstructure during processing, with improved properties in the final products [11,12]. The SPS technique is widely used for the fabrication of MMCs [11,13]. However, the SPS equipment is too expensive to meet the demands of mass production in the industrial sector. Furthermore, with long-term continuous operation, pulsed electric current generators are easily destroyed. Therefore, it is required to develop a super-fast consolidation, simple and low-cost sintering technique [13]. Previous works by researchers on non-conventional sintering showed that ERS is a faster sintering technique than microwave and SPS [9,14,15]. S Maki et al. [16] fabricated Cu-Al 2 O 3 composites by the ERS technique applying power input up to 12 kW and Voltage of 8-10 V. A. H. Ansari et al. [17] sintered brass by applying DC of 0.4-1.0 kA on voltage 12 V. M. A. Lagos et al. [15] developed an ERS process for the sintering of hard metals. They fabricated WC-Co composite applying current 35 kA on 40 V. F. G. Cuevas et al. [18] investigated the effect of different ceramic dies in ERS. M. B. N. Shaikh, et al. [9] sintered Al-SiC composite by applying the DC of 0.8-1 kA at a low voltage of range 3-5 V using a steel die lined with an alumina tube to insulate the green compact.
Thus, previous works show that a DC of very high intensity, in the range of 1-10 kA, having a low voltage of the range of 3-12 V is required for ERS. A ceramic die is generally used in ERS [18], in which powder or green compact is placed for sintering by applying DC [9]. The resistance of green compact causes to generate heat according to Joule's heating [9,15]. Sometimes, the porosity of the loose powder kept in the ceramic die causes a problem, and the current of lower intensity on low voltage does not flow. In the current work, a high-density graphite die is used in place of a ceramic die, and it gives a favorable result. In tribological applications, hybrid AMCs reinforced with micro, and nano-sized particles are being explored due to their advantages, such as micro-sized reinforcements supporting the skeleton of soft Al matrix and nano-sized reinforcements improving mechanical characteristics by promoting fine grain structure and preventing dislocation movements [6]. In comparison to micron-sized reinforcements, the inclusion of a small proportion of nanoparticles can increase the characteristics of Al without affecting its ductility [7]. Automobile components made of AMCs utilizing SiC, Al 2 O 3 , B 4 N, TiC, etc., are preferred in vehicles so that weight can be reduced and fuel efficiency can be increased, along with providing good wear resistance and high cycle fatigue resistance at moderately elevated temperatures [19]. Researchers have paid little attention to the use of TiO 2 as reinforcement in AMCs, despite its high hardness, and modulus, as well as excellent wear and corrosion resistance [20,21]. Rutile is a natural form of TiO 2 mineral that is economical and readily available [22]. The incorporation of any solid lubricant is advantageous under dry sliding conditions because it forms a thin layer of lubricant that reduces wear loss [6]. For further enhancement of tribological performance, graphite is the most suitable and frequently used material because its inclusion in AMCs also causes improved wear resistance and machinability, excellent anti-seizure effects, low thermal expansion, and high damping capacity in the product [6,23]. Therefore, in this work, the Al matrix is reinforced with 9 wt. % TiO 2 (rutile) nanoparticles and 3 wt. % graphite micro particles to fabricate a self-lubricating Al-TiO 2 -Gr hybrid composite which would be useful for bearings and bushing production.
According to the best of the authors' knowledge, there has been no research on ERS using the high-density graphite die in powder metallurgy. In addition, there has been no work on the fabrication of an Al-TiO 2 -Gr composite having the above-mentioned composition prepared by this new approach in ERS. L25 array Taguchi models were used for the optimization of the process parameters of ERS. Further, the mechanical properties such as density, hardness, and wear loss are also investigated as well as the effect of sintering temperature, holding time, and sintering pressure on these properties are also analyzed. Worn surfaces and wear debris are analyzed through SEM images to understand the mechanism of wear.

Raw Materials and Their Characteristics
The pure Al and graphite powder was purchased from Central Drug House Pvt. Ltd., India. Nano-sized particles of TiO 2 (rutile) were obtained from Sisco Research Laboratories India. Some usual properties of these powders are shown in Table 1. These powders were used with the following composition: 88 wt. % Al, 9 wt. % TiO 2 and 3 wt. % Gr. To achieve the above composition, the weighing of powders was done carefully on a digital balance (Precisa, Model No ES225SM-DRSCS, Dietikon, Switzerland) with readability up to 5 decimal places in grams. Weighted powders were sealed air-tight in a stainless-steel container and mixed properly at an ambient temperature of 25 • C in a low-energy vibratory ball mill (Fritsch, Pulverisette MM-1552, Germany) for 30 min at a constant rotational speed of 300 rpm. High stainless steel (HSS) balls of 10 mm diameter were used as grinding media in this blending process. Ball-to-powder ratio (BPR) was kept at 10:1 to reduce the chance of clustering in these powders. To avoid the excessive cold welding of the aluminium particles among themselves as well as onto the surface of balls and vials, 0.5 wt. % of stearic acid (C 18 H 36 O 2 ) was also added to the mixture, which works as a process control agent (PCA) during this mechanical mixing process. The morphological characteristics and elemental composition of powders were investigated using scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectrometry (EDS) (Jeol, JSM 6510LV-Japan). Different phases present in Al-TiO2-Gr composite powder were identified using an x-ray diffractometer (XRD, Bruker, D8 ADVANCE, USA).

Preparation of Al-TiO 2 -Gr Composites
Mechanically mixed composite powders were compacted through a single-acting uni-axial hydraulic press (Type KE, Sr No. 1327, Kimaya Engineers, India) within a high carbon steel pallet die having a 12 mm internal diameter at an applied pressure of 300 MPa and room temperature of 25 • C. Generally, the cold welding of some Al metal particles along the die wall takes place due to friction during compaction; therefore, to avoid this problem as well as for easy removal of the sample from the die, the curved surfaces of the die cavity and punches were lubricated properly with zinc stearate (C 36 H 70 O 4 Zn) after every compaction. For each run, the sample was kept in the die under pressure for 25 s approximately to minimize the effects of back-pressure produced during compaction.
After cold compaction, green compacts were pre-sintered before ERS. The pre-sintering operation was carried out in a tubular furnace (Ants Innovations Pvt. Ltd. India) at 200 • C for 30 min with a heating rate of 10 • C/minute under the controlled atmosphere of N 2 gas. The pre-sintering facilitates in burning off moisture, additives, etc. and the integrity of the compacts for the sintering is also improved [9]. The heating curve adopted in the pre-sintering is shown in Figure 1. Green compacts were allowed to cool naturally in the tubular furnace. For the ERS operation, the pre-sintered compact was placed in high-density graphite die between the upper and lower graphite plunger attached to the copper electrode. The schematic diagram of the ERS setup is shown in Figure 2. Available AC (220-230 V) is converted into low voltage and high current with the help of a continuously variable autotransformer (Make-Automatic Electric Limited, Mumbai, India) by simple transforming action further, the output is supplied to the rectifier to convert alternating current (AC) into plain direct current (DC). Hence, a plain DC in the range of 200-400 A of low voltage (3-5 V) was obtained that was applied for the sintering of the green compact. The resistance of green compact is caused to generate heat according to Joule's heating [16].
At the same time, the resistance of graphite die, and punches also produced heat when the current passed through it, and hence the graphite die also worked as a heating element. Thus, the sample was heated due to the conduction of heat from the graphite die to the AMC sample, along with resistance heating of the AMC. Hence the sintering happened in both ways simultaneously at a lower intensity of the current (300-500 A). The sintering cabin is shown in Figure 3. The experiments were performed according to the L25 array of the Taguchi Model. Following these three factors were selected as input process parameters: (i) sintering temperature, (ii) holding time during sintering, and (iii) sintering pressure. Each of these factors had five levels for the analysis of their effect on the properties of the composites. The design of the experiment based on the Taguchi model is shown in Table 2.

Microstructural Characterization (SEM, XRD, EDS & Optical Micrographs)
SEM images were captured at 15 kV in secondary electron mode at various magnifications. To verify the presence of reinforcing particles (TiO 2 and graphite) in AMCs, a structural phase study of the fabricated composites was performed using an X-ray diffractometer operating at 40 mA current and 20 kV accelerating voltage using CuKα radiation (wavelength λ = 1.5406 Å). To determine the present phase, data were collected at a slow scan speed of 0.02 steps/s with a step size of 0.01 • within the scanning angle range 20 • ≤ 2θ ≤ 80 • . DIFFRAC ® plus (Bruker AXS Inc., MA, USA) software was used to process the obtained data. Dispersion of reinforcements in matrix material and homogeneity of AMCs were performed using EDS elemental mapping. Sample preparation is the first step to taking the optical micrographs through a microscope. Sintered compact was mounted with a resin mixed with hardener (Araldite) that facilitates a better and more stable holding during polishing. Initially, the mounted samples were polished with the help of emery papers of grit sizes 180, 220, 320, 400,600, 800, 1000, 1200, and 2000.
Finally, these samples were polished on a single disc polishing machine on synthetic velvet cloth using alumina as grinding media. To reveal the microstructure, polished samples were etched with NaOH solution. The optical micrographs of the polished sample were taken by an inverted metallurgical microscope (Truemet, India).

Theoretical Density
It is the maximum achievable density without any voids. The density of each component and its mass fraction should be known to calculate it. The theoretical density of the composite is calculated analytically according to the following formula [24] ρ theo = 1 m Al where 'm' is the mass fraction of a particular component, and 'ρ' denotes the density.
Applying the above formula, the theoretical density of the composite is calculated as below:

Green Density
The density of the composite just after compaction is termed green density. It is always less than the sintered density and theoretical density.

Sintered Density
Archimedes's principle (ASTM: B962-17) was applied to measure the sintered density of fabricated composites. The density of the composite after the sintering process is termed sintered density. The porosity of the sample gets eliminated during the sintering operation when the mass transportation takes place, and hence the density of the composite gets increased. The efficiency of the sintering process is usually evaluated by sintered density.

Hardness and Wear Characteristics
Metal hardness is a property that influences how well a metal resists abrasion and wear. The hardness of the fabricated composites was measured by an analog Rockwell hardness tester (Meta Test instrument, New Delhi, India). according to ASTM E18-17 standard. The hardness test was performed as per the condition provided in Table 3. The wear test of the fabricated composites was carried out on a pin-on-disc wear machine (Model: TR-201 CL, Ducom, India) under ASTM G99-95a (reapproved 2000). Dry sliding wear tests, of sample size as per ASTM G99 standard, were conducted. Wear specimen of 12.5 mm dia., and 6 mm thickness was polished with 600 emery paper. A cleaned specimen of fabricated AMC was pressed against a rotating EN32 steel disc (hardness 65HRC) of diameter 80 mm and thickness of 8 mm by applying the 10 N load at the room temperature of about 28 • C on a rotating speed of 450 rpm.

Characterizations of the Powders and Composites
The microstructure of the aluminium, TiO 2 (rutile), and graphite powders were investigated using SEM images. SEM micrographs of these powders are presented in Figure 4. Aluminium powders show irregular, cylindrical, droplet, and ligamental morphology, while TiO 2 nanoparticles possess mostly spherical shapes and some irregular shapes with wide size distribution.
The microparticles of graphite have an irregular shape with sharp edges. This variety in the shapes and sizes of these powders will be favorable for the powder consolidation processes [9].
The XRD graphs of composite powder and fabricated composites are shown in Figure 5. From XRD plots, the presence of Al, TiO 2 , and graphite powders is confirmed from their respective peaks in sintered composites.   Figure 6 presents the EDS of the Al-TiO 2 -Gr composite sintered at 600 • C for 10 min of holding time. The peaks of Al, Ti, C, and O are evident from the EDS report. The wt. % of Al was taken at 88% in the composition, but at a specific location, the EDS result showed 70 wt. % Al in the sample. On the other hand, the result showed 10.02 wt. % C (graphite) while graphite was taken 3 wt. % in the composition of the powder during mixing. The reason behind this variation in wt. % may be the lack of proper mixing of the composite powder. To avoid this deficiency, the milling time should be increased. The atomic percentage of oxygen and Ti elements were 15.54% and 2.93%, respectively. It must be noticed that TiO 2 was the only reinforcement added to AMCs having oxygen. According to the chemical formula of TiO 2 , the atomic ratio between Ti and O is 1:2. Hence, for 2.43% of Ti, there should be 4.86% of oxygen. This increased atomic % of oxygen element dictates that oxidation of aluminium has occurred. Figure 7 shows the elemental mapping of the fabricated AMCs. Elemental mapping ensures that there was a lack in the homogeneous mixing of graphite while TiO 2 was distributed uniformly in the matrix material.

Optical Micrographs
Two samples sintered at 450 • C and 600 • C through ERS technique were selected for polishing to take optical micrographs. The two micrographs depict that the sample sintered at 450 • C has more porosity than the sample sintered at 600 • C. Thus, higher temperature causes more consolidation of particles. Some large grains were observed in a later one which revealed that grain coarsening has also taken place at the higher sintering temperature. The optical micrographs of these two samples are shown in Figure 8a,b.  Figure 9 depicts the trend of sintered density for sintering temperature, holding time of sintering, and sintering pressure. It was noticed that for lower sintering temperatures, e.g., 400 • C and 450 • C, the achieved densification in the pellets was very low; even for 400 • C, the improvement in the density was negligible. The reason for this slight change in the sintered density at lower temperatures may be due to lower diffusion rates. Similar findings have been reported by Choudhury et al. [25]. The possible reason for the slight change in the sintered densities could be associated with the fact that the presence of TiO 2 affected the sintering adversely at lower temperatures [26,27]. Further, the densification rate increases for the higher sintering temperature. The maximum sintered density was achieved at 550 • C sintering temperature. This maximum achievement in sintered density is due to fast pores reduction at high temperatures [28]. Lokendar Ram et al. [29] reported that the optimum sintering temperature was 620 • C for Al alloy powder compacted at 350 MPa. Maximum sintered density was obtained in the range of 610-625 • C sintering temperature; they also reported that the rate of densification was highest between 600 • C and 620 • C after 620 • C there was a decrease in the sintered density.

Sintered Density
The sintered density was found to be increasing with the holding time of sintering. It may be due to the consolidation of the particles with the initiation of the sintering. It causes an increase in the density of the composites. A longer holding time causes more consolidation. The rate of densification was high in the range of 1 to 5 min of holding time. The rapid change in sintered density in the first 5 min may be attributed to the lower abundance of pores in the composites. Further increase in holding time reduced the rate of densification. It would be due to the following reasons: (a) a larger amount of porosity reduction in the initial stage and, (b) the increase in relative density of the alloy slowed due to grain growth [28]. Figure 9 also depicts the variation in sintered density due to isostatic pressure during processing. The rate of densification is slow in comparison to the densification rate due to the variation in temperature and holding time. From the figure, it is evident that up to 4 MPa applied pressure, sintered density increases sharply. This sharp rise in the density is because of the rearrangement of poorly packed powders and the plastic deformation of soft aluminium particles [30]. Further, high densification may be achieved beyond 4 Mpa may be due to an increase in compaction pressure leading to an increase in the particle contact area, which alleviates sintering phenomena [31].  Figure 10 shows the variation in Rockwell hardness of sintered composite fabricated at various levels of input parameters. The hardness was found to be increasing with the increase in sintering temperature, and the maximum hardness was obtained for the Al-TiO 2 -Gr composite sintered at 600 • C. Here it was noticed that the hardness was increasing rapidly between 400 • C and 500 • C; after this range, the slope of hardness was lowered. The rise in the hardness of composites due to the temperature rise may be due to the following reasons: (a) fast reduction of pores at elevated temperature, (b) improvement in the sintered density of the composites [32]. The trend of variation of hardness for holding time is also positive. This positive gain in the hardness of composites is exhibited due to improved diffusion phenomena at large time intervals [33,34]. Further, the maximum hardness is achieved at a holding time of 10 min, and beyond this, there is a decline in hardness value; it would be due to grain coarsening that started to take place for a longer duration of sintering time [35]. Sintering pressure also caused to increase in the hardness of the AMCs. However, this change in hardness is almost insignificant, as suggested by the mean line depicted in Figure 10. In the present study, the holding time and sintering temperature were observed to be critical parameters to improve the hardness of the composites.

Wear Loss
The effect of sintering temperature, holding time, and sintering pressure on wear loss was examined and shown in Figure 11. The wear test was performed in dry sliding conditions at the ambient temperature of 27 • C for the sliding parameters of load 10 N, speed 2.0 m/s, and distance 750 m for all samples. Figure 11 reveals the behavior of wear loss of AMCs sintered at various pre-decided process parameters. Wear loss was found to be decreasing with an increase in sintering temperature. AMCs sintered at 600 • C showed minimum wear loss. This maximum resistance in the wear of the composites may be due to the following reasons: (a) proper densification at high temperatures, and (b) maximum gain in hardness at 600 • C [36]. It was noticed that the holding time range from 5 to 15 min was optimum for developing wear resistance in the AMCs. In addition to it, sintering pressure was also a factor that affected the wear loss. As this pressure increased, the wear loss was found to be decreasing. Higher sintering temperature facilitates better inter-particle fusion, which restrains the pull-out of TiO 2 and graphite particles. Therefore, lesser reinforcement particles crammed between surfaces during sliding, reducing third body abrasion wear along nanoparticles of TiO 2 (rutile). Higher temperatures facilitate a strong interfacial bonding with the matrix material and work as a barrier to resist the movement of particles during wear. It also creates a mechanically mixed layer at the surface, working as a loadbearing constituent [9]. Graphite works as a solid lubricant that helps to reduce wear loss during friction [6]. Figure 12 dictates the behavior of wear loss for the hardness of AMCs. The variation in wear loss was found to be according to Archard's law which states that wear resistance is proportional to hardness in other words, wear loss decreases with an increase in hardness [9,37].

Worn Surface Analysis
SEM images were taken of two samples fabricated at different parameters for the analysis of the worn surfaces. Figure 13a shows the worn surface of the Al-TiO 2 -Gr composite sintered at 450 • C for 10 min of holding time. Delamination is the most dominant wear mechanism for this sample, as suggested by SEM micrographs. Delamination, adhesive, abrasive, and fretting are some wear mechanisms that are related to the wear behavior of AMCs [38]. Delamination is characterized by excessive material loss from the surface in the form of thin layers. A few significant signs of abrasion are as follows flake-like debris, deep grooves, pits, and craters [37,39]. Figure 13a shows craters, large pits, and fretting wear at various locations. A series of interconnected wear marks and ploughing are also evident on worn-out tracks due to the poor bonding of reinforcing particles in the matrix material. Further, fractured debris in the form of flakes was also seen due to plastic deformation of the contact surface. The heat produced due to sliding also softens the rubbing surface, which further leads to material loss due to the following reasons; (a) severe plastic deformation and (b) reinforcement particles pullout form the embedding locations. [6].  Figure 13b depicts the worn surface of the same aluminium hybrid composite, which was sintered at 600 • C for 10 min. In this figure, a lesser amount of wear marks, such as ploughing, micro pits, craters, and wider grooves, are seen in comparison to Figure 13a. However, narrow longitudinal grooves and shallower scratches parallel to each are visible, which confirms that abrasion wear has taken place here. Adhesive wear is also identified by the display of plastic deformation at a few locations. The smooth appearance of the worn surface, along with the restriction of grooves, validate the wear resistance of this composite. The reason behind the high resistance against wear is as follows (a) a higher level of sintering temperature, which resulted in better interfacial bonding between the particles and reinforcement, (b) enough amount of surface energy to accomplish diffusion between the particles and mass transfer [40].

Wear Debris
As shown in Figure 14a, large flakes like wear debris were formed due to plastic deformation and delamination. Irregular and flaky chips confirm that the adhesive wear mechanism is dominant. Similar findings have been reported in past studies [41,42]. Nanosized particles of TiO 2 and micro-sized graphite particles were ejected from their place due to poor bonding and were seen in debris, causing a high wear loss. On the other hand, Figure 14b depicts the wear debris of smaller size with some sharp edges in comparison to the previous one. Abrasion wear, in this case, caused a smaller size of debris with fewer amounts of equiaxed flakes. Shaikh et al. [9] reported that densely packed Al-SiC composite sintered through ERS produced small-size wear debris during the wear test in comparison to pure aluminium.

Conclusions
In this experimental work, AMCs reinforced with 9 wt. % nano sized TiO 2 (rutile) and 3 wt. % micro-sized graphite was produced successfully using ERS method. The main findings of the experimental work are as follows:  Sintering temperature and holding time are the most significant parameters over sintered density. However, the effect of sintering pressure over density is insignificant. The maximum density is achieved at 550 • C with a holding time of 15 min.

I
The main effect plot of the Taguchi model suggested that the maximum hardness of composites is achieved at 600 • C and 10 min of holding time.

I
The main effect plot of the Taguchi model suggested that the minimum wear loss of composites is achieved at 600 • C, 10 min holding time, and 7 MPa sintering pressure. I SEM of worn surfaces exhibits that micro-cutting, crates, grooves, and smaller pits are the dominant reason for the wear loss of the composite under a dry sliding environment.