Processing of MMC through conventional sintering and spark plasma sintering process: A review

Spark plasma sintering (SPS) is one of the advanced methods to produce MMC with the desired quality. The high amount of heat was achieved through direct pulse heating to enhance the diffusion mechanism and it’s involved in the grain growth. Many of the researchers are doing the development of the sintering process for attaining better substance properties with lesser time. SPS was used to fabricate all kinds of materials including ceramics, glass, etc., Direct heat was applied to the graphite die and compact unit. The rate of heat was an essential role in this process to control the densification of powder particles. In the present situation, aluminium MMC plays an essential role in every field such as automobile, aerospace, manufacturing, and electrical sectors. It has superior substance properties such as specific strength, creep, toughness, and corrosion resistance. These properties are enhanced through the addition of reinforcements to the base metal.


Introduction
Demand for MMC in proven corrosion resistance environment applications has increased in recent days [1]. MMCs are more resistant to wear and have a much smaller weight, greater strength, and stiffness [2][3][4][5]. In SPS, the thermal gradients were controlled and the uniform heating was maintained. It was one of the hybrid heating systems which were suitable for refractory powder particles. The possibility of densifying powders was also controlled through SPS. This is the best method to fabricate ceramics with good properties such as magnetic [6], piezoelectric [7], thermo-electric [8], and optical [9]. The homogeneous material structure, high density, and low defects were attained for the use of ceramics [9][10][11]. The coarse microstructure was appeared in sintered based components due to longer heating and soaking methods. The structural properties have been improved through SPS. The uniform AC or DC pulsed current was applied to heat the components [12]. The plasma was generated between the contacts of powder particles [13][14]. The SPS has unique advantages like as low power consumption, surface cleaned due to electric discharges, and good quality of the components. The applications of Al-MMC have been increased in all sectors like the structural and automotive industry due to greater and mechanical performances and resistance of wear [15][16][17][18][19][20][21][22][23][24]. The two reinforcements were regularly used in Al MMC such as silicon carbides and aluminium oxides [25]. The wear resistance and toughness have been improved through boron carbide reinforcements [26]. The better mechanical properties and low thermal expansion were achieved in Al MMC through the reinforcement of silicon carbides and aluminium oxides [27]. The surface roughness of the Al MMC was measured during the machining process. The cutting speed and weight percentage of SiC were affecting the surface roughness [28][29]. The present investigation was used to provide the idea of processing of MMC through conventional sintering and spark plasma sintering process. The different kinds of literature have been collected related to the conventional sintering and spark plasma sintering process.

Materials and role of reinforcements
The quality and structure of the MMC mainly depended on the composition of the alloying elements, the weight percentage of the reinforcements, material processing techniques, and the finishing process. The accumulation of reinforcements was used to strengthening materials through the variation of material properties. The different reinforcements were used to improve the quality of the aluminium MMC such as SiC, B4C, TiC, aluminium oxides, fibers, tungsten carbides, and fly ash. The cutting properties that have been evaluated in SiC reinforced Al 6061and abrasive wear on the tool was investigated [30]. The aluminium reinforced with its oxides was prepared through an electromagnetic stir casting route. It was used to enhance the strength of the material and grain size [31]. Through the hot pressing methods, titanium and boron tri oxides were reinforced to the Al and its characteristics have been discussed [32]. The surface quality characteristics were performed in boron carbide reinforced Al MMC and its quality mainly depended on material removal [33]. The zircon glass particles were reinforced with Al through the powder metallurgy route and its compressive strength was discussed [34]. The fly ash was used for the production of Al 2024 MMC and a larger accumulation of fly ash was reduced the tensile strength of the material [35]. The corrosion resistance was decreased due to the accumulation of fly ash in aluminium composite [36]. The strength and nature of the materials were related to the reinforcement of the particles [37][38][39][40][41][42]. It has provided a better material structure during the sintering process.

Conventional Sintering Process
The materials were filled inside the die cavity and then it was closed. The punching pressure was applied through a compression testing machine with different loads with respect to the materials. Then it was moved to the sintering process and it was processed at a particular temperature to the nature of the workpiece. The rate of heat was utilized up to 5°C/min. During these periods, the interdiffusion takes place and the powder particles were bonded with adhesion properties. The size of the particle and its weight percentage was decided on the microstructure of the materials [43]. The SiC-based Al MMC was produced through a spark plasma sintering process. The matrix properties and their characteristics were examined [44]. The blending of powder and compaction pressure was played an important role in material composites. The sintering temperature was used to strengthen the composite and it's enhancing the hardness of the MMC. The heating and cooling rate were varied in the conventional sintering process. The controlling of temperature was also difficult.

Spark plasma sintering method
The SPS process was shown in Fig.1. It consists of a die, punch, vacuum chamber, and electrodes. The metal powders were placed in die units. The pressure was applied through punches. The sparks have been generated when two electrodes were connected to the DC power supply. The DC pulsed current has produced the heat and it was focused on the metal powders through punches. The sintering temperature, current, and oil pressure were controlled through the proper system. The SPS process and its characteristics were investigated [45]. The plasma was generated between the particles and it was used to improve the interfacial strength of the materials [46]. The densification, plastic deformation, activation, and refining of the powder were the important factors that were used for material processing [47][48][49]. The SPS input parameters such as temperature, holding time, and heating rate and its effect on the quality of the material were reported [50]. The sintering temperature was the most influential factor in the density and yield strength of the materials [51]. The process factors affected the substance properties and it was concluded that the compaction pressure a provided good relation to the responses [52]. The modern technologies used in SPS for the processing of powder materials based on pulsed electricity [53][54].

Effect of parameters
The effect of graphene and silicon carbide nanoparticle reinforcement and its mechanical and tribological properties were investigated in spark plasma sintered magnesium matrix composites [55]. The particle size and effects were related to the Densification mechanisms [56]. The particle effect was studied in the microwave sintering process [57][58]. The film oxides on aluminium powders were observed during the electrical sintering process [59]. Carbon nanotubes and their effects on mechanical properties were analyzed [60][61][62][63]. The ceramics-based particles were processed under a high temperature-high pressure method [64]. The quality of the composite was depended on the mixing of metal powder particles [65]. The effect of sintering temperature on the microstructure of the materials and its behaviors was studied [66]. The parametric effect on hot pressing was reported in SPS [67]. The porosity effect on spark sintered titanium was studied [68][69]. The sintering temperature has been played an essential role in the material structure [70]. The alloying elements and sintering behaviors were closely related to the surface quality of the composite [71][72].

Conclusions
The following summary of conclusions is drawn from the study of the above kinds of literature: • The metal matrix, particularly aluminium, and its reinforcements were discussed.
• The characteristics of the reinforcements were studied.
• The effect of reinforcement with MMC was investigated.
• The sintering and SPS operation details and features were discussed. • The comparison of conventional and SPS process and its processing of materials have been studied. • The different effects of particles on material quality were studied.
• The sintering temperature was the main parameter which affects the microstructure of the material. • The strength of the material depends on the alloying elements and its SPS parameters.