The densification behavior of metals and alloys during spark plasma sintering: A mini-review

As of today, metals and alloys are widely utilized as structural, mechanical,
 electrical, and magnetic materials in every aspect of technology and
 industry. The complexity and diversity of metals applications emphasize the
 need for a new production method to quickly and easily manufacture metallic
 components. Spark plasma sintering as a new solid-state sintering method has
 been recently developed to respond to these needs. Up to now, numerous
 papers and researches have been published in the field of spark plasma
 sintering and its ability to manufacture metallic samples. Density is a
 significant property that has a great influence on the physical, mechanical,
 and functional properties of metallic parts. To the best of our knowledge,
 there is no concrete paper that reviews the densification behavior of metals
 and alloys during spark plasma sintering and the effect of its parameters on
 the density of metallic samples. As a result, this article dedicated to
 addressing this need. In this mini-review, after a short description of
 spark plasma sintering method, the effects of each sintering parameters on
 densification behavior of metals and alloys are studied, and possible
 physical and microstructural factors which have the effect of densification
 behavior of metallic samples at various sintering conditions are reviewed.
 Finally, the recent advances in finite element modeling, which study the
 temperature and stress inhomogeneity in the SPS process, have been reviewed
 in this paper.


Introduction
Spark plasma sintering (SPS) is a comparatively new sintering method for densification of various ceramic [1] or metallic [2] systems at lower sintering temperature and shorter sintering time, in comparison to conventional sintering methods such as hot press (HP) or hot isostatic press (HIP). SPS process is Similar to HP method. However, instead of external heating source, SPS utulizes a pulsed and low voltage direct current during the heating process tosinter the powders, which enables the SPS to produce highly densified materials at comparatively low temperature and shorter dwell time, in comparison to other methods [3][4][5][6]. Fig. 1 shows a schematic view of SPS apparatus. The powders are initially poured in a die, and vaccum pumps evacuate the air from inside the die. At this stage, an inert gas isutulized as sintering atmosphere. Then, as shown in Fig. 1, an axial pressure is applied and an electric current is passed through the die via pressure block (and the powders, in the case of conductive samples) during the sintering process. During this process, the die is heated up by the electeric current, warming up the sample. In addition,when the electric current passes through the conductive sample will also increase the temperature, acting as a heating source and may generate plasma between particles (explained in section 2.1). Such an advantage along with a fast heating and cooling rates enables the SPS to densify the powders at low sintering temperatures and shorter sintering times [7,8]. It is worth mentioning that an external heating source may also be used in special cases. The SPS apparatus has 5 mainparameters such as electric current, temperature, time, heating rate, and pressure, which are usuallyaltered by the experts to set up the sintering conditions for their desired properties. Temperature and time are the most important parameters, greatly affecting the physicomechanical properties of materials. A comprehensive survey on literatures conducted by the authors showed that these parameters affect the densification behavior of metals and alloys.There are numerous papers in the field of sintering metallic samples by SPS [2,9,10].
However, there is not a concrete publishedpaper to review the effect of the SPS parameters on densification behavior of metals or alloys. This review focused on densification behavior of metals and alloys during the SPS process. These effects may not be restricted to metals or alloysand may be valid for ceramics and composites, as well.

Densification Behavior of Metals and Alloys During SPS Process
Densification is one of the most effective characteristic of materials influencing their physical and mechanical properties.Increasing in densification means lower porosity and higher material cohesion, which bring about enhanced mechanical properties such as high hardness, tensile/ and compressive strength, and strain, as well as superior physical properties such as elevated electrical and thermal conductivity. In this regard, each aforementioned SPS parameters have its own effect on densification, which are surveyed in this section.

SPS Electric Current
Electric field or current applied during the SPS is one of the major differences between SPS and other sintering methods,enabling it to produce highly dense metallic or alloyed parts. In this regard, electric current can increase the mass transformation (diffusion) by increasing point defects concentration, improving defects mobility, applying electron wind effect (electromigration), and decreasing the activation energy for defects migration [11][12][13]. As a result, increment in SPS electric current enhances the densification of metals or alloys due to better mass transportation. In fact, the effect of electric field on mass transportation can be explaniedby electromigration theory.
D and i C are the total flux of diffusing of ith species, the species diffusity, and spiecies concentration, respectively. Moreover, F, Z * , E, R and T are Faraday's constant, the effective charge on the diffusing species, electric field, gas constant, and temperature, respectively [14]. As seen in Eq. 1, the increment in electric current increases the diffusion flux.
Electric field has also a cleaning effect on metal powders and breaks the oxide layers present on surface of metal particles. Additionaly, electric current may idonize the gas entrapped between two metal powders and transform it to plasma, which enhances the neck formation and the densification [15][16][17]. In fact, the plasma formation at the contact points or in the gap between metal particles increases the local temperature up to ten thousends centigrades for a moment, resulting in melting and evaporation of metal particle surfaces. The melted or evaporated atoms solidifed at the vicinity of contact pointswill enhance the neck formation [9]. Electric current that passes through the powders also increases the temperature by joule heating, resulting in better densification [10].

SPS Temperature
Similar to other sintering methods,incremental changes in SPS temperature brings about higher relative density for the materials. Such an enhancement in metals and alloys is due to the following facts that: 1. The elevated grain growth at higher SPS temperatures decreases the pores initially existed between metals particles and increases the inter-particle contacts [18][19][20]. 2. High SPS temperatures form better necks between metallic and alloyed particles [21]. 3. Diffusion processes are faster at high SPS temperatures. 4. High SPS temperatures results in high plastic flow rates [22]. 5. Increment in SPS temperature decreases the strength of metallic powders, which enhances the creep flow [23].
Beside aforementioned causes, there are also other reasons and phenomena in special cases which alter the effect of SPS temperature on the densification behavior of the metallic or alloyed parts. For instance, viscosity in glassy metals or alloys is one of the crucial factors. In this respect, as seen for Cu 47 Ti 33 Zr 11 Ni 6 Sn 2 Si 1 alloy in Fig. 3, amorphous metals and alloys spark plasma sintered (SPSed) at higher temperatures have better densification due to lowered viscosity at such temperatures [27].
Phase transformation is another factor can alter the densification behavior of metals and alloys during sintering process. Titanium (commercial grade 1) is an example, in which phase transformation can occur. On incrementing SPS temperature by more than 890 °C, its microstructure changes from α (HCP) to β (BCC), where α phase is denser in comparison to β phase. As a result, there is more space for diffusion in β phase, bringing higher densification for Ti, as illustrated in Fig. 3 [28].  Formation of liquid phase (partial melting) is one kind of phase transformation which occurs in some metallic systems during spark plasma sintering at high temperatures. This phenomenon may enhance or degrade the densification. For instance, formation of a liquid phase containing W, Ni, Fe, Cu, or Co in tungsten heavy alloys (WHAs) or melting Pb in Cu-Pb alloys results in higher densifications at elevated SPS temperatures, as shown in Fig. 2, due to wetting ability of the liquid phases and enhancement in diffusion [29][30][31]. However, de-wetting may occur in some metallic systems at specific SPS temperatures [31,32]. For instance, Pb melt is unable to wet Cu particles in nanostructured Cu-Pb system when the SPS temperature exceeds 350 °C. Such phenomenon decreases the densification of metals and alloys [31].
The densification behavior of metallic parts may be affected by the presence of interstitial atoms. For instance, the presence of nitrogen atoms in interstitial sites of iron lattice decreases the density of the system at temperatures above 700 °C. N 2 gas forms at such temperatures according to Eq. 2, resulting in pores and reduced density, as shown in Fig. 1 [33,34].
The SPS temperature also has effects on the oxide surface of metallic powders. Incremental changes in SPS temperature during the sintering of metallic powders such as Al and Mg, which are susceptible to oxidation, brings higher densification by breaking the oxide layer and pave the way for diffusion as seen for Al and Mg in Fig. 3

SPS Time
The studies have shown that incremental changes in SPS time generally enhances the densification of metals and alloys due to enhanced mass transfer [36], improved inter-particle bonding, reduced porosities [18], and enhanced heat flux [37], as depicted in Fig. 4, for Aluminum [35], and stainless steel 430L [19] as examples. As a matter of fact, incremental changes in SPS time results in coalescence of intergranular pores and formation of closed pores. On the other hand, metallic particles with different sizes agglomerate and form coarse grains. Such phenomena bring enhanced densification and expanded grains for the SPSed metals or alloys [38]. Moreover, incremental changes in SPS dwell time decreases the viscosity of glassy metal powders and results in some additional structural relaxations, enhances the density of glassy metals. Such a trend is shown for Cu 47 Ti 33 Zr 11 Ni 6 Sn 2 Si 1 in Fig. 4 [27].
The incremental change in SPS time does not always increase the density of metallic systems. For example, the incremental changes in SPS time more than 10 minutes has not any effect on densification of M2 steel as seen in Fig. 4 [21]. Moreover, studies have revealed that the incremental changes in SPS dwell time does not increase the densification of nanostructured grade 2 Ti [39] as seen in Fig. 4. In such system, high SPS pressure (80 MPa) during the sintering may bring about the maximum densification of the system at early times of sintering. As a result, further sintering did not increase the densification. The low difference between 3 and 5 min may also be another reason for remaining the relative density stable at this region [39].
Moreover, some studies have reported a reduction in densification at high SPS dwell times. For instance, it was observed that the incremental changes in SPS dwell time slightly degrades the densification of pure copper, as illustrated in Fig. 4, due to expanding and obturation of the gases absorbed on the surface of Cu powders during the plastic deformation in the early stage of the sintering [40,41].

SPS Heating Rate
The incremental changes in SPS heating rate has both positive and negative effects on densification of metals and alloys, which depend on metallic systems.Studies have shown that the incremental changes in SPS heating rate result in higher densification in some metallic systems, while in other may reduce the densification. Generally, increasing in heating rate reduces the relative density of metallic and alloyed parts. This is due to the fact that the sample remains longer in any specific temperature in lower heating rates, resulting in higher sintering time and better densification [37,42]. Moreover, high heating rate brings about forming local hot spots and melting via arcing (i.e., dielectric breakdown) at initial stages, which causes immature necking. Such necking leads to entrapments of porosities and lowering the density [39].
The premature necking and inhomogenization mayalso be generated by the fact that there are minimal time for neck formation at early stage of sintering at higher heating rates [43]. These reasons are responsible for decrement in relative density of various metallic systems such as Fe-18Cr-2Si, Ti, and WHA (W-based alloy),as shown in Fig. 5. On the other hand, higher heating rate may result in higher densification. High heating rate brings about better surface activation and neck formation between metallic or alloyed powders. As a result, the powders superiorly weld together and the densification will be improved. This trend is shown in Fig. 5 for 316L stainless steel. Such improvement is not monotonic. The incremental changesof densification is higher at lower heating rate increments. While the heating rate increases, the densification improvement rate decreases [44]. Moreover, sintered samples by higher heating rates that experiences higher temperatures at early stages of SPS, may enhance their densification [35].
Heating rate also has both reduction and incremental effect on relative density of metals. For instance, increasing the heating rate up to a specific value would increase the relative density of nickel alloys by reducing the amount and size of the porosities, resulting in better particle bonding. However, increasing the heating rate more than a critical value (175 o C/min in nickel alloy) has induced lower densification [45].
Such increment in densification is related to two effects: (i) mechanical and (ii) intrinsic effect of SPS pressure. In fact, the mechanical one is related to decreasing in space between particles by increasing in pressure, which enhances the diffusion. Moreover, increase in SPS pressure results in better breakage of agglomerated particles. The intrinsic effect is related to increase in diffusion driving force [39]. In addition, the metallic/alloys powders are weaker at higher temperatures. As a consequence, applying pressure at these stages significantly results in plastic deformation of powders and more densification [35].

Temperature and Stress Gradients
Beside aforementioned advantages of SPS process for metals, alloys [52], and ceramics [53] in comparison with conventional sintering methods, SPS has drawbacks relating to temperature and stress gradients in the sample as well as SPS apparatus parts such as the die and punches [54]. Such non-uniform distributions bring about inhomogeneity in the densification and functional properties of the samples. On the other hand, any uniformity in densification accelerates inhomogeneous distribution in the electrical current and temperature [54][55][56]. However, experimental restrictions such as enclosed chamber and high heating rates make it difficult to control the temperature as well as stress distribution in the sample. As a result, finite element modeling (FEM) is recently used for studying the temperature and electrical fields in the sample and SPS tools with consideration of thermo-electric coupling and studying the stress distribution with consideration of integrated mechanical coupling [57]. These studies confirm a complex distribution of temperature within the sample and SPS tools which mainly depends on (1) thermophysical properties of the sample and tool materials such as thermal and electrical conductivity, density, mechanical strength and emissivity; (2) the tool and sample geometry such as height and diameter of the sample and die; and (3) SPS parameters such as heating rate [12,[57][58][59]. Due to the strong effect of electrical properties of materials on the temperature and stress distribution, both the electrical conductive (metals and alloys) and non-conductive (most of ceramics) materials are taken into consideration in the following sections. Moreover, studying the temperature and stress gradients in nonconductive samples alongside those in conductive ones results in comprehensive understanding the nature of the process and gradients evolution.

Temperature Distribution
To study the temperature distribution, the electric pass in the sample and die is an essential parameter. Depending on the electrical conductivity of sample material and SPS tool, the heated components are various. If the sample is consisted of materials with low electrical resistance and the die is also composed of low electrical resistant materials such as graphite, both the die and the materials are heated up (Fig. 7a). If the sample has high electrical resistance (non-conductive materials) such as Alumina, the die is only heated and the sample will be sintered by thermal mechanisms without current-assisted mechanisms (similar to HP) (Fig. 7b) [12]. In this case, studies have shown that the radial distribution of electric current has a peak at the sample/die interface, and the current density gradually decreases to the periphery of die [57]. On the other hand, if the sample is a conductive material, the best results can be attained by utilizing a electrical resistant die, because the electrical current focuses on the sample and the highest possible heat will be generated in the sample (Fig. 7c). As a result, the electrical resistance of the SPS tools, depending on the size and material of the tools, has a considerable effect on heat distribution within the sample and SPS apparatus. In order to sinter samples with large dimensions and complex geometries, a reliable modeling method such as FEM is needed [12]. high electrical resistant die [12].
The simulations have shown that the temperature in the center of conductive materials such as metals or alloys is higher than the temperature of the sample/die interface. In fact, there is a radial temperature distribution in the conductive sample, the maximum value of which is located at the center of the conductive sample ( Fig. 8a and b) [16,[60][61][62][63]. For instance, Hwan-tae et al. [64] observed a temperature difference more than 130 o C between the center and periphery surface of nickel sample at the maximum temperature of 930 o C. In the SPS tools, the temperature of the die is usually measured by a thermocouple [66] or pyrometer [53]. In fact, there is no scalable method to measure the exact temperature in various regions of the sample and validate the simulation results. However, Molenat et al. [67] validated the simulation results by measuring the approximate temperature in the TiAl alloy based on the microstructure transformation. These alloys are usually consisted of circular or polyhedral grains with γ structure and lamellar grains with α+γ structure. When the temperature exceeds 1330 °C, a transformation of α to α+ γ occurs, the products of which has a lamellar structure. As seen in Fig. 9a, the microstructure of the center part of the TiAl sample had full lamellar microstructure with bigger grains, indicating the high temperature in this part exceeds 1330 °C, while the temperature in the periphery part of the sample was lower than that of the center as illustrated by γ circular grains (Fig. 9b). Similar results are also observed by Voisin et al. [63]. Regarding the densification, in another study, Shongwe et al. [68] revealed that the tungsten samples with larger diameters have lower relative density and hardness due to such inhomogeneity in temperature distribution. On the other hand, the trend of temperature distribution in non-conductive materials depends on experimental conditions and tools size [55]. In many cases, the trend of temperature distribution in the non-conductive ceramics is similar to that of the metals and alloys (Fig. 8c) [57,65,69,70]. For instance, Achenani et al. [57] have shown that in the case of Alumina, the temperature is maximal at the center of the Alumina sample and decrease along with the outer side of the die [57]. However, Olevsky et al. [55] showed that distribution pattern of temperature (the position of the warmer and cooler sides in the sample) in Alumina can be changed by altering tools size. Moreover, Vanmeenselet al. [62] reported that the inner part of ZrO 2 sample had a lower temperature than outer part of it in a steadystate temperature distribution (Fig. 8c). On the other hand, Mondalek et al. [71] showed that the center temperature of the Alumina specimen was lower than the edge temperature before 80 seconds, and it reversed after 80s. They suggest that the changes in thermal conductivity of graphite die and the Alumina are responsible for this phenomenon.
However, the temperature gradient and distribution in the materials with high electrical resistance is still questionable because of the considerable effects of the sample materials and SPS tools. There are very rare experimental researches to study the temperature distribution in the samples with high electrical resistance. This is mainly due to the fact that it is a very difficult task to find such a sample with high sensitivity to temperature, especially at high SPS temperatures applied for sintering non-conductive materials such as high electrical resistant ceramics [72]. In this regard, Shijia Gu et al. [72] showed the radial and vertical temperature inhomogeneity in the SPS sample by studying the optical properties of Zeolite samples. Such materials transform from opaque to transparent when the temperature exceeds a critical value (1250 °C). For instance, Spark Plasma Sintering (SPSing) a Zeolite sample at 1325 °C resulted in a disk with transparency to the sun light at the center and opaque to it at the edge. This proves the existence of a temperature gradient in the Zeolite sample, which has its maximum value at the center part of the sample.
Generally, such inhomogeneity in the conductive and non-conductive materials increases by incremental changes in heating rate or sample size [55-57, 59, 61, 68]. Moreover, the incremental changes in SPS temperature results in more inhomogeneity in temperature distribution [7,73].
Regarding the uniformity of temperature distribution in conductive and nonconductive materials, there is a conflict between the researchers. Some papers revealed that the temperature gradient inside the non-conductive samples is more uniform than that inside the conductive sample [60,62]. For instance, Vanmeensel et al. [62] observed a higher radial temperature difference (79 o C) inside a TiN sample than a 25 °C temperature difference existed in a 3Y-ZrO 2 sample in the final step of sintering at SPS temperature of 1500 °C. The different behaviors of these materials in the electric field are responsible for the discrepancy in their temperature distribution. In the materials with low electrical resistivity such as TiN, a considerable portion of electric current passes through the sample and heats the sample by joule-heating, resulting in a temperature difference between the sample and the die. However, by utilizing samples with high electrical resistivity such as 3Y-ZrO 2 , the electric current passes through the die. In this situation, the joule-heating generated in the die compensate the heat losses caused by radiation at the outer side. Moreover, the heat swiftly transmitted to the non-conductive sample due to small size of the sample in comparison to the size of SPS tools and high thermal conductivity of the graphite die [62].
On the other hand, some papers showed that temperature distribution is more homogeneous in conductive sample than non-conductive materials. For instance, Munoz et al. [65] observed 22 °C and 4 °C radial temperature differences in the Alumina and Copper samples with 19 mm diameter, respectively. The temperature distribution inside the sample is governed mainly by its thermal conductivity: the lower thermal conductivity of Alumina leads to a non-homogeneous temperature distribution with significant gradients, while the higher thermal conductivity of Copper leads to a homogeneous temperature distribution inside the sample [65]. However, it seems that the temperature gradient difference between conductive and non-conductive materials depends on the thickness of the die. If the SPS has a thick die, the temperature gradient in conductive materials will be probably higher than that of nonconductive material. In fact, by utilizing thin die, more heat, which is produced in the conductive sample by joule-heating, can pass out, and consequently more homogeneity in temperature can be reached. On the contrary, utilizing a thin die for non-conductive sample causes the current flows through the die in the vicinity of the sample, leading more heat to the sample and consequently results in higher temperature difference. As a result, a thin die should be utilized for SPSing conductive materials, and thick dies must be used for sintering non-conductive samples [59].
Diatta et al. [74] numerically calculated the relative density of Alumina sample versus its radial distance from the center of the sample. The calculated radial distribution of relative density was in correspondence to radial temperature and hardness distribution, indicating the effect of temperature distribution on physicomechanical properties of materials.
Beside radial temperature inhomogeneity, studies have shown that there is also a vertical temperature difference in axial center of non-conductive or conductive materials, which increases by increasing in sample dimensions (radial and axial dimensions) [57,73,75]. For instance, such inhomogeneity can be negligible (<2 °C) for small Alumina samples (diameter: 20 mm and thickness: 2.5 mm) which increases to 14 °C for large Alumina samples (diameter: 40 mm and thickness: 10 mm) as reported by Achenani et al. [57].
The most of attempts have been done to decrease the thermal inhomogeneity by optimizing the size and shape of SPS tools and the samples [57]. Beside these attempts, Achenani et al. [57] have shown that vertical displacement of die can decrease or increase the axial temperature inhomogeneity, but it did not have any effect on axial temperature distribution. They also showed that inserting a graphite felt as a thermal insulator around the outer surface of the die can inverse the thermal distribution and decrease the thermal inhomogeneity in small Alumina samples. However, they observed that this increases the thermal inhomogeneity in large samples. They proposed that implementing partial insulators with optimized height and thickness can decrease the thermal inhomogeneity even in large Alumina samples without considerable effect on axial temperature distribution. Similar results are also observed by Munoz et al. [69].
It is worth to mention that there is also temperature difference in the SPS tools. Such differences can reach to 300 °C for a typical die in temperature range of sintering of the carbides (1900-2100 °C) [73]. This indicates that the temperature distribution is important not only for physicomechanical properties of the samples, but also for the maintenance of the SPS tools. Overheating may occur in SPS tools due to the focus of electrical current in specific regions, which results in temperature raise. Overheating usually occurs at the top punch and make it red hot few minutes after starting the device, which can be avoided by optimization of the SPS tools geometry and size [76].

Stress Distribution
The thermomechanical computations have shown that the combination of the effects of expansions caused by uniaxial load applied during the SPS process according to the Poisson's ratio and the mismatch between the thermal expansion of sample and tools materials can generate displacements and stress distribution in the sample and the SPS tool [60,73,74]. Fig. 10 shows the vertical, radial and angular stress distributions in the Zirconium Oxycarbide sample and the SPS die, simulated by Guy Antou et al. [73]. As expected, there is a constant vertical compressive stress in the sample, which is approximately equivalent to the pressure applied during the sintering. Moreover, the graphite felt located at the sample/die interface reduces the mechanical friction and consequently inhibit any transmitting of vertical stress from the sample to the die [73]. Fig. 10. Vertical, angular, shear, and radial stress distributions in a Zirconium Oxycarbide sample sintered by SPS process at temperature of 1950 °C (the temperature of outer surface of the die) [73].
Regarding to the radial stress, there is a compressive radial stress along the sample and the die, the magnitude of which in the sample is higher than that in the die. Regarding to the angular stress, the sample experienced a compressive angular stress, which turns to tensile stress in the die. Such radial and angular stress distributions are formed in the sample and in the die due to the coefficient of thermal expansion (CTE) mismatch between the sample and the die. By incremental changes of temperature, the sample exhibit more expansion than the die and induce pressure to the die, resulting in compressive stress to both the sample and the die. Such stresses have larger magnitude at higher SPS temperatures. On the other hand, there would be no radial and angular stresses if the CTE of the die was larger than that of the sample, and the mechanical friction at the sample/die interface was negligible (as here due to graphite felt). Moreover, the larger magnitudes of radial and angular stresses at the center of the sample than that in the sample edge are resulted from the temperature gradient along the sample diameter. In addition, the results show that the radial and angular stresses can exceed vertical stress in the SPS sample. In addition, radial stress inhomogeneity can also be greater than radial temperature inhomogeneity [73]. Regarding metallic samples, Wang et al. [60] observed a similar result for Copper. They observed 33 % stress gradient in the Copper sample, which had only 1 % temperature gradient by SPSing at 700 o C with 200 °C/min heating rate due to its large thermal expansion. Generally, these stress gradients depend on both the sample mechanical properties (CTE and Young's modulus) and the temperature.
In summary, by utilizing FEM, it is possible to choose the best sample and tool geometry as well as SPS parameters for achieving the lowest thermal or stress non-uniformity as much as possible. It also helps sintering samples with complex shapes and geometry [12].

Conclusion
The present paper has reviewed a numerous research works about the densification behavior of pure metals and alloys sintered by SPS process, and provided the readers with a broad overview of physical and microstructural features and phenomena which affect the densification behavior of metallic systems during SPS process.
Densification is a fundamental property that alters the physicomechanical properties of metallic or alloyed samples. The effects of each SPS parameters including electrical current, temperature, time, pressure, and heating rate on density and relative density of metallic parts are reviewed in details. We have discussed that an increase in electric current or pressure normally enhances the densification of metals and alloys. On the other hand, incremental changes in temperature, time, and heating rate may degrade the densification of metallic systems in some cases.These reviewed trends are summarized in Table I, where each physical factor, which governs these trends, is addressed by different authors.
The densification of the SPS sample is also under the influence of temperature and stress distributions, which are affected by electromechanical properties of SPS tool and samples materials, SPS tools and sample geometries and sizes, and SPS parameters. As a result, it is crucial to choose proper SPS conditions and geometry as can be simulated by FEM.
The researchers may refer this mini review to choose the best SPS conditions for manufacturing metallic/alloyed samples. The authors believe that there is a profound need in experimental works to survey the microstructural and physical phenomena during the SPS process and their effects on densification behavior of materials. Moreover, the developments in modeling tools and conducting experimental researches to study the real temperature in the SPS samples are of great importance.
Tab. I Summery of the effect of SPS parameters on the densification behavior of alloys and metals.

Enhancement
Some metals and alloys such as 316L SS [18].