Trans Indian Inst Met DOI 10.1007/s12666-016-1030-5

TECHNICAL PAPER

Effect of Particle Size on the Deformation Behaviour of Sintered Al–TiC Nano Composites M. Sivaraj1 • N. Selvakumar2

Received: 1 March 2016 / Accepted: 20 December 2016  The Indian Institute of Metals - IIM 2017

Abstract Cold upsetting experiment was impeccably carried out on sintered Al–TiC preform to evaluate their deformation characterization. Effects of TiC content and aspect ratio of the preform on deformation behaviour were completely investigated by using Zinc stearate as a lubricant. Cylindrical preforms with different particle size at 5% (2 lm and B200 nm) and different aspect ratios (1.00 and 0.75) were prepared by using suitable die, on a 1.0 MN capacity hydraulic press and sintered in electrical muffle furnace for 1.5 h and followed by cooling the furnace to its room temperature itself. Analysis of the experimental data have proved the power  law relationship between fractional q theoretical density q f and strain factor eðez eh Þ . Further, it th was found that the preforms with low size of TiC content showed higher value of deformation properties such as axial stress and the Poisson’s ratio than high size of TiC preform, provided that the initial fractional density taken was kept constant. Keywords Compaction  Process  Sintering  Densification  Preform  Fracture  SEM

& M. Sivaraj [email protected] 1

Department of Mechanical Engineering, Ponjesly College of Engineering, Nagercoil, Tamilnadu 629 003, India

2

Department of Mechanical Engineering, Mepco Schlenk Engineering College, Virudhunagar, Tamilnadu 626 005, India

1 Introduction Aluminium metal matrix ceramic composite are broadly utilized in the automotive, aerospace and electronics industry because of their light weight, high strength and trouble-free of fabrication [1]. Powder metallurgy (P/M) has been described as the art and science of producing metal powders and manufacturing semi finished and finished objects from individual, mixed or alloyed powders with or without the addition of nonmetallic elements. Production of P/M parts are engaged by mixing of elemental or alloy powders with additives and lubricants, compacting the mixture in a proper die and heating the green compacts in a furnace so as to bond the particles. The developing parts are solid bodies of materials with sufficient strength and density for making advantage in various fields of consumptions. Powder metallurgy, moreover assists in attaining high production rates, precision forming among close tolerance requirements in addition to near-net shape products requiring minimum or no machining [2]. Al-based MMCs with ceramics (SiC, Al2O3, TiC and TiB2) as the reinforcement phases have been broadly revised in various research works [3–5]. Many researchers have detailed the experimental and methodical investigation work on upsetting of cylindrical preform. Upsetting test is utilized for finding out the workability and the effect of the relative density on the forming limit of powder metallurgy [6]. Lower aspect ratio preform provides uniform density resulting from quick load transfer ensuing in exclusive hardening process. Conversely, it is not exact for the higher aspect ratio preform [7]. The reduction in the level of porosity throughout upsetting, increases the density with load bearing crosssectional area [8]. Poisson’s ratio being 0.50 for all materials that adapt to volume constancy and in the plastic

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deformation of sintered P/M preform, density variation resulting in Poisson’s ratio approaching but not equal to 0.50, is required for attaining theoretical density [9]. The compressive analysis is an important aspect for various engineering applications. The workability of several metallic alloys have broadly been investigated by many authors [10–14], but the analysis in Al–TiC with nano TiC is fairly investigated. The present investigation is to try to find out the forming behavior of sintered Al–TiC composite preforms with different particle size of TiC at 5% (2 lm, 200 nm) and different aspect ratio (1.00, 0.75); while the cold upset test takes place. This has been carried out due to its industrial importance attached to the material. An effort has been made in the current investigation to evaluate the particle size variation of TiC particle reinforced in the composite based on the fractional theoretical density, Poisson’s ratio, and other parameters such as stress, strain, and strain factor under Al–5TiC preform composition. The effect of aspect ratio variation in the composite by analysing the fracture zone using microstructural studies has been studied. This composite has broad scope in structural material for aerospace and automobile industries as they have high strength, high elastic modulus and advanced resistance to wear, creep and fatigue when compared to unreinforced metals.

2 Materials and Methods 2.1 Materials In the present investigation, aluminium and titanium carbide powders were used. These powders were collected from Alfa Aesar Pvt. Ltd., England. Atomized aluminium and titanium carbide were obtained with 100 and 99.6% purity respectively. The characteristics of the powders were noted in Table 1. 2.2 Powder Preparation The entity powders were blended and mixed in a high energy ball mill using tungsten carbide as crushing medium by way of ball to powder ratio (BPR) 1:20 [11]. Milling

Table 1 Characteristics of Al–TiC powders Aluminium

Titanium carbide

Purity (%)

100

99.60

Particle size (lm)

45.00

2.00

Melting point (C)

660

3140

Density (g cm-3)

2.70

4.93

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was done in wet condition, because wet mixing was used to produce more fine and uniform mixture of powder particles and also prevent oxidation. Toluene was used as milling medium in wet milling. The TiC powder was pulverized in a high energy ball mill for 12 h and the obtained particle size was approximately below 200 nm. The individual powders (Al, TiC) were grained finely in a high energy ball mill for further 1.5 h and later it was blended on a mass basis with 5% titanium carbide and rest aluminium powder. Then the combined Al-5TiC powders were mixed systematically in the high energy ball mill for another 15 min to attain homogeneous mixture. A referral to various technical articles [14–16], revealed that, 5% TiC was the best choice. Hence for analyzing the properties, 5% TiC was selected. 2.3 Compacting and Sintering A compact was prepared using different size (2 lm and B200 nm) of TiC at 5% and different aspect ratio (1.00 and 0.75) with 15 mm diameter. Die compaction was generally done by using hydraulic press with capacity range of 1.0 MN. In room temperature, the compaction employed pressures in the range of 470–660 MPa. Metal powders were mixed with zinc–stearate (ZnS) lubricants prior to compaction. Die compaction by high pressure was an important process, where densification and shaping arose concurrently. After compaction, the density could be as high as 85%. After compacting, the compact was removed on the spot from the die and inserted into the furnace at 550 C for a period of 1.5 h for sintering. The inert gas circulating the electrical muffle furnace was used to prevent undesirable reactions (oxidation, decarburization or carburization) during sintering. After the process, the sintered compact in the furnace was cooled to the atmospheric temperature. Once the process was over, a wire brush was utilized for cleaning up the sintered compacts. After sintering, the density could be as high as above 90%. 2.4 Characterization Scanning Electron Microscope (SEM) micrograph was utilized to found out the particles before and after milling and also used to confirm the uniform distribution of reinforced particles after sintering. XRD studies and analysis were performed on as-fabricated materials and as-sintered samples. They revealed the absence of any intermetallic and reaction phase in the composites. And also in this present study, EDS pattern was used to identify the elements present in the particular system.

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2.5 Deformation Test

3 Results and Discussion

During deformation test, the preform was deformed at atmospheric temperature between the flat dies. The load (step of 0.005 MN) was applied until visible fine cracks appeared on the preform. Lubricant (ZnS) was applied on the end of preform and contacting surface of flat die. As soon as the loading schedule got over, height, contact diameter (up and down), bulged diameter and density were determined. The Archimedes principle was utilized for finding out the density. Cold deformation experimental data were used to find out the various parameters (Poission’s ratio, stresses and strain). Figure 1 shows an illustrative photo image of the specimen before and after deformation. By means of detailed analysis [6, 17], the consecutive constants were established as follows:    True axial strain; ez ¼ ln ho hf ð1Þ     2D2B þ D2C True hoop strain; eh ¼ ln ð2Þ 3D2O   DC 0 ð3Þ Conventional hoop strain, eh ¼ ln DO

3.1 Evaluation of Micrograph

Fractional   theoretical   density ratio; qf qo ðez eh Þ ¼ e qth qth New ðorÞ instantaneous Poisson’s ratio;

ð4Þ

u¼

eh 2ez

ð5Þ

Hoop stress to bulged and contact diameter; with respect  aþc ðrh Þ ¼ rz 1 þ ac ð6Þ True axial stress; Hydrostatic stress;

ðrz Þ ¼

Load Contact surface area

1 ðrm Þ ¼ ðrh  rz Þ 3

ð7Þ ð8Þ

Fig. 1 Illustrative photo image of the upset specimen. a Before and b after deformation

SEM has been utilized to find out the microstructure of the particles before milling and has been shown in Fig. 2a, b. Figure 2a shows the micrograph of aluminium powder, the particles are assumed to be spherical and irregular in shape. Figure 2b illustrates the micrograph of TiC powder, the particles are assumed to be of pancake and dendritic structure. The SEM microstructure of milling powders are shown in Fig. 3a, b. Figure 3a shows the fine particles of TiC extremely grinded after 8 h of milling. It clearly reveals that large particles are in fact agglomerates of much smaller particles especially after 8 h of milling which may be due to the following reasons; the particles are deformed, cold welded and fractured due to high energy collision. Those events may change particle shape and also decrease particle size and form layered structure. In addition, the aluminum is generally a ductile material and it shows a highly non-linear stress–strain relationship up to the maximum strength with lower Young’s modulus. Figure 3b shows the mixed powders of Al–TiC. EDS analysis of the TiC reinforced composite is shown in Fig. 4a, b. In the figure Al, Ti, C and oxygen peaks are observed. The EDS analysis confirms the presence of TiC particles within the composites. From the above analysis it has been asserted that TiC particles are successfully incorporated in the Al matrix composites. Figure 4a shows an enhanced incidence of Al and O in the pure aluminium specimen. Figure 4b shows the incidence of Ti, Al and C particles in the fabricated Al–TiC composite. Figure 5a, b illustrates the SEM micrographs of the sintered preform. The above micrographs points out the even allotment of TiC particles in the aluminium matrix and Al–TiC (B200 nm) composites compared to Al–TiC (2 lm) which demonstrates minor agglomeration. Similar trends has been observed in certain other research papers [1–3]. The small particles are distributed homogenously between the big particles (agglomeration of many small particles together) and the particle size of the small particles are uniform. However, some pores can be observed. The formation of pores are mainly due to the non uniformity of the initial powder particles. Also, it is clear from Fig. 5a, b that the reinforcement particles of the composites are embedded in the aluminum matrix. A small agglomeration of TiC particles in the aluminum matrix is noticed and this is mainly due to non-homogeneity involved in the mixing and blending process carried out before sintering. The microstructure evaluation also shows that, for a given series of composites, the size of the TiC particles in the composites increases as the TiC content increases.

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Fig. 2 SEM micrograph of as received powders. a Al, b TiC

Fig. 3 SEM images of a Fine particles of TiC extremely grinded after 8 h of milling, b mixed powders of Al–TiC

Fig. 4 EDS spectrum of a aluminium preform after sintering, b Al–5TiC composite after sintering

Grey regions imply Al matrix and dark grey and cornered particles imply the reinforcement component of TiC. It is observed that TiC particles are all well dispersed and uniformly distributed in Al matrix. The uniform

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distribution of TiC particles can be attributed to the time and the method of mixing. For the composite materials, it is very important to obtain homogeneous reinforcement in the matrix to enhance the mechanical properties.

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Fig. 5 SEM micrograph of preform for an aspect ratio 0.75. a Al–5% TiC (2 lm), b Al–5% TiC (B200 nm)

Fig. 6 XRD pattern of composite. a Al–5TiC (2 lm), b Al–5TiC (B200 nm)

In the XRD patterns of Al and TiC, a few main peaks are clearly visible, as shown in Fig. 6a. Diffraction patterns relative to Al powder exhibit peaks at 39.5, 45.2, 65.3 and 79.5 respectively corresponding to the (111), (200), (220) and (311) reflections of fcc Al Diffraction patterns relative to TiC powder exhibit peaks at 36.5, 42.5, 61.5 and 73.5 respectively corresponding to the (111), (200), (220) and (311). The peaks in these patterns indicate a good match with the references patterns (JCPDS). Figure 6a, b show the diffraction pattern of Al–TiC composites and also show that all the composite have no impurities. The XRD results for the prepared composites are indicated by the presence of aluminium (in the largest peaks), and the presence of titanium carbide particles are indicated by minor peaks. Both the peaks are clearly visible in the composites. The increase in the intensity of the TiC peaks with the decreasing particle size of TiC content of the composite is also evident. It shows that there is no oxygen reaction in the samples during the sintering process.

X-ray mapping in electron microscopes with EDS help in qualitative X-ray micro analysis by providing a representation of the elements present. Mapping routines have long identified elements within a sample and displays the elemental distribution in an image map of the sample area. Often, such comparisons of different elemental maps, side-by-side or overlaid the locations where combinations of elements occur. These combinations of elements together in a map give a deeper understanding of the chemical nature of the material. Figure 7 displays the SEM micro-graphs and the corresponding Al, Ti and C composition maps obtained by EDS analysis of the composites reinforced with 5 wt% of TiC particles. The elemental distribution map clearly reveals the homogeneous distribution of TiC particle in Al matrix. A map overlay of all three elements shows where the combination of Al (purple) has occurred with Ti (red) and where it has occurred with C (green). It is also apparent that there are some areas where the elements are not in combination with any of the others.

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Fig. 7 Elemental distribution map of the composite with 5 wt% of the TiC reinforcement. a Al–TiC, b Al, c Ti, d C

3.2 TiC Particle Size Variation on Axial Stress Figure 8a illustrates the effect of TiC on the axial stress, that are aligned along the axial strain direction, for the aspect ratio (0.75) but with various size of TiC at 5% (2 lm, B200 nm) in the preform. The graph shows that all the lines are nearly having same characteristics irrespective of TiC size. The axial stress value shows rapid increase initially, followed by gradual increase with the rise of axial strain until the preform is on the verge of collapse. Further, it has been found that a preform with lower size (B200 nm) TiC content exhibits improved load bearing capacity compared to the higher size (2 lm) of TiC content, with the given initial aspect ratio kept constant. Figure 8b exhibits the same effect of TiC substances for an aspect ratio (1.00). The development of the points are similar to those in Fig. 8a. It is also observed from the Fig. 8a, b, that the size of pores become small for the lower size (B200 nm) of TiC substance reinforced preform and

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therefore, value of axial stress is higher for both aspect ratio (0.75, 1.00). Further, it is found that a preform having low aspect ratio (0.75) exhibits improved load bearing capacity compared to the high aspect ratio (1.00), by keeping the given initial particle size of TiC constant. The result is in good agreement with [6, 7, 9]. 3.3 TiC Particle Size Variation on Poisson Ratio Figure 9 depicits the poisson ratio derived from the only contact diameter [Eq. (5)] with respect to fractional theoretical density [14] which have been obtained during the cold deformation test. These curves are drawn based on (1) aspect ratio and (2) various size of TiC particle. The curves also reveal that the small aspect ratio (0.75) and lower size (200 nm) of TiC preforms achieves more densification than higher aspect ratio (1.00) and higher size (2 lm) of TiC preform. The characteristic feature of these curves can be categorised into three different

Trans Indian Inst Met Fig. 8 Correlation between axial stress and axial strain for an aspect ratio. a 0.75 and b 1.00

sections. Section 1 describes a rapid rise in the value of poisson ratio with little densification. The little densification is due to the material offering initial resistance to deformation. Section 2 describes, the densification occurring with a small increase in the poisson ratio. Here, densification occurs mostly in the lateral and the axial direction, due to the elimination of pores. Section 3 describes a rapid increase in the poisson ratio taking place without many enhancements in the density values in the deforming preforms. Here, the lateral spread is higher compared to the peak strains. However, the tendency is seen to approach a limiting value of 0.5, which is the theoretically feasible value of the poisson ratio. The result is in good agreement with [6, 7, 9].

3.4 TiC Particle Size Variation on Relative Density Figure 10 depicts the fractional theoretical density with respect to strain factor derived from [Eq. (4)]. This graph demonstrates the correlation for the aspect ratios (1.00, 0.75) and various size of TiC at 5% (2 lm, B200 nm). Construction of all curves are nearly similar in character with slight variation based on the aspect ratio and size of TiC. The TiC size variation provides enormous variation into the performance of density ratio. Further, it has been found that lower size (B200 nm) of TiC and low aspect ratio (0.75) exhibits increased densification compared to that for higher size (2 lm) of TiC and high aspect ratio (1.00) preform, for the given initial fractional density that

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Trans Indian Inst Met Fig. 9 Correlation between Poisson’s ratio and relative density

Fig. 10 Correlation between relative density and strain factor

was kept constant. At this moment, a gradual rise in density ratio related to the strain factor are established. The result is in good agreement with [6, 7, 9]. 3.5 TiC Particle Size Variation on Various Stresses The stresses increase by increasing stage of axial strain shown in Fig. 11a. Naturally, Hoop stress is tensile because, when the compressive loading takes place, the bulge diameter expands. For every deformation, the enhancement in hoop stress during the loading is less related to axial stress. The hydrostatic stress (rm) is greatly less than other stresses (rz ; rh ) and has a compressive nature for different strain level. As the low size

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of TiC particle diffuses into the Al–TiC preform, the axial and the hoop stresses also increases for a given aspect ratio of 0.75. Figure 11b exhibits the response of TiC substance on the development curve with the aspect ratio of 1.00. Formation of the graph is similar to Fig. 11a. It is also observed from the graphs of Fig. 11a, b which shows that the size of pore becomes lesser for B200 nm TiC content composite and therefore axial and hoop stresses value increases. Hardly, axial and hoop stresses decreases when the aspect ratio increases from 0.75 to 1.00, because of the larger pores size. This indicates that 0.75 shows superior densification when compared to other. The result is in good agreement with [7, 9].

Trans Indian Inst Met Fig. 11 Correlation between various stresses and axial strain for an aspect ratio. a 0.75 and b 1.00

3.6 Effect of Aspect Ratio on Fracture Figure 12a, b demonstrates the fracture analysis of Al–5TiC relating to aspect ratio (1.00, 0.75). Because of the ductile characteristics of the composite, the fractures possibly acts similar, and grain boundaries are clearly visible in the micrograph. Also the micrograph exhibits pore joining and fracture throughout the process. The cracked nuclei are formed at sites where the glide processes are impeded, like for example at precipitates (Fig. 12a) or grain boundaries. Because the grains have different orientations with respect to each other, the crack generally divides into terrace-like steps

when crossing a grain boundary in Fig. 12b. The newly created different crack planes join together during further crack propagation producing a characteristic river-like pattern. The flatness of cleavage facets suggests the idea that only the two atomic planes forming the fracture surfaces are involved in the crack process. But in cleavage, a plastic zone is also formed in front of a running crack. This plastic zone consumes the main part of the work of fracture. Depending on the lateral extent of the plastic zone, the material near the crack planes is plastically deformed. Additionally, it is also noticed that due to the aspect ratio (0.75), the preform density becomes higher, the fracture thickness becomes lower which

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Fig. 12 SEM micrograph for fracture of Al–5% TiC sintered preform after upsetting for an aspect ratio. a 1.00 and b 0.75

shows smart cracks. This defines that the crack movement is delayed for low aspect ratio.

•

In future, it will be worthwhile to test the Al–10TiC composite for upset and mechanical analysis.

4 Conclusions References The conclusions derived from the present investigations are as follow: •

•

•

•

•

•

Powder metallurgy route was employed to produce aluminium based nanocomposites reinforced with titanium carbide of varied levels. XRD pattern and EDS confirmed the functional element present in the Al–TiC (2 lm, B200 nm) composites through the different peak intensity using JCPDS and the elemental peaks of aluminium and titanium carbide were also observed. A powerlaw  relationship between fractional theoretical qf density q and strain factor (eðez eh Þ ) was established. th Further, it was found that the Al–TiC (B200 nm) preform had increased densification mechanism compared to that of Al–TiC (2 lm) preforms. The preforms of Al–TiC (B200 nm) attained higher value of deformation properties namely stress, strain, and Poission’s ratio than other preforms provided, that the initial fractional density was kept constant. Rarely, axial stress decreased when the aspect ratio increased from 0.75 to 1.00, because of the larger pore size. It was noticed that the low aspect ratio had dominant work-hardening behaviour. So, Al–TiC composites prepared by this method were suitable for structural and industrial applications, similar to other Al based MMCs.

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