Evaluation of Mechanical behaviour of Powder Metallurgy

IJRMET Vol. 6, IssuE 1, NoV 2015-ApRIl 2016 ISSN : 2249-5762 (Online) | ISSN : 2249-5770 (Print)

w w w . i j r m e t . c o m 120 INterNatIONal JOurNal Of reSearch IN MechaNIcal eNgINeerINg & techNOlOgy

Evaluation of Mechanical behaviour of Powder Metallurgy-Processed Aluminium Self Lubricating Hybrid

Composites with B4C Gr Additions1K. Sunil Ratna Kumar, 2Ch. Ratnam, 3C. Lakshmi Poornima

1,3Dept. of Mechanical Engineering, Sir CRR College of Engineering, Vatlur, Eluru, AP, India2Andhra University Women’s College of Engineering, Andhra University, Visakhapatnam, AP, India

AbstractIn this experimental study, aluminium (Al)-based self-lubricating hybrid metal matrix composite materials were fabricated with Boron carbide (B4C) and graphite (Gr) as the reinforcement by powder metallurgy technique. The required composition of powders (Al-B4C & Gr) were ball milled in a RESTECH100 ball milling machine and then the milled powder were compressed in an UTM to get green pellets and then the pellets were sintered at an elevated temperature in a muffle furnace filled with orgon gas to get required composites. The mechanical property like hardness of these composite materials was investigated. The result of the test shows that the B4C-reinforced hybrid composites exhibited high hardness values compared to the unreinforced alloy and Al-Gr composites. It was found that with an increase in the B4C content, there is an increase in hardness and decrease in density, various weight based composites like Al-B4C-Gr ( 94.5% - 1.5% - 4%, 93% - 3% - 4%, 91.5% - 4.5%- 4%, 90% - 6% - 4% ) were fabricated by powder metallurgy technique. For the fabrication of aluminium composites, various reinforcements like Sic, B4C, Al2O3 and Gr were used to improve the mechanical properties. One of the reinforcing materials which is B4C having high hardness value and less density when compared with Al2O3 and Sic, the other reinforcing material which is Gr has less density, softer than the other materials and it can be used for the lubricating purpose. The hardness of the composite will increases with the increase of B4C and then it is very difficult to machine, to overcome this effect, small amount of Gr can be used for the fabrication of composites.

KeywordsHybrid Composite, Self Lubricating, Powder Metallurgy, Sintering, Compression, Green Pellets

I. IntroductionNow a day’s, the mechanical properties of the elements which were used in the design of various components were improved by the use of composite. These composites can be used as the substitutute for the existing materials. These composites give good strength and sometimes less denser than the parant material [4,6,14]. Aluminium material is an encouraging one to give better properties with low density and also cheaper than the other materials Mg and Ti [6]. There is a potential to use Aluminium composites in place of steel and cast iron components [27-28]. Al 2024 can be used for the aerospace and automobile application due to its high strength to weight ratio and other mechanical properties. A key challenge for lightweight materials is the ability to produce an efficient component at acceptable mechanical properties [27, 28]. Alumina (Al2O3), titanium carbide (TiC), Boron carbide (B4C) and graphite (Gr) in the form of particles and whiskers [3] can be used as reinforcing materials for aluminium matrix to give good properties. In this context,

Aluminium Powder Metallurgy (P/M) can provide components with excellent mechanical and fatigue properties, low density, corrosion resistance, high thermal and electrical conductivity and excellent machinability [8,26]. The primary driver for the use of aluminium P/M is the unique properties of aluminium relating to the ability to produce complex net or near net-shaped parts, which can eliminate the operational and capital costs associated with complex machining operations. At the present time, aluminium metal matrix composites (AMMCs) are well recognized and steadily improving due to their advanced engineering properties, such as wear resistance, low density, specific strength and stiffness [8,26]. Among all of these superior properties, the improved wear resistance of AMMCs has attracted significant attention in the field of tribology [8,26]. Wear is one of the most commonly encountered industrial problems leading to the replacement of components and assemblies in engineering. However, wear reduces the operating efficiency by increasing material losses, fuel utilisation and the rate of component replacement [26]. Thus, assessment of the wear behaviours of engineering materials is essential. In this context, both the mechanical strength and the wear resistance of composites increase with the addition of hard B4C particles to the aluminium matrix alloy. However, the consequent increase in hardness makes machining difficult [28]. Additional problems associated with these hard particles are their tendency to detach from the matrix and act as third-body abrasives, leading to an increase in wear [29, 30]. Furthermore, the use of a single reinforcement in an aluminium matrix may sometimes compromise the values of its physical and tribological properties [13, 29,30]. Thus, it is essential to identify ways to retain the advantageous influence of B4C while simultaneously attending to the problems of machining B4C -reinforced composites. Graphite particulates are well suited for this application, as their addition improves the machinability and wear resistance of Al– B4C composites. Al–B4C composites reinforced with graphite particulates are known as Al– B4C –Gr hybrid composites [13,28]. Recent investigations [31–38] have revealed that the addition of hard, solid lubricant particles improves the tribological properties of these composites under sliding wear conditions, which is due to strengthening of the B4C to matrix and lubrication by the scattering of Gr on the two counter faces. One of the advantages of Al/B4C/ graphite composites is that they are self-lubricating materials, and yet, their strength is improved by the presence of the B4C ceramic particles. Basavarajappa [29] investigated the influence of sliding speed on the dry sliding wear behaviour and subsurface deformation of hybrid metal matrix composites (Al-SiC-Gr) using a liquid metallurgy technique. In addition, Riahi and Alpas [39] showed that the formation of a tribolayer delayed the transition from mild to severe wear. Rohatgi et al. [40]. have reported that the reduction in the friction coefficient of Al–10SiC–6Gr is caused by the combination of an increase in the bulk mechanical properties as a result of the addition of SiC and the formation of a graphite film. Biswas and Pramila Bai

IJRMET Vol. 6, IssuE 1, NoV 2015-ApRIl 2016

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ISSN : 2249-5762 (Online) | ISSN : 2249-5770 (Print)

[41] showed that unreinforced Al–Si composites had better wear properties than those with 2.7–5.7% graphite. Ted Guo and Tsao [42] observed that the wear of Al–10SiC–(2–8)Gr increases as the graphite content approaches 5% because of the reduced fracture toughness and then decreases due to the formation of a thick solid .lubricant film that overrides the effect of the reduced fracture toughness. Hybrid composites have been studied with respect to different combinations of reinforcements and matrices and their contribution to the tribological properties of the materials. The authors of this report have previously described the application of factorial techniques to study the wear of Al hybrid composites [13]. And also depicted tribological behaviour of powder metallurgy-processed aluminium hybrid composites with the addition of graphite solid lubricant [26]. But no systematic attempt has been made to study the influence of the hybridisation of B4C on the tribological properties of aluminium-based composites prepared by powder metallurgy route. Furthermore, it is evident from these studies that the majority of the alloys chosen as matrices have been the A356, 6xxx and 7xxx series alloys. Although some studies have been reported on the 2xxx series alloys reinforced with both silicon carbide and graphite particulates, much less attention has been given to the Al 2024 alloy matrix composites, which, has the highest hardness among all Al alloys [43]. Therefore, in the present study, the variation of hardness with the effect of the B4C content in Al 2024 hybrid composites is investigated.

II. Experimentation

A. Specimen PreparationThe aluminium composites were fabricated by Powder metallurgy technique. Aluminium 2024 was used as the matrix and the composition as follows:

Table 1: Ingredients Present in Al 2024S.No COMPONENT %OF WEIGHT

1 Aluminium 90.7-94.72 Copper 3.8-4.93 Chromium Max 0.14 Ferrous Max 0.55 Magnesium 1.2-1.86 Manganese 0.3-0.97 Silicon Max 0.58 Titanium Max 0.159 Zinc Max 0.2510 Other total Max 0.3

Table 2: Details of ReinforcementsReinforcement Average size Density(g/cm3)

B4C 50 μm 2.52Gr 45 μm 2.09-2.33

Investigation, the details of its composition are given in Table 1. This matrix was chosen because it provides an excellent combination of strength and damage tolerance at elevated and cryogenic temperatures. It can be used for both automobile and aerospace application. It was received from perfect metal works, Bangalore,India in the form of plate, filing operation was done to get different size of particles and then the obtained particles were sieved in sieving machine to get an average size of 60 μm.

The reinforcement particles which were B4C with average size of 50μm together with flake Gr particles with average size of 45 μm were received from sigma aldritch, india. The size distribution of the Al 2024 and Gr particles was measured using particle size analyzer. The results were shown in fig. 1 and fig. 2. The images of all the powers for the fabrication of the composites were shown in fig 3 and the SEM micrographs of these powders are shown in fig. 4. To carry out the study, five types of composites were prepared (Refer Table 3). Table 2 provides the details of the B4C and Gr particulates, which were used as reinforcements. Generally, the wear resistance of metal matrix composites increases with increasing size and shape of the reinforcement particles [26]. According to literature study, size selection of the reinforcement particles (B4C, Gr) was based on its potential applications and tribological features [28-29,32,36]. Table 3 gives the details of the hybrid composites. The mixing of the powder was performed in a planetary tumbler mixer using tungsten carbide balls with a diameter of 10 mm and a ball to powder weight ratio of 10:1 at a speed of 400rpm for about 24 hours. The uniformly mixed powder is then kept in air for about 24 hours to evaporating the volatile matter present in the mixture. The mixed powders were pressed in a uniaxial press at 400 Mpa to form green compacts. Before each run, die wall lubrication was performed manually using silicon spray.

Fig. 1: Particle Size Distribution of as-received Aluminium Particle

Fig. 2: Particle Size Distribution of as-received Graphite Powder



Fig. 3: Particle Size Distribution of as-received B4C Powder

Fig. 4: Powders Before Composition (a) Aluminium Powder (b) Gr Powder (c) B4C Powder

Fig. 5: SEM Micrographs of as-received: (a) Aluminium Powders, (b) B4C Particles, (c) Gr Particles

III. Fabrication of Specimens

A. Powder FormationAluminium powder which was used as the matrix for the fabrication of aluminium hybrid composites was obtained by filing Al 2024 plate and then sieving the obtained particles in a sheave shaker to collect the required powder of an average size of 60μm and the used reinforcement in this composition was Boron Carbide of an average size of 50μm was obtained from Metal Powder Company Ltd, Vijayawada, for the lubrication purpose another

used reinforcement was graphite, which was obtained from sigma aldrich,Bangalore.

B. Powder Mixing

1. Steps of Ball Milling ProcessBall milling variables like time, speed, process control agents will influence the final results of the mechanical alloy and were chosen from the literature, A Retsch100 Ball Mill, Germany was used for mechanical activation and alloying. High energy ball milling experiment was conducted on Al 2024, B4C and Graphite. A set of operation parameters must be chosen in order to obtain the optimum values. The ball milling time, speed and process control agents were selected. Proper ball diameter was chosen to mix the powder and for analyse the optimal performance. The numbers of balls were varied to find the proper ball to powder weight ratio, BPR. The best routine was obtained with 24 balls with diameter of 10 mm. For simplicity no combinations of small and large balls were used to randomize the motion. After number of balls and their diameter were selected, the charge Al- B4C -Gr to be milled was set to ensure at least 50% empty space volume in the container that guarantees proper balls movement. The jar capacity used was 45cm3.The empty free space in the jar was 25%. This provides enough space for ball movement. The ball-to-powder weight ratio BPR was 10:1. In wet milling best medium must be chosen in order to avoid agglomeration. The best performance (less cold welding occurrence) and wettability was obtained with B4C, Graphite by using the ball diameter as 10mm and the process was carried out in the toluene medium.

Fig. 6: Ball Mill Machine Used to Mix Powders

Fig. 7: Mixed Al 2024 & B4C and Graphite Powders




C. Preparation of PelletsIn this step a die-punch was used for compaction of the blended and mixed powders using cold compaction by the hydraulic press (UTM). Pellets of 12mm diameter and approximately 15mm height were made by compacting powders in die-punch using a hydraulic press. The compaction pressure for each pellet was 400Mpa. The well-mixed powders of a particular sample were first put in the die. The die was filled with approximately same quantity of powder for every pellet to be made. A Silicon spray was sprayed into the die before the powder was put in the dye so as to provide proper lubrication between the die walls and the powder used. Then the punch was placed over the powder in the dye, filled up to the brim. A pressure of 400Mpa was then applied on the die-punch by a hydraulic press. The pressure was applied for about 1 minute and was released after that the pellets were ejected out after compaction was over. For each composition 5 pellets were made for various tests. The dimensions length of the various pellets was measured using a Vernier Calliper. Three different readings were taken for each pellet at various sites of the pellet and then an average of these was taken as the length of a particular pellet.

Fig. 8: Pellets Obtained After Compaction

D. SinteringSintering of powder sequentially involves the establishment and growth of bonds between the particles of powder at their areas of contact and migration of the grain boundaries formed at the bonds [30]. The pellets were sintered in an electric muffle furnace at a closely regulated temperature of 6300C for about 120 min in an organ atmosphere as suggested by Yamagushi [44] and samples were allowed to get cooled to room temperature in the furnace itself. At last, in order to reach to the proper shape and size, all the compacts were trimmed to the exact size with a diameter of 12 mm and a height of 15 mm. The ends of the specimens were sequentially polished with abrasive paper of grades 600, 800 and 1000. The density of the composite specimens was determined using a high precision digital electronic weighing balance with an accuracy of 0.0001 g by using Archimedes’ principle. Hardness of sintered compacts was performed in Rockwell scale B with a ball diameter of 1/16 inches and a load of 100 kgf. Each test was repeated six times, and the average results were taken.

Fig. 9: Equipment Used for Sintering

Fig. 10: Pellets Placed Inside Muffle Furnace

IV. Results and Discussions

A. Morphology of Aluminium, Graphite and Boron Carbide PowderThe size distribution of the B4C and Gr particles was shown in fig. 2 and 3. The average size of the particles were determined

B. X-ray Diffraction AnalysisThe X-ray diffraction (XRD) results for the prepared composites are shown in fig. 11. These results indicate the presence of aluminium (in the largest peaks), and the presence of boron carbide particles and carbon is indicated by minor peaks. A clearly visible carbon peak can be observed in the hybrid composites. The increase in the intensity of the B4C peaks with the increasing boron carbide content of the composite is evident. Fig. 11 also shows that there is no oxygen reaction in the samples during the sintering process. The XRD pattern confirmed the presence of aluminium, Gr (C) and B4C particles in the hybrid composite. These XRD facilities were taken from Central Electrochemical Research Institute, Karaikudi, Tamil Nadu, India, using HITACHI SU-6600.

Fig. 11: XRD Results of Various Compositions



C. Characterization of Al-B4C Hybrid Composites

1. SEM – EDS StudiesMicrostructure examinations were carried out using Scanning Electron Microscopy (SEM). Kevex energy dispersive x-ray spectroscopy (EDS) system and X-ray mapping were also utilized to characterize chemical elements and their distribution In order to verify the composition of the composite, EDS analysis was used the spectrum of composites. Aluminium, boron carbide and graphite were detected in the analysis. It seems that the process was protected quite well since small quantity of oxygen was detected. Since the average size of the particles is less than 60 μm, it is very difficult to use EDS spot analysis on a single particle due to the limitation of the e-beam resolution in this instrument. Therefore, mapping scanning was employed. Fig. 11 shows the distribution of the elements aluminium (Al), Boron carbide (B4C), graphite(Gr) respectively. The results show that aluminium and Boron carbide is distributed uniformly, which probably indicates a good dispersion of B4C particles in the matrix.

Fig. 12: Optical Micrographs of the Produced Composites. (a) Al 2024, (b) Al 2024-Gr 4%, (c) Al 2024-Gr 4%-B4C 1.5%, (d) Al 2024- Gr 4%-B4C 3%, (e) Al 2024-Gr 4%-B4C 4.5%,(f) Al 2024-Gr 4%-B4C 6%.

The grain structures and reinforced particles’ size, shape and their distribution were observed in micrographs. The more important use of optical microscope in microstructure examination was in the analysis of reinforced particles in aluminium matrix. Metallographic specimens of sintered preforms were prepared using standard hand polishing using 240, 600, 800 and 1000-grit silicon carbide papers. The specimens were then finish-polished using 1 lm diamond paste suspended in distilled water to obtain mirror-like surface finish. To expose the microstructural features, the polished specimens were etched with Keller etching solution. The etch–polish–etch procedures were used to attain good microstructure. These microstructure investigations show the presence of Gr and B4C in each hybrid composites as shown in Fig. 17. These graphite and B4C particles are uniformly distributed throughout the Al 2024 matrix phase. The absence of cracks can also be observed from the micrographs.

2. XRD StudiesFor XRD studies of micro and nano aluminium composites (Model: 2036E201; Rigaku, Ultima IV, Japan) the 10x10mm solid

specimens are prepared for the characterization. These samples are set into the XRD sample holder. Generally there are two types of sample holders, one is for characterizing the powder sample which is having dimensions of 20x20mm with 0.3mm depth and the other is for solid sample which is having a centred hole with 20x20mmThe solid composite sample is placed into the solid sample holder and set it in the XRD machine.The surface of the composite sample must be in plane with the sample holder .The practical X-ray diffractometer (XRD) machine is shown in the The XRD is power on and the voltage and the current values are set to standard values i.e. 40kV and 20mA. Now the XRD pattern is generated with a speed of 2 deg per minute and the pattern is generated from 2 deg to 90 deg. The generated XRD pattern is now analyzed with JADE 7.0 Elevation software for structural information. From this we can obtain crystallinity of the composite sample, phase identification, crystallite size of the phase identified, planes orientation, crystal structure of the phase and residual strains of the crystals.

3. Density and HardnessFig. 12 shows the variation of density of hybrid composites at B4C content. It was observed that there is a decrease in density with increase in B4C reinforcement. This can be attributed to the addition of lower ensity reinforcements which are B4C and Gr.

Table 3: Density of Various CompositesS.NO COMPOSITION (Wt%) Density(g/cm3)

1 Al 2024 2.8302 Al 2024/ 4 wt% of Gr composite 2.8243 Al 2024/ 4 wt% of Gr/ 1.5 wt% 2.8204 Al 2024/ 4 wt% of Gr/ 3 wt% 2.8155 Al 2024/ 4 wt% of Gr/ 4.5 wt% 2.8106 Al 2024/ 4 wt% of Gr/ 6 wt% 2.800

Fig. 13: Variation of Density With Increase in the Reinforce-ment

The density of the above composites were decreased with the increase in the % of composition of the reinforcement, Whenever only 4% of graphite was added to the aluminium, there is sudden decrease in the density due to less denser property of the graphite as compared with the other two materials, after which, the density gradually decreased with the increase in B4C, after the reinforcement reached 5%, thereafter, again sudden decrease in the density due to the decrease in volume of the aluminium which is having high density and this volume was occupied by the less denser reinforcement of B4C.




Table 5: Variation of Hardness of Al- B4C composites

S. No Composition by weight Rockwell hardness

1 Al 98.5%- B4C 1.5% 79

2 Al 97%- B4C 3% 81

3 Al 95.5%- B4C 4.5% 83

4 Al 94%- B4C 6% 85

Fig. 14: Variation of Hardness of Al- B4C Composites

Table 6: Variation of Hardness of Al- B4C -Gr Hybrid Composites With Reference to Al2024

S.No Composition by weight Rockwell hardness1 Al 2024 782 Al- 4% Gr 763 Al-1.5% B4C-4% Gr 77

4 Al-3%B4C-4%Gr 82

5 Al-4.5%B4C-4%Gr 85

6 Al-6% B4C-4%Gr 88

Fig. 15: Variation of Hardness of Al- B4C-Gr Hybrid Composites

From the above fig. 14, the Rockwell hardness of the composite increases with the increase in the reinforcement, whenever the reinforcement is only B4C then it is very difficult to machine the specimen due to the hardness of the composite.

Table 7: Variation of Hardness of Al-B4C- Gr hybrid composites

S.No Composition by weight Rockwell hardness1 Al 97.5%- B4C 1.5%-Gr 4% 772 Al 96%- B4C 3%- Gr 4% 823 Al 94.5%- B4C 4.5%- Gr 4% 854 Al 93%- B4C 6%- Gr 4% 88

Fig. 16: Comparison of Composites With and Without the Addition of Gr

Fig. 14 also shows the variation of hardness of the aluminium composite with increased B4C content. It can be understood from Fig. 14 that the hardness of the composites was improved with the increase in weight percent of B4C reinforcements. The variation of hardness with increase of B4C and fixed amount of Gr can be shown in Fig. 15 and the comparision between hybrid and non hybrid composites were shown in Fig. 16. The increase in hardness of hybrid composite is owing to the following reasons:

High hardness of B4C reinforcement particles.• Uniform distribution of B4C in the composites.•

ConclusionAluminium self lubricating metal matrix hybrid composites were fabricated successfully by using powder metallurgy technique, the obtained results showed that the hybrid composite material had less denser than the parent material which is Al 2024. The hardness of the obtained hybrid composites is higher than the unreinforced composites; however the graphite content can be used to soften the material for the machining purpose.

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K Sunil Ratna Kumar received his Bachelor of Engineering (BE) degree from Andhra University, Visakhapatnam, Andhra Pradesh, India, in 2000 and received Master of Engineering in the specialization of Machine Design in 2002 from same university. Now he is doing his research work in the area of Composite materials at Andhra University, Visakhapatnam. He is working as an Assistant Professor since 2003, in the

department of mechanical engineering of Sir C R R College of engineering, Eluru, Andhra Pradesh, India.

Documents

Evaluation of Mechanical behaviour of Powder Metallurgy