Physical Properties and Microstructure of Green Aluminium Matrix Composite

Transcription

1 Physical Properties and Microstructure of Green Aluminium Matrix Composite Zamri Yusoff 1*, Shamsul Baharin Jamaludin 2 and Mazlee Mohd Noor 2 1 Mechanical Engineering Department, Politeknik Kota Bharu, 16450, Ketereh, Kelantan. 2 Sustainable Engineering Research Cluster, Universiti Malaysia Perlis, Kangar, Perlis. *irmazai07@yahoo.com.my ABSTRACT A study of physical properties and microstructure was carried out on the pure aluminium reinforced with biomass by product such as palm shell activated carbon (PSAC), slag and PSAC/slag particles in order to develop the green aluminium matrix composite (GAMC). The aims of the work are to correlate the influence of reinforcement (PSAC and slag) content with physical properties of the three systems of GAMC which are PSAC/Al, slag/al, and PSAC/slag/Al composites and to identify the potential and ability of PSAC and Slag as reinforcement materials and justify the preferable reinforcement. This work used wt. % aluminium (70 µm) mixed with 0 20 wt. % of PSAC (65 µm) and slag (65 µm), to fabricate PSAC/Al un-hybrid GAMC, slag/al un-hybrid GAMC, and PSAC/slag/Al hybrid GAMC via conventional powder metallurgy that compacted into pin sample (Ø8 mm) followed by sintering at C/2 hr. Physical and mechanical properties such as porosity, density and macro-hardness test have been determined on as-sintered pin samples of GAMC. The microstructure was observed by scanning electron microscope with EDX for determination of particles distribution and bonding between reinforcement/matrix. Result appears that the slag/al un-hybrid GAMC had the highest values of bulk density and macro-hardness. The PSAC/slag/Al hybrid GAMC gave intermediate values and the PSAC/Al un-hybrid GAMC gave the lowest values. Porosity of PSAC/Al un-hybrid GAMC was the highest, slag/al un-hybrid GAMC was the lowest and PSAC/slag hybrid GAMC was the intermediate comparatively. Based on microstructural evaluation, micro-pores are observed in PSAC particles, but no micro-pores in the Slag particles. However, micro-crack is observed in the Slag particles. Micro-cracks and micro-pores have influenced the density, porosity and macro-hardness as well as bonding between reinforcement /matrix of GAMC. The slag/al un-hybrid GAMC is preferable due to good bonding between slag/al, lowest porosity, highest density and macro-hardness compared to others. Keywords: green aluminium matrix composite, microstructure, palm shell activated carbon, slag

2 INTRODUCTION Aluminium Matrix Composite (AMC) is a kind of material having special function and structures. It is formed by spreading reinforcement in matrix of aluminium alloy. AMC has many advantages such as isotropic properties, better specific strength, high stiffness and hardness as well as better wear resistance. Therefore, AMC are applied to automotive industry such as cylinder liners, brake disc and drums, wear resistance materials (cemented carbides) and sporting equipment (bicycle frame, golf head and chain wheels). Technologically, almost all AMCs have difficulties in mixing or combining the reinforcing phase with the matrix. Many of the techniques were taken to overcome the processing problems have resulted in high material costs, leading to extremely limited use [1]. However, the emerging mixing techniques have improved the fabrication technique for particulate composite with differing reproducible structures and properties. Fabrication techniques like vortex technique under vacuum, compocasting, infiltration process, spray process and powder metallurgy have been developed in the production of AMC. Currently, one of the major driving forces for the technological development of aluminium matrix composite is a result of superior wear resistance and hence potential candidate for a number of tribological applications [2]. Much effort has been given to produce more durable materials and techniques to reduce the wear of tools and engineering components. These include modification of bulk properties of the materials through fabrication of AMCs. Over 30 years, researches have been focused on understanding the behaviour of surfaces in sliding contact and wear mechanism in AMCs. It can be seen from the previous works, the main particulate reinforcements used in aluminium matrix composites are silicon carbide [3, 4] and graphite [5]. In fact, many other possible reinforcements are readily available or naturally renewable at affordable cost such as coconut shell char, mica, palm-kernel shell char and zircon [6] as well as fly ash [7]. Palm shell activated carbon (PSAC) and slag are also potential to be used as reinforcement in aluminium matrix composite due to the similarity with graphite that contain of carbon content. PSAC and slag are biomass by-products from palm oil factory and can be obtained locally. The idea proposed is to reduce the cost of starting material in fabrication of novel aluminium composite reinforced with local waste materials. From the practical perspective, the utilisation of PSAC as reinforcement in fabrication of metal composite is also important for the development of palm oil industry in Malaysia in order to reduce the waste materials. It can help the government to formulate the right and appropriate strategies to promote sustainable approaches in generating higher value-added products from palm biomass and adopting of zero waste strategy. Based on the current issues, there are problems need to be solved in order to make aluminium matrix composites more competitive and environmental friendly. The first problem is low density and high thermal conductivity of aluminium. However, aluminium by itself exhibits poor mechanical properties that lead to the problems during application [8]. Improvisation of wear resistance of the aluminium by using particulate reinforcement strengthening is necessary due to isotropic properties of particulate form in particular. The second problem is fabrication aspect due to highly cost of common reinforcement such as silicon carbide, alumina and graphite. It is possible, to reduce the cost of reinforcement by using natural resources [6] such as biomass (PSAC) and by-product (slag) that can be obtained locally at affordable cost or no cost. In Malaysia, palm shell is one of the main agriculture wastes from the palm oil industries lead to abundant and environmental pollution problems due to hard clinker. Several studies have been initiated to utilise palm shell in order to reduce the abundant problems [9, 10]. Therefore, to overcome the abundant and pollutions problem through sustainable engineering concept, the utilisation of the palm shell as reinforcement can be proposed to support the green technology government policies. This work attempted to correlate the influence of reinforcement (PSAC and slag) content on the physical properties of the three systems of aluminium matrix composite which are PSAC/Al, slag/al, and PSAC/slag/Al composites as well as to identify the potential and ability of PSAC and Slag as reinforcement materials and justify the preferable reinforcement.

3 METHODOLOGY The composites used in this work were prepared by powder metallurgy technique. The matrix was pure aluminium and the reinforcement was palm shell activated carbon particles and palm shell clinker (named as slag) particles. The pure aluminium was supplied by the Sigma- Aldrich German, the PSAC particle was supplied by the KD Technology Malaysia and the Slag particle was supplied by the Felda Jerangau Malaysia (Palm Oil factory). The Malvern particle size analyser was used to analyse particle size distribution of the pure aluminium powder, palm shell activated carbon (PSAC) and Slag particles. Along the particle size distribution, the mean sizes of the powders are 70, 65 and 65 µm for the pure aluminium, PSAC and Slag powder respectively. The chemical composition of aluminium powder is 0.5 wt. % Fe, 0.03 wt. % Pb and 99.7 wt. % aluminium. The chemical composition of PSAC particle is 0.05 ppm Fe, 0.01 ppm Cu, 0.05 ppm As and ppm C. The chemical composition of Slag is 4.8 wt. % Si, 5.48 wt. % O and wt. % C. Table 1 shows the composition of the specimens. Table 1: The Composition of the Specimens No. Composition PSAC (wt.%) Slag (wt.%) Al (wt.%) 1 Pure Aluminium wt.% PSAC/Al wt.% PSAC/Al wt.% PSAC/Al wt.% PSAC/Al wt.% Slag/Al wt.% Slag/Al wt.% Slag/Al wt.% Slag/Al wt.%psac/2.5 wt.% Slag/Al wt.%psac/5 wt.% Slag/Al wt.%psac/7.5 wt.% Slag/Al wt.%psac/10 wt.% Slag/Al Fabrication was started by powder mixing using a horizontal mill at 95 rpm (diameter container 125 mm) for 10 minutes of mixing time without ball and control agent in order to prevent agglomerated of mix powder that due to heat from collisions of ball mill. The compaction method was uni-axial compacting using a Universal Testing Machine. The pressure used was 200 MPa. The specimens were cold pressed using a die into a pin of 8 mm in diameter and mm in height. Compacted specimens were sintered at C in 2 hours. Density and Porosity test The bulk density of all prepared specimens was determined to examine the possibility of correlations between reinforcement content and bulk density. Water displacement (Archimedes) method was used for bulk density and porosity measurement. A microbalance with accuracy of 10-4 g was used in order to weighing the specimens. Three readings were collected during specimens i.e. weighed dry (w d ), immersed in water (w i ) after vacuumed for 1 hr and right after the specimens removed from water (w s ). Surface Hardness Test In order to study the changes of hardness with increasing reinforcement content, the Vickers hardness tester (Mitutoyo) was used to determine macrohardness. The result was obtained from an average of 5 measurements at different location for each specimen. The measurement used Vickers diamond indenter with applied load was N and the indentation time was

4 10 s. The load speed was 20µm/s. The specimens were fine ground with 600 grit abrasive paper before testing. Microstructural Evaluation To investigate the microstructure of the specimens, the microstructure of the monolithic matrix, the composite and the hybrid composites was examined by SEM microscope. Specimens for microscopic examination were prepared by standard metallographic procedure. RESULTS AND DISCUSSION Raw Material Characterisation Figure 1 shows the SEM micrographs of the three starting materials; aluminium powder, PSAC particles and Slag particles that reveal the shape of particles in flaky shape for the aluminium powder as shown in Figure1(a). Whereas the irregular shape particles for the PSAC as shown in Figure 1(b) and the Slag as shown in Figure 1(c). Figures 1(d) and 1(e) show the SEM micrograph at high magnification of PSAC and Slag particles respectively. It reveals that the presence of micro-pores in PSAC particles as shown in Figure 1(c), but no micro-pores in the Slag particles as shown in Figure 1(d). However, micro-cracks can be observed in the Slag particles. d Micro-pores e Micro-cracks No micro-pores Figure 1: The SEM micrographs of three starting materials; (a) Aluminium powder; (b) PSAC particles; (c) Slag particles;(d) high magnification of PSAC particles and (e) high magnification of Slag particles. Bulk Density and Apparent Porosity Figure 2(a) shows the variation of bulk density of the specimens when the weight percent of reinforcement particles increased. The density of the composites decreases with increasing the reinforcement content consequently the porosity of the composites is increased as shown in Figure 2(b). As an example, for the composite Slag/Al, the porosity increases from 11.76% to 14% as a result of dispersion of 20 wt. % of Slag as shown in Figure 2(b). Similar findings were also reported by other researchers for 6061Al/graphite [11]. They found that the porosity of the sintered 6061 Al/Graphite composite increased from 6.0% to 13.0% as a result of dispersion of 14 vol. % of graphite. The porosity of the sintered Al/PSAC composite increases from 11.76% to 45.78% as a result of dispersion of 20 wt. % of PSAC as shown in Figure 2(b). The variation of density and porosity of the composites is due to reinforcement particles disturb the diffusion bonding during sintering and presence of pores during compaction as well as irregularity of shape and size reinforcement particles. Similar finding were reported by others [12]. They found that measured density of the aluminium composite with 6 wt. % multi-walled carbon nanotubes (MWCNTs) is far below the theoretical expectation. They concluded that MWCNTs may

5 interrupt diffusion bonding among powder during hot working process and lot of pores might be induced during consolidation of powder containing high volume of MWCNTs. Surface Hardness Figure 2(c) shows the relationship between the weight percent of reinforcement particles and the hardness of the composites. The result shows that the hardness decreases slightly with the increasing weight percent of PSAC until 5 wt. % and decreases drastically after that. The hardness of the PSAC/Al composite is reduced around 70% as compared to the hardness of the pure Al. It is obvious that the surface hardness of aluminium is reduced when reinforced with PSAC particles especially when PSAC particle content above than 5 wt. %. For 10 wt. % to 20 wt. % of PSAC content, decreasing of the hardness is not significant. Figure 2(c) shows that the hardness decreases slightly with the increasing weight percent of Slag particles. The hardness of the Slag/Al is reduced around 40% as compared to the hardness of pure Al. Similarly, the decreasing of hardness with the addition of soft particle reinforcement such as graphite was also reported by some researchers [11, 13, 14, 15, 16, 17]. The increase amount of the brittle graphite particles will increase tendency of crack initiation and propagation at the graphite/metal interface [14]. The lower percentage of reduced hardness of the Slag/Al composite compared to the PSAC/Al composite is due to the interface bonding between slag particle/aluminium as shown in Figure 2(c). The hardness of Slag/Al is higher than PSAC/Al due to good bonding between matrix and slag particles in the slag/al composite. a b c Figure 2: Variation of (a) bulk density (b) porosity (c) hardness results of aluminium matrix composite Microstructural Evaluation Figure 3(a) shows the SEM micrograph of the compacted pure aluminium after sintering at C. There is parallel grain structure that can be observed owing to the flaky shape of pure aluminium powders. Figure 3(b) shows the SEM micrograph of 20 wt. % of PSAC/Al respectively reveals the distribution of PSAC particles (black phase) throughout the aluminium matrix. Figure 3(c) shows the SEM micrograph of 20 wt. % of Slag/Al respectively reveals the distribution of slag particles (white phase) throughout the aluminium matrix Figure 3(d) shows the SEM micrograph of the 10 wt. % PSAC/10 wt. % Slag/Al respectively reveals the distribution of PSAC particles (black phase) and Slag (white phase) throughout the aluminium matrix. a b c d Figure 3: SEM observation of after sintered at C (a) the pure aluminium (b) 20 wt.% PSAC; (c) 10 wt.% Slag; (c) 20 wt.% Slag; (d) 10 wt.% PSAC/10 wt.% Slag. Figure 4 shows the SEM micrograph at high magnification that focused on the reinforcement particles embedded in Al matrix. Figure 4(a) shows that Slag particle has a good interface

6 bonding with the Al matrix, whereas PSAC particle shows a poor interface bonding as shown in Figure 4(b). Figure 4: The SEM micrograph at high magnification (a) interface of Slag/Al (b) interface of PSAC/Al CONCLUSIONS 1) The physical properties of GAMC were influenced by PSAC and slag content whereby the slag/al un-hybrid GAMC had the highest values of bulk density ( g/cm 3 ) and macro-hardness ( VHN). The PSAC/slag/Al hybrid GAMC gave intermediate values ( g/cm 3 and VHN) and the PSAC/Al un-hybrid GAMC gave the lowest values ( g/cm 3 and VHN). The Porosity of PSAC/Al un-hybrid GAMC was the highest (33.8%), slag/al un-hybrid GAMC was the lowest (12.9%) and PSAC/slag hybrid GAMC was the intermediate (21.1%) comparatively. 2) Based on the microstructural evaluation, it reveals good interface bonding between Slag/Al and poor interface bonding between PSAC/Al. The microstructure influenced the density, porosity and macro-hardness of the composites. REFERENCES [1] J.J. Yeh, L.D. Chen and C.B. Lin: 11 th International Conference on Composite materials. Woodhead Publishing Limited (1997), p. 699 [2] M. N. Mazlee, J. B. Shamsul, K. R. Ahmad, M. W. M. Fitri & C. M. Ruzaidi. Journal of Microscopy, Vol. 2(2005), p [3] R.N. Rao and S. Das: Materials & Design, Vol. 32 (2011), p [4] Y.Q. Wang, A.M. Afsar, J.H. Jang, K.S. Han and J.I. Song: Wear, Vol. 268(2010), p. 863 [5] R. Dequino-Lara: Journal of Alloys and Compounds, Vol. 509(2010), p. 284 [6] J.U. Ejiofor and R.G. Reddy: Journal of Metals, Vol. 49(1997), p. 31 [7] D. Luong, N. Gupta and P. Rohatgi: Journal of Metal (Febuari 2011) [8] B. Venkataraman and G. Sundarajan: Wear, Vol. 245(2000), p. 22 [9] Y. B. Zamri, J. B. Shamsul, M. M. Amin. International Journal of Mechanical and Materials Engineering, Vol.6 (2011), p [10] Y. Zamri and J. B. Shamsul. Kovove Mater., Vol. 49(2011), pp [11] A.K. Jha, S.V. Prasad and G.S. Upadhyaya: Wear, Vol. 133(1989), p. 163 [12] H.J. Choi, S.M. Lee and D.H. Bae: Wear, 270(2010), p.12 [13] S. Suresh and B.K. Sridhara: Material and Design, Vol. 31(2010), p.1804 [14] H. Akhlaghi and A.Z. Bidaki: Wear, 266(2009), p. 37 [15] M.L. Ted Guo and C.Y.A. Tsao: Composite Science Technology, Vol. 60(2000), p. 65 [16] B.C. Pai and P.K. Rohatgi: Journal of Material Science, Vol. 13(1978), p. 329 [17] B.P. Krishnan,N. Raman, K. Narayanaswamy and P.K. Rohatgi:Wear, Vol. 60(1980), p. 205