Tuesday, 18 October 2016

Study of tensile properties, fractography and morphology of aluminium (1xxx)/coconut shell micro particle composites

Published Date
Available online 19 October 2015, doi:10.1016/j.jksues.2015.10.001
Open Access, Creative Commons license, Funding information
In Press, Corrected Proof — Note to users

Author 
  • Sefiu Adekunle Bello a,
  • Isiaka Ayobi Raheem b
  • Nasir Kolawole Raji c
  • aDepartment of Materials Science and Engineering, College of Engineering and Technology, Kwara State University, Malete, Nigeria
  • bDepartment of Metallurgical and Materials Engineering, Faculty of Engineering, University of Lagos, Lagos, Nigeria
  • cDepartment of Metallurgical Engineering, Federal Polytechnic Yaba, Lagos, Nigeria
Metal matrix composites are characterised with high specific mechanical properties such as high strength and ductility per unit volume. Their demands in structural applications for replacement of heavy legacy alloys are now gaining global acceptance. Among non-ferrous metals aluminium attains dominance because of its inherently excellent properties which include high ductility, ease of formability and resistance to atmospheric corrosion. Aluminium alloys and composites have found use in processing industries as packaging materials; transportation as vehicles’ structural parts, automobile engine blocks and in architectural applications (Labberton, 2014 and Harwood, 2008). Increasing demand of aluminium alloys/composites in various applications has called for research all over the world which focuses on aluminium (Al-Haidary and Jabur Al-Kaaby 2007). The only primary production of aluminium from bauxites is considered as a high energy consuming process. Also, the worse environmental issues associated with the primary production of aluminium places barriers on the process. However, volumes of discarded aluminium products due to their life span expiry/failure in service are on increase, littering our environments. Recycling of discarded aluminium products can serve as an alternative to aluminium production from bauxite. This recycling technique offers two advantages which include great amount of energy saving and closes the chance of greenhouse gas emissions peculiar to primary aluminium production from its natural ore.
Previous studies revealed the use of aluminium scraps for the green production of aluminium alloys/composites for engineering applications. Results from such studies revealed that the properties of the produced alloy/composites are comparable with those from primary aluminium. Hassan and Aigbodion (2015) used discarded high purity aluminium wire for the production of Al–Cu–Mg/eggshell particulates composites. Their results revealed that the use of carbonised eggshells gave better physical and mechanical properties than uncarbonised eggshell particles. Bello (2014) studied the development and characterisation of aluminium can–coconut shell composites. Results of his findings showed an improvement in the wear and mechanical properties of the matrix. Adewuyi and Omotoyinbo (2008) produced aluminium alloys through sand and die casting techniques from aluminium scrap. Their results showed that samples obtained from air cooled dies had better mechanical properties than those produced by sand casting. Also hardness values can be varied by changing the cooling media.
High cost of production of conventional composites due to expensiveness of synthetic fillers has limited their versatile applications (Bello, 2014 and Faye et al., 2009). This has given room for research in the production of low cost reinforcement from agricultural wastes. Hassan and Aigbodion, 2015Agunsoye et al., 2014Bello, 2014Ikpambese et al., 2014Sarki et al., 2011Hassan et al., 2013Idris et al., 2015Hassan et al., 2012 and Sapuan et al., 2003 and Hassan et al. (2008) have used reinforcements obtained from agricultural wastes for the production of polymer and metal matrix composites. Reports declared the better distribution of the fillers within the matrix and their recovery.
In this present work, metal matrix composites have been produced using aluminium (1xxx) as the matrix and 150 μm sized uncarbonised coconut shell microparticles as reinforcing fillers. This work was aimed at studying effects of %weight (wt) of coconut shell microparticle (CMPs) additions on the tensile properties and morphology of Al/CMP composites. Although a lot of research has been carried out on improvement of aluminium properties, reinforcement of aluminium (1xxx) with CMPs is very rare.

2 Materials and equipment

Coconut shells (CSs) used in this work were obtained from coconut chip centre, Ibadan, Oyo State Nigeria. Al (1xxx) used was soured from Al cans packed from event centres, Ibadan Oyo State Nigeria. CSs were sundried for two months after which they were manually broken into smaller pieces using a mortar and pestle. Broken CSs were classified using a 5 mm sized sieve vibrated manually with hands. Those CS pieces retained below the sieve were pulverised using a two hardened steel disc grinder to obtain CS powder/particles. CS particles were sorted into different mesh sizes ranging from 600 to 75 μm size using a set of sieves placed in descending order of grain fineness number (GFN), vibrated by a sine shaker for 40 min. The CS microparticles (CMPs) that were retained in a 150 μm sized sieve were used as reinforcement in this work. Equipment used in this work includes oil fired crucible furnace, scanning electron microscope with attached energy dispersive X-ray spectroscopy, Instron extensometer, Avery Denison Universal Impact tester, X-ray diffractometer and a cope and drag die cavity mould.

3 Methodology

The composites used in this study were Al–CMP composites containing 2–10% uncarbonised 150 μm CMPs at an interval of 2%. The samples were produced by compo cast technique in accordance with Bello, 2014 and Agunsoye et al., 2014. The samples were produced by varying CMP additions to the melt of Al and then stirred to ensure homogenous distribution of CMPs prior to pouring into the preheated metallic mould. A control sample without CMP additions was also produced.
The produced samples were shaped into standard samples for the purpose of tensile property investigation and impact energy determination. The morphology examination and elemental composition analysis of phases present in the produced composite samples were carried out using scanning electron microscope (ASPEX 3020), model SIRIUS50/3.8 with the attached energy dispersive X-ray spectroscopy.
The chemical formulae and compound names of the phases were determined using a Panalytical Empyrean X-ray diffractometer with Pixcel detector. The phases were identified using X’Pert High score plus software; PAB-ICSD and ICDD (2014) databases. Samples were placed on a zero background sample holder. The density of 60 × 10 × 10 mm of each sample was also measured using direct mass–volume method. A pioneer weighing scale of 210 g capacity and 0.0001 readability by Ohaus Corporation was used for mass measurement.
Tensile samples each of 40 mm gauge length and 5 mm diameter were gripped by jaws of the Instron extensometer, (model: Instron 3369; system ID: 3369S3457). They were subjected to tensile force and this resulted in the gradual stretching of the composite samples at a strain rate of 10−3 s−1 with the increasing applied stress until the fracture occurred after a considerable amount of plastic deformation. The impact energies of the produced samples were measured with the aid of Avery Denison Universal Impact Testing Machine. Before the test, the pendulum was released to set the scale to zero. The 60 × 10 × 10 impact samples of 1 mm notch depth and 0.02 notched radius at an angle of 45° were subjected to a striking energy of 300 J by a pendulum released from the upper position equivalent to charpy impact test. The impact energy absorbed by each sample was noted and recorded. In order to study the mode of fracture, the fractured surfaces of the unfilled Al and Al/10CMPs composite were analysed using SEM and Baskar optical microscope (Resolution: 640 × 840 pixels; Magnification: 10×–300×).

4 Results and discussions

4.1 SEM micrographs/EDS spectrographs

Plate 1 presents the SEM micrograph and EDS spectrograph of the 150 μm sized CMPs. The microstructure reveals the differences in geometry of the particles. Shapes of the CMPs can be classified into dendritic (tree like), prismatic, oval and other polygonal shapes. The EDS elemental analysis declares the presence of C, O, Si, Al and Cl in the CMPs. Plate 2 presents the SEM micrograph and EDS spectrograph of the unfilled Al sample. The microstructure is very plain. This is evident from EDS which reveals aluminium with maximum peak. The tiny white particles may represent some alloying elements present in the Al matrix in an amount so small that they cannot have distinct maximum peaks on the EDS spectrograph. Indistinguishable peaks around the Al peak confirm the presence of those elements. The shoulders around the Al peak may be attributable to presence of Al in different forms.
Plate 1. SEM/EDS of the bulk CS.
Plate 2. SEM/EDS of the control Al sample.
Plate 3Plate 4 and Plate 5 show the SEM micrograph/EDS spectrograph of the Al/CMP composites. Plate 3Plate 4 and Plate 5 display very fine microstructures with good interfacial bonding between Al matrix and CMP reinforcements. CMPs are well and evenly distributed and there is no CMP segregation. The microstructures are free from shrinkage cavities. This is an indication of effective stirring of the melt of the composite mixture prior to pouring and rapid solidification/cooling of the composite melt to room temperature. The rapid solidification is attributable to excellent heat conductivity of the mould material which transferred heat quickly from the melt and dissipated it to its immediate environment. The increased packing density is attributable to increment in %wt of CMPs additions. Comparison of EDS spectrographs on Plate 3Plate 4 and Plate 5 with that in Plate 2, showed that there were new elements present in the Al matrix. This indicated a chemical reaction between constituting elements in the Al matrix and those in CMPs, leading to formation of new compounds represented by the presence of those new elements on EDS spectrographs.
Plate 3. SEM/EDS of the Al/2%CMP composite.
Plate 4. SEM/EDS of the Al/4%CMP composite.
Plate 5. SEM/EDS of Al/10%CMP composite.

4.2 XRD diffractograms

XRD diffractograms of the unfilled Al presented in Fig. 1 show that the major diffraction peaks occurred at diffracting angles (2θ) of 44.94°, 52.39° and 52.39° with inter-planar distance of 2.34, 2.03 and 2.03 Å and relative X-ray diffracting intensities of 22, 60 and 62. The respective phases at these peaks are BeO, MnO and Al. This agrees with the EDS spectrograph on Plate 2. Presence of Ta2H and CO3Fe7 in Fig. 2indicated by peaks at respective diffracting angles of 44.12° and 51.86° with X-ray diffracting intensities 42 and 29 and inter-planar distances of 2.38 and 2.05 Å and Mg2Al3, C2H8ClN, CoFe and Mg2N2O3·14H2O in Fig. 3 indicated by peaks at respective diffracting angles 21.31°, 27.88°, 52.71° and 77.50° with intensities of 9, 3, 57 and 36 and respective inter-planar distances of 4.84, 3.72, 2.02 and 4.84 Å is attributable to chemical reaction between Al matrix and CMP reinforcements in Al/2%CMP and Al/10%CMP composites respectively.
Figure 1. XRD of the control cast of aluminium can sample.
Figure 2. XRD of the Al can/2%CMP composite.
Figure 3. XRD of Al can/10%CMP composite.

5 Densities

Fig. 4 display the variation in densities of the produced Al/CMPs composites as the %wt of CMPs increased. It is observed from Fig. 4 that the densities decreased as the %wt of CMPs increased. This indicates that CMPs is lighter than Al (1xxx). The density decreased from 2.69 g cm−3 of the Al with 0%CMPs addition to 2.196 g cm−3of Al/10%CMP composites equivalent to about 18.36% reduction in density. However, it can be inferred that lighter aluminium based composites can be fabricated with Al (1xxx) and CMPs.
Figure 4. Densities of Al can/CMP composites with %wt of CMPs.

6 Tensile properties

Fig. 5 show the stress–strain curves of the unfilled Al and Al/CMP composites. Different curves in Fig. 5 can be interpreted by Figure 6 and Figure 7Fig. 6 presents an increase in ultimate tensile stress (UTS) as the %wt of CMPs addition increased. Fig. 7 described an irregular behaviour with respect to tensile strain as %wt of CMP addition increased. There is a minimum tensile strain at 4%CMP addition. The enhancement in the UTS is attributable not only to the high diffusion rate of CMPs and their deep penetration into the Al matrix, leading to a good interfacial adhesion between the Al matrix and CMPs but also increased packing density of CMPs which were evenly dispersed throughout the Al matrix. Even dispersion of CMPs within the matrix of Al gave room for chemical reaction leading to the formation of 2nd phase compounds such as Ta2H, CO3Fe7, Mg2Al3, C2H8ClN, CoFe and Mg2N2O3·14H2O (see Figure 2 and Figure 3) occupying interstices between Al lattice sites. The presence of the newly formed phases caused an elastic straining effect as evident from an increment in the inter-planar spacing from 2.34 of a phase in the unfilled Al to 2.54 Å of one of the phases in the Al/CMP composites. This straining effect coupled with barriers to dislocation movement contributed to enhancement in the UTS. The barrier/hindrance to dislocation movement became more pronounced as the packing density of the CMPs increased. This led to pile-up of dislocation at the grain boundaries and new compounds’ sites such that any further dislocation movement would be induced by additional force application. Hence the composites became more strengthened. Therefore, Al/4%CMP composites at 150 μm size of CMPs can be recommended for application requiring a combination of rigidity and moderate ductility, especially in the car brake padding system and automobile engine block.
Figure 5. Stress–strain curves of Al-can/CMP composites.
Figure 6. Tensile stress with %wt of CMP additions.
Figure 7. Tensile strain (%) with %wt of CMP addition.

7 Impact energy

Fig. 8 presents a decrease in the charpy impact energies (IE) of the Al/CMP composites with an increment in %wt of CMP additions. The reduction in IE is gradual up to 6 %wt of CMPs. Above 6 %wt of CMP addition, there was a drastic reduction in the IE. The decrease in IE may be attributable to extreme hardness of the new phases which imparts brittleness to the Al matrix.
Figure 8. Impact energy with %wt of CMP addition.

8 Fractography

Plate 6 and Plate 7 show the SEM and optical micrographs of the fractured surfaces of the unfilled Al and Al/10%CMP composite samples respectively. Plate 6 and Plate 7reveal that there are differences in the morphology and grain sizes of the microstructures. Grains on Plate 7 are much finer than those on Plate 6Plate 6 and Plate 7 show that both fractured surfaces appeared goose and dimple, fibrous and dull which indicates that the fracture occurred in a ductile manner with a crack initiation. The porosity free-coarse fibrous grain microstructure of the unfilled Al (without CMP addition) inhibited crack growth, leading to absorption of higher impact energy than Al/10%CMP composites. Hence, the addition of CMPs to the Al matrix enhanced the strength and grain refinement but reduced the impact energy. Hence, this indicates a reduction in fracture toughness of the Al/CMP composites.
Plate 6. (a) SEM and (b) optical micrographs of the cast of Al can fractured surface.
Plate 7. (a) SEM and (b) optical micrographs of Al can/10%CMP composite fractured surface.

9 Conclusions

From results of investigation and discussion of this research work, the following inferences can be made:
  • [1]
    Newly light metal matrix composites suitable for use in transportation, construction, architectural design and packaging have been produced from Al (1xxx) sourced from Al cans and CS.
  • [2]
    Incorporation of CMPs in Al (1xxx) matrix led to a grain refinement in the microstructure of the Al/CMP composites.
  • [3]
    There was a proportionate increment in tensile stresses as %wt CMPs increased.
  • [4]
    There was an irregular increase in tensile strains of the Al/CMP composites as the %wt of CMP filling increased.
  • [5]
    Minimum tensile strain observed with Al/CMP composites at 4 %wt of CMPs additions indicates a critical reinforcement level that gave rise to Al/CMP composites characterised by high rigidity.

Acknowledgements

Authors wish to appreciate Mrs Bello Haneefah for her financial support in making this research worthwhile. Also appreciated are Rasheed Abiodun Jimoh and Abdul Wahab Abass Olayinka of Department of Materials Science and Engineering for their assistance in carrying out microstructural analysis.

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