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JPE (2016) Vol.19 (2) Original Scientific Paper

Ambedkar, B. R., Naidu, G. S., Lalitha, V.S.S., Krishna, D. M.

MICROSTRUCTURAL AND MECHANICAL PROPERTIES OF ALUMINIUM AND MAGNESIUM BIMETAL EXTRUDED COMPOSITE

Received: 23 June 2016 / Accepted: 10 September 2016 Abstract: The purpose of this paper is to investigate the microstructural and mechanical behaviour of aluminium-magnesium (Al-Mg) bimetal extruded composite. Bimetal Extrusion is a process of simultaneously extruding two or more materials to form a single entity. A new bimetal (Al/Mg)composite containing Mg reinforcement is made through hot extrusion process. The metallurgical bond between the materials gave rise to a fully integrated and void free interface. Mg ribbons are inserted in the holes drilled in Al rod. The sample thus made is extruded at 300°C. Characterization studies of the composite revealed that there is an appropriate bonding of the matrix and reinforcement resulting in a fine grain morphology. Hardness, wear and compression tests results are presented in this paper. Key words: Bimetal Extrusion, Aluminium, Magnesium, Interface-Bonding, Hardness Mikrostrukturne i mehaničke osobine ekstrudiranog aluminijum-magnezijum bimetalnog kompozita. Cilj ovog rada je da ispita mikrostrukturno i mehaničko ponašanje aluminijum-magnezijum (Al-Mg) ekstrudiranog bimetalnog kompozita. Bimetalna ekstruzia je proces istovremene ekstruzije dva ili više materijala u cilju formiranja jednog entiteta. Novi bimetalni kompozit (Al/Mg) sadrži Mg ojačanja i dobijen je putem tople ekstruzije. Metalurška veza između materijala je omogućila kompaktnu i besprekidnu vezu. Mg trake su umetnute u rupe izbušene u Al šipki. Uzorak napravljen ovakvim postupkom je ekstrudiran na 300°C. Studija karakteristika kompozita je otkrila da postoji odgovarajuća veza matrice i ojačanja što rezultira finom morfologijom zrna. Rezultati ispitivanja tvrdoće, habanja i testova sabijanja su prikazani u ovom radu. Ključne reči: Ekstruzija bimetala, aluminijum, magnezijum, spajanje, tvrdoća 1. INTRODUCTION Aluminium is the most popular metal matrix used in the fabrication of MMCs. AMCs has a wide range of applications in aerospace industry, automotive industries, in the high speed machinery, electronic packing and defence field. A composite can be said to have distinctive properties that are not obtained from any single component [2-5].Some of the major additions to aluminium include Silicon, magnesium, manganese and so on. Addition of small amounts of Magnesium enhances the properties of Aluminium.Magnesium is lighter than aluminium and is less ductile due to its hexagonal close packed crystal structure. Mg has a lower modulus of elasticity (40–45 GPa) than Al (69.6 GPa) [1]. It also has fatigue resistance and high temperature-creep resistance.A composite can be said to have distinctive properties that are not obtained from any single component [2-5].Addition of small amounts of Magnesium enhances the properties of Aluminium.

Literature studies shows that addition of Al in Mg can lead to enhanced toughness by mechanical interlocking mechanism[6]. It is also observed that rolling magnesium and aluminium to 50% reduction at 400°C followed by ARB (Accumulative Roll Bonding) resulted in the formation of Mg17Al12 and Al3Mg2 intermetallics[7]. Also, screw extrusion was used for producing a bimetal composite Al/Mg from granules containing aluminium alloy 6063 (AA6063) and

commercial pure magnesium. The rate of growths of intermetallic compounds in different ways was found out [8]. The bimetal interface has high hardness [9-11], non-uniform deformation being adapted by the softer metal at the interface based on chemical and mechanical interactions [12,13]. 2. METHODOLOGY Sample preparation for extrusion: An aluminium slab of 50mm diameter and 30mm height is taken and holes are drilled randomly into which magnesium ribbons are inserted. The sample is then heated in a muffle furnace above the recrystallization temperature (3500C) and is subjected to hot extrusion at 3000C in an extrusion.

Fig. 1. Specimen before extrusion

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The experiments were conducted at temperatures in the extrusion chamber high enough for generating satisfactory friction between the parts in the extruder set-up and the input materials, but at the same time low enough to avoid local melting.

2.1. Mechanical characterization:

To check the hardness profile of the extrusion product, Samples were put under Rockwell hardness testing apparatus. For Aluminium samples, 1/16th inch diameter steel ball is used as indenter. The depth of indentation calculated, gives the Rockwell Hardness Number on B-scale.

Also, the sample hardness is examined under Vicker’s hardness testing apparatus. The sample is mounted on the platform. Loading time and Load are preset as requires. Basing on the load applied, the area of indentation varies. Thus, the area of indentation gives the Vickers Hardness Number ((VHN). Wear and compression tests were also conducted for the extruded specimens. The wear test is conducted using a pin-on-disc apparatus with 20KN and 30KN loads. The compression test is conducted using hydraulic compression testing equipment.

2.2. Microstructural analysis: The extruded Al-Mg composite is machined to a cylinder of 20mm diameter and 30mm height. It is then polished with SiC abrasive papers of various grades. To give a scratch free surface, it is finally polished using a disc polishing machine and etched with Keller’s reagent (HNO3 5ml, HCl 3ml, HF 2ml and 190ml distilled water). The sample surface thus obtained is observed under an optical microscope. For further information regarding the grain morphology, the sample is subjected to SEM and XRD studies. 3. RESULTS&DISCUSSIONS 3.1. Microstructure:

In order to observe the surface features, the extruded composite is observed under an optical microscope.

Fig. 2. Al-Mg composite at a magnification of 100X

under optical microscope

Fig. 3. Al-Mg composite at a magnification of 100X

under optical microscope

Fig. 4. Optical microscope Microstructures at a

magnification of 450X 3.2 Discussion on optical microscope image results: The composite of aluminium containing 2.5% magnesium when viewed under optical microscope at a magnification of 10X is as shown above in fig... from the figure, it is evident that there is an even distribution of magnesium in the extruded composite giving rise to a uniform structure. At a higher magnification of 450X, the microstructures observed indicates the presence of very fine grains in the extruded composite. This implies that there should be an increase in the hardness of the composite due to the presence of fine grains after extrusion, which is observed in the hardness profile in the previous section. 3.3 Scanning Electron Microscope (SEM)images: The surface of sample containing 6.75% magnesium, as revealed by the plan-view SEM images, is smooth). In thiscase, very small particles are observed throughout the surface. 3.4 Discussion on SEM image results: The surface of sample containing 6.75% magnesium, as revealed by the plan-view SEM images, is smooth. In thiscase, very small particles are observed throughout the surface. At a higher resolution of 20µm, the surface is relatively smooth and there is an even distribution of magnesium particles in angular form in aluminium matrix.The sample surface when viewed at various magnifications is illustrated in the following figure. Lower magnifications indicate the presence of

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finely dispersed magnesium particles. At higher magnifications, the particles appear to be spherical and evenly distributed. 3.5 Chemical composition and XRD analysis: By mixing the constituents at elevated temperatures through extrusion,intermetallic phases can be formed through diffusion. Anoverview of the relevant phases is included in the figure. From the intensity vs. diffraction angle graph, it is evident that the aluminium is the matrix in which magnesium particles are reinforced. Also, the intermetallic phase formed by magnesium with aluminium matrix is rich in Al5.15Mg3.15phase. The Al2O3phase formed is due to the reaction of aluminium with oxygen in the environment.

Fig. 5. SEM images at a resolution of a)20µm and b)20

µm

Fig. 6. SEM images at a resolution of 10 µm and 20 µm

Fig. 7. SEM images at magnifications of (a) 270x, (b)

550x and (c) 1000x

Fig. 8. Intensity versus 2 graph

a

b

a

b

c

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3.6 Mechanical properties and Wear Analysis : Al-2.5% Mg composite is subjected to Rockwell hardness test to find out the overall hardness of composite before and after extrusion. The test is conducted at a load of 60Kg and the results indicate a 34% increase in the hardness of composite. Also, Vickers hardness test of the same indicated an 80.7% increase in the hardness of the composite when compared to parent metal (aluminium). Thus addition of magnesium to aluminium matrix in small amounts has brought up a huge rise in hardness profile of the parent metal. The compression test results displayed 23.8% increase in the compressive strength of the extruded specimen. The wear test results display that there is an increase in wear resistance of the composite with a corresponding increase in time span and frictional force. The reduction in weight (wear) is found to be 0.05g for 20KN load and 0.52g for 30KN respectively.

Fig. 9. Wear as a function of frictional force and time

for a load of 20KN

Fig. 10. Wear as a function of frictional force and

time for a load of 30KN

4. CONCLUSIONS From the results and discussions, the following conclusions are drawn. The production of bimetals through hot extrusion is possible.A well-mixed Al/Mg material was produced. Running extrusion at relatively high temperatures yielded a material with a quite homogeneous structure. It is observed that there is an overall increase in the hardness, compressive strength and wear resistance of the composite compared to base metal. From the SEM and XRD results, it can be concluded that there is a fine grain morphology in the extruded bimetallic composite and the respective intermetallic phases formed indicate uniform metallurgical bond between the aluminium matrix and magnesium reinforcement. It may be assumed that the corrosion resistance of the composite may decrease in a very percent due to the presence of magnesium in the matrix. Increasing the content of Mg to an Al gives promising results in regard to mechanical properties, and motivates further investigations with this novel method.

5. REFERENCES

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[11] A.G. Mamalis, A. Szalay, N.M. Vaxevanidis, D.I. Pantelis, Mater. Sci. Eng.A 188 (1994) 267–275.

[12] B.V. Krishna, P.Venugopal, K.P. Rao, Mater. Sci. Eng.A407 (2005) 77–83.

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Author: Assistant Professor on contract, B. Ratna kumar Ambedkar, Dr G. Swami Naidu, V. S. S. Lailitha, D. Murali Krishna. Department of Metallurgical engineering, JNTUK-UCEV, Vizianagaram, Phone:+918121332574 E-mail: ambi3639@gmail.com