INTRODUCTION:

Aluminum and its alloys are used extensively in numerous fields due to their low densities and high strength to weight ratio [1]. With conventional surface modification techniques it is difficult to improve the dispersion of reinforcement particles on the metal surface [2]. Earlier researches [3-4] reported that thermal spraying and laser beam techniques were utilized to prepare surface composites, in which it degrades the properties due to creation of unfavorable phases. So, Friction stir processing is solid state technique most suitable with aluminium and its alloys. It was developed by Mishra et al. as a generic tool for micro structural modification based on the basic principles of FSW and also for improving the surface modification [5]. Distribution of nano reinforcement particles on Al alloy surface and its control is complex to achieve in conventional surface modification methods [6]. As wear and corrosion are surface-dependent degradation, they can be improved by a suitable modification of surface microstructure and/or composition. A FSP technique can be employed to refine the microstructure and homogeneous dispersion of reinforcements on metallic surface [7]. Nevertheless, their poor resistance to wear and corrosion causes certain limitations for their application [8]. This exhibited better mechanical properties (hardness, tensile strength and percentage of elongation) than the base material [9]. These composites are a new type of material that exhibit good wear and corrosion resistance properties as compared to the matrix [10]. These Al Matrix Composites exhibit higher properties than that of parent alloy such as stiffness, improved tribological characteristics, weldability and high strength. Further these properties can be enhanced by using nano scale ceramic materials [11-12]. Sharma et al [13] have studied that pin on disc wear testing machine was used to carry out the tribological tests. Das. et.al [14] have demonstrated dry sliding wear behaviour, aluminum alloy reinforced with reinforced with SiCp-Graphite. The pin on disc wear test was conducted to examine the wear behavior of aluminum alloy which exhibits wear rate. A detailed analysis is carried out using SEM in order to find the influence of SiC particles on Al 6061.This investigation is helps to fabricate and study the influence of nano sized SiC (average size is 35 nm) reinforcement particles with their volume percentages accordingly on microstructural and mechanical behaviour of 6061-T6 Al alloy surface nano-composites by using FSP.

EXPERIMENTATION:

Materials:

The base material used in the present study is Al 6061 alloy plate with 6mm thickness. Aluminium 6061 alloy after artificial aging (temper T6) it exhibits full strength, good mechanical properties and weldability along with excellent corrosion resistance, 6061 alloy has a density of 2.70 g/cm³and its composition is shown in Table 1.

Al6061 is used as the base material and SiC particulates in powder form fabricated through powder metallurgy process method of an average particle size of 35 nm at a required volume percentage (such as 1.5%, 3% and 4.5% ) was chosen as the reinforcement materials. Silicon Carbide is one of the most commonly used non-metallic reinforcements, combined with Aluminum, Magnesium etc., to obtain composites. It provides unique combination of properties such as high strength-to-weight ratio, stiffness, hardness, wear resistance.

Table 1: The chemical composition of Al 6061 alloy.

CHEMICAL ELEMENT

Manganese

(Mn)

Iron (Fe)

Copper

(Cu)

Magnesium

(Mg)

Silicon

(Si)

Zinc

(Zn)

Chromium

(Cr)

Titanium

(Ti)

Aluminium

(Al)

AA 6061 wt %

0.15 Max

0.70 Max

0.15 –0.40

0.80 – 1.20

0.40–0.80

0.25 Max

0.04 – 0.35

0.15Max

BALANCE

Fabrication of Composites:

Friction stir processing is effective for improving the mechanical properties and eliminates casting defects and refines microstructures, thereby improving strength and ductility. Earlier researches [15] reported that thermal spraying and laser beam techniques were utilized to prepare surface composites, in which it degrades the properties due to creation of unfavorable phases so in order to avoid the problems FSP technique is used as best solid state technique for processing Al 6061 plates.

These plates held in position with the help of a clamp. The entire setup was kept in the vice of the converted milling machine and accordingly as shown in Figure:1. A non-consumable tool made from JIS-SKH 57 was used as the tool material, EN31 tool was used for the process. A concave shaped tool with a shoulder diameter of 20 mm was selected , pin dia. and pin length of 5 mm and 3.2 mm, respectively. [16]

The plates were shear cut to avoid any misalignment. The aluminium plate AA6061 - 100mm x 70mm x 6mm of required dimensions shown in Figure:2 are taken and grooves of different breadth and depth (1.5*1.5, 2*2, 2.5*2.5mm) are cut. The grooves are filled with nano sized SiC (The average size is 35 nm) reinforcement particles at a required volume percentage (such as 1.5%, 3% and 4.5% ) and the region is processed with three different tools according to their requirement. Here the emphasis is laid on various combinations of the parameters like the plunge depth, cooling rate and backing plates. Tool RPM – 1120 rpm; Tool traverse speed – 80 mm/min.

Microstructure Study:

Microstructural study was carried out by using optical microscope for getting different regions of friction stir processed specimen. The processed sample was cut as per dimensions and the cross section of processed zone was taken as test specimen. The samples were cold mounted, mirror polished with Lavigated polishing alumimium grade-2 and properly etched. The etchant used was modified Keller‘s reagent. Modified Keller’s HCl, HNO₃, Ethanol in equal proportions and 1 drop of HF acid. Grain size evolution is primarily affected by these two variables degree of deformation and peak temperature and also had a major advantage with FSP. Microstructure observations are carried out at the cross sections of Nugget zone [17] of the material Al 6061 plate normal to FSP direction.

Hardness Test:

Hardness is the resistance of a material to localized deformation. The hardness tests were carried out according to test procedure IS 1501:2002 ASTM standards using Vickers hardness testing machine. The principle in this test is that a defined shaped indenter is pressed into the material. The indenting force is applied for a certain decided amount of time. Microhardness tests were carried out at the cross section normal to the FSP direction. The test was conducted at room temperature (28 ^◦ C) and the measurement of hardness was taken at three different places on each sample to obtain an average value of hardness.

Tensile Test:

A Tensile test mostly named as tension test is probably the fundamental type of mechanical test that can perform on material to determine/verify material properties. It is an ability to predict the loads that will cause a part to fail depends upon both material properties and the machine part geometry. The tensile specimen of Al plate with dimensions of 120 mm in length, 20mm in breadth and 6 mm in thickness was prepared. The emery papers were used to polish the test specimens in order to decrease the machining scratches and the effects of surface defects on the sample.

Figure 3: Schematic sketch of the specimen

Wear Test:

Wear is a process of removal of material from one or both sides of solid surfaces in solid state contact, occurring when two solid surfaces are in sliding or rolling motion together. The wear tests are conducted by using pin-on Disc tri-bometer as per ASTM: G99-05 standards. The wear rate of specimen was found by weight loss method. Wear testing was carried out under dry sliding condition constant load of 40N, disc rotational speed of 650 rpm and sliding speed of 3.4 m/s to find the Mass loss, Frictional force, co-efficient of friction. A cylindrical pin of size 8 mm diameter and 30mm length was loaded through a vertical specimen holder against horizontal rotating disc. The rotating disc EN31 was made of carbon steel of diameter 120 mm and hardness of 62 HRC. Standard wear pin specimens of 8mm diameter and 6mm thickness for wear test was prepared through wire cut EDM process are shown in Figure-4. The weights were measured before and after each test segment to determine the abrasive wear loss of each sample. Wear rate is determined by wear rate (mm³/m) is equal to (volume loss/sliding distance).

Figure-4: Specimen prepared through wire cut EDM process.

Scanning electron microscopy (SEM):

The wear mechanisms of the composites were established by scanning electron microscopic (SEM) analysis of the surface morphology of the test samples. The Scanning Electron Microscope (SEM) today is extending into an ever wider field of applications. The S3700N has a huge sample chamber capable of accommodating a 300mm diameter sample with a maximum height of 110 mm which has a secondary electron detector, a variable pressure (VP) mode and a five segment backscattered electron (BSE) detector that has a large sample chamber and can place samples upto 300 mm in diameter and 110 mm tall. The scanning electron microscopy was used in order to evaluate the morphological changes and the elemental analysis of composites.

X-Ray Diffractometer (XRD):

X-ray powder diffraction is most widely used for the identification of unknown crystalline materials (e.g. minerals, inorganic compounds). The structural characterization of ultrafine-grained (UFG) samples of aluminum 6061 alloy produced by severe plastic deformation (SPD) was performed using X-ray diffraction analysis. The X-ray diffraction measurements were carried out with the help of a Maxima model 2036E201 Kα radiation (Kα= 1.54056) at an accelerating voltage of 40 kV and a current of 30 mA. In this test the sample was in stationary condition, only the arms of the X- ray tube was rotating in the opposite direction of 2θ during the test. The samples were scanned with a scan rate of 2^o/ min.

RESULTS AND DISCUSSIONS:

Microstructure Characterization:

The optical micrographs of Al 6061 with reinforcement particles of SiC were observed to be dispersed uniformly in the NZ for all the conditions of composites made by FSP as the rotating tool gives sufficient heat generation and a circumferential force to distribute the reinforcement particles to flow in a wider area. The stirred zones were about the size of rotating pin with width 8mm and depth 4mm. The OP micro-graphs of SiC particles on Al 6061after FSP are shown in Figure: 5.From specimen Figure it can be observed that, the distributions of reinforcements particles are fairly uniform.

Figure 5 : Optical microstructures of all the nano surface composites (a),(b),(c),(d),(e),(f),(g),(h),(i).

Microhardness:

The Vickers hardness test results of Al 6061 material and their composites containing SiC particles (1.5–4.5 wt. %) showing the relationship between the hardness values and % of reinforcement of SiC at cast condition and heat treated (T₆ ) condition in the Figure.. The micro hardness value is depends on the presence of SiC particles and their homogeneous dispersal. It is seen that expansion over volume rate of SiC, microhardness increments up to 73 Hv. It is also observed that the increasing the volume percentage of SiC particles immensely increases the microhardness due to the presence of the SiC particles. Vickers hardness testing machine with a 10 mm diamond indenter and 5 kg load for 25 s is used to calculate the microhardness. The resulting indentation diagonals are measured and recorded. The hardness number is calculated by dividing the force by the surface area of the indentation which is shown in Equation 1. Readings on 3 different locations are taken and average reading of each sample is calculated that are shown on Figure: 6.

…………Equation (1).

Figure 6: Micro hardness survey of Al/SiCsurface nano composites and as-received Al alloy

Tensile Test:

Tensile strength is a measurement of the force required to pull something to the point before it breaks. Tensile test was done using (UTM) Universal Testing Machine (FIE/UTN-40). The Specimen used is of ASTM E8 standard. (a) and (b) shows the specimens before and after tensile testing. As per the ASTM B 557:2006 standard, the tensile strength was evaluated on the specimen as-cast and heat treated condition (T₆ ). The tensile specimen of Al plate with dimensions of 120 mm in length, 20mm in breadth and 6 mm in thickness was prepared. The emery papers were used to polish the test specimens in order to decrease the machining scratches and the effects of surface defects on the sample. Ultimate tensile strength was observed in Figure: 7 at the stir zone of Al 6061weldments at constant tool rotation speed and traverse speeds such as 900, 1120, 1400rpm and 40 mm/min respectively.

Figure 7. Comparison of Tensile properties of Al/SiCsurface nano composites As-received Al alloy

Wear Test:

A wear and friction monitor shown in Figure. was used to investigate the dry sliding wear behavior of AMMCs. Standard wear pin specimens of 8 mm diameter and 30 mm height for wear test were prepared from the above composites and were retrieved through wire cut EDM process as shown in Figure and polished metallographically. The wear tests have been con-ducted under heat treated and cast condition at a fixed sliding velocity of 2.00 m/s. Each wear test has been carried out for a total sliding distance of about 2 km. The sliding wear tests were conducted on track diameter 100mm with load 40N, sliding speed 0.314 m/s speed 650rpm and sliding distance 113.097 m. The dry sliding wear was observed by measuring weight loss. Weight loss of pins was converted into volume loss using density of specimens. The results are shown in Table: 2.

Table 2: Wear test on Pin-on-disk Report.

SAMPLE	LOAD (N)	WEAR TRACK (mm)	SPEED (RPM)	TIME (min)	TEMP (C)	WEAR (microns)	F.F (N)	COF	INITIAL WEIGHT OF TEST SAMPLE (gm)	FINAL WEIGHT OF TEST SAMPLE (gm)	WEIGHT LOSS (gm)
1	40	100	650	5	ambient	36	15	0.375	0.81575	0.080945	0.00633
4	40	100	650	5	ambient	1168	24.36	0.609	0.76296	0.68601	0.07695
5	40	100	650	5	ambient	1040	27.52	0.688	0.78672	0.71414	0.07258
6	40	100	650	5	ambient	1084	27.72	0.693	0.78446	0.71065	0.07381
7	40	100	650	5	ambient	70	14.24	0.356	0.77795	0.77447	0.00348

Scanning Electron Microscope (SEM):

The SEM image of the sample reveals that the SiC particles dispersion on the surface of Al 6061 plates at Nugget Zone. SEM image the agglomeration of particles was observed at higher because of aging. Parameters for the computer-assisted SEM analysis employed in this project were chosen as those found to be reasonable for characterization of environmentally significant particle. The SEM fractography of Al-SiC-1.5 vol. %, Al-SiC-3 vol. % and Al-SiC-4.5 vol. % surface nano sized composites and base metal Al alloy shown in Figure: 8. By observing the fracture surface of base metal Al alloy consist of voids and dimple which reveals the ductile fracture . It is also observed that the extracting out of the reinforcement particles and minute dimples are seen in the rupture surface of Al-Sic-4.5 vol % surface nano-composites.

Figure 8: SEM micrographs of Sic reinforcement particles after FSP.

X-Ray Diffractometer:

The X Ray Diffraction of nano sized SiC reinforcement particles shown in Figure. The average size of the nanoparticles has been calculated for the highest peak using the Debye-Scherrer formula, where, λ is the x-ray wavelength (1.504 nm), θ is the Bragg diffraction angle, and β is the full width at half maximum such that the average size of nanoparticles was to be found. The general view of X-ray patterns was taken with a scanning step 0.05° and exposure time in every point equal to 5 seconds in the range of 2ɵ angles from 10^o to 80^o. Precise measurements of the chosen X-ray peaks were performed with a step 0.02^o and collection time 4 seconds/point. An X-ray diffraction pattern of al alloy matrix composites and al alloys were shown in the Figure 9.

SAMPLE NO: 01 - LOW WEAR

SAMPLE NO: 04 HIGH WEAR

Figure 9. XRD Test graphs of Al 6061-T6/SiC_p Samples

The X-ray diffraction of the MMC which shows that the presence of the Al and many other components. The peaks in the pattern can be indexed to a mixture of different compounds and other remaining minor peaks attributed to impurity.

Wear Surface Analysis:

Figures show SEM micrographs at 100 magnifications of the worn surfaces of Al 6061 matrix material SiC composites in as cast and heat treated conditions. Wear rate is a function of the amount of SiC particles in composites and are effective to enhance the wear resistance of the composites in both the conditions.

The interactions between dislocations and SiC particles resist the propagation of cracks during sliding wear. Strain fields are created around SiC particles due to the thermal mismatch between the aluminum alloy and SiC particle during solidification. Those strain fields offer resistance to the propagation of the cracks and subsequent material removal. The grain refining action of SiC particles can further be considered to play a role in lowering the wear rate. By increasing the weight percentage of SiC from 1.5 wt% to 3 wt% and 4.5 wt.%, less deep grooves can be observed and the surface is much smoother compared to the one it was observed. The surface of Al 6061 reinforced by SiC of 4.5 wt. % demonstrates rougher worn surface than Al 6061 3 wt% SiC. This can be the main reason for increasing of wear loss and COF at high weight percentage of SiC.

Figure 10: SEM micrographs of SiC particles after wear test.

CONCLUSION:

The nano surface composite surface layer by reinforcing SiC particles on 6061-T6 Aluminum Alloy via FSP successfully fabricated. Effect of nano sized reinforcement particles such as SiC (average size is 35 nm) on microstructure and mechanical properties of 6061-T6 Aluminum alloy based surface nano composites fabricated via FSP was studied and the following conclusions are to be obtained.

It is observed that increase in volume percentage of SiC, microhardness decreases.

It had been that all the tensile properties of Al surface nano composites were decreased as compared with the as-received Al alloy.

It is seen that at 4.5 volume percentage higher tensile properties exhibited higher than the tensile properties as compared with the 1.5 vol. % and 3 vol. %.

The optical micrograph and SEM images revealed that SiC and alumina particulates are fairly distributed in Aluminium alloy matrix.

XRD shows the dispersion of SiC in aluminium matrix improves the wear properties of the composites.

It is found that wear rate tends to decrease with increasing weight percentage, which confirms that the silicon carbide is beneficial for reducing wear rate.

ACKNOWLEDGEMENT:

The author(s) would like to express thankful to the University college of Technology Laboratory, Osmania University, Hyderabad. DUCOM Instruments PVT LTD, Bangalore and also for management of S R Engineering college, Warangal for their constant support during this work. providing the facilities to carry out this work.

REFERENCES:

1. Bakes, H.; Benjamin, D. Metals Handbook; ASM international: Metal Park, OH, USA, 1979.

2. Budinski, K.G. Surface Engineering for Wear Resistance; Prentice Hall: Upper Saddle River, NJ,USA, 1988.

3. Gupta M, Mohamed F A, Lavernia E J. Solidification behaviour of Al-Li-SiCp MMCs processed using variable co-deposition of multi-phase materials. Materials and Manufacturing Processes, 5: 2: 165-196, 1990.

4. Mabhali L A B, Pityana S L, Sacks N. Laser Surface Alloying of Aluminum (AA1200) with Ni and SiC Powders. Materials and Manufacturing Processes, 25:12: 1397-1403, 2010.

5. R.S. Mishra, M.W. Mahoney, S.X. McFadden, N.A. Mara, A.K. Mukherjee, High Strain Rate Superplasticity In A Friction Star Processed 7075 Al Alloy, 42 (2000).

6. Rabinowicz E. Friction and wear of materials. New York: JohnWiley and Sons 1965.

7. Shafiei-Zarghani, A.; Kashani-Bozorg, S.F.; Zarei-Hanzaki, A. Microstructures and mechanicalproperties of Al/Al2O3 surface nano-composite layer produced by friction stir processing.Mater. Sci. Eng. A 2009, 500, 84–91.

8. Ravi, N.; Sastikumar, D.; Subramanian, N.; Nath, A.K.; Masilamani, V. Microhardness and,Microstructure Studies on Laser Surface Alloyed Aluminum Alloy with Ni-Cr. Mater. Manuf. Process. 2000, 15, 395–404.

9. Mechanical and tribological properties of AA7075–TiC metal matrix composites under heat treated (T₆) and cast conditions- Veeravalli Ramakoteswara Rao, Nallu Ramanaiah ^b,Mohammed Moulana Mohiuddin Sarcar ^c-2 0 1 6 ; 5 ( 4 ) : 377–383.

10. Miracle, D.B. Metal matrix composites–From science to technological significance. Compos. Sci.Technol. 2005, 65, 2526–2540.

11. Ravi N, Sastikumar D, Subramanian N, Nath A K, Masilamani V. Microhardness and microstructure studies on laser surface alloyed aluminium alloy with Ni-Cr. Materials and Manufacturing Processes, 15: 395–404, 2000.

12. Clyne T W, Withers P J. An Introduction to Metal Matrix Composites. Cambridge University Press: Cambridge, 1993.Rabinowicz E. Friction and Wear of Materials. JohnWiley and Sons: New York, 1965.

13. S.C. Sharma “The Sliding Wear Behaviorof Al6061–Garnet Particulate Composites”: Wear 249 (2001) 1036–1045.

14. S.Das, “Development Of Aluminium Of Alloy Compositefor Engineering Application”; Trans. Indian Inst. Met. Vol.57, No. 4, August 2004, Pp. 325-334.

15. Gupta M, Mohamed FA, Lavernia EJ. Solidification behaviour of Al-Li-SiCp MMCs processed using variable co-deposition of multi-phase materials. Mater Manuf Process 1990;5(2):165e96.

16. K. Elangovan, V. BalasubramanianInfluences of pin profile and rotational speed of the tool on the formationof friction stir processing zone in AA2219 aluminium alloy, Materials and Design 29 (2008) 362 – 373.

17. Aruri Devaraju and V Kishan. Preparation of nano surface layer composite (TiB2)p on 6061-T6 Aluminum Alloy via Friction Stir Processing. Materials Today: Proceedings 4 (2017) 4065–4069.

Received on 02.10.2017 Accepted on 09.12.2017

Research J. Engineering and Tech. 2018;9(1): 27-36

DOI: 10.5958/2321-581X.2018.00005.3