Investigation of Friction Stir Processed Al7075 Aluminium Plate Reinforced with Al2O3 / 1 % Gr Hybrid Surface Nanocomposite

Dr. T. Prakash, Professor & Head,
Department of Mechanical Engineering,
SNS College of Technology, Coimbatore,
Tamil Nadu, India.
Abstract
Hybrid Metal matrix composites provide huge opportunities to optimize the engineering enforcement in metal matrix composites for applicable in the automotive and aerospace industries, where unique filler content and selective protected areas of reinforcements are needed. There are various efforts have been carried out to study the mechanical action of particle reinforced aluminium based composites. In this present study, Friction stir processing (FSP) is used to carry out reinforcement study on the commercially available Al7075 Al sheet metals reinforced with aluminium oxide of different sizes (75μm,150μm & 200μm) and percentage (2%, 4%, 6% & 8 % – Al₂O₃) and 1 % graphite (Gr). It has been described that these composites showed significant development in hardness, strength and related properties, as well as improved wear performance compared to fundamental alloys. It is well known that MMC’s containing small-size reinforcements which are ideal materials for application in high performance and wear resistant components due to their appealing physical and mechanical properties. To meet the above requirements, the samples were subjected to microstructure examination and hardness study. It was noted that there is significant improvement in the hardness value and the grain size was considerably reduced.
Keywords: Friction stir processing; microstructure; mechanical properties.

1. Introduction

Mishra et al. [1,2] developed the concept of Friction Stir Processing (FSP) derived from the Friction Stir Welding (FSW) process and it is used as tool for studying the microstructural modification (Fig.1). Aluminium based composites due to their wide alloy range, heat treatment capability and processing flexibility has been chosen as the matrix material for the present study [4-5]. In particle reinforced composites, the fracture mode has been observed to depend on reinforcement purity, reinforcement particle size and nature of interface, volume fraction of the reinforcement, fabrication route adopted and the presence of any intermetallic precipitates [1-3]. In this study, FSP is carried out in the DRO vertical milling machine (Fig 3). As shown in the figure the High Carbon High Chromium (HcHr) pinned tool is used to localize the microstructural modification in the workpiece [6] and property enhancement in cast aluminum alloys [7].

Fig. 1. Schematic Drawing of Friction Stir Processing
In the Friction Stir Process, the non-consumable rotating HcHr pinned tool is plunged into the workpiece to be processed and traversed along the line of the workpiece. During this process, the material undergoes severe plastic deformation and localized heating is produced by the friction between the rotating tool and the workpiece causing the material to the range where it can be plastically deformed and it also results in significant grain refinement [8, 11-16]. The FSP method is only the method to produce the surface metal matrix nano composite (SMMNC) and it is the only method to produce nanosized reinforcement on the metallic substrate with controlled distribution of reinforcement over the surface that cannot be achieved by conventional surface treatment [9-10]. There are many surface modification techniques like plasma spraying and electron beam treatment which can be used only for light metal alloys [17]. There is no work related to Al based MMCs sheet metals reinforced with various sizes and % Al₂O₃ / 1 % Gr as a surface modification technique through FSP, which can be applicable in aerospace and automotive Industries.
The main objective of the present study is to increase the hardness of Al7075 alloy, to get fine grained microstructure and to study the mechanical and tribological behavior of Al7075 alloy using friction stir processing (FSP). FSP is carried out using the Al7075 alloy with the high carbon high chromium (HcHcr) tool by adding reinforcements as Al₂O₃ (2%, 4%, 6% & 8%), particle size (75µm, 150µm & 200µm) with Gr (1%) mixed at various percentages and using process parameters such as traverse speed 800 rpm, rotational speed 60 mm/min and axial force as 7 KN.

1. Experimental Procedure
   • Process Description

Friction stir processing (FSP) is one of the new innovative thermo mechanical processing techniques that alter the micro structural and mechanical properties of the material. FSP is done on Al7075 alloy using nano and hybrid composites. The Al7075 plate with a nominal composition as shown in the Table 1 is clamped in the fixture table and the tool is made to run in between the grooves in the Al plate. The required feed rate, spindle speed, direction are kept in constant level for both processing of nano and hybrid composites. Due to effect of friction the surface of the Al plate is processed. The first cycle of operation is carried out using nano composites (Al₂O₃) at various level of percentage with variations in size. The hybrid composite graphite is introduced in the processed zone in combination with Al₂O₃ and once again the process is carried out in various percentage and sizes. The process parameters are to be followed as shown in the Table 2 for processing the hybrid composites. The physical properties such as mechanical (hardness and microstructure) are determined.
Table 1. Chemical Composition (wt %) of Aluminium Alloy Al7075

Table 2. Process Parameters

• Setup of Friction Stir Processing (FSP)

The rolled plates of 6 mm thickness, Al7075 aluminium alloy were cut into the required sizes 150×50 mm by grinding machine. A groove of 1.5×1.5mm is made by using EDM wire cutting machine as shown in the Fig 2. After the groove is made, without using any reinforcement the work piece is clamped on the fixture and the FSP (Fig 3) is carried out by using tool High Carbon High Chromium steel (HcHcr) with  threaded cylindrical probe of 4.5mm diameter pin, shank diameter of 18.5mm,shoulder diameter of 15mm as shown in the Fig 4.

Fig. 2. Sample of Al7075 Aluminium Alloy with Grooved One

Fig. 3. Macrograph showing the FSP Tracks on the Al7075 Plate with the FSP Tool

Fig. 4. Threaded Cylindrical Tool
The reinforcement of x wt. % Al₂O₃ – 1% Gr (x = 0, 2, 4, 6, 8 wt.%) with various particle size (75µm, 150µm & 200µm) is mixed and filled by using thickness gauge and work is clamped on the fixture and by using tool High Carbon High Chromium steel (HcHcr) with straight cylindrical probe of 4.5mm is made to move on the workpiece. As tool descended to the work piece, the rotating tool with pin contacts the surface that produces the friction between tool pin and the metal surface causing heat to dissipated from the metal surface which softens the metal column. The rotating tool provides a continuous heating of work piece which plasticizes the metal and transports the metal from the face of the pin to top edge. Additional heat is produced in the metal surface when the shoulder contacts it and plasticizes a huge amount of metal column around the cylindrical tool. The shoulder also provides an additional forging force that holds the metal to flow from the tool during. During FSP, work piece and the tool are moved relative to each other to process the required area. The processed zone cools forming a defect-free recrystallized and fine grain microstructure. After that the work piece is been removed from the fixture and material sample is cut and polished by graphite paste and different grades of  emery sheets to get mirror image and testing is done. This process is repeated for 2, 4, 6 & 8% of Al₂O₃ with various combinations of particle size 75µm, 150µm & 200µm and constant 1% of Gr as reinforcements. All FSP studies were carried out at room temperature.

• Characterization

After FSP, the surface of the sheet was cleaned and samples were cut for microstructural study from the different locations of the processed sheet. They were cold moulded and mechanically and electro polished to study under a polarized light optical microscope which would enable us to qualitatively comment on the grain refinement by friction stir processing.

• Mechanical Property Evaluation

The hardness tests were carried out to study the effect of FSP on the Al7075 Al alloy sheet metal. Vickers micro hardness tests were carried out using a micro hardness tester by employing a load of 50 g with dwell time of 20 s.

2. Results and Discussion

The effect of friction stir processing on the microstructure and strength in terms of hardness for Al7075 aluminium alloy reinforced with 2%, 4%, 6% & 8% of Al₂O₃ with various combinations of particle size 75µm, 150µm & 200µm and constant 1% of Gr were discussed here below:
3.1. Microstructural Evaluation
From the microstructure [Fig. 5] it is clearly found that the more refined grains occur in the matrix of the processed metal when compared to the parent metal. From the Fig. 5 (a), it can be clearly seen that the average grain sizes of Al7075 was around 100 µm for unprocessed samples. The grain sizes of matrix material in the processed zone were in the order of 55, 45, 35 and 40  µm for the reinforced Al₂O₃(150μm) sample of  2%, 4%, 6% & 8% [ see Fig. 5(b), 5(c), 5(d) & 5(e)]. The matrix grain size was decreased significantly with respect to addition of reinforcement up to 6 % and then increases due to the reason that the material reaches saturated condition. These results clearly explained that the grain refinement in the matrix was due to severe plastic deformation engendered by the FSP tool and 6 % Al₂O₃ (150μm) / 1 % Gr having minimum grain size compared to the other particle size and % due to the fact that increase or decrease in the particle size of Al₂O₃ causes the particle to be escaped from the groove causing no melting of reinforcement. So, normally the particle size is maintained in mid-way to use as a reinforcement in FSP process.

Fig. 5. Micro Structural Modifications of Al7075 with Reinforcement of Al₂O₃ (150µm) for (a) 0% (b) 2% (c) 4% (d) 6% & (e) 8% with 1% Gr
3.2. Micro Hardness Evaluation
The variation of hardness with different areas of Al7075 Al alloy processed by FSP is shown in Table 3 – 5. From the result it can be observed that the hardness of processed zone of samples were around 30 %, 35 %, 45 % & 40 % greater than unprocessed zone with respect to process parameter. This was occurred due to the grain refinement in the structure obtained by the FSP process. Further it can be noted in the sample that as the particle size of Al₂O₃ and % is increased the hardness of stirred zone increases rapidly first (up to sample 4 &150μm) and then increases slightly or decreases. This is due to the fact that the strengthening of the material reaches saturated condition up to sample 4 &150μm due to the grain refinement by FSP tool and wt. % Al₂O₃ / 1 % Gr. So in this work the samples were processed via FSP up to 8 % of Al₂O₃ with 1 % Gr and particle size up to 200µm. The hardness variation as function of process parameters for Al7075 Al alloy reinforced by 0%, 2%, 4%, 6 % & 8% Al₂O₃ with particle size 75µm, 150µm & 200µm / 1 % Gr hybrid surface composite for centre of stirred zone is shown in Fig.6. From the Figure 6 it was clear that the reinforcement 6% & particle size 150μmof Al₂O₃ and Gr 1% having very hardness value compared to the other samples and this sample can be employed in the production of many automobile and industrial components.
Table 3. Variation of Hardness with different Areas of Al7075 Processed by FSP with a Particle Size of Al₂O₃ (75 µm) with Gr 1%

 Table 4. Variation of Hardness with different areas of Al7075 processed by FSP with a particle size of Al₂O₃ (150 µm) with Gr 1%

 Table 5. Variation of Hardness with different Areas of Al7075 Processed by FSP with a Particle Size of Al₂O₃ (200 µm) with Gr 1%

Fig.6. Variation of Hardness for Al7075 Al Alloy Reinforced with x-wt. % of Al₂O₃&Particle Size 75µm, 150µm & 200µm / 1 % GrHybrid Surface Composite (centre of stirred zone)
3.3 Tribological Behaviour of Al7075 – x wt. %Al₂O₃ – 1 % Gr Hybrid Surface Composite with various Particle Size
The wear testing has been taken under certain condition such as the velocity5m/s, speed955rpm, and it has run for about 2mins of time. Fig.7 shows the variation of wear rate with various percentage and particle size of Al₂O₃ in Al7075-x wt. % Al₂O₃-1% Gr hybrid surface composite. It was observed here that as the percentage & particle size of Al₂O₃ increases the wear rate was decreased up to 6% &150µmof Al₂O₃ and then it started to increase. This was due to that homogeneous distribution of reinforcement over the surface of Al7075 alloy was expected to occur up to 6% / 150µmof Al₂O₃ and then non-homogeneous or clustering of reinforcement was expected to occur over the same alloy surface. These clustering of reinforcement would reduce the wear rate of Al7075 alloy beyond 6% / 150µmof Al₂O_3.
The wear rate of 6% / 150µmof Al₂O₃-1%Gr was 0.7 g/s and the maximum wear rate was obtained in the 0% reinforcement and it was 1.4 g/s respectively. These results indicated that the wear rate for 6% / 150µmof Al₂O₃ – 1%Gr sample was decreased by 1.5 times than unreinforced one.

Fig.7. Wear Rate Vs % &Particle Size of Reinforcements
3.4 SEM Analysis of Al7075 Alloy with various % &Particle Size of Al₂O₃and Gr 1%
The scanning electron micrographs (SEM) of the worn surface of Al7075 for 0%, 2%, 4%, 6% and 8% Al₂O₃ (150µm) sample is shown in Fig. 8. It can be observed that the wear track was covered with the compacted debris and delaminations. Surface projections like debris &delaminations or grooves were observed in the contact surface of the processed zone which is caused by high local pressure during the relative motion between the contact surfaces (pin and disc). From Fig. 8 (a) it is clear to view that deep grooves were formed due to no graphite content and Fig. 8 (b-d) shows that tribo film are formed due to proper mixing of reinforcement of (Al₂O₃ – Gr). Fig. 8 (e) shows that there is no formation of tribo film because of the clustering of the reinforcements and hence more delaminations was observed. Further, Fig. 8 (b-d) shows that there is no formation of deep grooves due to the presence of graphite content as a thin lubrication film.