A Review of friction stir processing and behavioral study of surface modified Al-Si-Mg alloy done by this technique

A Review of friction stir processing and behavioral study of surface modified Al-Si-Mg alloy done by this technique Amit Kumar 1*, Satyawan 2 1 Department of Mechanical engineering, UIET MDU, Rohtak- 124001(Haryana) 2 Foreman Instructor at GPES Manesar, Gurgaon- 122051 (Haryana) Abstract Silicon is a good metallic alloy for Aluminum alloys as it increases the fluidity, reduces the melting temperature, decreases the shrinkage during solidification, and is very inexpensive as a raw material. Recently use of cast Al Si alloys has been expanding widely in military, automobile and general engineering industry. The wear behavior of these alloys depends on material related parameters and service conditions. Surface treatments are generally carried out to improve their service life. Out of various means available for surface modification, friction stir processing (FSP) is being widely used these days. This paper comprises of an elaborative study of friction stir processing for surface modification of Al-Si-Mg alloy system. 1. INTRODUCTION Aluminum alloys are distinguished according to their major alloying elements. Silicon is good in metallic alloys. This is because it increases the fluidity of the melt, reduces the melting temperature, decreases the shrinkage during solidification and is very inexpensive as a raw material. Use of cast Al Si alloys as a tribological component in recent years has been expanding widely in military, automobile and general engineering industry. The wear behavior of these alloys depends on a number of material related parameters i.e., shape, size, composition and distribution of micro constituents in addition to the service conditions such as load, sliding speed, temperature, environment and counter surface. The presence of silicon as alloying elements in these alloys improves wear resistance significantly [1]. Silicon also has a low density which may be an advantage in reducing the total weight of the cast component. Silicon has a very low solubility in aluminium; it therefore precipitates as virtually pure silicon, which is hard and hence improves the abrasion resistance. Aluminium-silicon alloys form a eutectic at 12.6 % silicon, the eutectic temperature being 577 C. This denotes a typical composition for a casting alloy because it has the lowest possible melting temperature [2]. Aluminium-Silicon system is a simple binary eutectic with limited solubility of aluminium in silicon and limited solubility of silicon in aluminium. There is only one invariant reaction in this diagram. L α + β (eutectic) In above equation, L is the liquid phase, α is predominantly aluminium, and β is predominantly silicon. It is now widely accepted that the eutectic reaction takes place at 577 C and at a silicon level of 12.6%. Aluminium-Silicon casting alloys are the most useful of all common foundry cast alloys in the fabrication of pistons for automotive engines. Depending on the Si concentration in weight percentage, the Al-Si alloy systems are divided into three major categories [2-4]: 1) Hypoeutectic (<12 % Si) 2) Eutectic (12-13 % Si) 3) Hypereutectic (14-25 % Si) 33

Figure 1.1 Al-Si equilibrium diagram [2]. Surface treatments are generally carried out to tailor the surface properties without affecting the bulk properties to improve strength, toughness, corrosion and wear resistance. Surface modification techniques can be classified as: 1) Surface modification by adding new material onto the surface a) Welding i) Gas welding ii) Arc welding iii) Plasma welding b) Thermal spraying i) Flame spraying ii) Arc spraying iii) Plasma spraying iv) Detonation spraying c) Cladding i) Brazing ii) Explosive bonding iii) Diffusion bonding d) Vapor deposition i) Chemical ii) Physical 2) Surface modification by changing surface chemically a) Thermo chemical diffusion process b) Electrochemical process c) Chemical conversion coatings(carburizing, Nitriding, Sulphurising) 3) Surface modification without changing the material chemically a) Mechanical treatment i) Peening ii) Deep rolling iii) Shot blasting iv) Friction stir processing 34

b) Thermal treatment i) Ion implantation ii) Laser beam treatment iii) Electron beam treatment 2. FSP DESCRIPTION FSP is accomplished by modifying technology developed for friction stir welding (FSW). Friction stir welding, a solid state joining process invented at TWI in 1991, is a viable technique for joining Al alloys that are difficult to fusion weld. FSP uses the same methodology as friction stir welding (FSW), but FSP is used to modify the local microstructure and does not join metals together. FSP is an emerging surface-engineering technology that can locally eliminate casting defects, refine microstructures, improving strength and ductility, and improve other properties like fatigue and resistance to corrosion and enhance formability. Friction-stir processing can also produce fine-grained microstructures through the thickness to impart super plasticity. FSP provides the ability to thermo mechanically process selective locations on the structure s surface and to some considerable depth (>25mm) to enhance specific properties [3-4]. The technology involves plunging a rapidly rotating, non-consumable tool, comprising a profiled pin and larger diameter shoulder, into the surface and then traversing the tool across the surface. Large surface areas can be traversed rapidly by using the appropriate tool. Frictional heating and extreme deformation occurs causing plasticized material (constrained by the shoulder) to flow around the tool and consolidate in the tool s wake. FSP zones can be produced to depths of 0.5 to 50mm, with a gradual transition from a fine-grained, thermodynamically worked microstructure to the underlying original microstructure. In friction process a location within a plate or sheet, a specially designed cylindrical tool is rotated and plunged into the selected area. The tool has a small diameter pin with a concentric larger diameter shoulder. When descended to the part, the rotating pin contacts the surface and rapidly friction heats and softens a small column of metal. The tool shoulder and length of entry probe control the penetration depth [5]. When the shoulder contacts the metal surface, its rotation creates additional frictional heat and plasticizes a larger cylindrical metal column around the inserted pin. The shoulder provides a forging force that contains the upward metal flow caused by the tool pin. During FSP, the area to be processed and the tool are moved relative to each other such that the tool traverses, with overlapping passes, until the entire selected area is processed to a fine grain size. The rotating tool provides a continual hot working action, plasticizing metal within a narrow zone, while transporting metal from the leading face of the pin to its trailing edge. The processed zone cools, without solidification, as there is no liquid, forming a defect-free recrystallized, fine grain microstructure. Essentially, FSP is a local thermo mechanical metal working process that changes the local properties without influencing properties in the remainder of the structure [1, 4]. 2.1 Tool geometry and materials Tool geometry is the most influential aspect of the process development. It plays a critical role in material flow and in turn governs the traverse rate at which friction stir welding can be conducted. The tool consists of a shoulder and pin. In the initial stage of tool plunge, the heating results primarily from the friction between pin and work piece. The tool is plunged till the shoulder touches the work piece. The friction between shoulder and work piece results in the biggest component of heating. From the heating aspect, the relative size of the pin and shoulder is important. The shoulder also provides confinement for the heated volume of material. The second function of tool is to stir and move the material [4]. Tools consist of a shoulder and a probe which can be integral with the shoulder or as a separate insert possibly of a different material. The design of the shoulder and of the probe is very important for the quality of the weld. The probe of the tool generates the heat and stirs the material being welded but the shoulder also plays an important part by providing additional frictional treatment as well as preventing the plasticized material from escaping from the weld region. The plasticized material is extruded from the leading to the trailing side of the tool but is trapped by the shoulder which moves along the weld to produce a smooth surface finish. Clearly, different materials and different thicknesses will require different profile probes and welds can be produced from just one side or by welding half the thickness then turning over to complete the other side. There are three types of FSW/P tools, i.e. fixed, adjustable and self-reacting, as illustrated in Figure 1.3. 35

2.2 Process parameter Figure1.3 (a) fixed, (b) adjustable and (c) bobbin type tools [4] For FSW, two parameters are very important: tool rotation rate (w, rpm) in clockwise or counter clockwise direction and tool traverse speed (v, mm/min) along the line of joint. The rotation of tool results in stirring and mixing of material around the rotating pin and the translation of tool moves the stirred material from the front to the back of the pin and finishes welding process. Higher tool rotation rates generate higher temperature because of higher friction heating and result in more intense stirring and mixing of material. The process parameters of friction stir welding are tool rotation speed (RPM), welding speed (mm/min), downward force (kn), tool pin dimensions and shape. Tool tilt angle is also an important welding parameter. A suitable tilt of the spindle towards trailing direction ensures that the shoulder of the tool holds the stirred material by threaded pin and move material efficiently from the front to the back of the pin. 3.1 Structure Refinement 3. LITERATURE SURVEY During FSP, the rotating pin with a threaded design produces an intense breaking and mixing effect in the processed zone, thereby creating a fine, uniform, and densified structure. Therefore, FSP can be developed as a generic tool for modifying the microstructure of heterogeneous metallic materials such as cast alloys, metal matrix composites, and other aluminum alloys. Alloys of Al-Si-Mg are widely used to cast high strength components in the aerospace and automobile industries, because they have good cast ability and can be strengthened by artificial aging. The as-cast structure of Al-Si-Mg alloys is characterized by porosity, coarse acicular Si particles, and coarse primary aluminum dendrites. These micro structural features limit the mechanical properties of cast alloys, in particular, toughness and fatigue resistance. Eutectic modifiers and high-temperature heat treatment are widely used to refine the microstructure of cast Al-Si alloys, to enhance the mechanical properties of the castings. However, none of these approaches can heal the casting porosity effectively and redistribute the Si particles uniformly into the aluminum matrix. In an investigation carried out by Sima Ahmad et al. [2] on the microstructure of as cast A356 consists of primary α- Al dendrites and inter dendritic irregular Al Si eutectic regions and FSP results in a significant breakdown of coarse and acicular Si particles and Al dendrites, and creates a uniform distribution of fine and near spherical Si particles in the Al matrix. This is attributed to thermal exposure and intense plastic deformation and high-temperature exposure during FSP results in generation of fine and equiaxed recrystallized grains due to dynamic recrystallization. Z.Y. Ma et al. [5] conducted FSP on sand-cast A356 plates under wide FSP parameters. Their results indicated that FSP resulted in the significant breakup of coarse acicular Si particles and coarse primary aluminum dendrites, the closure of casting porosities, and the uniform distribution of broken Si particles in the aluminum matrix (Figure 2.1). Increasing the rotation rate and number of FSP passes resulted in a decrease in the size and aspect ratio of the Si particles and the porosity level, due to the intensified stirring effect. 36

Figure 2. Optical micrographs showing morphology and distribution of Si particles in A356 samples: (a) as-cast and (b) FSP at 900 rpm and 203 mm/min [5] Similarly, Santella et al. [6] showed that FSP destroyed the coarse and heterogeneous cast structure of A319 and A356 sand castings and created a uniform distribution of broken second-phase particles. The TEM observations revealed the generation of a fine-grained structure of 5 to 8 lm in FSP A356. Furthermore, TEM examinations revealed that the coarse Mg2Si precipitates in the as-cast A356 sample disappeared after FSP, indicating the dissolution of most of the Mg2Si precipitates during FSP. After FSP, most of the solutes were retained in solution, due to rapid cooling from the FSP temperature, thereby forming a supersaturated aluminum solid solution. T.S. Mahmoud et al. [7] showed the effect of the tool rotational and traverse speeds as well as the number of passes on the microstructure of the modified surfaces was investigated. The as-cast A390 alloy exhibited mean size and aspect ratio of Si particulates of about 59 ± 24 μm and 3.56 ± 1.9, respectively. FSP significantly reduced both the mean size and aspect ratio of the Si particulates. The mean size of the Si particles increases with increasing the tool rotational and reducing the tool traverse speeds, but reduced by increasing the number of passes. Samples of friction stir (FS) processed at 1200 rpm, 20 mm/min and three passes exhibited the minimum mean size (4.39 ± 1.9 μm) and aspect ratio (1.18 ± 0.4) of the Si particulates. FSP eliminates the cavities, refines the grain structure and refines the coarse acicular Si particles in the eutectic structure as well as the primary Si particles. In an another study A.G. Rao, et al. [8] demonstrated the effect of two pass overlap friction stir processing on micro structural refinement of Al 30Si alloy, which delineates significant reduction in size and aspect ratio of silicon particles from average 200 to 2 μm and 4.93 to 1.75 μm respectively. Double pass FSP with 100% overlapping on the top of the first pass itself has a pronounced effect on size, shape and distribution of Si particles. The stirring action of the tool during FSP resulted in breakup of coarse primary silicon particles, thereby causing extensive refinement of both the flake type coarse primary silicon as well as the needle type eutectic silicon. It is difficult to distinguish eutectic silicon and primary silicon in the stir zone after one pass and two passes of FSP. In a continuation with the previous investigation the author found that the micro structural evolution and related dynamic recrystallization phenomena were investigated in overlapping multi pass FSP of hypereutectic Al-30 Si alloy [8]. The initially coarse primary silicon particles in the cast plate with a mean primary silicon particle size of 188 µm were refined to 1.3 µm after six passes of FSP. The particle size after three passes (1.4 µm) was found to remain nearly the same even after six pass FSP (1.3 µm). However, the aspect ratio does change from the initial value of 4.93 as it does with a deceasing trend after each subsequent FSP pass up to 1.42 in six passes. The optical micrographs in Figure 2.2 clearly show the effect of FSP on the refinement of coarse primary silicon, its redistribution, and elimination of porosities present in cast plate. 3.2 Mechanical Properties FSP results in significant micro structural evolution within and around the stirred zone, i.e., nugget zone, TMAZ, and HAZ. This leads to substantial change in post weld mechanical properties. In the following sections, typical mechanical properties, such as strength, ductility, fatigue, and fracture toughness are briefly reviewed. 37

The improvement in the mechanical properties of FSP Al-Si alloys is attributed to the following factors. First, the breakup of coarse acicular Si particles reduced the Si particle cracking under low stress and, consequently, minimized the possibility of the void initiation associated with damaged Si particles, thereby improving the ductility and increasing the strength. Furthermore, the ultra-fine Si particles produced by FSP exerted additional strengthening effects on the aluminum matrix through dislocation/particle interaction [1]. Second, the fundamental elimination of porosity reduced the possibility of void initiation at such porosities, thereby improving the strength and ductility of Al-Si alloys. Third, the room-temperature natural aging of the supersaturated aluminum solid solution produced by FSP resulted in an increase in the strength of the FSP samples. Table 2.1 Summary of ultrafine-grained microstructures produced via FSW/FSP in aluminum alloys [5] Material Plate thickness (mm) Tool geometry Speed (rpm) Traverse speed (mm/min) Grain size ( m) 2024Al 6.5 Cylindrical 650 60 0.5-0.8 1050Al 5 Conical 560 155 0.5 7075Al 2 N/R 1000 120 0.1 Cast Al-Zn-Mg 6.7 Cylindrical 400 25.4 0.68 Figure 3. Optical microstructure showing (a) interface between stirred zone and base metal, (b) large primary silicon particles, (c) (f) stir zone of single pass, two pass, three pass, and six pass FSP, respectively [9] 38

In an investigation carried out by S. Jana, R.S. Mishra et al. [9] showed the enhancement in the fatigue life of hypoeutectic Al-Si alloy. In the present study FSP led to a five times improvement in fatigue life of an investment cast Al 7Si 0.6 Mg hypoeutectic alloy. The reason for such an enhancement was linked to the closure of casting porosities, which acted as crack nucleation sites in the as cast condition. Porosities acted as notches in the as cast alloy and led to an order of magnitude higher crack growth rate. As FSP eliminated the porosities and refined the Si particles the crack growth rate dropped, due to elimination of the notch effect. Effect of maximum bending stress (S) on number of cycles to failure (N) is as shown in the figure 2.3. Figure2.3Effect of maximum bending stress (S) on number of cycles to failure (N) [10] In another study M. Jayaraman et al. [11] demonstrated the effect of FSW process parameters on the tensile strength of cast A356 aluminium alloy. The joint fabricated using the process parameters of tool rotation speed 1000 r/min, welding speed 75 mm/min and axial force 5 kn yielded a higher tensile strength compared to other joints. Defectfree weld region, higher hardness of weld region and very fine, uniformly distributed eutectic Si particles in the weld region are found to be the important factors attributed for the higher tensile strength of the above joints. M L Santella et al. [6] showed that Micro hardness distributions were more uniform. The ultimate tensile strengths, ductility, and fatigue lives of both alloys (A319 and A356) were increased by the friction stir processing. In contrast to the as-cast conditions, microstructures in the stir zones were characterized by relatively uniform distributions of second-phase particles that were also relatively uniform in shape. Visible porosity and dendritic microstructures were eliminated. Table 2.2 Comparison of tensile properties for cast and FSP A356 and A319 [5] Condition Cast A319 FSP A319 Cast A356 FSP A319 Yield strength (MPa) - 151.5 157.2 163.6 167.6 157.5 99.1 100.2 101.8 86.7 88.2 86.5 Tensile strength (MPa) 154.8 151.5 175.6 300.2 300.4 288.3 139.5 137.4 123.2 173.6 172.8 172.6 Uniform elongation (%) 0.5 0.5 0.9 8.5 8.6 7.0 2.5 2.4 1.2 12.8 13.9 12.4 In another study L. Karthikeyan et al. [12] demonstrated the effect of FSW process parameters on the mechanical properties of cast A319 aluminium alloy. Processing was done at three different traverse feed rates, viz. 22.2 mm/min, 40.2 mm/min and 75 mm/min, and five tool rotational speeds of 800, 1000, 1200, 1400 and 1600 rpm. Single stir passes were used. At all feed rates employed in the present experiments, the best mechanical properties were obtained at a tool rotational speed of 1200 rpm. Ductility increases ranging from 1.5 to 5 times the value for 39

the as-cast A319 alloy could be achieved in the processed specimens. In most of the processed specimens the hardness was greater, with the maximum increase being about 20%. FSP increased the yield strength of the starting material by about 13% (maximum), while the enhancement in the tensile strength varied between 20% and 50%. In further study M. Jayaraman et al. [13] demonstrated the effect of FSW process parameters on the tensile strength of cast A319 aluminium alloy. Processing was done at four different traverse feed rates, viz. 22 mm/min, 40 mm/min 75 mm/min and 100 mm/min, and four tool rotational speeds of 900, 1000, 1200 and 1400 rpm and four different axial force 2,3, 4, and 5 kn. Single stir passes were used. The joint fabricated with a 1200rpm tool rotation speed, 40 mm/min welding speed, and 4 kn axial force showed superior tensile strength. T.S. Mahmoud et al. [14] showed the effect of the tool rotational and traverse speeds as well as the number of passes on mechanical characteristics of the modified surfaces was investigated. The FS-processed samples exhibited less scattered and higher hardness values than the as cast alloy. The as-cast A390 alloy showed hardness values between 62.5 and 94.6 VHN. The minimum hardness values were between 95.3 and 99.5 VHN for samples FS-processed at 1800 rpm and 12 mm/min. The hardness of the FS-processed region was found to be increased by increasing the tool traverse speed and the number of FSP passes, but reduced by increasing the tool rotational speed. Samples FSprocessed at 1200 rpm, 20 mm/min and three passes exhibited the maximum hardness values between 114.66 and 119.34 VHN. 3.3 Wear Behavior The progressive loss of material from the surface to solid body due to mechanical action that is the contact relative motion against a solid liquid or gaseous counter body. Wear is caused by disintegration of integrating components as the result of overstressing of the material in the immediate vicinity of the surface. Wear may result in dimensional changes of components or surface damage causing secondary problems such as vibration or misalignments. Generation of wear debris may cause even serious problem. Sima Ahmad et al. [2] showed that Wear resistance of the FSP samples of as-cast A356 with varying tool rotation speed. The magnitude of improvement in wear resistance of the FSP samples over the as-cast A356 increases with increasing the rotation rates. In other words, the difference in wear rate is more pronounced at higher tool rotation rates. T.S. Mahmoud et al [7] showed the effect of the tool rotational and traverse speeds as well as the number of passes on tribological characteristics of the modified surfaces was investigated. The FS-processed samples exhibited lower wear rates than the as-cast A390 hypereutectic Al Si alloy. The wear rates were found to be reduced by reducing the tool rotational speed and increasing the tool traverse speed. Increasing the number of passes reduces the wear rate as well as the coefficient of friction. While the minimum average coefficient of friction was about 0.33 for samples Friction Stir-processed for three pass at 1200 rpm and 20 mm/min. increasing the tool traverse speed and reducing the tool rotational speed improved the wear resistance of the FS-processed samples. Increasing the number of FSP passes significantly improved the wear resistance of the FSP samples. In another investigation T S Mahmoud et al [14] showed the effect of the friction stir processing (FSP) on the microstructural, mechanical and tribological characteristics of as-cast A413 aluminum alloy. The amount of wear (measured by weight loss) for the Friction Stir Processed samples increases with increasing sliding distance, but the weight loss of the as-cast A413 Al alloy is significantly higher than weight loss of the Friction Stir Processed samples. The variation of the wear rate with the tool rotational speed at different traverse speeds is shown in Figure 2.5. Figure2.5. Variation of the wear rate with tool rotational speed at several traverse speeds [14]. 40

3.4 Corrosion Behaviour The corrosion of aluminum alloy friction stir welds is commonly investigated using methods such as immersion tests, polarization techniques, an agar gel exposure, the droplet cell method or cyclic spray tests. For high-strength alloys, the heat-affected zones of the weld exhibit the highest susceptibility to inter granular corrosion, which correlates with copper depletion along the grain boundaries. Surekha et al. [15] investigated the effect of processing parameters (rotation speed and traverse speed) on the corrosion behaviour of friction stir processed high strength precipitation harden able AA 2219-T87 alloy Only the rotation speed has influence on the corrosion behaviour, while the traverse speed does not show any influence. The corrosion resistance increased with the increase in rotation speed. The effect of parameters on corrosion behaviour in friction stir processed alloys is similar to friction stir welded samples. In a continuation with the previous investigation the author found that the corrosion resistance is almost linearly proportional to the extent of reduction/redistribution of second phase, irrespective of the technique used for their dissolution. In an another study Senthil Kumar et al. [16] showed the corrosion resistance of friction stir welded alloy was studied by polarization and electrochemical impedance spectroscopy in 3.5% NaCl. The heat-affected zones of the weld exhibited the highest susceptibility to inter-granular corrosion. Corrosion resistance decreases with the increase of traverse speed from 0.37 to 0.76 mm/s at a rotary speed of 800 rpm. Corrosion resistance at rotary speed 1000 rpm is lower than that of 1200 rpm, An increase in the corrosion resistance may also be reached by the breaking down and dissolution of the inter-metallic particles. 4. VARIOUS FINDINGS AND ANALYSIS FSP is used to modify the local microstructure of the work piece. FSP is an emerging surface-engineering technology that can locally eliminate casting defects improves mechanical and metallurgical properties. Friction-stir processing can also produce fine-grained microstructures through the thickness to impart super plasticity. The probe is the part of the tool which is plunged below the surface of the work piece. Its shape depends on the application. The shoulder of the tool rests on the surface of the material being processed. The design of the shoulder and of the probe is very important for the quality of the work piece after processing. The probe of the tool generates the heat and stirs the material and shoulder prevents the plasticized material from escaping from the stir region. The plasticized material is extruded from the leading to the trailing side of the tool but is trapped by the shoulder. The process parameters of FSP are tool rotation speed (RPM), welding speed (mm/min), downward force (kn), tool pin dimensions and shape. Tool tilt angle is also an important welding parameter. FSP has successfully evolved as an alternative technique of fabricating metal matrix composites. Microstructure refinement study was carried by many researchers. From the study of their work it is clear that FSP has great impact on microstructure. Micro structural features limit the mechanical properties of cast alloys, in particular, toughness and fatigue resistance. Eutectic modifiers and high temperature heat treatment are widely used to refine the microstructure of cast Al-Si alloys, to enhance the mechanical properties of the castings. However, none of these approaches can heal the casting porosity effectively and redistribute the Si particles uniformly into the aluminum matrix. Sima Ahmad et al. investigated A356 and found that thermal exposure and intense plastic deformation and high temperature exposure during FSP resulted in generation of fine and equiaxed recrystallized grains due to dynamic recrystallization. Z.Y. Ma et al. found that increasing the rotation rate and number of FSP passes resulted in a decrease in the size and aspect ratio of the Si particles and the porosity level, due to the intensified stirring effect. In an another study A.G. Rao et al. demonstrated the effect of two pass overlap friction stir processing on micro structural refinement of Al 30Si alloy, which delineated significant reduction in size and aspect ratio of silicon particles from average 200 to 2 μm and 4.93 to 1.75 μm respectively. FSP results in significant micro structural evolution within and around the stirred zone, i.e., nugget zone, TMAZ, and HAZ. This leads to substantial change in post weld mechanical properties. The improvement in the mechanical properties of FSP Al-Si alloys is attributed to the breakup of coarse acicular Si particles and it minimized the possibility of the void initiation associated with damaged Si particles, thereby improving the ductility and increasing the strength. Furthermore, the ultra-fine Si particles produced by FSP exerted additional strengthening effects on the aluminum matrix through dislocation/particle interaction. Second, the fundamental elimination of porosity reduced the possibility of void initiation at such porosities, thereby improving the strength and ductility of Al-Si alloys. Third, the room-temperature natural aging of the supersaturated aluminum solid solution produced by FSP resulted in an increase in the strength of the FSP samples. M. Jayaraman et al. demonstrated the effect of FSW process 41

parameters on the tensile strength of cast A356 aluminium alloy. He concluded that the joint fabricated using the process parameters of tool rotation speed 1000 r/min, axial force 5 kn yielded a higher tensile strength compared to other joints. Defect-free weld region, higher hardness of weld region and very fine, uniformly distributed eutectic Si particles in the weld region are found to be the important factors attributed for the higher tensile strength of the above joints. The progressive loss of material from the surface to solid body due to mechanical action is called wear. Wear is caused by disintegration of integrating components as the result of overstressing of the material in the immediate vicinity of the surface. Wear may result in dimensional changes of components. Sima Ahmad et al. showed that wear resistance of the FSP samples of cast A356 with varying tool rotation speed. The magnitude of improvement in wear resistance of the FSP samples increases with increasing the rotation rates. T.S. Mahmoud et al. showed the effect of the tool rotational and traverse speeds as well as the number of passes on tribological characteristics of the modified surfaces. The FS-processed samples exhibited lower wear rates than the as-cast A390 hypereutectic Al Si alloy. There are various techniques to study the corrosion behaviour of aluminum alloy such as immersion tests, polarization techniques, an agar gel exposure, the droplet cell method or cyclic spray tests. For high-strength alloys, the heat-affected zones of the weld exhibit the highest susceptibility to inter granular corrosion, which correlates with copper depletion along the grain boundaries. Surekha et al. investigated the effect of processing parameters (rotation speed and traverse speed) on the corrosion behaviour of friction stir processed high strength precipitation harden able AA 2219-T87 alloy. Only the rotation speed has influence on the corrosion behaviour, while the traverse speed does not show any influence. The corrosion resistance increased with the increase in rotation speed. In a continuation with the previous investigations of various authors, there is large scope of further work. REFERENCES [1]. Z.Y. Ma; Friction Stir Processing Technology: A Review; The Minerals, Metals & Materials Society and ASM International 2008. [2]. Sima Ahmad Alidokht, Amir Abdollah-zadeh, Soheil Soleymani, Tohid Saeid, Hamid Assadi; Evaluation of microstructure and wear behaviour of friction stir processed cast aluminum alloy; Materials Characterization 63(2012) 90 97. [3]. R.S. Mishra, Z.Y. Ma; Friction stir welding and processing; Materials Science and Engineering R 50 (2005) 1 78. [4]. Y. N. Zhang, X. Cao, S. Larose and P. Wanjara; Review of tools for friction stir welding and processing; Canadian Metallurgical Quarterly 51 (2012) 250-261. [5]. 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Kao; Improvement of mechanical properties of a cast Al Si base alloy by friction stir processing; Materials Letters 80 (2012) 40 42. [11]. M. Jayaraman, V. Balasubramanian; Effect of process parameters on tensile strength of friction stir welded cast A356 aluminium alloy joints; Transactions of Nonferrous Metals society of China 23(2013) 605 615. [12]. L. Karthikeyan, V.S. Senthilkumar, K.A. Padmanabhan; On the role of process variables in the friction stir processing of cast aluminum A319 alloy; Materials and Design 31 (2010) 761 771. [13]. M. Jayaraman, R.Sivasubramanian, V. Balasubramanian and S.Babu; Effect of process parameters on tensile strength of friction stir welded cast A356 aluminium alloy joints;met.mater.int.vol.15,no.2 (2009),313-320. [14]. T.S.Mahmoud, S.S.Mohamed; Improvement of microstructural, mechanical and tribological characteristics of A413cast Al alloys using friction stir processing; Materials Science & Engineering A 558 (2012) 502 509. [15]. K. Surekha, B.S. Murty, K. Prasad Rao; Effect of processing parameters on the corrosion behaviour of friction stir processed AA 2219 aluminum alloy; Solid State Sciences 11 (2009) 907 917. [16]. Senthil Kumar; Corrosion Analysis of Friction Stir Welded AA 7075 Aluminum Alloy; Journal of Mechanical Engineering (2009). 42