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Influence of multi-pass friction stir processing on wear behaviour and machinability of an Al-Si hypoeutectic A356 alloy
Accepted Manuscript Title: Influence of multi-pass friction stir processing on wear behaviour and machinability of an Al-Si hypoeutectic A356 alloy Author: Sandeep Kumar Singh R.J. Immanuel S. Babu S.K. Panigrahi G.D.Janaki Ram PII: DOI: Reference:
S0924-0136(16)30160-1 http://dx.doi.org/doi:10.1016/j.jmatprotec.2016.05.019 PROTEC 14821
To appear in:
Journal of Materials Processing Technology
Received date: Accepted date:
23-3-2016 18-5-2016
Please cite this article as: Singh, Sandeep Kumar, Immanuel, R.J., Babu, S., Panigrahi, S.K., Ram, G.D.Janaki, Influence of multi-pass friction stir processing on wear behaviour and machinability of an Al-Si hypoeutectic A356 alloy.Journal of Materials Processing Technology http://dx.doi.org/10.1016/j.jmatprotec.2016.05.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of multi-pass friction stir processing on wear behaviour and machinability of an Al-Si hypoeutectic A356 alloy
Sandeep Kumar Singh1, R.J. Immanuel1, S. Babu1, S.K. Panigrahi1*, G.D. Janaki Ram2
1
Department of Mechanical Engineering
2
Department of Metallurgical & Materials Engineering
Indian Institute of Technology Madras, Chennai- 600036, India
*Corresponding author: Tel.: +91-44-22574742, Email: [email protected]
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Abstract A356 is a widely used Al-Si alloy in automotive and aerospace industries for its good combination of mechanical and tribological properties. When these materials are to be considered for advanced engineering applications, machining becomes inevitable. However, presence of the coarse and non-uniformly distributed silicon particles greatly influence the material properties imposing great challenge to its machinability. Our present work is focussed to improve the wear resistance and machinability of this alloy using multi pass friction stir processing (FSP). The wear behaviour of FSPed materials is characterised against metallic and abrasive medium and the machining studies are done by drilling experiments in dry condition. Under metallic wear condition, the wear resistance is maximum for the material subjected to 3 pass FSP and in case of abrasive wear, the maximum wear resistance is observed in 2 pass FSPed material. A decrease in both drilling force and surface roughness is observed with increase in FSP passes upto 2 pass beyond which they increased. Study on edge burr formation during drilling suggests that the entry and exit burrs are minimal for 3 pass FSPed material. A detailed investigation on the observed results is done in correlation with the microstructural evolution and mechanical properties.
Keywords: Multi-pass friction stir processing; Severe plastic deformation; Al-Si cast alloy; Wear behaviour; Machinability; Built-up-edge formation.
1. Introduction Excellent castability, high strength to weight ratio, good wear resistance and low thermal coefficient make Al-Si cast alloys a suitable replacement for conventional Fe alloys in automotive and many other engineering applications. However, presence of micro-pores, dendritic microstructure with sharp acicular shaped second phase Al-Si eutectic particles (Silicon particles, hereafter) degrade their mechanical and tribological properties. (Finkin, 1979) states that a significant operating cost of any industry rises from the damage incurred due to wear. Wear resistance becomes an important parameter in selecting material for applications which involve relative motion between the contact surfaces such as pistoncylinder pair in combustion engines. 2
Tribological performance of any material is generally done in two ways. In one case, the material is held in motion against a metal counterface to study the performance of the material against metallic wear. In the other case, suitable abrasive medium will be used as the counterface to analyse the materials resistance against abrasion. Al-Si alloys are found to have good tribological properties under both conditions. Though the alloy in its as-cast condition has poor wear resistance owing to its dendritic microstructure with sharp silicon particles, modification of microstructure using suitable thermal/thermo-mechanical treatment is found to enhance the wear properties. (Yasmin et al., 2004) observed that Spheroidization of silicon particles by solution heat treatment tend to increase the wear resistance of an Al – 7 % Si alloy. The post solution-treatment artificial aging also found to enhance the wear properties further. (Chandrashekharaiah and Kori, 2009) asserts that refining the microstructure by addition of alloying elements like Sr, Ti, etc. have positive influence on the wear behaviour. Severe plastic deformation (SPD) is another technique in which the microstructure is refined by imposing large plastic strain. The SPD modified materials are found to possess excellent mechanical properties (Valiev et al., 2000). However, the effect of SPD methods did not always yielded positive results on the wear properties always. (Zhong Han and Yusheng Zhang, 2008) reviewed the wear behaviour of various materials subjected to different SPD processes and found numerous contradicting results showing both enhancement and degradation in the wear resistance of the materials after SPD processes. Friction stir processing (FSP) is one of the recent SPD techniques, first reported by (Mishra et al., 1999). During FSP, the zone over which the FSP tool travels is subjected to high temperature and plastic deformation. The imposed plastic deformation induces large amount of dislocations and the thermal energy provides the driving energy for dynamic recrystallization leading to fine/ultra-fine grains in the processed zone. (Ma et al., 2006) subjected a cast A356 alloy to FSP and observed complete elimination of casting porosity and fine distribution of silicon particles with enhanced strength and ductility. The effect of FSP on the tribological behaviour of these alloys is also studied by various researchers. (Reddy and Rao, 2010) observed a tremendous increase in the wear resistance, when A356 is subjected to FSP. Both the uniform distribution of silicon particles and fine grained aluminium matrix after FSP strengthen the material and increase the wear resistance. Machinability is an important factor in considering any material for practical implications as this may have a strong impact in the production and fabrication cost. As mentioned by 3
(Dwivedi et al., 2008), about 80 % of the materials are subjected to any of the machining operations before being put into its application. Though, cast products are made to near-net shape, machining in one form or the other is essential in most of the engineering application. This calls for an important consideration in studying the problems associated with machining in cast alloy systems too. Machinability of Al-Si cast alloy systems are greatly affected by the shape and distribution of the silicon particles, hardness of the matrix, casting porosity and additional alloying elements as mentioned by (Tash et al., 2006). The work on machinability of Al-Si alloys subjected to any of the SPD techniques is rarely reported in the literature. In our earlier work (Guru et al., 2015), single pass of FSP was imposed on A356 material to enhance the machinability by microstructural modification. FSP resulted in refinement and uniform distribution of silicon particles along with grain refinement of α-aluminium matrix leading to low cutting and better surface finish. With this background, our present research work aims at: (a) Imposing multi pass FSP in an Al-Si hypo-eutectic alloy, A356 to develope ultrafine grained microstructure with refined and uniformly distributed silicon particles (b) studying the influence of multiple passes of FSP on wear behaviour of A356 under both metallic and abrasive wear conditions, (c) analysing the machinability of multi-pass FSPed materials by drilling in terms of the drilling force, surface roughness and the burr formation.
2. Experimental procedures 2.1 Material A commercially available Al-Si hypoeutectic alloy, A356 was procured as a cast ingot from Sargam metals, Chennai. The compositional analysis was done using optical emissive spectroscopy (OES) as per ASTM E1251, which is shown in Table 1. 2.2 Friction stir processing (FSP) Plates for FSP were sliced from the ingot and milled to the final dimension of 300 mm X 60 mm X 8 mm. FSP was carried out on these plates along the length at 120 mm/min traverse speed with 9 kN applied vertical force and 800 rpm tool rotation speed using an indigenously developed friction stir processing machine as shown in Fig. 1.
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A tapered pin FSP tool made of H13 steel was used in the present study. The tool has a shoulder diameter of 15 mm and the tapper pin is 4 mm long with initial and final diameters of 6 mm and 4 mm respectively. The FSPed zone is subjected to multiple pass of FSP upto a maximum of 3 pass with 100 % overlap and same process parameters (120 mm/min traverse speed, 9 kN vertical force, 800 rpm tool rotation). The materials subjected to 1 pass, 2 passes and 3 passes of FSP are hereafter termed as FSP 1P, FSP 2P and FSP 3P materials respectively. 2.3 Microstructural characterisation The microstructural studies of the base and multi-pass FSPed materials were carried out using a Quasmo MR 500 optical microscope. The samples for microstructural study were prepared by grinding the samples using a series of emery in increasing order of grit number from 180 to 2000 followed by 3 µm and 0.5 µm diamond polishing. The polished samples were then etched in 10 % NaOH solution. In case of FSPed materials, the processed plates are sliced along the transverse direction and the central part of the nugget zone (Fig. 2) is subjected to microstructural characterisation. 2.4 Wear studies The wear behaviour of the as-cast and FSPed materials were studied by pin-on-disc method using a Ducom-TR20 tribometer as per ASTM G 99 standards. Pins of 6 mm diameter were machined from base and processed materials. In case of the FSPed material, the pins for wear study were machined from the central processed zone (nugget zone) as shown in Fig. 3. Wear test was conducted in two modes: 1) metal over metal (metallic mode hereafter) and 2) metal over abrasives (abrasive mode hereafter). In the first mode, a hardened EN31 steel with 65 HRC is used as disc material. The steel disc is hardened by following standard heat treatment procedures. In the second mode, Abrasive sheet with 400 grit size of silicon carbide is used as disc material. The various test parameters used in the wear test are tabulated in Table 2. The weight loss during wear test is considered in evaluating the wear rate. The volumetric loss of material is evaluated by dividing the weight loss by the density of the material and the wear rate is then calculated as the volumetric loss per unit time. The frictional force, F is obtained dynamically during the test from the load cell of the test-rig. The coefficient of friction, µ is calculated using the expression, µ = F/N where N is the normal load applied.
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The surface morphology of the worn pin is studied using a FEI-Quanta 200 Scanning electron microscope (SEM). 2.5 Machinability studies To study the effect of FSP on machinability, drilling process is chosen since drilling is an inevitable manufacturing process required in any machinery that has multi-components assembled in it. Machining (drilling) was performed using two-flute HSS twist drill bits of 3 mm diameter in an AMS DTC-300 Vertical machining centre. Both the as-cast and FSPed materials were pre-machined to plates of thickness 6 mm. Through-holes were then made on these pre-machined samples with a feed and speed of 0.25 mm/rev and 45 m/min respectively. In case of FSPed materials, samples for machinability study were machined from the FSP plates as shown in Fig. 3. For each material condition, new drill bit was used for drilling and a total of 12 holes were drilled per material condition. A Zeiss Stemi 2000-CS stereo microscope was used to analyse the drill tool for BUE formation. The surface roughness of the drilled holes was measured using a Bruker 3D Non-contact Profiler.
3. Results 3.1 Microstructural evolution The effect of friction stir processing (FSP) passes on the microstructural evolution of A356 alloy is analysed by optical microscopy and is shown in Fig. 4. The material in its as-cast form shows dendritic microstructure (Fig. 4 (a)) with coarse silicon particles accumulated along the dendritic boundaries of α-aluminium phase. A closer look on the silicon particles (inset in Fig. 4 (a)) shows that these particles are large in size with sharp edges. When the as-cast A356 alloy is subjected to one pass of FSP (FSP 1P material), the silicon particles are fragmented and are redistributed in the α-aluminium matrix (Fig. 4 (b)). However, the particle refinement is not efficient with single pass of FSP. Large number of coarse silicon particles are seen which is marked by black arrow heads in Fig. 4 (b). In-order to minimize/eliminate the presence of these particles, the material is subjected to multiple FSP passes. With increase in FSP passes i.e. in FSP 2P and FSP 3P materials (Fig. 4 (c) and (d)), the silicon particles are found to be refined further and their distribution in the αaluminium phase tend to be more uniform. After 3 pass of FSP, the silicon particles are refined to a great extent and are uniformly distributed in the α-aluminium phase (Fig. 4 (d)).
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3.2 Tribological studies The experimental results of the wear experiment under metallic mode are shown in Fig. 5. There is an increasing trend in the wear rate from the cast material till FSP 2P after which a sudden decrease in the wear rate is seen in FSP 3P material. Fig. 5 (b) shows the variation in coefficient of friction, where the base as-cast material possess the highest friction coefficient. After 1 pass FSP, the variation in friction coefficient is not significant, but a decrease in the friction coefficient is observed in FSP 2P material and it decreased further in FSP 3P material. The worn surface morphology is characterised using SEM and is shown in Fig. 6. The surface of the as cast material (Fig. 6 (a)) is characterised by micro grooves and adhesion marks. After 1 pass of FSP, the adhesion marks got reduced (Fig. 6 (b)) and the micro grooves became deeper than as-cast material. In FSP 2P material (Fig. 6 (c)), the adhesion marks are reduced further and the depth of the micro grooves are found to be deeper than the cast material but shallower than that of FSP 1P material. After 3 pass of FSP, there is a transformation of wear mechanisms (Fig. 6 (d)) where the adhesion marks are dominating with shallow micro grooves. Further, the materials performance against abrasive media has been studied and the experimental results are shown in Fig. 7. A significant difference in the material’s performance against steel and abrasive grits counterface is observed. In abrasive wear mode, the base material shows a moderate wear resistance (Fig. 7 (a)). After 1 pass FSP, the wear resistance of the material increased and the maximum wear resistance is observed for FSP 2P material. FSP 3P material which yielded maximum wear resistance against steel counterface shows the least wear resistance against abrasive counterface. There is no much variation in the friction coefficient among the materials (Fig. 7 (b)). However, the as-cast material possesses least friction coefficient and the FSP 3P material shows the maximum. The surface morphology of the wear samples subjected to abrasive wear is shown in Fig. 8. The worn surface is characterised by micro-grooves. The grooves are wider with evidence for plastic flow along the edges of grooves in the as-cast material (Fig. 8 (a)). After 1 pass FSP (Fig. 8 (b)), a mixture of fine and coarse grooves are observed. However, the grooves are not as deep as observed in as-cast material. In FSP 2P material (Fig. 8 (c)), the micro-grooves are very fine and sharp with no plastic flow along the edges. The worn surface morphology of FSP 3P material is similar to that of the as-cast material with wider grooves.
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3.3 Machinability studies Drilling experiments were performed to assess and compare the machinability of the as-cast material and FSPed materials subjected to multiple passes, which are shown in Fig. 9. The surface roughness and cutting force were chosen as machinability index to judge machinability. The cutting force for the as-cast material is observed as 2.2 kN which is the maximum among the four material conditions. After 1 pass FSP, there is a drastic decrease in the cutting force to 1.7 kN. A further decrease is observed after 2 pass FSP and reaches the minimum of 1.6 kN. However, in case of FSP 3P material, a sudden increase in the cutting force is observed and led to 2 kN. The surface roughness results of the drilled holes (Fig. 9) also show similar trend to that of the drilling force. The surface roughness is maximum in ascast material with Ra of 6.6 µm. A decrease in the roughness value is observed with increasing FSP passes upto 2 pass in which the Ra value is found to be 2.9 µm. The surface roughness value then increased for FSP 3P material to 5.9 µm. The effect of FSP passes on the edge finish of the drill holes has been studied in terms of the entry and exit burrs and the micrographs of the cross section of the drilled holes are shown in Fig. 10. In the as-cast material (Fig. 10 (a) & (b)), both the tool entry and exit zone (top and bottom surface respectively, hereafter) is characterised by thick burrs of about 132 µm and 170 µm height respectively. After FSP, the burr formation at the top surface is significantly reduced. The burr at the top face of FSP 1P material is about 48 µm height and of FSP 2P material is 46 µm (Fig. 10 (c) & (d)). However, the trend is reverse in case of bottom surface burrs. Both FSP 1P and FSP 2P materials had burrs of height 292 µm and 194 µm respectively at the bottom face. FSP 3P material showed better performance against drilling burrs with almost no burr in the top face and 80 µm high burr in the bottom face. The cutting edge of drill bits after drilling 12 holes in as cast and three multi-pass FSPed materials were further studied using stereo-microscope in order to evaluate the BUE formation and is shown in Fig. 11. Among all the four material conditions, maximum BUE formation is observed in the as-cast material (Fig. 11 (a)). The extent of BUE formation is reduced after 1 pass FSP (Fig. 11 (b)) and virtually no BUE formation is observed in the tool used for drilling FSP 2P material (Fig. 11 (c)). However, large BUE formation is found in the tool used for drilling FSP 3P material.
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4. Discussions A356 alloy in its as-cast state has dendritic microstructure with sharp needle shaped silicon particles filling the inter-dendrite region (Fig. 4 (a)). During FSP, the material is subjected to intense plastic deformation because of which the silicon particles are fragmented and redistributed (Fig. 4 (b)). The microstructural modification has a significant impact on the mechanical properties. The quantitative data on the particle refinement and mechanical properties has been adapted from our earlier work (Meenia et al., 2016) and presented in Table 3 for discussion. With a 30 MPa increase in strength, the observed increase in the ductility in FSP 1P material is almost fourfold. However, the statistical data on the silicon particles morphology (Table 3) shows that a large scatter in the particle size and the same is also evident from Fig. 4 (b). With increasing FSP passes, the silicon particles are refined further and redistributed more uniformly in the α- aluminium matrix because of intense plastic deformation during FSP. The enhanced particle refinement and uniform distribution yields significant enhancement in the mechanical properties after 3 pass of FSP with a tensile strength of 200 MPa and a ductility of 36.5 %. A more detailed study on the microstructural evolution and mechanical properties was reported in our earlier work (Meenia et al., 2016). The effect of multi-pass FSP on the tribological and machining behaviour is discussed in detail in our present work with the aid of these microstructural evolution and the enhancement in mechanical properties. 4.1 Tribological behaviour In the as-cast material, the large silicon particles provide resistance for wear against the hardened steel. At the same time, the soft aluminium matrix adheres and gets fused to the steel surface because of the exerted pressure. The fused zones are then detached by the relative motion giving rise to the adhesion marks as seen in Fig. 6 (a). Along with the adhesion mechanism, the silicon particles get pulled out from the matrix and they abrade the surface causing large and deep grooves increasing the wear rate. In FSP 1P material, the decrease in the particle size reduces the load bearing capacity of these particles and hence they get out of the matrix easily. Also, the improvement in the material’s overall yield strength is not much significant (Table 3). Therefore the pulled out silicon particles get into the wear zone (pin-disk interface) and leads to more abrasive wear. The severe abrasion is observed as deep grooves in Fig. 6 (b). The wear resistance of FSP 2P material is almost similar to that of FSP 1P material owing to their similar strength and ductility. In FSP 3P material, the material’s strength and ductility are increased to 200 MPa and 36.5 % 9
respectively (Table 3). Higher strength after 3 passes of FSP led to enhanced resistance against abrasion. Further, the toughness of the material has increased because of the simultaneous enhancement of strength and ductility in FSP 3P material which enhanced the wear resistance of the material further by absorbing more strain energy during wear. The observed result is also supported by the friction coefficient data (Fig. 5 (b)). A lower friction coefficient of FSP 3P material proves that the material is more homogenous with uniform load distribution because of which the wear resistance is increased. The material’s behaviour against abrasive grits seems to be completely different. As asserted by (Shah et al., 2007), the dominating mechanisms under abrasive condition are ploughing and micro-cutting. In the as-cast material, the silicon particles size are found to vary between 22 µm and 4 µm with an average of 13 µm (Table 3) whereas, the abrasive paper used for the current study carries abrasive grits of an average size of 20 µm. Since the size of the hard silicon particles are of comparable size with that of the abrasive grits, they actively resist the abrasion. However, these particles are non-uniformly distributed and are present along the dendritic boundaries because of which, their resistance against the abrasion is not efficient. Also the aluminium matrix of the as-cast material is very soft and hence the abrasive grits easily penetrate them causing deep grooves which are evident from Fig. 8 (a). Therefore, the dominating wear mechanism is found to be micro-cutting in as-cast material. Both FSP 1P and 2P material showed almost similar wear resistance which is higher than that of the as-cast material. The possible reason may be due to its similar microstructure and mechanical properties. The average size of the silicon particles in both FSP 1P and FSP 2P materials are 2.5 µm and 2.4 µm respectively. Since the particles are refined and uniformly distributed, the abrasion resistance of the material is increased. Also the aluminium matrix is strengthened by grain refinement as observed by (Meenia et al., 2016) due to which the overall wear resistance of FSP 1P and 2P materials has increased. In case of FSP 3P material, a reduced wear resistance is observed under abrasive wear condition. The intense silicon particles refinement to an average particle size of 2.14 µm has led to high strength after 3 pass of FSP which is highly favourable for the material’s wear resistance. However, the enhancement in ductility (36.5%) of the material increases the material’s susceptibility towards continuous chip formation when the abrasive grits interact with the aluminium matrix of FSP 3P material. With this increased ductility, the fine particles lose its ability to withstand the abrasion and hence are easily cut away along with the ductile 10
chips increasing the wear loss. The increased micro-cutting phenomenon increases the friction coefficient and this is evident from Fig. 7 (b). 4.2 Machinability The machinability of the as-cast material is observed to be poor in every aspect. Because of the non-uniform distribution of silicon particles, the drilling tool is subjected to intermittent loading. Since the particles are large in size, large force is required to shear them and so the drilling force is high for as-cast material (Fig. 9). The aluminium matrix is soft and leads to diffusion of tool material giving rise to BUE formation (Fig. 1 (a)). The combined effect of the BUE formation and the non-uniform distribution of silicon particles tend to increase the surface roughness of the drilled hole in as-cast material. The reduction in the drilling force and surface roughness in FSP 1P material is attributed to the refinement and redistribution of the silicon particles during FSP. With further decrease in the particle size after 2 pass, a decreasing trend is observed in both drilling force and surface roughness. Though both the materials having similar mechanical properties (Table 3), there is a significant change in the chip morphology of both these materials. One possible reason is the materials’ behaviour towards BUE formation. The tool used for drilling FSP 1P material showed large BUE whereas a very minimal BUE formation is observed in tool used for FSP 2P material. Therefore, in FSP 1P material, the BUE formed alters the tool geometry thereby altering the chip morphology. Since the BUE formation is minimum in case of FSP 2P material, they yielded curly and ductile chips with uniform shear bands. In case of FSP 3P material, increased ductility made the material more prone to BUE formation. The BUE greatly influence the cutting edge geometry and hence an increase in the cutting force is observed in FSP 3P material. Also, the combined effect of ductility and BUE yielded long and thick chips as shown in Fig 12. The interaction of these long strain hardened chips with the drilled surface increased the surface roughness. From the above analysis, it may be obvious that the FSP 2P material yields better machining performance. However, a study on the edge burr formation shows something different, where the FSP 3P material showed better performance with almost no burr. The burr formation at the tool entry side is significantly reduced in all FSPed materials. Since the toughness of the material is enhanced after FSP, the plastic flow of the material at its surface is suppressed and hence the entry burr is reduced in all the FSPed conditions. However, the burr formed at the 11
tool exit side is large in both FSP 1P and FSP 2P material. Though the complete mechanism behind this is unknown, one possible solution is the increased ductility with a moderate strength in both FSP 1P and FSP 2P materials. Since a significant increase in the strength is observed after 3 pass FSP, the exit burr formation is drastically reduced in FSP 3P material.
Conclusions In the present work, the Al-Si cast alloy A356 is subjected to multi pass FSP and the following conclusions are drawn regarding its tribological and machining performance: 1. Under metallic wear condition, the material subjected to 3 pass FSP yielded better wear resistance with lower coefficient of friction. The dominant wear mechanism is found to be abrasive and adhesive. 2. The wear resistance in abrasive wear condition is maximum for the materials subjected to both 1 pass and 2 pass FSP. Enhanced ductility in FSP 3 Pass material made it more prone to micro-cutting thereby decreasing its wear resistance. The friction coefficient is almost similar in all material conditions 3. Machinability studies showed that the cutting force and the surface roughness of the drilled hole are minimum for FSP 2P material. No BUE formation is observed in the drill tool used for drilling FSP 2P material whereas the tools used for all the other material conditions showed significant amount of BUE formation in it. 4. The edge burrs are characterized both at the tool entry and exit zone and it is observed that the FSP 3P material showed a trace amount of burr both at the entry and exit zone whereas the other material conditions showed significant amount of burr formed at its surface. 5. Acknowledgement The authors wish to thank Dr. P Ramkumar of Machine Design Section, Department of Mechanical Engineering, Indian Institute of Technology Madras, for providing the wear test facility to carry out the present research work.
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References Chandrashekharaiah, T.M., Kori, S. a., 2009. Effect of grain refinement and modification on the dry sliding wear behaviour of eutectic Al–Si alloys. Tribol. Int. 42, 59–65. doi:10.1016/j.triboint.2008.05.012 Dwivedi, D.K., Sharma, aluminium
alloys:
a., Rajan, T. V., 2008. Machining of LM13 and LM28 cast Part
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doi:10.1016/j.jmatprotec.2007.05.032 Finkin, E.F., 1979. Adhesive wear: a general review of the state of experimental knowledge and theory. Int. J. Mater. Eng. Appl. 1, 154–161. doi:10.1016/0141-5530(79)90004-9 Guru, P.R., Khan MD, F., Panigrahi, S.K., JanakiRam, G.D., 2015. Enhancing strength, ductility and machinability of a Al–Si cast alloy by friction stir processing. J. Manuf. Process. 18, 67–74. doi:10.1016/j.jmapro.2015.01.005 Ma, Z.Y., Sharma, S.R., Mishra, R.S., 2006. Effect of multiple-pass friction stir processing on microstructure and tensile properties of a cast aluminum-silicon alloy. Scr. Mater. 54, 1623–1626. doi:10.1016/j.scriptamat.2006.01.010 Meenia, S., Khan MD, F., Babu, S., Immanuel, R.J., Panigrahi, S.K., JanakiRam, G.D., 2016. Particle refinement and fine-grain formation leading to enhanced mechanical behaviour in a hypo-eutectic Al-Si alloy subjected to multi-pass friction stir processing. Mater. Charact. Accepted manuscript. Mishra, R.., Mahoney, M.., McFadden, S.., Mara, N.., Mukherjee, A.., 1999. High strain rate superplasticity in a friction stir processed 7075 Al alloy. Scr. Mater. 42, 163–168. doi:10.1016/S1359-6462(99)00329-2 Reddy, G.M., Rao, K.S., 2010. Enhancement of wear and corrosion resistance of cast A356 aluminium alloy using friction stir processing. Trans. Indian Inst. Met. 63, 793–798. Shah, K.B., Kumar, S., Dwivedi, D.K., 2007. Aging temperature and abrasive wear behaviour of cast Al–(4%,12%,20%)Si–0.3% Mg alloys. Mater. Des. 28, 1968–1974. doi:10.1016/j.matdes.2006.04.012 Tash, M., Samuel, F.H., Mucciardi, F., Doty, H.W., Valtierra, S., 2006. Effect of metallurgical parameters on the machinability of heat-treated 356 and 319 aluminum alloys 434, 207–217. doi:10.1016/j.msea.2006.06.129
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Valiev, R.. Z., Islamgaliev, R.. K., Alexandrov, I.. V, 2000. Bulk nanostructured materials from severe plastic deformation. Prog. Mater. Sci. 45, 103–189. doi:10.1016/S00796425(99)00007-9 Yasmin, T., Khalid, A. a., Haque, M.M.M., 2004. Tribological (wear) properties of aluminum–silicon eutectic base alloy under dry sliding condition. J. Mater. Process. Technol. 153-154, 833–838. doi:10.1016/j.jmatprotec.2004.04.147 Zhong HAN, Yusheng ZHANG, K.L., 2008. Friction and Wear Behaviors of Nanostructured Metals. J. Mater. Sci. Technol 24, 483–494.
Fig. 1 (a) Experimental setup used for friction stir processing. (b) A defect free processed plate.
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Fig. 2 Macrograph of FSPed (FSP 1P material) sample showing the region of microstructural characterisation.
Fig. 3 A Schematic of FSPed material showing the procedure for cutting wear and machining samples
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Fig. 4 Optical micrographs of (a) As-cast (b) FSP 1P (c) FSP 2P (d) FSP 3P materials.
Fig. 5 Tribological behaviour of the as-cast and FSP processed materials against steel counterface. a) Wear rate b) Coefficient of friction.
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Fig. 6 SEM micrographs showing the surface morphology of samples subjected to metallic wear. a) As-cast b) FSP 1P c) FSP 2P d) FSP 3P.
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Fig. 7 Tribological behaviour of the as-cast and FSP processed materials against abrasive counterface. a) Wear rate b) Coefficient of friction.
Fig. 8 SEM micrographs showing the surface morphology of samples subjected to abrasive wear: a) As-cast b) FSP 1P c) FSP 2P d) FSP 3P. 18
Fig. 9 Effect of FSP passes on cutting force and surface roughness of the drilled holes.
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Fig. 10 Cross section of drilled holes showing the entry and exit burrs in (a) & (b) as cast; (c) & (d) FSP 1P; (e) & (f) FSP 2P; (g) & (h) FSP 3P materials. (a), (c), (e) & (g) show the tool entry zone and (b), (d), (f) & (h) show the tool exit zone.
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Fig. 11 Micrographs showing the built-up-edge formation at the cutting edge of the tool after drilling of a) As-cast b) FSP 1P c) FSP 2P d) FSP 3P materials.
Fig. 12 Chip morphology in drilling of FSP 3P material.
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Table 1 Elemental composition of A356 (wt. %). Si
Mg
Mn
Fe
Cu
Ti
Zn
Others
Al
6.62
0.357
0.092
0.324
0.0783
0.0329
0.0233
0.03
Bal.
Table 2 Test parameters used in pin-on-disc wear study. Test mode Metallic
Normal load, Total distance, Speed, Track diameter, Velocity, kg m rpm mm m s-1 3 1000 250 80 1.04
Abrasive
1
100
250
80
1.04
Table 3 Quantified data of silicon particles morphology, porosity content and mechanical properties like strength and ductility (Meenia et al., 2016).
Material
As Cast FSP 1P FSP 2P FSP 3P
Particle size, µm 13.03 8.89 2.55 1.19 2.40 1.12 2.14 1.05
Aspect ratio ± 10.24 6.99 ± 1.705 0.61 ± 1.655 0.47 ± 1.585 0.46
± ± ± ±
Porosity volume fraction, %
Strength Hardness, YS, UTS, Hv MPa MPa
Ductility, %
2.68
60.26
107.44
151.91
4.79
0.75
61.20
109.97
181.60
22.81
0.61
65.10
114.52
190.40
22.38
0.25
73.90
127.92
200.56
36.51
22
S0924-0136(16)30160-1 http://dx.doi.org/doi:10.1016/j.jmatprotec.2016.05.019 PROTEC 14821
To appear in:
Journal of Materials Processing Technology
Received date: Accepted date:
23-3-2016 18-5-2016
Please cite this article as: Singh, Sandeep Kumar, Immanuel, R.J., Babu, S., Panigrahi, S.K., Ram, G.D.Janaki, Influence of multi-pass friction stir processing on wear behaviour and machinability of an Al-Si hypoeutectic A356 alloy.Journal of Materials Processing Technology http://dx.doi.org/10.1016/j.jmatprotec.2016.05.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of multi-pass friction stir processing on wear behaviour and machinability of an Al-Si hypoeutectic A356 alloy
Sandeep Kumar Singh1, R.J. Immanuel1, S. Babu1, S.K. Panigrahi1*, G.D. Janaki Ram2
1
Department of Mechanical Engineering
2
Department of Metallurgical & Materials Engineering
Indian Institute of Technology Madras, Chennai- 600036, India
*Corresponding author: Tel.: +91-44-22574742, Email: [email protected]
1
Abstract A356 is a widely used Al-Si alloy in automotive and aerospace industries for its good combination of mechanical and tribological properties. When these materials are to be considered for advanced engineering applications, machining becomes inevitable. However, presence of the coarse and non-uniformly distributed silicon particles greatly influence the material properties imposing great challenge to its machinability. Our present work is focussed to improve the wear resistance and machinability of this alloy using multi pass friction stir processing (FSP). The wear behaviour of FSPed materials is characterised against metallic and abrasive medium and the machining studies are done by drilling experiments in dry condition. Under metallic wear condition, the wear resistance is maximum for the material subjected to 3 pass FSP and in case of abrasive wear, the maximum wear resistance is observed in 2 pass FSPed material. A decrease in both drilling force and surface roughness is observed with increase in FSP passes upto 2 pass beyond which they increased. Study on edge burr formation during drilling suggests that the entry and exit burrs are minimal for 3 pass FSPed material. A detailed investigation on the observed results is done in correlation with the microstructural evolution and mechanical properties.
Keywords: Multi-pass friction stir processing; Severe plastic deformation; Al-Si cast alloy; Wear behaviour; Machinability; Built-up-edge formation.
1. Introduction Excellent castability, high strength to weight ratio, good wear resistance and low thermal coefficient make Al-Si cast alloys a suitable replacement for conventional Fe alloys in automotive and many other engineering applications. However, presence of micro-pores, dendritic microstructure with sharp acicular shaped second phase Al-Si eutectic particles (Silicon particles, hereafter) degrade their mechanical and tribological properties. (Finkin, 1979) states that a significant operating cost of any industry rises from the damage incurred due to wear. Wear resistance becomes an important parameter in selecting material for applications which involve relative motion between the contact surfaces such as pistoncylinder pair in combustion engines. 2
Tribological performance of any material is generally done in two ways. In one case, the material is held in motion against a metal counterface to study the performance of the material against metallic wear. In the other case, suitable abrasive medium will be used as the counterface to analyse the materials resistance against abrasion. Al-Si alloys are found to have good tribological properties under both conditions. Though the alloy in its as-cast condition has poor wear resistance owing to its dendritic microstructure with sharp silicon particles, modification of microstructure using suitable thermal/thermo-mechanical treatment is found to enhance the wear properties. (Yasmin et al., 2004) observed that Spheroidization of silicon particles by solution heat treatment tend to increase the wear resistance of an Al – 7 % Si alloy. The post solution-treatment artificial aging also found to enhance the wear properties further. (Chandrashekharaiah and Kori, 2009) asserts that refining the microstructure by addition of alloying elements like Sr, Ti, etc. have positive influence on the wear behaviour. Severe plastic deformation (SPD) is another technique in which the microstructure is refined by imposing large plastic strain. The SPD modified materials are found to possess excellent mechanical properties (Valiev et al., 2000). However, the effect of SPD methods did not always yielded positive results on the wear properties always. (Zhong Han and Yusheng Zhang, 2008) reviewed the wear behaviour of various materials subjected to different SPD processes and found numerous contradicting results showing both enhancement and degradation in the wear resistance of the materials after SPD processes. Friction stir processing (FSP) is one of the recent SPD techniques, first reported by (Mishra et al., 1999). During FSP, the zone over which the FSP tool travels is subjected to high temperature and plastic deformation. The imposed plastic deformation induces large amount of dislocations and the thermal energy provides the driving energy for dynamic recrystallization leading to fine/ultra-fine grains in the processed zone. (Ma et al., 2006) subjected a cast A356 alloy to FSP and observed complete elimination of casting porosity and fine distribution of silicon particles with enhanced strength and ductility. The effect of FSP on the tribological behaviour of these alloys is also studied by various researchers. (Reddy and Rao, 2010) observed a tremendous increase in the wear resistance, when A356 is subjected to FSP. Both the uniform distribution of silicon particles and fine grained aluminium matrix after FSP strengthen the material and increase the wear resistance. Machinability is an important factor in considering any material for practical implications as this may have a strong impact in the production and fabrication cost. As mentioned by 3
(Dwivedi et al., 2008), about 80 % of the materials are subjected to any of the machining operations before being put into its application. Though, cast products are made to near-net shape, machining in one form or the other is essential in most of the engineering application. This calls for an important consideration in studying the problems associated with machining in cast alloy systems too. Machinability of Al-Si cast alloy systems are greatly affected by the shape and distribution of the silicon particles, hardness of the matrix, casting porosity and additional alloying elements as mentioned by (Tash et al., 2006). The work on machinability of Al-Si alloys subjected to any of the SPD techniques is rarely reported in the literature. In our earlier work (Guru et al., 2015), single pass of FSP was imposed on A356 material to enhance the machinability by microstructural modification. FSP resulted in refinement and uniform distribution of silicon particles along with grain refinement of α-aluminium matrix leading to low cutting and better surface finish. With this background, our present research work aims at: (a) Imposing multi pass FSP in an Al-Si hypo-eutectic alloy, A356 to develope ultrafine grained microstructure with refined and uniformly distributed silicon particles (b) studying the influence of multiple passes of FSP on wear behaviour of A356 under both metallic and abrasive wear conditions, (c) analysing the machinability of multi-pass FSPed materials by drilling in terms of the drilling force, surface roughness and the burr formation.
2. Experimental procedures 2.1 Material A commercially available Al-Si hypoeutectic alloy, A356 was procured as a cast ingot from Sargam metals, Chennai. The compositional analysis was done using optical emissive spectroscopy (OES) as per ASTM E1251, which is shown in Table 1. 2.2 Friction stir processing (FSP) Plates for FSP were sliced from the ingot and milled to the final dimension of 300 mm X 60 mm X 8 mm. FSP was carried out on these plates along the length at 120 mm/min traverse speed with 9 kN applied vertical force and 800 rpm tool rotation speed using an indigenously developed friction stir processing machine as shown in Fig. 1.
4
A tapered pin FSP tool made of H13 steel was used in the present study. The tool has a shoulder diameter of 15 mm and the tapper pin is 4 mm long with initial and final diameters of 6 mm and 4 mm respectively. The FSPed zone is subjected to multiple pass of FSP upto a maximum of 3 pass with 100 % overlap and same process parameters (120 mm/min traverse speed, 9 kN vertical force, 800 rpm tool rotation). The materials subjected to 1 pass, 2 passes and 3 passes of FSP are hereafter termed as FSP 1P, FSP 2P and FSP 3P materials respectively. 2.3 Microstructural characterisation The microstructural studies of the base and multi-pass FSPed materials were carried out using a Quasmo MR 500 optical microscope. The samples for microstructural study were prepared by grinding the samples using a series of emery in increasing order of grit number from 180 to 2000 followed by 3 µm and 0.5 µm diamond polishing. The polished samples were then etched in 10 % NaOH solution. In case of FSPed materials, the processed plates are sliced along the transverse direction and the central part of the nugget zone (Fig. 2) is subjected to microstructural characterisation. 2.4 Wear studies The wear behaviour of the as-cast and FSPed materials were studied by pin-on-disc method using a Ducom-TR20 tribometer as per ASTM G 99 standards. Pins of 6 mm diameter were machined from base and processed materials. In case of the FSPed material, the pins for wear study were machined from the central processed zone (nugget zone) as shown in Fig. 3. Wear test was conducted in two modes: 1) metal over metal (metallic mode hereafter) and 2) metal over abrasives (abrasive mode hereafter). In the first mode, a hardened EN31 steel with 65 HRC is used as disc material. The steel disc is hardened by following standard heat treatment procedures. In the second mode, Abrasive sheet with 400 grit size of silicon carbide is used as disc material. The various test parameters used in the wear test are tabulated in Table 2. The weight loss during wear test is considered in evaluating the wear rate. The volumetric loss of material is evaluated by dividing the weight loss by the density of the material and the wear rate is then calculated as the volumetric loss per unit time. The frictional force, F is obtained dynamically during the test from the load cell of the test-rig. The coefficient of friction, µ is calculated using the expression, µ = F/N where N is the normal load applied.
5
The surface morphology of the worn pin is studied using a FEI-Quanta 200 Scanning electron microscope (SEM). 2.5 Machinability studies To study the effect of FSP on machinability, drilling process is chosen since drilling is an inevitable manufacturing process required in any machinery that has multi-components assembled in it. Machining (drilling) was performed using two-flute HSS twist drill bits of 3 mm diameter in an AMS DTC-300 Vertical machining centre. Both the as-cast and FSPed materials were pre-machined to plates of thickness 6 mm. Through-holes were then made on these pre-machined samples with a feed and speed of 0.25 mm/rev and 45 m/min respectively. In case of FSPed materials, samples for machinability study were machined from the FSP plates as shown in Fig. 3. For each material condition, new drill bit was used for drilling and a total of 12 holes were drilled per material condition. A Zeiss Stemi 2000-CS stereo microscope was used to analyse the drill tool for BUE formation. The surface roughness of the drilled holes was measured using a Bruker 3D Non-contact Profiler.
3. Results 3.1 Microstructural evolution The effect of friction stir processing (FSP) passes on the microstructural evolution of A356 alloy is analysed by optical microscopy and is shown in Fig. 4. The material in its as-cast form shows dendritic microstructure (Fig. 4 (a)) with coarse silicon particles accumulated along the dendritic boundaries of α-aluminium phase. A closer look on the silicon particles (inset in Fig. 4 (a)) shows that these particles are large in size with sharp edges. When the as-cast A356 alloy is subjected to one pass of FSP (FSP 1P material), the silicon particles are fragmented and are redistributed in the α-aluminium matrix (Fig. 4 (b)). However, the particle refinement is not efficient with single pass of FSP. Large number of coarse silicon particles are seen which is marked by black arrow heads in Fig. 4 (b). In-order to minimize/eliminate the presence of these particles, the material is subjected to multiple FSP passes. With increase in FSP passes i.e. in FSP 2P and FSP 3P materials (Fig. 4 (c) and (d)), the silicon particles are found to be refined further and their distribution in the αaluminium phase tend to be more uniform. After 3 pass of FSP, the silicon particles are refined to a great extent and are uniformly distributed in the α-aluminium phase (Fig. 4 (d)).
6
3.2 Tribological studies The experimental results of the wear experiment under metallic mode are shown in Fig. 5. There is an increasing trend in the wear rate from the cast material till FSP 2P after which a sudden decrease in the wear rate is seen in FSP 3P material. Fig. 5 (b) shows the variation in coefficient of friction, where the base as-cast material possess the highest friction coefficient. After 1 pass FSP, the variation in friction coefficient is not significant, but a decrease in the friction coefficient is observed in FSP 2P material and it decreased further in FSP 3P material. The worn surface morphology is characterised using SEM and is shown in Fig. 6. The surface of the as cast material (Fig. 6 (a)) is characterised by micro grooves and adhesion marks. After 1 pass of FSP, the adhesion marks got reduced (Fig. 6 (b)) and the micro grooves became deeper than as-cast material. In FSP 2P material (Fig. 6 (c)), the adhesion marks are reduced further and the depth of the micro grooves are found to be deeper than the cast material but shallower than that of FSP 1P material. After 3 pass of FSP, there is a transformation of wear mechanisms (Fig. 6 (d)) where the adhesion marks are dominating with shallow micro grooves. Further, the materials performance against abrasive media has been studied and the experimental results are shown in Fig. 7. A significant difference in the material’s performance against steel and abrasive grits counterface is observed. In abrasive wear mode, the base material shows a moderate wear resistance (Fig. 7 (a)). After 1 pass FSP, the wear resistance of the material increased and the maximum wear resistance is observed for FSP 2P material. FSP 3P material which yielded maximum wear resistance against steel counterface shows the least wear resistance against abrasive counterface. There is no much variation in the friction coefficient among the materials (Fig. 7 (b)). However, the as-cast material possesses least friction coefficient and the FSP 3P material shows the maximum. The surface morphology of the wear samples subjected to abrasive wear is shown in Fig. 8. The worn surface is characterised by micro-grooves. The grooves are wider with evidence for plastic flow along the edges of grooves in the as-cast material (Fig. 8 (a)). After 1 pass FSP (Fig. 8 (b)), a mixture of fine and coarse grooves are observed. However, the grooves are not as deep as observed in as-cast material. In FSP 2P material (Fig. 8 (c)), the micro-grooves are very fine and sharp with no plastic flow along the edges. The worn surface morphology of FSP 3P material is similar to that of the as-cast material with wider grooves.
7
3.3 Machinability studies Drilling experiments were performed to assess and compare the machinability of the as-cast material and FSPed materials subjected to multiple passes, which are shown in Fig. 9. The surface roughness and cutting force were chosen as machinability index to judge machinability. The cutting force for the as-cast material is observed as 2.2 kN which is the maximum among the four material conditions. After 1 pass FSP, there is a drastic decrease in the cutting force to 1.7 kN. A further decrease is observed after 2 pass FSP and reaches the minimum of 1.6 kN. However, in case of FSP 3P material, a sudden increase in the cutting force is observed and led to 2 kN. The surface roughness results of the drilled holes (Fig. 9) also show similar trend to that of the drilling force. The surface roughness is maximum in ascast material with Ra of 6.6 µm. A decrease in the roughness value is observed with increasing FSP passes upto 2 pass in which the Ra value is found to be 2.9 µm. The surface roughness value then increased for FSP 3P material to 5.9 µm. The effect of FSP passes on the edge finish of the drill holes has been studied in terms of the entry and exit burrs and the micrographs of the cross section of the drilled holes are shown in Fig. 10. In the as-cast material (Fig. 10 (a) & (b)), both the tool entry and exit zone (top and bottom surface respectively, hereafter) is characterised by thick burrs of about 132 µm and 170 µm height respectively. After FSP, the burr formation at the top surface is significantly reduced. The burr at the top face of FSP 1P material is about 48 µm height and of FSP 2P material is 46 µm (Fig. 10 (c) & (d)). However, the trend is reverse in case of bottom surface burrs. Both FSP 1P and FSP 2P materials had burrs of height 292 µm and 194 µm respectively at the bottom face. FSP 3P material showed better performance against drilling burrs with almost no burr in the top face and 80 µm high burr in the bottom face. The cutting edge of drill bits after drilling 12 holes in as cast and three multi-pass FSPed materials were further studied using stereo-microscope in order to evaluate the BUE formation and is shown in Fig. 11. Among all the four material conditions, maximum BUE formation is observed in the as-cast material (Fig. 11 (a)). The extent of BUE formation is reduced after 1 pass FSP (Fig. 11 (b)) and virtually no BUE formation is observed in the tool used for drilling FSP 2P material (Fig. 11 (c)). However, large BUE formation is found in the tool used for drilling FSP 3P material.
8
4. Discussions A356 alloy in its as-cast state has dendritic microstructure with sharp needle shaped silicon particles filling the inter-dendrite region (Fig. 4 (a)). During FSP, the material is subjected to intense plastic deformation because of which the silicon particles are fragmented and redistributed (Fig. 4 (b)). The microstructural modification has a significant impact on the mechanical properties. The quantitative data on the particle refinement and mechanical properties has been adapted from our earlier work (Meenia et al., 2016) and presented in Table 3 for discussion. With a 30 MPa increase in strength, the observed increase in the ductility in FSP 1P material is almost fourfold. However, the statistical data on the silicon particles morphology (Table 3) shows that a large scatter in the particle size and the same is also evident from Fig. 4 (b). With increasing FSP passes, the silicon particles are refined further and redistributed more uniformly in the α- aluminium matrix because of intense plastic deformation during FSP. The enhanced particle refinement and uniform distribution yields significant enhancement in the mechanical properties after 3 pass of FSP with a tensile strength of 200 MPa and a ductility of 36.5 %. A more detailed study on the microstructural evolution and mechanical properties was reported in our earlier work (Meenia et al., 2016). The effect of multi-pass FSP on the tribological and machining behaviour is discussed in detail in our present work with the aid of these microstructural evolution and the enhancement in mechanical properties. 4.1 Tribological behaviour In the as-cast material, the large silicon particles provide resistance for wear against the hardened steel. At the same time, the soft aluminium matrix adheres and gets fused to the steel surface because of the exerted pressure. The fused zones are then detached by the relative motion giving rise to the adhesion marks as seen in Fig. 6 (a). Along with the adhesion mechanism, the silicon particles get pulled out from the matrix and they abrade the surface causing large and deep grooves increasing the wear rate. In FSP 1P material, the decrease in the particle size reduces the load bearing capacity of these particles and hence they get out of the matrix easily. Also, the improvement in the material’s overall yield strength is not much significant (Table 3). Therefore the pulled out silicon particles get into the wear zone (pin-disk interface) and leads to more abrasive wear. The severe abrasion is observed as deep grooves in Fig. 6 (b). The wear resistance of FSP 2P material is almost similar to that of FSP 1P material owing to their similar strength and ductility. In FSP 3P material, the material’s strength and ductility are increased to 200 MPa and 36.5 % 9
respectively (Table 3). Higher strength after 3 passes of FSP led to enhanced resistance against abrasion. Further, the toughness of the material has increased because of the simultaneous enhancement of strength and ductility in FSP 3P material which enhanced the wear resistance of the material further by absorbing more strain energy during wear. The observed result is also supported by the friction coefficient data (Fig. 5 (b)). A lower friction coefficient of FSP 3P material proves that the material is more homogenous with uniform load distribution because of which the wear resistance is increased. The material’s behaviour against abrasive grits seems to be completely different. As asserted by (Shah et al., 2007), the dominating mechanisms under abrasive condition are ploughing and micro-cutting. In the as-cast material, the silicon particles size are found to vary between 22 µm and 4 µm with an average of 13 µm (Table 3) whereas, the abrasive paper used for the current study carries abrasive grits of an average size of 20 µm. Since the size of the hard silicon particles are of comparable size with that of the abrasive grits, they actively resist the abrasion. However, these particles are non-uniformly distributed and are present along the dendritic boundaries because of which, their resistance against the abrasion is not efficient. Also the aluminium matrix of the as-cast material is very soft and hence the abrasive grits easily penetrate them causing deep grooves which are evident from Fig. 8 (a). Therefore, the dominating wear mechanism is found to be micro-cutting in as-cast material. Both FSP 1P and 2P material showed almost similar wear resistance which is higher than that of the as-cast material. The possible reason may be due to its similar microstructure and mechanical properties. The average size of the silicon particles in both FSP 1P and FSP 2P materials are 2.5 µm and 2.4 µm respectively. Since the particles are refined and uniformly distributed, the abrasion resistance of the material is increased. Also the aluminium matrix is strengthened by grain refinement as observed by (Meenia et al., 2016) due to which the overall wear resistance of FSP 1P and 2P materials has increased. In case of FSP 3P material, a reduced wear resistance is observed under abrasive wear condition. The intense silicon particles refinement to an average particle size of 2.14 µm has led to high strength after 3 pass of FSP which is highly favourable for the material’s wear resistance. However, the enhancement in ductility (36.5%) of the material increases the material’s susceptibility towards continuous chip formation when the abrasive grits interact with the aluminium matrix of FSP 3P material. With this increased ductility, the fine particles lose its ability to withstand the abrasion and hence are easily cut away along with the ductile 10
chips increasing the wear loss. The increased micro-cutting phenomenon increases the friction coefficient and this is evident from Fig. 7 (b). 4.2 Machinability The machinability of the as-cast material is observed to be poor in every aspect. Because of the non-uniform distribution of silicon particles, the drilling tool is subjected to intermittent loading. Since the particles are large in size, large force is required to shear them and so the drilling force is high for as-cast material (Fig. 9). The aluminium matrix is soft and leads to diffusion of tool material giving rise to BUE formation (Fig. 1 (a)). The combined effect of the BUE formation and the non-uniform distribution of silicon particles tend to increase the surface roughness of the drilled hole in as-cast material. The reduction in the drilling force and surface roughness in FSP 1P material is attributed to the refinement and redistribution of the silicon particles during FSP. With further decrease in the particle size after 2 pass, a decreasing trend is observed in both drilling force and surface roughness. Though both the materials having similar mechanical properties (Table 3), there is a significant change in the chip morphology of both these materials. One possible reason is the materials’ behaviour towards BUE formation. The tool used for drilling FSP 1P material showed large BUE whereas a very minimal BUE formation is observed in tool used for FSP 2P material. Therefore, in FSP 1P material, the BUE formed alters the tool geometry thereby altering the chip morphology. Since the BUE formation is minimum in case of FSP 2P material, they yielded curly and ductile chips with uniform shear bands. In case of FSP 3P material, increased ductility made the material more prone to BUE formation. The BUE greatly influence the cutting edge geometry and hence an increase in the cutting force is observed in FSP 3P material. Also, the combined effect of ductility and BUE yielded long and thick chips as shown in Fig 12. The interaction of these long strain hardened chips with the drilled surface increased the surface roughness. From the above analysis, it may be obvious that the FSP 2P material yields better machining performance. However, a study on the edge burr formation shows something different, where the FSP 3P material showed better performance with almost no burr. The burr formation at the tool entry side is significantly reduced in all FSPed materials. Since the toughness of the material is enhanced after FSP, the plastic flow of the material at its surface is suppressed and hence the entry burr is reduced in all the FSPed conditions. However, the burr formed at the 11
tool exit side is large in both FSP 1P and FSP 2P material. Though the complete mechanism behind this is unknown, one possible solution is the increased ductility with a moderate strength in both FSP 1P and FSP 2P materials. Since a significant increase in the strength is observed after 3 pass FSP, the exit burr formation is drastically reduced in FSP 3P material.
Conclusions In the present work, the Al-Si cast alloy A356 is subjected to multi pass FSP and the following conclusions are drawn regarding its tribological and machining performance: 1. Under metallic wear condition, the material subjected to 3 pass FSP yielded better wear resistance with lower coefficient of friction. The dominant wear mechanism is found to be abrasive and adhesive. 2. The wear resistance in abrasive wear condition is maximum for the materials subjected to both 1 pass and 2 pass FSP. Enhanced ductility in FSP 3 Pass material made it more prone to micro-cutting thereby decreasing its wear resistance. The friction coefficient is almost similar in all material conditions 3. Machinability studies showed that the cutting force and the surface roughness of the drilled hole are minimum for FSP 2P material. No BUE formation is observed in the drill tool used for drilling FSP 2P material whereas the tools used for all the other material conditions showed significant amount of BUE formation in it. 4. The edge burrs are characterized both at the tool entry and exit zone and it is observed that the FSP 3P material showed a trace amount of burr both at the entry and exit zone whereas the other material conditions showed significant amount of burr formed at its surface. 5. Acknowledgement The authors wish to thank Dr. P Ramkumar of Machine Design Section, Department of Mechanical Engineering, Indian Institute of Technology Madras, for providing the wear test facility to carry out the present research work.
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References Chandrashekharaiah, T.M., Kori, S. a., 2009. Effect of grain refinement and modification on the dry sliding wear behaviour of eutectic Al–Si alloys. Tribol. Int. 42, 59–65. doi:10.1016/j.triboint.2008.05.012 Dwivedi, D.K., Sharma, aluminium
alloys:
a., Rajan, T. V., 2008. Machining of LM13 and LM28 cast Part
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197–204.
doi:10.1016/j.jmatprotec.2007.05.032 Finkin, E.F., 1979. Adhesive wear: a general review of the state of experimental knowledge and theory. Int. J. Mater. Eng. Appl. 1, 154–161. doi:10.1016/0141-5530(79)90004-9 Guru, P.R., Khan MD, F., Panigrahi, S.K., JanakiRam, G.D., 2015. Enhancing strength, ductility and machinability of a Al–Si cast alloy by friction stir processing. J. Manuf. Process. 18, 67–74. doi:10.1016/j.jmapro.2015.01.005 Ma, Z.Y., Sharma, S.R., Mishra, R.S., 2006. Effect of multiple-pass friction stir processing on microstructure and tensile properties of a cast aluminum-silicon alloy. Scr. Mater. 54, 1623–1626. doi:10.1016/j.scriptamat.2006.01.010 Meenia, S., Khan MD, F., Babu, S., Immanuel, R.J., Panigrahi, S.K., JanakiRam, G.D., 2016. Particle refinement and fine-grain formation leading to enhanced mechanical behaviour in a hypo-eutectic Al-Si alloy subjected to multi-pass friction stir processing. Mater. Charact. Accepted manuscript. Mishra, R.., Mahoney, M.., McFadden, S.., Mara, N.., Mukherjee, A.., 1999. High strain rate superplasticity in a friction stir processed 7075 Al alloy. Scr. Mater. 42, 163–168. doi:10.1016/S1359-6462(99)00329-2 Reddy, G.M., Rao, K.S., 2010. Enhancement of wear and corrosion resistance of cast A356 aluminium alloy using friction stir processing. Trans. Indian Inst. Met. 63, 793–798. Shah, K.B., Kumar, S., Dwivedi, D.K., 2007. Aging temperature and abrasive wear behaviour of cast Al–(4%,12%,20%)Si–0.3% Mg alloys. Mater. Des. 28, 1968–1974. doi:10.1016/j.matdes.2006.04.012 Tash, M., Samuel, F.H., Mucciardi, F., Doty, H.W., Valtierra, S., 2006. Effect of metallurgical parameters on the machinability of heat-treated 356 and 319 aluminum alloys 434, 207–217. doi:10.1016/j.msea.2006.06.129
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Valiev, R.. Z., Islamgaliev, R.. K., Alexandrov, I.. V, 2000. Bulk nanostructured materials from severe plastic deformation. Prog. Mater. Sci. 45, 103–189. doi:10.1016/S00796425(99)00007-9 Yasmin, T., Khalid, A. a., Haque, M.M.M., 2004. Tribological (wear) properties of aluminum–silicon eutectic base alloy under dry sliding condition. J. Mater. Process. Technol. 153-154, 833–838. doi:10.1016/j.jmatprotec.2004.04.147 Zhong HAN, Yusheng ZHANG, K.L., 2008. Friction and Wear Behaviors of Nanostructured Metals. J. Mater. Sci. Technol 24, 483–494.
Fig. 1 (a) Experimental setup used for friction stir processing. (b) A defect free processed plate.
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Fig. 2 Macrograph of FSPed (FSP 1P material) sample showing the region of microstructural characterisation.
Fig. 3 A Schematic of FSPed material showing the procedure for cutting wear and machining samples
15
Fig. 4 Optical micrographs of (a) As-cast (b) FSP 1P (c) FSP 2P (d) FSP 3P materials.
Fig. 5 Tribological behaviour of the as-cast and FSP processed materials against steel counterface. a) Wear rate b) Coefficient of friction.
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Fig. 6 SEM micrographs showing the surface morphology of samples subjected to metallic wear. a) As-cast b) FSP 1P c) FSP 2P d) FSP 3P.
17
Fig. 7 Tribological behaviour of the as-cast and FSP processed materials against abrasive counterface. a) Wear rate b) Coefficient of friction.
Fig. 8 SEM micrographs showing the surface morphology of samples subjected to abrasive wear: a) As-cast b) FSP 1P c) FSP 2P d) FSP 3P. 18
Fig. 9 Effect of FSP passes on cutting force and surface roughness of the drilled holes.
19
Fig. 10 Cross section of drilled holes showing the entry and exit burrs in (a) & (b) as cast; (c) & (d) FSP 1P; (e) & (f) FSP 2P; (g) & (h) FSP 3P materials. (a), (c), (e) & (g) show the tool entry zone and (b), (d), (f) & (h) show the tool exit zone.
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Fig. 11 Micrographs showing the built-up-edge formation at the cutting edge of the tool after drilling of a) As-cast b) FSP 1P c) FSP 2P d) FSP 3P materials.
Fig. 12 Chip morphology in drilling of FSP 3P material.
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Table 1 Elemental composition of A356 (wt. %). Si
Mg
Mn
Fe
Cu
Ti
Zn
Others
Al
6.62
0.357
0.092
0.324
0.0783
0.0329
0.0233
0.03
Bal.
Table 2 Test parameters used in pin-on-disc wear study. Test mode Metallic
Normal load, Total distance, Speed, Track diameter, Velocity, kg m rpm mm m s-1 3 1000 250 80 1.04
Abrasive
1
100
250
80
1.04
Table 3 Quantified data of silicon particles morphology, porosity content and mechanical properties like strength and ductility (Meenia et al., 2016).
Material
As Cast FSP 1P FSP 2P FSP 3P
Particle size, µm 13.03 8.89 2.55 1.19 2.40 1.12 2.14 1.05
Aspect ratio ± 10.24 6.99 ± 1.705 0.61 ± 1.655 0.47 ± 1.585 0.46
± ± ± ±
Porosity volume fraction, %
Strength Hardness, YS, UTS, Hv MPa MPa
Ductility, %
2.68
60.26
107.44
151.91
4.79
0.75
61.20
109.97
181.60
22.81
0.61
65.10
114.52
190.40
22.38
0.25
73.90
127.92
200.56
36.51
22