Journal of Materials Processing Technology

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1 Journal of Materials Processing Technology 211 (2011) Contents lists available at ScienceDirect Journal of Materials Processing Technology journal homepage: Surface modification of aluminium by friction stir processing Adem Kurt a, Ilyas Uygur b,, Eren Cete c a Gazi University, Technical Education Faculty, 06500, Ankara, Turkey b Duzce University, Faculty of Engineering, 81620, Duzce, Turkey c Gazi University Institute of Science, 06500, Ankara, Turkey article info abstract Article history: Received 31 March 2010 Received in revised form 15 September 2010 Accepted 30 September 2010 Keywords: Friction stir processing Aluminium Plate bending Hardness Microstructure In this study, SiC particles were incorporated by using Friction Stir Processing (FSP), into the commercially pure aluminium to form particulate surface layers. Samples were subjected to the various tool rotating and traverse rates with and without SiC powders. Microstructural observations were carried out by employing optical microscopy of the modified surfaces. Mechanical properties like hardness and plate bending were also evaluated. The results showed that increasing rotating and traverse rate caused a more uniform distribution of SiC particles. The hardness of produced composite surfaces was improved by three times as compared to that of base aluminium. Bending strength of the produced metal matrix composite was significantly higher than processed plain specimen and untreated base metal Elsevier B.V. All rights reserved. 1. Introduction Friction stir welding (FSW) and its variation of FSP, a solid-state joining and surface modification technique were invented at The Welding Institute of UK in 1991 (TWI, 2009). Application of FSW showed great promise in joining many ferrous and non-ferrous alloys that were joined by using conventional welding techniques with great difficulty. The convenience of the FSW as a joining technique has been demonstrated in a number of studies, especially for commercially pure aluminium (Kurt et al., 2006, 2007). The technique was energy efficient, environmentally friendly and versatile. Recently, a new processing technique, FSP was developed by Mishra et al. (2003) Microstructural modification and fabrication of metal matrix composites are based on the basic principles of FSW. In this case, a rotating tool with or without pin and shoulder was inserted into a single piece of material, for localized microstructural modification and specific property enhancement. The FSP caused intense plastic deformation, material mixing, and thermal exposure, resulting in significant microstructural refinement, densification, and homogeneity of processed zone. This technique has been successfully applied in the production of fine grained structure and surface composite (Ma, 2008). FSP has great advantages including: solidstate microstructural evaluation, adjusting mechanical properties by optimizing tool design and process parameters, the depth of Corresponding author. Tel.: ; fax: address: ilyasuygur@duzce.edu.tr (I. Uygur). processed zone and location which can be identified by the operator. Moreover, the process does not produce any deleterious gas, noise and materials distortion. Two types of material modification have been developed; the in-volume FSP (VFSP) consisting on the modification of the full thickness and the surface FSP (SFSP) consisting of the modification of the surface of processed materials up to 2 mm depth. For the VFSP, the overlap ratio was an important factor for the evolution of microstructure and alteration of mechanical properties (Nascimento et al., 2009). Aluminium and its alloys are used commonly in aerospace and transportation industries because of their low density and high strength to weight ratio. Especially Al-based Metal Matrix Composites (MMC) exhibited high strength, high elastic modulus, and improved resistance to fatigue, creep and wear; which make them promising structural materials for many industries (Uygur, 1999). However, these composites suffered from low ductility and toughness due to incorporation of ceramic reinforcements, which limited their wide applications to a certain extent. Generally surface properties determined the useful life of components where wear and fatigue resistance were main concern. In these situations, only surface layer of components was reinforced by reinforcement particles while the bulk of component retained the original properties. Although most of surface modifications consisted of melting treatments, such as plasma spraying, laser and electron beam treatments, solid-state surface modification of light metal alloys are also available in literature (Wang et al., 2009). The objective of this paper was to investigate the possibility of incorporation of reinforcement particles into surface layer of /$ see front matter 2010 Elsevier B.V. All rights reserved. doi: /j.jmatprotec

2 314 A. Kurt et al. / Journal of Materials Processing Technology 211 (2011) Table 1 The effect of processing parameters on the hardness distribution. Rotation speed (rpm) Travelling speed (mm/min) Hardness (Hv) [plain] Hardness (Hv) [SiC added] Fig. 1. Schematic illustration of FSP tool. commercially pure aluminium to form metal matrix composites by means of FSP technique. Also, the influence of tool rotation speed and traverse rate on the mechanical properties were experimentally investigated. 2. Experimental procedure The starting materials were monolithic cold-rolled plates of 1050 aluminium alloy with a nominal composition of 0.4 wt.% Fe, 0.12 wt.% Si, 0.03 wt.% Cu, 0.02 wt.% Ti, 0.02 wt.% Mg, 0.02 wt.% Mn, and the balance Al. The surface of plates was cleaned with grinding paper before processing. The average size of the SiC particles was about 10 m. SiC particles were added into a small amount of methanol and mixed, and then applied to surface of the plates to form a thin SiC particle layer. No binder was added to the mixture. The dimensions of the workpiece were 50 mm 100 mm 5 mm. Fig. 1 showed a schematic illustration tool designed for FSP, which is composed of a plain cylindrical shoulder without pin. The tool was made from AISI 1050 steel. The tool was rotated clockwise at rotation speed (ω) of rpm, with the rotating shoulder 0.1 mm inserted into the workpiece. The travelling speeds (v) were mm/min. The tool spindle angle of 2 was used, which helps in forging action at the travelling edge of shoulder. All FSP studies were carried out at room temperature with only a single pass. Microstructural observations and Vickers microhardness test were performed on the treated surfaces. The transverse sections of as-processed aluminium plates were mounted and then mechanically polished. The obtained samples from the FSP were etched with Keller reagent to distinguish the grain boundaries, identify precipitates and difference in composition. The hardness tests were carried out under a load of 100 g. The average microhardness measurements were made on the FSP treated surfaces. Samples with 10 mm 100 mm 5 mm were tested in an Instron 5566 equipped with load of 50 kn at constant deformation speed of 2 mm/min for three point bending tests. Three test samples for each condition were tested. 3. Results 3.1. Effects of travelling and rotation speed on microstructure The effect of travelling speed on the compressed surfaces without SiC particle addition was shown in Fig. 2a and b. The shoulder was the main heat source in this study. It was clearly seen that the thickness of the modified surface significantly decreased due to the heat input. The thickness of the surface was 100 m for 15 mm travel speed, but it was just 30 m for the 30 mm travel speed. The effect of rotation speed on the FSP processed sections of commercially pure aluminium was shown in Fig. 3a and b. Again, the thickness of the processed zone was affected by rotation speed. An increase in rotation speed caused thicker surface thickness compared with slow rotation speed rates, due to the amount of the heat input during surface treatment. The thickness was 40 m for 500 rpm and it is 50 m for the 1000 rpm rotation speed. From these results, it can be said that the travelling speeds have more crucial effect on the microstructure compared with rotational speed. The effects of travelling speed on the sectioned specimens with the SiC particulate reinforced surfaces were shown in Fig. 4a and b. The thickness layer of the composite was significantly high compared with the low travelling speed specimen. The surface layer was about 150 m and 75 m for 15 and 30 mm/min travelling speed respectively. Again, heat input of the process is the main governing mechanism due to the extensive plastic deformation which was related with elevated temperature. At high travelling speed (30 mm/min), the surface composite layer was usually weakly bonded with the aluminium alloy substrate. Similar observations and effects were also observed for different rotational speeds (Fig. 5a and b) Hardness profile The effect of processing parameters on the surface microhardness of plain and SiC particle added specimens was shown in Table 1. Increased travelling speeds caused significant increment on the hardness values for all rotational speeds for the plain specimens. The hardness of as received Al has an average of 67. The highest hardness value was 80 and the lowest was 37 for the plain specimens. However, increased rotation speeds resulted in lower hardness values at the same travelling speeds. Contrary to these, an increase in travelling speed causes the dramatic reduction of hardness values of all rotational speeds for the SiCp added specimens. The highest hardness values were obtained from high rotation and low travelling speeds for SiCp added specimens. More than twice increment of the hardness of the parent alloy was achieved by 1000 rpm rotational and 15 mm/min travelling speed since shoulder supplied sufficient heat and force, and they were dispersed well in the base metal. As a result, the hardness was enhanced with uniformly SiCp additions. In some cases, hardness profile showed a general softening and reduction of hardness, especially with high rotational speeds in spite of smaller grain size. The result means that the hardness distribution was not combined with the Hall Petch relationship. Therefore, other factors may govern the mechanical properties of stirred zone, like precipitations and dislocation density Bending test The results of the room temperature bending test were shown in Fig. 6. Both plates were deformed at an angle of 30. In both cases the FSP zone was in the top surface of the plates where plain strain was the greatest. An increase of formability in the plain specimen was proportional to the increase in the ductility which resulted from the decreasing grain size. As expected, the plain FSP treated surface was much more ductile and had a lower yield strength as compared with SiCp added specimen. The yield strength of 1050 Al-alloy was 28 MPa (Matweb, 2010). Three point yield strength of

3 A. Kurt et al. / Journal of Materials Processing Technology 211 (2011) Fig. 2. The effect of travelling speed on the microstructure of FSP plain specimens at constant ω = 500 rpm, (a) = 15 mm/min, (b) = 30 mm/min. Fig. 3. The effect of rotation speed on the microstructure of FSP plain specimens with constant = 20 mm/min, (a) ω = 500 rpm, (b) ω = 1000 rpm. Fig. 4. The effects of travelling speed on the microstructure of FSP SiC added specimens at constant ω=500 rpm, (a) = 15 mm/min, (b) = 30 mm/min. Fig. 5. The effects of rotation speed on the microstructure of FSP plain specimens with constant = 20 mm/min, (a) ω = 500 rpm, (b) ω = 1000 rpm. 315

4 316 A. Kurt et al. / Journal of Materials Processing Technology 211 (2011) Fig. 6. Three point bending specimens ω = 1000 rpm and = 15 mm/min, (a) just FSP treated, (b) SiC particle added. plain specimen was 60 MPa for plain specimen, but it was 84 MPa for the SiCp added specimen. It can be said that surface modification resulted in significant enhancement of the strength of tested specimens, but SiCp addition causes low ductility, as failure was visible in Fig. 6b. 4. Discussions As seen in Figs. 2 5, the microstructure varies significantly with rotational and travelling speeds. It can be said that slow travelling speed rates caused more heat input; hence it significantly changes the microstructure beneath the surface where extremely small grains were evident. The structure was very homogenous, with the large precipitates and defects a part broken. Intense plastic deformation and heat input during FSP cause the break up of coarse dendrites and second-phase particles, the refinement of matrix grains, the closure of porosity, and the dissolution of precipitates, thereby creating a fine, uniform, and defect free structure. This type of structure appears to enhance the ductility (Figs. 2 and 3). Ma et al. (2002) obtained similar microstructure of 7075 Al-alloy which exhibited high strain rate of super plasticity at 490 C. Also, Nascimento et al. (2009) observed homogeneous and fine equiaxied grains due to dynamic recrystallisation of highly deformed grains during processing in the nugget zone for AA7022-T6 alloy. In this area, depletion of precipitates was also evident due to the rise of temperature during process. As seen in Figs. 4 and 5, there are three distinct regions on the microstructure: (i) Upper surface where SiC particles are homogenously distributed. Finely dispersed SiC particles can limit the grain growth and resulted in an ultrafine grain size. It was reported that (Asadi and Givi, 2009) increasing rotational speed produced more heat input and so the grain growth enhanced. The thickness of this region can be adjusted by rotational and travelling speed or pin height. (ii) Lower surface where extremely small fine grains, broken inclusions and redistributed fine precipitations occur as a result of recrystallization. (iii) Beneath the surface where elongated grains can be seen due to the material flow caused by the rotating shoulder that includes high plastic deformation in this zone. (iv) Base metal, where no microstructural changes occurred. Also, it can be said that high friction causes more local heat input per volume and results in elevated temperature which leads to severe plastic flow and more uniform SiC particle distributions compared with counterparts as seen in Figs. 4a and 5b where surface composite layers appear to be very well bonded to the aluminium alloy substrate, and no defects were visible. Several researchers (Kwon et al., 2003; Colligan, 1999) have suggested that there is a difference in plastic flow behaviour of materials during FSP. It is therefore likely that these microstructural differences resulted from the different flow behaviour and heat input. It was reported that higher rotational speed and lower travelling speed caused more heat input, and the tool supplied shear force to make the SiC particles flow and disperse in wider region (Wang et al., 2009). In order to supply enough frictional and shear force for covered particles and avoid them to agglomerate, the shoulder and process parameters play a dominant role. During the experiments, a novel way to the surface treatment of pure aluminium and Al based MMCs through FSP was applied successfully. By changing rotational and travelling speeds, well bonded and uniformly distributed SiC particulate composites can be produced. As seen in Table 1, welding parameters significantly alters the hardness values of the treated surfaces of the materials. It is known that SiCp takes its advantage of hardness (3200 Hv). Effective enhancement role in the MMCs can be obtained even by little percentage of SiCp (Wang et al., 2009) and increased as the SiCp volume fraction would cause further improvements in the hardness values and the strength (Ma, 2008). The possible strengthening mechanism can be attributed to grain and subgrain structure and dislocation distribution of the modified surfaces. However, presumably the governing strengthening mechanisms for the SiCp surfaces can be attributed to dislocation density and formation of work hardening, due to the strain misfit between the elastic reinforcing particles and the plastic matrix. According to the characteristics of the microstructure, the major contributions to hardness of the surface composite layers fabricated by FSP are 1) the fine grain size of the Al matrix, and 2) the Orowan strengthening due to the reinforcement particles (Zarghani et al., 2009). This significant hardness improvement can be attained by controlling the grain size and heat input via the tool rotation and travelling speeds. The enhanced strength found in FSP aluminium is most likely caused by recrystallised grains, residual stress due to the shoulder compression, work hardening and SiCp additions into the soft matrix where thermal mismatch occurs between hard ceramic particles and soft matrix. It was reported that tensile strength of the 2124 Al-alloy composites significantly changed in accordance with SiCp volume fraction, particle size and distribution. The modulus, yield strength and UTS of all composites were dramatically enhanced with respect to the unreinforced base alloy (Uygur, 2004). Uygur et al. (2004) also showed that coarse reinforcing particles and high volume fraction of these resulted in high dislocation density hence increasing the tensile and fatigue properties of MMCs. 5. Conclusions In the present study, the plain Al surface and Al/SiCp composites were successfully fabricated by FSP. The microstructure, microhardness and bending behaviour were evaluated observing the matrix grains and dispersion of the reinforcement particles. The following results were obtained: 1. It has been demonstrated that FSP was an appropriate method to modify the microstructure and mechanical properties of 1050 Al-alloy. In general, FSP decreased the grain size and increased the hardness of processed material. 2. Increased rotation speed and low travelling speeds caused more heat input which affects the thickness of the surface layer, grain size and distribution of the precipitates and reinforcing particles. A good dispersion of SiCp can be obtained for the composite layer produced by ω= 1000 rpm and v = 20 mm/min. 3. Good interfacial conditions between particles and base metal can be formed during this solid-state process which avoids the chemical reactions on the interface.

5 A. Kurt et al. / Journal of Materials Processing Technology 211 (2011) The depth of the surface layer can be tailored by welding parameters or probe design which could be used with pin or without pin. 5. FSP treatments improve the formability of plain samples; hence they could be used for super plastic applications. 6. The microhardness of the plain surface of Aluminium increased significantly with increasing travelling speeds. The highest microhardness value was obtained 80 Hv for the plain specimen by ω = 500 rpm and = 30 mm/min 7. The microhardness of the SiCp added composites surface increases significantly with increasing rotation speed. The highest microhardness value was obtained 150 Hv for the plain specimen by ω = 1000 rpm and = 15 mm/min 8. The high microhardness of Al/SiCp composite can be attributed to the presence of reinforcement particles, which also improved the bending strength. 9. With further research efforts and increased understanding, FSP could be conducted for mechanical behaviour of these composites, like fatigue and creep response and new tool design for uniform distribution of reinforcement particles into the matrix materials. References Asadi, P., Givi, M.K.B., Effect of friction stir processing on microstructure and hardness of AZ91-SiC composite. In: Kurt, A., Turker, M. (Eds.), 1st Int. Conference on Welding Technologies June, Ankara, Turkey, p. 244 (proceedings). Colligan, K., Material flow behavior during friction stir welding of aluminium. Weld. J. 78, Kurt, A., Uygur, I., Ates, H., Effect of porosity content on the weldability of powder metal parts produced by friction stir welding. Mater. Sci. Forum , Kurt, A., Gulenc, B., Uygur, I., Ates, H., The effect of rotation speed on mechanical properties of commercially pure Aluminium joined by friction stir welding. In: Materials and Technology Conference, October., Fundamentals and Characterisation vol. 1, Ohio, USA, pp Kwon, Y.J., Shigematsu, I., Saito, N., Mechanical properties of fine-grained aluminium alloy produced by friction stir process. Scrip. Mater. 49, Ma, Z.Y., Mishra, R.S., Mahoney, M.W., Superplastic deformation behaviour of friction stir processed 7075Al alloy. Acta Mater. 50, Ma, Z.Y., Friction stir processing technology: a review. Metall. Mater. Trans. 39A, Matweb, Mishra, R.S., Ma, Z.Y., Charit, I., Friction stir processing: a novel technique for fabrication of surface composite. Mater. Lett. A341, Nascimento, F., Santos, T., Vilaça, P., Miranda, R.M., Quintino, L., Microstructural modification and ductility enhancement of surfaces modified by FSP in aluminium alloys. Mater. Sci. Eng. A506, TWI, Uygur, I., Environmentally assisted fatigue response of Al Cu Mg Mn with SiC particulate metal matrix composites. PhD thesis, University of Wales, SWANSEA, UK. Uygur, I., Tensile behaviour of P/M processed (Al Cu Mg Mn)/SiCp composites. Iranian J. Sci. Technol. B28, Uygur, I., Evans, W.J., Bache, M., Gulenc, B., The fatigue behaviour of SiCp reinforced 2124 aluminium matrix composites. Metallofiz. Nove. Tekhnol. 26, Wang, W., Shi, Q., Liu, P., Li, H., Li, T., A novel way to produce bulk SiCp reinforced Al-MMC by FSP. J. Mater. Process Technol. 209, Zarghani, A.S., Boorg, S.F.K., Hanzaki, A.Z., Microstructure and mechanical properties of Al/Al 2O 3 surface nano-composite layer produced by friction stir processing. Mater. Sci. Eng. A500,