Effect of Pin Profile on Friction Stir Welded Aluminum Matrix Composites

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1 This article was downloaded by: [ ] On: 20 December 2012, At: 04:22 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Materials and Manufacturing Processes Publication details, including instructions for authors and subscription information: Effect of Pin Profile on Friction Stir Welded Aluminum Matrix Composites Adel Mahmood Hassan a, Tarek Qasim a & Ahmed Ghaithan a a Department of Industrial Engineering, Jordan University of Science and Technology, Irbid, Jordan Accepted author version posted online: 26 Jun 2012.Version of record first published: 26 Nov To cite this article: Adel Mahmood Hassan, Tarek Qasim & Ahmed Ghaithan (2012): Effect of Pin Profile on Friction Stir Welded Aluminum Matrix Composites, Materials and Manufacturing Processes, 27:12, To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 Materials and Manufacturing Processes, 27: , 2012 Copyright # Taylor & Francis Group, LLC ISSN: print= online DOI: / Effect of Pin Profile on Friction Stir Welded Aluminum Matrix Composites Adel Mahmood Hassan, Tarek Qasim, and Ahmed Ghaithan Department of Industrial Engineering, Jordan University of Science and Technology, Irbid, Jordan To clarify the role of pin profile geometry on some properties of friction stir welded of the considered aluminum matrix composites (Al-4 wt% Mg, reinforced with 1 wt% SiC and 1 wt% graphite particles) plates of 8 mm thickness were fabricated by compocasting method then annealed at 400 C for 2 h. Tools with different pin profiles (square, hexagonal, and octagonal) were manufactured to be used for friction stir welding (FSW) of aluminum matrix composites plates at four different levels of welding (transverse) and rotational speeds. The effects of these pin profiles on microstructure and some mechanical properties of the friction stir welded joints were studied. The results show that the plates welded by square head pin have better properties compared to the other pin profiles. This pin seems to cause better grain refinement and redistribution of SiC and graphite particles in the welded nugget zone (NZ) than the other two types. This has led to better improvements in the considered mechanical properties. Also, these properties were improved by increasing welding speed, while increasing the rotational speed has a diverse effect on them. Keywords Aluminum; Compocasting; Composites; Friction; Properties; Welding. INTRODUCTION Friction stir welding (FSW) was introduced in 1991 by The Welding Institute (TWI) in Cambridge, England, as a solid-state metal joining process [1, 2]. In the FSW, process parts to be joined must be tightly clamped to backing plate in order to prevent them from moving during the welding process. A rotating pin tool is forced down into a hole along the weld line until the shoulder of the tool comes into contact with the parts to be joined. The rotating tool travels along the joint line direction with a constant welding (traverse) speed. During the welding process, the material along the joint undergoes intense plastic deformation due to frictional elevated temperature, resulting in fine and equiaxed recrystallized grains, which in turns enhances the mechanical properties of the welded joint [3, 4]. The friction stir weld joint consists of three distinct zones: the nugget zone (NZ) in the middle of the joint, followed by the thermomechanically affected zone (TMAZ), and the third zone is the heat-affected zone (HAZ). At the NZ, the plastic deformation will produce a recrystallized, equiaxed, and fine-grain microstructure. TMAZ exposes to lower plastic deformation (less than the NZ). Therefore, this zone consists of relatively large grains. The HAZ is not subjected to any plastic deformation only; it is exposed to thermal affect, which results in some modification and coarsening the grains. During the FSW process, because of the rotation of the profiled pin of the welding tool nearly concentric rings are developed in the NZ, which is called the onion rings structure [5]. The process can be used in many applications, such as the joining of similar metals, dissimilar metals [6], high-strength aerospace aluminum alloys, and composite materials that have limitations to be welded by conventional fusion welding processes [7]. More details of the advantages and limitations of the FSW process can be found in [8]. In the FSW process, the microstructure evolution and the mechanical properties of the weld joints is influenced by the material flow in the weld zone. The most significant parameter affects the material flow is the tool geometry [9]. Among other parameters affecting the material flow are the friction rotational speed and welding (transverse) speed. All these parameters have a remarkable influence on grain size of the NZ microstructure, which, in turn, will affect the mechanical properties of the weld zone [10]. In general, it can be stated that FSW is a combination of extruding, forging, and stirring of the material [9]. Most of the previous studies in the recent developed field of FSW have focused on the effect of welding (transverse) speed and rotational speed on the properties of welded joints [11]. Little work has been done to study the effect of the welding pin profile tool on properties of friction stir welded joints [12], especially on composite materials. Accordingly, the present work was concentrated on studying the effect of pin profile geometry of the welding tools on mechanical properties, utilizing aluminum matrix composites. Received August 22, 2011; Accepted September 23, 2011 Address correspondence to Adel Mahmood Hassan, Department of Industrial Engineering, Jordan University of Science and Technology, P. O. Box 3030, Irbid 22110, Jordan; adel@just.edu.jo EXPERIMENTAL WORK Materials Commercial pure aluminum alloyed with 4 wt% Mg as wetting agent reinforced by 1 wt% SiC and 1 wt% graphite 1397

3 1398 A. M. HASSAN ET AL. particles were used in fabrication the aluminum matrix composites plates. Silicon carbide powder having a diameter of 200 mm and a density of 3.21 g=cm 3 was chosen as reinforcement particles because it has a high wear resistance. In addition, graphite particles having a density of 2.1 g=cm 3 were used as second reinforcement particles to improve the machinability and wear resistance of the considered composite, graphite acts as a lubricating agent [13]. Processing the Plates The processing of the composite plates (100 mm 75 mm 8 mm) used in the present study was manufactured by compocasting method. More details about this method can be found in [14]. All plates produced were annealed at 400 C for a period of 2 h, before they were butt-welded by FSW process. Prior to welding the annealed plates properties were tested and recorded for comparative reasons. The annealed plates before welding have a tensile strength of 130 MPa and Vickers hardness of 71.2 HV. Welding Tool Fabrication Tools with square, hexagonal, and octagonal pin profiles were fabricated from 0.4% C plain carbon steel using conventional milling process. The steel was oil hardened to reach a hardness of 63 HRC. The schematic diagram for the square head pin tool is shown in Fig. 1. The hexagonal and octagonal head tools are identical in their design to the square head tool. Welding Procedure The fabricated and annealed plates were butt welded by FSW process using a conventional milling machine. The plates were clamped firmly to a specially designed fixture, which was mounted and fixed tightly on the milling machine. For each pin profile tool, four welding (transverse) speeds of 35, 45, 55, 65 mm=min and four rotational speeds 630, 800, 1,000, 1,250 rpm were utilized in the present study. Metallurgical and Mechanical Tests Microstructure analysis of the weld joints was carried out using an optical microscope. Vickers hardness was conducted using a universal hardness testing machine. Tensile test specimens were welded, then prepared by a CNC milling machine, so that the welded joint was exactly in the middle of the specimen. FIGURE 2. (a) Base composite microstructure together with the friction stir welded NZ microstructures of (b) square, (c) hexagonal, and (d) octagonal head pin tools at rotational speed of 630 rpm and welding transverse speed of 65 mm=min. Magnification 500X. The wear tests were carried out at a normal load of 50 N and rotational speed of 100 rpm using a pin-on-disk type test machine at dry conditions. Wear specimen with 25 mm length and 4 mm in diameter pin was prepared from the center of the NZ of the weld joint. The wear rate can be calculated using the following equation [15]: FIGURE 1. Square head pin friction stir welding (FSW) tool. W ¼ M=ðD SÞ; ð1þ

4 EFFECT OF PIN PROFILE ON FRICTION STIR WELDS 1399 FIGURE 3. Effect of welding (transverse) speed and pin profile tool geometry on average Vickers hardness at rotational speed of 630 rpm. FIGURE 5. Effect of welding (transverse) speed and pin profile tool on the tensile strength at rotational speed of 630 rpm. FIGURE 4. Effect of rotational speed and pin profile tools on average Vickers hardness at a welding (transverse) speed of 65 mm=min. where W is wear rate expressed in (cm 3 =m), M is mass loss during wear in (g), S is sliding distance in (m), and D is density of the respective composite in (g=cm 3 ), which is equal to 2.67 g=cm 3, as determined by the rule of mixture method. mechanical properties are expected to be changed relevance to the microstructural changes [16]. Hardness Figures 3 and 4 show that the square head tool has the highest effect on HV hardness values at the same welding transverse speed and rotational speed, higher than the hardness obtained by other profiled tools. Tensile Strength Figures 5 and 6 indicate that the used of the square pin profiled pin has given the highest values for both welding (transverse) speed and rotational speed. Again, the highest improvement of the tensile strength was encountered with square profile pin. Wear Resistance Results are obtained for both welding transverse speed and rotational speed as shown in Figs. 7 and 8, respectively, where the wear resistance in both figures RESULTS Microstructure Analysis The effects of pin profile geometry on the microstructure of the friction stir welded NZ at a rotational speed of 630 rpm and a welding transverse speed of 65 mm=min min are shown in Fig. 2. The microstructure of the base composite is shown in the figure, together with the microstructure of the NZs produced by the considered three profiled pins of the welding tool. The square pin profile tool produces weld joints with small and fine grains than the other two profiled pins of the welding tool, as shown in the figure. The microstructure of the friction stir weld joint is affected by the pin profile tool type, and the FIGURE 6. Effect of rotational speed and pin profile tool on the tensile strength at welding (transverse) speed of 65 mm=min.

5 1400 A. M. HASSAN ET AL. FIGURE 7. Effect of welding (transverse) speed and pin profile tools on the wear rate at rotational speed of 630 rpm. FIGURE 8. Effect of rotational speed and pin profile tools on the wear rate at welding (transverse) speed of 65 mm=min. is higher for the square profiled pin than the other types of profiled pins. DISCUSSION Microstructure evolution in the friction stir weld joints were resulted from the intensive plastic deformation which causes grain refinement in the weld zone. In addition to that, there is a breaking up and uniform redistributions of the SiC and graphite particles within the NZ. Pin profile geometry plays an important role in material flow at the weld zone [17]. In general, the pin stirs the material to avoid voids and to make complete joint. These actions will lead to an improvement in the mechanical properties, such as hardness, tensile strength and wear resistance (see Figs. 3 8). The higher improvement in the above-mentioned mechanical properties is encountered by using square pin profile geometry, since the square head pin tool has the smallest cross-sectional area followed by hexagonal head than the octagonal head for the same circle diameter in which these profiles are drawn. Thus, the frictional heat during the welding tool rotation of this smaller cross-sectional area of the square head pin will cause less heat input in the weld zone. This has significant importance in terms of properties such as fatigue, wear, and even corrosion [18]. The highest frictional heat input will be caused by the octagonal head pin. Accordingly, the microstructure of the NZ welded by the square head tool will have fine grains, because less frictional heat is encountered by this type of profiled tool, and when it is cooled by the surrounding air, there will not be enough time for the grains to grow, in contrast to the other two types of pins. Larger grain size will be found in the NZ welded by the octagonal head pin, as more frictional heat input will be developed, since, there is more time for the grain to cool to room temperature. This argument can also be applied to the hexagonal head pin, where the grains of the NZ are larger than those obtained by the square head pin but smaller than those obtained by the octagonal head pin, as its cross-sectional area is intermediate between the square and the octagonal head pins. According to the Hall Petch relationship [19], it can be stated that the smaller the grain size is, the better the improvement in the hardness, tensile strength, and wear resistance will be. The above discussion can be considered true with other welding parameters, i.e., welding speed and rotational speed, as the NZ will have smaller grain size when the welding speed is increased at a constant rotational speed as there will be smaller frictional heat input encountered within the weld causing small grains to be formed and an improvement in the considered properties (see Figs. 3, 5, and 7). But the increase in the rotational speed at constant welding speed causes more frictional heat to form within the NZ, and the material will take a rather long time to cool to room temperature, so that the grains will have time to grow. Thus, relatively large grains will be formed causing a reduction in the values of hardness, tensile strength, and wear resistance (see Figs. 4, 6, and 8). CONCLUSIONS The microstructure of the friction stir weld joint has great affect on the considered mechanical properties, i.e., hardness, tensile strength, and wear resistance, as the reduction of the grain size will cause an improvement in them, according to the Hall Petch relationship. In addition, the heat input caused by frictional forces is lower in the square head pin rather than the other two profiles of the welding tools, so that less growth in the grain of the NZ structure will occur during the cooling to room temperature. This means that the square head pin have more influence on the considered mechanical properties. It is important to note that smaller heat input developed in the NZ when there was an increase in the welding speed and=or a decrease in the rotational speed. So that, less time will be required to cool the NZ to room temperature, causing its structure to develop

6 EFFECT OF PIN PROFILE ON FRICTION STIR WELDS 1401 smaller grain size, which in turn increases the considered mechanical properties. The implications of the current study go beyond showing the ability of FSW method to join successfully aluminum matrix composites, but, also, studying the process parameters and understanding the effect of pin profile on the joints welded by FSW are of importance to many industrial applications. ACKNOWLEDGMENTS This work was supported by a grant from the Deanship of Scientific Research at Jordan University of Science and Technology (Grant No. 2010=195). The authors also would like to acknowledge all members of the Industrial Engineering Department workshops and laboratories for their help in using the machines and other available facilities. REFERENCES 1. Kallee, S.W. Friction Stir Welding at TWI; The Welding Institute (TWI): Cambridge, England, Thomas, W.M.; Dolby, R.E. Friction stir welding developments. Proceedings of the Sixth International Trends in Welding Research, Materials Park, ASM International, USA, 2003, pp Jata, V.; Semiatin, S.L. Continuous dynamic recrystallization during friction stir welding of high strength aluminum alloys. Scripta Materialia 2000, 43 (8), Mishra, R.S.; Ma, Z.Y. Friction stir welding and processing. Materials Science and Engineering 2005, 50, Krishnan, K.N. On the formation of onion rings in friction stir welds. Materials Science and Engineering 2002, A327, Kwon, Y.J.; Shigematsu, I.; Saito, N. Dissimilar friction stir welding between magnesium and aluminum alloys. Materials Letters 2008, 62, Storjohann, D.; Barabash, O.M.; Babu, S.S.; David, S.A.; Sklad, P.S.; Bloom, E.E. Fusion and friction stir welding of aluminum metal matrix composites. Metallurgical and Materials Transactions A 2005, 36, Nandan, R.; DebRoy, T.; Bhadeshia, H.K. Recent advances in friction-stir welding process, weldment structure and properties. Progress in Materials Science 2008, 53, Zhang, Z.; Zhang, H.W. Numerical studies on the effect of transverse speed in friction stir welding. Materials and Design 2009, 30, Zhao, Y.H.; Lin, S.B.; Qu, F.X.; Wu, L. Influence of pin geometry on material flow in friction stir welding process. Materials Science and Technology 2006, 22, Jones, M.J.; Heurtier, P.; Desrayaud, C.; Montheillet, F.; Allehaux, D.; Driver, J.H. Correlation between microstructure and microhardness in a friction stir welded aluminium alloy. Scripta Materialia 2005, 52, Kumar, K.; Kailas Satish, V.; Srivatsan, T.S. The role of tool design in influencing the mechanism for the formation of friction stir welds in aluminum alloy Materials and Manufacturing Processes 2011, 26, Suresha, S.; Sridhara, B.K. Effect of addition of graphite particulates on the wear behaviour in aluminum silicon carbide graphite composites. Materials and Design 2010, 31, Hassan, A.M.; Hayajneh, M.; Alrashdan, A.; Mayyas, A.T. Prediction of density, porosity and hardness in aluminum-copper based composite materials using artificial neural network. J. Materials Processing Technology 2009, 209, Zhang, S.; Wang, F. Comparison of friction and wear performances of brake material dry sliding against two aluminum matrix composites reinforced with different SiC particles. Materials Processing Technology 2007, 182, Mahmoud, R.I.; Takahashi, M.; Shibayanagi, T.; Ikeuchi, K. Effect of friction stir processing tool probe on fabrication of SiC particle reinforced composite on aluminium surface. Science and Technology of Welding and Joining 2009, 5, Zeng, W.M.; Wu, H.L.; Zhang, J. Effect of tool wear on microstructure, mechanical properties and acoustic emission of friction stir welded 6061 Al alloy. Acta Metallurgica Sinica 2006, 19 (1), Dehghani, K.; Mazinani, M. Forming nanocrystalline surface layers in copper using friction stir processing. Materials and Manufacturing Processes 2011, 26, Newell, J. Essentials of Modern Materials Science and Engineering; Wiley: Hoboken, NJ, 2009.