Effect of Processing Parameters on Friction Stir Welded Aluminum Matrix Composites Wear Behavior

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1 This article was downloaded by: [Jordan Univ. of Science & Tech], [Mohammed Almomani] On: 28 November 2012, At: 21:40 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 Processing Parameters on Friction Stir Welded Aluminum Matrix Composites Wear Behavior Adel Mahmood Hassan a, Mohammed Almomani 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: 21 Jun To cite this article: Adel Mahmood Hassan, Mohammed Almomani, Tarek Qasim & Ahmed Ghaithan (2012): Effect of Processing Parameters on Friction Stir Welded Aluminum Matrix Composites Wear Behavior, 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 Processing Parameters on Friction Stir Welded Aluminum Matrix Composites Wear Behavior Adel Mahmood Hassan, Mohammed Almomani, Tarek Qasim, and Ahmed Ghaithan Department of Industrial Engineering, Jordan University of Science and Technology, Irbid, Jordan In the present work, the aluminum matrix composites reinforced with both SiC and graphite particles are joined using a friction stir welding (FSW) process. The wear characteristics of the welded joints were investigated at constant load of 50 N and a rotational speed of 1,000 rpm using a pin-on-disk wear testing apparatus. This study focuses on the influences of the FSW processing parameters (tool geometry, rotational speed, and welding speed) on the wear characteristics of the welded joint of the considered hybrid aluminum matrix composite under dry sliding conditions. The experimental results indicate that the wear resistance of the joint increases at high welding speeds (>45 mm/min) and/or low value of rotational speeds. Different tool pin profiles (square, octagonal, and hexagonal) are developed to perform the welding process, and the effects of the tool pin profile on the weldments were studied. It is found that joints welded with square pin profile have better wear resistance compared to the other pin profiles. The results demonstrate that the FSW processing parameters greatly affect on the wear resistance of the welded joints due to various microstructural modifications during welding that cause an improvement in the welded zone hardness and wear properties. Keywords Aluminum; Composite; Friction; Wear; Welding. INTRODUCTION At present, there is continuously increasing demand for high strength and light weight materials in transport and aerospace industries. These materials are more energy efficient in terms of reducing fuel consumption. Aluminum matrix composite (AMCs) is one of the promising candidate materials which have great potential in the applications where a weight reduction is critical [1, 2]. They have outstanding attractive properties, i.e., inexpensive, low specific density, high strength, good mechanical, and wear properties [3, 4]. Both fibers and particles reinforcement can be used in the aluminum matrix of AMCs. SiC, Al 2 O 3, SiO 2, BN, and graphite are examples of the reinforcement particulates that used commonly in AMCs [5]. Even though, the great advantages of the AMCs, the difficulties associated with joining them together or with different materials using the conventional fusion welding techniques limit their use in the industrial applications [6]. Fortunately, a new solid state joining process called friction stir welding (FSW) succeed to overcome many welding difficulties associated with the conventional fusion welding processes [7]. Nowadays, welding and joining of AMCs to themselves or other materials become possible, and the market share for metal matrix composites (MMCs) has increased extensively [8]. Many of the products and the components made from AMCs are moving and sliding parts, e.g., brake drums, Received February 28, 2012; Accepted May 16, 2012 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 cylinder blocks, and drive shafts [9]. Due to the fact that the production of these components involves the use of various secondary operations, e.g., machining, joining, etc. Then, it is very important to fully establish the wear characteristics of the weld joint of AMCs in order to build a solid understanding of their behavior in service condition. The studies on the dry sliding wear behavior of aluminum alloys illustrate that the MMCs reinforced with hard particles are more resistive to wear than the corresponding monolithic alloy [9]. It was observed that the addition of SiC particulate to the aluminum matrix makes it harder, stronger, and more resistive to wear. This increase in the hardness will be at the expense of machinability. Therefore, graphite particles are used to maintain the gain achieved by SiC addition and concurrently improve machining as well [1]. These considerations were an encouragement to select the considered hybrid aluminum matrix composite with the two reinforcements particulate (SiC and graphite) in the present study. Therefore, the present work differs from most other earlier studies that focused on AMCs reinforced by only one type of reinforcement by ceramics particles. Optimization of the effect of the FSW processing parameters on the weld joint performance has been of the interest of many researchers [10]. Meran and Canyurt [11] observed that fine grain structure formed at higher welding speeds due to a lower heat input to the welding zone. In contrast, coarser grains generated at lower welding speeds as a result of the increased heat input [11]. Furthermore, several researchers studied the influence of the welding tool design parameters on the strength of the weld joint [12 15]. 1419

3 1420 A. M. HASSAN ET AL. Few researchers studied the wear resistance of FSW weld zone for Al based MMCs [16]. Hence, the present investigation is an attempt to assess the effect of the FSW processing parameters (tool profile, rotational speed, and welding speed) on the wear resistance of the considered welded hybrid composite joints under dry sliding conditions. EXPERIMENTAL PROCEDURE Plates of AMC with chemical composition shown in Table 1 (100 mm x 75 mm x 7.5 mm) were butt friction stir welded using a special fixture. This fixture was fixed firmly on the table of a conventional milling machine. The machine spindle equipped with a welding tool. The working parameters used for FSW are shown in Table 2. Samples from the nugget zone of the friction stir welded plates were cut in a length of 25 mm in the welding direction and 20 mm in the transverse direction. All samples were grounded with emery paper 800 and 1,200 grit size, and washed with water and alcohol to get rid off any left contamination. The effects of the FSW processing parameters (rotational speed, welding speed, and tool pin profile) on the dry sliding wear rate of the welded zone of the considered hybrid composite were examined. A pin-on-disk setup was used to investigate the dry sliding wear characteristics of the FSW of the aluminum metal matrix composites according to ASTM G99-95 standards. The size of wear specimens was 4 mm diameter and length of 25 mm, which were machined from the already taken samples of the nugget zone. The tests were conducted at constant load of 50 N and a rotational speed of 1,000 rpm. An electronic digital balance with 0.1 mg accuracy was used to measure the initial and final mass of the specimen. The masses of all specimens were measured before and after running. The test was carried out for three hours; then the specimens were removed from the wear testing machine and cleaned with alcohol, in order to remove all the attached worn particles, and then the mass of the specimen was measured to determine the mass loss. The wear rates were determined using the volume loss method, as indicated by the following equation [17]: W ¼ M q:s ; ð1þ where W ¼ volume loss during test period (cm 3 =m), M ¼ mass loss during wear test (g), s ¼ sliding distance (m), and q ¼ density of the composite as computed from the rule of mixture ¼ 2.7 g=cm 3. TABLE 2. Process parameters of friction stir welding (FSW). Pin profile tool Welding (transverse) speed (mm=min) Rotational speed (rpm) Square 35, 45, 55, , 800, 1,000, 1,250 Hexagonal 35, 45, 55, , 800, 1,000, 1,250 Octagonal 35, 45, 55, , 800, 1,000, 1,250 RESULTS Influence of the Tool Geometry on Wear Rate The wear resistances of the friction stir welded composite joints were examined using three different tool profiles (square, hexagonal, and octagonal). For every tool profile, the wear tests were conducted at four different welding speeds (35, 45, 55, and 65 mm=min), at different rotational speeds of 630, 800, 1,000, and 1,250 rpm. Figure 1 shows the variation of the wear rate with welding speed for various tool pin profiles when the rotational speed is maintained constant at 630 and 1,250 rpm. The horizontal line shown in Fig. 1 is drawn only for the purpose of comparison between the as-cast base AMCs and the welded zone. This line represents the nominal value of the wear rate at the base AMCs measured at various points. The results demonstrate that the tool geometry have significant influences on the wear resistance of the welded joint, as obtained by the analysis of variance (ANOVA) method [18]. In all considered cases, the welded joints produced by the square pin profile tool exhibits superior wear resistance to those joints welded by octagonal pin profile tools. The welded joints produced by hexagonal pin profile tool shows intermediate resistance values for wear. Accordingly, it was decided to use the square head welding pin in all the following works of this paper. Influence of the Rotational Speed on Wear Rate The effect of the rotational speed on the dry sliding wear resistance of the friction stir welded joints of the considered composite was investigated using the square pin profile tool, as mentioned earlier. The wear rates were measured at different rotational speeds 630, 800, 1,000, and 1,250 rpm, and at different constant welding (transverse) speeds of 35, 45, 55, 65 mm=min. The results, shown in Fig. 2, indicate that the wear rate increases with the increase in the rotational speed, for all the examined welding speeds. The joint welded at 630 rpm and 65 mm=min experiences the highest wear resistance among the other examined specimens. However, ANOVA results indicate the changes of the wear rate produced from different rotational speeds are not statistically significant [18]. TABLE 1. Chemical composition of the considered aluminum matrix composite (AMC). Cu Mg Si Fe Mn Ni Zn Ti Cr S C Al <0.01 <

4 FRICTION STIR WELDED ALUMINUM MATRIX COMPOSITES 1421 FIGURE 1. Wear rate (mm 3 =m) vs. welding speeds (mm=min) for square, hexagonal, and octagonal pin profiles at rotational speed: (a) 630 rpm and (b) 1,250 rpm. Influence of the Welding Speed on Wear Rate A set of experiments was conducted to measure the wear rate at different welding speeds (35, 45, 55, and 65 mm=min), while keeping the rotational speed constant at 630, 800, 100, and 1,250 rpm. Figure 3 shows the results obtained from the wear test. It seems that the maximum wear rate of the welding zone was achieved by applying welding (transverse) speed of 45 mm=min, with a rotational speed of 1,250 rpm. The lower and higher values of 45 mm=min cause a decrease in the wear rate of the weld zone. The wear rate increased 6% due to an incremental increase of the transverse speed from 35 mm=min to 45 mm=min, FIGURE 2. Wear rate (mm 3 =m) vs. rotational speed (rpm) at different welding speeds for FSW AMCs welded joint (square head pin tool). FIGURE 3. Wear rate (mm 3 =m) vs. welding speed (mm=min) at different rotational speeds for FSW AMCs welded joints (Square head pin tool). respectively. When the transverse speed was raised from 45 mm=min to 65 mm=min, the wear rate decreased dramatically. The sample welded using 630 rpm and 65 mm= min shows the lowest wear rate among the other examined specimens. In our previous study, ANOVA method shows that the variations of the wear rate due to changes of the welding speed are statistically significant [18]. Accordingly, it can be said, that the wear rate decreases with the increase in welding speed at constant rotational speed, and it increases with the increase in rotational speed at constant welding speed. DISCUSSION Influence of the Tool Profile on Wear Rate The results show that the wear resistance of the friction stir welded joint of the considered composite is affected by the use of different tool pin profiles. The welded joint produced by the square head shows the highest wear resistance. This could be attributed to the mechanical stirring employed, and the total frictional heat input by different tool pin profiles developed in the weld zone. Since the square pin cross-section is the smallest among the other considered pin profile, when they are all drawn in the same circle, so that less heat input will be developed in the weld zone, and thus, a fine grained microstructure will form due to recrystallisation [19], further heat will cause grain growth stage to occur during the recrystallization process, causing the formation of a coarser grain microstructure, when the other two profiles are used, which is consistent with our findings as shown in Fig. 4, in which superior grain refinement is achieved upon using square pin profile. The development of the fine structure will result in an increase in the strength and hardness, and the weld zone will be more resistive to wear as stated by Archrd s law [20]. Influence of the Rotational Speed on Wear Rate It was found that wear rate of the friction stir welded joints of the considered composite increases with the increase in the rotational speed. This increase is a consequence of the microstructure coarsening in the weld zone, as the process of recrystallisation occurs, due to

5 1422 A. M. HASSAN ET AL. FIGURE 4. Effect of tool pin profiles on FSW zone microstructure using (a) hexagonal, (b) square, and (c) octagonal pin profile tools at rotational speed of 800 rpm and welding (transverse) speed of 55 mm=min with magnification of 500. FIGURE 5. Effect of rotational speed on the zone microstructure at rotational speed of 630, 800, 1,000, 1,250 rpm and welding (transverse) speed of 55 mm=min with magnification of 500 (color figure available online). FIGURE 6. Effect of welding (transverse) speed on the zone microstructure at welding (transverse) speed of 35, 45, 55, 65 mm=min and rotational speed of 800 rpm with magnification of 500 (color figure available online). the relatively high frictional heat input generated at high rotational speed; this coarsening effect is also observed in our experiments, where substantial grain refinement in weld zone is obtained upon using low rotational speed, as shown in Fig. 5. It is believed that the stage of grain growth will be reached during the recrystallization process; due to the high temperature developed by increasing the rotational FSW speed leading to a relatively coarse grain microstructure. Also, the high stirring rate will lead to the formation of some voids and cracks at the coarse matrix=sic and graphite particle interface [9], which assist the releasing and detaching of the reinforcement particles during sliding conditions, causing the wear resistance to deteriorate. Influence of the Welding Speed on Wear Rate The dry sliding wear resistance of the friction stir welded joints of the composite is improved by conducting the welding process at higher welding speed. This improvement results from the reduction of the developed heat per unit area of the joints during welding with the increase in the welding speed, and the increase in the cooling rate which leads to the formation of a fine grain structure after recrystallisation, therefore hardness increases, and the wear resistance improved. The change in the microstructure with the welding speed is shown in Fig. 6. For example, the average grain size is reduced almost into half of its size as the welding speed changes from 35 mm=min to 65 mm=min. CONCLUSIONS The present study investigated the influence of some process parameters of FSW technique on the dry sliding wear characteristics of the considered hybrid composite

6 FRICTION STIR WELDED ALUMINUM MATRIX COMPOSITES 1423 weld joint. The main conclusions are summarized as follows: 1. The wear resistance of the friction stir welded joints is improved compared to the base composite. 2. The wear rate of the welded joint increases at high rotational speeds. The microstructure coarsening is responsible on the reduction of the wear resistance of the welded joints at high rotational speeds. Also, the high stirring rate will lead to the formation of some voids and cracks at the coarse matrix=sic and graphite particle interface, which assist the releasing and the detaching of these particles during sliding causing a reduction in the wear resistance of the composite. 3. The welding speed has a great influence on the wear characteristics of the welded joint. As the welding speed increases less heat will be encountered in the zone to be welded causing a fine grained microstructure to be formed. This will lead to an increase in the strength, hardness, and a reduction in wear rate of the weld zone. 4. The dry sliding wear characteristics of the friction stir weld joint is affected by the pin profile tool type, where the joints fabricated using the square pin profile tool reveals higher resistance against wear than joints fabricated using either hexagonal or octagonal pin profile tool. This tool profile induces less frictional heat than the other two profiles, which, in turn, enhances fine grains structure formation, causing an increase in the strength, hardness and the wear resistance of the welded joint. ACKNOWLEDGMENT 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. 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