The effect of welding parameters on the mechanical and microstructural properties of friction stir welded dissimilar AA 3003-H 24 and 2124 / SiC / 25 pT 4 alloy joints

Assemblies consisting of aluminum and particle reinforced aluminum composite alloys have wide spread applications in construction and automotive industries. For a successful use of such dissimilar joints, the welds must have sufficient strength and formability. In the present study, 3003-H24 aluminum alloy and 2124/SiC/25p-T4 composite plates were successfully friction stir butt joined by offsetting the high strength plate to retreating side of the tool under tool rotational speed of 900 to 1400 rpm and traversing speed of 40 to 125 mm/min, keeping other parameters constant. Ultimate tensile strength, percentage elongation and joint efficiency values of the welded joints and hardness variations across the weld interface were determined. The integrity of the joints was investigated using optical microscopy, scanning electron microscopy and energy dispersive spectroscopy. Results indicated that the high quality welded joints could be obtained when a tool rotational speed of 900 rpm and a traverse speed of 125 mm/min were employed. The maximum tensile strength value obtained was about 182 MPa, which corresponds to a joint efficiency of 104% of that of the aluminum alloy base metal.


INTRODUCTION
Aluminum based metal matrix composites (Al-MMCs) possess improved specific stiffness and strength-toweight ratio at room and elevated temperatures, excellent fatigue properties, high formability and improved wear resistance, and greater thermal stability in respect to the corresponding unreinforced matrix alloys, thus finding applications mainly in the aerospace, automotive and motorsport fields (Uzun, 2007;Rotundo et al., 2010).Similarly, aluminum and its alloys are important metallic materials used in various areas.Due to its low cost, high specific strength, high thermal conductivity as well as *Corresponding author.E-mail: ybozkurt@marmara.edu.tr.Tel: +90 216 336 57 70/357.Fax: +90 216 337 89 87.good corrosion resistance, 3003 Al alloy has been widely used for automotive heat exchangers to replace more traditional materials like stainless steels and copper alloys (Mei-jun et al., 2010).Joining of these dissimilar materials is becoming increasingly important in industrial applications due to their numerous advantages.These include not only technical advantages, such as desired product properties, but also benefits in terms of production economics.Therefore, the existence of suitable joining techniques for dissimilar materials is indispensable.However, dissimilar metals are difficult to be joined with conventional fusion welding methods due to their different chemical and physical characteristics, thus solid state joining methods have received much attention in recent years (Xue et al., 2010).The conventional fusion welding of aluminum and its alloys has always been a great challenge for designers and technologists.The difficulties associated with fusion joining are mainly related to the presence of a tenacious oxide layer, high thermal conductivity, high coefficient of thermal expansion, solidification shrinkage and high solubility of hydrogen and other gases in molten state.
The fusion welding of aluminum alloys leads to the melting and re-solidification of the fusion zone which results in the formation of brittle inter-dendritic structure and eutectic phases.The formation of brittle structures in the weld zone leads to a drastic decrease in the mechanical properties, such as in hardness, strength and ductility (Shanmuga and Murugan, 2010;Çam et al., 1999, 2000;Çam and Koçak, 2007) in fusion welding processes.These problems are not encountered in friction stir welding because it is a solid-state process where the maximum process temperature does not reach the melting point of the welded materials (Leal and Loureir, 2008).Wert (2003) and Sharifitabar and Nami (2011) observed formation of grain boundary films during dissimilar friction stir welding of 2024 to 2014 AA reinforced with 20 vol% particulate Al 2 O 3 and 2024 aluminum alloy to Al/Mg 2 Si metal matrix cast composite, respectively.Wert stated that overheating may be attributed to the unusually high tool rotation rate used in the dissimilar welds for the purpose of adequately softening of two base materials.In addition, the heat input from friction/plastic deformation increases with increasing flow stress.He concluded that because the flow stress of the MMC is higher than that of the monolithic alloy, the heat input for the welds is likely to be higher than in the case of two monolithic 2024 aluminum alloy plates welded using the same conditions.Both factors favor higher temperatures in the dissimilar 2024AA to 2014 AA/Al 2 O 3 composite weld.So, in FSW of dissimilar welds, some liquid metal may forms in some situations.Friction stir welding (FSW) patented in 1991 by the Welding Institute in the United Kingdom (Liu et al., 2009), has several advantages such as a lower welding temperature and shorter welding time than conventional welding methods.Therefore, FSW is potentially a practicable joining process for dissimilar materials (Tanaka, 2009a).In FSW, a rotating tool is fed through the joint between the two plates to be welded.The tool rotation promotes mechanical (solid-state) mixing of the materials on the advancing and retreating sides of the weld.In this process, joining of the materials is facilitated by severe plastic deformation in the solid state, involving dynamic recrystallization of the base material (Somasekharan and Murr, 2004).Recently, some reports have been available on the friction stir welding of dissimilar alloys by FSW, such as joining of different Al alloys (Shigematsu et al., 2003;Wert, 2003;Ghosh et al., 2010;Khodir and Shibayanagi, 2008;Cavaliere et al., 2009a), aluminum to steel (Chen and Kovacevic, 2004;Tanaka, 2009a), Ti and aluminum to stainless steel (Najafabadi et al., 2010;Uzun, 2005), aluminum to magnesium (Yong et al., 2010), aluminum to copper (Liu et al., 2008), copper to brass alloy (Barlas and Uzun, 2010).However, a study on dissimilar joining of AA3003-H24 to AA2124 containing 25 vol% SiC particles reinforcement metal matrix composite (AA2124/SiC/25p-T4 MMC) by FSW has not been reported to date.
In the present study, these dissimilar two plates were joined successfully by FSW.The mechanical properties and microstructural evolutions in the dissimilar Al/MMCs joints obtained by employing different parameters were determined.The results obtained provide an important fundamental for further studying of FSW Al/MMCs dissimilar materials.

MATERIALS AND METHODS
The cold rolled 3003 aluminum alloy produced by ASSAN (TR) and AA2124/SiC/25p MMC plates produced by Aerospace Metal Composites Ltd (UK), with dimensions of 50 × 150 × 3 mm (3003 in H24 and 2124 in the T4 condition) were used in this study.The chemical compositions of the plates are given in Table 1.3003-H24 Al alloy and AA2124/SiC/25p-T4 MMC were successfully butt joined by friction stir welding with different tool rotation speeds (900,1120 and 1400 rpm) and traverse speeds (40-80-125 mm/min) with a constant tilt angle of 2° which were chosen according to the optimum results obtained for AA2124/SiC/25p-T4 MMC plates (Bozkurt et al., 2011a).The dissimilar joints were produced with the high strength alloy (AA2124/SiC/25p-T4) positioned on the retreating side and the AA3003-H24 (the softer one) on the advancing side.This plate positioning was chosen based on the consideration that the microstructure and the mechanical properties of the stirred zone are influenced by the retreating side of the joint in order to obtain defect-free joints.Such dependence of the strength on the material position was previously observed for dissimilar AA6082/AA2024 (Cavaliere et al., 2009a) and AA6061/A356 alloys (Lee et al., 2003).The max. tensile properties were obtained for these joints with the high strength Al alloys positioned on retreating side of the weld.All the FSW trials were performed with an FSW-adapted universal milling machine.The FSW tool was made of high speed steel with shoulder diameter 20 mm, pin diameter 6 mm and cylindrical pin length 2.8 mm. Figure 1 shows the dissimilar FSW process.The rotating tool pin along with the shoulder during its travel produces frictional heat with Figure 1.Dissimilar friction stir welding process: a) rotating tool prior to penetration into the butt joint; b) tool shoulder makes contact with the part, creating heat; c) restricting further penetration while expanding the hot zone and moving parts under the tool, creating a friction stir weld nugget; d) retraction of the tool from the joining zone.temperature below the melting point and plastic deformation due to the stirring of the material around the pin, forming the weld (Gopalakrishnan and Murugan, 2011).After FSW process, the joints were sectioned perpendicular to the welding direction by wire electric discharge machining device for metallographic and mechanical investigations.
The welded specimens were polished with 9 and 1 µm diamond paste according to ASTM E3 (ASTM E3-01, 2007), standard and chemically etched with Keller's reagent.Microhardness (HV0,5) tests were performed on the cross-sections of the welded joints both at the top and root locations with a 1 mm interval using 500 g load for 20 s to determine the hardness variations from base material to stir zone.The tensile tests were also carried out at room temperature according to ISO/TTA2 standard (ISO/TTA2, 1997) using universal type tensile test machine to determine the tensile properties of the joints.At least three specimens were tested under the same conditions to guarantee the reliability of the results.Microstructural and chemical analyses were performed from the cross-sections of FSWed joints by optical microscope (OM) and scanning electron microscope (SEM).The distributions of chemical elements in the stir zone were analyzed by energy dispersive X-ray spectroscopy (EDS) system.

Appearances
In the present study, 3003-H24 Al alloy and AA2124/SiC/25p-T4 MMC plates were successfully buttjoined by the FSW.Although some prosity was formed in the weld area of the dissimilar joint produced at low transverse speed (that is 40 mm/min), the other joints exhibited no visible superficial defects, which are generally observed in the welded zone of the composite and Al joined using conventional fusion arc welding processes (such as porosity, reinforcement segregation  or carbide formation) (Ceschini et al., 2007a).In addition, the surface morphology of FSW zone became smoother with increasing tool transverse speed.Figure 2 shows the top and root surfaces after FSW of dissimilar Al alloy and MMCs with different tool rotation and welding speeds.

Microstructure
The macroscopic cross-sections of different welded zones are shown in Figure 3.In these cross-sections, typical features of the stir zones of dissimilar Al/MMC FSW joints are observed.The weld zone consists of several characteristic areas.All the dissimilar welds exhibit different microstructural zones from the other welding techniques (Sarsilmaz et al., 2010a) as shown in Figures 3a and b. Figure 3 shows the weld regions of two of the joints obtained, one exhibiting the highest mechanical properties (that is, 900 rpm tool rotation speed and 125 mm/min tool transverse speed: 900/125 rev/mm welding parameters) and the other displaying the lowest results (that is, 1400/40 welding parameters).forward movement extrudes metal from advancing side (AS) to the retreating side (RS) of the tool.Other authors have reported a similar behaviour and related it to a partial recrystallization in the stirred zone (Ceschini et al., 2007b).Feng et al. (2008) found that the onion rings in the stir zone of the friction stir welded AA2009/SiCp composite included small particle rich-bands and large SiC particles did not segregate in the onion rings.Nami et al. (2011) showed that in frction stir welding of Al/Mg 2 Si cast composite, onion rings in the stir zone consisted of alternating bands of needle shape eutectic Mg 2 Si phase and Mg 2 Si particles.There was no large Mg 2 Si particle in the bands of one pass welded samples.Figures 5 and 6 show SEM micrographs of the dissimilar Al/MMC FSW joints produced at 900/125 and 1400/40 welding parameters at different locations of the joint.The microstucture of AA2124/SiC/25p-T4 BM, as shown in Figure 5g, consists of the AA2124 aluminum alloy matrix with an almost uniform distribution of coarse (<6 µm) and fine grained (<3 µm) silicon carbide particles (SiC p ) (Bozkurt et al., 2011a).
The stir zone which appeared to be composed of different regions of both the Aluminum and MMC alloys experienced the highest strain and undergoes recrystallization (Cavaliere et al., 2009a) exposure to high stresses and the heat produced by stirring tool is responsible for the plastic deformation which results in the rearrangement of the particles.Aluminum and SiC particles were homogeneously distributed in the stir zone and the microstructure of this zone was characterized fine equiaxed recrystallized grains (Bozkurt et al., 2011a) as shown in Figures 5d and 6e.This recrystallized microstructure is formed as a result of the mechanical action of the tool probe that generates a continuous dynamic recrystallization process.From this point of view, some researchers demonstrated that the microstructure in the stir zone evolves through a continuous dynamic recrystallization process (Cerri and Leo, 2010).The higher temperature and the severe plastic deformation during the weld in the stirred zone result in a recrystallized equiaxed grained structure with a grain size much finer than BM.Although tool wear took place in the similar butt joining of AA2124/SiC/25p-T4 MMC plates at the welding parameters of 900/100 causing the formation of Cu 2 FeAl 7 phase (Bozkurt et al., 2011b), no evidence of porosity and particle cracking or tool wear was observed in the stir zones of dissimilar joints, (Figures 5d and 6e).TMAZ regions (Figures 5c and e, 6b and d), which are adjacent to the stir zone from both the advancing and the retreating sides, were also plastically deformed and thermally affected.The strong stirring effect can be advantageous on the homogeneity of the particles distribution.However, the TMAZ region of 3003-H24 Al alloy in the joint produced at 1400/40 welding parameters showed a non-uniform distribution of SiC particulates.
The HAZ (Figures 5b, f, 6a and c), between TMAZ and BM regions both at the advancing and retreating sides, exhibited a microstructure similar to the BM.

Microhardness of FSWed dissimilar Al/MMCs plates
The hardness profiles were evaluated by taking averages of both the top and root side of the FSW joints as shown a lower hardness value in the stir zone in comparison to MMC base material because of differences in the thermal expansion between Aluminum and MMC alloys rapid heating and cooling will give rise to the generation of dislocations at the matrix-SiC interface (Bozkurt et al., 2011a).The hardness decrease from the base material to the FSW zone was also observed in the unreinforced 6061 aluminium alloy, even if with a grain refinement of the aluminium alloy matrix (Liu et al., 1997).The refinement of the SiC particles and the grains contributed to the increase up to 170 HV in the hardness at the retreating side and 70 HV at the advancing side of the stir zone (Figure 7).The hardness values at the retreating side of the weld was also determined for the similar FSW joints of MMCs (Bozkurt et al., 2011a).The maximum hardness value was obtained in the joints produced at the traverse speed of 125 mm/min and tool rotation rates of 900-1400 rpm as shown in Figures 7 to 9. In all the conditions, the minimum hardness value is measured at the retreating side of the HAZ region due to the differences in the dislocation density (Storjohann et al., 2005), accelerated ageing and recovering processes caused by the weld thermal cycle as reported by Threadgill (1997) and an annealing effect (Cavaliere et al., 2009a).This is because of the fact that the HAZ region has been deformed very slightly, and has different thermo-mechanical behaviour with respect to the stir zone and the TMAZ.Some researchers showed such effect by analyzing the reinforcing particles behaviour about dimension and aspect ratio in the FSW Al alloy (Cavaliere et al., 2009a).The hardness at the advancing side (AA3003-H24 side) of the HAZ region is higher than TMAZ because of thermal softening and frictional heating in this region, with the except in sample produced with 1400/40 rev/mm welding parameter as shown in Figure 9.In the joints produced with this welding parameter, the hardness increased up to 80 HV at the advancing side of the TMAZ region.This is attributed to the thermal effect, microstructural modifications and orientation of SiC particulates from stir zone to Al side (Figure 6b).The thermal effect and microstructural modifications occurring during the FSW joints depend on the different welding circumstances.(Sarsilmaz et al., 2010a;Cerri and Leo, 2010).

Tensile test results
The tensile test results and fracture locations of the dissimilar FSWed joints produced with different welding parameters are summarized in Table 2. Consistent and repetitive results were obtained from the tensile tests conducted.Most of the specimens extracted from the joints produced with the 900/40-80 and 1120-1400/80-125 welding parameters fractured in the HAZ region of AA3003-H24 side and the others fractured within the BM of AA3003-H24, except the joint obtained with 1400/40 welding parameter, which is fractured along TMAZ of AA3003-H24 side.The fracture surfaces of the joints produced with 900/125 and 1400/40 welding parameters after the tensile testing are shown in Figures 10 and 11.The tensile strength and percent elongation values increased with the increasing tool transverse speeds at constant tool rotational speeds.The maximum tensile joint efficiency value was obtained from the joint produced at 900/125 welding parameters, that is 104% (that is UTS FSW /UTS base material x 100), which was higher than the tensile strength of AA3003-H24 BM but much lower than MMC (40.2%).On the other hand, the elongation value of this joint was higher than both base plates especially MMC.
The lowest tensile joint efficiency and percent elongation values were obtained from the joint produced with 1400/40 welding parameters.The tool transverse speed had a strong effect on the weldability of FSWed dissimilar alloys.The softened area was narrower for the high tool transverse speed than for the lower tool transverse speed.Thus, the strength of joints is strongly affected by transverse speed as the case for simmilar MMC joints (Sarsilmaz et al., 2010a).So, it can be concluded that the welding parameters are very influential on the joining efficiency.The reasons for obtaining different joint efficiency values for the Al/MMC dissimilar joints are obviously complex.One of the reasons is the different peak temperatures the material experiences during FSW as a result of various welding parameters.Another reason is the temper condition, that is aged (Bozkurt et al., 2009) or solution treated.The temper condition plays a significant effect on the mechanical behaviour of the joints (Çam and Koçak, 1998;Çam, 2011).It is well known that the reinforcement particles have a significant influence on the strength of the MMCs.Because of the SiC particulates nonuniform distributed at the TMAZ region of AA3003-H24 in this zone ultimate tensile strength and elongation is reduced down to 162 MPa and 8.4%, respectively.

Fracture surface analysis
The fracture surfaces of two dissimilar joints, one being with the highest joint efficiency and the other with the lowest joint efficiency, were examined using SEM micrographs to identify the fracture mechanisms as shown in magnifications of these fracture surfaces in these two samples.The results of EDS analysis conducted on the joints produced are also presented.SEM analysis results indicated that ductile fracture of the Al matrix took place, which had an excellent cohesion with the reinforcement particles (Ceschini et al., 2010).The specimens extracted from the joint with the highest joint efficiency exhibited a typical ductile mode of failure characterized by the presence of dimples known as microvoid coalescence.
The specimens extracted from the joint with the lowest efficiency, on the other hand, displayed a mixed mode of fracture, consisting of ductile and brittle fracture, but generally brittle fracture.

Conclusions
The effect of the welding parameters on the microstructural and mechanical properties of FWSed dissimilar aluminium alloys joints was investigated in this study.The results obtained can be summarized as follows: 1) AA 3003 H24 and 2124/SiC/25-T4 dissimilar alloys plates were successfully joined by FSW without any visible superficial porosity or macroscopic defects on the top and bottom regions of welded samples, except the one produced with a traverse speed of 40 mm/min.
2) In the SEM examination, no evidence of porosity and article cracking or tool wear was observed in the stir zones of the welds.
3) All the microhardness profiles of the dissimilar Al/MMCs FSW joints exhibited a lower hardness in the stir zone.The hardness in the advancing side (AA3003-H24 Al side) of the HAZ region is higher than TMAZ because of thermal softening and frictional heating apart the dissimilar Al/MMCs FSW joint produced with 1400/40 welding parameters.In this welding parameter, the hardness increased up to 80 HV at the advancing side of the TMAZ region and thus a weaker joint efficiency value was obtained.4) Tensile test results indicated that high quality dissimilar Al/MMC joints could be obtained by FSW with 900/125 welding parameters.5) The joint with the highest joint efficiency exhibited a typical ductile mode of failure.The joint with the lowest joint efficiency, on the other hand, displayed a mixed mode fracture containing both ductile and brittle fracture, but generally brittle.

Figure 2 .
Figure 2. Top and root surfaces after FSW of dissimilar Al alloy and MMCs.

Table 2 .
Tensile test results and fracture locations of the dissimilar FSWed joints.