Effect of Modified Pin Profile and Process Parameters on the Friction Stir Welding of Aluminum Alloy 6061-T6

This work deals with the effect of a new “bolt-head” pin profile on the friction stir welding performance of the aluminum alloy 6061-T6, compared to traditional pin profiles. Friction stir welding parameters such as the tool rotation speed and the welding speed were investigated together with the different pin profiles; the results show that the new “bolt-head” pin profile leads to better mechanical properties of welded specimens.The pin profiles used in this work were the straight square (SS), straight hexagon (SH), taper cylindrical (TC), and the straight hexagon “bolt-head” (SHBH). It was found that the last pin profile improves the material flow behavior and the uniform distribution of plastic deformation and reduces the formation of macroscopic defects on the welded zone. Mechanical tensile tests on welded specimens were performed to determine the tensile strength: the specimens welded with the SHBH pin profile have shown the highest mechanical properties. An approach is presented for material flow on this aluminum alloy using the SHBH pin profile, which is related to the improvement on the resulting mechanical properties.


Introduction
The alloy 6061-T6 is the most popular and used of all aluminum alloys; it presents an adequate balance of mechanical properties, cost, and weight. This alloy is used for aerospace and automotive industries under joining and welding requirements [1][2][3][4]. The traditional fusion welding methods, such as MIG and TIG, are characterized by high heat dissipation and the transformations of microstructural phases, leading to a number of defects (microporosities, less fine equiaxed recrystallized grain) and thermic instabilities (hardening precipitates), on the joining elements [5,6]. For aeronautical application of the 6061-T6 alloy, the laser welding leads to high mechanical properties (reduction of FZ and HAZ, even if porosity may be promoted) and good finishing [7,8]; however, the cost is a principal constraint for this welding method.
Friction stir welding is a solid state welding technique which has been developed at the end of 1991 by "The Welding Institute" in the United Kingdom [9]. A nonconsumable rotating tool communicates energy to the two facing surfaces, which may be of the same metallic alloy or different metallic alloys. The rotating movement and the related friction generate heat dissipation between the two facing surfaces, leading to softening of this region and facilitating the high shear plastic deformation and mixing of the material [10][11][12].
It has been recognized that the principal parameters affecting the bonding strength of FSW and the resulting microstructure are the rotational speed of tool, the welding speed, the axial tool force, and the angle of tool in regard to the welding surface (tilt angle) [13,14].
The application of this joining technique has been widely used for the aluminum alloy 6061-T6 [15][16][17][18][19]; nevertheless, in this work is presented the effect of a new profile of the pin tool 2 Advances in Materials Science and Engineering to improve the mechanical properties on this material, after the friction stir welding.
The pin profile is a parameter which plays important role during the FSW process, affecting the welding temperature, plastic deformation, the material mixing, the grain size, the welding defects, and so forth [20][21][22]. Some different pin profiles have been used to investigate their effects on the mechanical properties of welded materials using FSW [23][24][25]. Furthermore, recent investigations are oriented to the determination of the physical and mechanical effects of pin profiles: a model has been proposed for a taped cylindrical pin profile [22], which proposes equations for the heat generation: (a) from the shoulder surface, (b) from the taper probe surface, and (c) from the pin tip surface. The heat generation integral expression for the taper probe surface is with the solution where is the angular velocity, contact is the contact stress, probe is the vertical length of taper cylindrical pin, is the taper angle, PS is the bottom radius of the pin, PT is the top radius of the pin, and is the inclined length of the pin: = probe / cos .
Equation (1) is obtained considering independent of (along the vertical axis of the pin), which is not the case. Here, we propose as a function of z: = PT − tan ; the integral equation (1) becomes With probe tan = PT − PS , the solution is Equation (4) gives values slightly lower compared to (2) for the taper probe surface. The principal conclusion of [22] is that temperature registered during FSW process decreases with the taper angle.
Analytical models for heat generation were obtained for the pins: triangular, square, pentagonal, and hexagonal profiles [26], with the following principal conclusions: (a) heat generation increases from the triangular to square profile; then, it decreases to hexagonal pin profile; (b) increase on the tool rotation speed under constant weld speed induces heat generation increase; (c) the increase on the weld speed, under constant tool rotation speed, leads to decrease on heat generation. These conclusions are not in the same direction in regard to the results obtained for straight and taper cylindrical pin profiles [27], in which heat generation increases with angular speed (the tool rotation speed) and  with the tangential speed (welding speed). Recently, small tool pin profiles have been investigated using FSW on the 6061 aluminum alloy [28], with better results on mechanical properties for the triangular pin profile compared to the threaded tapered cylindrical and square pin profiles.

Experimental Set-Up and Procedure
The chemical composition in weight and the principal mechanical properties of aluminum alloy 6061-T6 are shown in Tables 1 and 2, respectively. Plates of dimensions 200 mm × 150 mm × 9.5 mm were used for the FSW purpose. The classification of the FSW tests according to the welding parameters for this work is shown in Table 3, whereas in Table 4 are listed the pins profiles and their dimensions.
Heat generation is increased with the shoulder diameter because the area of friction is increased; the effect of this parameter was not studied in this work. Figure 1 presents the four pin profiles used in this investigation; the PSHBH nomination represents the "bolt-head" pin, previous to manufacture of the bolt-head at the end of the pin. The dimensions of the "bolt-head" pin are shown in Figure 2, whereas in Figure 3 are depicted the welding plates rigidly clamped to the machine bed without any obstruction for the rotation and translation motion of the pin. The alignment of welding line and pin center was maintained close to zero deviation and all tests were carried out with 0 degrees for the tool tilt angle. The FSW process was implemented in a vertical machining center: Dyna Myte/DM-4800 equipped     with Mitsubishi controls and a maximum tool rotation of 6000 rpm.
The testing specimens were obtained from the welded plates in perpendicular direction in regard to the FSW line for the tests: tensile, hardness, and the microstructural observation by optical microscope. Tensile specimens were manufactured according to the norm ASTM-E8M, with a gauge length of 64 mm, a gauge width of 6 mm, and a thickness of 9.5 mm, as shown in Figure 4.
All tensile tests were carried out on a tension machine model Zwick/Roell Z100 with 100 KN of maximum test load and at constant test displacement of 6 mm/min. The microhardness Mitutoyo model HM 200 was used to obtain the hardness profiles along the welding zone with an applied load of 0.1 Kg and 10 sec of time application. In addition, the FSW welding process was obtained under the following condition: the distance between the tip of pin and the bottom of the welding workpiece was 0.3 mm.

Results and Discussions
Plates of thickness 9.5 mm of aluminum alloy 6061-T6 were joined by FSW technique using classical pin profiles (SS, SH, and TC) and the new pin profile "bolt-head" to compare the results on welding and mechanical properties [29]. In Figure 5 are illustrated the different zones, from 1 to 8, along the welding line from which the tensile specimens have been machined. This figure shows the welding direction as well as 4 Advances in Materials Science and Engineering    the advancing and retreating sides and the direction of tool rotation. The tensile specimens were obtained from the zones 2 to 4; the zones 1 and 8 were eliminated for this purpose. Figure 6 presents the macrostructure of the transverse welding zone for the different used pin profiles in function of the rotational  last two velocities and using the pin profiles TC and SH, the ultimate tensile stress decreases as shown on the same table.
The maximum ultimate tensile stress is obtained with the SHBH pin profile (176-177 MPa), using high values for the two weld speeds.
Concerning the tunnel defect in the welded specimens, it is present for the three pin profiles, SS, TC, and SH, as shown in Figure 6; nevertheless, this defect is eliminated using the SHBH pin profile for all rotating and welding used speeds, as depicted in the same figure. An attempt to explain the causes that induce the elimination of tunnel defect with the use of SHBH pin profile is undertaken in the last section. These results seem to agree with the physical conditions close to the contact zone between the pin and the welding material: the heat generation increases when the tool rotation speed increases; then, the flow of material across the welding thickness increases too. For tapered pin profiles particularly, the contact area of pin decreases with the depth of the welding thickness and the flow of material increases across the thickness, contributing to tunnel defect.
In Table 5 are listed the tensile tests for the welded specimens using the four pin profiles, as well as the tensile tests on the base material (last three specimens in this table). The ultimate tensile stress for the welded specimens ranges from 108 to 177 MPa, whereas for the base material these values range from 308 to 316 MPa [30,31]. Figure 7 presents the tensile tests graphs for the welded specimens and the base material with the following identification in regard to Table 5 . In all cases for tensile tests on welded specimens, the higher values for UTS were obtained with the SHBH pin profile. Furthermore, the fracture sites under tensile tests of welded SHBH specimens were located outside the welding zone, as shown in Figure 8. Figure 9 presents the Vickers microhardness measured along the weld cross-section comprising the zones: the nugget zone (NZ), the thermomechanical heat zone (TMHZ), the heat affected zone (HAZ), and the base metal (BM), for the welded SHBH specimens. The higher values of this mechanical property have been obtained for low rotation speed during the FSW process and intermediate values for welding speed; this tendency has been mentioned by some authors [32,33], for aluminum alloys.
Fracture surfaces for the tensile specimens are shown in Figure 10: the corresponding fracture surface for specimens containing the tunnel defect is depicted in Figure 10(a), whereas the fracture surface for specimen without the tunnel defect is presented in Figure 10(b). Figure 10(c) shows the fracture surface for the base material. Low plastic deformation characterizes the separation of fracture surfaces for specimens with tunnel defect, revealing that low resistance is opposed to tensile loading: this behavior is represented by the tensile tests W2-3 and W3-3 in Figure 7.
The increase of plastic strain is related to the ductile fracture and the microvoids coalescence, which is observed on this aluminum alloy for tensile specimens presenting higher resistance, as illustrated on Figures 10(b) and 10(c). In this case, the peripheral plastic deformation zone increases with the UTS of tensile test on this material, as it is observed in the last two figures. The central zone of fracture surfaces for specimens without tunnel defect is typified by low or null plastic deformation and a granular nondeformed microstructure [34].
The material flow during the FSW process is a principal factor influencing the presence of welding defects, such as cavities, root flaws, and kissing bond [35]. Generally speaking, the material flow in FSW is determined by three different motions: (a) the material flow beneath and deeper of the tool shoulder, from and to the root of the probe, (b) the flow of material along the pin surface, downward in the vertical direction, and (c) the material flow from the bottom of pin towards the shoulder, away from the pin surface; this flow interacts with the two precedents flows [36,37].
A swirl is present in the advancing side which increases with the welding speed and induces the radial motion illustrated, whereas in the retreating side the radial motion is considerably reduced and may change direction according to the welding parameters and the testing material [38,39].    In Figure 11 are represented the three mentioned flows, contributing to the flow of material during the welding process by FSW.
The flow of material along the pin surface and in the downwards vertical direction is reduced using the SHBH pin  bolt-head at the end of the pin reduces considerably the material flow in vertical direction: the mechanical effect is the reduction of discontinuities on the material with the rotating and the advancing motion, which leads to reduction of tunnel and other defects in this aluminum alloy. The vertical flow is associated with the continuity of material along the vertical direction during local FSW stirring; the reduction of this flow by the pin bolt-head induces the reduction of material discontinuity and hence welding defects.

Conclusion
In this work is presented a new pin profile denominated bolthead, for the friction stir welding of the aluminum alloy 6061-T6. The tensile and hardness tests show that this pin profile improves such mechanical properties of the welded alloy. Additional conclusions related to this work are as follows: (a) No tunnel defect was observed on the welded specimens using the SHBH pin profile, for the three rotating speeds and two welding speeds of experimental testing. (b) A modified expression is proposed to calculate heat generation from the taper probe surface. (c) The fracture for tensile specimens, using the SHBH pin profile, took place systematically outside the welding zone. (d) The Vickers microhardness along the welding zone for the SHBH welded specimens was higher when using low rotational speeds of tool, for the testing welding speeds [40]. (e) Low plastic deformation is observed for the fracture surfaces of specimens with tunnel defect, indicating that low resistance is opposed during the tensile tests. The fracture surface of tensile specimens without tunnel defect shows appreciable plastic deformation around the peripheric rectangular area, indicating resistance of material to tensile loading. (g) Further investigations will be undertaken in the future to assess the effect of the bolt-head profile on the mechanical properties of joining materials by the FSW method, particularly, the heat generation, the material flow, modification of the bolt-head profile, the welding of other materials than aluminum alloys, and so forth.