An Experimental Approach for Friction Stir Welding A Case Study for Aa 2024T351
* Author to whom correspondence should be addressed.
Sigma Journal of Engineering and Natural Sciences 2020, Vol. 38, Issue 4, pp. 1999-2011; doi.org/10.62051/ytu.sigma-journal-of-engineering-and-natural-sciences-an-experimental-approach-for-friction-stir-welding-a-case-study-for-aa-2024t351
Abstract
Keywords: Friction stir welding; overlapping of weld seams; mechanical properties; microstructure; AA 2024 – T351.
1. Introduction
Aluminum alloys are functional materials used in automotive, aircraft, shipbuilding and transportation industries [1]. The welding applications in those industries require special care; it is worth noting that some of the materials are accepted to be welded problematically using conventional fusion welding techniques. Even 2XXX and 7XXX series of aluminum alloys are accepted to be the most popular alloys in the relevant industries namely automotive and aircraft, the welding procedure of those alloys is well-known to be welded problematically using fusion welding techniques [2-6] due to the tendency of formation of defects; e.g. hot cracking [2, 4, 6], solidification defects, porosity [4, 6] and hydrogen solubility [7]. Those defects are located in the fusion zone originating from melting and subsequent fast solidification, thus deteriorating the mechanical strength of the welded joints. The defects, observed in fusion welding processes, could be eliminated using the friction stir welding (FSW) technique. FSW technique is one of the solid-state welding techniques in which no melting and re-solidification of a metal occurs during and following the welding process. The most important characteristic feature of the FSW *
Corresponding Author: e-mail: sefika.kasman@deu.edu.tr, tel: (232) 301 92 17 1999
technique is the formation of plastic deformation, originating from the movement of a rotary tool with the high frictional heat. A sufficient amount of plastic deformation and heat input provide an effective bonding between the two plates to be welded. While the rotating tool governs the intensity of frictional heat and plastic deformation; notably, heat input per unit area is determined by the welding speed (WS). The tool, used in a FSW process, is consisted of two characteristic parts, namely pin and shoulder. With the rotation of the tool, the spinning pin is pressed against the abutted surface of the plates to be welded, relocating the plasticized material from the advancing side (AS) towards the retreating side (RS) along the weld line [7]. Thus, a weld joint is formed by the rotating tool which moves along the weld line. It is worth noting that, such a solid phase bonding between the two plates to be joined makes the FSW process popular among the welding techniques preferred in the joining processes of aluminum alloys [8]. AA 2024 aluminum alloy is a widely used alloy in aircraft structures [9-13]; it exhibits superior corrosion resistance, relatively high mechanical strength [10-12] and fatigue strength [12, 13]. In this study, AA 2024-T351 alloy plates were subjected to a special FSW technique in order to improve tensile properties. The overlapping phenomenon of weld seams is the characteristic feature of this special FSW technique. In this study, the effects of overlapping phenomenon on the mechanical properties of welded joints, produced via pin offset technique, were investigated by taking FSW parameters and pin offset direction into consideration. The overlapping process of weld seams is performed via offsetting the pin with a predetermined distance towards the AS or RS following the obtainment of the first weld seam. Welded joints, produced using this technique, are accepted to be distinctly different from the welded joints consisting of single FS weld seam. The schematic illustration of a welded joint, produced via pin offsetting is shown in Fig. 1. To the best of our knowledge, in the available and open literature, there is no study focusing on FSW process of AA 2024 using pin offset technique. Notably, researchers produced overlapped weld seams in the joining process of AA 7075 [14-16], AA 6013 [17] and dissimilar alloys AA 6013/AA 7075 [18, 19]. This study aimed to provide a new approach to the FSW technique in order to improve the mechanical and microstructural properties of FS welded AA 2024-T351 aluminum alloy joints.
Figure 1. Schematic illustration of weld seam overlapping in a FSW process
2. Experimental Procedures
AA 2024 – T351 plates, with a nominal composition of 3.91% Cu, 1.23% Mg, 0.52% Mn, 0.14% Fe and balance aluminum, were 225 mm in length, 100 mm in width and 6 mm in thickness. The abutting faces of the work-piece plates were machined in order to provide an effective contact, thereby preventing the separation with the effect of vertical force of tool pin. The welding processes were performed using universal milling machine. The work-piece plates were placed on a backing plate; support plates were subsequently positioned on the work-piece plates and clamped with studs as shown in Fig. 2a. The tool pin, used in the weld applications, were made of H13 hot-work tool steel. A tool for FSW is configured with pin shape. Two different pins, namely pentagonal (P) and tapered threaded (TT), were selected in the present study. The tool and pin configurations are given in Figs. 2b and c. The plunge depth for pin was fixed to be 5.90 mm. While the WS was fixed at 63 mm/min, the tool rotational speed (TRS) was set at two different rates, namely 400 and 630 rpm. The tool rotation axis is the normal to the rolling direction and it was tilted by 3°. The experiments were set at two different FSW conditions considering two different pin profiles. The FSW conditions are given in Table 1. All the FSW experiments were performed by considering the parameter’s combinations given in Table 2. Table 1. The FSW process parameters and their levels Parameters (unit) Tool pin type Pin offset Tool rotation speed (rpm) Welding Speed (mm/min)
Microstructural observations were performed on the cross-section of the welded joints. The surface of the samples for microstructural observation and micro-hardness test were prepared according to the standard metallographic procedures; polished samples were subsequently etched using the Poulton’s reagents. The microstructure of FS welded joints were observed using optical microscope. The micro-hardness measurements across the weld zone were performed with a load of 0.98 N and a dwell time of 15 s. The micro-hardness measurements were performed at 1 mm intervals. Tensile tests were performed with a cross-head speed of 2 mm/min in accordance with the ASTM E8/E8M [20]. The ultimate tensile strength and elongation at rupture of AA 2024T351 is 456 MPa and 22 %, respectively. The grain size in the stir zone (SZ) was measured using the linear intercept method in accordance with the ASTM E112-13 [21]. In this study, the mechanical properties of FS welded joints were characterized using tensile strength and elongation at rupture (%) values. Three different test samples were prepared for each welding condition, and the average of the test results obtained from three different tests represented the average tensile strength and elongation at rupture (%) of the FS welded joint. The mechanical properties of FS welded joints were analyzed using one-way ANOVA method. The Tukey HSD post hoc test revealed whether there was a significant difference between the groups and if ρ < 0.05, the differences between the samples were considered statistically significant.
Figure 2. A sample FSW process and the tool pin profiles used in FSW applications Table2. Experimental layout for FSW applications Exp. No W1 W2 W3 W4 W5 W6 W7 W8 W9 W10 W11 W12
Process Parameters PO TRS WS 0 400 63 0 630 63 AS 400 63 AS 630 63 RS 400 63 RS 630 63 0 400 63 0 630 63 AS 400 63 AS 630 63 RS 400 63 RS 630 63
3.1. Macro-structural Investigation of FSW Joints
The macro-structural observation was performed on the transverse cross-section of each welded joints. A detailed observation, given in Fig. 3, confirmed that all the welded joints were consisted of three distinct zones, namely SZ, thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ). The dark region, revealed as a consequence of etching process, was evaluated to be the base metal (BM). The confined intermediate region consisting of three different zones between two dark regions exhibited different shades as a consequence of etching. Moreover, the wideness of intermediate region exhibited differences. The main reason of the differences is attributed to the change in plastic deformation and the amount of frictional heat 2002
input. The overlapping phenomenon of weld seams resulted in different regions, namely SZ-1 and SZ-2, in the SZ. Representative microstructures of SZ are shown in Fig. 4-W3, W5 and W6. While W3 was produced by overlapping two weld seams via shifting the pin towards AS, the W5 and W6 were produced via shifting the pin towards RS. This type of structure occurred in case two weld seams were overlapped, causing complex microstructure to occur. It was observed that an increase in TRS caused the wideness of weld zone consisting of SZ and TMAZ to increase. In addition to the effect of TRS on the width of the weld zone, shifting the pin towards AS or RS resulted in the weld zone to be expanded. Macrostructure images exhibiting the regions marked with yellow arrows and circles in Fig. 4 at W7-W12 belong to defective joints; notably, cavity and tunnel-type defects were found to occur. The tunnel-type defect was encountered in joints welded with the TRS of 630 rpm using a tapered threaded pin profile. The defects were observed to be located at the root of weld seam. The tunnel type defects are occurred under the conditions of improper flow of the material and insufficient heat input as reported elsewhere [7].
Figure 3. The transition zones for W1 The region, referred SZ, was located on the contact surface of BMs; it was exposed to common effect of the intense plastic deformation and frictional heat, causing changes to occur in the grain structure. Fig. 3a exhibits three distinct zones taken from W1. Fig. 3-A was obtained from the SZ and the grain size was found to be relatively finer and equiaxed in comparison to that of the TMAZ; the finer grain size is attributed to dynamical recrystallization process with the effect of intense plastic deformation and frictional heat. During the FSW process, the weld seam is under the effect of both extrusion and forging processes. It is considered that the downward force of a tool, TRS and WS determine the formation of those two above-mentioned processes and the capability of obtaining defect-free welded joints. The TMAZ is a region, located adjacent to the SZ; however, recrystallization does not occur in this region [3]. The effect of frictional heat is accepted not to be high enough to initiate a transformation on grain structure. This region is characterized by the grains which are oriented towards the direction of maximum shear stress [7]; it is worth noting that it exhibits finer grain size than that of the HAZ. This characteristic structure
is seen in Fig. 3a and Fig. 3a-B. A distinct border was found to occur between TMAZ and SZ as reported elsewhere [3]. Birol et. al. [7] reported that the distortion within the grains was revealed to be intensive between TMAZ and SZ and on the region, under tool shoulder, originating from the intensive effect of plastic deformation. It is worth noting that the stirring effect and frictional heat control the features of TMAZ.
3.2. Micro-structural Investigation of FSW Joints
The effect of intensive plastic deformation and cooling rate on the microstructural evolution of FS welded joints were investigated using optical microscopy. Fig. 5 exhibits the microstructural images of SZ; it was revealed that the microstructure was consisted of finer equiaxed grains in comparison to that of BM as reported elsewhere [3]. The SZ was revealed to be divided into three region which were under the shoulder (Fig. 5a), at the center of the SZ (Fig. 5b) and at the root of the SZ (Fig. 5c). The average grain size in Figs. 5a-c was measured to be 3.49 µm ± 0.35, 4.68 µm ± 0.34 and 2.92 µm ± 0.29, respectively. The average grain size in SZ of W1, W2, W3, W4, W5 and W6 was measured to be 4.68 ± 0.34 µm, 2.73 ± 0.26 µm, 4.77 ± 0.44 µm, 3.30 ± 0.50 µm, 4.04 ± 0.46 µm, 3.83 ± 0.32 µm. The grain size in the SZ of W1-W6 was evaluated to be related to the TRS; it was revealed that any increase in TRS caused the grain size to be finer as reported elsewhere [8, 22]. The overlapping phenomenon of weld seams is evaluated to be a complex process, creating a different deformation rate in SZ. This phenomenon may result in a differentiation on the grain size of SZ.
Figure 5. The change in the grain size for W1 (a: under the shoulder; b: center of the SZ; C: root of the SZ)
Small segments were found to occur as seen in Figs. 6b, bʹ, cʹ and cʹʹ. While the magnified images of the segments in Figs. 6b and bʹ are shown in Fig. 6bʹʹ, the other segments are shown in Figs. 6cʹ and cʹʹ. It was revealed that those segments were consisted of precipitates; notably, they were located through a zig-zag line. The precipitates were found to be oriented towards the deformation way and tool rotation direction as seen in Figs. 6a and aʹ. The EDS results revealed that those precipitates could be (Al,Cu)xMn, Al(Cu,Fe,Mn), (Al,Cu)x(Fe,Mn)ySi and Al2CuMg as reported elsewhere [2, 4, 9, 23-25].
3.3. Tensile properties of the FS welded joints
Tensile test results of FS welded joints are given in Fig. 7a and b. Experimental studies were performed under two groups considering the type of pin profile, namely pentagonal (Fig. 7a) and tapered threaded (Fig. 7b). The welding processes, performed on W1-6 samples, were carried out using a tool with pentagonal pin profile; however W7-12 joints were welded using a tool with tapered threaded pin profile. According to the tensile test results, the highest tensile strength and elongation at rupture (%) values were achieved using pentagonal pin profiled tool with the process parameters of 400 rpm TRS and 0 mm pin offset. In case the pin offset value was 0 mm, the increase in TRS value from 400 rpm to 630 rpm led to a decrease in tensile strength. Tensile strength values of W3 and W5 joints, joined with the process parameters of 400 rpm and 2 mm pin offset value, were measured to be 380 ± 8 and 379 ± 13 MPa, respectively. Notably, there was no significant difference (ρ > 0.05) between the tensile strength of W3 and W5. Increasing the TRS value from 400 rpm to 630 rpm for welded joints, produced using a pentagonal pin profiled tool, led to a decrease in tensile strength and elongation at rupture (%). When a sequence is made 2006
in terms of tensile strength values of FS welded joints produced using a tool with pentagonal pin profile, it was revealed that FS welded joints are ranked W1 > W3 > W5 > W2 > W6 > W4. On the other hand, the elongation at rupture (%) of W1, W3 and W5 joints was measured to be 5.2%, 3.8% and 3.8%, respectively. W1, W3 and W5 joints, joined with the process parameter of 400 rpm TRS exhibited significantly higher (ρ < 0.05) elongation at rupture (%) value than W2, W4 and W6 joints welded with the process parameter of 630 rpm TRS. W9 joint exhibited the highest tensile strength with a value of 341 ± 22 MPa among the joints welded using a tool with tapered threaded pin profile. Notably, W9 joint was produced under the process parameters of 400 rpm TRS and 2 mm pin offset on the AS. When a general evaluation is made; the increase of TRS from 400 rpm to 630 rpm for pentagonal and tapered threaded pin profiles, resulted in a decrease in tensile strength and elongation at rupture (%). It is worth noting that there was no significant differences (ρ > 0.05) between the tensile strength as well as elongation at rupture (%) of W7 and W8 welded joints. The geometry of the tool, used in the FSW process, was revealed to affect the mechanical properties of the joints. Tensile strength and elongation at rupture (%) of FS welded joints were measured to be in the range of 271-409 MPa and 0.4-5.2%, respectively. The lowest elongation at rupture (%) was obtained when using a tool with tapered threaded pin profile under the process parameters of 630 rpm TRS and 2 mm pin offset towards AS. Images showing the fracture location of the samples following the tensile test are given in Fig. 8. W1, 3, 5 and 6 joints, welded using pentagonal pin profiled tool, were observed to be fractured outside the SZ. W3 and W5 welded joints were observed to be fractured between HAZ and BM. This fracture location is quite close to the BM in the W5 joint, occurring on the AS of the welded joint. It was revealed that the fracture location was found to be between HAZ and TMAZ in W1 and W6 welded joints. Notably, the fracture location of W6 joint was observed to be closer to TMAZ. Fractures were observed to occur in the SZ of W2 and W4 joints. Notably, all welded joints, produced using a tapered threaded pin profiled tool, were detected to be fractured from the SZ.
Figure 7. The change in the UTS and Ɛp according to the change in FSW condition It is worth noting that a different type of pin profile, namely straight-helical has been used in our previous study [26].
3.4. Micro-Hardness Measurements
The change in hardness profile across the weld zone for each welded joint is seen in Fig. 9. The microstructural characteristics, mentioned above, were evaluated to be consistent with the micro-hardness measurements as reported elsewhere [25]. It was revealed that micro-hardness measurements of welded joints exhibited similar trend. The micro-hardness values increased gradually from BM to SZ. The highest micro-hardness was measured to be 155 HV in SZ of W1. The micro-hardness distributions in SZ of overlapped weld seams were revealed to be nonuniform originating from the higher deformation rate and different cooling behavior of weld zones as reported elsewhere [25]. It is considered that this non-uniformity is attributed to the overlapped weld seams, formation of second phase particles and precipitates [3]. The W10 joint was produced with the TRS of 630 rpm; however, the pin was offset towards AS following the formation of first weld seam. The micro-hardness values in overlapped region were measured to be in the range of 126-142 HV. The coarser grains are attributed to the overlapping effect, causing a decrease in micro-hardness values. Taking the differentiation in the micro-hardness into account, it was revealed that the micro-hardness values in SZ and TMAZ were high due to overlapped weld seams, intensive plastic deformation and heat input.
Figure 9. The change in hardness profile across the weld zone
4. Conclusions
In this study, pin overlap method was used in order to improve the mechanical properties of welded joints. TRS, tool pin profile were associated with weld seam overlap. According to the results obtained from the experiments, following conclusions were drawn; a) The highest tensile strength and elongation at rupture (%) were obtained from the joint welded using pentagonal profiled pin with the process parameters of 400 rpm TRS and zero pin offset value. The tensile properties of the welded joints were deteriorated in case the TRS was increased from 400 rpm to 630 rpm. b) The average grain size in SZ of a welded joint, produced using pentagonal profiled pin, was measured to be in the range of 2.73-4.77 µm. The increase in TRS caused the grain size in SZ to be finer. The coarsest grain size was measured in SZ of the welded joint which was produced with the process parameters of 400 rpm TRS and 2 mm pin offset value towards the AS. c) The volumetric tunnel-type defect was observed in SZ of the welded joint produced using tapered threaded pin with the process parameters of 630 rpm TRS and 2 mm pin offset value towards the AS. Another defective weld seam, produced with tapered threaded pin, was found to be consisted of cavity-type defects. The cavity-type defect was found to be located on the AS and RS of welded joint produced with the process parameter of zero pin offset value.
Acknowledgement
The present study was supported by Dokuz Eylul University under project no. 2017.KB.FEN.002. The authors would like to acknowledge this financial support.
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KASMAN, Ş.; OZAN, S. An Experimental Approach for Friction Stir Welding A Case Study for Aa 2024T351. Sigma Journal of Engineering and Natural Sciences 2020, Vol. 38, pp. 1999-2011. https://doi.org/10.62051/ytu.sigma-journal-of-engineering-and-natural-sciences-an-experimental-approach-for-friction-stir-welding-a-case-study-for-aa-2024t351