Adaptive Torque Control for Process Optimization in Friction Stir Welding of Aluminum 6061-T6 Using a Horizontal 5-Axis CNC Machine

Abstract

The research presented herein investigates the impact of axial force and feed rate in the Friction Stir Welding (FSW) of aluminum alloy 6061-T6 in a GROB G552 horizontal 5-axis CNC machine with adaptive torque control enabled. The purpose of this study is to further advance the performance and characteristics of FSW aluminum alloys in 5-axis CNCs, particularly in conjunction with adaptive torque control. The Taguchi and ANOVA methods were utilized to define parameter tables and analyze the resulting data. Optical microscopy and tensile tests were performed on the welded samples to evaluate weld quality. The results from this study provide clear evidence that axial force has a significant effect on tensile strength in FSW AA6061-T6. The maximum UTS found in this study, welded with an axial force of 9.4 kN, retained 69% tensile strength of the base material. Conversely, a decrease in strength and an increase in void formation was found at higher feed rates with this force. Ideal welds, with minimal defects across all feed rates, were performed with an axial force of 8.3 kN. A feed rate of 300 mm/min at this force resulted in a 67% base metal strength. These findings contribute to improving joint strength and application efficiency in FSW AA6061-T6 performed in a horizontal 5-axis CNC machine where adaptive torque control is enabled.

1. Introduction

Friction Stir Welding is a solid-state joining process that was originally developed in 1991 at The Welding Institute in Cambridge, UK [1]. This process utilizes a rotating tool, plunged into a base material and traversed along a path at a specified speed. FSW has gained popularity recently as its use in industry has become more practical. Applications in the automotive, aerospace, marine, and rail industries have garnered interest for further research and the development of this process [2]. The ability to perform adjacent milling and joining of AA6061-T6 in a horizontal 5-axis CNC further expands the possibilities of FSW in manufacturing settings, which is the goal of this study. To accomplish acceptable welds on AA6061-T6, much higher axial forces are required than have previously been studied on a machine of this configuration. This study will determine the effects axial force and feed rate have on the final weld quality in a horizontal 5-axis CNC. The mechanical and microstructural effects of using adaptive torque control to monitor and regulate axial force will also be studied. Given the critical role of torque in determining weld quality, process stability, and tool wear, studies on FSW must provide a thorough analysis of torque control strategies, as these significantly influence the optimization and repeatability of the welding process [3].

1.1. Applications and Use of AA6061-T6

AA6061-T6 is widely used in industrial applications due to its high strength-to-weight ratio, weldability, and corrosion-resistant properties [4]. It is a key candidate for FSW in a 5-axis CNC as it is widely used in parts that require complex machining operations, followed by welding in a separate operation through processes such as MIG, TIG, laser, etc. As industries strive to improve the strength-to-weight ratio of load-bearing products, the use of aluminum alloys has continued to increase [5]. With the birth of aviation in 1903, aluminum naturally became a staple in aircraft structures and continues to be used in the aerospace industry today. Since its inception as Alloy61s in 1935, AA6061 has become one of the most extruded alloys in use [6]. The nominal composition of AA6061-T6 is shown in

Table 1. Chemical and mechanical properties of Al-6061-T6 [7,8].

Chemical Composition
Mg	Si	Cr	Cu	Mn	Ti	Fe	Zn	Other
1.00	0.60	0.20	0.28	0.15	0.15	0.70	0.25	0.20
Physical and Mechanical Properties
Density	Elastic Modulus	Thermal Conductivity			Tensile Strength		Hardness	Yield
(kg/m3)	(GPa)	(W/m·K)			(MPa)		(Brinell)	(MPa)
2700	69	167			310		95	275

.

1.2. Five-Axis Machining

In 5-axis machining operations, the optimization of the tool path is used to improve the overall process efficiency. This requires manipulating the part to provide the ideal cutting orientation that will provide the best surface finish and prevent tool gouging [9]. Tool path optimization in 3-axis machining requires longer tools to be used in order to machine the sides of the part, while a 5-axis CNC allows the part to be rotated more optimally, so a shorter tool can be utilized to machine the sides. This is key in reducing vibrations, which help extend tool life and improve the surface finish as well [10]. The cost benefits of increasing tool life are apparent, while an improved surface provides opportunities for industrial sectors such as the medical, aerospace, and mold and die sectors that require higher surface finishes and tighter tolerances. To avoid the overheating of the cutting tool, cooling must also be considered. A loss of accuracy and tool life may occur if thermal input from machining of the part is not controlled [11]. For this reason, a cooling system is used to maintain a thermal steady state. The additional dynamic of placing the Z-axis horizontally in a 5-axis CNC, as opposed to vertically, opens up even more capabilities, such as upside-down machining, allowing contactless chip evacuation for improved surface finish [12]. The importance of surface finish and trim is particularly highlighted when machining parts to be welded, ensuring proper joint fit-up.

1.3. Previous Work on FSW 6061

When welding a heat-treatable alloy such as AA6061, thermal input must be controlled to ensure mechanical integrity is maintained. Heat created from friction is dissipated into the material, leading to lower hardness in the heat-affected zone (HAZ) and the thermo-mechanically affected zone (TMAZ) as compared to the stir zone (SZ) [13]. This reduction in hardness can be attributed to the Hall–Petch effect, which explains that larger grain sizes will result in lower hardness [14]. When the heat is sufficient enough to lose the temper properties of AA6061-T6, the material, in essence, becomes annealed [15]. The effects of annealing 6061 were studied by Nasir et al. and found that a Brinell hardness of “as-received” AA6061 dropped from an average of 94.6 to 90 HB when annealed [16]. Research conducted by Hussein et al. agrees with these findings and also notes that the yield stress of AA6061 in the annealed condition is 42% lower than the T6 condition [17]. In a study identifying optimal temperature ranges when welding AA6082-T6 and AA5083-H111, Abboud et al. found that a temperature equivalent to 65% of the solidus temperature of the base material should be maintained at the weld for adequate weld quality and freedom from defects such as voids [18]. The main factor in a loss of strength, where excessive heat is input into the material, is the dissolution of β′-phase precipitates that are key to the mechanical strength of AA6061-T6 [19]. For these reasons, in conventional FSW of AA6061-T6, the level of heat input must be controlled by optimized process parameters.

Some examples of FSW process parameters are the feed rate, the axial force, and the tool rotation speed. These parameters control the speed at which the tool travels through the weld seam, the force on the tool at the workpiece, and the rate at which the tool spins while welding, respectively. The effects that these parameters have on mechanical properties have been widely studied and continue to be optimized for alloys such as AA6061-T6. In a study by Sefene et al., the tool rotation speed, the feed rate, and the tool pin profile were assessed for their effects on the mechanical properties of 5 mm thick AA6061-T6 [20]. It was found that a feed rate of 30 mm/min had a higher UTS and hardness than the 40 mm/min and 50 mm/min feed rates. The feed rates were also found to have the greatest effect on the level of heat generation in the weld. The tool rotational speed can play a significant role in the hardness of the weld at speeds of 2000 rpm and higher [21]. In a study by Zhang et al., higher tool rotation speeds were found to enlarge grains in 6 mm-thick AA6061-T4. The weld nugget size grew as the tool rotation speed increased. Adversely, the hardness decreased when the tool rotation speed exceeded 1000 rpm, and the heat generated by the tool increased greatly above 2000 rpm.

The role of the FSW tool is to generate heat in the weld zone and plastically deform the base materials into a solid-state bond. The shoulder generates frictional heat through sliding and sticking forces, while the pin mixes the material around it. A balance of heat and mixing defines the quality and integrity of the weld, as most defects are due to a lack of material flow or excessive heat inducing grain growth in the stir zone [14]. The effects of tool shoulder geometry on FSW joint strength have been previously studied. In a study on the effects of shoulder geometry on FSW AA6061, Trueba et al. note that cavitation along the centerline of the weld, due to poor material flow, is directly related to shoulder geometry. Shoulder geometry was found to not only play a role in weld strength but also to improve surface finish when the shoulder had a raised spiral face [22]. A nominally flat surface on the base metal has also been found to reduce weld flash. This is noted by Lader et al. in another study where weld flash was exacerbated by the additional heat created from excess material built up around the shoulder [23]. Controlling the heat generated during FSW can also be managed by way of internal and/or external cooling. Processes such as Underwater Friction Stir Welding (UFSW), where the surrounding water acts as a cooling agent, reducing heat input into the material, can improve the microstructure and tensile strength of the weld as compared to conventional FSW [24]. In recent years, the optimization of FSW processes has received increasing attention, encompassing various enhancements such as liquid CO₂-assisted cooling FSW [25], the combined use of flowing coolants with ultrasonic vibrations [26], UFSW [24,27], Stationary Shoulder FSW (SSFSW) [28,29], Level-Compensation FSW (LCFSW) [30], Vertical Compensation FSW (VCFSW) [31], and force control in FSW [32]. Processes such as UFSW and SSFSW aid in reducing heat input, while VCFSW has been found to improve joint quality where gaps may be present. Testing with LCFSW was found to improve ductility and joint strength in AA6061-T6. Force and temperature control were achieved by adjusting the tool rotation speed and the plunge depth in FSW AA6061-T6 to AA7075-T6 [32]. Improved welds were made by monitoring and controlling both variables when compared to welds made by conventional FSW. However, the system was limited by low updating frequency due to using an external system for monitoring and controlling feedback.

1.4. Benefits of FSW in 5-Axis CNC

Required and resulting forces during FSW limit the flexibility of application to machines that can withstand several kilonewtons of force across the machine axes. Trimble et al. note that the forces generated from FSW are considerably larger than the forces generated during milling operations [33]. However, harmonic frequencies imposed on the machine tend to be lower in FSW compared to milling processes, notably due to the absence of cutting teeth on the FSW tool, which leads to smoother lobed sinusoidal waves [34]. Three-axis CNCs, or robotic gantry systems, with fewer axes and simpler kinematics, are often utilized for FSW, but limit welding to be performed in the flat planar orientation and limit the geometry of parts able to be machined as well [35]. In a study on 5-axis CNC development, Breaz et al. noted the benefit of 5-axis kinematics that allow for simultaneous motion in all five axes. In this way, translational and rotational cuts can be performed on much more complex parts than are able to be performed on a 3-axis CNC [36]. This is mostly critical when machining parts to be friction stir welded and preparing the joint before welding. In FSW, part fit-up and gap management are paramount in ensuring weld integrity [37]. In a study by Inada et al. the effects of butt joint gap distances were tested on 5 mm nominal thickness AA1050-H24. Joint gaps over 20% material thickness caused microscopic internal defects and the deterioration of joint strength [38].

This research examines the mechanical and microstructural effects of AA6061-T6 FSW in a horizontal 5-axis CNC where adaptive torque control is utilized as a variable for parameter optimization. Adaptive torque control offers a more suitable method for maintaining constant axial force at the tool than conventional positional control, along with more favorable joint attributes [39]. While different methods of torque and force control have been investigated previously [32,39], this study aims to address the gap in joint strength optimization where the axial force, by way of adaptive control, and the feed rate are variables in the optimization of FSW AA6061-T6. The implications of continuous adaptive force control under varied feed rates and their relationship to joint strength in FSW AA6061-T6 are not widely studied to the best of the authors’ knowledge. Further developments can be made with the results from this study serving as a foundation for process optimization in future tests.

2. Testing Methodology

A methodical approach was used to define the experimental parameters in order to set up a procedure for testing and determining the results of this experiment. The AA60601-T6 weld samples, with a nominal thickness of 6.35 mm, were laser cut to the dimensions of 100 × 200 mm² using a Trumpf TruFlow 4 kW CO₂ laser. The weld sample dimensions, in mm, are shown in Figure 1. Holes were also laser cut into the plates for fastening to an aluminum backing plate for welding in the butt-joint configuration. The FSW tool used had a smooth, cylindrical, tapered pin profile with a 19 mm shoulder and was machined from AISI A2 tool steel. The FSW pin had a length of 5 mm with a major diameter of 5.5 mm, tapering down to 3 mm at the tip. Based on the ideal parameters found in previous research on FSW AA3003-H14 in a horizontal 5-axis CNC, a tool rotation speed of 1200 rpm was used for all welds in this study [40]. The parameter table for this experiment is shown in

Table 2. Parameter table.

Level	Axial Force (kN)	Feed Rate (mm/min)
1	7.3	100
2	8.3	200
3	9.4	300

. Axial forces of 7.3, 8.3, and 9.4 kN and feed rates of 100, 200, and 300 mm/min were tested. Adaptive torque control was used to maintain a constant axial force while welding. The axial force was calculated based on motor torque in the Z-axis. Increments of 5 Nm of motor torque equated to 1.04 kN of axial force enacted onto the FSW tool. This explains the force range of 7.3, 8.3, and 9.4 kN. Initial tests at lower force ranges did not yield successful welds. Feed rates were selected based on previous research on FSW AA3003-H14 in a horizontal 5-axis CNC machine [40]. The purpose of controlling force, as opposed to position control in the Z-direction, is to use the adaptive control of tool position to maintain constant pressure on the FSW tool. The advantage of this is noted by Safeen et al. in a study that found a larger acceptance range among various weld parameters with force control due to the adaptability to overcome variations in material form, specifically in thinner material [41]. Additionally, Longhurst states that volumetric defects are reduced when using force control, as opposed to position control [39].

The adaptive torque control system used in this study measured torque on the Z-axis motor, in relation to a pre-defined parameter, to monitor and control axial force on the FSW tool. A diagram of the Z-axis components in the CNC machine configuration used in this study is shown in Figure 2. In this configuration, the Z-axis motor controlled the Z-axis ball screw which drove the Z-axis and attached spindle either forward in the positive (+) direction or backwards in the negative (−) direction. The ball screw pitch and motor torque were used to calculate the axial force at the tool. The ball screw pitch was a constant based on machine configuration, while the motor torque was an input parameter. A flow chart of the adaptive torque control process is shown in Figure 3. The motor torque (value in Nm) was entered as an input parameter into the machine controller. The torque on the Z-axis motor was monitored while driving the Z-axis carriage along with the spindle by way of the Z-axis ball screw, as it approached the pre-defined motor torque value. Variations in the motor torque were measured and adjusted at a frequency of 500 Hz. By monitoring and adjusting the torque on the Z-axis motor, the axial force at the tool was controlled and maintained throughout the welding process to within 2.3% of the programmed value.

The workpiece setup in the machine is shown in Figure 4, where the FSW tool and the tool holder can be seen welding in the horizontal butt position. Welding was performed in a GROB G552 horizontal 5-axis CNC machine with a Siemens 840D controller. In a horizontal 5-axis CNC of this configuration, the feed rate, or the weld direction, is in the +X direction while axial plunge is in the +Z direction in relation to the three-dimensional Cartesian plane. Tensile specimens were cut from the workpiece and machined to a reduced thickness of 4 mm at the zone of welding, as seen in Figure 5. Non-standard geometries were used for the tensile specimens in this study. Testing was part of an industrial request that did not require ASTM E8M standard specimens to be used. Due to the presence of an unwelded zone below the weld pin, the tensile specimens were machined to a nominal thickness of 4 mm to validate the joint strength of only the welded material. Two tensile samples were cut for each weld set in this study. The samples prepared for testing are shown in Figure 6. Weld sample numbers are labeled below each sample. A complete list of weld conditions for each sample # is shown in

Table 3. Weld conditions by sample #.

Sample #	Axial Force (kN)	Feed Rate (mm/min)
1	7.3	100
2	7.3	100
3	7.3	200
4	7.3	200
5	7.3	300
6	7.3	300
7	8.3	100
8	8.3	100
9	8.3	200
10	8.3	200
11	8.3	300
12	8.3	300
13	9.4	100
14	9.4	100
15	9.4	200
16	9.4	200
17	9.4	300
18	9.4	300

.

3. Results

The specimens were tensile tested to failure using a Zwick-Roell 100 kN ProLine test frame with a Zwick/Roell Type 8406 50 kN load cell used in conjunction with Zwick/Roell testXpert^® III software. No signal conditioning or amplification was used with this software in these tests. The specimens were secured center and square in the tensile stand jaws. Weld samples are shown after tensile testing in Figure 7, with a white dotted line representing the original center of the weld seam. Both brittle and ductile failures were observed in the destructive tests. The tensile results for all of the samples in this study are shown in Figure 8. Weld sample #s are labeled for each result. On average, the welds performed at 8.3 kN axial force had a higher tensile strength than other axial forces tested in this experiment. These results show an improved UTS compared to previous work on the conventional FSW of AA6061-T6 [15,22,42]. Joint strength in VCFSW 4 mm AA6061-T6 was found to be comparable to the results from this study [43]. Specimens exhibiting ductile failures breaking outside of the weld zone had, on average, an 18% higher UTS than specimens with brittle fractures within the weld. Kalinenko et al. explain this phenomenon in FSW AA6061-T6 with regard to continuous recrystallization and grain refinement under the FSW tool. Higher levels of continuous recrystallization and grain refinement in the SZ are aspects of FSW that have been found to increase UTS. Additionally, the partial dissolution of β′ precipitates and annealing in the TMAZ have been found to retain up to 75% ductility in FSW AA6061-T6 [44,45]. In these studies, samples with a lower tensile strength were attributed to brittle fracture behavior due to crack propagation within the weld. In this study, parametric settings yielding higher ductility in tensile failures were found with a 7.3 kN axial force at lower feed rates and an 8.3 kN axial force at higher feed rates. However, higher feed rates welded with a 9.4 kN axial force exhibited a decrease in both ductility and UTS. It should be noted that with a 9.4 kN axial force at a lower feed rate, tensile strength was comparable to a 8.3 kN axial force at higher feed rates. When comparing the average UTS of the samples for each weld parameter, a similar trend can be seen in Figure 9. The Standard Deviation (σ) for each parameter is also plotted by the black line along the UTS values. It can be seen that while an axial force of 9.4 kN with a feed rate of 100 mm/min had higher UTS than most other weld parameters, the σ was also higher. Conversely, the welds performed with an axial force of 8.3 kN and 300 mm/min averaged a higher tensile strength with a drastically lower σ.

The lowest UTS found was sample #15, welded with a 9.4 kN axial force at a 200 mm/min feed rate and exhibiting a UTS of 147 MPa. Macroscopic cracks through the weld seam, shown in Figure 10, were observed before destructive testing in this specimen. The crack through the center of the weld seam created a stress riser, which led to brittle fracture propagation through the weld during tensile testing. Although this is a defect in the weld, the UTS was not outside of the data boundaries enough to be considered an outlier. This was the only sample with an external crack observed before destructive testing.

Specimens welded with 8.3 and 9.4 kN axial forces were prepared, cut, and polished before etching in sodium hydroxide for optical microscopy (OM). Microscopic imaging was performed with a Carl Zeiss Axio Imager Z2M. Samples welded with 7.3 kN axial force were tensile tested preemptively to micrographs, as this study was part of an industrial request for information on UTS. With the goal of achieving the highest UTS, the first samples welded with a 7.3 kN axial force were destroyed before OM was performed. Figure 11 shows a microscopic view of samples #14–18 before destructive testing (above) and after destruction (below). Voids can be seen, circled in red, along the advancing side of the welds. Etched welds are outlined by the yellow dotted line. Brittle fractures occurred in samples #14–18, mainly due to void formation within the weld root along the advancing side. Voids formed in all samples welded with a 9.4 kN axial force, which could indicate that a higher axial force has adverse effects on solid-state mixing. This could be in part due to excessive material flow causing voids under the pin, as also previously noted by Longhurst [39]. Manickam et al. also state, in an extensive review on the implications of process parameters in FSW AA6061-T6, that excessive mixing and material flow can cause tunneling and voids [46]. The location of the voids at the base of the pin is likely due to excessive heating and mixing at the higher axial force range. Trueba et al. note that the presence of voids, or lack thereof, has the greatest influence on joint strength and ductility [47]. Also visible in the microscopic images is the presence of a lazy “S”, or kissing bond, in the weld nugget. The lazy “S” defect is found when oxides from the surface of the aluminum are mixed into the weld, preventing a solid-state bond from forming along the oxidation layer [48].

A comparison of the before and after destruction images shows that fractures occurred along and through the lazy “S” but do not explicitly follow the “S” path. An enlarged image of the lazy “S” in sample 18 is shown in Figure 12. The lazy “S” also appears to become more prominent as the feed rate increases. This phenomenon was observed by Zhang et al., whereby increasing the feed rate, and in turn decreasing the heat input, caused a more pronounced lazy “S” defect [49,50]. Voids in the root of the SZ were also found to form in welds conducted with higher feed rates, correlating with the results found in this study as well. Higher feed rates resulted in lower plastic deformations and higher concentrations of the surface oxide within the weld. Meanwhile, lower feed rates, with higher levels of plastic deformation per unit length, reduced the continuity of the lazy “S”. Areas with unbonded material, such as along the lazy “S” continuity, have been found to drive crack transmission during destructive testing [51]. As the tensile samples experienced vertical load, the fracture path began at the voids in the root and propagated along and through the lazy “S” defect. Interestingly, this defect does not appear to be as prevalent in samples welded at 8.3 kN, as can be seen in Figure 13. Micrographs of welds performed at 7.3 kN were not collected in this study, as previously mentioned.

Taguchi Analysis/ANOVA

A Taguchi Analysis was run on the effects of axial force and the feed rate on UTS using the Minitab 21.1 software. The analysis included three levels of an orthogonal array across two factors (axial force/feed rate). The signal-to-noise ratio was plotted using a “Larger-is-better” formula for the UTS, as shown in Equation (1), where y is the UTS, and i is the nth test. Axial force had a greater influence on the UTS than the feed rate, as can be seen by the rank in

Table 4. Response table for S/N ratios and means.

	Response Table for S/N Ratios		Response Table for Means
Level	Axial Force	Feed Rate	Axial Force	Feed Rate
1	45.11	45.82	181.8	196.2
2	45.96	44.9	199.2	177.2
3	44.41	44.76	168	175.7
Delta	1.55	1.06	31.2	20.5
Rank	1	2	1	2

. The rank is an indication of which variable had the greatest effect on the results. In this study, the feed rate had a smaller difference, or delta, between max and min values than axial force across the S/N ratios and means, determining that the amount of variation in the UTS was lower due to changes in the feed rate than from axial force. Additionally, the F-values for axial force and the feed rate are 4.17 and 2.23, respectively, showing greater effects on the UTS due to changes from axial force. F-values calculated from the ANOVA test explain which variable had a more significant effect on the results. This conclusion can be further delineated in the ANOVA results for UTS in

Table 5. Analysis of variance results for UTS.

Source	DF	Adj SS	Adj MS	F-Value	p-Value
Axial Force	2	2926	1463.2	4.17	0.040
Feed Rate	2	1567	783.5	2.23	0.147
Error	13	4561	350.8
Lack-of-Fit	4	3088	771.9	4.72	0.025
Pure Error	9	1473	163.7
Total	17	9054

, where the p-value for axial force is lower than the alpha of 0.05. A p-value less than the predetermined alpha of 0.05 denotes significance in the results caused by changes to the variable being measured.

$${\displaystyle \frac{S}{N_{LB}}}=-10{\log }_{10}\left[{\displaystyle \frac{1}{n}}\sum _{i=1}^n\left({\displaystyle \frac{1}{y_i^2}}\right)\right]$$

(1)

Equation (1): Larger-is-better formula for UTS S/N ratio.

The Main Effects Plot for Means of each variable in this study can be seen in Figure 14. The Main Effects Plot for Means displays the average UTS across each variable and how changes to each variable affected the results. It can be seen that welds performed with an 8.3 kN axial force had the highest mean tensile strength, while a 9.4 kN axial force had the lowest. This phenomenon agrees with studies conducted by Ramamoorthi et al., where an increase in axial force from ideal parameter settings by 1 kN exhibited a decrease in tensile and bending strength of 6 mm AA5086/AA6063 FSW samples [52]. Additionally, Sreenivas et al. noted similar results in FSW 6 mm AA6082-T6, where a decrease in UTS was found when the axial force was increased by 1 kN from ideal parameters [53]. While the individual sample with the highest UTS was welded at 100 mm/min with a 9.4 kN force, higher levels of variation occurred at this force range. This was due to the presence of defects within the weld root, which decreased the UTS. The lazy “S” and voids in all samples welded at 9.4 kN resulted in a reduction in tensile strength at this axial force. Brittle fractures occurred in the SZ in samples with voids, which agrees with findings in the literature [44,45].

4. Conclusions

In this study, AA6061-T6 was FSW in a horizontal 5-axis CNC utilizing adaptive torque monitoring and control. It was found that changes made to axial force had a greater influence on tensile strength than changes to the feed rate. Welds performed with an 8.3 kN axial force had the highest UTS on average across all other axial force ranges. The presence of Lazy “S” defects and voids in welds performed with the highest axial force of 9.4 kN caused a 15% reduction in tensile strength compared to welds performed with an 8.3 kN axial force. The optimum settings found in this study were welded with an 8.3 kN axial force and a 300 mm/min feed rate. Both brittle and ductile failures occurred during destructive testing, with ductile failures yielding 18% higher tensile strength than samples exhibiting brittle fractures. These findings will contribute to continued research in a horizontal 5-axis CNC where adaptive torque control is enabled.

Key points from the work presented herein are listed below:

• The optimal parameters yielding the highest UTS with the fewest defects were welded with an 8.3 kN axial force at a 300 mm/min feed rate.
• A total of 67% of the base metal tensile strength was retained.
• Welds performed with higher axial force caused voids due to excessive heating and mixing under the FSW pin.
• Void creation and tunneling led to a decrease in UTS.
• Lazy “S” defects were found in micrographs across all welds performed with a 9.4 kN axial force.
• Higher feed rates increased the presence of the Lazy “S.”
• Taguchi analysis showed axial force had a greater effect on UTS than feed rate.

5. Future Work

Future work will involve investigations into FSW AA6061-T6 and dissimilar additively manufactured alloys.