Metallographic structure, mechanical properties, and process parameter
optimization of 5A06 joints formed by ultrasonic-assisted refill friction
stir spot welding
Xin-bo Liu1,2), Feng-bin Qiao2), Li-jie Guo2), and Xiong-er Qiu1)
1) Hunan Provincial Key Laboratory of Grids Operation and Control on Multi-Power Sources Area, Shaoyang University, Shaoyang 422000, China
2) Shanghai Aerospace Equipment Manufacturer, Shanghai 200245, China
(Received: 6 August 2016; revised: 13 October 2016; accepted: 24 October 2016)
Abstract: Novel hybrid refill friction stir spot welding (RFSSW) assisted with ultrasonic oscillation was introduced to 5A06 aluminum alloy joints. The metallographic structure and mechanical properties of 5A06 aluminum alloy RFSSW joints formed without ultrasonic assistance and with lateral and longitudinal ultrasonic assistance were compared, and the ultrasonic-assisted RFSSW process parameters were opti-mized. The results show that compared with lateral ultrasonic oscillation, longitudinal ultrasonic oscillation strengthens the horizontal bond-ing ligament in the joint and has a stronger effect on the joint’s shear strength. By contrast, lateral ultrasonic oscillation strengthens the ver-tical bonding ligament and is more effective in increasing the joint’s tensile strength. The maximum shear strength of ultrasonic-assisted RFSSW 5A06 aluminum alloy joints is as high as 8761 N, and the maximum tensile strength is 3679 N when the joints are formed at a tool rotating speed of 2000 r/min, a welding time of 3.5 s, a penetration depth of 0.2 mm, and an axial pressure of 11 kN.
Refill friction stir spot welding (RFSSW) is a new sol-id-phase welding technology, which is considered as the most suitable spot-connecting technologies for lightweight metals such as aluminum alloys due to the advantages of high-quality surfaces (no exit hole), high strength, good energy efficiency, and low costs. RFSSW technology was patented by GKSS GmbH & Co. KG in 2003 [1–2]. A schematic of the RFSSW process is shown in Fig. 1 [3]. RFSSW is assembled by clamping ring, sleeve, and pin, which are mounted coaxially and operated by three separate actuators; thus, the three parts can move up and down inde-pendently. Both the pin and sleeve are connected to the ac-tuators; the pin and sleeve therefore rotate in the clamping ring at the same speed [3]. As illustrated in Fig. 1, the RFSSW process can be explained through four distinct stages. (a) The sheets are clamped together by the clamping ring. Both the pin and sleeve start to rotate to produce fric-
tional heat on the upper sheet surface, leading to localized plasticization. (b) The pin and sleeve are moved axially in the opposite direction, following the sleeve driving into the material and the pin making space for the displaced material. (c) After a predetermined plunge depth is reached, the process is reversed. The sleeve is withdrawn, and the ma-terial is pressed into the space provided by the pin. (d) The joint is completed after the sleeve has returned to its original position, leaving a smooth surface. The clamping force is maintained until the joining process is completed [4].
Because of the complicated procedures and short life-span of tools, there are few applications of this welding process. Muci-Küchler et al. [5] proposed a fully coupled thermomechanical finite element model to predict the tem-perature, deformation, stress, and strain distributions in the resulting joints. Prakash and Muthukumaran [6] investigated Al–Mg–Si aluminum joints formed by RFSSW and found that they exhibited a higher strength than those made by the basic friction stir spot welding (BFSSW) technique developed
X.B. Liu et al., Metallographic structure, mechanical properties, and process parameter optimization of 5A06 joints... 165
Fig. 1. Schematic of the RFSSW processes: (a) rotating pin and sleeve; (b) moving pin and sleeve in the opposite direction; (c) pressing the material into the space provided by pin; (d) completing the joint.
by Mazda Motor Corporation in 1993 [7]. BFSSW issimilar in concept and equipment as its predecessor, friction stir welding (FSW), which offers advantages such as high-strength welds and good energy efficiency [8]. Mazda Motor Corporation successfully applies BFSSW in the pro-duction of rear door for 2003 RX-8 [9]. However, BFSSW leaves an obvious exit hole in the middle of joints, which substantially and adversely affects the mechanical properties of the joints. Corrosion can occur preferentially at BFSSW joints because of rainwater penetration into the exit hole, where body paint hardly reaches the bottom. Uematsu et al. [10] also re-ported that the RFSSW process improved tensile strength by 30% compared to BFSSW because the refilling process in-creased the effective cross-sectional area of the nugget. In fact, heat input is an important factor in forming RFFSW joints. Shen et al. [11] observed some void defects in the RFSSW joints of 6061–T4 alloy as a result of insufficient heat input. However, the amount of heat input necessitates the increase in tool rotating speed, axial force, and welding time. As a result, the RFSSW tool life can be substantially reduced. Frequent replacement of RFSSW tools leads to high production costs, low production rates, and low weld quality. Thus far, researchers have attempted to optimize the shape of RFSSW tool and the process parameters such as tool rotating speed, axial force, penetration depth, and welding time [11–12].
Since the 1950 s, the application of ultrasonic energy to the plastic deformation of metals has been widely investi-gated. Ultrasonic oscillations can reduce the tool static de-
formation forces, increase the processing speeds, and im-prove the product quality in various manufacturing processes such as machining, drilling, and welding [13–16].
In the present work, to demonstrate the feasibility of ul-trasonic-assisted RFSSW and provide a prospective ex-ploration of the applications of RFSSW with respect to heat input in high-temperature materials, the heat input of joints was strengthened by integrating ultrasonic oscilla-tions into the RFSSW process, the effect of ultrasonic os-cillations on RFSSW joints of 5A06 aluminum alloy was studied using metallography and comparison tests, and then, the ultrasonic-assisted RFSSW process parameters were optimized.
2. Experimental
Ultrasonic-assisted RFSSW welding experiments were carried out on a robotic arm or a machine tool consisting of RFSSW tool, RFSSW head, and ultrasonic device. The RFSSW tool produced by Shanghai Aerospace Equipments Manufacturer is as shown in Fig. 2(a). The tool is comprised of clamping ring, sleeve, and pin, whose outer diameters are 18, 9, and 5.2 mm, respectively. The tool is mounted in the end of RFSSW head, as shown Fig. 3. The ultrasonic device with an ultrasonic frequency of 20 kHz and a power output of 1.5 kW is composed primarily of an ultrasonic generator, a transducer, and an ultrasonic horn, as shown in Fig. 2(b).
The materials used for welding in this study were 2-mm-thick 5A06 aluminum plates, which are widely used
166 Int. J. Miner. Metall. Mater., Vol. 24, No. 2, Feb. 2017
in the aerospace and shipbuilding industries. All specimens were wiped with acetone to remove oil, coatings, and other impurities before welding. Fig. 2(c) illustrates the configura-tion of lap-shear and cross-tension specimens. The speci-mens were fabricated from two 30 mm × 150 mm sheets.
All the specimens were welded in the middle of the overlap area. Shear strength tests and tensile strength tests were per-formed according to Japanese Industrial Standard (JIS) Z 3136–1999 and Z 3137–1999, respectively, and were carried out on a CSS-44100 universal testing machine.
Fig. 2. Ultrasonic device and specimens: (a) RFSSW tool; (b) ultrasonic device; (c) dimensions of the test specimen.
Three welding experiments were carried out with lon-gitudinal ultrasonic assistance (UA), lateral UA, and no ul-trasonic oscillations. The method of longitudinal UA was performed using a special fixture to hold the ultrasonic vi-bration head under the welding head longitudinally, as shown in Fig. 3(a). The lateral UA is as shown in Fig. 3(b). When the longitudinal ultrasonic-assisted RFSSW experi-ment was performed, the ultrasonic oscillation head con-taining ultrasonic horn and transducer was mounted onto the RFSSW head directly under the RFSSW tool in 12-o’clock position. The RFSSW tool and ultrasonic horn were coaxial and facing each other. They were used to
perform the RFSSW process and to induce the ultrasonic oscillations on clamped specimens. For the lateral ultra-sonic-assisted RFSSW experiment, the ultrasonic oscilla-tion head was changed to 9-o’clock position below the RFSSW tool. The axis of the ultrasonic horn was ortho-gonal to that of the RFSSW tool. The specimens were clamped between the RFSSW tool and the backing anvil mounted on the RFSSW head. The RFSSW tool was used to carry out the RFSSW process, and the ultrasonic horn provided the ultrasonic oscillations. In the case of the no-ultrasonic-assisted RFSSW experiment, the ultrason-ic-assisted device was switched off.
Fig. 3. Ultrasonic-assisted welding head and the longitudinal (a) and horizontal (b) experiment models.
3. Analysis of comparison tests
In the comparison tests, the process parameters were as shown in Table 1. All of the values were the same for the no-ultrasonic-assisted RFSSW, lateral-assisted RFSSW, and longitudinal-assisted RFSSW for 5A06 aluminum alloys.
Fig. 4(a) shows the RFSSW joint cross-section of 5A06 aluminum alloy welded under the condition of no UA. Three obvious bonding ligaments are observed on the cross-sections
of the RFSSW weld, two vertical bonding ligaments and a horizontal bonding ligament. The vertical bonding ligament is located approximately where the sleeve operates in the upper sheet, and the horizontal bonding ligament is located between the upper and lower sheets. This phenomenon is explained by the relatively complex movement during the FFSSW process which causes a sharp change in weld ma-terial flow in the transition zone, followed by the formation of bonding ligaments.
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Table 1. RFSSW process parameters
Tool rotating speed / (r·min1)
Welding time / s
Penetration depth / mm
Axial pressure / kN
2000 2.5 0.20 11
Fig. 4(b) shows the RFSSW joint cross-section of 5A06 aluminum alloys welded under the condition of longitudinal UA. The vertical bonding ligaments are still obvious, whe-reas the horizontal bonding ligament is indistinct. The tran-sition zone between the upper and lower sheets is very
smooth. In the local horizontal bonding ligament area be-tween two sheets, the material microvibration caused by the longitudinal ultrasonic oscillation results in an increase in the local heat input. The material then plasticizes further, forges ahead, and backfills more fully in the joint. Hence, the horizontal bonding ligament is partly eliminated com-pared to that of RFSSW joints without UA. This phenome-non indicates that longitudinal ultrasonic oscillation strongly affects the horizontal bonding ligament whose surface is normal to the oscillation direction.
Fig. 4. Metallographic structures of joints without UA (a), with longitudinal UA (b), and with lateral UA (c).
Fig. 4(c) shows the RFSSW joint cross-section of 5A06 aluminum alloy welded under the condition of lateral UA. The horizontal bonding ligament is still obvious, whereas the vertical bonding ligament is indistinct. The transition zone around where the sleeve works in the upper sheet is very smooth. In the local vertical bonding ligament area around where the sleeve operates, the material microvibra-tion caused by the lateral ultrasonic oscillation results in an increase in the local heat input. The material then plasticizes further, forges ahead, and backfills more fully in the joint. Hence, the vertical bonding ligament is partly eliminated. This phenomenon indicates that the lateral ultrasonic oscilla-tion also strongly affects the vertical bonding ligament whose cylindrical surface is partly normal to the oscillation direction.
Fig. 5(a) shows the RFSSW joint shear strength of 15 lap-shear specimens, 5 with no UA, 5 with lateral UA, and 5 with longitudinal UA. The shear strength of the RFSSW joints with longitudinal UA increases by 32% on average
compared to that with no UA. Moreover, the joints with lat-eral UA only increases by 7% on average. Thus, longitudin-al ultrasonic oscillation can strengthen the horizontal bond-ing ligament between two sheets. It can be concluded that the horizontal bonding ligament plays a more important role in shear strength than in tensile strength of the RFSSW joints.
Fig. 5(b) shows the RFSSW joint tensile strength for 15 cross-tension specimens, 5 with no UA, 5 with lateral UA, and 5 with longitudinal UA. The tensile strength of the RFSSW joint with lateral UA increases by 46% on average compared to that with no UA, whereas that formed with longitudinal UA only increases by 16% on average. As evi-dent from these results, the lateral ultrasonic oscillation can strengthen the vertical bonding ligament around where the sleeve operates in the upper sheet. Thus, the vertical bond-ing ligament plays a more important role in the tensile strength than in the shear strength of the RFSSW joints.
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Fig. 5. Comparison of the shear strength (a) and tensile strength (b) of 5A06 joints.
4. Optimization of process parameters
In this article, the frequency and power output of the ul-trasonic device were constant at 20 kHz and 1.5 kW, respec-tively. The mechanical properties of the RFSSW joints with longitudinal/lateral UA changed as the RFSSW process pa-rameters (i.e., tool rotating speed (n), welding time (t), pe-netration depth (h), and axial pressure (F)) were varied.
Fig. 6 shows the shear strength and the tensile strength of RFSSW joints with longitudinal/lateral UA at six different tool rotating speeds (n = 500, 1000, 1500, 2000, 2500, and 3000 r/min) in the condition of t = 3.5 s, h = 0.2 mm, and F = 11 kN. The results in the figure clearly show that the
shear strength and tensile strength of the RFSSW joints both first increase and then decrease with increasing tool rotating speed. The shear strength reaches its maximum value at a tool rotating speed of 2000 r/min, the tensile strength of the joint with longitudinal UA reaches its maximum value when the tool rotating speed is 2500 r/min, and the tensile strength of the joint with lateral UA reaches its maximum value when the tool rotating speed is 2000 r/min.
Fig. 7 shows the shear strength and tensile strength of RFSSW joints with longitudinal/lateral UA at six different welding times (t = 0.5, 1.5, 2.5, 3.5, 4.5, and 5.5 s) in the condition of h = 0.2 mm, n = 2000 r/min, and F = 11 kN. As evident from the results, the shear strength and tensile
Fig. 6. Influence of rotating speed on the shear strength (a) and tensile strength (b) at h = 0.2 mm, t = 3.5s, and F = 11 kN.
Fig. 7. Influence of the welding time on the shear strength (a) and tensile strength (b) at h = 0.2 mm, n = 2000 r/min, and F = 11 kN.
X.B. Liu et al., Metallographic structure, mechanical properties, and process parameter optimization of 5A06 joints... 169
strength of the RFSSW joint both first increase and then de-crease with increasing welding time. The shear strength and tensile strength both reach their maximum values at t = 3.5 s.
Fig. 8 shows the shear strength and tensile strength of RFSSW joints with longitudinal/lateral UA at six different penetration depths (h = 0, 0.1, 0.2, 0.4, 0.6, and 0.8 mm) in the condition of n = 2000 r/min, t = 3.5 s, and F = 11 kN. As evident from the results, both the shear strength and the ten-sile strength of the RFSSW joints first increase and then de-crease with increasing penetration depth. The maximum
shear and tensile strength both reach their maximum values at h = 0.2 mm.
Fig. 9 shows the shear strength and tensile strength of RFSSW joints with longitudinal/lateral UA at seven differ-ent axial pressures (F = 5, 7, 11, 13, 16, 18, and 20 kN) in the condition of h = 0.2 mm, n = 2000 r/min, and t = 3.5 s. The results show that both the shear strength and tensile strength of the RFSSW joints first increase and then de-crease with increasing axial pressure. The shear and tensile strengths both reach their maximum values at F = 11 kN.
Fig. 8. Influence of penetration depth on the shear strength (a) and tensile strength (b) at n = 2000 r/min, t = 3.5 s, and F = 11 kN.
Fig. 9. Influence of axial pressure on the shear strength (a) and tensile strength (b) at h = 0.2 mm, n = 2000 r/min, and t = 3.5 s.
Figs. 6(a), 7(a), 8(a), and 9(a) reveal that the shear strength of the RFSSW joints formed with longitudinal UA is higher than that formed with lateral UA. Figs. 6(b), 7(b), 8(b), and 9(b) show that the tensile strengths of the RFSSW joints formed with longitudinal UA are lower than that formed with lateral UA.
On the basis of the aforementioned analysis, the shear strength of the RFSSW joint formed with longitudinal UA reached a maximum value of 8761 N, and the tensile strength of the RFSSW joint formed with lateral UA reached a maximum value of 3679 N when the frequency and power of the ultrasonic device output were constant at 20 kHz and 1.5 kW, respectively. The optimized processing parameters
were followed as a tool rotating speed of 2000 r/min, a welding time of 3.5 s, a penetration depth of 0.2 mm, and an axial pressure of 11 kN.
5. Conclusions
(1) Longitudinal ultrasonic oscillation can strengthen the horizontal bonding ligament in the RFSSW joints of 5A06 aluminum alloys and increase the joint shear strength by 32% on average compared to the joints formed without UA.
(2) Lateral ultrasonic oscillation can strengthen the ver-tical bonding ligaments in the RFSSW joints of 5A06 alu-minum alloys and increase the joint tensile strength by 46%
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on average compared to the joints formed without UA. (3) Longitudinal ultrasonic oscillation can strengthen the
horizontal bonding ligament in a joint and has a more pro-nounced effect on the joint shear strength than lateral ultra-sonic oscillation, whereas lateral ultrasonic oscillation can strengthen the vertical bonding ligaments in the joint and is more effective than longitudinal ultrasonic oscillation in in-creasing joint tensile strength.
(4) The maximum shear strength of the ultrason-ic-assisted RFSSW 5A06 aluminum alloy joints is 8761 N, and the maximum tensile strength is 3679 N when the joints are formed at a tool rotating speed of 2000 r/min, a welding time of 3.5 s, a penetration depth of 0.2 mm, and an axial pressure of 11 kN.
Acknowledgments
This work was financially supported by Hunan Science and Technology Research Projects (Nos. 2016GK2021 and 2016TP1023) and Hunan Provincial Natural Science Foun-dation of China (No. 2016JJ4082). The authors also would like to thank Dr. Axel Meyer from Application and Tech-nology Centre for Robotic Friction Welding who provided the assistance with the development of the spot welding de-vice.
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