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Friction Stir Welding assisted by electrical Joule effect
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Friction Stir Welding assisted by electrical Joule effect Telmo G. Santos a,∗ , R.M. Miranda a , Pedro Vilac¸a b a UNIDEMI, Departamento de Engenharia Mecânica e Industrial, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal b Department of Engineering Design and Production, School of Engineering, Aalto University, Finland
a r t i c l e
i n f o
Article history: Received 26 July 2013 Received in revised form 20 January 2014 Accepted 6 March 2014 Available online xxx Keywords: Friction Stir Welding Root defects Joule effect Electrical current FSW variant
a b s t r a c t This paper presents a variant of Friction Stir Welding (FSW) aiming to minimize or eliminate the root defects that still constitute a major constrain to a wider dissemination of FSW into industrial applications. The concept is based on the use of an external electrical energy source, delivering a high intensity current, passing through a thin layer of material between the back plate and the lower tip of the tool probe. Heat generated by Joule effect improves material viscoplasticity in this region, minimizing the root defects. The concept was validated by analytical and experimental analysis. For the later, a new dedicated tool was designed, manufactured and tested. Numerical simulations were performed to study the electrical current flow pattern and its effect on the material below the probe tip. The potential use of this variant was shown by reducing the size of the weld root defect, even for significant levels of lack of penetration, without affecting overall metallurgical characteristics of the welded joints. © 2014 Elsevier B.V. All rights reserved.
1. Introduction One of the major constrains to a wider application of Friction Stir Welding (FSW) in industry is the formation of root defects in butt welds, since it is difficult to control the penetration along the welds. Internal defects in a FSW joint (e.g. volumetric and planar) can be overcome with the correct development of shoulder and probe geometric features and processing parameters. The origin of the root defects in FSW is, usually, attributed to insufficient viscoplastic material flow below the probe tip due to an insufficient probe penetration. A thin layer of material below the probe is not fully processed by the tool rotation and displacement. This fact is even more critical for welding engineering materials where superficial oxides layer is very stable and the heat conductivity is high, as in aluminium alloys. A poor metallic continuity results when an oxide alignment remains in the bottom of the nugget, that is, when there is no recrystallization along the full thickness. In case the probe penetration is even lower, the nugget is not fully developed across the full thickness and LoP defects occur. In this case, the defect is coincident with the non-recrystallized zone. The most significant defects are oxides alignments and lack of penetration (LoP). Lammlein et al. (2011) reported on the
∗ Corresponding author. Tel.: +351 21 2948567. E-mail address: [email protected] (T.G. Santos).
significance of these defects in Al alloys and Cui et al. (2012) characterized them and studied their effect on the mechanical behaviour of friction stir welded AA6061-T4 joints in a T configuration, while Zhang et al. (2006) studied the formation procedure associated to the lack of penetration in Mg alloys. Extensive work has been developed on the influence of processing parameters on the microstructural characterization of root defects, as well as, on their effect on mechanical properties as the ones from Rodrigues et al. (2009) and Moreira et al. (2008). The later authors studied the fatigue behaviour of welded joints using notched samples and a remarkable drop in fatigue was noticed when there was a stress concentration point on the surface. More recently, similar studies are performed on steel due to the increasing importance of FSW in these alloys. Rosado et al. (2010) showed that the root defects, both oxide alignment and LoP, are of concern because they are difficult to prevent by controlling the process parameters, and also because they are difficult to detect by Non-Destructive Testing. The presence of these defects dramatically reduces the mechanical resistance of welded joints, especially under fatigue loading and corrosive environments, contributing to catastrophic failures of FSW constructions. Thus, they limit the application of FSW in components with high safety requirements, such as in aeronautic and naval industries. Several techniques exist to overcome this problem, namely machining the weld root or using dedicated FSW tools as the bobbin toolTM patented by Thomas et al. (1991). The first constitute
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additional reworks with material loss and increased costs. The second still has some technological problems related to the frequent cracking of the tool due to the highly demanding combination of loads involved. So, these techniques have not been systematically implemented in industry and there is still a need for developing technology to minimize, or eliminate root defects improving the joints performance and contributing to strengthen the industrial implementation of FSW. Several alternatives have been developed aiming to improve specific aspects of the FSW process and, thus, the overall mechanical behaviour. Examples are: underwater FSW developed by Liu et al. (2011) aiming to refine the microstructure and improve strength, hybrid FSW assisted by laser tested by Merklein and Giera (2008) to soften the material and improve its viscoplastic deformation behaviour and the use of an induction coil in front of the rotating tool with the same purpose patented by Midling (1999). Long and Khanna (2005) numerically modelled a variant of FSW assisted by an electric current with two objectives: reducing the forging force necessary for FSW and minimize tool wear. Ferrando (2008) experimentally tested this idea, while Park (2009) used ultrasounds with the same objective. In fact, tool wear is a relevant issue when welding hard materials as steel or titanium alloys, but the forging force can be negligible, since it does not consume energy and is ensured by the stiffness of the machines. None of these process variants focused on the need to prevent root defects, which is nowadays the key issue in FSW. This paper presents the results achieved so far on a process variant of FSW using assisted electric current to increase the viscoplasticity of the material in the root of a butt weld, aiming at reducing, or even eliminating, these defects. The concept of FSW assisted by electric Joule effect is presented and discussed. Experimental and analytical validation were performed and the results are quite promising in terms of increasing the welding process reliability.
2. The concept of FSW assisted by electric Joule effect The conventional FSW originates a viscoplastic material flow beneath the probe that is usually insufficient to fully recrystallize the material in this region. The root zone of the weld joint is the only region of the original abutting surfaces that do not undergo severe tri-dimensional viscoplastic deformation promoting the correct stiring effect of the base materials. This zone undergoes essentially severe distortion, since it is mainly deformed by shear stress acting in the tangential direction but not in the axial one. Moreover, the viscoplasticity of this zone is affected by the significant loss of heat via conductivity into the back plate. So, an improvement of the material viscoplasticity would be beneficial and, for this, an external energy source is proposed. The basic concept relies on the use of an external electrical energy source delivering a high intensity current passing through a thin layer of material between the back plate and the probe lower tip, generating heat by Joule effect (Fig. 1). This can find some similitude to an hybrid process involving the principles of FSW and continuous seam resistance welding. The heat generated under these conditions by Joule effect, raises the material temperature and, thus, its ductility increases, while reducing mechanical strength and improving the viscoplasticity. This promotes more efficient local stirring effect and uniform dynamic recrystallization, preventing oxide alignment and LoP defects in the weld root. In the conventional FSW, to increase the process temperature it is necessary to increase the rotation to travel speed ratio (˝/Vx ) [rev/mm], since to increase the viscoplastic material flow on the root it would be necessary to increase the overall heat input, e.g. by increasing the rotation to travel speed
ratio. This procedure would promote a wider HAZ with a higher loss of properties. This innovative concept represents a decoupling between the process temperature and the welding parameters. So, even in cold welding conditions, with low rotation to travel speeds ratios, the temperature in the root zone can be high. In fact, electrical assisted processes attracted the interest of several research groups working in manufacturing process development with the main objective of softening the material, as Maia et al. (2013) showed for the hot embossing technology and Zimniak and Adkiewicz (2008) in copper drawing. Hybrid processes that is, processes where a synergic effect is achieved from the individual processes involved, is increasingly used in industry, taking advantage of the benefit specificities of each process and minimizing the drawbacks they exhibit. The objective of the concept, developed within this work, relies on the benefits of assisting FSW by Joule effect in order to improve a defect free process in the bottom part of the plates where the tool can hardly process the material and a non-recrystallized zone occurs with poor material consolidation, especially in butt welds. Fig. 1 depicts a scheme of the concept. It considers a specially designed FSW tool to allow a high intensity electrical current to flow into the weld root aiming to improve the local energy efficiency of the process. A simple analytical model was used to validate the concept of FSW assisted by electrical current where all the generated heat is due to Joule effect. The model relies on two assumptions: (i) the electric current I (A) flows within the copper core of the probe as depicted in Fig. 1b with a diameter [m] at a distance h [m] from the back plate, closing the electric circuit; (ii) the increase of temperature due to Joule effect occurs under adiabatic conditions, that is, energy losses to adjacent materials (back plate, base materials and tool) are negligible. Under these conditions, all the heat generated by Joule effect due to the current flowing through the control volume (Fig. 1b) is used to raise the temperature according to Eq. (1). The heat generated due to Joule effect, is the product of the electric power and the time, while the heat to increase the temperature is given by the fundamental calorimetric equation (Eq. (2)), where: R [] is the material electrical resistance, I [A] is the current intensity, t [s] is the time for passing the current, m [kg] is the mass dissipating heat, Cp [J/kg ◦ C] is the specific heat of material and T [◦ C] is the increment in temperature. The electrical resistance can be calculated by Eq. (3), where: h [m] is the inherent gap distance, e [m] is the electrical resistivity and [m] the diameter of the conducting copper core. The interaction time t [s] is related to the welding speed (Vx ) [m/s] and can be calculated by Eq. (4), considering that there is no complete overlapping between the copper core and the material interaction area during the time t. The mass m [kg] is given by Eq. (5) considering the geometrical features of the copper core (), gap distance (h) and the material density [kg/m3 ]. QJoule
= QIncrease
effect
R · I 2 · t = m · Cp · T
temperature
(1) (2)
R=
h · e 2 /4
(3)
t=
Vx
(4)
m=·h·
2 4
(5)
Substituting Eqs. (3)–(5) into Eq. (2) and rearranging, Eq. (6) can be obtained and expresses the increment of temperature in
Please cite this article in press as: Santos, T.G., et al., Friction Stir Welding assisted by electrical Joule effect. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.012
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Fig. 1. Schematic representation of the concept. (a) Transversal view and (b) detail of the previous.
the weld root, simply due to the Joule effect created by passing an electric current. From this equation, four factors can be identified, affected by a constant, each one contributing to the increase of temperature and these are related to: (i) material properties (e /(Cp )); (ii) FSW travel speed (Vx ); (iii) geometric features of copper core diameter () and (iv) electric current intensity (I). T =
16 e 1 1 · · · · I2 2 · Cp Vx 3
(6)
It must be emphasized that this increment in temperature is only due to the electric current, and is added to the heat produced by plastic deformation plus friction between the tool shoulder and the material, and this is also valid for the layer below the probe tip and the back plate. So, it was perceived that the probe diameter () has to be as small as possible to increase the temperature in the material layer below the probe, and the current intensity (I) should be in the upper limits withstanded by copper and base materials. The model does not consider the decrease of the electrical conductivity with the increase in temperature. In fact, when the temperature increases, the electrical conductivity decreases, and consequently the Joule effect is more pronounced. Thus, the values predicted by the model are actually below the ones that would be verified in practice. Plotting Eq. (6) for different materials, assuming a travel speed Vx = 200 mm/min and a = 3 mm, Fig. 2 was obtained from which, it can be clearly seen that for an electrical current of 1000 A an increase in temperature of 300–450 ◦ C can be obtained for aluminium alloys, while this value decreases for copper, a highly conductivity material with an electrical resistivity e = 1.724 × 10−8 m and increases for titanium, which has a high electrical resistivity (e = 1.724 × 10−6 m). Analysing Fig. 2 for AA2024-T3 aluminium alloy, an electrical current of about 900 A generate an increase in temperature of about 400 ◦ C, which is in
the range of the values measured in conventional FSW, without electric current for the same alloy by Vilac¸a et al. (2007). Thus, a more uniform distribution of temperature in the weld is achieved when assisting FSW by an electrical Joule effect and this is another advantage of the process variant presented. 3. Numerical simulation In parallel to the analytical model described, a numerical simulation was performed in order to verify the current density distribution in the tool, in the layer beneath the probe and in the back plate, as shown in Fig. 3. The Finite Element Modeling software CST-EM Studio Suit was used according to the geometrical model shown in Fig. 3a with 3 × 106 hexaedric elements. In this simulation an electrical current (I) of 100 A was imposed, forcing it to flow from the top of the tool to the base plate. The results are presented in a logarithmic scale for a better reading of different electrical current density values in distinct areas of the material and the tool. Fig. 3b is a transversal cut of the tool showing the detail of current density distribution in the layer below the probe, in the copper core and the main base. Fig. 3(c) shows the intensity of the electrical current flow in a vectorial representation. Fig. 3(d) is a top view of the current density distribution in the layer below the probe, halfway between this and the base plate. This is an interesting projection from which it can be seen the electric current flowing from the central spot below the probe tip and the base plate, in a similar shape as in seam resistance welding. From the numerical values computed, about 70% of the total introduced current intensity flows through the layer below the probe tip, while the remaining 30% is deviated from the copper core into other parts, such as the steel probe and the shoulder. 4. Experimental 4.1. Materials Butt joints were produced in cold rolled plates of AA6082-T6 alloy with 4 mm thickness. Table 1 shows the physical properties of the material relevant for this study. 4.2. Equipment An adapted FSW machine was used equipped with a set of dedicated components designed and manufactured to validate the concept. Several constructive solutions could be assumed to implement the concept of FSW assisted by electric joule effect. In this study a modular system was designed, so that the system components could be improved or replaced to test different process parameters, such as the geometrical features of the tool, as well as the electric current intensity. The components designed comprised:
Fig. 2. Temperature increase with electrical current for FSW electrically assisted for different materials.
- a FSW tool (Fig. 4);
Please cite this article in press as: Santos, T.G., et al., Friction Stir Welding assisted by electrical Joule effect. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.012
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Fig. 3. Numerical simulation of the current density distribution. (a) Geometrical model and mesh, (b) transversal cut view, (c) vectorial representation of the previous and (d) top view.
Table 1 Material physical properties of AA6082-T6. Density () (kg/m3 )
Specific heat (Cp ) (J/kg ◦ C)
Electrical resistivity (e ) (m)
Melting temperature (Tm ) (◦ C)
2700
896
3.99 × 10−8
623
Fig. 4. Tool for FSW assisted by electric joule effect. (a) Longitudinal cut of the tool, (b) exploded view of the tool components and (c). Probe detail: (Legend: 1 – tool body; 2 – shoulder; 3 – probe; 4 – copper rod; 5 – copper ring; 6 – copper screws; 7 – probe backstop; 8 – teflon insulator; 9 – bakelite washer insulator).
- a base plate and a brush guide support (Fig. 5); - electronics and data acquisition system (Fig. 6).
4.2.1. Tool An FSW tool was developed in order to meet the following design criteria:
- conduct the current without interfering with the machine electrical circuit;
- confine the electric current to the layer below the probe, avoiding the current to flow through the tool main body and shoulder; - continuous adjustment of the probe length and facility of replacement; - guarantee the mechanical robustness of the tool body.
Fig. 4 depicts the different components of the tool. The main features were a copper rod (4) inserted tightened along the H13 steel probe axis (3) to constrain the electric current flow. The copper
Fig. 5. Base plate and brush guide support. (a) Overview of the base plate, (b) schema of brush guide support and (c) overall assembly on FSW machine. (Legend: 10 – CK45 base plate, 11 – copper insert, 12 – base cables, 13 – chassis of brush support, 14 – graphite brushes, 15 – insulator plate, 16 – brush cables).
Please cite this article in press as: Santos, T.G., et al., Friction Stir Welding assisted by electrical Joule effect. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.012
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Fig. 6. Electronics and data acquisition. (a) Experimental electrical apparatus and data acquisition, and (b) schematic representation of the overall installation.
ring (5) is connected to the graphite brush to deliver the electrical current to the tool. 4.2.2. Base plate and brush guide support Fig. 5 depicts the different components of the base plate and the brush guide support. The base plate (Fig. 5a) is fundamental to properly close the electrical circuit under the probe tip in the weld root. In order to achieve this, a copper ribbon (11) was embedded in a CK45 carbon steel base plate (10) under pressure. A brush system was used to deliver the current to the tool (Fig. 5b). Graphite brushes (14) enable electrical current to flow from the brush cables (16) as it slides in contact with the copper ring (5) allowing rotation. Fig. 5(c) depicts the set up into the FSW machine. 4.2.3. Electronics and data acquisition The overall system is depicted in Fig. 6 and comprises the FSW machine, the electric current supply which consists of a 12 V battery with 720 A nominal current intensity and a data acquisition system. The data acquisition system monitors the welding process at 0.2 s interval, measuring the current intensity through two independent and redundant electrical current transducers, and the voltage at the power supply terminals and between the tool body and the base plate. These data allows to calculate the electrical power delivered to the weld root. A multifunction data acquisition system (DAQ) was used to acquire the data and a dedicated software to show process parameters in a graphical user interface in real time and save data at the end of each test. 5. Results and discussion In order to validate the concept, several welds were performed in butt weld configuration. Fig. 7a shows a friction stir weld produced with a travel speed of 180 mm/min, a rotation speed of 1120 rev/min and a tilt angle of 1.5◦ with a probe plunging depth of 3.3 mm, to deliberately produce a lack of penetration (LOP) since the plate has 4 mm thickness. Friction Stir Welding was continuously produced along the plate with and without electrical current in order to analyze the effect of the last one on the dimension of the root defect, as well as, on the extension of the different zones common in FSW. For this, samples were removed for macrographic analysis in seven positions along the joint (M1–M7) as depicted in Fig. 7a. The weld started without electrical current for a length of 80 mm, and an electrical current of 800 A intensity was forced
to pass through the probe and stopped at 125 mm. Between this point, to a distance of 165 mm, no electrical current was introduced and after that 700 A was introduced again till the end of the weld (190 mm). Plots of current intensity and voltage are depicted in Fig. 7b with the same space scale. As far as the current intensity is concerned, the two transducers showed a good agreement with a minor deviation in the measured values. When analysing the voltage, it can be seen that when the electrical current starts, the voltage measured on the power supply terminal droped from 12 to 6.5 V, while the voltage measured between the tool body and the base plate increased to 4.5 V. This difference corresponds to the electrical resistance of the circuit, which is in this particular case of 2.5 m (R = (6.5–4.5)/800). Computing the mechanical power of a conventional FSW process (Eq. (7)), where Vx [m/s] is the travel speed, Fx [N] is the travel force, [rad/s] is the rotating tool speed and M [Nm] is the torque, using experimental data from Vilac¸a et al. (2007) a value of 5 kW was obtained for AA2024-T351 with 3.9 mm thick with a similar tool geometry and process parameters. Though these values were taken from tests performed without electrical current, it can be assumed that no significant variations exists, on both vertical and transverse forces, since, the processed volume is an order of magnitude larger than the control volume (Fig. 1). Similarly, the electrical power delivered by the electrical source was calculated by Eq. (8) where V [V] is the voltage and I [A] is the current intensity. Pmec = Vx × Fx + ˝ × M
(7)
Pelec = V × I
(8)
Fig. 8 depicts both the mechanical and the electrical powers. The mechanical power of FSW was calculated according to Eq. (7) and is represented by the dotted line. The electrical power supply by the battery is represented by the blue line with circular marks, while the effective electrical power on the FSW tool is shown the red line with triangular marks, both experimentally measured as described in Section 4.2.3. The latter is lower than the previous, since some electrical power was dissipated in the overall installations, mainly in cables by Joule effect. It can be seen that the electrical power delivered is almost of the same order of magnitude as the mechanical power associated to the FSW process, which is an interesting remark, since this additional power is very significant.
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Fig. 7. FSW with and without electrical current. (a) Top view of the weld and (b) plot of current intensity and voltage with the same space scale.
Fig. 8. Comparison of FSW mechanical and electrical powers involved.
The welds were sectioned at M1–M7 locations and macrographs taken from these sections for analysis and these are shown in Figs. 9 and 10. Comparing the macrographs from samples welded without electrical current (M1 and M5) with the ones with electrical current (M3, M4 and M7), no significant evidences exist of the electrical current effect on the extension and morphology of the nugget, as well as, on its structure, where typical geometrical and metallurgical features are observed. Looking at sections M1 and M2 welded
without electrical current, a lack of penetration is seen in the weld root with about 15 m wide and 0.6 mm depth. However, when the electrical current assisted the process (M3, M4 and M7) it is evident from Fig. 11 that the width of the LoP decreased from 15.5 to 3.3 m, and this is the most significant difference observed. This result shows that, even in such adverse welding conditions, as producing a large LoP, there is a clear beneficial effect of the assisted current in minimizing the width of the root defect. This means that the electrical current increase the
Fig. 9. Macrograph of butt weld at position M2 without electrical current.
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Fig. 10. Macrographs of butt welds. (a) and (b) With electrical current of 800 A at positions M3 and M4 respectively, (c) without electrical current at position M5 and (d) with electrical current of 700 A at position M7.
Fig. 11. Micrographs of butt welds. (a) Without electrical current at position M2, and (b) with electrical current of 800 A at position M3.
viscoplastic material flow creating the correct thermo-mechanical conditions to suppress this root defect.
6. Conclusions From the study performed it can be concluded that: - A concept of FSW assisted by electrical current was presented, tested and validated aiming at minimizing weld root defects in FSW. - The concept relies on forcing an electrical current to pass in the weld root to increase the local temperature, improving the material viscoplasticity. - An analytical model validated the concept and it was seen that, for currents of about 800 A, the temperature raised 200–300 ◦ C for the aluminium alloys under study. The numerical simulation allowed characterizing the current density distribution. - Experimental results showed that passing an electrical current through the weld root, LoP defects reduced in size from a width of 15.5 to 3.3 m, under the conditions tested. - Electrical current seems to increase the viscoplastic material flow creating the conditions to suppress LoP, without affecting the overall metallurgical properties of the FSW and thus improving the mechanical performance of FSW butt joints. The concept reduces the sensitivity of the FSW process to generate root defects supporting an easier industrialization of the process into high demanding applications.
Acknowledgment T.S. and R.M. acknowledge Pest OE/EME/UI0667/2011 from the Portuguese Fundac¸ão para a Ciência e a Tecnologia (FCT-MEC).
References Cui, L., Yang, X., Zhou, G., Xu, X., Shen, Z., 2012. Characteristics of defects and tensile behaviors on friction stir welded AA6061-T4 T-joints. Mater. Sci. Eng. A 543, 58–68. Ferrando, W.A., 2008. The Concept of Electrically Assisted Friction Stir Welding (EAFSW) and Application to the Processing of Various Metals. Technical Report (NSWCCD-61-TR-2008/13). Naval Surface Warfare Center Carderock Division. Lammlein, D., Trepal, N., Posada, M.A., 2011. Defect Significance and Detection in Aluminum Friction Stir Welds: A Literature Search. Technical Report NSWCCD-61-TR-2011/20. Naval Surface Warfare Center, Carderock Division, West Bethesda. Liu, H.J., Zhang, H.J., Yu, L., 2011. Effect of welding speed on microstructures and mechanical properties of underwater friction stir welded 2219 aluminum alloy. J. Mater. Des. 32, 1548–1553. Long, X., Khanna, S., 2005. Modelling of electrically enhanced Friction Stir Welding process using finite element method. Sci. Technol. Weld. Joi. 10, 482–487. Maia, J., Penga, L., Lai, X., Lin, Z., 2013. Electrical-assisted embossing process for fabrication of micro-channels on 316L stainless steel plate. J. Mater. Process Technol. 213, 314–321. Merklein, M.G., Giera, A., 2008. Laser assisted Friction Stir Welding of drawable steelaluminum tailored hybrids. Int. J. Mater. Form 1, 1299–1302. Midling, O., 1999. Modified Friction Stir Welding. In: International Patent Application WO 99/39861. Norsk Hydro, USA. Moreira, P.M.G.P., Oliveira, F.M.F., Castro, P.T.C., 2008. Fatigue behaviour of notched specimens of friction stir welded aluminium alloy 6063-T6. J. Mater. Process. Technol. 207, 283–292. Park, K., 2009. Development and Analysis of Ultrasonic Assisted Friction Stir Welding Process (Ph.D. thesis). University of Michigan, USA. Rodrigues, D.M., Loureiro, A., Leitão, C., Leal, R.M., Chaparro, B.M., Vilac¸a, P., 2009. Influence of Friction Stir Welding parameters on the microstructural and mechanical properties of AA 6016-T4 thin welds. Mater. Des. 30, 1913–1921. Rosado, L., Santos, T.G., Piedade, M., Ramos, P., Vilac¸a, P., 2010. Advanced technique for non-destructive testing of Friction Stir Welding of metals. Measurement 43, 1021–1030. Thomas, W.M., Nicholas, E.D., Needham, M.G., Templesmith, D.J., 1991. Friction stir butt welding. International Patent Application PCT/GB92/02203, GB Patent Application 9125978.8, US Patent 5.460.317. Vilac¸a, P., Quintino, L., dos Santos, J.F., Zettler, R., Sheikhi, S., 2007. Quality assessment of Friction Stir Welding joints via analytical thermal model, iSTIR. Mater. Sci. Eng. A 445, 501–508. Zhang, H., Lin, S.B., Wu, L., Feng, J.C., Ma, S.L., 2006. Defects formation procedure and mathematic model for defect free Friction Stir Welding of magnesium alloy. J. Mater. Des. 27, 805–809. Zimniak, Z., Adkiewicz, G., 2008. The electroplastic effect in the cold-drawing of copper wires for the automotive industry. Arch. Civil Mech. Eng. 8, 173–179.
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Friction Stir Welding assisted by electrical Joule effect Telmo G. Santos a,∗ , R.M. Miranda a , Pedro Vilac¸a b a UNIDEMI, Departamento de Engenharia Mecânica e Industrial, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal b Department of Engineering Design and Production, School of Engineering, Aalto University, Finland
a r t i c l e
i n f o
Article history: Received 26 July 2013 Received in revised form 20 January 2014 Accepted 6 March 2014 Available online xxx Keywords: Friction Stir Welding Root defects Joule effect Electrical current FSW variant
a b s t r a c t This paper presents a variant of Friction Stir Welding (FSW) aiming to minimize or eliminate the root defects that still constitute a major constrain to a wider dissemination of FSW into industrial applications. The concept is based on the use of an external electrical energy source, delivering a high intensity current, passing through a thin layer of material between the back plate and the lower tip of the tool probe. Heat generated by Joule effect improves material viscoplasticity in this region, minimizing the root defects. The concept was validated by analytical and experimental analysis. For the later, a new dedicated tool was designed, manufactured and tested. Numerical simulations were performed to study the electrical current flow pattern and its effect on the material below the probe tip. The potential use of this variant was shown by reducing the size of the weld root defect, even for significant levels of lack of penetration, without affecting overall metallurgical characteristics of the welded joints. © 2014 Elsevier B.V. All rights reserved.
1. Introduction One of the major constrains to a wider application of Friction Stir Welding (FSW) in industry is the formation of root defects in butt welds, since it is difficult to control the penetration along the welds. Internal defects in a FSW joint (e.g. volumetric and planar) can be overcome with the correct development of shoulder and probe geometric features and processing parameters. The origin of the root defects in FSW is, usually, attributed to insufficient viscoplastic material flow below the probe tip due to an insufficient probe penetration. A thin layer of material below the probe is not fully processed by the tool rotation and displacement. This fact is even more critical for welding engineering materials where superficial oxides layer is very stable and the heat conductivity is high, as in aluminium alloys. A poor metallic continuity results when an oxide alignment remains in the bottom of the nugget, that is, when there is no recrystallization along the full thickness. In case the probe penetration is even lower, the nugget is not fully developed across the full thickness and LoP defects occur. In this case, the defect is coincident with the non-recrystallized zone. The most significant defects are oxides alignments and lack of penetration (LoP). Lammlein et al. (2011) reported on the
∗ Corresponding author. Tel.: +351 21 2948567. E-mail address: [email protected] (T.G. Santos).
significance of these defects in Al alloys and Cui et al. (2012) characterized them and studied their effect on the mechanical behaviour of friction stir welded AA6061-T4 joints in a T configuration, while Zhang et al. (2006) studied the formation procedure associated to the lack of penetration in Mg alloys. Extensive work has been developed on the influence of processing parameters on the microstructural characterization of root defects, as well as, on their effect on mechanical properties as the ones from Rodrigues et al. (2009) and Moreira et al. (2008). The later authors studied the fatigue behaviour of welded joints using notched samples and a remarkable drop in fatigue was noticed when there was a stress concentration point on the surface. More recently, similar studies are performed on steel due to the increasing importance of FSW in these alloys. Rosado et al. (2010) showed that the root defects, both oxide alignment and LoP, are of concern because they are difficult to prevent by controlling the process parameters, and also because they are difficult to detect by Non-Destructive Testing. The presence of these defects dramatically reduces the mechanical resistance of welded joints, especially under fatigue loading and corrosive environments, contributing to catastrophic failures of FSW constructions. Thus, they limit the application of FSW in components with high safety requirements, such as in aeronautic and naval industries. Several techniques exist to overcome this problem, namely machining the weld root or using dedicated FSW tools as the bobbin toolTM patented by Thomas et al. (1991). The first constitute
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additional reworks with material loss and increased costs. The second still has some technological problems related to the frequent cracking of the tool due to the highly demanding combination of loads involved. So, these techniques have not been systematically implemented in industry and there is still a need for developing technology to minimize, or eliminate root defects improving the joints performance and contributing to strengthen the industrial implementation of FSW. Several alternatives have been developed aiming to improve specific aspects of the FSW process and, thus, the overall mechanical behaviour. Examples are: underwater FSW developed by Liu et al. (2011) aiming to refine the microstructure and improve strength, hybrid FSW assisted by laser tested by Merklein and Giera (2008) to soften the material and improve its viscoplastic deformation behaviour and the use of an induction coil in front of the rotating tool with the same purpose patented by Midling (1999). Long and Khanna (2005) numerically modelled a variant of FSW assisted by an electric current with two objectives: reducing the forging force necessary for FSW and minimize tool wear. Ferrando (2008) experimentally tested this idea, while Park (2009) used ultrasounds with the same objective. In fact, tool wear is a relevant issue when welding hard materials as steel or titanium alloys, but the forging force can be negligible, since it does not consume energy and is ensured by the stiffness of the machines. None of these process variants focused on the need to prevent root defects, which is nowadays the key issue in FSW. This paper presents the results achieved so far on a process variant of FSW using assisted electric current to increase the viscoplasticity of the material in the root of a butt weld, aiming at reducing, or even eliminating, these defects. The concept of FSW assisted by electric Joule effect is presented and discussed. Experimental and analytical validation were performed and the results are quite promising in terms of increasing the welding process reliability.
2. The concept of FSW assisted by electric Joule effect The conventional FSW originates a viscoplastic material flow beneath the probe that is usually insufficient to fully recrystallize the material in this region. The root zone of the weld joint is the only region of the original abutting surfaces that do not undergo severe tri-dimensional viscoplastic deformation promoting the correct stiring effect of the base materials. This zone undergoes essentially severe distortion, since it is mainly deformed by shear stress acting in the tangential direction but not in the axial one. Moreover, the viscoplasticity of this zone is affected by the significant loss of heat via conductivity into the back plate. So, an improvement of the material viscoplasticity would be beneficial and, for this, an external energy source is proposed. The basic concept relies on the use of an external electrical energy source delivering a high intensity current passing through a thin layer of material between the back plate and the probe lower tip, generating heat by Joule effect (Fig. 1). This can find some similitude to an hybrid process involving the principles of FSW and continuous seam resistance welding. The heat generated under these conditions by Joule effect, raises the material temperature and, thus, its ductility increases, while reducing mechanical strength and improving the viscoplasticity. This promotes more efficient local stirring effect and uniform dynamic recrystallization, preventing oxide alignment and LoP defects in the weld root. In the conventional FSW, to increase the process temperature it is necessary to increase the rotation to travel speed ratio (˝/Vx ) [rev/mm], since to increase the viscoplastic material flow on the root it would be necessary to increase the overall heat input, e.g. by increasing the rotation to travel speed
ratio. This procedure would promote a wider HAZ with a higher loss of properties. This innovative concept represents a decoupling between the process temperature and the welding parameters. So, even in cold welding conditions, with low rotation to travel speeds ratios, the temperature in the root zone can be high. In fact, electrical assisted processes attracted the interest of several research groups working in manufacturing process development with the main objective of softening the material, as Maia et al. (2013) showed for the hot embossing technology and Zimniak and Adkiewicz (2008) in copper drawing. Hybrid processes that is, processes where a synergic effect is achieved from the individual processes involved, is increasingly used in industry, taking advantage of the benefit specificities of each process and minimizing the drawbacks they exhibit. The objective of the concept, developed within this work, relies on the benefits of assisting FSW by Joule effect in order to improve a defect free process in the bottom part of the plates where the tool can hardly process the material and a non-recrystallized zone occurs with poor material consolidation, especially in butt welds. Fig. 1 depicts a scheme of the concept. It considers a specially designed FSW tool to allow a high intensity electrical current to flow into the weld root aiming to improve the local energy efficiency of the process. A simple analytical model was used to validate the concept of FSW assisted by electrical current where all the generated heat is due to Joule effect. The model relies on two assumptions: (i) the electric current I (A) flows within the copper core of the probe as depicted in Fig. 1b with a diameter [m] at a distance h [m] from the back plate, closing the electric circuit; (ii) the increase of temperature due to Joule effect occurs under adiabatic conditions, that is, energy losses to adjacent materials (back plate, base materials and tool) are negligible. Under these conditions, all the heat generated by Joule effect due to the current flowing through the control volume (Fig. 1b) is used to raise the temperature according to Eq. (1). The heat generated due to Joule effect, is the product of the electric power and the time, while the heat to increase the temperature is given by the fundamental calorimetric equation (Eq. (2)), where: R [] is the material electrical resistance, I [A] is the current intensity, t [s] is the time for passing the current, m [kg] is the mass dissipating heat, Cp [J/kg ◦ C] is the specific heat of material and T [◦ C] is the increment in temperature. The electrical resistance can be calculated by Eq. (3), where: h [m] is the inherent gap distance, e [m] is the electrical resistivity and [m] the diameter of the conducting copper core. The interaction time t [s] is related to the welding speed (Vx ) [m/s] and can be calculated by Eq. (4), considering that there is no complete overlapping between the copper core and the material interaction area during the time t. The mass m [kg] is given by Eq. (5) considering the geometrical features of the copper core (), gap distance (h) and the material density [kg/m3 ]. QJoule
= QIncrease
effect
R · I 2 · t = m · Cp · T
temperature
(1) (2)
R=
h · e 2 /4
(3)
t=
Vx
(4)
m=·h·
2 4
(5)
Substituting Eqs. (3)–(5) into Eq. (2) and rearranging, Eq. (6) can be obtained and expresses the increment of temperature in
Please cite this article in press as: Santos, T.G., et al., Friction Stir Welding assisted by electrical Joule effect. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.012
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Fig. 1. Schematic representation of the concept. (a) Transversal view and (b) detail of the previous.
the weld root, simply due to the Joule effect created by passing an electric current. From this equation, four factors can be identified, affected by a constant, each one contributing to the increase of temperature and these are related to: (i) material properties (e /(Cp )); (ii) FSW travel speed (Vx ); (iii) geometric features of copper core diameter () and (iv) electric current intensity (I). T =
16 e 1 1 · · · · I2 2 · Cp Vx 3
(6)
It must be emphasized that this increment in temperature is only due to the electric current, and is added to the heat produced by plastic deformation plus friction between the tool shoulder and the material, and this is also valid for the layer below the probe tip and the back plate. So, it was perceived that the probe diameter () has to be as small as possible to increase the temperature in the material layer below the probe, and the current intensity (I) should be in the upper limits withstanded by copper and base materials. The model does not consider the decrease of the electrical conductivity with the increase in temperature. In fact, when the temperature increases, the electrical conductivity decreases, and consequently the Joule effect is more pronounced. Thus, the values predicted by the model are actually below the ones that would be verified in practice. Plotting Eq. (6) for different materials, assuming a travel speed Vx = 200 mm/min and a = 3 mm, Fig. 2 was obtained from which, it can be clearly seen that for an electrical current of 1000 A an increase in temperature of 300–450 ◦ C can be obtained for aluminium alloys, while this value decreases for copper, a highly conductivity material with an electrical resistivity e = 1.724 × 10−8 m and increases for titanium, which has a high electrical resistivity (e = 1.724 × 10−6 m). Analysing Fig. 2 for AA2024-T3 aluminium alloy, an electrical current of about 900 A generate an increase in temperature of about 400 ◦ C, which is in
the range of the values measured in conventional FSW, without electric current for the same alloy by Vilac¸a et al. (2007). Thus, a more uniform distribution of temperature in the weld is achieved when assisting FSW by an electrical Joule effect and this is another advantage of the process variant presented. 3. Numerical simulation In parallel to the analytical model described, a numerical simulation was performed in order to verify the current density distribution in the tool, in the layer beneath the probe and in the back plate, as shown in Fig. 3. The Finite Element Modeling software CST-EM Studio Suit was used according to the geometrical model shown in Fig. 3a with 3 × 106 hexaedric elements. In this simulation an electrical current (I) of 100 A was imposed, forcing it to flow from the top of the tool to the base plate. The results are presented in a logarithmic scale for a better reading of different electrical current density values in distinct areas of the material and the tool. Fig. 3b is a transversal cut of the tool showing the detail of current density distribution in the layer below the probe, in the copper core and the main base. Fig. 3(c) shows the intensity of the electrical current flow in a vectorial representation. Fig. 3(d) is a top view of the current density distribution in the layer below the probe, halfway between this and the base plate. This is an interesting projection from which it can be seen the electric current flowing from the central spot below the probe tip and the base plate, in a similar shape as in seam resistance welding. From the numerical values computed, about 70% of the total introduced current intensity flows through the layer below the probe tip, while the remaining 30% is deviated from the copper core into other parts, such as the steel probe and the shoulder. 4. Experimental 4.1. Materials Butt joints were produced in cold rolled plates of AA6082-T6 alloy with 4 mm thickness. Table 1 shows the physical properties of the material relevant for this study. 4.2. Equipment An adapted FSW machine was used equipped with a set of dedicated components designed and manufactured to validate the concept. Several constructive solutions could be assumed to implement the concept of FSW assisted by electric joule effect. In this study a modular system was designed, so that the system components could be improved or replaced to test different process parameters, such as the geometrical features of the tool, as well as the electric current intensity. The components designed comprised:
Fig. 2. Temperature increase with electrical current for FSW electrically assisted for different materials.
- a FSW tool (Fig. 4);
Please cite this article in press as: Santos, T.G., et al., Friction Stir Welding assisted by electrical Joule effect. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.012
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Fig. 3. Numerical simulation of the current density distribution. (a) Geometrical model and mesh, (b) transversal cut view, (c) vectorial representation of the previous and (d) top view.
Table 1 Material physical properties of AA6082-T6. Density () (kg/m3 )
Specific heat (Cp ) (J/kg ◦ C)
Electrical resistivity (e ) (m)
Melting temperature (Tm ) (◦ C)
2700
896
3.99 × 10−8
623
Fig. 4. Tool for FSW assisted by electric joule effect. (a) Longitudinal cut of the tool, (b) exploded view of the tool components and (c). Probe detail: (Legend: 1 – tool body; 2 – shoulder; 3 – probe; 4 – copper rod; 5 – copper ring; 6 – copper screws; 7 – probe backstop; 8 – teflon insulator; 9 – bakelite washer insulator).
- a base plate and a brush guide support (Fig. 5); - electronics and data acquisition system (Fig. 6).
4.2.1. Tool An FSW tool was developed in order to meet the following design criteria:
- conduct the current without interfering with the machine electrical circuit;
- confine the electric current to the layer below the probe, avoiding the current to flow through the tool main body and shoulder; - continuous adjustment of the probe length and facility of replacement; - guarantee the mechanical robustness of the tool body.
Fig. 4 depicts the different components of the tool. The main features were a copper rod (4) inserted tightened along the H13 steel probe axis (3) to constrain the electric current flow. The copper
Fig. 5. Base plate and brush guide support. (a) Overview of the base plate, (b) schema of brush guide support and (c) overall assembly on FSW machine. (Legend: 10 – CK45 base plate, 11 – copper insert, 12 – base cables, 13 – chassis of brush support, 14 – graphite brushes, 15 – insulator plate, 16 – brush cables).
Please cite this article in press as: Santos, T.G., et al., Friction Stir Welding assisted by electrical Joule effect. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.012
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Fig. 6. Electronics and data acquisition. (a) Experimental electrical apparatus and data acquisition, and (b) schematic representation of the overall installation.
ring (5) is connected to the graphite brush to deliver the electrical current to the tool. 4.2.2. Base plate and brush guide support Fig. 5 depicts the different components of the base plate and the brush guide support. The base plate (Fig. 5a) is fundamental to properly close the electrical circuit under the probe tip in the weld root. In order to achieve this, a copper ribbon (11) was embedded in a CK45 carbon steel base plate (10) under pressure. A brush system was used to deliver the current to the tool (Fig. 5b). Graphite brushes (14) enable electrical current to flow from the brush cables (16) as it slides in contact with the copper ring (5) allowing rotation. Fig. 5(c) depicts the set up into the FSW machine. 4.2.3. Electronics and data acquisition The overall system is depicted in Fig. 6 and comprises the FSW machine, the electric current supply which consists of a 12 V battery with 720 A nominal current intensity and a data acquisition system. The data acquisition system monitors the welding process at 0.2 s interval, measuring the current intensity through two independent and redundant electrical current transducers, and the voltage at the power supply terminals and between the tool body and the base plate. These data allows to calculate the electrical power delivered to the weld root. A multifunction data acquisition system (DAQ) was used to acquire the data and a dedicated software to show process parameters in a graphical user interface in real time and save data at the end of each test. 5. Results and discussion In order to validate the concept, several welds were performed in butt weld configuration. Fig. 7a shows a friction stir weld produced with a travel speed of 180 mm/min, a rotation speed of 1120 rev/min and a tilt angle of 1.5◦ with a probe plunging depth of 3.3 mm, to deliberately produce a lack of penetration (LOP) since the plate has 4 mm thickness. Friction Stir Welding was continuously produced along the plate with and without electrical current in order to analyze the effect of the last one on the dimension of the root defect, as well as, on the extension of the different zones common in FSW. For this, samples were removed for macrographic analysis in seven positions along the joint (M1–M7) as depicted in Fig. 7a. The weld started without electrical current for a length of 80 mm, and an electrical current of 800 A intensity was forced
to pass through the probe and stopped at 125 mm. Between this point, to a distance of 165 mm, no electrical current was introduced and after that 700 A was introduced again till the end of the weld (190 mm). Plots of current intensity and voltage are depicted in Fig. 7b with the same space scale. As far as the current intensity is concerned, the two transducers showed a good agreement with a minor deviation in the measured values. When analysing the voltage, it can be seen that when the electrical current starts, the voltage measured on the power supply terminal droped from 12 to 6.5 V, while the voltage measured between the tool body and the base plate increased to 4.5 V. This difference corresponds to the electrical resistance of the circuit, which is in this particular case of 2.5 m (R = (6.5–4.5)/800). Computing the mechanical power of a conventional FSW process (Eq. (7)), where Vx [m/s] is the travel speed, Fx [N] is the travel force, [rad/s] is the rotating tool speed and M [Nm] is the torque, using experimental data from Vilac¸a et al. (2007) a value of 5 kW was obtained for AA2024-T351 with 3.9 mm thick with a similar tool geometry and process parameters. Though these values were taken from tests performed without electrical current, it can be assumed that no significant variations exists, on both vertical and transverse forces, since, the processed volume is an order of magnitude larger than the control volume (Fig. 1). Similarly, the electrical power delivered by the electrical source was calculated by Eq. (8) where V [V] is the voltage and I [A] is the current intensity. Pmec = Vx × Fx + ˝ × M
(7)
Pelec = V × I
(8)
Fig. 8 depicts both the mechanical and the electrical powers. The mechanical power of FSW was calculated according to Eq. (7) and is represented by the dotted line. The electrical power supply by the battery is represented by the blue line with circular marks, while the effective electrical power on the FSW tool is shown the red line with triangular marks, both experimentally measured as described in Section 4.2.3. The latter is lower than the previous, since some electrical power was dissipated in the overall installations, mainly in cables by Joule effect. It can be seen that the electrical power delivered is almost of the same order of magnitude as the mechanical power associated to the FSW process, which is an interesting remark, since this additional power is very significant.
Please cite this article in press as: Santos, T.G., et al., Friction Stir Welding assisted by electrical Joule effect. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.012
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Fig. 7. FSW with and without electrical current. (a) Top view of the weld and (b) plot of current intensity and voltage with the same space scale.
Fig. 8. Comparison of FSW mechanical and electrical powers involved.
The welds were sectioned at M1–M7 locations and macrographs taken from these sections for analysis and these are shown in Figs. 9 and 10. Comparing the macrographs from samples welded without electrical current (M1 and M5) with the ones with electrical current (M3, M4 and M7), no significant evidences exist of the electrical current effect on the extension and morphology of the nugget, as well as, on its structure, where typical geometrical and metallurgical features are observed. Looking at sections M1 and M2 welded
without electrical current, a lack of penetration is seen in the weld root with about 15 m wide and 0.6 mm depth. However, when the electrical current assisted the process (M3, M4 and M7) it is evident from Fig. 11 that the width of the LoP decreased from 15.5 to 3.3 m, and this is the most significant difference observed. This result shows that, even in such adverse welding conditions, as producing a large LoP, there is a clear beneficial effect of the assisted current in minimizing the width of the root defect. This means that the electrical current increase the
Fig. 9. Macrograph of butt weld at position M2 without electrical current.
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Fig. 10. Macrographs of butt welds. (a) and (b) With electrical current of 800 A at positions M3 and M4 respectively, (c) without electrical current at position M5 and (d) with electrical current of 700 A at position M7.
Fig. 11. Micrographs of butt welds. (a) Without electrical current at position M2, and (b) with electrical current of 800 A at position M3.
viscoplastic material flow creating the correct thermo-mechanical conditions to suppress this root defect.
6. Conclusions From the study performed it can be concluded that: - A concept of FSW assisted by electrical current was presented, tested and validated aiming at minimizing weld root defects in FSW. - The concept relies on forcing an electrical current to pass in the weld root to increase the local temperature, improving the material viscoplasticity. - An analytical model validated the concept and it was seen that, for currents of about 800 A, the temperature raised 200–300 ◦ C for the aluminium alloys under study. The numerical simulation allowed characterizing the current density distribution. - Experimental results showed that passing an electrical current through the weld root, LoP defects reduced in size from a width of 15.5 to 3.3 m, under the conditions tested. - Electrical current seems to increase the viscoplastic material flow creating the conditions to suppress LoP, without affecting the overall metallurgical properties of the FSW and thus improving the mechanical performance of FSW butt joints. The concept reduces the sensitivity of the FSW process to generate root defects supporting an easier industrialization of the process into high demanding applications.
Acknowledgment T.S. and R.M. acknowledge Pest OE/EME/UI0667/2011 from the Portuguese Fundac¸ão para a Ciência e a Tecnologia (FCT-MEC).
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Please cite this article in press as: Santos, T.G., et al., Friction Stir Welding assisted by electrical Joule effect. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.012