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Simultaneous measurement of tool torque, traverse force and axial force in friction stir welding
G Model JMP-203; No. of Pages 6
ARTICLE IN PRESS Journal of Manufacturing Processes xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Technical Paper
Simultaneous measurement of tool torque, traverse force and axial force in friction stir welding H. Su a , C.S. Wu a,∗ , A. Pittner b , M. Rethmeier b a b
MOE Key Lab for Liquid-Solid Structure Evolution and Materials Processing, and Institute of Materials Joining, Shandong University, Jinan 250061, China Federal Institute for Materials Research and Testing, Berlin 12205, Germany
a r t i c l e
i n f o
Article history: Received 30 July 2013 Received in revised form 28 August 2013 Accepted 5 September 2013 Available online xxx Keywords: Friction stir welding Measurement Traverse force Axial force Tool torque
a b s t r a c t Simultaneous measurement of the tool torque, traverse force and axial force during friction stir welding process is of great significance to the understanding of the underlying process mechanism and the optimizing of the process parameters. Different from the traditional measurement methods using load cell or rotating component dynamometer, an indirect but economical methodology is used in this study for the simultaneous measurement of the traverse force, axial force and tool torque by monitoring the output torques of the servo motors and main spindle three-phase AC induction motor inside the FSW machine. The values of the traverse force, axial force and tool torque are determined under different welding conditions, and the influencing factors are examined. The measured results in friction stir welding of AA2024-T4 aluminum alloys at different combinations of tool rotation speed and welding speed lay foundation for process optimization. © 2013 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
1. Introduction Friction stir welding (FSW) has got wide applications in joining high strength aerospace aluminum alloys, such as highly alloyed 2XXX and 7XXX series [1]. FSW is a solid-state joining process in which a non-consumable rotating tool with a specially designed pin and shoulder is inserted into the abutting edges of plates to be joined and traversed along the line of joint as shown in Fig. 1. The tool serves two primary functions: (a) heating of workpiece, and (b) movement of material to produce the joint. The heating is generated by friction between the tool and the workpiece and plastic deformation of the workpiece [2]. The localized heating softens the material around the pin and combination of tool rotation and translation transport material from the front of the pin to the back of the pin where it is forged into a joint [1,2]. During FSW process, the tool shoulder makes firm contact with the top surface of the workpiece. The tool rotation speed, tool traverse speed along the line of joint (welding speed), the vertical pressure on the tool, the tilt angle of the tool and the tool design are main independent variables that are used to control the FSW process [1]. The tool torque, traverse force and axial force generated by the simultaneous rotating and linear motion of the tool as well as
∗ Corresponding author at: Institute of Materials Joining, Shandong University, No. 17923 Jingshi Road, Jinan 250061, China. Tel.: +86 0531 8839 2711; fax: +86 0531 8839 2711. E-mail address: [email protected] (C.S. Wu).
the vertical pressure during FSW process are critical for the process parameters optimization [3–6], and the research & development of tool design [7–9]. Various methods have been used to measure the tool torque, traverse force and axial force during FSW process. The traditional load cells were used to measure the tangential load on the aluminum alloy sheets (tool torque) [10]. Cui et al. [11] used a specially designed LowStirTM device to measure the tool torque, and developed a satisfactory model relating the tool torque to the two major FSW parameters, tool rotation speed and welding speed. Kumar et al. [12] used load cells to detect the axial force and traverse force, and the tool power was measured by measuring the spindle motor current. Trimble et al. [13] employed the rotating component dynamometer to measure the torque and forces in the X, Y and Z directions. However, usage of such auxiliary devices is hugely expensive, and often needs an enormous modification of the welding machine, which is complicated and costly. During FSW process, the linear motion of the worktable and the tool, and the rotation of the tool are driven by the electrical motors. Therefore, the conditions of the electric motors are indirect response of the tool torque and process forces. Recently, the tool torque and forces were recorded directly or indirectly by monitoring the instantaneous power or current of the motors. Pew et al. [14] recorded the tool torque by monitoring the motor torque output. The methodology presented by Mehta et al. [15] were able to measure the torque and traverse force from the corresponding electrical power and current usage of the driving spindle motor and feed motor. Such a methodology is a more robust and economical route for indirect
1526-6125/$ – see front matter © 2013 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jmapro.2013.09.001
Please cite this article in press as: Su H, et al. Simultaneous measurement of tool torque, traverse force and axial force in friction stir welding. J Manuf Process (2013), http://dx.doi.org/10.1016/j.jmapro.2013.09.001
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ARTICLE IN PRESS H. Su et al. / Journal of Manufacturing Processes xxx (2013) xxx–xxx Table 1 Chemical composition of the workpiece (AA2024-T4).
Fig. 1. Schematic of friction stir welding.
Element
Si
Fe
Cu
Mn
Mg
Ni
Zn
Ti
Al
Wt.%
0.15
0.25
4.58
0.63
1.59
<0.10
0.20
<0.10
Balance
Fig. 4. Schematic view of main spindle conveyer system: (a) main spindle motor, (b) conveyor system, (c) FSW tool.
25 X-axis Torque Z-axis Torque Spindle Torque
20
Torque (N m)
15
10
5
0 Fig. 2. Schematic view of (a) workpiece and (b) profile of the FSW tool.
-5 0
50
100
150
200
250
300
Time (s) monitoring the tool torque and forces during FSW process. However, little work has been done for simultaneously measuring the tool torque, traverse force and axial force in a single welding process from the electrical parameters of the driving motors. The axial force is of critical importance to ensure the weld morphology and weld quality in friction stir welding, because the friction heat generation is directly proportional to the axial force applied on the tool. The measured date of the axial force will be used to calibrate the heat generation model for next step. Thus, measuring the axial force is of great significance. In this study, the electrical signals of the motors inside the FSW machine were used to detect the tool torque, traverse force (X-axis) and axial force (Z-axis) simultaneously. The output torques of the Xand Z-servo motors and the main spindle three-phase AC induction motor were monitored and recorded in real time, and the actual axial force, traverse force and tool torque were then determined. Finally, the influence of various parameters on the weld formation is discussed.
Fig. 5. Output torques of the motors during a FSW process. (Rotation speed ω = 1000 r/min, welding speed v = 40 mm/min.)
2. Experimentation The workpieces were AA2024-T4 aluminum alloys of the thickness 5.90 mm, and the chemical composition is listed in Table 1. The plates were 120 mm in length and 60 mm in width, as shown in Fig. 2(a). A taper screw threaded tool was used, and the crosssection profile is shown in Fig. 2(b). The diameter of tool shoulder is 15.00 mm, the top and bottom diameter of pin is 6.00 mm and 2.50 mm, respectively, and the length of pin is 5.50 mm. The tilt angle of tool toward trailing direction was kept constant at 2.5◦ during the welding process. The plunged depth was 0.1 mm. The welding procedure includes three stages: (1) the plunging stage with the tool plunging velocity 8 mm/min and dwelling time 10 s; (2) the welding stage with the welding velocity v; and (3) the pulling out stage of tool after dwelling 5 s.
Fig. 3. Schematic view of X-axis ball screw driving mechanism: (a) worktable, (b) ball screw, (c) planetary gear reducer, and (d) servo motor.
Please cite this article in press as: Su H, et al. Simultaneous measurement of tool torque, traverse force and axial force in friction stir welding. J Manuf Process (2013), http://dx.doi.org/10.1016/j.jmapro.2013.09.001
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35
35 40mm/min 80mm/min 120mm/min
30
20 15 10
600r/min 800r/min 1000r/min
30
Tool Torque (N m)
25
Tool Torque (N m)
3
25 20 15 10
5 5
0 0
0
50
100
150
200
250
300
40
80
120
Welding speed (mm/min)
Time (s)
(a) Rotation speed 600r/min
Fig. 7. The averaged tool torques under various welding conditions.
35 40mm/min 80mm/min 120mm/min
30
Tool Torque (N m)
25 20 15 10 5 0 0
50
100
150
200
250
300
Time (s)
(b) Rotation speed 800r/min 35 40mm/min 80mm/min 120mm/min
30
The experiments were conducted by using FSW-3LM-3012 machine. As schematically depicted in Fig. 3, the X-axis or Y-axis horizontal motion of the worktable is obtained by the servo motors which drive the ball screw units via the planetary gear reducers. Similarly, the Z-axis vertical motion of the tool is obtained by the servo motor which drives the ball screw unit, but Z-axis driving mechanism has an additional torque sensor. For the tool rotation, a three-phase AC induction motor is used to drive the main spindle via a conveyer system, as schematically shown in Fig. 4. During the welding process, the output torques of the X- and Zaxis servo motors and the spindle three-phase AC induction motor are monitored by the internal correlations of the electrical parameters and recorded once per second in real-time. Then, the specific formulas for the ball screw driving mechanism and the conveyer system can be used to calculate the actual forces and torque. It is mentionable that the transmission efficiencies of the ball screw driving mechanism and conveyer system can be up to 95% under normal circumstances, and are set to constants, i.e. 1.0, for convenience in this study. 3. Results and discussion
Tool Torque (N m)
25
3.1. Output torques of the motors during FSW process 20 15 10 5 0 0
50
100
150
200
250
300
Time (s)
(c) Rotation speed 1000r/min Fig. 6. Tool torque versus time for various welding conditions. (a) Rotation speed 600 r/min; (b) rotation speed 800 r/min; (c) rotation speed 1000 r/min.
Fig. 5 illustrates the output torques of the X- and Z-axis servo motors and the main spindle motor versus time in the FSW process. During the plunging stage (0–43 s) and the first dwelling stage (43–53 s), the output torque of the X-axis motor keeps steady as the moving velocity of the worktable is zero. It is worth pointing out that the torque value during these two stages is not zero because of the static torque of the ball screw, but it will not affect the measured value during the welding stage. On the other hand, during the plunging stage, the output torques of both the Z-axis motor and the main spindle motor rise quickly, reach the first peak values, and then decrease as the plunging depth increases and the local softening extent around the pin is improved. When the shoulder of the tool is immersed into the workpiece, the output torques of both the Z-axis motor and the main spindle motor rise again and reach the second peak values. As time goes on, they drop again due to the sufficient friction heating of the workpiece during the first dwelling stage. As soon as the welding starts, all three output torques of the Z-axis motor and the main spindle motor as well as the X-axis motor reach steady states promptly, and remain relative stable values during whole welding stage.
Please cite this article in press as: Su H, et al. Simultaneous measurement of tool torque, traverse force and axial force in friction stir welding. J Manuf Process (2013), http://dx.doi.org/10.1016/j.jmapro.2013.09.001
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10000
8000
X-axis Force (N)
6000
4000
2000
Fricon Resistance Force (N)
4400 40mm/min 80mm/min 120mm/min
40mm/min 80mm/min 120mm/min
4000
3600
3200
2800
0 0
50
100
150
200
250
0
300
20
40
60
80
100
Time (s)
Time (s)
(a) Rotation speed 600r/min
Fig. 9. Friction resistance force versus time.
10000 40mm/min 80mm/min 120mm/min
8000
and axial force) can be calculated by the special formulas of the conveyer system and the ball screw driving mechanism. 3.2. Tool torque
X-axis Force (N)
6000
For the spindle conveyor drive system, the transfer ratio of the conveyor is 2, so that the correlation between the tool torque and the main spindle motor torque is:
4000
MT = 2M0 2000
0 0
50
100
150
200
250
300
Time (s)
(b) Rotation speed 800r/min
10000
40mm/min 80mm/min 120mm/min
8000
(1)
where MT is the tool torque and M0 is the spindle motor torque. Fig. 6 shows the tool torque versus time under various welding conditions. It is clear that for the same rotation speed the tool torques are highly consistent during the plunging and the first dwelling stage. The rotation speed has a remarkable influence on the tool torque in the welding stage, while the effect of the welding speed on the tool torque is minor. Fig. 7 shows the averaged value of tool torque in the welding stage, the tool torques are about 24 N m, 13 N m and 10 N m at the rotation speed of 600 r/min, 800 r/min and 1000 r/min, respectively.
6000
During the welding process, the output torque of X-axis servo motor corresponds to two forces: the traverse force and the frictional resistance force caused by the relative movement between
4000
2000
5000
0
4000
0
50
100
150
200
250
300
Time (s)
(c) Rotation speed 1000r/min Fig. 8. X-axis force versus time. (a) Rotation speed 600 r/min; (b) rotation speed 800 r/min; (c) rotation speed 1000 r/min.
Traverse force (N)
X-axis Force (N)
3.3. Traverse force
600r/min 800r/min 1000r/min
3000
2000
1000
The above-mentioned observation matches well with the measurement results from the rotation component dynamometer [13]. Thus, this measurement methodology by monitoring the output torques of the driving motors is able to capture the characteristics of the FSW process. Based on the measured output torques of the driven motors, the actual tool torque and forces (traverse force
0 40
80
120
Welding speed (mm/min) Fig. 10. The averaged traverse forces under various welding conditions.
Please cite this article in press as: Su H, et al. Simultaneous measurement of tool torque, traverse force and axial force in friction stir welding. J Manuf Process (2013), http://dx.doi.org/10.1016/j.jmapro.2013.09.001
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20000
20000 40mm/min 80mm/min 120mm/min
16000
600r/min 800r/min 1000r/min
16000
12000
Axial force (N)
Z-axis Force (N)
5
8000
12000
8000
4000
4000 0 0
50
100
150
200
250
0
300
40
Time (s)
(a) Rotation speed 600r/min 40mm/min 80mm/min 120mm/min
16000
Z-axis Force (N)
120
Fig. 12. The averaged axial forces under various welding conditions.
20000
friction resistance force. The X-axis forces (f + F) can be calculated by Eq. (2), as shown in Fig. 8. The friction resistance force f can be determined by measuring the no-load (F = 0) output torque Mf0 of the worktable:
12000
Mf 0 = 8000
C ·f 2
(3)
Fig. 9 shows the friction resistance force under different welding speeds in 100 s. Then, based on Eqs. (2) and (3), the traverse force F can be written as:
4000
F=
0 0
50
100
150
200
250
300
Time (s)
(b) Rotation speed 800r/min 20000 40mm/min 80mm/min 120mm/min
16000
Z-axis Force (N)
80
Welding speed (mm/min)
8000
4000
0 50
100
150
200
250
C
(4)
Fig. 10 shows the averaged values of the traverse forces under different welding parameters. It is comprehensible that the traverse force is directly proportional to the welding speed. Moreover, the traverse force is much larger at high rotation speed (1000 r/min) than that at low rotation speed (600 r/min and 800 r/min). 3.4. Axial force
12000
0
2 · (Mf − Mf 0 )
300
Similar to the traverse force, the axial force (Z-axis) can be determined by monitoring the Z-axial output torque of the motor. Fig. 11 shows the axial force versus time for various welding parameters. During the plunging stage, the axial force undergoes a severer variation with two peak values. Compared to the tool torque and traverse force, the values of the axial force are not constant during the welding process. Fig. 12 demonstrates the averaged axial force under different welding conditions. Generally, a positive correlation is evident between the axial force and the welding speed. However, the relationship between the axial force and the rotation speed is not distinct. The axial force is between 7000 N and 13,000 N, which is much larger than the traverse force.
Time (s)
(c) Rotation speed 1000r/min
3.5. Weld morphology
Fig. 11. Z-axis forces versus time for various welding parameters. (a) Rotation speed 600 r/min; (b) rotation speed 800 r/min; (c) rotation speed 1000 r/min.
the worktable and the ball screw. For the ball screw driving unit, the relation between the forces and the output torque is given by: Mf =
C · (f + F) 2
(2)
where Mf is the load output torque, C is the helical pitch of the screw, is the transmission efficiency, F is the traverse force, and f is the
Fig. 13 shows the appearance of weld formation made at different welding parameters. As demonstrated in Fig. 13(g), the best weld morphology is achieved at low welding speed (40 mm/min) and high rotation speed (1000 r/min). Under such a set of welding parameters, the tool endures the least resistance and the tool torque is minimum, as shown in Fig. 6(c). Generally, the shear strength decreases significantly at high rotation speed, which means that the material is easier to flow around the tool. On the other hand, the tool forces are lower at
Please cite this article in press as: Su H, et al. Simultaneous measurement of tool torque, traverse force and axial force in friction stir welding. J Manuf Process (2013), http://dx.doi.org/10.1016/j.jmapro.2013.09.001
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Fig. 13. Weld formation for different parameters (welding speed v/rotation speed ω). (a) 40/600, (b) 80/600, (c) 120/600, (d) 40/800, (e) 80/800, (f) 120/800, (g) 40/1000, (h) 80/1000 and (i) 120/1000 (RS: retreating side, AS: advancing side).
low welding speed, as illustrated in Figs. 10 and 12. The weld formation in Fig. 13(d), (e) and (h) are also acceptable. However, the worst weld appearance is obtained at high welding speed and high rotation speed, as shown in Fig. 13(i). The burr indicates the inadequate and unsteady flow of the material caused by high welding speed, and the material is unable to fill in the cavity made by the pin, especially at the advancing side (AS). Similar phenomena are also found in Fig. 13(c) and (f) with the same welding speed as Fig. 13(i). 4. Conclusions (1) An indirect but economical and reliable methodology is used to experimentally measure the tool torque, traverse force and axial force simultaneously in real time through monitoring the output torques of the servo motors and spindle three-phase AC induction motor equipped inside the FSW machine. The measured results agree well with the published data captured by rotation component dynamometers and load cells. (2) The higher is the tool rotation speed, the lower is the tool torque, while the influence of the welding speed on the tool torque is negligible. (3) The traverse force is proportional to the welding speed. The higher is the tool rotation speed, the larger is the traverse force. The axial force is significantly larger than the traverse force. (4) The best weld appearance is obtained at low welding speed and high rotation speed, because the shear strength decreases significantly at high rotation speed and the tool forces are least at low welding speed. Acknowledgement The authors are grateful to the financial support for this research from the Sino-German Center for the Promotion of Science (Grant No. GZ-739).
References [1] Nandan R, DebRoy T, Bhadeshia HKDH. Recent advanced in friction stir welding – process: weldment structure and properties. Process Mater Sci 2008;53(6):980–1023. [2] Mishra RS, Ma ZY. Friction stir welding and processing. Mater Sci Eng 2005;50R(1-2):1–78. [3] Schmidt HB, Hattel JH. Thermal modeling of friction stir welding. Scripta Mater 2008;58(5):332–7. [4] Schmidt H, Hattel J, Wert J. An analytical model for the heat generation in friction stir welding. Model Simul Mater Sci Eng 2004;12(1): 143–57. [5] Khandkar MZH, Khan JA, Reynolds AP. Prediction of temperature distribution and thermal history during friction stir welding: input torque based model. Sci Technol Weld Joining 2003;8(3):165–74. [6] Colligan KJ, Mishra RS. A conceptual model for the process variables related to heat generation in friction stir welding of aluminum. Scripta Mater 2008;58(5):327–31. [7] Arora A, Mehta M, De A, DebRoy T. Load bearing capacity of tool pin during friction stir welding. Int J Adv Manufact Technol 2012;61(9-12): 911–20. [8] Arora A, De A, DebRoy T. Toward optimum friction stir welding tool shoulder diameter. Scripta Mater 2011;64(1):9–12. [9] Rai R, De A, Bhadeshia HKDH, DebRoy T. Review: friction stir welding tools. Sci Technol Weld Joining 2011;16(4):325–42. [10] Kumar K, Kalyan C, Kailas SV. An investigation of friction during friction stir welding of metallic materials. Mater Manufact Process 2009;24(4): 438–45. [11] Cui S, Chen ZW, Robson JD. A model relating tool torque and its associated power and specific energy to rotation and forward speeds during friction stir welding/processing. Int J Mach Tool Manufact 2010;50(12): 1023–30. [12] Kumar R, Singh K, Pandey S. Process forces and heat input as function of process parameters in AA5083 friction stir welds. Trans Nonferrous Met Soc China 2012;22(2):288–98. [13] Trimble D, Monaghan J, O’Donnell GE. Force generation during friction stir welding of AA2024-T3. Manufact Technol 2012;61(1):9–12. [14] Pew JW, Nelson TW, Sorensen CD. Torque base weld power model for friction stir welding. Sci Technol Weld Joining 2007;12(4):341–7. [15] Mehta M, Chatterjee K, De A. Monitoring torque and traverse force in friction stir welding from input electrical signatures of driving motors. Sci Technol Weld Joining 2013;18(3):191–7.
Please cite this article in press as: Su H, et al. Simultaneous measurement of tool torque, traverse force and axial force in friction stir welding. J Manuf Process (2013), http://dx.doi.org/10.1016/j.jmapro.2013.09.001
ARTICLE IN PRESS Journal of Manufacturing Processes xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Technical Paper
Simultaneous measurement of tool torque, traverse force and axial force in friction stir welding H. Su a , C.S. Wu a,∗ , A. Pittner b , M. Rethmeier b a b
MOE Key Lab for Liquid-Solid Structure Evolution and Materials Processing, and Institute of Materials Joining, Shandong University, Jinan 250061, China Federal Institute for Materials Research and Testing, Berlin 12205, Germany
a r t i c l e
i n f o
Article history: Received 30 July 2013 Received in revised form 28 August 2013 Accepted 5 September 2013 Available online xxx Keywords: Friction stir welding Measurement Traverse force Axial force Tool torque
a b s t r a c t Simultaneous measurement of the tool torque, traverse force and axial force during friction stir welding process is of great significance to the understanding of the underlying process mechanism and the optimizing of the process parameters. Different from the traditional measurement methods using load cell or rotating component dynamometer, an indirect but economical methodology is used in this study for the simultaneous measurement of the traverse force, axial force and tool torque by monitoring the output torques of the servo motors and main spindle three-phase AC induction motor inside the FSW machine. The values of the traverse force, axial force and tool torque are determined under different welding conditions, and the influencing factors are examined. The measured results in friction stir welding of AA2024-T4 aluminum alloys at different combinations of tool rotation speed and welding speed lay foundation for process optimization. © 2013 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
1. Introduction Friction stir welding (FSW) has got wide applications in joining high strength aerospace aluminum alloys, such as highly alloyed 2XXX and 7XXX series [1]. FSW is a solid-state joining process in which a non-consumable rotating tool with a specially designed pin and shoulder is inserted into the abutting edges of plates to be joined and traversed along the line of joint as shown in Fig. 1. The tool serves two primary functions: (a) heating of workpiece, and (b) movement of material to produce the joint. The heating is generated by friction between the tool and the workpiece and plastic deformation of the workpiece [2]. The localized heating softens the material around the pin and combination of tool rotation and translation transport material from the front of the pin to the back of the pin where it is forged into a joint [1,2]. During FSW process, the tool shoulder makes firm contact with the top surface of the workpiece. The tool rotation speed, tool traverse speed along the line of joint (welding speed), the vertical pressure on the tool, the tilt angle of the tool and the tool design are main independent variables that are used to control the FSW process [1]. The tool torque, traverse force and axial force generated by the simultaneous rotating and linear motion of the tool as well as
∗ Corresponding author at: Institute of Materials Joining, Shandong University, No. 17923 Jingshi Road, Jinan 250061, China. Tel.: +86 0531 8839 2711; fax: +86 0531 8839 2711. E-mail address: [email protected] (C.S. Wu).
the vertical pressure during FSW process are critical for the process parameters optimization [3–6], and the research & development of tool design [7–9]. Various methods have been used to measure the tool torque, traverse force and axial force during FSW process. The traditional load cells were used to measure the tangential load on the aluminum alloy sheets (tool torque) [10]. Cui et al. [11] used a specially designed LowStirTM device to measure the tool torque, and developed a satisfactory model relating the tool torque to the two major FSW parameters, tool rotation speed and welding speed. Kumar et al. [12] used load cells to detect the axial force and traverse force, and the tool power was measured by measuring the spindle motor current. Trimble et al. [13] employed the rotating component dynamometer to measure the torque and forces in the X, Y and Z directions. However, usage of such auxiliary devices is hugely expensive, and often needs an enormous modification of the welding machine, which is complicated and costly. During FSW process, the linear motion of the worktable and the tool, and the rotation of the tool are driven by the electrical motors. Therefore, the conditions of the electric motors are indirect response of the tool torque and process forces. Recently, the tool torque and forces were recorded directly or indirectly by monitoring the instantaneous power or current of the motors. Pew et al. [14] recorded the tool torque by monitoring the motor torque output. The methodology presented by Mehta et al. [15] were able to measure the torque and traverse force from the corresponding electrical power and current usage of the driving spindle motor and feed motor. Such a methodology is a more robust and economical route for indirect
1526-6125/$ – see front matter © 2013 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jmapro.2013.09.001
Please cite this article in press as: Su H, et al. Simultaneous measurement of tool torque, traverse force and axial force in friction stir welding. J Manuf Process (2013), http://dx.doi.org/10.1016/j.jmapro.2013.09.001
G Model JMP-203; No. of Pages 6 2
ARTICLE IN PRESS H. Su et al. / Journal of Manufacturing Processes xxx (2013) xxx–xxx Table 1 Chemical composition of the workpiece (AA2024-T4).
Fig. 1. Schematic of friction stir welding.
Element
Si
Fe
Cu
Mn
Mg
Ni
Zn
Ti
Al
Wt.%
0.15
0.25
4.58
0.63
1.59
<0.10
0.20
<0.10
Balance
Fig. 4. Schematic view of main spindle conveyer system: (a) main spindle motor, (b) conveyor system, (c) FSW tool.
25 X-axis Torque Z-axis Torque Spindle Torque
20
Torque (N m)
15
10
5
0 Fig. 2. Schematic view of (a) workpiece and (b) profile of the FSW tool.
-5 0
50
100
150
200
250
300
Time (s) monitoring the tool torque and forces during FSW process. However, little work has been done for simultaneously measuring the tool torque, traverse force and axial force in a single welding process from the electrical parameters of the driving motors. The axial force is of critical importance to ensure the weld morphology and weld quality in friction stir welding, because the friction heat generation is directly proportional to the axial force applied on the tool. The measured date of the axial force will be used to calibrate the heat generation model for next step. Thus, measuring the axial force is of great significance. In this study, the electrical signals of the motors inside the FSW machine were used to detect the tool torque, traverse force (X-axis) and axial force (Z-axis) simultaneously. The output torques of the Xand Z-servo motors and the main spindle three-phase AC induction motor were monitored and recorded in real time, and the actual axial force, traverse force and tool torque were then determined. Finally, the influence of various parameters on the weld formation is discussed.
Fig. 5. Output torques of the motors during a FSW process. (Rotation speed ω = 1000 r/min, welding speed v = 40 mm/min.)
2. Experimentation The workpieces were AA2024-T4 aluminum alloys of the thickness 5.90 mm, and the chemical composition is listed in Table 1. The plates were 120 mm in length and 60 mm in width, as shown in Fig. 2(a). A taper screw threaded tool was used, and the crosssection profile is shown in Fig. 2(b). The diameter of tool shoulder is 15.00 mm, the top and bottom diameter of pin is 6.00 mm and 2.50 mm, respectively, and the length of pin is 5.50 mm. The tilt angle of tool toward trailing direction was kept constant at 2.5◦ during the welding process. The plunged depth was 0.1 mm. The welding procedure includes three stages: (1) the plunging stage with the tool plunging velocity 8 mm/min and dwelling time 10 s; (2) the welding stage with the welding velocity v; and (3) the pulling out stage of tool after dwelling 5 s.
Fig. 3. Schematic view of X-axis ball screw driving mechanism: (a) worktable, (b) ball screw, (c) planetary gear reducer, and (d) servo motor.
Please cite this article in press as: Su H, et al. Simultaneous measurement of tool torque, traverse force and axial force in friction stir welding. J Manuf Process (2013), http://dx.doi.org/10.1016/j.jmapro.2013.09.001
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35 40mm/min 80mm/min 120mm/min
30
20 15 10
600r/min 800r/min 1000r/min
30
Tool Torque (N m)
25
Tool Torque (N m)
3
25 20 15 10
5 5
0 0
0
50
100
150
200
250
300
40
80
120
Welding speed (mm/min)
Time (s)
(a) Rotation speed 600r/min
Fig. 7. The averaged tool torques under various welding conditions.
35 40mm/min 80mm/min 120mm/min
30
Tool Torque (N m)
25 20 15 10 5 0 0
50
100
150
200
250
300
Time (s)
(b) Rotation speed 800r/min 35 40mm/min 80mm/min 120mm/min
30
The experiments were conducted by using FSW-3LM-3012 machine. As schematically depicted in Fig. 3, the X-axis or Y-axis horizontal motion of the worktable is obtained by the servo motors which drive the ball screw units via the planetary gear reducers. Similarly, the Z-axis vertical motion of the tool is obtained by the servo motor which drives the ball screw unit, but Z-axis driving mechanism has an additional torque sensor. For the tool rotation, a three-phase AC induction motor is used to drive the main spindle via a conveyer system, as schematically shown in Fig. 4. During the welding process, the output torques of the X- and Zaxis servo motors and the spindle three-phase AC induction motor are monitored by the internal correlations of the electrical parameters and recorded once per second in real-time. Then, the specific formulas for the ball screw driving mechanism and the conveyer system can be used to calculate the actual forces and torque. It is mentionable that the transmission efficiencies of the ball screw driving mechanism and conveyer system can be up to 95% under normal circumstances, and are set to constants, i.e. 1.0, for convenience in this study. 3. Results and discussion
Tool Torque (N m)
25
3.1. Output torques of the motors during FSW process 20 15 10 5 0 0
50
100
150
200
250
300
Time (s)
(c) Rotation speed 1000r/min Fig. 6. Tool torque versus time for various welding conditions. (a) Rotation speed 600 r/min; (b) rotation speed 800 r/min; (c) rotation speed 1000 r/min.
Fig. 5 illustrates the output torques of the X- and Z-axis servo motors and the main spindle motor versus time in the FSW process. During the plunging stage (0–43 s) and the first dwelling stage (43–53 s), the output torque of the X-axis motor keeps steady as the moving velocity of the worktable is zero. It is worth pointing out that the torque value during these two stages is not zero because of the static torque of the ball screw, but it will not affect the measured value during the welding stage. On the other hand, during the plunging stage, the output torques of both the Z-axis motor and the main spindle motor rise quickly, reach the first peak values, and then decrease as the plunging depth increases and the local softening extent around the pin is improved. When the shoulder of the tool is immersed into the workpiece, the output torques of both the Z-axis motor and the main spindle motor rise again and reach the second peak values. As time goes on, they drop again due to the sufficient friction heating of the workpiece during the first dwelling stage. As soon as the welding starts, all three output torques of the Z-axis motor and the main spindle motor as well as the X-axis motor reach steady states promptly, and remain relative stable values during whole welding stage.
Please cite this article in press as: Su H, et al. Simultaneous measurement of tool torque, traverse force and axial force in friction stir welding. J Manuf Process (2013), http://dx.doi.org/10.1016/j.jmapro.2013.09.001
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10000
8000
X-axis Force (N)
6000
4000
2000
Fricon Resistance Force (N)
4400 40mm/min 80mm/min 120mm/min
40mm/min 80mm/min 120mm/min
4000
3600
3200
2800
0 0
50
100
150
200
250
0
300
20
40
60
80
100
Time (s)
Time (s)
(a) Rotation speed 600r/min
Fig. 9. Friction resistance force versus time.
10000 40mm/min 80mm/min 120mm/min
8000
and axial force) can be calculated by the special formulas of the conveyer system and the ball screw driving mechanism. 3.2. Tool torque
X-axis Force (N)
6000
For the spindle conveyor drive system, the transfer ratio of the conveyor is 2, so that the correlation between the tool torque and the main spindle motor torque is:
4000
MT = 2M0 2000
0 0
50
100
150
200
250
300
Time (s)
(b) Rotation speed 800r/min
10000
40mm/min 80mm/min 120mm/min
8000
(1)
where MT is the tool torque and M0 is the spindle motor torque. Fig. 6 shows the tool torque versus time under various welding conditions. It is clear that for the same rotation speed the tool torques are highly consistent during the plunging and the first dwelling stage. The rotation speed has a remarkable influence on the tool torque in the welding stage, while the effect of the welding speed on the tool torque is minor. Fig. 7 shows the averaged value of tool torque in the welding stage, the tool torques are about 24 N m, 13 N m and 10 N m at the rotation speed of 600 r/min, 800 r/min and 1000 r/min, respectively.
6000
During the welding process, the output torque of X-axis servo motor corresponds to two forces: the traverse force and the frictional resistance force caused by the relative movement between
4000
2000
5000
0
4000
0
50
100
150
200
250
300
Time (s)
(c) Rotation speed 1000r/min Fig. 8. X-axis force versus time. (a) Rotation speed 600 r/min; (b) rotation speed 800 r/min; (c) rotation speed 1000 r/min.
Traverse force (N)
X-axis Force (N)
3.3. Traverse force
600r/min 800r/min 1000r/min
3000
2000
1000
The above-mentioned observation matches well with the measurement results from the rotation component dynamometer [13]. Thus, this measurement methodology by monitoring the output torques of the driving motors is able to capture the characteristics of the FSW process. Based on the measured output torques of the driven motors, the actual tool torque and forces (traverse force
0 40
80
120
Welding speed (mm/min) Fig. 10. The averaged traverse forces under various welding conditions.
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20000 40mm/min 80mm/min 120mm/min
16000
600r/min 800r/min 1000r/min
16000
12000
Axial force (N)
Z-axis Force (N)
5
8000
12000
8000
4000
4000 0 0
50
100
150
200
250
0
300
40
Time (s)
(a) Rotation speed 600r/min 40mm/min 80mm/min 120mm/min
16000
Z-axis Force (N)
120
Fig. 12. The averaged axial forces under various welding conditions.
20000
friction resistance force. The X-axis forces (f + F) can be calculated by Eq. (2), as shown in Fig. 8. The friction resistance force f can be determined by measuring the no-load (F = 0) output torque Mf0 of the worktable:
12000
Mf 0 = 8000
C ·f 2
(3)
Fig. 9 shows the friction resistance force under different welding speeds in 100 s. Then, based on Eqs. (2) and (3), the traverse force F can be written as:
4000
F=
0 0
50
100
150
200
250
300
Time (s)
(b) Rotation speed 800r/min 20000 40mm/min 80mm/min 120mm/min
16000
Z-axis Force (N)
80
Welding speed (mm/min)
8000
4000
0 50
100
150
200
250
C
(4)
Fig. 10 shows the averaged values of the traverse forces under different welding parameters. It is comprehensible that the traverse force is directly proportional to the welding speed. Moreover, the traverse force is much larger at high rotation speed (1000 r/min) than that at low rotation speed (600 r/min and 800 r/min). 3.4. Axial force
12000
0
2 · (Mf − Mf 0 )
300
Similar to the traverse force, the axial force (Z-axis) can be determined by monitoring the Z-axial output torque of the motor. Fig. 11 shows the axial force versus time for various welding parameters. During the plunging stage, the axial force undergoes a severer variation with two peak values. Compared to the tool torque and traverse force, the values of the axial force are not constant during the welding process. Fig. 12 demonstrates the averaged axial force under different welding conditions. Generally, a positive correlation is evident between the axial force and the welding speed. However, the relationship between the axial force and the rotation speed is not distinct. The axial force is between 7000 N and 13,000 N, which is much larger than the traverse force.
Time (s)
(c) Rotation speed 1000r/min
3.5. Weld morphology
Fig. 11. Z-axis forces versus time for various welding parameters. (a) Rotation speed 600 r/min; (b) rotation speed 800 r/min; (c) rotation speed 1000 r/min.
the worktable and the ball screw. For the ball screw driving unit, the relation between the forces and the output torque is given by: Mf =
C · (f + F) 2
(2)
where Mf is the load output torque, C is the helical pitch of the screw, is the transmission efficiency, F is the traverse force, and f is the
Fig. 13 shows the appearance of weld formation made at different welding parameters. As demonstrated in Fig. 13(g), the best weld morphology is achieved at low welding speed (40 mm/min) and high rotation speed (1000 r/min). Under such a set of welding parameters, the tool endures the least resistance and the tool torque is minimum, as shown in Fig. 6(c). Generally, the shear strength decreases significantly at high rotation speed, which means that the material is easier to flow around the tool. On the other hand, the tool forces are lower at
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Fig. 13. Weld formation for different parameters (welding speed v/rotation speed ω). (a) 40/600, (b) 80/600, (c) 120/600, (d) 40/800, (e) 80/800, (f) 120/800, (g) 40/1000, (h) 80/1000 and (i) 120/1000 (RS: retreating side, AS: advancing side).
low welding speed, as illustrated in Figs. 10 and 12. The weld formation in Fig. 13(d), (e) and (h) are also acceptable. However, the worst weld appearance is obtained at high welding speed and high rotation speed, as shown in Fig. 13(i). The burr indicates the inadequate and unsteady flow of the material caused by high welding speed, and the material is unable to fill in the cavity made by the pin, especially at the advancing side (AS). Similar phenomena are also found in Fig. 13(c) and (f) with the same welding speed as Fig. 13(i). 4. Conclusions (1) An indirect but economical and reliable methodology is used to experimentally measure the tool torque, traverse force and axial force simultaneously in real time through monitoring the output torques of the servo motors and spindle three-phase AC induction motor equipped inside the FSW machine. The measured results agree well with the published data captured by rotation component dynamometers and load cells. (2) The higher is the tool rotation speed, the lower is the tool torque, while the influence of the welding speed on the tool torque is negligible. (3) The traverse force is proportional to the welding speed. The higher is the tool rotation speed, the larger is the traverse force. The axial force is significantly larger than the traverse force. (4) The best weld appearance is obtained at low welding speed and high rotation speed, because the shear strength decreases significantly at high rotation speed and the tool forces are least at low welding speed. Acknowledgement The authors are grateful to the financial support for this research from the Sino-German Center for the Promotion of Science (Grant No. GZ-739).
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Please cite this article in press as: Su H, et al. Simultaneous measurement of tool torque, traverse force and axial force in friction stir welding. J Manuf Process (2013), http://dx.doi.org/10.1016/j.jmapro.2013.09.001