A Fully Coupled Thermomechanical Model of Friction Stir Welding (FSW) and Numerical Studies on Process Parameters of Lightweight Aluminum Alloy Joints

doi:10.1007/s40195-017-0658-4

Abstract

Abstract:

This paper presents a new thermomechanical model of friction stir welding which is capable of simulating the three major steps of friction stir welding (FSW) process, i.e., plunge, dwell, and travel stages. A rate-dependent Johnson-Cook constitutive model is chosen to capture elasto-plastic work deformations during FSW. Two different weld schedules (i.e., plunge rate, rotational speed, and weld speed) are validated by comparing simulated temperature profiles with experimental results. Based on this model, the influences of various welding parameters on temperatures and energy generation during the welding process are investigated. Numerical results show that maximum temperature in FSW process increases with the decrease in plunge rate, and the frictional energy increases almost linearly with respect to time for different rotational speeds. Furthermore, low rotational speeds cause inadequate temperature distribution due to low frictional and plastic dissipation energy which eventually results in weld defects. When both the weld speed and rotational speed are increased, the contribution of plastic dissipation energy increases significantly and improved weld quality can be expected.

Key words: Aluminum alloy, Friction stir welding, Temperature distribution, Plastic energy, Frictional energy, Rate-dependent model, Friction modeling

Saad B. Aziz, Mohammad W. Dewan, Daniel J. Huggett, Muhammad A. Wahab, Ayman M. Okeil, T. Warren Liao. A Fully Coupled Thermomechanical Model of Friction Stir Welding (FSW) and Numerical Studies on Process Parameters of Lightweight Aluminum Alloy Joints[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(1): 1-18.

Figures/Tables 35

Fig. 1 Schematic representation of friction stir welding (FSW) process [2] In FSW, heat generation occurs in three distinct stages: plunge, dwell, and travel. During the dwell stage, the position of the pintool remains the same at the end of the plunge stage without changing the rotation speed. Additional heat is generated from the shoulder-workpiece interface, which raises the temperature of the workpiece close to its melting temperature. The two main sources of heat generation are: (1) the friction between pintool and workpiece and (2) plastic deformation of the workpiece material.

Fig. 2 Dominant FSW process parameters and experimental setup

Table 1 Chemical compositions (wt%) of AA2219

Element	Ti	Zn	Fe	V	Cu	Mn	Zr	Si	Mg
wt%	0.10	0.10	0.30	0.15	6.8	0.40	0.25	0.20	0.02

Fig. 3 a Finite element model of the workpiece and pintool; b dimensions of the pintool

Fig. 4 Schematic representation of boundary conditions of thermal analysis and meshes of the finite element model (all dimensions are in millimeter)

Table 2 Johnson-Cook material plastic model input [27]

A (MPa)	B (MPa)	n	m	Melting temperature (°C) [33]	Reference temp (°C)
369	684	0.73	1.7	543	25

Fig. 5 Mechanical boundary conditions of workpiece 2.6 FEA Modeling

Table 3 Simulation details for three steps (Plunge, Dwell, and Traverse)

Step	Time duration of the step	Boundary condition
Plunging	15.2 s	Displacement in y-axis, rotation in y-axis
Dwelling	0.1 s	Rotation in y-axis
Traversing	20 s	Rotation in y-axis movement in x-axis

Table 4 Friction coefficient (temperature dependent) used in present model

Temperature (°C)	Friction coefficient
25	0.30
300	0.25
420	0.20
543	0.01

Fig. 6 Flowchart used to explain selection of friction coefficient used in this study [13] 2.6.2 Material Properties

Fig. 7 Modified Coulomb’s law for sticking and sliding conditions [21] 2.6.7 Mesh Size Study

Fig. 8 Arrangement of thermocouples (inserted within the surface) and thermographer (all dimensions are in millimeter)

Table 5 Weld schedule used in temperature validation

Weld schedule	Rotational speed, \({ N }\)(rpm)	Weld speed, v (mm/s)
Case-1	350	1.27
Case-2	350	2.54

Fig. 9 Temperature variation histories from experiment and FEA analysis at location 3 for Case-1 weld schedule

Fig. 10 Temperature variation histories from experiment and FEA analysis at location 3 for Case-2 weld schedule

Fig. 11 Temperature field from simulation (for Case-1 weld schedule)

Fig. 12 Temperature variations along transverse direction between experimental and simulation data (for Case-1 weld schedule)

Fig. 13 Temperature variations along transverse direction between experimental and simulation data (for Case-2 weld schedule)

Table 6 Error analysis for weld schedule Case-1 along transverse direction

Distance from weld center (mm)	Temperature from FEA (°C)	Temperature from experiment (°C)	Absolute error (%)
0	406.7	422	3.6
15	318.0	345	7.8
26	227.2	248	8.3
32	208.5	231	9.7
39	200.4	214	6.3
Average error			7.1

Table 7 Error analysis for weld schedule Case-2 along transverse direction

Distance from weld center(mm)	Temperature from FEA (°C)	Temperature from experiment (°C)	Absolute error (%)
0	450.5	458	1.6
15	335.1	364	7.9
26	257.6	280	8.0
32	228.5	251	8.9
39	208.2	228	8.6
Average error			7.0

Fig. 14 Ratio of kinematic and internal energies with time for Case-1 weld schedule

Fig. 15 Ratio of kinematic and internal energies with time for Case-2 weld schedule

Table 8 Plastic/total energy ratio of different weld schedules

Weld schedule	Rotational speed, \(N\)(rpm)	Weld speed, v (mm/s)	Total friction energy (J)	Total plastic energy (J)	Total energy (J)	\(\frac{{{\text{Total}}\,{\text{plastic}}\,{\text{energy}}}}{{{\text{Total }}\,{\text{energy}}}}\times\) 100%
Case-1	350	1.27	4.62 × 10⁴	4.76 × 10³	5.09 × 10⁴	9.4%
Case-2	350	2.54	4.92 × 10⁴	6.30 × 10³	5.55 × 10⁴	11.4%

Fig. 16 Variation of frictional energy with rotational speed (plunge rate = 0.4 mm/s, v = 1.27 mm/s)

Fig. 17 Variation of plastic energy with rotational speed (plunge rate = 0.4 mm/s, v = 1.27 mm/s)

Table 9 Synopsis of energies for various rotational speeds

Plunge rate (mm/s)	Rotational speed, \(N{ }\)(rpm)	Weld speed, v(mm/s)	Total frictional energy (J)	Change in frictional energy^a	Total plastic energy (J)	Change in plastic energy^a	Total energy	\(\left( {\frac{\text{Total plastic energy}}{\text{Total energy}}} \right)\times\) 100%
0.4	450	1.27	4.76 × 10⁴	3.0%	5.91 × 10³	24.2%	5.35 × 10⁴	11.04%
0.4	350	1.27	4.62 × 10⁴	Base1	4.76 × 10³	Base1	5.09 × 10⁴	9.35%
0.4	200	1.27	3.70 × 10⁴	19.9%	1.52 × 10³	68.1%	3.85 × 10⁴	3.94%

Fig. 18 Variation of \(\frac{{{\text{Total}}\,{\text{plastic}}\,{\text{energy}}}}{{{\text{Total}}\,{\text{energy}}}}\) with time for different rotational speeds (plunge rate = 0.4 mm/s, v = 1.27 mm/s)

Table 10 Synopsis of energies for various weld speeds

Plunge rate (mm/s)	Rotational speed, \({ }N{ }\)(rpm)	Weld speed, v(mm/s)	Total friction energy (J)	Change in frictional energy^b	Total plastic energy (J)	Change in plastic energy^b	Total energy	\(\left( {\frac{{{\text{Total}}\,{\text{plastic}}\,{\text{energy}}}}{{{\text{Total }}\,{\text{energy}}}}} \right)\times\) 100%
0.4	350	2.54	4.92 × 10⁴	0.8%	6.30 × 10³	20.7%	5.55 × 10⁴	11.35%
0.4	350	1.69	4.88 × 10⁴	Base2	5.22 × 10³	Base2	5.40 × 10⁴	9.66%
0.4	350	1.27	4.62 × 10⁴	5.3%	4.76 × 10³	8.8%	5.09 × 10⁴	9.35%

Fig. 19 Variation of frictional dissipation energy with welding speed (\(N\) = 350 rpm, plunge rate = 0.4 mm/s)

Fig. 20 Variation of plastic dissipation energy with welding speed (\(N\) = 350 rpm, plunge rate = 0.4 mm/s)

Fig. 21 Variation of \(\frac{{{\text{Total}}\,{\text{plastic}}\,{\text{energy}}}}{{{\text{Total }}\,{\text{energy}}}}\) with welding speed (\(N\) = 350 rpm, plunge rate = 0.4 mm/s)

Fig. 22 Variation of frictional energy with plunge rate (welding speed = 1.27 mm/s, \(N\) = 350 rpm)

Fig. 23 Variation of plastic energy with plunge rate (welding speed = 1.27 mm/s, \(N\) = 350 rpm)

Table 11 Synopsis of energies for various plunge rates

Plunge rate (mm/s)	Rotational speed, \(N\)(rpm)	Weld speed, v (mm/s)	Total friction energy (J)	Change in frictional energy^c	Total plastic energy (J)	Change in plastic energy^c	Total energy	\(\frac{\text{Total plastic energy}}{\text{Total energy}}\times\) 100%
0.3	350	1.27	2.89 × 10⁴	24.6%	5.53 × 10³	37.2%	3.44 × 10⁴	16.07%
0.4	350	1.27	2.32 × 10⁴	Base3	4.03 × 10³	Base3	2.72 × 10⁴	14.81%
0.6	350	1.27	1.67 × 10⁴	28.0%	2.88 × 10³	28.5%	1.95 × 10⁴	14.76%

Fig. 24 Effect of plunge rate on plastic energy ratio at the end of plunge stage (welding speed = 1.27 mm/s, \(N\) = 350 rpm)

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