- Email: [email protected]
Research of temperature and microstructure in friction stir welding of Q235 steel with laser-assisted heating
Results in Physics 11 (2018) 1048–1051
Contents lists available at ScienceDirect
Results in Physics journal homepage: www.elsevier.com/locate/rinp
Microarticle
Research of temperature and microstructure in friction stir welding of Q235 steel with laser-assisted heating
T
⁎
Xinjiang Feia, , Zhifeng Wub a b
College of Mechanical and Electrical Engineering, Shaoxing University, 508 Huancheng West Road, Shaoxing, Zhejiang Province 312000, People’s Republic of China New Oriental Training School Co., Ltd, Yuecheng Liberation North Road, Shaoxing, Zhejiang Province 312000, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser-heated Friction stir welding Thermal model Temperature Microstructure
In this paper, a three dimension heat source is developed in laser heated friction stir welding. The model was calculated by CFD software FLUENT, and the temperature field and flow field of the welding are obtained. The effect of absence or presence of laser beam on flow field and temperature field is discussed. The microstructure of nugget zone between laser assisted friction stir welding and traditional friction stir welding is analyzed by metallographic observation. Results show that welding quality will be improved if it is used laser assisted friction stir welding to weld Q235 steel plates instead of traditional friction stir welding.
Introduction Friction stir welding is a new welding method invented by the British Welding Institute of Cambridge in 1991. It utilizes the heat generated by mechanical friction and elastic deformation between the tool and the workpiece to achieve the joint of welding [1]. Friction stir welding can avoid the shortcomings such as porosity, crack, thermal deformation and so on, which are easy to be produced by melting welding. A lot of scholars have made extensive theoretical studies on the welding mechanism of friction stir welding. Fonda et al. [2,3] and Arora et al. [4] have studied the evolution of weld microstructure in friction stir welding. Based on the consideration of thermodynamic interaction, multi-objective optimization of friction stir welding of aluminum alloy was carried out by Tutum et al. [5,6], and welding parameters were optimized. Yousif et al. [7] used neural network to predict the characteristics of friction stir welding. Guerdoux et al. [8] simulated the friction stir welding. The residual stresses in friction stir welding process were numerically simulated and experimentally verified by Buffa et al. [9]. The heat transfer process of friction stir welding is experimentally and numerically studied by Chao et al. [10]. Besides, many scholars also have studied the microstructure and properties of joints in friction stir welding. Liu et al. [11,12] studied mechanical properties and crack distribution of aluminum alloy friction stir welding. Chen et al. [13] studied the strain and strain rate of aluminum alloy friction stir welding. Abbass et al. [14] studied the corrosion behavior of aluminum alloy friction stir welding joints. Katsas et al. [15] studied the factors of affecting the super-plasticity of aluminum alloy
⁎
during friction stir welding. So the research on friction stir welding mainly focuses on the light alloy with low hardness, such as aluminum alloy at present. However, the research on friction stir welding of materials with higher hardness, especially steel, is relatively few, and the welding technology is not perfect. Heat softening is an effective way to solve the problem of joining high hardness, high melting point and high strength materials by friction stir welding. Heating can not only reduce the hardness of the workpiece, improve its plasticity and flow performance, but also greatly reduce the mechanical force of the tool, thus reducing the strength requirements of the tool, improving the welding efficiency, and reducing the wear of the tool. In this paper, based on the conventional friction stir welding, a laser heating heat source is added to the front of the tool to realize the rapid heating and softening of the materials, and then the welding seam is formed by the friction heat between the tool and the workpiece, thus realizing the high quality connection of the workpiece as Fig. 1 shows. The temperature field and flow field of the welding are obtained. The effect of absence or presence of laser beam on flow field and temperature field is discussed. The microstructure of nugget zone between laser assisted friction stir welding and traditional friction stir welding is analyzed by metallographic observation. Mathematical model The friction stir welding process is divided into three stages: down pressure, welding and pull-out. In the welding stage, the tool rotates at a fixed speed, and the workpiece moves at a fixed speed relative to the
Corresponding author. E-mail address: [email protected] (X. Fei).
https://doi.org/10.1016/j.rinp.2018.11.039 Received 8 October 2018; Received in revised form 8 November 2018; Accepted 10 November 2018 Available online 14 November 2018 2211-3797/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 11 (2018) 1048–1051
X. Fei, Z. Wu
The governing equation In the process of the welding, plastic material is regarded as fluid. Fluid flow is dominated by the law of conservation of physics. It mainly involves the law of conservation of mass, momentum and energy. The law of conservation of mass:
∂u ∂v ∂w + + =0 ∂x ∂y ∂z
(1)
u , v , w is the component of velocity vector u in x, y and z directions. The law of conservation of momentum:
Fig. 1. Schematic diagram of laser-assisted friction stir welding.
tool. The heat production rate of the tool is constant, and the cross section of the workpiece has similar geometry, microstructure and properties. The friction stir welding process can be regarded as a quasisteady process. The flow of plastic metal in weld zone is only considered in the calculation. When the mathematical model of laser assisted friction stir welding is established, the following assumptions are taken into account:
ρ
∂p ∂ 2u ∂ 2u ∂ 2u ∂ (uu) ∂ (uv ) ∂ (uw ) + μd ⎛ 2 + 2 + 2 ⎞ +ρ +ρ =− ∂x ∂y ∂z ⎠ ∂x ∂y ∂z ⎝ ∂x
(2)
ρ
∂p ∂ 2v ∂ 2v ∂ 2v ∂ (vu) ∂ (vv ) ∂ (vw ) + μd ⎛ 2 + 2 + 2 ⎞ +ρ +ρ =− ∂y ∂ ∂ ∂z ⎠ ∂x ∂y ∂z x y ⎝
(3)
ρ
⎜
⎟
⎜
⎟
∂ 2w
∂ 2w
∂ 2w
∂p ∂ (wu) ∂ (wv ) ∂ (ww ) ⎞ + μd ⎛ 2 + + +ρ +ρ =− ∂z ∂y 2 ∂z 2 ⎠ ∂x ∂y ∂z ⎝ ∂x ⎜
⎟
(4)
ρ is the material density, P is the pressure on the fluid infinitesimal, μd is the kinetic viscosity The law of conservation of energy: ρCρ
(1) the energy distribution of laser output is stable, and the absorptivity of material does not change with time. (2) The laser irradiation range is approximately circular, not elliptical. (3) fluid is turbulent, incompressible and homogeneous, with constant density. (4) the fluid is a viscous non isotropic single-phase non Newtonian fluid. (5) thermal conductivity has no directional property for isotropic liquids. (6) The contact area between the material and the tool satisfies the non-slip boundary condition that the linear velocity of rotation is equal to the product of the rotational velocity and the radius of the tool.
∂ (Tu) ∂ (Tv ) ∂ (Tw ) ∂ 2T ∂ 2T ∂ 2T ⎞ +ρ = ST + k ⎛ 2 + + +ρ 2 ∂x ∂y ∂z ∂ ∂ ∂z 2 ⎠ x y ⎝ ⎜
⎟
(5)
Cρ is the specific heat capacity, T is the temperature, k is the heat transfer coefficient of the fluid, and ST is the viscous dissipation In friction stir welding, the viscosity of the workpiece is still very high at high temperature and high strain rate. The accurate prediction of the viscosity is directly related to the correctness of the numerical model. The viscoplastic calculation model of Perzyna is applied, and the formula of viscosity is as follows: μd =
σ 3ε
(5)
σ is the flow stress of materials, ε is strain rate. The formula for calculating the flow stress based on Sheppard and Wright:
Fig. 2. Temperature field of FSW and LFSW. 1049
Results in Physics 11 (2018) 1048–1051
X. Fei, Z. Wu
Fig. 3. Flow field of FSW and LFSW.
Fig. 4. Microstructure of FSW and LFSW.
σ=
1 Z 1/ m sinh−1 ⎡ ⎛ ⎞ ⎤ ⎢⎝ A ⎠ ⎥ α ⎦ ⎣
σ , A , α and m
λ1 (6)
boundary condition of tool pin:
λ1
2λp (x−m)2 + (y − h)2 exp(− ) 2 πR R2
∂T λ1 |Γ = h (Text − T ) ∂n
(7)
Heat generation of shoulder:
μπnp1 r = 30
μσz R2 πn 3 ησs R2 πn + (1 − δ ) 90 30
(8)
(13)
The experimental equipment is mainly composed of two parts: friction stir welding machine which plays a major role in welding and optical fiber laser which provides auxiliary heat source. The welded joint with size 24 mm * 10 mm * 3 mm was produced by WEDM as the observation sample. 4% HNO3 alcohol solution was used to corrode Q235 steel. The diameter of the shoulder is 20 mm, the diameter of the pin is 6 mm, and the length of the pin is 2.7 mm.
(9)
λ is laser absorption coefficient, p is laser power, R is laser radius, n is the rotation of the tool, R2 is pin radius, p1 is pressure in the shoulder, δ is the proportion of sliding friction, σz is normal stress, σs is yield strength, r is the distance from any point to the tool center.
Results and discussion In order to understand the role of laser heat source in laser-assisted friction stir welding better, the simulation results of friction stir welding (FSW) and laser-assisted friction stir welding (LFSW) are compared and analyzed. The differences between FSW and LFSW are analyzed from the point of welding temperature field and flow field. In the simulation analysis, the laser power is 800 W, the spot diameter of laser is 16 mm, the rotating speed is 1180 r/min, and the welding speed is 23.5 mm/
Boundary condition boundary condition of laser:
∂T = q (x , y ) λ1 ∂z
(12)
Experiment
Heat generation of pin:
qpin = δ
∂T |Γ = qpin ∂n The convective boundary condition of other side surfaces:
Heat generation of laser:
qshoulder
(11)
are material constants and Z is Zener-Hollomon parameter.
Heat generation
q (x , y ) =
∂T |Γ = qshoulder ∂n
(10)
boundary condition of tool shoulder: 1050
Results in Physics 11 (2018) 1048–1051
X. Fei, Z. Wu
assisted friction stir welding and traditional friction stir welding are discussed. It is found that laser assisted friction stir welding can promote the flow of materials and a good joint can be obtained. 3. The size of grains in laser assisted friction stir welding is made comparison with that in tradtional friction stir welding. It is found that the grains of laser assisted friction stir welding is much finer.
min. The distance between the center of laser beam and the center of tool shoulder is 20 mm. Temperature distribution in the upper surface obtained by simulation of friction stir welding and laser-assisted heating friction stir welding is shown in Fig. 2(a) and (b) It can be seen from Fig. 2 that the maximum temperature is 1408 K when the friction heat source is loaded only. While the maximum temperature is 1667 K when the friction heat source and laser heat source are loaded simultaneously. The temperature rises because the laser heat source is added, but the maximum temperature is not higher than the melting point of the material. The shoulder region is still the highest temperature region. It can be seen from the Fig. 2(b) that the laser heat source acts as the auxiliary heat source in front of the stirring tool to preheat the workpiece. The heat production is less than that of the stirring tool. It is beneficial to the flow of plastic material and the friction and wear of the stirring tool. Fig. 3 shows the flow field distribution of friction stir welding and laser assisted friction stir welding. It can be seen that the material fluidity of laser-assisted friction stir welding is better than that of single-loaded stirring tool. When the auxiliary heat source is added, the material around the stirring tool has a higher temperature and a larger heating area. Besides, more materials are softened by heat. Because of the stirring action of the stirring tool, the materials flow from the backward side to the forward side and it is easier to get a sound joint. Therefore, the introduction of laser heat source into friction stir welding can not only prolong the life of stirring tool, but also can promote the flow of materials in the welding. The microstructure of nugget zone in LFSW and FSW are compared and analyzed as Fig. 4 shows. It can be seen that the grains nugget zone in LFSW is finer than that in FSW as Fig. 4(a) and (b) shows. The main reason is that the grains in LFSW experienced higher strain and strain rate due to the role of laser heating. The material in this area contacts the stirring tool directly and forms the weld with the action of rotation and extrusion of the stirring tool. Dynamic recrystallization occurs in the weld nugget zone and equiaxed grain morphology is shown. The average grain size is decreased compared with the base metal.
Acknowledgments The paper is founded by the scientific research start-up project of Shaoxing University (20170516) and Key scientific research projects of Shaoxing University (2017LG1004). References [1] Thomas WM, Nicholas ED, Needham JC, et al. International patent application No. PCT/GB92/02203 and GB Patent application No. 9125978.8 and US Patent application No. 5460317, December 1991. [2] Fonda RW, Bingert JF, Colligan KJ. Microstructural development in friction stir welding. Chemical/biochemical research. NRL Review; 2005. p. 121–2. [3] Fonda RW, Wert JA, Reynolds AP, Tang W. Initial microstructural evolution during friction stir welding. Materials science and technology. NRL Review; 2007. p.175–7. [4] Arora KS, Pande S, Schaper M, Kumar R. Microstructure evolution during friction stir welding of aluminum alloy AA2219. J Mater Sci Technol 2010;26(8):747–53. [5] Tutum Cem Celal, Hattel Jesper Henri. A multi-objective optimization application in friction stir welding: considering thermo-mechanical aspects. IEEE; 2010. [6] Tutum CC, Schmidt HB, Hattel JH. Optimization of the process parameters for controlling residual stress and distortion in friction stir welding. J Nutr Health Aging 2008:1–4. [7] Yousif YK, Daws KM, Kazem BI. Prediction of friction stir welding characteristic using neural network. Jordan J Mech Ind Eng 2008;2(3):151–5. [8] Guerdoux S, Fourment L. A 3D numerical simulation of different phases of friction stir welding. Model Simul Mater Sci Eng 2009;17:51–8. [9] Buffa G, Fratini L, Pasta S. Residual Stresses in friction stir welding: numerical simulation and experimental verification. JCPDS-international centre for diffraction data. 2009. p. 444–53. [10] Chao Yuh J, Qi X, Tang W. Heat transfer in friction stir welding-experimental and numerical studies. J Manuf Sci Eng, Trans ASME 2003;125(February):138–45. [11] Liu HJ, Fujii H, Maeda M, Nogi K. Mechanical properties of friction stir welded joints of 1050–H24 aluminium alloy. Sci Technol Weld Joining 2003;8(6):450–4. [12] Liu Huijie, Fujii H, Maeda M, Nogi K. Tensile properties and fracture locations of friction-stir welded joints of 6061–T6 aluminum alloy. J Mater Sci Lett 2003;22:1061–3. [13] Chen ZW, Cui S. Strain and strain rate during friction stir welding processing of Al7Si-0.3Mg alloy. IOP conf. series: materials science and engineering, vol. 4. 2009. p. 1–6. [14] Abbass MK, Abdul-Hussein BA, Nawi SA. Study of corrosion behavior of friction stir welded aluminum alloy (2024-T3); 2016. [15] Katsas Stavros, Todd Graham, Jackson Martin, Dashwood Richard, Grimes Roger. The effect of friction stir processing and subsequent rolling on the superplastic behaviour of aluminum alloys. TMS Miner, Met Mater Soc 2005:291–8.
Conclusion 1. A Three dimension heat source model which is from laser, shoulder and pin is developed respectively. 2. The temperature field and flow field are calculated and analyzed. The differences of temperature field and flow field between laser
1051
Contents lists available at ScienceDirect
Results in Physics journal homepage: www.elsevier.com/locate/rinp
Microarticle
Research of temperature and microstructure in friction stir welding of Q235 steel with laser-assisted heating
T
⁎
Xinjiang Feia, , Zhifeng Wub a b
College of Mechanical and Electrical Engineering, Shaoxing University, 508 Huancheng West Road, Shaoxing, Zhejiang Province 312000, People’s Republic of China New Oriental Training School Co., Ltd, Yuecheng Liberation North Road, Shaoxing, Zhejiang Province 312000, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser-heated Friction stir welding Thermal model Temperature Microstructure
In this paper, a three dimension heat source is developed in laser heated friction stir welding. The model was calculated by CFD software FLUENT, and the temperature field and flow field of the welding are obtained. The effect of absence or presence of laser beam on flow field and temperature field is discussed. The microstructure of nugget zone between laser assisted friction stir welding and traditional friction stir welding is analyzed by metallographic observation. Results show that welding quality will be improved if it is used laser assisted friction stir welding to weld Q235 steel plates instead of traditional friction stir welding.
Introduction Friction stir welding is a new welding method invented by the British Welding Institute of Cambridge in 1991. It utilizes the heat generated by mechanical friction and elastic deformation between the tool and the workpiece to achieve the joint of welding [1]. Friction stir welding can avoid the shortcomings such as porosity, crack, thermal deformation and so on, which are easy to be produced by melting welding. A lot of scholars have made extensive theoretical studies on the welding mechanism of friction stir welding. Fonda et al. [2,3] and Arora et al. [4] have studied the evolution of weld microstructure in friction stir welding. Based on the consideration of thermodynamic interaction, multi-objective optimization of friction stir welding of aluminum alloy was carried out by Tutum et al. [5,6], and welding parameters were optimized. Yousif et al. [7] used neural network to predict the characteristics of friction stir welding. Guerdoux et al. [8] simulated the friction stir welding. The residual stresses in friction stir welding process were numerically simulated and experimentally verified by Buffa et al. [9]. The heat transfer process of friction stir welding is experimentally and numerically studied by Chao et al. [10]. Besides, many scholars also have studied the microstructure and properties of joints in friction stir welding. Liu et al. [11,12] studied mechanical properties and crack distribution of aluminum alloy friction stir welding. Chen et al. [13] studied the strain and strain rate of aluminum alloy friction stir welding. Abbass et al. [14] studied the corrosion behavior of aluminum alloy friction stir welding joints. Katsas et al. [15] studied the factors of affecting the super-plasticity of aluminum alloy
⁎
during friction stir welding. So the research on friction stir welding mainly focuses on the light alloy with low hardness, such as aluminum alloy at present. However, the research on friction stir welding of materials with higher hardness, especially steel, is relatively few, and the welding technology is not perfect. Heat softening is an effective way to solve the problem of joining high hardness, high melting point and high strength materials by friction stir welding. Heating can not only reduce the hardness of the workpiece, improve its plasticity and flow performance, but also greatly reduce the mechanical force of the tool, thus reducing the strength requirements of the tool, improving the welding efficiency, and reducing the wear of the tool. In this paper, based on the conventional friction stir welding, a laser heating heat source is added to the front of the tool to realize the rapid heating and softening of the materials, and then the welding seam is formed by the friction heat between the tool and the workpiece, thus realizing the high quality connection of the workpiece as Fig. 1 shows. The temperature field and flow field of the welding are obtained. The effect of absence or presence of laser beam on flow field and temperature field is discussed. The microstructure of nugget zone between laser assisted friction stir welding and traditional friction stir welding is analyzed by metallographic observation. Mathematical model The friction stir welding process is divided into three stages: down pressure, welding and pull-out. In the welding stage, the tool rotates at a fixed speed, and the workpiece moves at a fixed speed relative to the
Corresponding author. E-mail address: [email protected] (X. Fei).
https://doi.org/10.1016/j.rinp.2018.11.039 Received 8 October 2018; Received in revised form 8 November 2018; Accepted 10 November 2018 Available online 14 November 2018 2211-3797/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 11 (2018) 1048–1051
X. Fei, Z. Wu
The governing equation In the process of the welding, plastic material is regarded as fluid. Fluid flow is dominated by the law of conservation of physics. It mainly involves the law of conservation of mass, momentum and energy. The law of conservation of mass:
∂u ∂v ∂w + + =0 ∂x ∂y ∂z
(1)
u , v , w is the component of velocity vector u in x, y and z directions. The law of conservation of momentum:
Fig. 1. Schematic diagram of laser-assisted friction stir welding.
tool. The heat production rate of the tool is constant, and the cross section of the workpiece has similar geometry, microstructure and properties. The friction stir welding process can be regarded as a quasisteady process. The flow of plastic metal in weld zone is only considered in the calculation. When the mathematical model of laser assisted friction stir welding is established, the following assumptions are taken into account:
ρ
∂p ∂ 2u ∂ 2u ∂ 2u ∂ (uu) ∂ (uv ) ∂ (uw ) + μd ⎛ 2 + 2 + 2 ⎞ +ρ +ρ =− ∂x ∂y ∂z ⎠ ∂x ∂y ∂z ⎝ ∂x
(2)
ρ
∂p ∂ 2v ∂ 2v ∂ 2v ∂ (vu) ∂ (vv ) ∂ (vw ) + μd ⎛ 2 + 2 + 2 ⎞ +ρ +ρ =− ∂y ∂ ∂ ∂z ⎠ ∂x ∂y ∂z x y ⎝
(3)
ρ
⎜
⎟
⎜
⎟
∂ 2w
∂ 2w
∂ 2w
∂p ∂ (wu) ∂ (wv ) ∂ (ww ) ⎞ + μd ⎛ 2 + + +ρ +ρ =− ∂z ∂y 2 ∂z 2 ⎠ ∂x ∂y ∂z ⎝ ∂x ⎜
⎟
(4)
ρ is the material density, P is the pressure on the fluid infinitesimal, μd is the kinetic viscosity The law of conservation of energy: ρCρ
(1) the energy distribution of laser output is stable, and the absorptivity of material does not change with time. (2) The laser irradiation range is approximately circular, not elliptical. (3) fluid is turbulent, incompressible and homogeneous, with constant density. (4) the fluid is a viscous non isotropic single-phase non Newtonian fluid. (5) thermal conductivity has no directional property for isotropic liquids. (6) The contact area between the material and the tool satisfies the non-slip boundary condition that the linear velocity of rotation is equal to the product of the rotational velocity and the radius of the tool.
∂ (Tu) ∂ (Tv ) ∂ (Tw ) ∂ 2T ∂ 2T ∂ 2T ⎞ +ρ = ST + k ⎛ 2 + + +ρ 2 ∂x ∂y ∂z ∂ ∂ ∂z 2 ⎠ x y ⎝ ⎜
⎟
(5)
Cρ is the specific heat capacity, T is the temperature, k is the heat transfer coefficient of the fluid, and ST is the viscous dissipation In friction stir welding, the viscosity of the workpiece is still very high at high temperature and high strain rate. The accurate prediction of the viscosity is directly related to the correctness of the numerical model. The viscoplastic calculation model of Perzyna is applied, and the formula of viscosity is as follows: μd =
σ 3ε
(5)
σ is the flow stress of materials, ε is strain rate. The formula for calculating the flow stress based on Sheppard and Wright:
Fig. 2. Temperature field of FSW and LFSW. 1049
Results in Physics 11 (2018) 1048–1051
X. Fei, Z. Wu
Fig. 3. Flow field of FSW and LFSW.
Fig. 4. Microstructure of FSW and LFSW.
σ=
1 Z 1/ m sinh−1 ⎡ ⎛ ⎞ ⎤ ⎢⎝ A ⎠ ⎥ α ⎦ ⎣
σ , A , α and m
λ1 (6)
boundary condition of tool pin:
λ1
2λp (x−m)2 + (y − h)2 exp(− ) 2 πR R2
∂T λ1 |Γ = h (Text − T ) ∂n
(7)
Heat generation of shoulder:
μπnp1 r = 30
μσz R2 πn 3 ησs R2 πn + (1 − δ ) 90 30
(8)
(13)
The experimental equipment is mainly composed of two parts: friction stir welding machine which plays a major role in welding and optical fiber laser which provides auxiliary heat source. The welded joint with size 24 mm * 10 mm * 3 mm was produced by WEDM as the observation sample. 4% HNO3 alcohol solution was used to corrode Q235 steel. The diameter of the shoulder is 20 mm, the diameter of the pin is 6 mm, and the length of the pin is 2.7 mm.
(9)
λ is laser absorption coefficient, p is laser power, R is laser radius, n is the rotation of the tool, R2 is pin radius, p1 is pressure in the shoulder, δ is the proportion of sliding friction, σz is normal stress, σs is yield strength, r is the distance from any point to the tool center.
Results and discussion In order to understand the role of laser heat source in laser-assisted friction stir welding better, the simulation results of friction stir welding (FSW) and laser-assisted friction stir welding (LFSW) are compared and analyzed. The differences between FSW and LFSW are analyzed from the point of welding temperature field and flow field. In the simulation analysis, the laser power is 800 W, the spot diameter of laser is 16 mm, the rotating speed is 1180 r/min, and the welding speed is 23.5 mm/
Boundary condition boundary condition of laser:
∂T = q (x , y ) λ1 ∂z
(12)
Experiment
Heat generation of pin:
qpin = δ
∂T |Γ = qpin ∂n The convective boundary condition of other side surfaces:
Heat generation of laser:
qshoulder
(11)
are material constants and Z is Zener-Hollomon parameter.
Heat generation
q (x , y ) =
∂T |Γ = qshoulder ∂n
(10)
boundary condition of tool shoulder: 1050
Results in Physics 11 (2018) 1048–1051
X. Fei, Z. Wu
assisted friction stir welding and traditional friction stir welding are discussed. It is found that laser assisted friction stir welding can promote the flow of materials and a good joint can be obtained. 3. The size of grains in laser assisted friction stir welding is made comparison with that in tradtional friction stir welding. It is found that the grains of laser assisted friction stir welding is much finer.
min. The distance between the center of laser beam and the center of tool shoulder is 20 mm. Temperature distribution in the upper surface obtained by simulation of friction stir welding and laser-assisted heating friction stir welding is shown in Fig. 2(a) and (b) It can be seen from Fig. 2 that the maximum temperature is 1408 K when the friction heat source is loaded only. While the maximum temperature is 1667 K when the friction heat source and laser heat source are loaded simultaneously. The temperature rises because the laser heat source is added, but the maximum temperature is not higher than the melting point of the material. The shoulder region is still the highest temperature region. It can be seen from the Fig. 2(b) that the laser heat source acts as the auxiliary heat source in front of the stirring tool to preheat the workpiece. The heat production is less than that of the stirring tool. It is beneficial to the flow of plastic material and the friction and wear of the stirring tool. Fig. 3 shows the flow field distribution of friction stir welding and laser assisted friction stir welding. It can be seen that the material fluidity of laser-assisted friction stir welding is better than that of single-loaded stirring tool. When the auxiliary heat source is added, the material around the stirring tool has a higher temperature and a larger heating area. Besides, more materials are softened by heat. Because of the stirring action of the stirring tool, the materials flow from the backward side to the forward side and it is easier to get a sound joint. Therefore, the introduction of laser heat source into friction stir welding can not only prolong the life of stirring tool, but also can promote the flow of materials in the welding. The microstructure of nugget zone in LFSW and FSW are compared and analyzed as Fig. 4 shows. It can be seen that the grains nugget zone in LFSW is finer than that in FSW as Fig. 4(a) and (b) shows. The main reason is that the grains in LFSW experienced higher strain and strain rate due to the role of laser heating. The material in this area contacts the stirring tool directly and forms the weld with the action of rotation and extrusion of the stirring tool. Dynamic recrystallization occurs in the weld nugget zone and equiaxed grain morphology is shown. The average grain size is decreased compared with the base metal.
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Conclusion 1. A Three dimension heat source model which is from laser, shoulder and pin is developed respectively. 2. The temperature field and flow field are calculated and analyzed. The differences of temperature field and flow field between laser
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