- Email: [email protected]
Research on Laser Peening of TC21 Titanium Alloy with High Energy Laser
Rare Metal Materials and Engineering Volume 43, Issue 12, December 2014 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2014, 43(12): 2962-2965.
ARTICLE
Research on Laser Peening of TC21 Titanium Alloy with High Energy Laser Che Zhigang1, 1
Yang Jie2,
Tang Nan3,
Gong Shuili1
Science and Technology on Power Beam Processes Laboratory, Beijing Aeronautical Manufacturing Technology Research Institute, Beijing
100024, China;
2
3
North China University of Water Resources and Electric Power, Zhengzhou 450045, China; The High School Affiliated to
Renmin University of China, Chaoyang School, Beijing 100028, China
Abstract: Laser peening (LP) also known as laser shock processing (LSP), is a surface enhancement technique that induces intensive plastic deformation, high dislocation density and deeper compressive residual stress to improve the surface performance of materials. The microhardness, surface profiles, roughness, and the residual stress of TC21 alloy were tested by LSP with high energy laser. The results show that the microhardness of the shocked area is improved apparently compared with that of unshocked area. The magnitude of dent is related with the diameter and the distribution of laser spot. The roughness Ra of shocked area is less than 0.8 μm. The compressive residual stress is enhanced greatly. The investigations show that the technology of LSP could further improve the performance of TC21 alloy. Key words: laser peening; TC21 titanium alloy; residual stress; microhardness; surface profile
Laser shock processing or laser peening is an advanced and promising surface treatment technique. It is an effective way to increase the wear resistance, the corrosion resistance and high-cycle fatigue (HCF) of metallic components, through high density dislocation and imparting compressive residual stress on the surface of a number of metals and alloys. The higher laser energy/power density could induce the higher density dislocation, deeper effect layer, larger action area and better treatment effect. TC21 titanium alloy possesses the level of 1000 MPa damage tolerance. As it has many advanced properties such as high strength and high fracture toughness corresponding with Ti-6-22-22S in America, next to Ti-6Al-4V used in F-22 fighter. As the material of aircraft structure, TC21 is superior to Ti-6Al-4V. TC21 titanium alloy has a wide application prospect in the manufacture of aviation, aerospace and related industries structural parts [1]. The research efforts to date have mainly been focused on the phase and properties of TC21 titanium alloy under heat treatment. However, only a little
work has been done on improving the surface performance of TC21 titanium alloy, which is important for its further application. The damage tolerance shows the behavior under the combined actions of load and defect. Therefore, it’s very important to improve the performance of titanium alloys. The objective of this work is to examine the effect of laser shock processing on the TC21, including hardness, surface profile and roughness. The properties of samples were improved after LSP.
1
Principle of LSP
The principle of LSP is shown in Fig.1. The surface of target is plastered with an absorbing layer (also called sacrificial layer, normal organic paint or metallic foil, such as tape, zinc or aluminum) after polishing[2]. The laser pulse with high peak value power (>1 GW/cm2), and short duration (nano second level) irradiates on the surface of coating layer traveling transparent confining layer (also called confined medium, water or glass) by focusing lens. The coating layer
Received date: December 24, 2013 Foundation item: National Natural Science Foundation of China (50975268) Corresponding author: Che Zhigang, Ph. D., Senior Engineer, Beijing Aeronautical Manufacturing Technology Research Institute, Beijing 100024, P. R. China, Tel: 0086-10-85701486, E-mail: [email protected] Copyright © 2014, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
2962
Che Zhigang et al. / Rare Metal Materials and Engineering, 2014, 43(12): 2962-2965 Pressure shockwave
a
b High pressure plasma
Laser pulse
Workpiece
Water confining layer Absorbing layer (Al foil)
Workpiece
Water confining layer Absorbing layer (Al foil)
Fig.1 Schematic illustrating of LSP and shockwave generation by confined plasma expansion before (a) and after (b) application of the laser pulse
absorbs the laser energy, resulting in an exploding and plasma is formed in very short time. The plasma is restrained by the confined medium during expanding and high pressure shock wave is generated [3]. When the peak value of stress wave exceeds the HEL (Hugoniot Elastic Limit) of the target for a suitable time, the dense and stable dislocations (or twin crystal) are formed. The surface strain hardening is produced at the same time. The elastic deformation energy with shock wave is greater than or equal to the plastic and yielding deformation energy of target material. The compressive residual stress is generated on the surface and in material due to the plastic deformation layer restraining the resuming of the elastic deformation energy. The existence of compressive residual stress alters the distribution of stress field and improves the fatigue performance of material surface. So the technique of LSP can evidently improve the performance of corrosion resistance and fatigue resistance [4,5]. The heat effect can be ignored due to the existence of absorbing layer and the very short time of interaction and protecting the target from thermal damage. The confined medium is used to enhance the pressure of shock wave and prolong the action time [6]. The HEL of the material depends on the properties [7]: λ + 2μ dyn 1 −ν dyn (1) HEL = σY = σY 2μ 1 − 2ν
μ=
Eν E λ= (1 + ν )(1 − 2ν ) 2(1 + ν ) ;
where λ and μ are the lame constant related with E (Young’s modulus)and ν (Poisson’s ratio). σ Ydyn is the dynamic yield strength at high strain rate (about 106 s-1). The pressure calculation of LSP is the foundation of determining the technique parameters. The pressure model used here can be obtained as follows [8] considering the left confining layer:
α (2) ⋅ Z ⋅ I0 （ 3 2α + 3） where p denotes pressure of shock wave, GPa; I0 denotes power density of laser pulse, GW/cm2; α denotes the fraction of the internal energy devoted to the thermal energy (typically, α=0.25); Z is the combined shock wave impedance (g/cm2·s) P = 0.01
relating with the shock wave impedance of target material (Z1), the confined medium (Z2) and the left absorbing layer (Z3). Among them the relationship could be defined as follows: 1 1 1 1 (3) = + + Z Z1 Z2 Z3 The pressure model is accordant with the practical situation and has more accurate calculation results than previous model due to the left coating layer being considered. The pressure value obtained by this model is accurate in pressure prediction and simulation calculation of FEM.
2
Experiment
TC21 titanium alloy was used for preparing samples. The setup for the present experiment was Q-switch Nd:glass (LSP-1) laser with infrared (1064 nm) radiation and 30 ns pulse width 30 ns pulse width and infrared (1064 nm) radiation. The energy of 20 J was used. The pulse laser was focused to a diameter of 4 ~5 mm onto the samples. Optical profilometer was used to dent geometry after LSP.
3
Results and Discussion
3.1 Microhardness analysis Due to the intensive plastic deformation and high peak pressure, the high density dislocation and fine deformation twins are produced in the laser shock processed surface layers of TC21. The microhardness could increase within the shocked area. The shocked areas are illustrated in Fig.2. The left circle is single spot with 4 mm in diameter. And right circle is two spots with 5 mm in diameter. The numbers show the test point on the surface of sample. The distributions of Vickers hardness are shown in Fig.3 under 4.9 N load and 15 s keeping load time. The HV hardness before LSP (21-0 line) are between 2710 MPa to 2880 MPa, while it reaches 3360 MPa after LSP (circles area) shown in Fig.4. Particularly on the shocked area, from No.2 to No.9 and No.12 to No.19, the HV hardness increase significantly. The maximum value reaches 3360 MPa. The irregular distributions of hardness value from No.12 to No.19 are due to shocking by overlapping two spot. The irregular distributions of single spot from No.2 to No.9 is due to the Gauss distribution of laser spot energy. As shown in Fig.3, the shock hardness effect decreases with increasing of distance from the center of laser beam, near to the edge of the shock area. The hardness is higher near to the center of laser beam. This is attributed to the higher pulse energy near to the center, which results in greater dislocation generation and motion. The surface hardness is improved apparently comparing with that of unshocked region. 3.2 Surface profile To quantitatively characterize the deformation, the interferometry with a vertical resolution of 50 nm was used to profile deformed regions. Fig.4 shows the measurement results of
2963
Che Zhigang et al. / Rare Metal Materials and Engineering, 2014, 43(12): 2962-2965
Residual Stress/MPa
100 0 –100 –200
Without LSP
–300 –400 –500 –600 0
Shocked region
With LSP
1
2
3
4
5
6
7
Test Number Fig.2 Distribution of measurement point on the sample
Fig.5 Residual stresses of TC21 without and with LSP vs test number
4000 HV4.9 N/MPa
3000 21-0 21-1
2000 1000 0 0
5
10
15
20
25
Distance/mm Fig.3 HV hardness vs the distance
Para.
Value
X
2.61
-
- mm
Unit
Y
2.02
-
- mm
Ht
–5.65
-
-
Dist
μm
- mm -
Para.
Value
X
2.44
Unit -
- mm
Y
2.22
-
- mm
Ht
–6.47
-
-
Dist Angle
μm
- mm -
Dent Depth/μm
Dent Depth/μm
Angle
Dent Depth/μm
Dent Depth/μm
dent geometry using optical profilometer after LSP on the surface of TC21. The cross-sectional measurements of the a X profile 2 0 –2 –4 –6 0.00 1.00 2.00 3.00 4.00 2 0 –2 –4 –6 0.00
Y profile
4
1.00 2.00 3.00 Distance/mm
b X profile 2 0 –2 –4 –6 –8 0.00 1.00 2.00 3.00 4.00 2 0 –2 –4 –6 0.00
dents are also shown in Fig.4 with X Profile and Y Profile. As shown in Fig.4a, the diameter of dents is about more than 4 mm near to the size of laser spot 4 mm. The difference between measurement value and laser spot diameter lies in the boundary effect and the existence of absorbing layer. In fact, the dimension of dent by LSP is correlative directly with the diameter and the energy of laser spot. The depth of the dent is 5.6 μm. As illustrated in Fig.4b, the two overlapping spot is shown. The depth reaches 6 μm due to the effect of the overlapping compared with that of Fig.4a. The roughness Ra of shocked region is less than 0.8 μm. 3.3 Residual stress The residual stresses were tested using XRD without and with LSP as shown Fig.5. Comparing with the measurement of without LSP, the residual stresses are all compressive and exceed 400 MPa. The technology of LSP changes the distribution of residual stress and improves the residual stress level of TC21 surface.
Y profile
1.00
2.00
3.00
Distance/mm
Conclusions
1) HV hardness increases after LSP in some extent. Higher HV hardness is in the center of laser spot. The shock hardness effect decreases with increasing of distance from the center of laser beam, especially near to the edge of the shock area. The surface hardness by LSP is improved apparently comparing with that of unshocked region. 2) The surface profiles are correlative directly with the diameter and the distribution of spots. The better surface roughness is kept within the shock region. 3) The technology of LSP improves the surface residual stress level of TC21, and thus the performance of fatigue resistance for this material is further improved.
References 1 Liu H J, Feng X L. Trans Nonferrous Met Soc China[J], 2011, 21: 58
Fig.4 Typical optical micrograph of dent geometry using optical profilometer of different radius: (a) left circle in Fig.2, single spot; (b) right circle, two overlapping spots in Fig.2
2 Szabó P J, Réti T, Czigány T. Mater Sci Forum[J], 2008, 589: 379 3 Wang Y N, Kysar J W, Yao Y L. Mechanics of Materials[J], 2008,
2964
Che Zhigang et al. / Rare Metal Materials and Engineering, 2014, 43(12): 2962-2965
40: 100 4 King A, Evans A D, Withers P J. Mater Sci Forum[J], 2005, 490-491: 340 5 Fairand B P, Wilcox B A, Gallagher W J et al. J Appl Phys[J], 1972, 43: 3893
[M]. Beijing: Chemical Industry Press, 2004 (in Chinese) 7 Ding K, Ye L. Laser Shock Peening: Performance and Process Simulation[M]. Boca Raton: CRC Press, 2006: 173 8 Che Z G, Gong S L, Cao Z W et al. Rare Metal Materials and Engineering[J], 2011, 40(S3): 235 (in Chinese)
6 Zhang Y K, Ye Y X et al. The Technology of Laser Machining
2965
ARTICLE
Research on Laser Peening of TC21 Titanium Alloy with High Energy Laser Che Zhigang1, 1
Yang Jie2,
Tang Nan3,
Gong Shuili1
Science and Technology on Power Beam Processes Laboratory, Beijing Aeronautical Manufacturing Technology Research Institute, Beijing
100024, China;
2
3
North China University of Water Resources and Electric Power, Zhengzhou 450045, China; The High School Affiliated to
Renmin University of China, Chaoyang School, Beijing 100028, China
Abstract: Laser peening (LP) also known as laser shock processing (LSP), is a surface enhancement technique that induces intensive plastic deformation, high dislocation density and deeper compressive residual stress to improve the surface performance of materials. The microhardness, surface profiles, roughness, and the residual stress of TC21 alloy were tested by LSP with high energy laser. The results show that the microhardness of the shocked area is improved apparently compared with that of unshocked area. The magnitude of dent is related with the diameter and the distribution of laser spot. The roughness Ra of shocked area is less than 0.8 μm. The compressive residual stress is enhanced greatly. The investigations show that the technology of LSP could further improve the performance of TC21 alloy. Key words: laser peening; TC21 titanium alloy; residual stress; microhardness; surface profile
Laser shock processing or laser peening is an advanced and promising surface treatment technique. It is an effective way to increase the wear resistance, the corrosion resistance and high-cycle fatigue (HCF) of metallic components, through high density dislocation and imparting compressive residual stress on the surface of a number of metals and alloys. The higher laser energy/power density could induce the higher density dislocation, deeper effect layer, larger action area and better treatment effect. TC21 titanium alloy possesses the level of 1000 MPa damage tolerance. As it has many advanced properties such as high strength and high fracture toughness corresponding with Ti-6-22-22S in America, next to Ti-6Al-4V used in F-22 fighter. As the material of aircraft structure, TC21 is superior to Ti-6Al-4V. TC21 titanium alloy has a wide application prospect in the manufacture of aviation, aerospace and related industries structural parts [1]. The research efforts to date have mainly been focused on the phase and properties of TC21 titanium alloy under heat treatment. However, only a little
work has been done on improving the surface performance of TC21 titanium alloy, which is important for its further application. The damage tolerance shows the behavior under the combined actions of load and defect. Therefore, it’s very important to improve the performance of titanium alloys. The objective of this work is to examine the effect of laser shock processing on the TC21, including hardness, surface profile and roughness. The properties of samples were improved after LSP.
1
Principle of LSP
The principle of LSP is shown in Fig.1. The surface of target is plastered with an absorbing layer (also called sacrificial layer, normal organic paint or metallic foil, such as tape, zinc or aluminum) after polishing[2]. The laser pulse with high peak value power (>1 GW/cm2), and short duration (nano second level) irradiates on the surface of coating layer traveling transparent confining layer (also called confined medium, water or glass) by focusing lens. The coating layer
Received date: December 24, 2013 Foundation item: National Natural Science Foundation of China (50975268) Corresponding author: Che Zhigang, Ph. D., Senior Engineer, Beijing Aeronautical Manufacturing Technology Research Institute, Beijing 100024, P. R. China, Tel: 0086-10-85701486, E-mail: [email protected] Copyright © 2014, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
2962
Che Zhigang et al. / Rare Metal Materials and Engineering, 2014, 43(12): 2962-2965 Pressure shockwave
a
b High pressure plasma
Laser pulse
Workpiece
Water confining layer Absorbing layer (Al foil)
Workpiece
Water confining layer Absorbing layer (Al foil)
Fig.1 Schematic illustrating of LSP and shockwave generation by confined plasma expansion before (a) and after (b) application of the laser pulse
absorbs the laser energy, resulting in an exploding and plasma is formed in very short time. The plasma is restrained by the confined medium during expanding and high pressure shock wave is generated [3]. When the peak value of stress wave exceeds the HEL (Hugoniot Elastic Limit) of the target for a suitable time, the dense and stable dislocations (or twin crystal) are formed. The surface strain hardening is produced at the same time. The elastic deformation energy with shock wave is greater than or equal to the plastic and yielding deformation energy of target material. The compressive residual stress is generated on the surface and in material due to the plastic deformation layer restraining the resuming of the elastic deformation energy. The existence of compressive residual stress alters the distribution of stress field and improves the fatigue performance of material surface. So the technique of LSP can evidently improve the performance of corrosion resistance and fatigue resistance [4,5]. The heat effect can be ignored due to the existence of absorbing layer and the very short time of interaction and protecting the target from thermal damage. The confined medium is used to enhance the pressure of shock wave and prolong the action time [6]. The HEL of the material depends on the properties [7]: λ + 2μ dyn 1 −ν dyn (1) HEL = σY = σY 2μ 1 − 2ν
μ=
Eν E λ= (1 + ν )(1 − 2ν ) 2(1 + ν ) ;
where λ and μ are the lame constant related with E (Young’s modulus)and ν (Poisson’s ratio). σ Ydyn is the dynamic yield strength at high strain rate (about 106 s-1). The pressure calculation of LSP is the foundation of determining the technique parameters. The pressure model used here can be obtained as follows [8] considering the left confining layer:
α (2) ⋅ Z ⋅ I0 （ 3 2α + 3） where p denotes pressure of shock wave, GPa; I0 denotes power density of laser pulse, GW/cm2; α denotes the fraction of the internal energy devoted to the thermal energy (typically, α=0.25); Z is the combined shock wave impedance (g/cm2·s) P = 0.01
relating with the shock wave impedance of target material (Z1), the confined medium (Z2) and the left absorbing layer (Z3). Among them the relationship could be defined as follows: 1 1 1 1 (3) = + + Z Z1 Z2 Z3 The pressure model is accordant with the practical situation and has more accurate calculation results than previous model due to the left coating layer being considered. The pressure value obtained by this model is accurate in pressure prediction and simulation calculation of FEM.
2
Experiment
TC21 titanium alloy was used for preparing samples. The setup for the present experiment was Q-switch Nd:glass (LSP-1) laser with infrared (1064 nm) radiation and 30 ns pulse width 30 ns pulse width and infrared (1064 nm) radiation. The energy of 20 J was used. The pulse laser was focused to a diameter of 4 ~5 mm onto the samples. Optical profilometer was used to dent geometry after LSP.
3
Results and Discussion
3.1 Microhardness analysis Due to the intensive plastic deformation and high peak pressure, the high density dislocation and fine deformation twins are produced in the laser shock processed surface layers of TC21. The microhardness could increase within the shocked area. The shocked areas are illustrated in Fig.2. The left circle is single spot with 4 mm in diameter. And right circle is two spots with 5 mm in diameter. The numbers show the test point on the surface of sample. The distributions of Vickers hardness are shown in Fig.3 under 4.9 N load and 15 s keeping load time. The HV hardness before LSP (21-0 line) are between 2710 MPa to 2880 MPa, while it reaches 3360 MPa after LSP (circles area) shown in Fig.4. Particularly on the shocked area, from No.2 to No.9 and No.12 to No.19, the HV hardness increase significantly. The maximum value reaches 3360 MPa. The irregular distributions of hardness value from No.12 to No.19 are due to shocking by overlapping two spot. The irregular distributions of single spot from No.2 to No.9 is due to the Gauss distribution of laser spot energy. As shown in Fig.3, the shock hardness effect decreases with increasing of distance from the center of laser beam, near to the edge of the shock area. The hardness is higher near to the center of laser beam. This is attributed to the higher pulse energy near to the center, which results in greater dislocation generation and motion. The surface hardness is improved apparently comparing with that of unshocked region. 3.2 Surface profile To quantitatively characterize the deformation, the interferometry with a vertical resolution of 50 nm was used to profile deformed regions. Fig.4 shows the measurement results of
2963
Che Zhigang et al. / Rare Metal Materials and Engineering, 2014, 43(12): 2962-2965
Residual Stress/MPa
100 0 –100 –200
Without LSP
–300 –400 –500 –600 0
Shocked region
With LSP
1
2
3
4
5
6
7
Test Number Fig.2 Distribution of measurement point on the sample
Fig.5 Residual stresses of TC21 without and with LSP vs test number
4000 HV4.9 N/MPa
3000 21-0 21-1
2000 1000 0 0
5
10
15
20
25
Distance/mm Fig.3 HV hardness vs the distance
Para.
Value
X
2.61
-
- mm
Unit
Y
2.02
-
- mm
Ht
–5.65
-
-
Dist
μm
- mm -
Para.
Value
X
2.44
Unit -
- mm
Y
2.22
-
- mm
Ht
–6.47
-
-
Dist Angle
μm
- mm -
Dent Depth/μm
Dent Depth/μm
Angle
Dent Depth/μm
Dent Depth/μm
dent geometry using optical profilometer after LSP on the surface of TC21. The cross-sectional measurements of the a X profile 2 0 –2 –4 –6 0.00 1.00 2.00 3.00 4.00 2 0 –2 –4 –6 0.00
Y profile
4
1.00 2.00 3.00 Distance/mm
b X profile 2 0 –2 –4 –6 –8 0.00 1.00 2.00 3.00 4.00 2 0 –2 –4 –6 0.00
dents are also shown in Fig.4 with X Profile and Y Profile. As shown in Fig.4a, the diameter of dents is about more than 4 mm near to the size of laser spot 4 mm. The difference between measurement value and laser spot diameter lies in the boundary effect and the existence of absorbing layer. In fact, the dimension of dent by LSP is correlative directly with the diameter and the energy of laser spot. The depth of the dent is 5.6 μm. As illustrated in Fig.4b, the two overlapping spot is shown. The depth reaches 6 μm due to the effect of the overlapping compared with that of Fig.4a. The roughness Ra of shocked region is less than 0.8 μm. 3.3 Residual stress The residual stresses were tested using XRD without and with LSP as shown Fig.5. Comparing with the measurement of without LSP, the residual stresses are all compressive and exceed 400 MPa. The technology of LSP changes the distribution of residual stress and improves the residual stress level of TC21 surface.
Y profile
1.00
2.00
3.00
Distance/mm
Conclusions
1) HV hardness increases after LSP in some extent. Higher HV hardness is in the center of laser spot. The shock hardness effect decreases with increasing of distance from the center of laser beam, especially near to the edge of the shock area. The surface hardness by LSP is improved apparently comparing with that of unshocked region. 2) The surface profiles are correlative directly with the diameter and the distribution of spots. The better surface roughness is kept within the shock region. 3) The technology of LSP improves the surface residual stress level of TC21, and thus the performance of fatigue resistance for this material is further improved.
References 1 Liu H J, Feng X L. Trans Nonferrous Met Soc China[J], 2011, 21: 58
Fig.4 Typical optical micrograph of dent geometry using optical profilometer of different radius: (a) left circle in Fig.2, single spot; (b) right circle, two overlapping spots in Fig.2
2 Szabó P J, Réti T, Czigány T. Mater Sci Forum[J], 2008, 589: 379 3 Wang Y N, Kysar J W, Yao Y L. Mechanics of Materials[J], 2008,
2964
Che Zhigang et al. / Rare Metal Materials and Engineering, 2014, 43(12): 2962-2965
40: 100 4 King A, Evans A D, Withers P J. Mater Sci Forum[J], 2005, 490-491: 340 5 Fairand B P, Wilcox B A, Gallagher W J et al. J Appl Phys[J], 1972, 43: 3893
[M]. Beijing: Chemical Industry Press, 2004 (in Chinese) 7 Ding K, Ye L. Laser Shock Peening: Performance and Process Simulation[M]. Boca Raton: CRC Press, 2006: 173 8 Che Z G, Gong S L, Cao Z W et al. Rare Metal Materials and Engineering[J], 2011, 40(S3): 235 (in Chinese)
6 Zhang Y K, Ye Y X et al. The Technology of Laser Machining
2965