- Email: [email protected]
Study on fatigue crack growth performance of EH36 weldments by laser shock processing
Accepted Manuscript
Study on fatigue crack growth performance of EH36 weldments by laser shock processing Wang Yun , Baidoo Philip , Xu Zhenying , Wu Junfeng PII: DOI: Reference:
S2468-0230(18)30312-2 https://doi.org/10.1016/j.surfin.2018.10.009 SURFIN 252
To appear in:
Surfaces and Interfaces
Please cite this article as: Wang Yun , Baidoo Philip , Xu Zhenying , Wu Junfeng , Study on fatigue crack growth performance of EH36 weldments by laser shock processing, Surfaces and Interfaces (2018), doi: https://doi.org/10.1016/j.surfin.2018.10.009
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Study on fatigue crack growth performance of EH36 weldments by laser shock processing
WANG Yun1, BAIDOO Philip1*, XU Zhenying1, Wu Junfeng1
[email protected], [email protected], [email protected], [email protected]
CR IP T
Corresponding author e-mail address; [email protected], Telephone number; +86 18605242722 School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, China Abstract: EH36 ship steel plates are widely used under harsh environment of ships and offshore platforms because of high specific strength, good low temperature impact toughness, good weldability, and good corrosion resistance. However, welded ship steel plates produce unforeseen crack and fatigue failure during service. In order to improve the fatigue resistance of EH36 weldments, the influence of
AN US
impact pressure, spot diameter, and laser shock processing (LSP) on the fatigue crack growth (FCG) rate of EH36 weldment was studied. LSP experiments and fatigue tests on EH36 compact tension (CT) weldments were carried out. The surface compressive residual stress and the microhardness in the welding zone (WZ) with LSP-3 are 230MPa and 270HV, and grain size in the surface layer of WZ was refined. Fatigue life of EH36 weldments with LSP-3 increases by 270% compared to that of them without LSP. FCG rate obviously decreases because of the secondary cracks and narrow and dense
M
fatigue striations occurring in the fatigue fracture. Moreover, large and deep dimples on the fatigue fracture effectively improve the plasticity of weldments. The proposed research provides the instruction
ED
to predict the fatigue life of EH36 weldments by LSP. Keyword: Laser shock processing (LSP), EH36 weldments, Compressive residual stress, Microhardness, FCG rate, Fatigue fracture
PT
crack growth normal to the welding or laser scan direction, a retardation crack growth disappeared for laser welds subjected to a stress relief [3-4]. When the peak pressure of LSP wave exceeds
1 Introduction
CE
Ships are under harsh environment with alternating dynamic forces of inevitable welding-induced residual tensile stress for thick steel plates, welded ship steel plates produce unforeseen crack fatigue failure at service. For the betterment life increment on both fatigue and corrosion of weldments, some treatments are used such as weld grinding, Tungsten inert gas (TIG) dressing and weld peening. However, the above treatments have limitation of application to ship structures. The LSP is a novel and promising technique for fatigue life improvement of weldments due to high residual compressive stress induced by shock wave [1]. A large responds of literature confirmed to LSP has been described improving fatigue performance and stress corrosion of aluminum alloy, nickel base alloy and steel [2-6]. As a
dynamic yield strength on the material, useful residual compressive stress is produced on the
AC
surface layer, which delays the fatigue and crack initiation [7]. Ren et al. [8] showed that LSP improved on “7050-T7451Al” alloy fatigue intensity by inhibiting fatigue crack initiation and growth of it. Spanrad et al. [9] revealed that LSP delayed the onset instituting of crack and damage features impact depends on both angle and velocity. Zhang et al. [10] evaluated that different shock roots effect on property of fatigue on 7050-T7451Al alloy. Chahardehi et al. [11] studied the fatigue crack growth (FCG) 1
ACCEPTED MANUSCRIPT 1. Cutting weldments of expected dimensions by electro-discharge machined (EDM). 2. Boring two holes with the diameter of (Φ) 12.5mm. 3. Grinding and polishing samples with SiC paper at different grades of roughness. 4. Cleaning samples in treated solution water and saving in drying box. 5. Eliminating machined surface residual stress of CT samples by naturally aging treatment for a period of time. 6. Preparing pre-cracked samples in the center of WZ of 2.5mm from GB/T 6398-2000 standard. After preparations and treatments, the EH36 metal plate had/obtained width of WZ 2mm, HAZ 10mm and length of welding 37.5mm and with LSP zone of 35mm respectively.
of LSP on stainless steel plates where the impact of residual compressive stress was induced. Again, it was found that few studies were made to the effect of LSP on the FCG performance of
CR IP T
EH36 weldments with the high tensile strength. Therefore, the objective of this paper is to investigate the impact pressure, spot diameter and LSP mechanism on FCG of EH36 weldments and fatigue fracture morphology analyzed by scanning electron microscopy (SEM) without forgotten the enhancement mechanism of the FCG were all considered and discussed. 2 Experimental procedures
AN US
2.1 Material and sample preparation Thick ship steel plates EH36 are widely used under harsh environment of ships and offshore platforms having a high specific strength, good low temperature impact toughness, and specific properties of mechanical and chemical as indicated in table 1 and 2. However, improving fatigue property of EH36
weldment is our ultimate objective and it’s LSP
technology. standard
is
a
novel
According
and
to
experimental
and
anti-fatigue
GB/T6398-2000 conditions,
ED
which
M
uniquely prolonging material service life of
the
compact tension (CT) samples were prepared with the dimensions shown in Fig.1 to explore
PT
the effect of the LSP on the FCG rate of EH36 weldments. The pointed out area of welding
CE
zone (WZ), heat-affected zone (HAZ) and base metal (BM) for EH36 weldments were carefully marked by marking pen in order to distinguish
AC
from different zones weldments. The CT samples
prepared
procedures
Fig. 1. Dimension of CT samples subjected to
respectfully
followed:
LSP.
Table 1 show chemical composition (Wt. %) of EH36 high strength steel Grade Wt. (%)
C 0.15
Mn 1.45
P 0.01
S 0.006
Si 0.606
Sources: https://wenku.baidu.com/view/33ac56ed4afe04a1b071dee7.html
2
Cu 0.193
Al 0.125
ACCEPTED MANUSCRIPT
Table 2 show mechanical properties of EH36 high strength steel Steel Grade
YS*(MPa)
UTS*(MPa)
Total EL(%)
Wt. (%)
430
540
34
Ballistic work(J)/-40 153,143,173
Sources: https://wenku.baidu.com/view/33ac56ed4afe04a1b071dee7.html
Table 3. Welding electrode composition of EH36 weldments C 0.08
Si 0.90
Mn 1.45
S 0.015
P 0.013
Ni 0.01
Mo 0.04
V 0.01
AC
CE
PT
ED
M
AN US
CR IP T
Element Wt.%
3
ACCEPTED MANUSCRIPT
ED
M
AN US
CR IP T
2.2 Principle of LSP HE36 weldment and experimental parameters The LSP utilized high energy pulse to hit the surfaces of the material and then formed plasma. The restrain plasma created at a higher surface pressure propagated into the material as a shock wave. Fig.2. showed the diagrammatical principle of the LSP. The massive LSP impacts in the WZ crack tip of EH36 weldments were carried out using a Qswitched Nd: YAG (Neodymium doped Yttrium Aluminum Garnet) laser system and numerical control workbench. Shock path was illustrated in Fig.1, welding electrode composition of weldment (wt%) parameters was shown in Table3. All samples were submerged into the water bath when they were processed by LSP. A layer of water thickness of 1mm was used as the transparent confining layer and the professional aluminum foil with a thickness of 0.1mm was used as an absorbing layer to protect the sample surface from the thermal effect. The LSP parameters of HE36 weldment were 1 impact time, 2 impact time and 3 impact times respectively. The laser energy, pulse width, spot diameter, spot spacing of 12J, 10ns, 3mm, 2mm were used respectively. However laser welding was done on one top side of the specimens and the Q-switched Nd: YAG power Laser beam was used. The laser light was transmitted via a water-cooled glass fiber and the beam projected onto the specimens by collimating and focusing optics. When the laser beam hits the surface of the specimen, the spot is heated up to vaporization temperature, and a vapor cavity is formed in the weld specimen due to the escaping metal vapor. The energy-flow density of the freely burning arc was 17kW/cm and frequency of 1Hz. The fiber laser power was 4kw, the above spot diameter of the laser beam and a welding speed of 40mm/s was conducted during welding.
PT
Fig. 2. Schematic principle of LSP
AC
CE
2.3 Measurements of residual stress The outward layer of the specimen residual stress measurement shown in Fig.1 on the WZ with and without LSP impacts were done using X-ray diffraction (XRD) machine with the sin2Ψ method. The X-ray beam diameter and source were about 1mm and CrKa ray diffraction plane was α phase (310) plane in the stress calculation. The feed angle of the ladder scanning was 0.1degs-1. The scanning started angle and terminated angle were 167°and 157 ° respectively. The arithmetic average value of three repeated measurements was taken as the residual stress of each measurement point. 2. 4 Micro-structural observations Firstly, the WZ with and without LSP were prepared by EDM to research on microstructure. In addition, the samples were embedded in a fixture, and then were subjected to several successive steps of grinding and polishing. After that, they were etched by a solution of 4% Nital to reveal the microstructure of WZ, which was characterized by cross-sectional optical microscopy (OM).
2.5 Microhardness measurement The Vickers hardness on the WZ surface was tested by using HXD-1000TM digital microhardness
4
ACCEPTED MANUSCRIPT
instrument. The arithmetic average values of three measurement points were taken as the microhardness of the determined region. The WZ surface was cleaned with 99% ethanol material before the measurement. During the hardness measurement, 200gf force was applied for 10s. The corresponding microhardness value was calculated by measuring the diagonal length of indentation.
CR IP T
2.6 FCG test FCG tests were performed to study the effect of LSP on FCG performance of EH36 weldments on a MTS-809 servo-hydraulic system operating with 10Hz frequency in sine wave at room temperature. The stress ratio R (R= min max ) was maintained at 0.1. The EH36 weldments with and without LSP were tested with maximum tensile force of 15kN. Crack lengths were measured at a magnification of 10x using a CCD camera. Crack termination length was 30mm.
AN US
2.7 Fatigue fracture analysis The fatigue fracture morphology of EH36 weldments without LSP and with LSP-3 impacts was analyzed to explore the mechanism of fatigue life improvement. The fatigue fracture of EH36 weldments were prepared by EDM and then cleaned by using ultrasonic cleaning machine to prevent fracture from oil pollution. The fatigue fracture was characterized by scanning electron microscope (SEM).
3. Results and discussion
AC
CE
PT
ED
M
3.1 Residual stress distribution The surface residual stress profiles of the WZ with LSP impacts and without LSP were shown in Fig.3. The direction of the crack tip is perpendicular to the direction of welding on the X axis was also shown in Fig.1. It was notice from Fig.3 that large tensile stress was existed in the WZ surface of EH36 weldments because of non-uniform temperature field and local plastic deformation produced by welding (as shown in No LSP direction in the line of Fig. 3. The surface compressive residual stress in the WZ of EH36 weldments for LSP-1, LSP-2 and LSP-3 shown 110MPa, 180MPa and 230MPa, respectively. The surface compressive residual stress in the WZ of EH36 weldments was increased by 63.6% and 27.8% when the impact time increased from 1 to 2 and 2 to 3. Since compressive residual stress induced by LSP in near-surface layer caused the changes of the actual stress situation in the WZ of EH36 weldments bearing a larger outside force on compressive residual stress, have a significant role in retarding the fatigue crack growth of EH36 weldments. And compressive residual stress in near-surface layer was the key to cause the great increase of notch fatigue limit and the significant reduction of fatigue notch sensitivity on them. The surface and depth compressive residual stress in the WZ of EH36 weldments with LSP increases the crack closure and reduces the actual stress intensity factor of the crack tip, which can efficiently reduce the negative effect of tensile stress accelerating crack extension and causing new crack. Therefore, fatigue of the crack growth resistance and life of EH36 weldments were improved.
5
CR IP T
ACCEPTED MANUSCRIPT
Fig. 3. Surface residual stress distribution in the WZ with different impact times in X axis
AN US
3. 2 Effect of LSP on Microstructure Fig.4 and Fig.5 show the microscopic structure in the WZ with LSP-3 and without LSP. The WZ microstructure was mainly composed of slender proeutectoid ferrite with slender body and acicular ferrite (as shown in Fig.4). The BM with high carbon content dissolves in molten pool during the welding process, which improves carbon content in the WZ. Some granular bainite exists in the WZ of EH36 weldments. The generated granular bainite after laser process in referenced to fig. 4 and 5 respectively, the existing of generated granular bainite in the material contribute to layer and structure higher strengthen, hard and durable by the way of absorbing shocks when external load or force applied on it which improved the mechanical property of the weld zone (wz).
AC
CE
PT
ED
M
Fig.5 suggests that LSP had no effect on the microscopic structure in the weld zone (WZ) of EH36 weldments because of an absorbing layer with aluminum foil avoided a thermal effect from heating of the samples surface by LSP and non-uniform microstructure in the weld zone (WZ) of EH36 weldments. However, these cell structures in the WZ of EH36 weldments were the refined grains by dislocation multiplication and plastic deformation induced by LSP. After LSP of 3, the micro structural characteristics deformations in the hardening layers were shown in Fig.7. According to dislocation theory in [12] and HallPetch theory in [13], grain refinement plays an important role in the improvement of tensile strength and fatigue strength in the WZ of EH36 weldments. The black grains which mean pearlite are the solidified grains reinforced carbon black gains. As a result, LSP clearly improves the tensile strength and the fatigue of the EH36 weldments.
Fig. 4. Microstructure in the WZ without LSP Fig. 4. Microstructure in the WZ without LSP
6
CR IP T
ACCEPTED MANUSCRIPT
M
AN US
Fig. 5. Microstructure in the WZ with LSP-3
AC
CE
PT
ED
Fig. 6. Microstructure characteristics of depth direction in the hardening layer after LSP-3
Fig.7. sectional welding indentation points
3.3 The microhardness of the WZ Fig.8 clearly shows the microhardness in the WZ with and without LSP. The microhardness in the WZ
7
ACCEPTED MANUSCRIPT
CR IP T
of EH36 weldments were 230HV, 250HV, 260HV and 270HV for No LSP, LSP-1, LSP-2 and LSP-3 with 12J. The microhardness in the WZ was increased by 8.7%, 13% and 17.4%, when the impact times increased from 0 to 1, 1 to 2, 2 to 3 respectfully. Because the microhardness of the material increases with the decrement of grain size according to Hall-Petch law [14] and LSP makes the grain refinement in the outward body layer of the weldment zone as shown in Fig.6 and fig.8 with regards to sectional welding indented points in fig. 7 respectively. Therefore, LSP effectively improves the surface microhardness of WZ. The grain refinement was associated with the formation of the dislocation lines induced by LSP. A larger amount of dislocation motions form dislocation walls and dislocation tangles. When dislocation density of the dislocation walls and dislocation tangles reaches a certain value, the dislocation begins to rearrange to form new dislocation walls, which makes the grain refinement on the surface layer.
ED
M
AN US
From Fig.8, it was observed that the microhardness increases with the increase of impact times. However, the increasing rate of the microhardness decreases as the impact time increases. A larger number of dislocation tangles and dislocation walls form the resistance of the dislocation movement resulting in the work-hardening in the near surface. The work-hardening introduces the decrement of microhardness amplitude with the increment of impacts times and hinders the crack initiation.
Fig. 8. Impact of LSP impacts on the microhardness in the WZ of EH36 weldments
AC
CE
PT
3.4 Effect of LSP on FCG rate Fig.9 shows influence of different LSP impacts on the FCG rate da/dN of EH36 CT weldments. In the FCG stage, FCG rate of EH36 weldments after LSP-1 decreases compared to FCG rate of EH36 weldments without LSP. The decrements of FCG rate of EH36 weldments after LSP-2 and LSP-3 are more evident, which was consistent with LSP induced compressive residual stress values on the crack tip of the weldment zone (WZ). When delta K was small, LSP would obviously decreased FCG rate da/dN of EH36 weldments. And FCG rate of EH36 weldments after LSP of 3 was minimum, which was associated with large compressive residual stress in the surface layer of WZ with LSP of 3. Compressive residual stress induced by LSP could brought about the crack closure and invited the decreased of effective driven force, which was beneficial of the reduction for stress intensity factor range of FCG rate. Dynamic FCG causes compressive residual stress relaxation on the crack tip on WZ with increment of delta K and then compressive residual stress could not offset the effected of tensile stress on the LSP effect decreases. In the final rupture stage, FCG speed greatly accelerates and LSP effect was significantly weakened because the crack driven stress was greater than LSP-induced compressive residual stress.
8
ACCEPTED MANUSCRIPT
1E-5
No LSP LSP-1 LSP-2 LSP-3
1E-6
500
600
700
Delta K Applied (N/mm^1.5)
800
CR IP T
da/dN (mm/cycle)
1E-4
Fig. 9. FCG rate of EH36 weldments with different LSP impacts
CE
PT
ED
M
AN US
Fig.10. shows the curves of the crack length versus cycle index on EH36 weldments before LSP and after LSP impacts. When fatigue life of EH36 weldments without LSP is 12354, fatigue lives of EH36 weldments are 32460, 43090 and 45820 after LSP-1, LSP-2 and LSP-3. The fatigue lives of EH36 weldments are increased by 162%, 32% and 6.3% when the impact times increase from 0 to 1, 1 to 2 and 2 to 3. The cycle index of EH36 weldments with LSP obviously increases with the increment of impact times, but the increasing rate of the cycle index reduces gradually. Because compressive residual stress on the surface layer of WZ with LSP diminishes the effective stress intensity factors of FCG. Furthermore, EH36 weldments with LSP-3 got the highest fatigue life, which is attributed to the largest compressive residual stresses distribution and the most evident grain refinement induced by the LSP.
Fig. 10. Relationship between the crack length and the cycle index
Fatigue fracture analysis The fatigue lives of CT samples under the cyclic force were determined by the fatigue crack initiation and propagation. Most metallic materials under the cyclic force experience FCG stage and enter into final rupture stage. Fatigue fracture morphology of material really reflects the characteristics of FCG process, so the fatigue fracture analysis was of great importance. Fig.11 shows the macroscopic fracture morphology in the WZ of EH36 weldments before LSP and after LSP-3. Fatigue fracture morphology in the WZ mainly consists of Pre-crack region, FCG region and final rupture region. Cracks formed by crack sources and inclusions extend in radiation shape in the precrack region. In the FCG region, fatigue fracture morphology had obvious characteristics of shells or beach pattern. The final rupture region was fibrous and dark gray, which was rougher compared with other regions. Compared to FCG region in Fig.11 (a), FCG region was longer and its fracture morphology was
AC
3.5
9
ACCEPTED MANUSCRIPT
CR IP T
smoother and denser (as shown in Fig.11 (b)). And crack arrests with semicircular or fan curved line were perpendicular to the FCG direction in FCG region. Because compressive residual stress induced by LSP in the surface layer restrains fatigue crack growth of EH36 weldments and FCG rate of EH36 weldments decreased.
AN US
Fig. 11. Fatigue fracture macrostructure morphology in the WZ of EH36 weldments before and after LSP, (a) No LSP (b) LSP-3
AC
CE
PT
ED
M
Fig.12 illustrates the microstructure morphology of FCG region in the fatigue fracture of WZ without LSP and with LSP-3. In Fig.12(a), fatigue fracture surface was not very smooth and shown that terraces form. Cracks were given priority to with the intragranular expansion and secondary cracks were few. However, fatigue fracture surface was relatively smooth and more secondary cracks extend along the broken crystal direction (as shown in Fig.12(b)). Because surface compressive residual stress in the surface layer of WZ with LSP-3 offsets partial tensile stress and secondary cracks consume large amounts of energy, fatigue crack extension resistance was increased and FCG rate was effectively delayed for CT samples with LSP-3. Fig.12(c) and (d) were partial enlarged drawings of (a) and (b). The fatigue striations spacing were 0.84 µm before LSP and 0.46 µm after LSP. The fatigue striations in Fig.12(d) became narrower and denser compared to the fatigue striations in Fig.12(c), which indicated that stress intensity factor of the crack front area for CT samples with LSP-3 was relatively smaller and FCG rate was slower. Because grains refinement and compressive residual stress induced by LSP-3 in the WZ result in the plasticity and toughness of CT samples. The results shown that LSP could efficiently alleviate the negative effects of the crack initiation and propagation of EH36 weldments.
Fig. 12. Fatigue fracture microstructure morphology of FCG region in the WZ of EH36 weldments, (a) No LSP (b) LSP-3, (c) and (d) the partial enlarged drawing of (a) and (b) Fig.13 presents the microstructure morphology of final rupture region in the fatigue fracture of WZ without
10
ACCEPTED MANUSCRIPT
AN US
CR IP T
LSP and with LSP-3. The white teare ridge and a lot of dimples appear on the surfaces of toughness fracture characteristic. Because micro cracks were formed at the interfaces of inclusions, second phase particles and substrate. Furthermore, the coalescence of adjacent micro cracks produces microvoids. Microvoids produce in the second phase, grain boundaries and subgrain boundaries, and then slowly grow up with the increase of stress. Finally, connection of the microvoids forms fracture and leaves trace. The size and depth of dimples depends on the plasticity of the material and the number of microvoids nucleate. Because the appearance of dimples, shear deformation of the material occurs in the local area. There were more, larger and deeper dimples in Fig.13 (b). In addition, there were some deeper holes caused by drawn type, which indicates the ductility of EH36 weldments was better. In conclusion, the ductility and plasticity of EH36 weldments with LSP-3 were superior to that of EH36 weldments without LSP.
Fig. 13. Microstructure morphology of final rupture region of WZ without LSP and with LSP-3, (a) Without LSP (b) With LSP-3 4. Conclusions
AC
CE
PT
ED
M
The impact of LSP on the FCG rate of EH36 welds was investigated in this work. The surface microhardness and residual stress in the WZ, the FCG rate and the fatigue fracture characteristic of EH36 weldments before and after LSP were all discussed and analyzed. The following conclusions have been drawn as follows: （1）Surface compressive residual stress in the WZ was increased by 63.6% and 27.8% when the impact time increases from 1 to 2 and 2 to 3. Compressive residual stress induced by LSP increased the crack closure and reduced the actual stress intensity factor of FCG, which improved FCG resistance of EH36 weldments. （2）LSP-induced grain refinement increased the microhardness of the WZ. The microhardness in the WZ with LSP-3 was increased by 17.4%. However, LSP-induced work hardening introduced the decrement of microhardness increase amplitude with the increment of impact times. （3）The compressive residual stress induced by LSP in the WZ diminishes the effective stress intensity factors of FCG process. The fatigue lives of EH36 weldments were increased by 162%, 32% and 6.3% when the impact times increased from 0 to 1, 1 to 2 and 2 to 3. （4）The fatigue striations spacing were 0.84µm before LSP and 0.46µm after LSP-3 in FCG region. Narrow and dense fatigue striations and more secondary cracks in FCG region after LSP-3 increase the resistance of FCG and delays FCG rate. Large and deep dimples in the final rupture region after LSP-3 improved the ductility and plasticity of EH36 weldments. Acknowledgement The financial support provided by NSFC (51575245) and the Key Project of Jiangsu Province (BE2015134) is especially acknowledged and also grateful to Priority Academic Program Development of Jiangsu Higher Education Institutions.
11
ACCEPTED MANUSCRIPT
References:
ED
M
AN US
CR IP T
[1] Brennan FP, Ngiam SS, Lee CW. An experimental and analytical study of fatigue crack shape control by cold working. Eng Fract Mech. 2008; 75(3):355-363. [2] Rankin JE, Hill MR, Hackel LA. The effects of process variations on residual stress in laser peened 7049 T73 aluminum alloy. Materials Science and Engineering: A. 2003; 349(1):279-291. [3] Tsay LW, Young MC, Chen C. Fatigue crack growth behavior of laser-processed 304 stainless steel in air and gaseous hydrogen. Corros Sci. 2003; 45(9):1985-1997. [4] Rubio- González C, Gomez-Rosas G, Ocaña JL, Molpeceres C, Banderas A, Porro J, Morales M. Effect of an absorbent overlay on the residual stress field induced by laser shock processing on aluminum samples. Appl Surf Sci. 2006; 252(18):6201-6205. [5] Sanchez-Santana U, Rubio- González C, Gomez-Rosas G, Ocana JL, Molpeceres C, Porro J, Morales M. Wear and friction of 6061-T6 aluminum alloy treated by laser shock processing. Wear. 2006; 260(7):847-854. [6] Rubio-Gonzalez C, Ocana JL, Gomez-Rosas G, Molpeceres C, Paredes M, Banderas A, Porro J, Morales M. Effect of laser shock processing on fatigue crack growth and fracture toughness of 6061-T6 aluminum alloy. Materials Science and Engineering: A. 2004; 386(1):291-295. [7] Srinivasan S, Garcia DB, Gean MC, Murthy H, Farris TN. Fretting fatigue of laser shock peened Ti-6Al-4V. Tribol Int. 2009; 42(9):1324-1329. [8] Ren XD, Zhang YK, Yongzhuo HF, Ruan L, Jiang DW, Zhang T, Chen KM. Effect of laser shock processing on the fatigue crack initiation and propagation of 7050-T7451 aluminum alloy. Materials Science and Engineering: A. 2011; 528(6):2899-2903. [9] Spanrad S, Tong J. Characterisation of foreign object damage (FOD) and early fatigue crack growth in laser shock peened Ti-6Al-4V aerofoil specimens. Materials Science and Engineering: A. 2011; 528(4):2128-2136. [10] Zhang L, Lu JZ, Zhang YK, Luo KY, Zhong JW, Cui CY, Kong DJ, Guan HB, Qian XM. Effects of different shocked paths on fatigue property of 7050-T7451 aluminum alloy during two-sided laser shock processing. Mater Design. 2011; 32(2):480-486. [11] Chahardehi A, Brennan FP, Steuwer A. The effect of residual stresses arising from laser shock peening on fatigue crack growth. Eng Fract Mech. 2010; 77(11):2033-2039. [12] Di Schino A, Kenny JM. Grain size dependence of the fatigue behaviour of a ultrafine-grained AISI 304 stainless steel. Mater Lett 2003; 57:3182. [13] Chen M, Ma E, Hemker KJ, Sheng H, Wang Y, Cheng X. Deformation twinning in nanocrystalline aluminum. Science 2003; 300:1275. [14] Petch NJ. The cleavage strength of polycrystals. J. Iron Steel Inst. 1953; 174:25. Biographical notes
CE
PT
WANG Yun, born in 1975, is currently a professor at Die and Mould Technology Institute,School of mechanical Engineering, Jiangsu University, China.He received his PhD degree from HefeiUniversityofTechnology, China, in 2003.He has awarded First Prize ofNational Award for Science and Technology Progress. His research interests include marine engineering equipment manufacturing research. Tel:+86-138-15483092; E-mail: [email protected]
AC
BAIDOO Philip, born in 1974, is currently a Ph.D candidate at Die and Mould Technology Institute, School of mechanical Engineering, Jiangsu University, China. His research interests include fatigue performance improvement of weldments by laser shock processing. Tel: +86-18605242722; E-mail: [email protected]
XU Zhenying, born in 1977,is currently an associate professoratIndustrial Nondestructive Testing Institute, School of mechanical Engineering, Jiangsu University, China. She received his PhD degree from HefeiUniversityofTechnology, China, in 2003.She has awarded First Prize ofNational Award for Science and Technology Progress. Her research interests include precision forming and precision controlof micro device. Tel: +86-137-75372507; E-mail: [email protected]
12
ACCEPTED MANUSCRIPT
AC
CE
PT
ED
M
AN US
CR IP T
WU Junfeng, born in 1988, is currently a master candidate at Die and Mould Technology Institute, School of mechanical Engineering, Jiangsu University, China. His research interests include fatigue performance improvement of weldments by laser shock processing. Tel: +86-187-06102013; E-mail: [email protected]
13
Study on fatigue crack growth performance of EH36 weldments by laser shock processing Wang Yun , Baidoo Philip , Xu Zhenying , Wu Junfeng PII: DOI: Reference:
S2468-0230(18)30312-2 https://doi.org/10.1016/j.surfin.2018.10.009 SURFIN 252
To appear in:
Surfaces and Interfaces
Please cite this article as: Wang Yun , Baidoo Philip , Xu Zhenying , Wu Junfeng , Study on fatigue crack growth performance of EH36 weldments by laser shock processing, Surfaces and Interfaces (2018), doi: https://doi.org/10.1016/j.surfin.2018.10.009
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Study on fatigue crack growth performance of EH36 weldments by laser shock processing
WANG Yun1, BAIDOO Philip1*, XU Zhenying1, Wu Junfeng1
[email protected], [email protected], [email protected], [email protected]
CR IP T
Corresponding author e-mail address; [email protected], Telephone number; +86 18605242722 School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, China Abstract: EH36 ship steel plates are widely used under harsh environment of ships and offshore platforms because of high specific strength, good low temperature impact toughness, good weldability, and good corrosion resistance. However, welded ship steel plates produce unforeseen crack and fatigue failure during service. In order to improve the fatigue resistance of EH36 weldments, the influence of
AN US
impact pressure, spot diameter, and laser shock processing (LSP) on the fatigue crack growth (FCG) rate of EH36 weldment was studied. LSP experiments and fatigue tests on EH36 compact tension (CT) weldments were carried out. The surface compressive residual stress and the microhardness in the welding zone (WZ) with LSP-3 are 230MPa and 270HV, and grain size in the surface layer of WZ was refined. Fatigue life of EH36 weldments with LSP-3 increases by 270% compared to that of them without LSP. FCG rate obviously decreases because of the secondary cracks and narrow and dense
M
fatigue striations occurring in the fatigue fracture. Moreover, large and deep dimples on the fatigue fracture effectively improve the plasticity of weldments. The proposed research provides the instruction
ED
to predict the fatigue life of EH36 weldments by LSP. Keyword: Laser shock processing (LSP), EH36 weldments, Compressive residual stress, Microhardness, FCG rate, Fatigue fracture
PT
crack growth normal to the welding or laser scan direction, a retardation crack growth disappeared for laser welds subjected to a stress relief [3-4]. When the peak pressure of LSP wave exceeds
1 Introduction
CE
Ships are under harsh environment with alternating dynamic forces of inevitable welding-induced residual tensile stress for thick steel plates, welded ship steel plates produce unforeseen crack fatigue failure at service. For the betterment life increment on both fatigue and corrosion of weldments, some treatments are used such as weld grinding, Tungsten inert gas (TIG) dressing and weld peening. However, the above treatments have limitation of application to ship structures. The LSP is a novel and promising technique for fatigue life improvement of weldments due to high residual compressive stress induced by shock wave [1]. A large responds of literature confirmed to LSP has been described improving fatigue performance and stress corrosion of aluminum alloy, nickel base alloy and steel [2-6]. As a
dynamic yield strength on the material, useful residual compressive stress is produced on the
AC
surface layer, which delays the fatigue and crack initiation [7]. Ren et al. [8] showed that LSP improved on “7050-T7451Al” alloy fatigue intensity by inhibiting fatigue crack initiation and growth of it. Spanrad et al. [9] revealed that LSP delayed the onset instituting of crack and damage features impact depends on both angle and velocity. Zhang et al. [10] evaluated that different shock roots effect on property of fatigue on 7050-T7451Al alloy. Chahardehi et al. [11] studied the fatigue crack growth (FCG) 1
ACCEPTED MANUSCRIPT 1. Cutting weldments of expected dimensions by electro-discharge machined (EDM). 2. Boring two holes with the diameter of (Φ) 12.5mm. 3. Grinding and polishing samples with SiC paper at different grades of roughness. 4. Cleaning samples in treated solution water and saving in drying box. 5. Eliminating machined surface residual stress of CT samples by naturally aging treatment for a period of time. 6. Preparing pre-cracked samples in the center of WZ of 2.5mm from GB/T 6398-2000 standard. After preparations and treatments, the EH36 metal plate had/obtained width of WZ 2mm, HAZ 10mm and length of welding 37.5mm and with LSP zone of 35mm respectively.
of LSP on stainless steel plates where the impact of residual compressive stress was induced. Again, it was found that few studies were made to the effect of LSP on the FCG performance of
CR IP T
EH36 weldments with the high tensile strength. Therefore, the objective of this paper is to investigate the impact pressure, spot diameter and LSP mechanism on FCG of EH36 weldments and fatigue fracture morphology analyzed by scanning electron microscopy (SEM) without forgotten the enhancement mechanism of the FCG were all considered and discussed. 2 Experimental procedures
AN US
2.1 Material and sample preparation Thick ship steel plates EH36 are widely used under harsh environment of ships and offshore platforms having a high specific strength, good low temperature impact toughness, and specific properties of mechanical and chemical as indicated in table 1 and 2. However, improving fatigue property of EH36
weldment is our ultimate objective and it’s LSP
technology. standard
is
a
novel
According
and
to
experimental
and
anti-fatigue
GB/T6398-2000 conditions,
ED
which
M
uniquely prolonging material service life of
the
compact tension (CT) samples were prepared with the dimensions shown in Fig.1 to explore
PT
the effect of the LSP on the FCG rate of EH36 weldments. The pointed out area of welding
CE
zone (WZ), heat-affected zone (HAZ) and base metal (BM) for EH36 weldments were carefully marked by marking pen in order to distinguish
AC
from different zones weldments. The CT samples
prepared
procedures
Fig. 1. Dimension of CT samples subjected to
respectfully
followed:
LSP.
Table 1 show chemical composition (Wt. %) of EH36 high strength steel Grade Wt. (%)
C 0.15
Mn 1.45
P 0.01
S 0.006
Si 0.606
Sources: https://wenku.baidu.com/view/33ac56ed4afe04a1b071dee7.html
2
Cu 0.193
Al 0.125
ACCEPTED MANUSCRIPT
Table 2 show mechanical properties of EH36 high strength steel Steel Grade
YS*(MPa)
UTS*(MPa)
Total EL(%)
Wt. (%)
430
540
34
Ballistic work(J)/-40 153,143,173
Sources: https://wenku.baidu.com/view/33ac56ed4afe04a1b071dee7.html
Table 3. Welding electrode composition of EH36 weldments C 0.08
Si 0.90
Mn 1.45
S 0.015
P 0.013
Ni 0.01
Mo 0.04
V 0.01
AC
CE
PT
ED
M
AN US
CR IP T
Element Wt.%
3
ACCEPTED MANUSCRIPT
ED
M
AN US
CR IP T
2.2 Principle of LSP HE36 weldment and experimental parameters The LSP utilized high energy pulse to hit the surfaces of the material and then formed plasma. The restrain plasma created at a higher surface pressure propagated into the material as a shock wave. Fig.2. showed the diagrammatical principle of the LSP. The massive LSP impacts in the WZ crack tip of EH36 weldments were carried out using a Qswitched Nd: YAG (Neodymium doped Yttrium Aluminum Garnet) laser system and numerical control workbench. Shock path was illustrated in Fig.1, welding electrode composition of weldment (wt%) parameters was shown in Table3. All samples were submerged into the water bath when they were processed by LSP. A layer of water thickness of 1mm was used as the transparent confining layer and the professional aluminum foil with a thickness of 0.1mm was used as an absorbing layer to protect the sample surface from the thermal effect. The LSP parameters of HE36 weldment were 1 impact time, 2 impact time and 3 impact times respectively. The laser energy, pulse width, spot diameter, spot spacing of 12J, 10ns, 3mm, 2mm were used respectively. However laser welding was done on one top side of the specimens and the Q-switched Nd: YAG power Laser beam was used. The laser light was transmitted via a water-cooled glass fiber and the beam projected onto the specimens by collimating and focusing optics. When the laser beam hits the surface of the specimen, the spot is heated up to vaporization temperature, and a vapor cavity is formed in the weld specimen due to the escaping metal vapor. The energy-flow density of the freely burning arc was 17kW/cm and frequency of 1Hz. The fiber laser power was 4kw, the above spot diameter of the laser beam and a welding speed of 40mm/s was conducted during welding.
PT
Fig. 2. Schematic principle of LSP
AC
CE
2.3 Measurements of residual stress The outward layer of the specimen residual stress measurement shown in Fig.1 on the WZ with and without LSP impacts were done using X-ray diffraction (XRD) machine with the sin2Ψ method. The X-ray beam diameter and source were about 1mm and CrKa ray diffraction plane was α phase (310) plane in the stress calculation. The feed angle of the ladder scanning was 0.1degs-1. The scanning started angle and terminated angle were 167°and 157 ° respectively. The arithmetic average value of three repeated measurements was taken as the residual stress of each measurement point. 2. 4 Micro-structural observations Firstly, the WZ with and without LSP were prepared by EDM to research on microstructure. In addition, the samples were embedded in a fixture, and then were subjected to several successive steps of grinding and polishing. After that, they were etched by a solution of 4% Nital to reveal the microstructure of WZ, which was characterized by cross-sectional optical microscopy (OM).
2.5 Microhardness measurement The Vickers hardness on the WZ surface was tested by using HXD-1000TM digital microhardness
4
ACCEPTED MANUSCRIPT
instrument. The arithmetic average values of three measurement points were taken as the microhardness of the determined region. The WZ surface was cleaned with 99% ethanol material before the measurement. During the hardness measurement, 200gf force was applied for 10s. The corresponding microhardness value was calculated by measuring the diagonal length of indentation.
CR IP T
2.6 FCG test FCG tests were performed to study the effect of LSP on FCG performance of EH36 weldments on a MTS-809 servo-hydraulic system operating with 10Hz frequency in sine wave at room temperature. The stress ratio R (R= min max ) was maintained at 0.1. The EH36 weldments with and without LSP were tested with maximum tensile force of 15kN. Crack lengths were measured at a magnification of 10x using a CCD camera. Crack termination length was 30mm.
AN US
2.7 Fatigue fracture analysis The fatigue fracture morphology of EH36 weldments without LSP and with LSP-3 impacts was analyzed to explore the mechanism of fatigue life improvement. The fatigue fracture of EH36 weldments were prepared by EDM and then cleaned by using ultrasonic cleaning machine to prevent fracture from oil pollution. The fatigue fracture was characterized by scanning electron microscope (SEM).
3. Results and discussion
AC
CE
PT
ED
M
3.1 Residual stress distribution The surface residual stress profiles of the WZ with LSP impacts and without LSP were shown in Fig.3. The direction of the crack tip is perpendicular to the direction of welding on the X axis was also shown in Fig.1. It was notice from Fig.3 that large tensile stress was existed in the WZ surface of EH36 weldments because of non-uniform temperature field and local plastic deformation produced by welding (as shown in No LSP direction in the line of Fig. 3. The surface compressive residual stress in the WZ of EH36 weldments for LSP-1, LSP-2 and LSP-3 shown 110MPa, 180MPa and 230MPa, respectively. The surface compressive residual stress in the WZ of EH36 weldments was increased by 63.6% and 27.8% when the impact time increased from 1 to 2 and 2 to 3. Since compressive residual stress induced by LSP in near-surface layer caused the changes of the actual stress situation in the WZ of EH36 weldments bearing a larger outside force on compressive residual stress, have a significant role in retarding the fatigue crack growth of EH36 weldments. And compressive residual stress in near-surface layer was the key to cause the great increase of notch fatigue limit and the significant reduction of fatigue notch sensitivity on them. The surface and depth compressive residual stress in the WZ of EH36 weldments with LSP increases the crack closure and reduces the actual stress intensity factor of the crack tip, which can efficiently reduce the negative effect of tensile stress accelerating crack extension and causing new crack. Therefore, fatigue of the crack growth resistance and life of EH36 weldments were improved.
5
CR IP T
ACCEPTED MANUSCRIPT
Fig. 3. Surface residual stress distribution in the WZ with different impact times in X axis
AN US
3. 2 Effect of LSP on Microstructure Fig.4 and Fig.5 show the microscopic structure in the WZ with LSP-3 and without LSP. The WZ microstructure was mainly composed of slender proeutectoid ferrite with slender body and acicular ferrite (as shown in Fig.4). The BM with high carbon content dissolves in molten pool during the welding process, which improves carbon content in the WZ. Some granular bainite exists in the WZ of EH36 weldments. The generated granular bainite after laser process in referenced to fig. 4 and 5 respectively, the existing of generated granular bainite in the material contribute to layer and structure higher strengthen, hard and durable by the way of absorbing shocks when external load or force applied on it which improved the mechanical property of the weld zone (wz).
AC
CE
PT
ED
M
Fig.5 suggests that LSP had no effect on the microscopic structure in the weld zone (WZ) of EH36 weldments because of an absorbing layer with aluminum foil avoided a thermal effect from heating of the samples surface by LSP and non-uniform microstructure in the weld zone (WZ) of EH36 weldments. However, these cell structures in the WZ of EH36 weldments were the refined grains by dislocation multiplication and plastic deformation induced by LSP. After LSP of 3, the micro structural characteristics deformations in the hardening layers were shown in Fig.7. According to dislocation theory in [12] and HallPetch theory in [13], grain refinement plays an important role in the improvement of tensile strength and fatigue strength in the WZ of EH36 weldments. The black grains which mean pearlite are the solidified grains reinforced carbon black gains. As a result, LSP clearly improves the tensile strength and the fatigue of the EH36 weldments.
Fig. 4. Microstructure in the WZ without LSP Fig. 4. Microstructure in the WZ without LSP
6
CR IP T
ACCEPTED MANUSCRIPT
M
AN US
Fig. 5. Microstructure in the WZ with LSP-3
AC
CE
PT
ED
Fig. 6. Microstructure characteristics of depth direction in the hardening layer after LSP-3
Fig.7. sectional welding indentation points
3.3 The microhardness of the WZ Fig.8 clearly shows the microhardness in the WZ with and without LSP. The microhardness in the WZ
7
ACCEPTED MANUSCRIPT
CR IP T
of EH36 weldments were 230HV, 250HV, 260HV and 270HV for No LSP, LSP-1, LSP-2 and LSP-3 with 12J. The microhardness in the WZ was increased by 8.7%, 13% and 17.4%, when the impact times increased from 0 to 1, 1 to 2, 2 to 3 respectfully. Because the microhardness of the material increases with the decrement of grain size according to Hall-Petch law [14] and LSP makes the grain refinement in the outward body layer of the weldment zone as shown in Fig.6 and fig.8 with regards to sectional welding indented points in fig. 7 respectively. Therefore, LSP effectively improves the surface microhardness of WZ. The grain refinement was associated with the formation of the dislocation lines induced by LSP. A larger amount of dislocation motions form dislocation walls and dislocation tangles. When dislocation density of the dislocation walls and dislocation tangles reaches a certain value, the dislocation begins to rearrange to form new dislocation walls, which makes the grain refinement on the surface layer.
ED
M
AN US
From Fig.8, it was observed that the microhardness increases with the increase of impact times. However, the increasing rate of the microhardness decreases as the impact time increases. A larger number of dislocation tangles and dislocation walls form the resistance of the dislocation movement resulting in the work-hardening in the near surface. The work-hardening introduces the decrement of microhardness amplitude with the increment of impacts times and hinders the crack initiation.
Fig. 8. Impact of LSP impacts on the microhardness in the WZ of EH36 weldments
AC
CE
PT
3.4 Effect of LSP on FCG rate Fig.9 shows influence of different LSP impacts on the FCG rate da/dN of EH36 CT weldments. In the FCG stage, FCG rate of EH36 weldments after LSP-1 decreases compared to FCG rate of EH36 weldments without LSP. The decrements of FCG rate of EH36 weldments after LSP-2 and LSP-3 are more evident, which was consistent with LSP induced compressive residual stress values on the crack tip of the weldment zone (WZ). When delta K was small, LSP would obviously decreased FCG rate da/dN of EH36 weldments. And FCG rate of EH36 weldments after LSP of 3 was minimum, which was associated with large compressive residual stress in the surface layer of WZ with LSP of 3. Compressive residual stress induced by LSP could brought about the crack closure and invited the decreased of effective driven force, which was beneficial of the reduction for stress intensity factor range of FCG rate. Dynamic FCG causes compressive residual stress relaxation on the crack tip on WZ with increment of delta K and then compressive residual stress could not offset the effected of tensile stress on the LSP effect decreases. In the final rupture stage, FCG speed greatly accelerates and LSP effect was significantly weakened because the crack driven stress was greater than LSP-induced compressive residual stress.
8
ACCEPTED MANUSCRIPT
1E-5
No LSP LSP-1 LSP-2 LSP-3
1E-6
500
600
700
Delta K Applied (N/mm^1.5)
800
CR IP T
da/dN (mm/cycle)
1E-4
Fig. 9. FCG rate of EH36 weldments with different LSP impacts
CE
PT
ED
M
AN US
Fig.10. shows the curves of the crack length versus cycle index on EH36 weldments before LSP and after LSP impacts. When fatigue life of EH36 weldments without LSP is 12354, fatigue lives of EH36 weldments are 32460, 43090 and 45820 after LSP-1, LSP-2 and LSP-3. The fatigue lives of EH36 weldments are increased by 162%, 32% and 6.3% when the impact times increase from 0 to 1, 1 to 2 and 2 to 3. The cycle index of EH36 weldments with LSP obviously increases with the increment of impact times, but the increasing rate of the cycle index reduces gradually. Because compressive residual stress on the surface layer of WZ with LSP diminishes the effective stress intensity factors of FCG. Furthermore, EH36 weldments with LSP-3 got the highest fatigue life, which is attributed to the largest compressive residual stresses distribution and the most evident grain refinement induced by the LSP.
Fig. 10. Relationship between the crack length and the cycle index
Fatigue fracture analysis The fatigue lives of CT samples under the cyclic force were determined by the fatigue crack initiation and propagation. Most metallic materials under the cyclic force experience FCG stage and enter into final rupture stage. Fatigue fracture morphology of material really reflects the characteristics of FCG process, so the fatigue fracture analysis was of great importance. Fig.11 shows the macroscopic fracture morphology in the WZ of EH36 weldments before LSP and after LSP-3. Fatigue fracture morphology in the WZ mainly consists of Pre-crack region, FCG region and final rupture region. Cracks formed by crack sources and inclusions extend in radiation shape in the precrack region. In the FCG region, fatigue fracture morphology had obvious characteristics of shells or beach pattern. The final rupture region was fibrous and dark gray, which was rougher compared with other regions. Compared to FCG region in Fig.11 (a), FCG region was longer and its fracture morphology was
AC
3.5
9
ACCEPTED MANUSCRIPT
CR IP T
smoother and denser (as shown in Fig.11 (b)). And crack arrests with semicircular or fan curved line were perpendicular to the FCG direction in FCG region. Because compressive residual stress induced by LSP in the surface layer restrains fatigue crack growth of EH36 weldments and FCG rate of EH36 weldments decreased.
AN US
Fig. 11. Fatigue fracture macrostructure morphology in the WZ of EH36 weldments before and after LSP, (a) No LSP (b) LSP-3
AC
CE
PT
ED
M
Fig.12 illustrates the microstructure morphology of FCG region in the fatigue fracture of WZ without LSP and with LSP-3. In Fig.12(a), fatigue fracture surface was not very smooth and shown that terraces form. Cracks were given priority to with the intragranular expansion and secondary cracks were few. However, fatigue fracture surface was relatively smooth and more secondary cracks extend along the broken crystal direction (as shown in Fig.12(b)). Because surface compressive residual stress in the surface layer of WZ with LSP-3 offsets partial tensile stress and secondary cracks consume large amounts of energy, fatigue crack extension resistance was increased and FCG rate was effectively delayed for CT samples with LSP-3. Fig.12(c) and (d) were partial enlarged drawings of (a) and (b). The fatigue striations spacing were 0.84 µm before LSP and 0.46 µm after LSP. The fatigue striations in Fig.12(d) became narrower and denser compared to the fatigue striations in Fig.12(c), which indicated that stress intensity factor of the crack front area for CT samples with LSP-3 was relatively smaller and FCG rate was slower. Because grains refinement and compressive residual stress induced by LSP-3 in the WZ result in the plasticity and toughness of CT samples. The results shown that LSP could efficiently alleviate the negative effects of the crack initiation and propagation of EH36 weldments.
Fig. 12. Fatigue fracture microstructure morphology of FCG region in the WZ of EH36 weldments, (a) No LSP (b) LSP-3, (c) and (d) the partial enlarged drawing of (a) and (b) Fig.13 presents the microstructure morphology of final rupture region in the fatigue fracture of WZ without
10
ACCEPTED MANUSCRIPT
AN US
CR IP T
LSP and with LSP-3. The white teare ridge and a lot of dimples appear on the surfaces of toughness fracture characteristic. Because micro cracks were formed at the interfaces of inclusions, second phase particles and substrate. Furthermore, the coalescence of adjacent micro cracks produces microvoids. Microvoids produce in the second phase, grain boundaries and subgrain boundaries, and then slowly grow up with the increase of stress. Finally, connection of the microvoids forms fracture and leaves trace. The size and depth of dimples depends on the plasticity of the material and the number of microvoids nucleate. Because the appearance of dimples, shear deformation of the material occurs in the local area. There were more, larger and deeper dimples in Fig.13 (b). In addition, there were some deeper holes caused by drawn type, which indicates the ductility of EH36 weldments was better. In conclusion, the ductility and plasticity of EH36 weldments with LSP-3 were superior to that of EH36 weldments without LSP.
Fig. 13. Microstructure morphology of final rupture region of WZ without LSP and with LSP-3, (a) Without LSP (b) With LSP-3 4. Conclusions
AC
CE
PT
ED
M
The impact of LSP on the FCG rate of EH36 welds was investigated in this work. The surface microhardness and residual stress in the WZ, the FCG rate and the fatigue fracture characteristic of EH36 weldments before and after LSP were all discussed and analyzed. The following conclusions have been drawn as follows: （1）Surface compressive residual stress in the WZ was increased by 63.6% and 27.8% when the impact time increases from 1 to 2 and 2 to 3. Compressive residual stress induced by LSP increased the crack closure and reduced the actual stress intensity factor of FCG, which improved FCG resistance of EH36 weldments. （2）LSP-induced grain refinement increased the microhardness of the WZ. The microhardness in the WZ with LSP-3 was increased by 17.4%. However, LSP-induced work hardening introduced the decrement of microhardness increase amplitude with the increment of impact times. （3）The compressive residual stress induced by LSP in the WZ diminishes the effective stress intensity factors of FCG process. The fatigue lives of EH36 weldments were increased by 162%, 32% and 6.3% when the impact times increased from 0 to 1, 1 to 2 and 2 to 3. （4）The fatigue striations spacing were 0.84µm before LSP and 0.46µm after LSP-3 in FCG region. Narrow and dense fatigue striations and more secondary cracks in FCG region after LSP-3 increase the resistance of FCG and delays FCG rate. Large and deep dimples in the final rupture region after LSP-3 improved the ductility and plasticity of EH36 weldments. Acknowledgement The financial support provided by NSFC (51575245) and the Key Project of Jiangsu Province (BE2015134) is especially acknowledged and also grateful to Priority Academic Program Development of Jiangsu Higher Education Institutions.
11
ACCEPTED MANUSCRIPT
References:
ED
M
AN US
CR IP T
[1] Brennan FP, Ngiam SS, Lee CW. An experimental and analytical study of fatigue crack shape control by cold working. Eng Fract Mech. 2008; 75(3):355-363. [2] Rankin JE, Hill MR, Hackel LA. The effects of process variations on residual stress in laser peened 7049 T73 aluminum alloy. Materials Science and Engineering: A. 2003; 349(1):279-291. [3] Tsay LW, Young MC, Chen C. Fatigue crack growth behavior of laser-processed 304 stainless steel in air and gaseous hydrogen. Corros Sci. 2003; 45(9):1985-1997. [4] Rubio- González C, Gomez-Rosas G, Ocaña JL, Molpeceres C, Banderas A, Porro J, Morales M. Effect of an absorbent overlay on the residual stress field induced by laser shock processing on aluminum samples. Appl Surf Sci. 2006; 252(18):6201-6205. [5] Sanchez-Santana U, Rubio- González C, Gomez-Rosas G, Ocana JL, Molpeceres C, Porro J, Morales M. Wear and friction of 6061-T6 aluminum alloy treated by laser shock processing. Wear. 2006; 260(7):847-854. [6] Rubio-Gonzalez C, Ocana JL, Gomez-Rosas G, Molpeceres C, Paredes M, Banderas A, Porro J, Morales M. Effect of laser shock processing on fatigue crack growth and fracture toughness of 6061-T6 aluminum alloy. Materials Science and Engineering: A. 2004; 386(1):291-295. [7] Srinivasan S, Garcia DB, Gean MC, Murthy H, Farris TN. Fretting fatigue of laser shock peened Ti-6Al-4V. Tribol Int. 2009; 42(9):1324-1329. [8] Ren XD, Zhang YK, Yongzhuo HF, Ruan L, Jiang DW, Zhang T, Chen KM. Effect of laser shock processing on the fatigue crack initiation and propagation of 7050-T7451 aluminum alloy. Materials Science and Engineering: A. 2011; 528(6):2899-2903. [9] Spanrad S, Tong J. Characterisation of foreign object damage (FOD) and early fatigue crack growth in laser shock peened Ti-6Al-4V aerofoil specimens. Materials Science and Engineering: A. 2011; 528(4):2128-2136. [10] Zhang L, Lu JZ, Zhang YK, Luo KY, Zhong JW, Cui CY, Kong DJ, Guan HB, Qian XM. Effects of different shocked paths on fatigue property of 7050-T7451 aluminum alloy during two-sided laser shock processing. Mater Design. 2011; 32(2):480-486. [11] Chahardehi A, Brennan FP, Steuwer A. The effect of residual stresses arising from laser shock peening on fatigue crack growth. Eng Fract Mech. 2010; 77(11):2033-2039. [12] Di Schino A, Kenny JM. Grain size dependence of the fatigue behaviour of a ultrafine-grained AISI 304 stainless steel. Mater Lett 2003; 57:3182. [13] Chen M, Ma E, Hemker KJ, Sheng H, Wang Y, Cheng X. Deformation twinning in nanocrystalline aluminum. Science 2003; 300:1275. [14] Petch NJ. The cleavage strength of polycrystals. J. Iron Steel Inst. 1953; 174:25. Biographical notes
CE
PT
WANG Yun, born in 1975, is currently a professor at Die and Mould Technology Institute,School of mechanical Engineering, Jiangsu University, China.He received his PhD degree from HefeiUniversityofTechnology, China, in 2003.He has awarded First Prize ofNational Award for Science and Technology Progress. His research interests include marine engineering equipment manufacturing research. Tel:+86-138-15483092; E-mail: [email protected]
AC
BAIDOO Philip, born in 1974, is currently a Ph.D candidate at Die and Mould Technology Institute, School of mechanical Engineering, Jiangsu University, China. His research interests include fatigue performance improvement of weldments by laser shock processing. Tel: +86-18605242722; E-mail: [email protected]
XU Zhenying, born in 1977,is currently an associate professoratIndustrial Nondestructive Testing Institute, School of mechanical Engineering, Jiangsu University, China. She received his PhD degree from HefeiUniversityofTechnology, China, in 2003.She has awarded First Prize ofNational Award for Science and Technology Progress. Her research interests include precision forming and precision controlof micro device. Tel: +86-137-75372507; E-mail: [email protected]
12
ACCEPTED MANUSCRIPT
AC
CE
PT
ED
M
AN US
CR IP T
WU Junfeng, born in 1988, is currently a master candidate at Die and Mould Technology Institute, School of mechanical Engineering, Jiangsu University, China. His research interests include fatigue performance improvement of weldments by laser shock processing. Tel: +86-187-06102013; E-mail: [email protected]
13