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Effects of laser shock processing on high cycle fatigue crack growth rate and fracture toughness of aluminium alloy 6082-T651 Zoran Bergant, Uro Trdan, Janez Grum Faculty of Mechanical Engineering, University of Ljubljana, Askerceva 6, 1000 Ljubljana, Slovenia a r t i c l ei n f o Article history: Received 25 November 2015 Received in revised form 10 February 2016 Accepted 16 February 2016 Available online 3 March 2016 Keywords: Laser shock processing Fatigue crack growth Aluminium alloy Residual stresses Fracture toughness a b s t r a c t The effects of laser shock processing without protective coating on high-cycle fatigue crack growth and fracture toughness were investigated. Laser shock peening treatment was performed on compact tension specimens from both sides perpendicular to the crack growth direction, followed by subsequent grinding. Fatigue crack growth tests were performed at frequencies between 116 and 146 Hz, at R = 0.1 and a con- stant stress intensity range during the fatigue crack initiation phase and K-decreasing test. A lower num- ber of cycles was required to initiate a fatigue precrack, and faster fatigue crack growth was found in tensile residual stress fi eld of LSP-treated specimens. The crack growth threshold decreased by 60% after LSP treatment. The fracture toughness decreased by 2833% after LSP treatment. The fatigue-to-ductile transition boundary on fractographic surfaces show linear fatigue crack fronts in non-treated specimens and curves after LSP treatment. ? 2016 Elsevier Ltd. All rights reserved. 1. Introduction Laser shock processing (LSP) is a relatively new surface treat- ment technology to extend the lifetime of dynamically loaded components. Several studies 13 have already demonstrated that LSP treatment decreases the fatigue crack growth (FCG) rate through the generation of large amplitude compressive stresses as a result of material local plastic deformation induced by the pressure shock wave. Since the fatigue cracks normally initiate from the surface, locked compressive stresses will, under external load, be superimposed on the applied stress, leading to smaller stress intensity factors at the crack tip and possible crack closure to incur the reduction of effective driving force for the FCG 1,2. LSP can also improve fatigue strength and fatigue crack initia- tion life due to a considerable densifi cation of dislocations and microstructural grain refi nement. Huang et al. 3 confi rmed a reduced FCG rate due to the highly tangled and dense dislocation arrangements in the LSP-processed surface of an AlMgSi alloy (6061-T6). In another study Lu et al. 4 reported obvious microstructure refi nement in AlCuMg alloy (LY2) after LSP. The results confi rmed LSP to be a promising method to obtain a high density of dislocations to improve fatigue resistance, whereas the minimum grain size in the top surface after multiple LSP impacts was about 100200 nm. Furthermore, residual stress resulting from laser processing can be signifi cantly higher with much deeper effective penetration in the material compared to conventional shot peening 5. Huang et al. 1,3 reported benefi cial effects of LSP under differ- ent process setups on the fatigue crack growth properties of 6061 T6 aluminium alloy. Their results indicate that the FCG rate decreases with increasing laser energies 1 and LSP coverage areas 3, especially in the initial fatigue crack growth stage. However, the strengthening effects are weak in the fi nal period of fatigue crack growth since the residual stresses release as the crack grows. In another study, Hatamleh et al. 5 compared laser and shot peening effects on FCG in friction-stir-welded AlZn alloy (7075- T7351) joints, reporting reductions of the FCG rate in comparison with the base material for the LSP-treated specimen, whereas shot peening revealed negligible improvement of the FCG rate. However, despite the fact that the LSP process in which a pro- tective coating is used has already been proven as an advanced, competitive surface treatment for improving the fatigue life, corro- sion resistance, hardness and wear resistance of various metals and alloys 69, several negative points exist that limit the widespread application of this process. Open discussion on the last, i.e. 5th international conference on laser peening and related phenomena in Cincinnati, 2015 10 pointed out that new/combined strategies are needed in the industrial fi eld applications. Basically, two main approaches laser shock treatment to achieve considerable merging of dislocations and generation of compressive residual stresses are /10.1016/j.ijfatigue.2016.02.027 0142-1123/? 2016 Elsevier Ltd. All rights reserved. Corresponding author. Tel.: +386 1 477 1203; fax: +386 1 477 1225. E-mail addresses: zoran.bergantfs.uni-lj.si (Z. Bergant), uros.trdanfs.uni-lj.si (U. Trdan), janez.grumfs.uni-lj.si (J. Grum). International Journal of Fatigue 87 (2016) 444455 Contents lists available at ScienceDirect International Journal of Fatigue journal homepage: used; (i) fi rst regime, uses high-energy laser pulses (100 J), longer pulse duration up to 100 ns combined with the protective coating, (ii) the second approach uses lower laser energies in the order of few joules or fewer, increased overlap, smaller spots and no coat- ing (also known as LSPwC). A detailed description of both princi- ples was reported previously 11. Although, the protective coating on the processed part prevents surface ablation, while maintaining high surface quality, it is a time-consuming affair since the overlay is damaged severely dur- ing the LSP process and requires frequent replacements, making it slow and expensive in industrial applications 12. Furthermore, since some parts being peened might not be accessible for the application of protective overlay, much attention was focused on LSP process without a need for a protective ablative overlay. How- ever, after this process typical surface craters are obtained with increased surface roughness and waviness due to the local surface ablation, which in turn creates a high-pressure plasma that, con- tained by a thin layer of water fl owing on the specimen surface, generates a pressure wave propagating into the specimen 13. Based on the open literature review, very few comprehensive stud- ies have been conducted on laser shock processing without protec- tive coating on fatigue crack growth, and none of them have evaluated fatigue characteristics after subsequent grinding proce- dure to eliminate the downward effect on uncoated LSP-induced surface topography. Rubio-Gonzalez et al. 14,15 investigated the effects of a two- sided uncoated LSP process on compact tension (CT) specimens of 2205 duplex stainless steel and 6061-T6 Al alloy. The results confi rmed that the FCG rate decreased while fracture toughness increased with the increased pulse density. Although there are numerous investigations of laser processing effect on material fati- gue crack growth properties 1,5,14, there is limited literature on the possible detrimental effect of laser shock processing. Further, all the above studies have been performed after pre-existing fati- gue pre-crack, under low sine wave frequencies ( 106) 16. To obtain a quantitative comparison between the fracture toughness of untreated and LSP specimens in the fi nal rupture stage under static load will be deter- mined as well. In addition, the possible softening effect due to the local surface melting, ionization, and re-solidifi cation during laser process will be carefully investigated by means of dislocation arrangements, residual stress, and microhardness in depth distri- bution. All LSP specimens under investigation were analyzed after subsequent grinding procedure in order to obtain a proper surface fi nish. 2. Experimental methods 2.1. Material and specimen preparation Plates of commercial wrought AlMgSi aluminium alloy (EN AW 6082-T651) were machined to obtain the specimens. The over- all heat treatment T-651 procedure (homogenization, solution treatment, aging, etc.) is detailed in Ref. 17. Its chemical compo- sition in wt% was 0.87 Si, 0.72 Mg, 0.42 Mn, 0.35 Fe, 0.15 minor ele- ments (Cu, Cr, Ni, Zn, Ti) and balance Al. In order to obtain mechanical properties for the given material in the specifi c rolling direction, three cylindrical tensile specimens were machined in longitudinal and three in the transversal direction of rolling with a test diameter of 5 mm and a measuring length of 20 mm (Fig. 1). The average measured tensile strength, yield stress, and elongation are given in Table 1. A total of six C(T) specimens were tested from the plate in the LT direction for fatigue crack growth and fracture toughness tests. Three specimens were tested in not- treated conditions, and three specimens were LSP-treated with dif- ferent parameter sets: LSP 1, LSP 2 and LSP 3. Notches were made using electrical discharge machining (EDM) using a 0.35 mm wire, which resulted in a notch with a radius of 0.19 mm. The geometry of the C(T) specimen, adopted for testing with Rumul Cracktronic unit and Krak-gages AMF-5, is given in Fig. 2. The applied stress intensity rangeDK was used, calculated according to the equation 18: DK DP B ffi ffi ffi ffi ffiffi W p? Y;1 whereDP = Pmax? Pminis the alternating force, B is the specimen thickness, W is the length from the load centerline to the edge of specimen, Y is a geometrical factor, which is derived for linear elas- tic material for compact-tension specimens: Y 2 a 1 ?a3=2 0:886 4:64a? 13:32a2 14:72a3? 5:6a4;2 wherea= a/W, a is the crack length. 2.2. Laser shock processing Laser shock processing was performed using a Spectra-Physics Q-switched Nd:YAG laser with an irradiation wavelength of 1064 nm. The maximum laser beam energy was 2.8 J/pulse, whereas the FWHM (Full Width Half Maximum) of the generated Gaussian intensity pulses was 10 ns. Focused laser beam diameters were varied via modifi cation of convergent lens. Beam diameters, used during LSP 1, LSP 2 and LSP 3 treatments, were set to 1.5, 2.0 and 2.5 mm, respectively. In this study, a predefi ned overlap- ping pulse density of 1600 pulses/cm2was chosen, with the laser-advancing direction parallel to the plate-rolling direction (L). Specimens were irradiated using laser shock processing method without any protective coating in a water confi nement regime, whereas laser pulses were overlapped and scanned in a zig-zag pattern (Fig. 3). A water jet set up was employed to create a thin water layer and to avoid the formation of water bubbles or the con- centration of impurities resulting from the material ablation, thus, constantly ensuring a pure lasermatter interaction. ThetreatedareaontheC(T)specimenswasapprox. 10 ? 10 mm2on both sides of the specimens. For TEM microstruc- tural analysis, additional LSP specimens were prepared using the Fig. 1. Tensile and C(T) specimen orientation in Al-plate. Z. Bergant et al./International Journal of Fatigue 87 (2016) 444455445 same processing conditions as for C(T) specimens. Prior to fatigue crack growth testing, C(T) specimens were subsequently ground to the fi nal thickness of 9.8 mm to remove the rough asperities after LSP treatment (Fig. 4). Prior to laser shock processing, no pre-crack was initiated. However, specimens for TEM analysis were not ground in any way to obtain the representative microstruc- tures produced by the laser peening treatment. 2.3. Surface roughness After the surface near pre-machined notch was laser surface treated, the surface profi le and the roughness were measured using Surtronic 3+ profi lometer (Taylor Hobson) and the x-y micrometer sliding table stroke. TalyProfi le Lite v.3 software was used with micro-roughness fi ltering ratio of 2.5l m and a Gaussian fi lter to extract the roughness parameter. The selected parameter for eval- uation of surface roughness is Ra, which is the average arithmetic deviation from the mean line on the measuring length. The mea- suring profi le length for the roughness parameter Radetermination Table 1 Average measured tensile properties of 6082-T651 plate. Direction of loadLT Tensile strength Rm(MPa)311310 Yield stress Rp0.2(MPa)296295 Elongation A (%)9.78.5 Fig. 2. Geometry of C(T) specimens and measuring resistance gage AMF-5. Fig. 3. Laser shock processing setup and treated C(T) specimen. Fig. 4. C(T) specimens (a) no treatment, (b) after LSP 1 treatment, (c) LSP 1 after grinding. 446Z. Bergant et al./International Journal of Fatigue 87 (2016) 444455 of LSP-treated surfaces was 16 mm, while the measuring length used for measuring roughness on crack surfaces was 2.5 mm. 2.4. Transmission electron microscopy The LSP effect on the material microstructure was characterized using a JEOL 2000-FX transmission electron microscope (TEM) operated at a voltage of 200 kV. In order to obtain proper insight into LSP effects on microstructure and dislocation confi gurations, we chose not to grind the LSP surfaces in any way, due to a possible loss of information. Instead, we prepared cross-sectional thin foils for TEM analysis in the following steps: (i) bonding two pieces of the LSP-treated specimens face-to-face with GatanTMglue (treated surfaces in the middle), (ii) cutting a cross-sectional specimen into 1.0 mm thick sheets, (iii) grinding it carefully to about 100lm thick; (iv) punching out to 3 mm diameter discs, (v) ion thinning by Ar+ bombardment. On average, 80 microstructural images for each specimen condition were taken from the depth 0 to about 500lm from the original surface. For the sake of convenience, images with the highest dislocation densities were chosen as the representative ones for each LSP-processing condition. 2.5. Microhardness The microhardness was measured with a micro-Vickers test using a small load of 0.071 N, Leitz Wetzlar gage, Leica Microsys- tems. Indentations were made in the vertical direction through the entire cross section of C(T) specimen. In order to obtain proper information about LSP effects on hardness, the surface was not ground, since the hardened material could have been completely removed. The fi rst and last 10 measurements below the surface were made using a spacing of 32lm to increase the number of measurements in the subsurface region of interest. Then, the spac- ing over the rest of the material thickness increased to approx. 214lm in order to reduce the experimental time. 2.6. Residual stresses The analysis of residual stresses was performed using the hole- drilling relaxation method in accordance with the Standard Test Method for Determining Residual Stresses by the Hole-Drilling Strain-Gage Method ASTM E 837-08 19. Residual stresses were determined with measurement equipment, a product of the Vishay Measurements Group (Vishay Intertechnology Inc., Malvern, PA, USA) with the resistance gage CEA-06-062UM-120. The hole was drilledusingadepthfeederwithmicrometer(resolution 0.01 mm). The hole diameter was measured after drilling and aver- aged on three locations using an optical Olympus macroscope, a ColorView Camera, and AnalysisDocu software. Maximum and minimum principal residual stresses and the orientation angle were calculated using H-drill v3.10 software and the integral method of stress calculation with a strain deviation error estima- tion of 3.2lm. 2.7. Fatigue crack growth test Fatigue testing was performed on the electromagnetically dri- ven testing device, Cracktronic 8204 with Fractomat, by Russen- berger Prfmachinen AG. Testing was performed according to the available testing space on Cracktronic and Standard Test Method for Measurement of Fatigue Crack Growth Rates ASTM E647 20. The basic module of Cracktronic has separate static and dynamic drives. The static load is generated by a ball screw spindle driven by a DC-motor and couples over the torsion rod into the oscillating system. The dynamic load is generated by means of electromag- netic driven resonator. The electromagnet is integrated in a closed loop system and excites the oscillating system in its natural reso- nant frequency, where the operating point is situated at the peak of the resonance curve with typical frequencies between 50 and 200 Hz. The dynamic and static parts are controlled by separate circuits, connected with the Credo control unit and computer soft- ware, and allow any combination of stress ratios with high accu- racy. The alternating force is measured using Rumul load cells (piezoelectric) with the maximum load of 8000 N and a resolution of 1 N. Furthermore, the specimen with its elasticity is part of the system 21. The crack lengths were continuously monitored using the indirect electric potential drop method. In this method, the electric potential value is taken off the thin (5lm) constantan resistance foil Krak-gages (type RMF-A5), as opposed to the direct potential drop method”, in which the potential is taken off directly from the specimen. The Krak-gages were attached on both sides of the specimen to monitor and average the crack lengths. The crack in the gage is growing simultaneously with the real crack of the specimen. The measurement of crack length is recorded in a reso- lution of 0.001 mm and the accuracy is higher that 2%. The main source of error is gage misalignment when gluing the gage on spec- imen, which amounts to 0.2 mm from the actual notch tip radius. The gluing of Krak-gages was performed using cyanoacrylate- based glue (Vishay Measurements) under optical macro-scope at 1520? magnifi cation with the same technique as strain gage glu- ing. The error of positioning was measured by spatially calibrated microscope. The error length of positioning from actual EDM notch was taken into account at starting notch length value input in Frac- tomat. In order to performDK = const. tests at a constant R-ratio, the relationship in Eq. (1) is integr