54-60 Al-Khwarizmi Engineering Journal,Vol. 11, No. Laser Peening on Aluminum Alloy 7 Alalkawi H. J. M.* *Departement of **, *** College of (Received Abstract Black paint laser peening (bPLP) technique is currently applied for many engineering materials , especially for aluminum alloys due to high improvement in fatigue life and strength . Constant and variable bending fatigue tests have been performed at RT and stress ratio R= surface work hardening which generated high negative improvement factor (FLIF) for bPLP constant fatigue behavior was from 2.543 to 3.3 compared to and the increase in fatigue strength at 10 1.786 at L-H and 1.55 at H-L due to black paint coating . Keywords: Constant and variable fatigue behavior , Black paint laser peening , 7049 Al 1. Introduction The failure of metalic material structure especially the light structures ( Aluminum alloys ) is totally difficult to control , experimentally when the qpplied loads behaver in variable manner[1]. The estimation of the fatigue life companents subjected to random loading (cumulative amplitude conditions) subject [2]. However, in practice ma or components are subjected to complex fatigue loading which is happened due to changing in the applied stress amplitude. field of variable amplitude loading, it is an important to assess the fatigue damage due to variable amplitude loading conditions [3]. Damage increases when cyclic stresses increase a cumulative way which may lead to fracture. Fatigue damage analysis in variable loading very important factor in the fatigue of structures and components[4]. Khwarizmi Engineering Journal,Vol. 11, No. 3, P.P. 54-60 (2015) Aluminum Alloy 7049 Using Black Paint Surface Coating * Basim A. Sadkhan** Noor N. Electromechanical Engineering/ University of Technology College of Engineering/ University of Almustansiriyah. *Email: Alalkawi2012@yahoo.com **Email: drbasimajel@yahoo.com ***Email: nawara_najm@yahoo.com (Received 9 September 2014; accepted 7 May 2015) Black paint laser peening (bPLP) technique is currently applied for many engineering materials , especially for aluminum alloys due to high improvement in fatigue life and strength . Constant and variable bending fatigue tests nd stress ratio R= -1 . The results of the present work observed which generated high negative residual stresses in bPLP specimens .The fatigue life improvement factor (FLIF) for bPLP constant fatigue behavior was from 2.543 to 3.3 compared to and the increase in fatigue strength at 107 cycle was 21% . The bPLP cumulative fatigue life behavior was L due to black paint coating . Constant and variable fatigue behavior , Black paint laser peening , 7049 Al- alloy . The failure of metalic material structure especially the light structures ( Aluminum alloys ) difficult to control , experimentally when the qpplied loads behaver in variable manner[1]. of the fatigue lifetime of companents subjected to random loading is a complex in practice many structural are subjected to combined or happened due to stress amplitude. In the field of variable amplitude loading, it is an fatigue damage occurred conditions [3]. when cyclic stresses increase in which may lead to fracture. in variable loading plays life evaluation Laser peening (LP) is which works to create stresses at the surface and to extend metal surface [5]. LP has been effective in improving the properties of many metals and alloys combination of laser and shot peening treatment was investigated in 7075 AA gave an optimum residual stress profile at the surface resulting in the improvement in fatigue life and strength of 7075 AA[7]. Everett et al [ and 2024-T3 AA under fatigue tests using shot peening and laser peening treatments.The experimental results showed that after peening the fatigue life and crack growth rates were noticeably improv life was increase by a factor of 2 than the results of the average un pe .Comparision between laser peening (LP) and shot peening (SP) residual stresses of 7049 made by Rankin et al [ residual stress 0.1 mm from the surface due to LP Al-Khwarizmi Engineering Journal (2015) 49 Using Black Paint Surface Noor N. Abed*** f Technology. Black paint laser peening (bPLP) technique is currently applied for many engineering materials , especially for aluminum alloys due to high improvement in fatigue life and strength . Constant and variable bending fatigue tests that the significance of the bPLP specimens .The fatigue life improvement factor (FLIF) for bPLP constant fatigue behavior was from 2.543 to 3.3 compared to untreated fatigue cycle was 21% . The bPLP cumulative fatigue life behavior was improved by alloy . P) is a surface treatment which works to create compressive residual surface and to extend below the P has been known to be very effective in improving the mechanical and fatigue many metals and alloys [6] .The combination of laser and shot peening treatment investigated in 7075 AA gave an optimum residual stress profile at the surface resulting in the improvement in fatigue life and strength of Everett et al [8] examined4340 steel under fatigue and crack growth tests using shot peening and laser peening treatments.The experimental results showed that after peening the fatigue life and crack growth rates were noticeably improved and the fatigue life was increase by a factor of 2-4 times greater than the results of the average un peened results laser peening (LP) and shot peening (SP) residual stresses of 7049-T3 AA was made by Rankin et al [9] .They observed that residual stress 0.1 mm from the surface due to LP Alalkawi H. J. M Al-Khwarizmi Engineering Journal, Vol. 11, No. 3, P.P. 54- 60(2015) 55 was far greater than for SP. AA type LY2 was tested under LP in two ways and they compared .The results show that the LP improved the fatigue life by 131.4 % compared to untreated condition [10]. Laser peening treatment without protective coating (LPwC) was carried out by Yasuo Ochi et al[11] to improve fatigue strength ,the result showed the LPwC treatment was effective for the fatigue strength improvement in fatigue lives regime before 2∼3×106 cycles, but the treatment reduced the strength after the cycles at the both stress ratios conditions. Duplex stainless steel alloy used to study the effect of laser treatment on fatigue charistrestice . The condaded remarks were increasing pulse density reducing the growth of the cracks which resulting in increasing the fatigue life[12]. Different laser peening were used to study the laser effect on lifetime of (3003-H18) AA using different (LP). The results observed that the fatigue lifetime increment over the life of samples without (LSP) in range (12%) for 1-spot LSP, 18%for 2-spots LSP and 77%for 3-spots LSP[13]. Al-6061-T6 surface topography was studied using energy Nd: YAG laser with 300mJ. They concluded that laser peening without coating (LPWC) can significantly improve the surface topography i-e compressive stress , and microhardness but the surface roughness showed an increase. The compressive residual stress was improved by 27% and the hardness was increased by 10 HV [14].45 specimens of 7049 AA were examined under constant rotating fatigue at R= -1 using 7049 AA to establish the S-N curve for three types of surface coatings .15 specimens for air without peening ,15 specimens for air laser peening (ALP) and the third group 15specimens was tested under water laser peening (WLP). The results indicated that no effect of laser peening at low cycle fatigue ( LCF ) above 300 MPa . But this effect appeared clearly at (HCF). Also it was concluded that the fatigue life improvement factor is 32.6 and 8.97 under WLP for 200MPa and 250 MPa respectively[15]. Date in the literture observed the benficial effects on fatigue life , an increase in fatigue life by as much as a factor of 10 at a given stress level for aluminum alloys[16] . 2. Experimental Details 2.1. Material and Testing The material used was 7049 AA supplied as a round bar of 10mm diameter with chemical composition in weight percentage as given in table (1). The mechanical properties of the material were obtained using an instron machine . The average results of three tests are presented in table (2). Table1, Chemical composition in wt% of 7049 AA. Element Si% Fe % Cu% Mn % Mg% Cr% Zn% Ti% Al% Standard Max. 0.25 Max. 0.35 1.2-1.9 Max. 0.2 2-2.9 0.1-0.22 7.2-8.2 Max. 0.1 Bal. Experimental 0.21 0.29 1.52 0.14 2.5 0.18 7.8 0.08 Bal. Table 2, Mechanical properties of 7049 AA. Property Yield Stress, �� (MPa) Ultimate Stress, �� (MPa) Elongation% Modulus of elasticity, ϻ E (GPa) HB Experimental 312 515 19 74 0.32 131 Standard 317 520 20 74 0.33 135 Alalkawi H. J. M 2.2. Fatigue Specimen Fatigue specimen , shown in figure(1) employed to carryout all the fatigue tests laser peening and without laser treatment . fatigue specimens were manufactured using programmable (CNC) turning machine specimen is: Fig. 1. Fatigue specimen dimensions ( (all dimnsions in mm). The detailed dimnsions of specimen are shown in fig. (1) . The samples were then numbered and grinded , first , with grades 200,400,600,1000,1200 emery paper. After grinding , one sample was polished firstly using polishing cloth with diamond pastes with 1/3 micron,for 5 minutes and then etched in Keller solution for a 2-3 minutes . 2.3. Fatigue Test Procedure Fatigue analysis are normally based on the results obtained from S-N curve then the first step was to established the constant continuous cycling S-N curve .Fifteen specimens were tested under room temperature control stress with zero mean stress .The second step was to find the S with black paint laser peening (bpLp) in order to do a comparison in life and strength . The third group of testing was 12 specimens , 6 specimens under untreated cumulative fatigue and the other 6 specimens under (bpLp) cumulative fatigue . 2.4. Fatigue Test Rig. A fatigue test machine of type (SC PUNN rotating bending is employed the fatigue test, as illustrated in figure (4): Al-Khwarizmi Engineering Journal, Vol. 11, No. 56 shown in figure(1) , was employed to carryout all the fatigue tests under laser peening and without laser treatment .All the fatigue specimens were manufactured using machine ,the test (DIN 50113) nsions of specimen are shown were then numbered and grinded , first , with grades 200,400,600,1000,1200 emery paper. After polished firstly using with diamond pastes and alumina for 5 minutes and then etched in Fatigue analysis are normally based on the curve then the first step was to established the constant continuous cycling specimens were tested under room temperature control stress with zero mean stress .The second step was to find the S-N curve pLp) in order to do a comparison in life and strength . The third specimens , 6 specimens cumulative fatigue and the other 6 specimens under (bpLp) cumulative fatigue . f type (SCHENCK) PUNN rotating bending is employed to execute fatigue test, as illustrated in figure (4): Cycles counter specimen Motor Load Fig. 2. Fatigue test machine. 2.5. Laser Surface Treatment The laser system used in this work was (Q switched Nd–YAG laser) with wavelength is about 1.065 µm ,the energy ns) shock with black paint for each specimen around the minimum diameter of fatigue specimens were selected. Figure (2) shows (Q-switched Nd system ) used in the present study . Fig. 3. Nd–YAG laser peening device of Technology-Baghdad . Fig (4) illustrates the laser ring around the minimum diameter. Laser spots Fig. 4. Laser spots around min. diameter of fatigue specimen. Khwarizmi Engineering Journal, Vol. 11, No. 3, P.P. 54- 60(2015) specimen Load switch . Fatigue test machine. Surface Treatment The laser system used in this work was (Q- YAG laser) with wavelength is m ,the energy of pluse (300)mj . (16 shock with black paint for each specimen around the minimum diameter of fatigue switched Nd –YAG laser system ) used in the present study . laser peening device at university ig (4) illustrates the laser ring around the 4. Laser spots around min. diameter of fatigue Min Dia. of shaft of shift Alalkawi H. J. M Al-Khwarizmi Engineering Journal, Vol. 11, No. 3, P.P. 54- 60(2015) 57 3. Results and Discussion Table (3) gives the results of the three groups mentioned above . Table 3, Constant and variable fatigue tests with and without (bpLp). Condition Dry fatigue S-N curve results Specimens No. Applied stress (σƒ)( MPa ) Cycles to failure Nƒ (Cycles) Nƒ Average 1,2,3 400 2000,2200,2500 2233 4,5,6 300 6000,7000,8000 7000 7,8,9 250 33000,31800,32600 32467 10,11,12 200 63000,57500,66000 62167 13,14,15 150 642000,664300,686600 664300 bplp fatigue S-N curve results 16,17,18 400 4200,5000,3800 4333 19,20,21 300 9000,11200,10000 10067 22,23,24 250 119000,130000,110000 119667 25,26,27 200 205000,198000,180000 194333 28,29,30 150 2479000,2655000,2303000 2479000 Dry cumulative fatigue results L-H 31,32,33 200-300 12700,16000,11600 13433 H-L 34,35,36 300-200 10800,11600,14000 12133 bplp cumulative fatigue results L-H 37,38,39 200-300 27000,20000,25000 24000 H-L 40,41,42 300-200 21000,18600,17000 18867 3.1. Basic S-N Curve Fig (5) describes the results obtained from constant stress amplitude tests at room temperature under stress control condition and zero mean stress with and without bpLp . The bending stress was calculated from the bending moment using the equation : σb(N/mm 2 ) = 125.7*32*P(N) / π d³ ...(1) Where P is the applied load (N) and the arm of the force is equal to (125.7mm) and d is the minimum diameter of the specimen in mm . Fig. (5) illustrates the effect on the fatigue lifetimes of specimns that have been pened under (bpLP) . As shown by the experimental date , the fatigue life of (bpLP) specimens is improved compared to unpeened specimens and this improvement can be described by table (4) below for different stress levels . The bPLP creates compressive residual stresses, there by ofering improved resistance to the growth of near- surface , macroscopic crack . This characteristic can , therefore , lead to signficant improvements in the fatigue life of treated specimens , which leads to high FLEF given in the above table . This finding is in good agreement with Ref [11 ]. Table 4, Fatigue life improvement factor due to bpLP for different stress levels. Fatigue life improvement factor (FLIF) 0.4 σu 0.5 σu 0.6 σu 0.7 σu 0.8 σu 208 MPa 260 MPa 312 MPa 364 MPa 416 MPa 3.3 3.03 2.833 2.674 2.543 Alalkawi H. J. M Al-Khwarizmi Engineering Journal, Vol. 11, No. 3, P.P. 54- 60(2015) 58 Fig. 5. Convential basic line S-N curve for untreated and bplp fatigue . The stress amplitude and life (Nƒ) were estimated from the relation known as the Basques equation , gives the fatigue strength properties and may take the following formula. σƒ = A(Nƒ) α ...(2) Where A : is the fatigue strength coefficient. α : is the fatigue strength exponent. The value of the parameters in equation (2) are listed in table (5) Table 5 Fatigue properties of 7049 AA. α A Condition -0.145 981 Untreated fatigue -0.135 1064 bpLP fatigue The bending fatigue properties of 7049 AA that has received bplp and untreated , respectively are compared in fig (5) . The bpLP gave a 21% rise in endurance fatigue limit, compared with the untreated specimens . Peyre et al [14] have found that an increase of 22% in fatigue strength of 7075-T7351 AA after treating by LSP ( 3.8 G W/cm2 ) compared to the unpeened specimens . The constant fatigue life of 7049 AA are improved, as illustrated in table (4), based on stress level applied . This improvement treated due to greater plastiaty affected depth in the matrials as well as the preservation of surface roughness . Clauer etal [17]concluded that the constant fatigue life of 2024 –T3 specimens with solid laser spot has a fatigue life about 40 times longer than the annular laser spot and a life about three times greater than the as-recived ones . Referring to table (3). It can be obtained a cumulative fatigue life improvement factor (CFLIF) for cumulative fatigue results in table (6). Table 6, Cumulative constant fatigue life improvement factor . Stress sequences MPa Cumulative fatigue life improvement factor ( CFLIF ) 200-300 1.786 300-200 1.555 It can be seen that : The Cumulative fatigue life was increased by a factor of 1.786 at low –high stress sequences while this factor reduced to 1.555 at high –low stress sequences . The fatigue life of 7049 was extended by bpLP due to greating compressive stress at the surface. This finding is in good agreement with Peyre etal [14] . 4. Conclusions The bending fatigue behaviour of 7049 Al- alloy were stuided , the following remarks are drawn from the work described here in : 1- The fatigue strength at 107 cycles was improved by a 21% increase due to bpLP . 2- The fatigue life improvement factor ( FLIF) was obtained to be from 2.543 to 3.3 due to bpLP based on stress level applied . 3- The lives of sequence loading (L-H) were higher than lives of sequence loading (H-L) either with bpLP or without . 4- The fatigue lives were improved by afactor of 1.786 at ( L-H ) , while this factor was 1.555 at ( H-L ) due to bpLp . σƒ = 1064N ƒ -0.135 for bplp σƒ = 981N ƒ -0.145 Untreated fatigue 100 150 200 250 300 350 400 450 0 500000 1000000 1500000 2000000 2500000 3000000 A p p li e d S tr e s s (M P a ) Number of Cycle to Failure \ Alalkawi H. J. M Al-Khwarizmi Engineering Journal, Vol. 11, No. 3, P.P. 54- 60(2015) 59 Notation AA Aluminum alloy Bplp Black paint laser peening CFLIF Cumulative fatigue life improvement factor FLIF Fatigue life improvement factor HCF High cycle fatigue H-L High-Low Stresses LCF Low cycle fatigue L-H Low-High Stresses LPWC Laser Peening Without Coating LP Laser peening R Stress ratio SP Shot peening 5. References [1] Pavlou, D.G., A Phenomenological Fatigue Damage Accumulation Rule Based on Hardness Increasing, for the 2024-T42 Aluminum, Engineering Structures, 2002, Vol. 24 , pp. 1363-1368. [2] Franke, L. and Dierkes, G., A Non-Linear Fatigue Damage rule with an Exponent Based on a Crack Growth Boundary Condition, International Journal of Fatigue, 1999, Vol. 21, pp. 761-767. [3] Kawai, M., and Hachinole, A., Two-Stress Level Fatigue of Unidiretional Fiber-Metal Hybrid Composite: Glare 2, International Journal of Fatigue, 2002, Vol. 24, pp. 567- 580. [4] Fatemi, A. and Yang, L., Cumulative Fatigue Damage an Life Prediction Theories: A Survey of the State of the Art for Homogeneous Materials, International Journal of Fatigue, 1997, Vol. 20, No. 1, pp. 9-34. [5] Hu, Y., Yao, Z., Hu, J.: 3-D FEM Simulation of Laser Shock Processing. In: Surface & Coatings Technology 201 (2006), p. 1426- 1435. [6] Rubio-Gonzalez, C., Ocana, J.L.,Gomez- Rosas, G., et al.: Effect of Laser Shock Processing on Fatigue Crack Growth and Fracture Toughness of 6061-T6 Aluminum Alloy. In: Materials Science and Engineering A386 (2004), p. 291-295. [7] P.Peyre , R.Fabbro ,P.Merrien , H.P.Lieurade , et al. :Laser shock processing of aluminum alloys,Application to high cycle fatigue behavior . In Matrial Science and Engineering A210 ,pp102-113 (1996). [8] Everett R.A. , W.T.Matthews , et al. : The effects of shot and laser peening on fatigue life and crack growth in 2024 aluminum alloy and 4340 steel . In NASA/TM-2001- 210843 ,ARL – TR-2363 (2001) . [9] Rankin J.E , Michael R.H , Lloyd A.H. et al. : The effects of process variations on residual stress in laser peened 7049-T73 aluminum alloy Materials Science and Engineering pp279-291 (2003). [10] Zhang, Y.K., Lu, J.Z., Ren, X.D., et al.: Effect of Laser Shock Processing on the Mechanical Properties and Fatigue Lives of the Turbojet Engine Blades Manufactured by LY2 Aluminum Alloy. In: Materials and Design 30 (2009), p. 1697-1703. [11] Yasuo Ochi , Takashi Matsumura , Kiyotaka Masaki , Toshifumi Kakiuchi , Yuji Sano , Takafumi Adachi, et al. : Effect of laser peening treatment without protective coating on axial fatigue property of aluminum alloy. InScience Direct 2 ,pp491-498(2010). [12] Rubio-Gonzales, C., Felix-Martinez, C., Gomez-Rosas, G., et al.: Effect of Laser Shock Processing on Fatigue Crack Growth of Duplex Stainless Steel. In: Materials Science and Engineering A 528 (2011), p. 914-919. [13] Alalkawi H.J., Abdul-Jabar H.Ali, Saisaban A.Fahad ,et al. :Effect of Laser Shockpeening on fatigue life of aluminum- alloy (3003-H18). In: Mechanical Engineering 61(2013), p. 17107-17109 . [14] S.Sathyajith , S.Kalainathan , S.Swaroop , et al. :laser peening without coating on aluminum alloy Al-6061-T6 using low energy Nd:YAG laser )) Optics and laser Technology 45,pp 389-394(2013). [15] Alalkaw H.J. , Bedaiwi B.A. ,Adel A.M. et al. : Constant fatigue behavior under different conditions of laser peening Proceeding of the ASME 2014 . International mechanical congress and Exposition (IMECE ) , Montereal , Canada 14-20 November 2014 . [16] Berns H. , Weber , et al. : Influence of residual stresses on crack growth .In Impact surface treatment , pp 33-44 (1984). [17] [17] Clauer A.H., Dulaney R.C., Rice R.C. , Koucky J.R. , et al. : Laser shock processing for treating fastener holes in aging aircraft . 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