خير الله سلمان Al-Khwarizmi Engineering Journal Al-Khwarizmi Engineering Journal, Vol. 8, No.4, PP 90- 95 (2012) Analysis of the Effects of Aggressive Shot Peening on Fatigue Life of 7075 – T6 Aluminum Alloy Khairallah S. Jabur Department of Mechanical/ Institute of Technology- Baghdad Email: a_Khairallah@yahoo.com (Received 29 November 2011; accepted 7 August 2012) Abstract For many years controlled shot peening was considered as a surface treatment. It is now clear that the performance of control shot peening in terms of fatigue depends on the balance between its beneficial (compressive residual stress and work hardening) and beneficial effects (surface hardening). The overall aim of this paper is to study the effects of aggressive shot peening on fatigue life of 7075 – T6 aluminum alloy. The fatigue life reduction factor (LRF) due to the aggressive shot peening was established and empirical relations were proposed to describe the behavior of LRF, roughness and fatigue life. The benefits of shot peering in terms of fatigue life are dependent on the shot peening time (SPT). The higher SPT is the lower the benefit is. Higher roughness results in lower fatigue life. Keywords: aggressive shot peening, fatigue life , LRF , 7075 – T6 Al-alloy. 1. Introduction and Review Shot peening treatments are widely used in mechanical and aeronautical engineering to improve the fatigue performance of components . There have however been reports of extensive variations in the fatigue life results for peened components and , in some cases a decrease in fatigue life has been observed [1] . Peening attempts to spread material near the impact point against the resistance of neighboring material , thus introducing a complex sub- surface residual stress distribution in which generally , the surface is in elastic compression . There is a rapid transition to elastic tension at a deeper level , produced by the "enlarged" surface layer . This tension decays in deeper regions towards zero as shown in Figure (1) [2] . Significant effect of peening intercity on fatigue life is also been noted ; there appears to be an optimum peening intensity to achieve the longest improvement in fatigue life .This intensity varies with the type of aluminum and heat treatment [3] [4] . Fig. 1. Schematic of Residual Stress Distribution below a Peened Surface. Compressive Layer Generally Extends 200 – 400 µ m below the Surface [2]. Alalkawi et al. studied the effect of shot peening on two aluminum alloys 2024 and 5052 . The cumulative fatigue life of 2024 aluminum alloy is increased by shot peening while the mailto:a_Khairallah@yahoo.com Khairallah S. Jabur Al-Khwarizmi Engineering Journal, Vol. 8, No.4, PP 90- 95 (2012) 91 fatigue life of 5052 aluminum alloy is reduced due to high surface roughness and consequently high tensile residual stresses [5] . The predictions of the continuum damage modeling (CDM) were compared to the experimentally observed mechanical response and to the micromechanical characterization of damage. The comparison showed good agreement for 2024 – T351 aluminum alloy plate subjected to multi – acid stress states [6] . Aggressive shot peening tends to increase the surface roughness which can be detrimental to fatigue performance . The higher roughness creates short crack growth and reduce the fatigue life [7] [8] . This paper discusses the fatigue behavior of 7075 – Al – alloy under different shot peening times (SPT) and summarizes the improvements in peening arisen from this research. 2. Material Aluminum alloy AA 7075 – T6 is used in the present work which has low specific weight and high strength to weight ratio. This alloy is widely used in industry and in particular in aircraft structure and pressure vessels [9] is presented in Table (1) chemical composition of 7075 – T6 Al – alloy wt% [10] ; While the mechanical properties were effected at the center of standardization and quality control are illustrated in Table (2) . Table 1, Chemical Composition of 7075 – T6 Al – alloy wt%. Si Fe Cu Mn Mg Zn Cr Al 0.16 0.32 1.8 0.1 2.3 5.4 0.2 Rem Table 2, Mechanical Properties of 7075 – T6 Al – alloy. Properties σ u (MPa) σ y (MPa) EI % HB Experimental 600 521 11 158 Standard 570 505 9 150 3. Fatigue Testing Specimens All fatigue specimens were prepared according to DIN 50113. The shape of the fatigue specimen is hour – glass type in order to obtain stress concentration in the middle of the specimens (minimum diameter). Fig. (2) Shows the fatigue specimen configuration Fig. 2. Fatigue Specimen (All Dimensions in mm) According to DIN 50113. 4. Shot Peening Shot peening was carried out for different times using spherically ball of 1mm in diameter at constant distance between the nozzle and the specimen of 10 cm. The specimen is rotating continuously during peening to ensure 100% coverage. Shot peening technique was performed using spherical steel balls of 1mm diameter. The ball hardness is 48 – 50 HB 12 bar average blasting pressure, the ball speed is 40 m/s , 100% coverage . The shot peening device used was (shot tumblast control model STB – OB) machine No. 03008 05 type. Fig (3) shows the shot peening device with shot balls used. Khairallah S. Jabur Al-Khwarizmi Engineering Journal, Vol. 8, No.4, PP 90- 95 (2012) 92 (a) (b) Fig. 3. (a) Shot Peening Device. (b) Shot Balls. 5. Roughness Measurement Surface roughness measurements were carried out at the labs, Of Malaysia Pahang University using surface roughness testing machine type Perthometer 52. More details can be found in Ref [11] . Table (3) gives the average roughness (Ra) for different conditions of shot peening time (SPT). Table 3, Average Roughness Before and After (SPT) for 7075 – T6 Al. alloy. SPT (min.) Ra (µm)* SPT (min) Ra (µm) Zero** 0.25 35 4.6 15 2.5 45 5.5 25 3.8 55 6.7 * The data are an average of 5 readings. ** Zero SPT i . e before shot peening or as received. 6. Fatigue Life Tests 12 specimens are tested under constant rotating bending amplitude stress , 0.6 σ u (360 MPa) , and at room temperature with stress ratio R = -1 . Table (4) shows the experimental results versa surface roughness (Ra) and SPT . Table 4, Comparison of Fatigue Life and Surface Roughness for a Number of SPT. Spec. No. SPT (min) Surface roughness Ra ( mµ ) Fatigue life , Nf cycles 1 , 2 0 0.25 62800 , 59200 3 , 4 15 2.5 200600 , 188000 5 , 6 25 3.8 52800 , 57900 7 , 8 35 4.6 41200 , 39600 9 , 10 45 5.5 30800 , 32600 11 , 12 55 6.7 28000 , 26700 The results appear to show a direct correlation between SPT and fatigue life of peened 7075 – T6 Al. alloy. As SPT increases (from zero to 15 SPT) the fatigue life increases. But as roughness increases, fatigue life decreases (from 25 to 55 SPT). It is therefore considered that it might be possible to quantify the effects of peening on fatigue life by measuring surface roughness. The conclusion, at this stage is that some correlation exists between roughness and fatigue life for SPT from 25 to 55 , but where Khairallah S. Jabur Al-Khwarizmi Engineering Journal, Vol. 8, No.4, PP 90- 95 (2012) 93 the history of the part may have included some peening , from zero to 15 SPT , this relationship is unreliable . Fig (4) shows the roughness against fatigue life for SPT from 25 to 55. The best fit formula which deseribes the above relation can be written by the empirical equation Ra =22803 Nf-0.8 …(1) Fig. 4. The Relationship between Measured (Ra) and Fatigue Life Nf For 25 To 55 SPT. Based on the results of Table (4), the mean life improvement factor (LIF) for SPT 15 minutes is 3.185 . This value is obtained from Table (4) as Nf average 15SPT 194000 LIF= -------------------------- = --------- = 3.185 Nf average zero SPT 61000 which obays the work of Sharp , et al. [12] . They proposed formula which described the relation between LIF and applied stress based on the experimental results for aluminum alloys as: LIF=6*1015(Stress)-5.8749 …(2) At 400 MPa Sharp et al. found that the LIF was 3.192 at the same SPT used in this work. For SPT 25 to 55 the effect of peening is inversially proportional to fatigue life, i . e when SPT increases the fatigue life decreases . Because of the greater damage to the material, the surface finish (roughness and defects) is also recognized as a critical influence on fatigue life. The LRF (Life reduction factor) for specimens peened at SPT 25 to 55 is tabulated in Table (5) Table 5, LRF due to High SPT. Spec. No. Average life cycles Average (Ra) µm LRF * 5 , 6 55350 3.8 1.102 7 , 8 40400 4.6 1.509 9 , 10 31700 5.5 1.924 11 , 12 27350 6.7 2.230 * LRF is calculated based on unpeened Average fatigue life 61000 (specimens 1,2). The results of Table (5) appear to a direct relation between surface roughness and LRF of peened aluminum alloy 7075 – T6 . As (Ra) increases, LRF increases as shown in Figure (6). It is clear that the relation between LRF and Nf is inversely proportional as shown in the formula: LRF = 89743 Nf-1.037 …(3) 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 Series1 Theo Fig. 6. Roughness against LRF for 7075–T6 Al – Alloy. The problem of excessive or aggressive peening leads to defects buried insub – surface and to very deep laps , which do influence the surface roughness measurement , while greatly reduces fatigue life [13] . The relation which describes the surface roughness with LRF can be written as Ra(µm)=3.443LRF0.771 …(4) Su rf ac e r ou gh ne ss (R a) µ m Nf (Cycles) Ra =22803 Nf -0.8 LRF Exp Ra = 3.443 LRF0.771 R a (µ m ) Khairallah S. Jabur Al-Khwarizmi Engineering Journal, Vol. 8, No.4, PP 90- 95 (2012) 94 7. Conclusions In this work the effects of aggressive SPT are analyzed and modeled according to their effects on fatigue life. Surface roughness is modeled with fatigue life reduction factor (LRF). The analysis reveals that: 1- The benefits of shot peening in terms of fatigue life are dependent on the SPT. The higher the SPT is the lower the benefit is for time 0 < t < 15 min. 2- The fatigue life improvement factor (LIF) is limited up to 15 SPT and equals to 3.185 . 3- After 25 SPT, fatigue life is reduced with time and the LRF is appeared. 4- After 25 SPT , increasing LRF leads to increasing the average roughness (Ra) and reducing the average number of cycles for failure (Nf) 5- An empirical formula which described the relation between (Ra) and LRF is proposed as: 771.0)(443.3 LRFRa = 8. References [1] Clayton J. Q. and Clark G. " The effect of steel shot and Glass Bead Peening treatments on the fatigue resistance of 7075 – T6351 Aluminum alloy " Proc. Aust. Fract. Group. Fracture mechanics in engineering practice , Melbourne , pp. 44-51 , 1988 . [2] P. K. Sharp and G. Clark " The effect of peening on the fatigue life of 7050 Aluminum alloy" DSTO , 2001 . [3] Luo W, Noble , B and Waterhouse R. B. " The interaction between shot peening and heat treatment on fatigue and fretting fatigue properties of the high strength aluminum alloy 7075" Proc. 2nd Int. Conf. on impact treatment procedure , 1986 . [4] Luo W, Noble B and Waterhouse R. B. , "The effect of shot peening intensity on the fatigue and fretting behavior of an aluminum alloy " , Advances in surface treatment and surface finshing , vol . 5 , pp. 145 – 153 , 1987 . [5] Alalkawi H. J. M , Qusay Kh . M., Al – Nuami Waleed S. " Evelation of residual stress using shot cumulative fatigue damage " J. of Eng. And Tech. , 2009 . [6] S. Khan A. Vyshnevskyy J. Mosler "low cycle lifetime assessment of Al 2024 alloy” Inter. J. of fatigue 32 , 1270 – 1277 , 2010 . [7] M. A. Wahab , J. H. Park , M. S. Alam , SS. Pang. “Effect of corrosion prevention compounds on fatigue life in 2024 – T3 aluminum alloy “ Jounal of materials processing Technology 174 – 211 – 217 , 2006 . [8] Y. Fouad and Mostafa M. El Metalally “ Effect of shot peening on high cycling fatigue of Al – 2024 – T4 , International conference on advanced materials engineering IPCSIT vol. 15 , 2011 . [9] G. H. Majzoobi , J. Nemati , A. J. Novin Rooz , G. H. Farrahi " Modification of fretting fatigue behaviors of Al 7075 – T6 alloy by the application of titanium coating using IBFD technique and shot peening " Tribology inter. 42 , 121 – 129 , 2009 . [10] Alakawi H. J. W. , Talal Abdul – Jabbar , Safaa H. Alokaidi , Basil A. Selman , Ali Yousif Khenyab , "The effects of dry and wet shot peening on the mechanical and fatigue properties of 7075 – T6 aluminum – alloy" , International Journal of Engineering research and industrial applications to be published , 2012 . [11] Safaa H. A. Alokaidi " Modeling the fatigue behavior of shot peened aluminum alloys under variable stress range conditions" phD thesis , university of Technology , 2011 . [12] Sharp P. K. , Barter S. A. and Clark G. " localized life extension specification for the F / A – 18 Y 470x19 Pocket " DSTO – TN – 0279 , 2000 . [13] Alakawi H. J. , Kadhim M. j. , Selman B. A. , Khengab A. Y. " Improvement of fatigue property in high strength aluminum alloy by shot peening " University umdarunn J. under publication , 2012 . )2012( 90- 95، صفحة 4، العدد8رزمي الھندسیة المجلد مجلة الخوا خیر اهللا سلمان 95 T6 – 7075تحلیل تأثیرات القذف بالكریات المفرط على عمر الكالل لسبیكة االلمنیوم اهللا سلمان جبر خير بغداد -معھد التكنولوجیا /قسم المیكانیك a-_Khairallah@yahoo.com االلكتروني البرید: الخالصة لكالل یعتمد لعدة سنوات عملیة القذف بالكرات المسیطرة كانت تعتبر معاملة سطحیة اما اآلن فتوضح بان اداء عملیة القذف بالكرات المسیطرة بداللة ا ) . لتصلید السطح(وعملیة التصلید السطحي والتأثیرات المفیدة ) الجھادات الضغطیة المتبقیةا(على الموازنة بین ما ھو مفید ت م ایج اد عام ل تخف یض عم ر الك الل . T6 – 7075الھدف الكلي لھذا البحث ھو دراس ة ت أثیرات الق ذف المف رط عل ى عم ر الك الل لس بیكة االلمنی وم )LRF ( نتیجة للقذف المفرط وتم اقتراح عالقات ریاضیة معتمدة على التجارب العملیة لتصف تصرف)LRF ( فوائد القذف بداللة . والخشونة وعمر الكالل .اما زمن القذف الكبیر یعطي فائدة قلیلة وخشونة عالیة والتي تعطي عمر كالل واطىء ) . SPT(عمر الكالل تعتمد على زمن القذف بالكریات mailto:a-_Khairallah@yahoo.com