EFFECTS OF MANUAL SHOT PEENING CONDITIONS ON HIGH CYCLE FATIGUE

Transcription

1 Proceedings of the SEM Annual Conference June 1-4, 2009 Albuquerque New Mexico USA 2009 Society for Experimental Mechanics Inc. EFFECTS OF MANUAL SHOT PEENING CONDITIONS ON HIGH CYCLE FATIGUE H. Bae, M. Ramulu and H. Diep* Department of Mechanical engineering, Box University of Washington, Seattle WA *The Boeing Company, Seattle ABSTRACT In the manual peening operation, the control of intensity and coverage is of vital importance to maintain quality assurance. This experimental investigation provides information to establish an optimal manual shot peening condition and develop a relationship between the fatigue life and peening process parameters such as coverage, saturation and intensity. Observations showed that the peening process is strongly dependent on the control of peening conditions in manual operation. The results of this work show that with proper controls, manual shot peening can be used to produce an optimum balance between surface hardening and surface roughness. Based on the results, the investigation evaluates the fatigue performance of manually peened 7050-T7451 aluminum alloy under high cycle loading. INTRODUCTION There are many processes used today to treat the surface of metals. Cold working of the surface of materials is a widely used method that has been around for centuries and shot peening is one of the many methods of this type of surface treatment [1-5]. Although the mechanism of shot peening is a simple concept, the process is complex. Shot peening produces several changes in the workpiece material, including changes to microstructure, residual stresses, and topography. Some of these changes are beneficial, and some are potentially detrimental [6-9]. However, the effectiveness of the shot peening process is dependent upon the uniformity of the induced compressive residual stresses and the energy transfer that occurs during the impact of the shot with the target surface. Although modern day shot peening is mostly a highly controlled, automated process, due to component shape and/or size in real world applications, it needs to employ manual shot peening treatment to induce the compressive residual stress in certain situations [3]. In manual peening operations, the control of intensity and coverage is of vital importance to maintain quality assurance [4]. In practice, process efficiency is established by means of coverage, intensity and saturation. Intensity and saturation can be found for varying input conditions. These are stand off distance (SOD), impingement angle, air pressure, shot size, shot properties and material properties. There is a need to understand the relationship between peening parameters and intensity. Limited manual peening studies and their impact on components have been published and there exists a real need for a much more extensive study to cover all the aspects that make for optimizing the manual peening process [10]. The purpose of this paper is to investigate manual shot peening process parameters (intensity and coverage) and also to evaluate the fatigue performance of manually peened 7050-T7451 aluminum alloy under high cycle loading.

2 EXPERIMENTAL METHOD The A type Almen strips are made from SAE 1070 CRS (cold rolled spring steel), with a standard hardness of 4450 HRC. The shot material is cast steel shot S230. The manual shot peening system, which is a vacuumblasting system from Vacublast, is employed with a 6 mm diameter nozzle. The A type Almen test strips meet the requirements of MIL-S-13165C, AMS-S-13165, SAE J442, SAE AMS 2430M and SAE AMS 2432B [11-14]. According to MIL-S-13165, cast steel shot S230 has 0.58mm (0.023in) nominal diameter. The range of applied pressure is from 55KPa to 242KPa (8psi to 35psi) and mass flow rate is found at each specific pressure, as shown in Table 1. A specially designed and built fixture is used to maintain constant stand off distance for a selected angle of impingement in this manual shot peening experimental study. Three standoff distances are considered and 304 mm is the highest standoff distance in the vacuum chamber. Almen test strips are verified by Almen gage. Maximum flatness +/ is employed [2] and Almen test strips with more or less maximum flatness tolerance values are rejected. Figure1 Shot peening system and experimental setup An image analysis system is introduced for coverage measurements. By this system, 100% coverage will be found on several conditions depending on three variables (air pressure, stand off distance and impingement angle). To obtain coverage data on engineering material, aluminum square blocks (100 mm X 100 mm) are utilized for this study. These blocks are made from Al 7050-T7451 and a total of 14 blocks were used for generating intensity and coverage data. The peening conditions are varied (air pressure (69-249kPa), stand off distance (-304 mm) and angle of impingement (30- degrees)) and design of experiments yield a total of 14 experimental combinations. All intensities are determined by the saturation curve process. To obtain uniform coverage areas, X-Y coordinated movement of the shot stream is applied to the flat blocks. Also the traverse rate for the XY coordinated movement of shot stream took 5 seconds per pass. In order to estimate the percentage of coverage area, Image Analysis [15] is utilized. A stereomicroscope with a digital camera is used. Photos recorded at 40 times and 60 times magnification is used for the image analysis. By changing the outside light beam s angle, the microscopic photos are taken so that dents from shots are represented as white area and the untreated areas are black. The optical micrographs are taken at nine preselected points, which represent coverage areas of the corners, edges and middle of the block at 40 times and 60 times magnification at each point. The percentage of coverage in each peening condition is determined by the average value of nine points percentage coverage. Surface topography is evaluated using optical microscopy. To characterize the effect of the manual shot peening on the fatigue life of the aluminum alloy (Al 7050-T7451), both peened and unpeened hour glass circular cross section specimens of Al 7050-T7451 are fatigue tested in completely reversed rotation bending (R=Smin/Smax=-1). Fatigue specimens are manually shot peened by four variables: air pressure, standoff distance, impingement angle and peening time. The fatigue test matrix is given in Table1. A group represents each condition of peening parameters and each group has 5 specimens. The fatigue life from 10^4 to 10^6 cycles is targeted and one maximum stress 310 MPa (45 ksi) is chosen. A commercial R. R. Moore rotating bending fatigue test machine is used at rotational speeds of 1,200 RPM. A schematic of the fatigue specimens and the rotational bending fatigue test machine is shown in Figure 2.

3 Figure 2 Spec of Al7050 specimen and Schematic of a rotational bending fatigue tester Table 1 Rotational bending fatigue test matrix for the manual shot peening Impingement angle ( ) SOD(mm) Peening Time (sec) Air Pressure (Kpa) Group As-machined A % 69 B 172 C 69 D 55 E 100% 69 F G 200% 69 H 172 I % 69 J 172 K 69 L 100% 55 M 69 N 172 O ) 69 P 200% 172 Q 80% 69 R 95% 69 S RESULTS AND DISCUSSION Intensity The shot peening parameters investigated are: air pressure, impingement angle and standoff distance (SOD). Each data point on a saturation curve is the average of three arc heights of three Almen test strips. Three Almen test strips are peened at each time point and averaged. By literature definition, saturation point (Intensity) is found as a point on the graph where a doubling of the peening time does not result in more than a 10% increase in arc height of the strip curvature. Figure 3 shows how a saturation curve and saturation point are generated. Each strip is measured prior to peening by using a flatness tolerance of +/ inch. All strips are also examined after peening to assure that a minimum of 100% coverage was achieved. All arc heights of the Almen test strips are measured by using an Almen gage. Almen Intensity results achieved are detailed in Table 2. Figure 3 Typical Almen Saturation Curve

4 Intensity results for air pressure, impingement angle and stand off distance (SOD) are presented graphically in Figure 4. Figure 4 (a) illustrates the relationship between Almen intensity and air pressure at mm (6 inch) (SOD) and 60 (impingement angle). Almen intensity increased from 0.131(A) (mm) to (A) (mm) when air pressure increased from 69 KPa to 242 KPa (10 psi to 35 psi). Air pressure exhibits a nearly linear increase in Almen intensity. An increase in impingement angles causes an increase in Almen intensity. Figure 4 (b) shows how the intensity changes at a different impingement angle for a given mm (SOD) and 172KPa (air pressure). Intensity varies from (A) (mm) to (A) (mm) when impingement angles change from 30 to. The gradient of the first linear curve is greater than the second one. Based on the result, it is clear that changes in impingement angles have a pronounced effect at low angles and very little effect at angles greater than 70. Stand off distance has a limited effect on intensity, and the effect is inversely proportional. The graph of Almen intensity versus SOD is shown in Figure 4 (c). It also illustrates how the intensity changes at different SODs for a given impingement angle. Increasing SOD, ranging from mm to mm, decreases the intensity and SOD at lower impingement angles has a greater effect on intensity than ones at a higher impingement angle. The effect of SOD on intensity is less than the effects of air pressure and impingement angle. Table 2 Almen Strip Intensity Pressure (KPa) SOD (mm) Impingement angle (Degree) Intensity (A)(mm) Coverage Coverage analysis is evaluated by using optical micrographs and the image J program. Typical coverage micrographs of shot peened Al 7050 flat block are shown in Figure 5. Figure 6 shows the coverage analysis by Image J and the development of coverage with exposure times for Almen test strips and Al 7050 flat block specimens. Original micrographs and binary images shown are for typical low coverage points and the relation between the coverage and the peening times deduced from this series of experiments are also shown in the plot. Table 3 summarizes the times to reach 100% coverage for the specimens. Higher intensity caused a reduction in the time to reach 100% coverage, as expected. Therefore, the energy is closely related to coverage time. Increasing air pressure and impingement angle from 30 to decreased the coverage time for a constant SOD and is consistent with others findings[16]. It is also observed that by decreasing SOD, for a given impingement angle, 100% coverage time is reduced.

5 Fatigue Life Figure 7 shows the micrographs of the as machined and shot peened surfaces of circular hourglass bending fatigue specimens. Note the machining feed marks on the as machined specimen. However, the shot peening process clearly suppressed and/or modified all the feed marks and generated a homogeneous peened surface. (a) Almen Intensity versus Air presssure (SOD: mm, Impingement angle: 60 ) (b) Almen Intensity versus Impingement angle (Air pressure:172kpa, SOD: mm) (c) Almen Intensity versus Standoffdistance (Air pressure: 172KPa, Impingement angle: ) Figure 4 Effect of Peening Parameters on Almen Intensity

6 Figure 5 Typical shot peened Al7050 flat block surfaces S230, p=69kpa, SOD=mm, Angle= S230, p=172kpa, SOD=mm, Angle=60 Figure 6 Examples of coverage analysis by Image J and the development of coverage with exposure time for Almen strip and Al 7050 flat block Table 3 Al 7050 Flat Surface Specimens Time to reach 100% coverage Mass flow rate(g/sec) Air pressure(kpa) Sample# SOD(mm) 203 Impingement Angle ( ) Intensity(A)(mm) Time to reach 100% Coverage (sec)

7 Figure 7 typical surface images of as machined and shot peened circular hour glass specimens The effect of manual shot peening on the fatigue life of Al7050 specimens is investigated by using rotational bending fatigue tests at a constant stress of 310MPa (45Ksi) and R=-1.0. Figure 8 shows the fatigue lives from group A to group S based on table 1. Group A is as machined test specimens. The bar marks in the figure represents the average, maximum and minimum values. It is clearly shown that fatiuge life of shot peened specimens is higher than those of as machined specimens. However, in test specimens, groups, I and Q, which were peened at higher intensity with 200% coverage, have consistently yielded low fatigue lives comparatively. Typical fracture surfaces of the fatigued specimens are shown in Figure 9. The fatigue cracks intiated on the surface for all the specimens tested in as-machined condition. As expected, in all shot peened specimens, fatigue cracks initiated in subsurface regions. Figure 8 Fatigue lives of as machined and shot peened Al 7050 specimen. Figure 9 Typical fracture surfaces of as machined and shot peened Al 7050 specimen

8 SUMMARY AND CONCLUSIONS A series of experiments were performed to characterize the manual shot peening process in terms of peening input parameters such as shot size and properties, air pressure, impingement angle, stand off distance, feed rate, and material properties. Intensity, saturation and coverage were determined experimentally by varying conditions. It is clear that intensity, saturation and coverage can be controlled by input parameters. Observation showed that the peening process is strongly dependent on control of peening conditions in manual operations. The research evaluated the fatigue performance of manually peened 750-T7451 aluminum under high cycle loading. The research found that intensity and coverage can be the means to control the manual shot peening process and manual shot peening can improve the fatigue life of Al 7050-T7451 alloy. It was also observed that high intensity and or high coverage does not yield high fatigue life due to surface micro-cracks and material folding. ACKNOWLEDGEMENTS The authors thank Mr. Jack Champaign of Electronics Inc. for his help in procuring test materials and for his encouragement during the course of this investigation. We also sincerely thank the Boeing Company for the financial support. REFERENCES 1. A. Niku-Lari, Shot-Peening, First Int l. Conf. on Shot Peening, 1981, pp J. Champaigne, Shot peening overview, Electronics Inc D. Kirk, Shot peening, Aircraft Engineering and Aerospace Technology, Vol71, 1999, pp D. Clarke and S. S. Birley, The Control Of Manual Shot Peening, Quality Assurance Directorate(Ordnance) London UK,1981,pp M. K. Tufft, Development of a Fracture Mechanics/ Threshold Behavior Model to Assess the Effects of Competing Mechanisms Induced by Shot Peening on Cyclic Life of Nickel-base Super alloy, Rene 88DT, Ph. D. dissertation, University of Dayton, A. Biggs, Analysis of factors affecting Almen strip arc height after shot peening, M. S. Thesis University of Washington, T. Dorr and L. Wagner, Effect of shot peening on residual life of fatigue pre-damaged 2024 Al, Sixth Int l. Conf. on Shot Peening, J. Champaigne and D. Kirk, The Curve Solver Program, The Shot Peener, Spring L. Wagner, Mechanical surface treatments on titanium, aluminum and magnesium alloy, Material Science and Engineering, Vol. A263, 1999, pp H.Diep, H. Bae and M. Ramulu, Characterization of Manual Shot Peening Process: Preliminary Results, ICSP10, SAE J442, Test Strip, Holder And Gage for Shot Peening, August SAE J443, Procedures for Using Shot Peening Test strip, January MIL-S-13165C, Shot peening of Metal Parts, AMS 2432, Shot Peening, Computer Monitored, M.D. Abramoff, Magelhaes, P.J., Ram, S.J. "Image Processing with ImageJ". Biophotonics International, volume 11, issue 7, pp , T. Ludian and L. Wagner, Coverage Effects in Shot Peening of 2024-T4, Proceedings of ICSP9, 2007, pp