PREDICTION AND XRD MEASURING OF RESIDUAL STRESS IN MACHINED WELDED PARTS

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1 Copyright -International Centre for Diffraction Data 21 ISSN PREDICTION AND XRD MEASURING OF RESIDUAL STRESS IN MACHINED WELDED PARTS Rafailov G. 1, Sariel J. 1, Dahan I. 1, Reuven R. 1, and Szanto M. 2 1 Nuclear Research Center, Negev, P.O. Box 91, Beer-Sheva, Mechanical Engineering Dept., Ben-Gurion University, Beer-Sheva, 8415 ABSTRACT In this study, residual stress distribution was measured in a spot welded Ti6Al4V disk before and after machining of the welded zone. The measurements were done using both X-Ray diffraction and Hole-Drilling (HD) methods. A numerical simulation of the distribution of residual stress was applied in order to predict the behavior of the stress. The distributions obtained by X-Ray diffraction, Hole-Drilling, and the numerical simulation were alike, and they were similar to the distribution expected according to the literature, i.e. high tensile stress near the fusion zone, which drops sharply to compressive stress when moving farther away from it, and then rises moderately towards zero at the edge of the disk. The tension residual stress near the hole's edge after machining was greater compared to the equivalent point after welding. On the other hand, both the value and position of the minimum compressive stress shifted to smaller values and to a larger radius. In general, the distributed stress becomes moderate due to the machining procedure. INTRODUCTION Welding processes usually result in residual stress, which are locked in the structural material or component. The effect of the stress can be either beneficial or detrimental, depending upon their magnitude, sign (tensile or compressive), and distribution with respect to the service induced stress. Thus, it is essential to be able to determine the residual stress, and consider them as a part of the service loads. Since the measurements taken apply only to the surface, a simulation is needed to accurately predict the bulk stress distribution. X-Ray diffraction (XRD) is a popular method to measure residual stress (Prevey, 1986), but still it is not the standard method. It needs special equipment, and there is a wide range of results from different laboratories even when measuring a very similar specimen (Grant, 22). In previous works, residual stress distribution in welded parts has been measured (Sariel et al., 25; Haupt and Astronautics, 1996). In this study, we measured residual stress obtained in a spot welded Ti6Al4V disk before and after machining out the welded zone using a universal X-Ray diffraction system, the results were then compared to those obtained by the Hole Drilling (HD) method, and by numerical simulation. EXPERIMENTAL A Ti6Al4V disk, 6 mm in diameter and with a thickness of 1.5 mm, was cut from a rod. The disk was polished using diamond paste up to 1 m, and cleaned before welding. Then it was autogenously spot welded at the center by the gas tungsten arc welding technique. The disk is seen in Fig. 1. The points on the right define the borders between the illuminated areas, and the holes at the left are holes for temperature thermocouples.

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 Copyright -International Centre for Diffraction Data 21 ISSN Fig. 2 represents a micrograph demonstrating the cross section micro-structure. At the center, the columnar grains are from the re-solidified fusion zone (bordered by dashed line in the fig. 2), and outside of its boarders is the heat affected zone with small equi-axed grains. Further from them is the original micro-structure of the material. FZ line FZ HAZ BM 2 cm Fig. 1: A disk after welding. Fig. 2: The cross section micro-structure. The diffraction measurements were performed by a universal Philips X'Pert X-ray diffraction system, using Cu anode ( =.154 nm), and a monochromator in front of the detector. The system was controlled by APD (Philips) software. In order to improve the quality of peaks, especially for the measurements under tilted conditions, the time per step was taken during tens of seconds. The measurement process was continued over a period of sixty-four hours. Measurements were made at several areas along the disk's radius, using a lead foil to exclude the unwanted regions from being exposed to the x-ray. Because of geometrical limitations only the tangential stress were measured. The peak examined was the (1,1,4) at about 2 =115, and the peak positions were found using the "fit profile" routine of the APD software. The measurements were made using the two exposure method (Prevey, 1986 and Cullity, 1978) at 3 and sometimes 4 tilts. The peak shift values were corrected using standard silicone powder from NIST, which was examined (offline) at the same tilt angles. The Hole Drilling (HD) technique is a standard residual stress measurement technique (ASTM E837), and it uses strain gauges to read the strain relief, while a small hole is drilled at the surface of the stressed area. In order to simulate the thermal induced residual stress distribution under welded conditions a conjugate thermal-structural transient computation was performed, using a commercial FEM code (ABAQUS and DEFORM in 8 node axi-symmetric elements. The disk was loaded with the external heat source at its center. The boundary conditions of heat convection between the disk's bottom and mounting plate, as well as convection combined with radiation to the surroundings at the disk's upper surface, were taken into consideration. Temperature dependent material properties were obtained from literature, however, the data regarding these properties is limited, and does not cover the whole temperature range of the process. In order to predict residual stress after machining the welded simulation results were used as a set-point. During the simulation the disk was held at three points, as it was during the actual machining process. The machined area was chosen to be at the disk's center, because the highest

4 Copyright -International Centre for Diffraction Data 21 ISSN residual stress distribution was predicated at this location. The machining process was completed in one step. The residual stress was calculated after the holder was removed. RESULTS AND DISCUSSION The stress was calculated using equation 1, where d and d are the planar spacing of the peaks, and and E are the Poison ratio and Young modulus of the material respectively. It should be noted that for Ti6Al4V the E effective (for the specific (h,k,l)), which was supposed to be used, is very close to the mechanical Young modulus (page 382 in ref. Error! Bookmark not defined.). (1) E ( d d 2 ( 1 )sin d ) WELDED DISK In fig. 3(a) the tangential stress distribution, which was predicted by the numerical simulation (Olabia et al., 27), is shown. In fig 3(b) a comparison between the numerical simulation and XRD measurements from the upper surface are shown. It can be seen that the nature of the tangential stress obtained by both methods is similar, i.e. high tensile stress near the fusion zone, sharp decrease to compressive stress, which then moderately rise towards zero close to the disk edge (Prev, 1996). Hence, we can assume that the bulk residual stress distribution is well represented by the simulation. 5cm a Stress (MPa) simulation of welding XRD of welding Radius (mm) Fig. 3 (a): Residual stress distribution obtained by numerical simulation for the whole disk. (b) Comparison of the results for the upper surface between the simulation and XRD measurements. The XRD results were also compared to those obtained by the HD technique (Fig. 4).The nature of the tangential stress obtained by both methods is also similar, and the stress values of the two methods seems alike. Those results similar to residual stresses of electron beam welded TC4 alloy plates. (Mei-juan, and Jin-he, 29). b

5 Copyright -International Centre for Diffraction Data 21 ISSN Stress [MPa] HD welded XRD welded Radius [mm] Fig. 4: Comparison of the stress distribution by XRD and by HD methods. As we see from fig. 3(b) and fig. 4, there are some discrepancies between the results of the two methods and simulation. The following are possible reasons. First, there are systematic errors in all the three methods. We assume that systematic errors fall between 1 and 15%. Another reason stems from the very sharp gradient of the stress distribution, where two measurements at a very small distance from each other can cause a large difference in the resulting stress. Third, since the XRD measurements were done by a system with line focus instead of microfocus, position error occurred. MACHINED DISK The distribution of residual stress obtained by numerical simulations after machining of the welded zone is shown in fig. 5 (a). In fig. 5 (b) a comparison between the numerical simulation and XRD measurements from the upper surface after machining of the welding zone is shown. It can be seen that the nature of the residual tangential-stress distribution obtained by the methods are alike.

6 Copyright -International Centre for Diffraction Data 21 ISSN cm a Stress (MPa) 8 7 simulation after machining 6 5 XRD after machining b Radius (mm) Fig. 5 (a): Residual stress distribution obtained by the numerical simulation for the whole disk after machining out the welding zone. (b): Comparison of the results for the upper surface between the simulation and XRD. A comparison of the XRD results to those obtained by the HD technique is shown in fig. 6. it can be seen that while close to hole's edge there is a close agreement between results. Far from the hole the results of the XRD and HD methods are not comparable due to poor statistics of HD. 3 2 HD XRD Stress [Mpa] Radius [mm] Fig. 6: Comparison of the stress distribution by XRD and HD methods. COMPARISON BETWEEN WELDED AND MACHINED CONDITIONS In fig. 7-9 comparisons between machined and welded conditions, using the three methods, are seen. Fig. 7 represents numerically simulated results for the welded and machined stages. The nature of residual stress distribution under both conditions is similar, with high tensile stress near the center of a disk decreasing to compressive stress, and then moderately rising towards zero

7 Copyright -International Centre for Diffraction Data 21 ISSN close to the disk's edge. However, the machining process resulted in the growth of tensile stress at the hole's edge and compressive stress reduction farther from it. Another difference between the two conditions is a shifting of the higher compressive stress (minimum) to a larger radius. Fig. 8 represents a comparison between both stages that is obtained by Hole Drilling. It can be seen that the behavior of residual stress as a function of the radius was also confirmed by Hole Drilling. The higher compressive stress (minimum) is shifted to a larger radius, as was predicted in the simulation, and tensile residual stress is higher than under welded conditions. Fig. 9 shows the comparison between both welded and machined conditions measured by XRD. The behavior of residual stress distribution was also confirmed by XRD. However, the value of the two conditions appears similar up to a 15 mm radius. The value of residual stress at higher radiuses is smaller in a machined disk. Since the simulation predicted small changes affected by the machining process, only precise and sensitive methods like HD or designated XRD can be applied, because, when using a conventional powder diffraction system small changes cannot be detected precisely. Stress (MPa) simulitation for welded disk simulation for machining disk Radius (mm) Fig 7: Comparison between the two conditions by numerical simulation. Stress [MPa] Radius [mm] HD machined HD welded Fig 8: Comparison between the two conditions by Hole Drilling.

8 Copyright -International Centre for Diffraction Data 21 ISSN after machining after welding Stress [Mpa] Radius [mm] Fig 9: Comparison between the two conditions by X Ray Diffraction. In this study, the residual stress distribution along the radius of spot welded and machined Ti6Al4V disks was measured and predicted. Under both conditions the distribution showed the same behavior discussed in literature regarding the subject, i.e. a high tensile stress close the center of the disk, which drops to compressive stress and reaches the minimum, and then rises back to about zero, at the edge of the disk. The machining process resulted in the growth of tensile stress near the hole's edge and compressive stress reduction farther from it. In addition, there is a shifting of the higher compressive stress (minimum) to a larger radius. This behavior was predict by the numerical simulation and confirmed using HD. Due to the low sensitivity of the XRD system the differences between the two conditions cannot be determined. ACKNOWLEDGEMENTS The authors wish to express their thanks to A. Michael for performing the spot welding, to S. Levi for the metallographic preparation, and T. Even-Hen for the XRD measurements. REFERENCES ABAQUS 25). (Computer software), Hibbit, Karlsson & Sorensen, Inc. Cullity, B.D. (1978). Elements of X-Ray Diffraction, (Adison-Wesley Publishing Company), 2nd. DEFORM (1991). (Computer software), Scientific Forming Technologies Corporation (SFTC), Ohio.

9 Copyright -International Centre for Diffraction Data 21 ISSN Grant, P. (22). Evaluation of Residual Stress Measurement Uncertainties Using X-Ray Diffraction and Hole Drilling via a UK Intercomparison Exercise. (Report MATC(MN)2). Teddington Middlesex, UK: National Physical Laboratory. Mei-juan, H. and Jin-he, H. L., (29) "Effects of zonal heat treatment on residual stresses and mechanical properties of electron beam welded TC4 alloy plates", Trans. Noneferros Met. Soc. China. 19, Olabia, A.G., Casalinob, G., Benyounisc, K.Y., and Rotondob, A., (27) "Minimization of the residual stress in the heat affected zone by means of numerical methods ", Materials & Design. 28(8), Prevey,P.S.(1986) "X-Ray Diffraction Residual Stress Techniques," in Metals Handbook, edited by G. M. Crankovic,( ASM International), Vol.1, p Prev 1996) Machining Deformation on the Development of Tensile Residual Stresses in Weld Fabricated Nuclear Components 5(1), Sariel, J., Dahan, I., Reuven, R., Szanto, M., and Stern, A. (25) "Residual Stresses Distribution in GTA Spot Welded Ti6Al4V disks", Advances in X Ray Analysis. 49, Haupt, C. W., and Astronautics,L.M., (1996) "Residual stress measurement of titanium weld certification rings" in 32nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference., Lake Buena Vista, FL: American Institute of Aeronautics and Astronautics.