RESIDUAL STRESS MEASUREMENTS OF CAST ALUMINUM ENGINE BLOCKS USING DIFFRACTION

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1 Copyright JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume RESIDUAL STRESS MEASUREMENTS OF CAST ALUMINUM ENGINE BLOCKS USING DIFFRACTION D.J. Wiesner a, T.R. Watkins b, T.M. Ely c, S. Spooner b, C.R. Hubbard b, J.C. Williams d a Dept. of Materials Science and Engineering, University of Tennessee, Knoxville, TN b Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN c Rigaku/MSC, Inc., The Woodlands, TX d Dept. of Materials Science and Engineering, Ohio State University, Columbus, OH ABSTRACT Residual stresses and strains in as-cast and annealed aluminum engine blocks were mapped in the main web area above the bearing split using diffraction. Automated coordinate-mapping of the surface and bulk strains was accomplished with X-ray and neutron diffraction, respectively. LabVIEW-based software was developed for improved efficiency, flexibility and quality of data analysis. In the hoop direction, an overall reduction in compressive surface stress level was found with increasing heat-treatment temperature. The radial surface stresses shifted similarly except that the final strains were slightly tensile. Residual strain mapping in the bulk was performed in two orthogonal directions with neutron diffraction. In the Y and Z (axial) directions, the bulk tensile strains were reduced from tensile to slightly tensile/compressive with increasing heat-treatment temperature. The opposite trend was observed in the surface. INTRODUCTION An engine casting plant identified the development of residual stresses in their cast aluminum engine blocks to be due to differential cooling during processing. Concern for the presence of these stresses derives from the distortion of critical features during machining, which increases the difficulty of maintaining dimensional tolerances, particularly for main web area. In an effort to remove these residual stresses, the engine blocks are subjected to a T5 stress relief treatment prior to machining. The stress relief treatment has added complexity to the process, and the stress relief oven has become a major production volume-limiting step. Alternate stress relief cycles, capable of allowing greater throughput in the stress relief oven, would be beneficial in this regard. To understand the problem better, the residual strains/stresses in a series of heat treated engine blocks were mapped using X-ray and neutron diffraction. EXPERIMENTAL Four engine blocks were supplied by an automotive manufacturer for study. The engine blocks are die cast from SAE (UNS A03830) alloy aluminum. This alloy is a silicon-copper alloy with compositional limits (wt %) [1]: 2.0 to 3.0 Cu, 0.1 Mg max, 0.50 Mn max, 9.5 to 11.5 Si, 1.3 Fe max, 0.30 Ni max, 3.0 Zn max, 0.15 Sn max, 0.50 others (total) max, bal Al. In Figure 1, a typical microstructure is displayed showing four distinct phases. One engine block was left in the as-cast condition, and three were heat treated (Alfe Heat Treating, Inc.) at 230 o C, 248 o C, and 270 o C for 4 hours.

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 JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume 48. Figure 1. Microstructure of the aluminum in the engine block. The X-ray system consists of a TEC Model 1600 X-ray stress analyzer mounted overhead to a gantry which forms the frame for a 15 x 8.2 x 9.2ft (L x W x H) shielded enclosure. This setup can accommodate small samples (100 lb. capacity) on small x-y-φ stages mounted to the bench top, large samples (250 lb. capacity, see Figure 2(a)) on large X-Y-φ stages mounted to the floor, or huge samples (+250 lb.) can be placed directly on the floor. Either sample stage can be computer controlled via LabVIEW for automated mapping of strains. In an effort to apply a more robust and efficient data analysis, LabVIEW software was developed to read the ASCII data files and fit the entire peak profile, allowing for improved accuracy of the residual stress calculation. (a) (b) Figure 2. (a) Enclosure, X-Y-φ sample stage and X-ray stress analyzer mounted to the gantry; (b) Area of interest for stress/strain mapping: Main web area above the bearing split. Note coordinate axes with the Z or axial direction normal to the surface. X-ray stress mapping was done on the main web area above the bearing split shown in Figure 2(b). The sample stages were leveled and the rotation (φ) axis was centered under the collimator. Gross sample positioning was achieved by marking the position of the engine block on the 137

4 Copyright JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume sample stage. Fine sample positioning was achieved through X and Y translation of the engine block to points on the 0, 90, and 180 degree lines. Table I lists the details of the experimental conditions for the X-ray measurements. Briefly, a single axis (ψ) goniometer [2] was employed for the stress measurements using the "Ωgoniometer geometry" [3(A)]. The (311) reflection from the Aluminum phase was utilized for the strain measurements. During scanning, the ψ axis was oscillated ±2 to improve particle statistics. Specimen alignment was accomplished using a contact probe, which was accurate to ±0.25 mm. Prior to data collection, goniometer alignment was ensured by examining an iron powder pellet standard. The maximum observed peak shift for the (211) reflection of Fe (156 2Θ) was less than Θ for Ω tilting as described in Table I. Table I. - Experimental conditions of the X-ray measurements made on the TEC large specimen stress analyzer. Parameter Condition Equipment TEC Model 1600 X-ray stress analyzer Position sensitive detector (PSD), 14 2Θ range Power 52.5 W; 35 kv, 1.5 ma Radiation Cr Kα 1, λ = Å Source to specimen distance 220 mm Specimen to detector distance 220 mm Tilt axis and angles 1 : Ψ = 0, ±9.6, ±13.6, ±16.8, ±19.5, ±21.9, ±24.1, ±26, ±28.1, ±30 (equal steps of sin 2 ψ) Scans: Θ/step; 60 sec/scan; o 2Θ Mapping: 33 points; radial distribution Direction: hoop and radial The X-ray data was analyzed with LabVIEW software based on the SaraTEC TM Windows TM software [2], which originated from James [4]. The OEM data analysis software, SaraTEC TM, assumes a biaxial stress state and fits a parabola to the top 20% of each peak to determine peak position. This method overemphasizes the top portion of the peak. Consequently minor perturbations of intensity can unduly influence the resulting peak position [5]. The LabVIEWbased software [6] addresses this by fitting the background and then fitting a Pearson VII function to the entire peak profile, accounting for the kα 1 -α 2 doublet. In addition to being able to handle kβ data, this software allows flexibility in background definition and data handling (i.e., rejection of data with poor profile fits and manual insertion of d 0 ). The stresses were calculated using the sin 2 ψ technique [3(B)], assuming a biaxial stress state. The fully expanded equation relating strain to stress, ε φψ = (d φψ -d 0 )/d 0 = (1+ν)/E {σ 11 cos 2 φ + σ 12 sin2φ + σ 22 sin 2 φ - σ 33 } sin 2 ψ + (1+ν)/E σ 33 - ν/e (σ 11 + σ 22 + σ 33 ) + (1+ν)/E {σ 13 cosφ + σ 23 sinφ} sin2ψ, (1) 1 All angles were used to measure the hoop stress. For the radial stress, Ψ values of were omitted due to clearance issues.

5 Copyright JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume may be reduced to ε φψ = (1+ν)/E σ φ sin 2 ψ - ν(σ 11 +σ 22 )/E (2) Alternatively, d φψ = (1+ν)/E σ φ sin 2 ψ d 0 - ν(σ 11 +σ 22 )/E d 0 + d 0 (3) assuming σ 13 =σ 23 =σ 33 =φ=0. ε, d, ν, E and σ are the strain, interplanar spacing, Poisson s ratio, Young s modulus and stress, respectively. The variables and subscripts φ, ψ and 0 refer to the azimuthal angle, tilt angle and strain-free, respectively. The y-intercept of the plot of d φψ vs. sin 2 ψ was taken as the strain free interplanar spacing, d 0. While this selection of d 0 is often not ideal, the effect on the calculated stress is <0.1% for the biaxial stress state [3(B)]. Here, E and ν were taken as 70 GPa and 0.33, respectively. Neutron scattering experiments were carried out on the HB-2 spectrometer at the High Flux Isotope Reactor (HFIR) of the Oak Ridge National Laboratory. The incident neutron beam wavelength was determined by the orientation of the (331) planes of a double-focusing silicon monochromator. The wavelength determination, 2θ zero error and 2θ/channel calibration of the position-sensitive detector was accomplished using a nickel powder standard. In this study, the (311) aluminum peak was located at around 83 2θ with the neutron wavelength of 1.61 Å. The diffracting volume was defined by a parallelepiped with 3 mm long sides and angles between the sides of 90, 90 and 83, for a volume of ~26.8 mm 3. The sample was mounted on an automated X-Y-Z translation table, and the coordinates of the sampling positions were referenced with respect to the surfaces of the sample. The locations of the external surfaces of the sample were verified with an intensity-position scan. Here, the sample is translated relative to the fixed gauge volume. When the scan displays a non-linear intensity decrease, the gauge volume has begun to exit the sample. Macro-residual strain was determined from the shift in the interplanar spacings of diffracting grains relative to that of a strain-free reference material (a free 0.25 inch cube of the parent material). This strain is the average from the diffracting grains within the sampling volume. Stress mapping was done on the main web volume above the bearing split shown in Figure 2(b). RESULTS Figure 3 shows the surface residual stresses in the hoop and radial directions. The applied color scheme was equally scaled for visual comparison of the different heat treatments. The surface hoop stresses show compressive residual stresses decreasing towards zero stress with increasing heat treatment. The reduced compressive residual stress can also be thought of as the addition of tensile residual stress. The stress profile is fairly non-uniform but becomes more uniform with increasing heat treatment. The radial stresses also show a trend of increasing of tension to 248 C then decrease towards zero after 4 hours at 270 C. No explanation can be offered for this behavior, particularly in light of the well-behaved hoop data from the same sample. As before, this stress profile becomes more uniform with increasing heat treatment.

6 Copyright JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume 48. Figure 3. Stress profiles from the surface. The XY grid is the same for each profile with X and Y in mm. The dots mark the measurement locations. Two strain components from the bulk of the as-cast and 230 C heat treated engine blocks were measured with neutron diffraction (see Figure 4). The height of each colored cylinder is proportional to the strain magnitude. Most of the strains are tensile with a maximum strain of +0.09%. Without the measurement of the third orthogonal strain in the X-direction, it was not possible to accurately calculate the residual stresses. In general, the strains are largest near the cylindrical bearing interface and show a left-to-right asymmetry, which is consistent with the fact that the web is connected to a thicker section at one end. This asymmetry is also clearly present in the radial X-ray data (see Figure 3) but of opposite sign. The tensile strains in the Y-direction are reduced with heat treatment, which is opposite to corresponding X-ray data. In the axial or Zdirection, the as-cast bulk strains are tensile and become compressive with 230 C heat treatment. For comparison purposes, the maximum strain crudely corresponds to +63 MPa, assuming the stresses and strains in orthogonal directions are zero. 140

7 Copyright JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume 48. Figure 4. The as-cast and heat-treated (230 C) strain profiles from the bulk. The arrows show the direction of the strain components. The Z direction corresponds to the axial direction while the Y-direction corresponds to a mixture of radial and hoop strains. Tensile strains are in red; compressive strains are blue. The maximum strain above was SUMMARY Residual stresses and strains in as-cast and annealed aluminum engine blocks were mapped using X-ray and neutron diffraction. Automated coordinate-mapping of the radial and hoop strains was accomplished with manual X-Y-Z gantry supporting a large sample X-ray stress analyzer, which is suspended above a computer controlled X-Y-φ sample stage. LabVIEW-based software was developed for improved efficiency, flexibility and quality of data analysis. Neutron strain mapping was performed in two orthogonal directions. Overall, the surface stresses went from compressive to near zero or slightly tensile with increasing heat treatment temperature. In concert, the bulk strains went from tensile to slightly tensile or compressive with increasing heat treatment temperature. Given the complex geometry of the engine block, complete stress relief may not be realistic given asymmetric constraints of the engine block structure. 141

8 Copyright JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume ACKNOWLEDGEMENTS Research sponsored by the Department of Energy, Office of Science Education through the SULI internship program and by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCAR and Vehicle Technologies, as part of the High Temperature Materials Laboratory User Program, Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy under contract number DE AC05-00OR REFERENCES [1] Properties of Cast Aluminum Alloys. ASM Handbook Volume 2, Properties and Selection: Nonferrous Alloys and Special Purpose Materials, ASM International: Materials Park, OH [2] TEC Model 1600 X-ray stress analysis system with SaraTEC TM Windows TM software v.1.64, Technology for Energy Corporation, Lexington Dive, Knoxville, TN 37932, [3] I. C. Noyan and J. B. Cohen, Residual Stress: Measurement by Diffraction and Interpretation, Springer-Verlag: New York, 1987, A, p. 101 and B, pp [4] M. R. James, An Examination of Experimental Techniques in X-ray Residual Stress Analysis, Ph.D. Thesis, Northwestern University: Evanston, IL [5] T. M. Ely, Investigation of the Repeatability of Stress Values Reported by TEC Software, Unpublished work, [6] D. J. Wiesner, Residual Stress Analysis 1.01, Labview based software, ORNL, January 2004.