«Overcoming Residual Stresses and Machining Distortion in the Production of Aluminum Alloy Satellite Boxes Mandy S. Younger and Kenneth H. Eckelmeyer ...»
Printed November 2007
Overcoming Residual Stresses and
Machining Distortion in the Production
of Aluminum Alloy Satellite Boxes
Mandy S. Younger and Kenneth H. Eckelmeyer
Sandia National Laboratories
Albuquerque, New Mexico 87185 and Livermore, California 94550
Sandia is a multiprogram laboratory operated by Sandia Corporation,
a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.
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Distortion frequently occurs during machining of age hardening aluminum alloys due to residual stresses introduced during the quenching step in the heat treatment process. This report quantifies, compares, and discusses the effectiveness of several methods for minimizing residual stresses and machining distortion in aluminum alloys 7075 and 6061.
ACKNOWLEDGMENTSMany colleagues participated in portions of this study. The authors appreciate the efforts of Gary Gallegos and Don Green, who conducted the heat treatment and quenching experiments; Jack Stephens, Roger Castillo, and Joe Nekoranec, who machined the test samples and measured subsequent distortions; and Tom Crenshaw and John Laing, who conducted the mechanical tests.
We also appreciate the comments and helpful reviews of Jim VanDenAvyle, Don Susan, and Curtis Gibson.
3. RESULTS AND DISCUSSION
3.1 Residual Stresses and Machining Distortion in Fully-Quenched Plates
3.1.1 7075 Aluminum Alloy
3.1.2 6061 Aluminum Alloy
3.2 Stress Relieving During Age Hardening and Overaging
3.2.1 Stress Relieving During Conventional Age Hardening
3.2.2 Stress Relieving During Overaging
3.3 Quench Rate Effects
3.3.1 Effect of Section Thickness on Age-Hardened Strength
3.3.2 Quenching In Warm Water to Reduce Quench-Induced Residual Stresses... 29
3.4 Mechanical Stress Leveling
3.5 The Effects of Cold Bending on Residual Stresses and Machining Distortion............ 39
4. ENGINEERING IMPLICATIONS FOR SATELLITE BOX DESIGNAND PRODUCTION
4.1 6061 Versus 7075 Aluminum Alloys
4.2 Process Options for Minimizing Residual Stresses: Warm Water Quenching Versus Mechanical Stress Relieving Versus Thermal Stress Relieving
4.3 Machining Strategies to Minimize Machining Distortion
4.4 Cold Straightening
FIGURESFigure 1. Distortion in 7075-T6 Plate Electro-Discharge Machined Along Its Centerplane....... 11 Figure 2. Balanced Residual Stresses Example.
Figure 3. Tensile Stress-Strain Curve of As-Quenched 7075
Figure 4. Tensile Stress-Strain Curve of As-Quenched 6061
Figure 5. Residual Stress Increases with Increasing Thickness Up to As-Quenched Yield Strength.
Figure 6. Peak Residual Stresses After Overaging 18 Hours (6061)
Figure 7. Peak Residual Stresses After Overaging 8 Hours (7075)
Figure 8. Calculated Effects of Overaging Temperature and Time on Peak Residual Stresses (6061).
Figure 9. Calculated Effects of Overaging Temperature and Time on Peak Residual Stresses (7075).
Figure 10. Effect of Overaging for 18 Hours on Yield Strength and Ultimate Tensile Strength (6061).
Figure 11. Effect of Overaging for 18 Hours on Yield Strength and Ultimate Tensile Strength (7075).
Figure 12. Effect of Plate Thickness on Yield Strength and Ultimate Tensile Strength (7075-T73 and 6061-T6)
Figure 13. Effect of Quench Water Temperature on Machining Distortion (7075-T73 and 6061-T6, 1 Inch Thick).
Figure 14. Effect of Quench Water Temperature on Peak Residual Stress (7075-T73 and 6061-T6, 1 Inch Thick).
Figure 15. Effect of Quench Water Temperature on Yield and Ultimate Tensile Strength (7075-T73 and 6061-T6, 1 Inch Thick)
Figure 16. Effect of Quench Water Temperature and Thickness on Peak Residual Stress (6061-T6)
Figure 17. Effect of Quench Water Temperature and Thickness on Distortion (6061-T6).
........ 33 Figure 18. Effect of Quench Water Temperature and Thickness on Yield and Ultimate Tensile Strength (6061-T6)
Figure 19. Effect of Quench Water Temperature and Thickness on Peak Residual Stress (7075-T73)
Figure 20. Effect of Quench Water Temperature and Thickness on Distortion (7075-T73).
...... 35 Figure 21. Effect of Quench Water Temperature and Thickness on Yield and Ultimate Tensile Strength (7075-T73)
Figure 22. Stress-Strain Curve
Figure 23. Residual Stress Development During Cold Straightening.
Figure 24. Comparative Residual Stress Profiles.
Figure 25. Maximum Surface and Internal Residual Stresses.
Figure 26. Deflections During and After External Loading.
Figure 27. Returning Distortion When Previously Cold Straightened Plates Are Finish Machined
TABLESTable 1. Measured Mid-Length Distortions and Calculated Peak Residual Stresses in As-Quenched Samples
Table 2. Effects of Age Hardening on Peak Residual Stresses
Table 3. Effects of Stress Leveling in Tension on Peak Longitudinal and Transverse Residual Stresses
Table 4. Effects of Stress Leveling in Compression on Peak Transverse Residual Stresses.
EXECUTIVE SUMMARYDistortion frequently occurs during machining of age-hardening aluminum alloys due to residual stresses that were introduced during the quenching step in the heat treatment process. Options for
reducing these residual stresses have been investigated in two extensively used aluminum alloys:
7075 and 6061. This report quantifies, compares, and discusses the effectiveness of several methods: 1) thermal stress relieving during traditional aging and non-traditional overaging,
2) mechanical stress relieving in tension and compression, and 3) quenching in warm water to minimize initial stress. The results are integrated into recommendations for designing, processing, and machining satellite boxes with various strength requirements.
Residual stress magnitudes in conventionally quenched aluminum alloys increase with increasing section thickness until they are eventually limited by as-quenched yield strength. The substantially higher as-quenched yield strength of 7075 makes it inherently more prone to residual stresses and machining distortion than 6061 in relatively thick sections (0.6 inch). Peak residual stresses are ~22 ksi in as-quenched 7075 plates thicker than 1.3 inches, and ~9 ksi in asquenched 6061 plates thicker than 0.6 inch.
Traditional age hardening is relatively ineffective in reducing quench-induced residual stresses.
Greater stress relief can be obtained by non-traditional overaging, but at the expense of substantial decreases in strength. Mechanical stress leveling in tension removes 80 to 95% of residual stresses with no strength penalty, but is difficult to apply to geometrically complex parts.
Mechanical stress relieving in compression is more applicable to complex shapes, but is less effective in reducing residual stresses. Quenching in pre-heated water reduces residual stresses, particularly at temperatures above 150°F. Hot water quenching can be applied regardless of geometric complexity, but moderate reductions in strength can occur in relatively thick sections of highly quench-rate-sensitive alloys, such as 7075.
Machining distortion occurs when material is removed asymmetrically with respect to the balanced pattern of residual stresses. Options for minimizing machining distortion are discussed, including machining strategies to reduce unbalancing the residual stress patterns characteristic of materials with various processing histories.
The residual stresses introduced by cold straightening are also described. Cold straightening is not recommended because it can result in very high residual stresses, which make the material exceptionally prone to distortion during finish machining as well as potentially susceptible to stress corrosion cracking.
1. INTRODUCTION Quench-induced residual stresses are a notorious cause of distortion during machining of agehardened aluminum alloys. These distortions often make it difficult or impossible to meet tight dimensional tolerances, particularly when large and/or complex parts are being manufactured. In this study, for example, mid-length bending deflections of 0.085 inch occurred when 10-inch-long plates of 1-inch-thick 7075-T6 aluminum alloy were split into two 0.5-inch-thick halves via wire electrical discharge machining. Figure 1 shows an edge-on photograph of a one-inch thick x 10-inch-long plate of 7075-T6 that was electro-discharge wire cut along its center plane. Note the 0.085-inch midlength distortions that occurred in each half when the plate was cut into two pieces and the dramatic extent of this distortion.
Figure 1. Distortion in 7075-T6 Plate Electro-Discharge Machined Along Its Centerplane.
Distortions such as these resulted in very high rejection rates during manufacturing of high strength aluminum alloy frames for satellite mechanical and electrical components. Reducing these rejection rates by overcoming excessive residual stress-induced machining distortion was the impetus for this study.
Residual stresses develop because of non-uniform cooling and the associated contractions that occur during the quench. When relatively thick parts are initially immersed in the quench bath, the surfaces cool first and thus contract more rapidly than the interior. At this time (early in the quench) the hot interior provides little resistance to the contraction of the surfaces – the soft interior plastically deforms to accommodate surface contraction. Later in the quench, however, when the interior cools and wants to contract, its contraction is resisted by the now cold and relatively strong near-surface material. As a result, tensile stresses develop in the interior. The material there wants to contract, but cannot. These tensile interior stresses are balanced by compressive stresses that develop near the surface. These represent the forces that resist contraction of the cooling interior. A symmetric pattern of residual stress develops with maximum compression on each surface and maximum tension along the centerline.
Figure 2 shows patterns of balanced residual stresses through the thickness of a quenched plate with maximum compressive (negative) stresses at each surface and maximum tensile (positive) stresses at the center plane. This illustrates a simple case of a quenched semi-infinite plate.
Residual stress patterns can be much more complex in less regular parts. This report will consider only semi-infinite quenched plates, presuming that these reasonably approximate the distorting elements in satellite boxes.
Figure 2. Balanced Residual Stresses Example.
The magnitudes of the peak compressive and tensile residual stresses in as-quenched material depend on the severity of the quench,1 the thickness of the plate,1 and as will be shown, the yield strength of the material in the as-quenched condition. Typically, residual stresses are quite low in thin sheets and plates, increase with increasing plate thickness, and reach as-quenched yield strength in very thick plates. This occurs because quench uniformity decreases with increasing thickness, i.e., thicker plates exhibit larger surface-to-center temperature differentials during quenching, leading to higher residual stresses.