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Research Papers

Reducing Residual Stress in 7050 Aluminum Alloy Die Forgings by Heat Treatment

[+] Author and Article Information
J. S. Robinson1

Department of Materials Science and Technology, University of Limerick, Limerick, Irelandjeremy.robinson@ul.ie

D. A. Tanner

Department of Manufacturing and Operations Engineering, Materials and Surface Science Institute, University of Limerick, Limerick, Irelanddavid.tanner@ul.ie

1

Corresponding author.

J. Eng. Mater. Technol 130(3), 031003 (May 22, 2008) (8 pages) doi:10.1115/1.2931150 History: Received June 25, 2007; Revised January 24, 2008; Published May 22, 2008

Aerospace aluminum alloy forgings can have the residual stresses arising from heat treatment reduced by modification to the quench cooling rates and subsequent aging treatments. A series of propeller hubs usually made from the alloy 2014 have been closed die forged from the less quench sensitive alloy 7050. These forgings have been subjected to various quenching and aging treatments in an attempt to improve the balance of mechanical properties with the residual stress magnitudes. These forgings were not amenable to stress relieving by cold compression or stretching. Warm water (60°C) and boiling water quenches are investigated in addition to quenching into molten salt (200°C) and uphill quenching from 196°C. Various dual aging treatments including retrogression and reaging have been evaluated in an attempt to optimize low residual stress magnitudes with mechanical properties. Residual stresses determined by the center hole-drilling strain-gauge method are reported in addition to electrical conductivity, stress corrosion cracking, fracture toughness, initiation fatigue, and tensile mechanical property variations. It was found that quenching into boiling water and salt at 200°C did substantially reduce the residual stress but had only a small detrimental effect on the majority of the properties measured. However, the influence of quench rate on fracture toughness was much more significant. This is attributed to both coarse grain boundary precipitation and heterogeneous precipitation of η on Al3Zr dispersoids within the grains, which promotes easier crack propagation.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Photograph of hub forging with hole-drilling assembly attached. The approximate location of the test pieces is indicated.

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Figure 2

Sample cooling curves for 7050 forgings quenched into boiling water, into water at 60°C (Q60), and into salt at 200°C

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Figure 3

Residual stress and deflection magnitudes plotted as a function of quench type for 7050 forgings heat treated according to Table 2. The asterisk ( *) refers to the forging number.

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Figure 4

Tensile properties for 7050 forgings quenched into water at 60°C and heat treated according to Table 3. A, percentage of elongation; Z, percentage of reduction in area. Errors bars are calculated as a standard deviation based on three samples.

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Figure 5

Tensile properties for 7050 forgings quenched into boiling water and salt and heat treated according to Table 3. A, percentage of elongation; Z, percentage of reduction in area. Errors bars are calculated as a standard deviation based on three samples.

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Figure 6

Fracture toughness properties for forgings quenched at 60°C (fast quench) and into boiling water and salt at 200°C (slow quench)

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Figure 7

Microstructure of 7050 quenched into water at 60°C (top image) and into molten salt at 200°C. The top image has had the contrast enhanced to highlight grain boundaries. This has caused the CuAl2 to appear black: orthophosphoric acid

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Figure 8

SSC data. Failure classification: A, no failures after 40days; B, failures observed between 20 and 40days of testing; C, failures observed within 20days of testing.

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Figure 9

Rotating bending fatigue results for aluminum alloy 7050 forgings heat treated to the conditions outlined in Table 3

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Figure 10

The variation of residual stress with hole depth for two quarter sections heat treated in accordance with 7050-1 and 7050-5

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