Tear resistance and stress relaxation behavior of high-strength AA7075-T6 aluminum alloy sheet at warm temperatures near 200 °C

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Date

2020-12-09

Authors

Nikolai, Daniel Edward

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Abstract

The use of lightweight metals is of great interest to the automotive industry for improving vehicle efficiency. The automotive industry is particularly interested in lightweight metals with high strength-to-density ratios that can perform well in crash scenarios. To this end, high-strength aluminum alloys are investigated for body-in-white applications. Currently, 6xxx series aluminum alloys are the most commonly used aluminum materials for the body-in-white. AA7075-T6 offers comparable density to and significantly higher strength than the 6xxx series aluminum alloys. However, AA7075-T6 demonstrates low ductility and formability at room temperature, which are barriers to producing and mechanically fastening components for the body-in-white. Prior investigations by Rader et. al. demonstrated that the ductility of AA7075-T6 sheet is approximately doubled at warm temperatures near 200 °C, allowing it to be stamped to complex geometries. However, joining of AA7075-T6 sheet remains a significant challenge. The present study investigates the material behaviors necessary to determine the potential for joining of AA7075-T6 sheet at warm temperatures by self-piercing riveting. Self-piercing riveting is commonly used with the lower strength 6xxx series aluminum alloys in the automotive industry. Tear-resistance and stress-relaxation experiments are conducted to evaluate the potential for successfully implementing of self-pierce riveting in AA7075-T6 sheet at warm temperatures. Tear energy measurements at warm temperatures appropriate for retrogression heat treatments in AA7075-T6 are compared to measurements at room temperature. A four-fold increase in the tear energy of AA7075-T6 at warm temperatures suggests a high possibility for success with self-piercing riveting at these temperatures. Rapid stress relaxation of up to 45% of the flow stress produced during deformation at warm temperatures indicates a potentially significant reduction in residual stresses after deformation, which should reduce spring-back after forming and reduce the chance of delayed cracking in AA7075-T6 sheet after the application of a self-piercing rivet. These experimental results support the potential to successfully apply a self-piercing rivet to AA7075-T6 sheet at a warm temperature while simultaneously retrogressing the material. Because a subsequent reaging heat treatment is known to restore full strength following a retrogression heat treatment, the retrogression riveting and reaging process is proposed as a method to successfully join AA7075-T6 sheet material while retaining the full strength of the T6 condition.

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