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

Access full-text files




Nikolai, Daniel Edward

Journal Title

Journal ISSN

Volume Title



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.


LCSH Subject Headings