In situ laser-ultrasonic monitoring of elastic parameters during natural aging in an Al-Zn-Mg-Cu alloy (AA7075) sheet
Georg Watzl, Clemens Grünsteidl, Aurel Arnoldt, Jürgen A. Nietsch, Johannes A. Österreicher
Abstract
Solution heat treatment increases the formability of 7xxx series aluminum alloys by dissolution of nanoscale hardening phases. However, subsequent natural aging at room temperature partly reverses this process within a few hours, posing a logistical challenge to industrial operations as formability decreases and springback increases. Hence, non-destructive monitoring of mechanical properties may help facilitate forming of parts from 7xxx series alloys, but no suitable non-destructive direct measurement technique has been established so far. Here we demonstrate an in situ laser ultrasonic method based on the generation and detection of plate resonances to continuously monitor small changes (<1%) of Young’s modulus, shear modulus, and Poisson’s ratio during natural aging of an age-hardenable aluminum alloy (AA7075) sheet. Additionally, we followed plastic mechanical properties by tensile testing and we intermittently measured electrical conductivity and performed differential scanning calorimetry to assess precipitation during natural aging. We find qualitative correlation between Guinier-Preston zone formation and an increase in elastic moduli and material strength.