Development of Precipitation Hardening Parameters for High Strength Alloy AA 7068

Julia Osten; Benjamin Milkereit; Michael Reich; Bin Yang; Armin Springer; Karina Nowak; Olaf Kessler

doi:10.3390/ma13040918

Development of Precipitation Hardening Parameters for High Strength Alloy AA 7068

Materials (Basel). 2020 Feb 19;13(4):918. doi: 10.3390/ma13040918.

Authors

Julia Osten¹, Benjamin Milkereit^{1

2}, Michael Reich¹, Bin Yang¹, Armin Springer³, Karina Nowak⁴, Olaf Kessler^{1

2}

Affiliations

¹ Materials Science, Faculty of Mechanical Engineering and Marine Technology, University of Rostock, Justus-von-Liebig-Weg 2, 18059 Rostock, Germany.
² Competence Centre °CALOR, Department Life, Light & Matter, Faculty of Interdisciplinary Research, University of Rostock, Albert-Einstein-Str. 25, 18059 Rostock, Germany.
³ Electron Microscopic Centre, University Medical Centre Rostock, Strempelstraße 14, 18057 Rostock, Germany.
⁴ Fraunhofer Research Institution for Large Structures in Production Engineering IGP, Albert Einstein-Str. 30, 18059 Rostock, Germany.

Abstract

The mechanical properties after age hardening heat treatment and the kinetics of related phase transformations of high strength AlZnMgCu alloy AA 7068 were investigated. The experimental work includes differential scanning calorimetry (DSC), differential fast scanning calorimetry (DFSC), sophisticated differential dilatometry (DIL), scanning electron microscopy (SEM), as well as hardness and tensile tests. For the kinetic analysis of quench induced precipitation by dilatometry new metrological methods and evaluation procedures were established. Using DSC, dissolution behaviour during heating to solution annealing temperature was investigated. These experiments allowed for identification of the appropriate temperature and duration for the solution heat treatment. Continuous cooling experiments in DSC, DFSC, and DIL determined the kinetics of quench induced precipitation. DSC and DIL revealed several overlapping precipitation reactions. The critical cooling rate for a complete supersaturation of the solid solution has been identified to be 600 to 800 K/s. At slightly subcritical cooling rates quench induced precipitation results in a direct hardening effect resulting in a technological critical cooling rate of about 100 K/s, i.e., the hardness after ageing reaches a saturation level for cooling rates faster than 100 K/s. Maximum yield strength of above 600 MPa and tensile strength of up to 650 MPa were attained.

Keywords: AA 7068; AlZnMgCu; aluminium alloys; differential fast scanning calorimetry (DFSC); differential scanning calorimetry (DSC); dilatometry; precipitation hardening.