Development of Precipitation Hardening Parameters for High Strength Alloy AA 7068.

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

doi:10.3390/ma13040918

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.

Highlights

AlZnMgCu alloys attain highest strength levels among aluminium alloys and are used in aviation applications [1]
The sequence of endothermic dissolution and exothermic precipitation reactions during heating of alloy 7068 has been analysed for various heating rates
At fast cooling of 100 K/s dilatometry still reveals quenched induced precipitation; While differential scanning calorimetry (DSC) only shows one very broad precipitation peak for a wide range of cooling rates, dilatometry indicates the overlap of several quench induced precipitation reactions; The physical upper critical cooling rate (CCR) at which quench induced precipitation is fully suppressed was determined by differential fast scanning calorimetry (DFSC) analysis to be about 600 to 800 K/s;

Summary

Introduction

AlZnMgCu alloys attain highest strength levels among aluminium alloys and are used in aviation applications [1]. The strength is typically adjusted through precipitation hardening, respectively, age hardening [2,3]. The age hardening process comprises three major steps: Solution annealing, quenching, and ageing [1]. The solution heat treatment intends to dissolve relevant alloying element atoms and is setting the initial structural condition for the following cooling or quenching. Due to quenching the alloying element atoms shall be retained in solution resulting in a supersaturation. From this unstable state, nanometre sized precipitates form during the final ageing treatment, which hinder the dislocation glide and thereby increase the strength of the alloy.

Methods

Results

Conclusion