Electron and optical microscopy, fractography, calorimetry, hardness, tensile tests and low cycle fatigue tests have been conducted to investigate the effects of final thermomechanical treatments(FTMT) on the metallurgical properties of high-strength weldable Al-Zn-Mg-Mn alloy which is newly developed in our laboratory.
From hardness test and calorimetric study, it has shown that application of cold rolling after the 1st step aging makes the 2nd step peak-aging time much shorter, but deformation more than 10% rolling makes little difference to precipitation process in the following heat treatment.
Results from tensile tests, based on transmission electron microscopy and fractographical studies, it was found that improved strength of FTMT material is attributed to the superposition of hardening effects, from aging precipitation and from dislocations and that fracture mode does not change with the FTMT. Predominatly transgranular fracture mode in its double step peak-aged(without FTMT) condition(T6) is considered to be due to the reduction in stress or strain concentration to the grain boundary by slip-homogenizing effect of Mn and Zr-dispersoids which are uniformly distributed in this alloy.
The FTMT materials showing much shorter LCF lives than the T6 treated condition have inherent defects to fatigue crack initiation, that is inclusions broken during the cold rolling process in the FTMT. The inclusions which are broken in perpendicular to the rolling direction play a significant role in the LCF crack initiation as already-cracked sites or accelerators of the crack initiation.
Nevertheless, for the same rolling degree(20%), higher temperature for the 2nd step peak-aging is desirable for longer fatigue lives due to its effect giving materials better cyclic-strain resistance, which may be originated from the microstructure characterized by precipitates-distribution made up with coarse precipitates and fine precipitates together.
