Uniaxial and biaxial deformation characteristics of AA7075-O friction stir welded joints

Abstract

The formability of friction stir welded (FSW) AA7075-O aluminum alloy sheet specimens was tested under a range of warm uniaxial and biaxial loading conditions. To study the effect of FSW process conditions on formability, four groups of specimens were prepared using a range of FSW parameters. Two rotational speeds (500 and 1000 rpm), four travel speeds (4.2, 5.1, 8.5, 12.7 mm/s), and four axial force levels (6895, 7006, 7117, 7346 N) were included in the test matrix. Each specimen had a butt joint centrally located and oriented parallel to the sheet rolling direction. Specimens from each of the four groups were then tested under different combinations of uniaxial (e.g., transverse tensile test) and biaxial loading conditions (bulge test), including at two different strain rates (0.0013 and 0.013 1/s) and three different temperature levels (25, 200, and 300 °C). From the flow curves obtained from each test combination, the relative effect that FSW parameters (rotational speed, travel speed, and axial force) and forming parameters (temperature and strain rate) had on formability was investigated in detail. Mechanical and structural variations of the weld zone were compared with those of the base material. Tool rotational speed was observed to have a major effect on the yield and tensile strength of FSW blanks, with strengths varying by as much as 20% over the range process and forming parameters tested. Stress-strain curves obtained by using hydraulic bulge tests yielded a 5–10% increase in strain values compared with uniaxial tensile test results. In the bulge tests, the fracture zone was only observed to occur at the apex of the dome along the joint line for specimens tested at 300 °C and a strain rate of 0.0013 1/s. Specimens tested under other FSW parameters and process combinations did not fail in this location. In tensile tests, the specimens were only fractured on the welding line under forming conditions of 300 °C and at either the low or the high strain rate. Otherwise, the specimens failed away from the joint line. The maximum dome height, as a measure of formability, was obtained under forming conditions of 200 °C temperature and at a 0.0013 1/s strain rate. This result correlated well with stress-strain curves obtained from uniaxial tension testing.