Accuracy in microstructure measurements in highly directional processes

Flowforming is an advanced forming technique to produce net shape axisymmetric components, with high dimensional accuracy, based on the simultaneous action of a rotating mandrel and multiple translational rollers. The deformation mechanism has great influence on the uniformity of microstructure, with a gradient in the grain size from the inner to the outer portion of the component cross section, with elongated grains in the rollers feed direction. The highly directional deformation of the microstructure, both along the radial and axial direction, may reach extreme values in the case of large reduction and this aspect may introduce significant uncertainties in the evaluation of the mechanical properties of the final parts (i.e. microhardness, maximum elongation, yield strength) due to the computational methodologies used to estimate the average grain size. The paper focuses on the most frequently used measurement methods to assess the measurement accuracy for the correlation with the local variations of mechanical properties. The reference case for the investigations is the flowforming of AA6082 tubes, processed at different process parameters to change the shape of the grain and the gradient along the radius. Comparisons were carried out with regards to methods that allow measuring the grain size using zero-dimensional features (points), one-dimensional features (lines), and two-dimensional features (areas), respectively with the triple-point count method, the Heyn intercept method and the Jeffries planimetric method.