The casting and quenching in the manufacture of automotive parts from cast aluminium alloys is a common process in components such as cylinder heads or electric motor housings. The final microstructure is typically composed of aluminium dendrites which can reach a size of up to some millimetres.

Non-destructive diffraction methods, such as neutron diffraction (ND) and synchrotron X-ray diffraction (SXRD), usually rely on good statistics (i.e. continuous Debye-Scherrer rings) to determine residual stresses (RS). Therefore, a dendritic microstructure always poses a challenge, which is more pronounced in the case of SXRD, where the gauge volumes are much smaller than in the case of ND (where the gauge volume can reach up to a few mm^3).

A monochromatic high-energy X-ray beam in combination with a conical slit cell (CSC) [1] enables the determination of the strain components over the whole sample thickness with sufficient spatial resolution. Helmholtz-Zentrum Hereon operates the P07-High Energy Materials Science beamline at DESY (Hamburg, Germany), offering the above-mentioned technique. The CSC has several concentric ring slits that are focused on a spot within the sample by their conical shape. The analysis of full diffraction rings enables the simultaneous determination of all strain components in the plane orthogonal to the beam by taking advantage of the 2D detector. This setup allows resolutions that can be customized depending on the beam energy and on the beam size. Nevertheless, the gauge volumes are usually well below 1 mm^3.

Although ND appears to be more suitable for determining RS in components with a dendritic microstructure, this work will demonstrate that the use of a monochromatic beam coupled with a CSC can be advantageous in determining RS of a 25 x 15 x 50 mm^3 cast aluminium block in T4 condition (water quenched) and resolving the stress components associated with the microstructure, which is not possible with ND.

References

[1] P. Staron, T. Fischer, J. Keckes, S. Schratter, T. Hatzenbichler, N. Schell, M. Müller, A. Schreyer, Depth-Resolved Residual Stress Analysis with High-Energy Synchrotron X-Rays Using a Conical Slit Cell, Materials Science Forum, 2014, 768-769, 72-75.