High resolution cone beam X-ray computed tomography of 3D-microstructures of cast Al-alloys

X-ray computed tomography (XCT) has become a very important method for non-destructive 3D-characterisation of materials. XCT systems with cone beam geometry, micro- or nano-focus tubes and matrix detectors are increasingly used in research and non-destructive testing. Spatial resolutions down to 1 μm can be reached with such XCT-systems for heterogeneities in metals with high absorption contrast. High resolution cone beam XCT is applied to five different Al-alloys: AlMg5Si7, AlCu4Mg1, AlZn6Mg2Cu2, AlZn8Mg2Cu2 and AlSi12Ni1. Up to four different types of inhomogeneities are segmented in one alloy using voxel sizes between (0.4 μm)³ and (2.3 μm)³. Target metallography and elemental analysis by energy dispersive X-ray analysis are used to identify the inhomogeneities. The possibilities and restrictions of XCT applied to Al-alloys are discussed. AlMg5Si7 XCT-data with a voxel size of (0.4 μm)³ show inhomogeneities with brighter grey-values than the Al-matrix identified as elongated Fe-aluminides, and those with lower grey-values identified as pores and Mg₂Si-particles with a "Chinese script-like" structure. Higher-absorbing interdendritic Al-Al₂Cu-eutectic regions appear brighter than the Al-dendrites in the CT-data of AlCu4Mg1 with (1.1 μm)3/voxel, whereas pores > 4 μm appear darker than the Al-matrix. The size and the 3D-structure of the α-Al dendrite arms with a diameter of 50-100 μm are determined in samples from chill cast billets of AlCu4Mg1 and AlZn6Mg2Cu2 alloys. The irregular interdendritic regions containing eutectic segregations with Cu- and Zn-rich phases are > 5 μm wide. Equally absorbing primary equi-axed Al₃(Sc, Zr) particles > 5 μm are distinguished in the centres of the dendrites by the level of sphericity values. The distribution of Ni- and Fe-aluminides in a squeeze cast AlSi12Ni1-alloy is imaged with (0.4 μm)3/voxel, but the Si-phase cannot be segmented.