Determining the Critical Fracture Stress of Al Dendrites near the Melting Point via Synchrotron X-ray Imaging

doi:10.1007/s40195-023-01531-w

Abstract

Abstract:

Dendrites are the most common microstructural features in the cast metals, significantly affecting the structure integrity and mechanical properties of the castings. In this study, the in situ synchrotron X-ray radiographic and tomographic imaging techniques were combined to evaluate the critical fracture stress of the growing dendrite tip during the solidification of an Al-15 wt% Cu alloy under an external electromagnetic force. Two dendritic 3D models have been proposed to simulate the dendrite 3D morphologic characteristics and thus revealed that the critical fracture stresses of the Al dendrites at temperatures close to its melting point were in the range of 0.5 kPa-0.05 MPa. The present results demonstrate the feasibility of measuring the high-temperature mechanical properties of the metallic dendrites.

Key words: Synchrotron X-ray radiography, Tomography, Al dendrite, Fragmentation, Critical fracture stress

Ling Qin, Zhiguo Zhang, Baisong Guo, Wei Li, Jiawei Mi. Determining the Critical Fracture Stress of Al Dendrites near the Melting Point via Synchrotron X-ray Imaging[J]. Acta Metallurgica Sinica (English Letters), 2023, 36(5): 857-864.

Figures/Tables 7

Fig. 1 a, b Time-evolved individual 3D dendrite morphology evolution and c, d the corresponding 2D cross-section slices selected from the adjacent in situ synchrotron X-ray tomographic scans at t = t0; and t = t0 + 120 s, respectively [17], indicating the progressive remelting and necking at the roots of the secondary dendrite arms without any external field

Fig. 2 Time-resolved X-ray radiographic images: a the typical dendrite growth under directional solidification with external electromagnetic fields; b the bending and breaking down of the dendrite tip by the external forces, i.e. the electromagnetic force and the force due to the density variation of the dendrite tip and the melt; c and d the oscillation of the dendrite tip in the horizontal direction

Fig. 3 Measured displacement and fitting curves of the dendrite tip after its detachment from the main trunk of the dendrite: a displacement versus time curve in the horizontal direction and b displacement versus time curve in the vertical direction

Fig. 4 Shape characteristics of the dendrite tip: a the 2D radiographic image for the dendrite with a detaching dendrite tip; b Model I: the real dendrite 3D morphology with a similar projection morphology of the dendrite tip to that in the 2D radiographic image; c Model II: the 3D morphology of the dendrite reconstructed from its 2D pixel intensities

Table 1 Three-dimensional shape characteristic parameters of the dendrite tip in the proposed models

Shape characteristics	Model I	Model II
3D volume V (μm³)	1.44 × 10⁵	1.66 × 10⁶
X-projection area A_p_x (μm²)	2.81 × 10³	4.63 × 10⁴
Y-projection area A_p_y (μm²)	3.30 × 10³	5.26 × 10⁴
Z-projection area A_p_z (μm²)	8.99 × 10³	8.99 × 10³
3D equivalent diameter (μm)$D = 2 \times \left( {\frac{3V}{{4\pi }}} \right)^{\frac{1}{3}}$	65	147
X equivalent diameter (μm)$d_{x} = \left( {\frac{{4A_{x} }}{\pi }} \right)^{\frac{1}{2}}$	60	243
Y equivalent diameter (μm)$d_{y} = \left( {\frac{{4A_{y} }}{\pi }} \right)^{\frac{1}{2}}$	65	259
Z equivalent diameter (μm)$d_{z} = \left( {\frac{{4A_{z} }}{\pi }} \right)^{\frac{1}{2}}$	107	107

Fig. 5 A geometric model for calculating the critical fracture stress of dendrite tip induced by the resultant force from buoyancy, gravity and electromagnetic force

Table 2 Calculated critical fracture stress under different conditions

Root diameter (μm)	Model I (Pa)	Model II (Pa)
5	4.0 × 10³	5.0 × 10⁴
10	5.0 × 10²	6.0 × 10³

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