Measurement of Mechanical Coherency Temperature and Solid Volume Fraction in Al-Zn Alloys Using In Situ X-ray Diffraction During Casting

Abstract

During solidification of metallic alloys, coalescence leads to the formation of solid bridges between grains or grain clusters when both solid and liquid phases are percolated. As such, it represents a key transition with respect to the mechanical behavior of solidifying alloys and to the prediction of solidification cracking. Coalescence starts at the coherency point when the grains begin to touch each other, but are unable to sustain any tensile loads. It ends up at mechanical coherency when the solid phase is sufficiently coalesced to transmit macroscopic tensile strains and stresses. Temperature at mechanical coherency is a major input parameter in numerical modeling of solidification processes as it defines the point at which thermally induced deformations start to generate internal stresses in a casting. This temperature has been determined for Al-Zn alloys using in situ X-ray diffraction during casting in a dog-bone-shaped mold. This setup allows the sample to build up internal stress naturally as its contraction is prevented. The cooling on both extremities of the mold induces a hot spot at the middle of the sample which is irradiated by X-ray. Diffraction patterns were recorded every 0.5 seconds using a detector covering a 426 9 426 mm 2 area. The change of diffraction angles allowed measuring the general decrease of the lattice parameter of the fcc aluminum phase. At high solid volume fraction, a succession of strain/stress build up and release is explained by the formation of hot tears. Mechanical coherency temperatures, 829 K to 866 K (556°C to 593°C), and solid volume fractions, ca. 98 pct, are shown to depend on solidification time for grain refined Al-6.2 wt pct Zn alloys.

Effect of Main Elements (Zn, Mg, and Cu) on Hot Tearing Susceptibility During Direct-Chill Casting of 7xxx Aluminum Alloys

“…Two kinds of experiments are often employed to investigate hot tearing: pilot-scale castings [5,16] and dedicated hot tearing apparatuses. [2,[17][18][19][20][21][22][23] The pilot-scale casting is the most direct approach to achieve goals. However, it is time-consuming and costly.…”

On the Prediction of Hot Tearing in Al-to-Steel Welding by Friction Melt Bonding

“…The mushy zone only starts to behave as a solid at the coalescence temperature. [18] Therefore, the predicted strain rates at higher temperatures are not reliable in the present model. Fig.…”

“…Sweet et al [16] argued that most of the pressure in the interdendritic liquid is developed at temperatures close to coalescence, and the choice of the upper temperature limit thus has a minor effect. Drezet et al [18] performed in situ synchrotron X-ray measurements of solidifying Al-Zn alloys. The coalescence solid fraction was found to be approximately 0.98 after the analysis of the lattice strains because of the macro-straining of the mushy zone.…”

Engineering Science at the I12 Beamline at Diamond Light Source

“…It ends at the rigidity temperature where the grains are sufficiently coalesced that the solid phase is able to transmit macroscopic tensile strains. During the ongoing cooling of the cast, this effect can lead to solidification cracking or so-called "hot tearing," as the rigidity of the structure prevents further thermal contraction [9,10].…”