In situ Neutron Diffraction during Casting: Determination of Rigidity Point in Grain Refined Al-Cu Alloys

Abstract

The rigidity temperature of a solidifying alloy is the temperature at which the solid plus liquid phases are sufficiently coalesced to transmit long range tensile strains and stresses. It determines the point at which thermally induced deformations start to generate internal stresses in a casting. As such, it is a key parameter in numerical modelling of solidification processes and in studying casting defects such as solidification cracking. This temperature has been determined in Al-Cu alloys using in situ neutron diffraction during casting in a dog bone shaped mould. In such a setup, the thermal contraction of the solidifying material is constrained and stresses develop at a hot spot that is irradiated by neutrons. Diffraction peaks are recorded every 11 s using a large detector, and their evolution allows for the determination of the rigidity temperatures. We measured rigidity temperatures equal to 557 °C and 548 °C, depending on cooling rate, for a grain refined Al-13 wt% Cu alloy. At high cooling rate, rigidity is reached during the formation of the eutectic phase and the solid phase is not sufficiently coalesced, i.e., strong enough, to avoid hot tear formation.

<i>In Situ</i> X-Ray Diffraction during Casting: Study of Hot Tearing in Al-Zn Alloys

“…The amount of straining on the hot spot is controlled by modulating cooling of the mould extremities, by preheating the mould with electric heating elements and by changing the length of the central part of the mould, i.e. by changing the amount of straining that is localized at the hot spot [16]. …”

“…With these studies, neutron diffraction at high flux sources appeared to be particularly well suited for stress measurements in aluminum alloys as this metal is very transparent to neutrons. Neutrons were then used to determine in situ the very moment when macroscopic strains and stresses appear in the mushy alloy [16]. As one diffraction peak was recorded only every 11 s, the cooling rate was reduced and rigidity temperatures were measured in grain refined Al-Cu alloys.…”

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

“…The amount of straining on the hot spot is controlled by modulating cooling of the mold extremities, by preheating the mold using electric heating elements and by changing the length of the central part of the mold, i.e., by changing the amount of straining that is localized at the hot spot. [3] B. In Situ X-ray Diffraction…”

“…At the macroscopic level, i.e., at the level of many randomly oriented grains, coalescence must be considered as a transition taking place between coherency (first contact between the grains) and rigidity (ability to transmit tensile strains and stresses), also called mechanical coherency [3] or tensile coherency. [4] If coalescence between some grains is slowed down, i.e., finishes at lower temperatures, the mushy structure becomes particularly sensitive to hot tearing or solidification cracking.…”

“…Neutrons were also used to determine in situ the very moment when macroscopic strains and stresses appear in the mushy alloy. [3] As one diffraction peak was recorded only every 11 seconds, the cooling rate was reduced and rigidity temperatures were measured in grain refined Al-Cu 13 wt pct alloys. The hot tearing tendency was linked to the rigidity temperature, higher or equal to the eutectic temperature, the cooling rate, and the degree of primary phase grain percolation.…”