Determination of Coherency and Rigidity Temperatures in Al-Cu Alloys Using In Situ Neutron Diffraction During Casting

Abstract

The rigidity temperature of a solidifying alloy is the temperature at which the solid phase is sufficiently coalesced to transmit tensile stress. It 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 an Al-13 wt.% Cu alloy using in situ neutron diffraction during casting in a dog-bone-shaped mold. This setup allows the sample to build up internal stress naturally as its contraction is not possible. The cooling on both sides of the mold induces a hot spot at the middle of the sample that is irradiated by neutrons. Diffraction patterns are recorded every 11 s using a large detector, and the very first change of diffraction angles allows for the determination of the rigidity temperature. We measured rigidity temperatures equal to 557°C and 548°C depending on the cooling rate for grain refined Al-13 wt.% Cu alloys. At a high cooling rate, rigidity is reached during the formation of the eutectic phase. In this case, the solid phase is not sufficiently coalesced to sustain tensile load and thus cannot avoid hot tear formation.

Solid Fraction Determination at the Rigidity Point by Advanced Thermal Analysis

“…As it can be observed in Figure 2, employing the SF values obtained, a regression coefficient of 0.81 was determined. The results obtained were compared with the scarce literature data [18,19,28,38,39] and were found to be within the range of solid fraction values previously described in the literature. We can observe how a variation of the composition of the alloy at the studied limits promotes a clear variation of the FRP values.…”

“…In the mechanical method, the force used to agitate an impeller inside a vessel where the metal is solidifying is continuously recorded until the impeller stops, and this is the moment that determines the RP [18]. In the in situ neutron diffraction determination, neutron diffraction is measured continuously, but it is an expensive method with less accurate results [19]. The twothermocouple method records the melt temperature at the core and at the inner wall of the test vessel, determining the RP from the differences between the core and wall temperatures versus the time curve.…”

I12: the Joint Engineering, Environment and Processing (JEEP) beamline at Diamond Light Source

“…The equipment is operated at the users' home laboratories, as well as at JEEP. Prominent examples include: a tomography rig for controlled heating, cooling and deformation of samples (Puncreobutr et al, 2012a,b); a gas gun for shock physics experiments (Eakins & Chapman, 2014); molten salt electrowinning apparatus (Styles et al, 2012); aluminium casting apparatus (Drezet et al, 2014); welding rigs; an internal combustion engine (Baimpas et al, 2013); chemical processing equipment (Lester et al, 2006); hydro-thermal reactors (Moorhouse et al, 2012); a magma chamber (Pankhurst et al, 2014); a custom furnace for studying solid oxide fuel cells (Robinson et al, 2014) and various mechanical test rigs (Evans et al, 2012;Horne et al, 2013;Mostafavi et al, 2013;Gussone et al, 2014).…”

A Comprehensive Analysis of Macrosegregation Formation During Twin-Roll Casting

“…This threshold is usually referred to as rigidity point in the literature. [33] On the other hand, the coherency point is defined as the moment at which the solid fraction is high enough such that bridges between crystals start to form, but no tensile loads can be sustained. [33] Such a threshold is not established…”