Determination of strain during hot tearing by image correlation

The evolution of strain and strain at the onset of localization resulting in a hot tear has been determined for alloys AA6111, AA3104, CA31218, and Al-0.5 wt pct Cu during solidification. A bar constrained at both ends was solidified with a window above the hot spot region to allow observation of hot tear formation and growth. A high-resolution camera was used to capture images of the surface during solidification. In addition, a thermocouple cast into the sample was used to acquire temperature data in proximity to the hot tear. The images were analyzed using digital image correlation software to measure the evolution of displacement and strain based on tracking the motion of discrete features on the as-cast surface of the sample. Strain data was determined starting from the point immediately after solidification shrinkage ends (the metal pulls away from the glass) and finishing when the first sign of a crack or hot tear is visible to the eye. The minimum strain at which strain localization occurs was found to be > 0.0069 in AA6111, > 0.0123 for AA3104, > 0.0047 for CA31218, and > 0.0021 for Al-0.5 pct Cu

Quantitative assessment of deformation-induced damage in a Semisolid aluminum alloy via X-ray microtomography

International audienceSemisolid tensile testing combined with X-ray microtomography (XMT) was used to characterize the development of internal damage as a function of strain in an aluminum-magnesium alloy, AA5182. Novel techniques were developed to allow the quantification of both the size evolution and orientation of the damage to determine mechanisms controlling the early stage growth and localization. During the initial stages of semisolid deformation, it was observed that strain was accommodated by both the growth of as-cast porosity and the detection of new damage-based voids. As the volume fraction of damage increases, the growth of voids occurs in an orientation perpendicular to the loading direction, both through expansion within the grain boundary liquid and void coalescence. The damage then localizes, causing failure

A Quest for a New Hot Tearing Criterion

Hot tearing remains a major problem of casting technology despite decades-long efforts to develop working hot tearing criteria and to implement those into casting process computer simulation. Existing models allow one to calculate the stress-strain and temperature situation in a casting (ingot, billet) and to compare those with the chosen hot tearing criterion. In most successful cases, the simulation shows the relative probability of hot tearing and the sensitivity of this probability to such process parameters as casting speed, casting dimensions, and casting recipe. None of the existing criteria, however, can give the answer on whether the hot crack will appear or not and what will be the extent of hot cracking (position, length, shape). This article outlines the requirements for a modern hot tearing model and a criterion based on this model as well as the future development of hot tearing research in terms of mechanisms of hot crack nucleation and propagation. It is suggested that the new model and criterion should take into account different mechanisms of hot tearing that are operational at different stages of solidification and be based on fracture mechanics, i.e., include the mechanisms of nucleation and propagation of a crack.Materials Science and EngineeringMechanical, Maritime and Materials Engineerin