A new hot-tearing criterion

Abstract

A new criterion for the appearance of hot tears in metallic alloys is proposed. Based upon a mass balance performed over the liquid and solid phases, it accounts for the tensile deformation of the solid skeleton perpendicular to the growing dendrites and for the induced interdendritic liquid feeding. This model introduces a critical deformation rate ( p,max ) beyond which cavitation, i.e., nucleation ⅐ ε of a first void, occurs. As should be expected, this critical value is an increasing function of the thermal gradient and permeability and a decreasing function of the viscosity. The shrinkage contribution, which is also included in the model, is shown to be of the same order of magnitude as that associated with the tensile deformation of the solid skeleton. A hot-cracking sensitivity (HCS) index is then defined as . When applied to a variable-concentration aluminum-copper alloy, this HCS ⅐Ϫ1 ε p,max criterion can reproduce the typical ''⌳ curves'' previously deduced by Clyne and Davies on a phenomenological basis. The calculated values are in fairly good agreement with those obtained experimentally by Spittle and Cushway for a non-grain-refined alloy. A comparison of this criterion to hot cracks observed in ring-mold solidification tests indicates cavitation depression of a few kilo Pascal and tensile stresses in the coherent mushy zone of a few mega Pascal. These values are discussed in terms of those obtained by other means (coherency measurement, microporosity observation, and simulation). Even though this HCS criterion is based only upon the appearance of a first void and not on its propagation, it sets up for the first time a physically sound basis for the study of hot-crack formation.

Process Optimization of Dual-Laser Beam Welding of Advanced Al-Li Alloys Through Hot Cracking Susceptibility Modeling

“…It is commonly assumed that the dendrites start to bridge together when the solid fraction reaches approximately 98 pct if the eutectic temperature is not yet reached. [5,15,16] Therefore, the integral limits for the A parameter in Eq. [5] are from the point where the solid fraction f s = 0.98, i.e., the so-called coherency temperature, to the liquidus temperature.…”

“…[5,15,16] Therefore, the integral limits for the A parameter in Eq. [5] are from the point where the solid fraction f s = 0.98, i.e., the so-called coherency temperature, to the liquidus temperature. The temperature-dependent fraction of solid can be determined from a thermodynamic calculation alone, using the Scheil-Gulliver assumption, which is known to well represent the solidification of aluminum alloys under standard conditions.…”

“…The most successful of these models for application to aluminum alloys is that developed by Rappaz, Drezet, and Gremaud (RDG). [5] This is a physics-based model that attempts to capture all of the key processes that lead to hot cracking. This model has been validated in a number of aluminum alloy systems.…”

Simulation of Semi-Solid Material Mechanical Behavior Using a Combined Discrete/Finite Element Method

“…[1][2][3][4][5][6][7] The formation of hot tears is similar to porosity formation in the sense that it is linked to a lack of liquid feeding in the mushy zone, but requires additionally shear or tensile deformation in order to separate, or pull apart, the solid network. These deformations occur because of the thermal gradients, solidification shrinkage, solid contraction, and mechanical constraints.…”

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

“…[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. [5] This defect is a spontaneous failure of semi-solid metallic alloys that results in an intergranular fracture profile. It forms during casting near the end of solidification where straining becomes localized and local liquid permeability is very low.…”