Relationship between tensile and shear strengths of the mushy zone in solidifying aluminum alloys

Abstract

Strength development in the mushy zone during solidification of three aluminum alloys (Al-4 wt pct Cu, Al-7 wt pct Si-1 wt pct Cu, and Al-7 wt pct Si-4 wt pct Cu) has been measured with two different techniques-horizontal tensile testing and direct shear cell testing. The strength results from the two methods correspond with one another to a much higher degree than suggested by the results presented in the literature. Tensile strength starts to develop at the maximum packing solid fraction, confirmed by the shear strength measurements. The maximum packing fraction represents the point where the internal network structure of the mushy zone is established and ligaments of the network must be broken to rearrange the dendrites. The data indicate a converging trend of the shear and tensile strength at high solid fractions, therefore indicating that the deformation mechanisms are also becoming similar. The results presented in this article suggest that it is possible to develop constitutive equations for the mechanical properties of the mushy zone over the entire solid fraction regime, i.e., from coherency to complete solidification. These equations could be used for the prediction of stress development as well as defect formation.

Rheological behavior of Al-Cu alloys during solidification constitutive modeling, experimental identification, and numerical study

“…Once the effective solid stress tensor is introduced, it is possible to formulate the viscoplastic potential that characterizes the solid skeleton behavior (the elastic response is not detailed here; refer to Reference 18). First, we assume an associated flow rule: [2] that relates the solid phase plastic strain rate to the viscoplastic potential ⍀(ˆs,g s ,T,C) via the volumetric solid fraction g s , the temperature T, and the internal variable C. Neglecting the effect of the third invariant of the effective solid stress tensor, we write the viscoplastic potential expression as [3] where and are the effective pressure (taken positive in compression) on the solid skeleton and the von Mises stress, respectively:…”

“…At this stage, the solid-liquid mixture is defined as a coherent mush (or "mushy zone") and develops shear strength due mostly to dendrite entanglement. [1,2,3] The solid skeleton deforms essentially by rearrangement of dendrites at this stage and shear strength is of the order of tens of kPa. [2,3] For higher solid fractions, dendrites become highly interlocked and deformation of the mush must proceed both by rearrangement and deformation of the dendrites.…”

“…[1,2,3] The solid skeleton deforms essentially by rearrangement of dendrites at this stage and shear strength is of the order of tens of kPa. [2,3] For higher solid fractions, dendrites become highly interlocked and deformation of the mush must proceed both by rearrangement and deformation of the dendrites. The resulting shear strength is of the order of 0.1 to 1 MPa and the mush also starts to develop tensile strength, although such strength is still very low.…”

“…The resulting shear strength is of the order of 0.1 to 1 MPa and the mush also starts to develop tensile strength, although such strength is still very low. [2,3] The solid fraction at which the mush develops such significant shear strength has been termed the "maximum packing solid fraction,"…”

Non-isothermal tensile tests during solidification of Al–Mg–Si–Cu alloys: Mechanical properties in relation to the phenomenon of hot tearing

“…A sharp transition exists between a displacement rate of 0.1 mm s À1 , where the stress reaches values characteristic of the fully solid state before fracture occurs (50 MPa), and higher displacement rates, where the maximum stress is one to two orders of magnitude smaller in the range 1-2 MPa. Such fracture stresses are characteristic of fracture in the semi-solid state (hot tearing) [23], where cracks propagate along thin liquid films constrained between solidifying grains. This is confirmed by observations of the fracture surfaces associated with the low and high displacement rates (Fig.…”