Observation of crack initiation during hot tearing

Davidson, Cameron; Viano, David M.; Lü, Liming; StJohn, David H.

doi:10.1179/136404606225023291

Cited by 54 publications

(23 citation statements)

References 5 publications

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“…As can be seen, M values at g s < 0.9 are insignificant, but increase sharply beginning at g s = 0.9; with increasing strain rate the rapid increase in M occurs at lower g s . As shown by many previous authors (see, for example, Davidson et al [31]), hot tearing susceptibility is significantly increased at g s > 0.9. These results confirm those findings, and indicate that it is the feeding ability of the mushy zone that determines hot tear formation.…”

supporting

confidence: 61%

Three-dimensional granular model of semi-solid metallic alloys undergoing solidification: Fluid flow and localization of feeding

et al. 2012

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This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues.Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. AbstractA three-dimensional (3-D) granular model which simulates fluid flow within solidifying alloys with a globular microstructure, such as that found in grain refined Al alloys, is presented. The model geometry within a representative volume element (RVE) consists of a set of prismatic triangular elements representing the intergranular liquid channels. The pressure field within the liquid channels is calculated using a finite elements (FEs) method assuming a Poiseuille flow within each channel and flow conservation at triple lines. The fluid flow is induced by solidification shrinkage and openings at grain boundaries due to deformation of the coherent solid. The granular model predictions are validated against bulk data calculated with averaging techniques. The results show that a fluid flow simulation of globular semi-solid materials is able to reproduce both a map of the 3-D intergranular pressure and the localization of feeding within the mushy zone. A new hot cracking sensitivity coefficient is then proposed. Based on a mass balance performed over a solidifying isothermal volume element, this coefficient accounts for tensile deformation of the semi-solid domain and for the induced intergranular liquid feeding. The fluid flow model is then used to calculate the pressure drop in the mushy zone during the direct chill casting of aluminum alloy billets. The predicted pressure demonstrates that deep in the mushy zone where the permeability is low the local pressure can be significantly lower than the pressure predicted by averaging techniques.

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confidence: 61%

Three-dimensional granular model of semi-solid metallic alloys undergoing solidification: Fluid flow and localization of feeding

et al. 2012

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“…The problem of the crack initiation is today solved by an educated guess, as only recently experimental observations on the crack nucleation upon natural hot tearing have started to emerge: first, for transparent analogues [16] and then for real metals. [58] Usually, the development of a hot crack is studied on samples with a notch, hence, with the artificial crack initiator. [46,64] Based on these observations and ''postmortem'' examination of hot tear surfaces, the following crack nuclei have been suggested: (1) liquid film or liquid pool; (2) pore or series of pores; (3) grain boundary located in the place of stress concentration; and (4) inclusions that can be easily separated from the surrounding liquid or solid phase, e.g., intermetallic particle or oxide film.…”

Section: Outline Of Hot Tearing Mechanisms As a Base Of A New Homentioning

confidence: 99%

A Quest for a New Hot Tearing Criterion

Eskin¹,

Katgerman

2007

Metall Mater Trans A

228

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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.

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“…[14,15,[23][24][25] There is still no general conclusion on the fraction solid at the onset of hot tearing. Further work is needed before such a conclusion can be drawn.…”

Section: Comparison With Reported Fraction Solid At Hot Tearingmentioning

confidence: 99%

“…A video camera was used to detect the onset of hot tearing, and with the Scheil model, the corresponding fraction solid in Al-0.5Cu was reported to be 0.93. [23] Acoustic emission was also used to detect the onset of hot tearing, and based on the evolution of the latent heat during solidification instead of the Scheil model, the fraction solid was found to be 0.71 to 0.99 in ring casting of 1050 Al. [25] The load cell has been most frequently used to detect the onset of hot tearing so far, and the corresponding fraction solid has been determined based on the Scheil model.…”

Section: Comparison With Reported Fraction Solid At Hot Tearingmentioning

confidence: 99%

Onset of Hot Tearing in Ternary Mg-Al-Sr Alloy Castings

Cao

Haygood

Kou

2010

Metall Mater Trans A

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Ternary Mg-Al-Sr alloys are the base of a few new creep-resistant, lightweight Mg alloys for automobiles. Hot tearing of Mg-Al-Sr alloys was studied by constrained rod casting (CRC) in a steel mold equipped with a load cell and a thermocouple. The alloys investigated included, in order of decreasing hot tearing susceptibility, Mg-4Al-1.5Sr, Mg-6Al-1.5Sr, Mg-8Al-1.5Sr, and Mg-8Al-3Sr. Two different molds were used, one for 8.7-mm-diameter rods and the other 7.9 mm. The cooling rate was varied by varying mold preheating from 523 K (250°C) to 658 K (385°C) and mold insulation. The load curve showed a clear peak when hot tearing occurred in all but Mg-8Al-3Sr due to its high resistance to hot tearing. From the peak and the cooling curve, the temperature at which hot tearing occurred was determined and found to decrease with increasing Al content from 4 to 8 pct. For a specific alloy, the hot tearing onset temperature did not change significantly with the rod diameter or mold preheating, at least within the experimental conditions used. The Scheil solidification model was used to estimate the fraction solid at the onset of hot tearing. It was found that hot tearing occurred near the end of primary solidification L fi a (Mg) and that the fraction solid at which hot tearing occurred decreased with increasing Al content. The validity of the Scheil model was discussed.

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Observation of crack initiation during hot tearing

Cited by 54 publications

References 5 publications

Three-dimensional granular model of semi-solid metallic alloys undergoing solidification: Fluid flow and localization of feeding

Three-dimensional granular model of semi-solid metallic alloys undergoing solidification: Fluid flow and localization of feeding

A Quest for a New Hot Tearing Criterion

Onset of Hot Tearing in Ternary Mg-Al-Sr Alloy Castings

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