HOT TEARING OF ALUMINUM ALLOYS A CRITICAL LITERATURE REVIEW


S. Li and D. Apelian Metal Processing Institute, Worcester Polytechnic Institute, Worcester, MA, USA Copyright © 2011 American Foundry Society Abstract


Hot tearing is a common and severe defect in casting and it is perhaps the pivotal issue defining an alloy’s castability. The subject has been extensively studied through various perspectives for decades and more recently computational models have been developed; however, despite these accomplishments there is confusion in the literature. The governing mechanisms and the control of hot tearing are not totally clear. Experiments show inconsistent results and different opinions exist about what roles processing or alloy factors play. WPI’s Casting Center (ACRC) initiated a major project devoted to developing a reliable experimental


Introduction


Hot tearing is a common and serious defect encountered during solidification of castings. It is also referred to as hot cracking, hot shortness or hot brittleness. Irrespective of the name, it is an irreversible defect that appears as cracks, ei- ther on the surface or inside the casting. Hot tears are gener- ally large and visible to the naked eye. Sometimes, they can also be small and only be observed using magnetic particle inspection and penetrating dyes, etc.1


It generally consists


of a main tear and numerous minor offshoots, which follow intergranular paths, and the failure surface reveals a dendrit- ic morphology.2


sively studied and many test techniques and computational models were developed. Novikov,3 al.5


The subject of hot tearing has been exten- Sigworth,4


and Eskin et have authored reviews on hot tearing. The various stud-


ies show that hot tearing is a complex phenomenon. It lies at the intersection of heat flow, fluid flow and mass flow, and various factors influence its formation. The variables in- clude alloy composition, mold properties, casting design and process parameters, etc.5


trolled casting process mitigate and limit hot tearing.4,6


A fine grain structure and a con- Over


the years much work has been devoted to understanding the mechanism of hot tearing. In general, it is accepted that hot tearing occurs due to shrinkage and thermal deformation developed during solidification. However, whether thermal stress or thermal strain, or strain rate is the controlling fac- tor is still not clear. Moreover, how hot tearing is measured (or controlled) is not standardized and a reliable predictive model is in need.


International Journal of Metalcasting/Winter 11


apparatus/methodology to quantitatively study hot tearing and to use it in studying the effects of various parameters on hot tearing. As part of this endeavor, a comprehensive literature review was conducted. The review focuses on: (i) Theories of hot tearing; (ii) Hot tearing variables; and (iii) Test methods. A summary of hot tearing criteria/models are also provided.


Keywords: hot tearing, aluminum alloys, process parameters, grain structure, hot tearing measurement, hot tearing criteria and models


Theories of Hot Tear Formation Theories Based on Stress, Strain, and Strain Rate


Hot tearing has been investigated since the beginning of 20th century. In the early days much was said about hot tears and how to prevent them, especially in steel castings, however, little was known of the mechanism of formation. The first at- tempt to understand the mechanism for hot tears was carried out in 1928 by Körber and Schitzkowski; they conducted a systematic study of hot tearing in carbon steels.7


In their


study, hot tears were intentionally produced in flanged steel bars by hindered contraction; it was found that hot tears most likely occurred in a temperature range from 1250 to 1300C (2282 to 2372F). Thereafter, it was believed for years that hot tears were formed at temperatures below solidus based on the experiments.8,9


In the early 1930s Briggs and Gezelius10,11 continued Körber


and Schitzkowski’s work and studied the stress evolution during solidification in a steel bar. The bar was designed in a way that there were no sharp thermal gradients in the casting therefore they did not break under contraction. Their work did not show the actual amount of stress necessary to cause hot tears, but intentionally showed the load-carrying ability of steel under hindered contraction.8


In 1936, Verö12 proposed that hot tearing in aluminum alloys


was caused by the stresses developed from the contraction of the primary crystals during solidification of the alloy. Dur-


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