A Quest for a New Hot Tearing Criterion
- PDF / 457,891 Bytes
- 9 Pages / 594 x 792 pts Page_size
- 81 Downloads / 196 Views
A Quest for a New Hot Tearing Criterion D.G. ESKIN and L. KATGERMAN 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. I.
INTRODUCTION: MECHANISMS OF HOT TEARING
VARIOUS defects of as-cast product are still frequently encountered in casting practice. One of the main defects is hot tearing or hot cracking, or hot shortness. Irrespective of the name, this phenomenon represents the formation of an irreversible failure (crack) in the still semisolid casting. Although in most works hot tearing is considered as a phenomenon linked to the inadequate compensation by melt flow of solidification shrinkage in the presence of thermal stresses, there are more factors that could be involved in the formation of cracks at supersolidus temperatures. From many studies[1–8] started already in the 1950s, and reviewed by Novikov[9] and Sigworth,[10] it appears that hot tears initiate above the solidus temperature and propagate in the interdendritic liquid film. In the course of solidification, the liquid flow through the mushy zone decreases until it becomes insufficient to compensate for the solidification shrinkage at which point the cavities (pores) are formed. Cracks can initiate on these pores. The fracture has a bumpy surface covered with a smooth layer and sometimes with solid bridges that connect or have connected both sides of the crack.[7,8,11–16] Industrial and fundamental studies show that hot tearing occurs in the late stages of solidification when the volume fraction of solid is above 85 to 95 pct and the solid phase is organized in a continuous network of grains. It is also known that a fine grain structure and controlled casting conditions (without large temperature and stress gradients) help to avoid hot cracking. During direct-chill (DC) casting of aluminum alloys, the primary and secondary cooling causes strong macroscopic D.G. ESKIN, Senior Sci
Data Loading...
