Hot Cracking Phenomena in Welds II

Abstract

Hot tearing remains a major problem of casting technology despite decades-long efforts to develop a working hot tearing criterion and to implement it into casting process computer simulation. Existing models allow one to calculate the stress–strain situation in a casting (ingot, billet) and to compare it with the chosen hot tearing criterion. Two kinds of hot tearing criteria are available in literature: mechanical and non-mechanical one. The mechanical criteria of hot tearing are derived based on mechanical behaviour semi-solid, and the non-mechanical one is based on other properties of semi-solid. In most successful cases, the simulation shows a relative probability of hot tearing and the sensitivity of this probability to such process parameters as casting speed, casting dimensions, and casting practice. None of the existing criteria, however, can give the quantitative answer on whether the hot crack will appear or not and what will be the extent of hot cracking (position, length, shape). This chapter outlines the requirements for a modern hot tearing criterion as well as the future development of hot tearing research in terms of mechanisms of hot crack nucleation and propagation. 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 semi-solid casting. in Aluminium Alloys Delft University of Technology, Netherlands Institute for Metals Research, Delft, The Netherlands 4 L. Katgerman, D.G. Eskin From many studies [1, 2, 3, 4, 5, 6, 7, 8] started already in the 1950’s, 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 fill initiated cavities so that they can grow further. 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, 12, 13, 14, 15, 16]. Research studies show that hot tearing occurs in the late stages of solidification when the volume fraction of solid is above 85–95% and the solid phase is organized in a continuous network of grains. It is also known that fine grain structures and controlled casting (without large temperature and stress gradients) help to avoid hot cracking. During direct-chill (DC) casting of aluminium alloys, primary and secondary cooling cause strong thermal gradients in the billet/ingot, resulting in uneven thermal contraction in different sections of the billet/ingot. As a result, macroscopic stresses cause distortion of the billet/ingot shape (e.g. butt curl and swell, rolling face pull-in) and/or may trigger hot tearing and cold cracking in the weak sections. The terms “hot” or “cold” refer to the temperature range where the cracking occurs – in the semi-solid mushy zone or below the solidus, respectively. In DC casting, the name “mushy zone” is frequently applied to the entire transition region between liquidus and solidus, which is misleading, as the semi-solid mixture in the top part of the transition region is actually a slurry. Only after the temperature has dropped below the coherency temperature, a real mush is formed. On the microscopic level solidification shrinkage and thermal contraction impose strains and stresses on the solid network in the mushy zone. The deformation behaviour of the mush is very critical for the formation of hot tears. The link between the appearance of hot tears and the mechanical properties in the semi-solid state is obvious and has been explored for decades; see for example reviews [9, 17]. Another important correlation – between the hot cracking susceptibility and the composition of an alloy – has been established on many occasions. A large freezing range of an alloy promotes hot tearing since such an alloy spends a longer time in the vulnerable state in which thin liquid films exist. A lot of efforts have been devoted to the understanding of the hot tearing phenomenon. Compilations of research in this field have been done by Novikov [9], Sigworth [10], and Eskin et al. [17]. Several mechanisms of hot tearing are already suggested in literature. Some of those are outlined in Table 1. In Search of the Prediction of Hot Cracking in Aluminium Alloys 5 Table 1. Summary of hot tearing mechanisms Mechanism Suggested and developed by Ref. Cause of hot tearing Thermal contraction Heine (1935); Pellini (1952); Dobatkin (1948) [18, 2, 19] Liquid film distribution Vero (1936) [20] Liquid pressure drop Prokhorov (1962); Niyama (1977) [37, 39] Vacancy supersaturation Fredriksson et al. (2005) [21]