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.

It is well known that ultrasonic melt treatment (UST) promotes grain refinement in aluminum alloys. Cavitation-aided grain refinement has been studied for many years; however, it is still not being applied commercially. The current article summarizes the results of experimental work performed on various alloying systems at different stages of solidification. The influence of UST parameters and solidification conditions on the final grain structure is analyzed. It was found that small additions of zirconium and titanium can significantly increase the efficiency of UST, under the stipulation that grain refinement is performed in the temperature range of primary solidification of Al₃Zr. The possible mechanisms for this effect are discussed.

Ultrasonic (cavitation) melt processing attracts considerable interest from both academic and industrial communities as a promising route to provide clean, environment friendly and energy efficient solutions for some of the core issues of the metal casting industry, such as improving melt quality and providing structure refinement. In the last 5 years, the authors undertook an extensive research programme into fundamental mechanisms of cavitation melt processing using state-of-the-art and unique facilities and methodologies. This overview summarises the recent results on the evaluation of acoustic pressure and melt flows in the treated melt, direct observations and quantitative analysis of cavitation in liquid aluminium alloys, in-situ and ex-situ studies of the nucleation, growth and fragmentation of intermetallics, and de-agglomeration of particles. These results provide valuable new insights and knowledge that are essential for upscaling ultrasonic melt processing to industrial level.