Like at Sylatech, every founder has known, that turbulence somehow degrades the quality of castings, especially aluminium alloy castings, but no-one really knew why.
Over the years since circa 1990 however, it became evident that the problem has arisen due to the folding of the surface oxide film, of liquid metal, into the liquid metal itself. This entrainment of the surface oxide into the melt, automatically creates doubled-over films, which cannot bond together, and therefore act as cracks in the liquid melt. Although the founder attempted to control everything during manufacture, including temperature, composition, and mould quality etc., turbulence was not on the list. The result has been a disappointing unreliability of castings, due to the content of cracks, varying from batch to batch, and from foundry to foundry. Consequently, the industry and the reputation of the casting process in general has suffered.
Assessing the root of the problem
If we accept all the evidence that turbulent pouring of castings can fill the liquid with a blizzard of cracks, we are then able to explain the numerous problems suffered by the founder, even before the casting has time to solidify.
The cracks (we shall call them ‘bifilms’ – the name is a reminder that they are double) float about in the liquid for minutes or hours. Hydrogen in solution in the metal, can diffuse into the gap between the films, inflating them to create visible porosity. Later, while the casting is still only half solidified, contraction strain can open these bifilms, creating the serious defect we know as ‘hot tearing’, which can scrap a casting. If these problems were not bad enough, when completely solid and tensile tested, the population of cracks lowers the strength and the ductility, and thus would fail the required specification, and so rejected back into the melt, or if accepted, risking failure in service.
These are problems which the foundry industry has attempted to grapple with for years, and part of the problem has been the fact that the bifilms are usually so thin, sometimes merely a few molecules thick, that they are often difficult to detect. Thus is a key reason for the defective behaviour of products being a mystery.
Addressing the casting challenge
At Sylatech, through the integration of simulation software in our casting process, we take a detailed engineering approach to address the technical challenge of casting. Using its modelling software, Sylatech can optimise the design of its running system, and through monitoring flow, it can make design modifications to minimise metal turbulence. Through the use of rapid prototyping and printing of patterns via Voxeljet, cast components can be designed and quickly trialled for optimal performance. Sylatech’s castings achieve reliability and properties which overtake those of forgings and wrought metals, at no extra cost, and customers can now enjoy castings that they can trust.
For further information or to discuss your casting requirements, please contact our engineering team.
