Dendrite fragmentation during solidification by stress induced remelting

Metals and alloys commonly form dendritic microstructures during solidification. These finely branched crystals seem to be fragile, and indeed occurrence of dendrite fragmentation is very common. Despite a great amount of experimental and theoretical work, dendrite fragmentation cannot be fully explained, experimental results are inconclusive. Current explanations are based on coarsening driven pinch off of dendrite side arms and thermal or constitutional re-heating. None of the established theories takes the influence of stress into account, although there are indications that this could be the deciding factor. Stresses can arise for several reasons, such as gravity acting on the highly branched structures or from shrinkage induced impingement of dendrites.

Why is it important to understand dendrite fragmentation? Casting of metals is still a crucial part of the manufacture industry in general, and in specifically in the UK. Dendrite fragmentation leads to non-conformity when occurring as stray crystals and freckle chains in single crystal turbine blades, or as non uniform grain structures in large vacuum arc re-melted (VAR) ingots of Titanium alloys. The financial loss due to scrap caused by these defects can only be estimated, but goes into the millions of pounds every year. Fragmentation is desired on the other hand when it leads to grain refinement in conventional casting and globular structures during semi-solid processing.

This project aims at developing new insight into the phenomenon of dendrite fragmentation. The research follows the hypothesis that stresses in the growing dendrites facilitate fragmentation. These stresses originate from gravity acting on the highly branched structure, or from external forces induced by fluid flow or other dendrites. Although small in magnitude - it is the sign and distribution of stresses that matters - these stresses change the local equilibrium at the solid-liquid interface. Dendrite side arms therefore detach from the main trunk by local re-melting of the joint. Fragmentation is not a fracture, but a local re-melting process.

This problem is addressed using a phase-field model to simulate the growth of alloy dendrites during solidification. The gravity induced stress distribution in the dendrite will be calculated and coupled to the driving force acting on the solid-liquid interface. This novel model does allow simulating the coupled growth-stress problem and will be used to study which solidification conditions lead to side arm melt-off. Insight from these simulations can then be used to guide the design of validation experiments. In the short term results will be compared against the vast amount of published experimental work on dendrite fragmentation.

The project addresses a problem in casting and solidification of alloys through development and application of modelling.
UK academia has traditionally been strong in solidification science of metals and alloys. This project will strengthen this position, by studying new aspects of the long standing problem of dendrite fragmentation. The developed models will be available and therefore continue to have impact in academia beyond the duration of this project. Future projects can build on these models and enhance them to test new hypothesis, devise new experiments and new solidification processes which are being developed at the moment. The latter refers especially to additive manufacturing, which involves solidification of alloys in previously unprecedented regimes. Here computational tools are of great importance for studying and understanding microstructure formation. This will then have further impact on manufacturing industry in the future.

This puts the manufacturing industry in the front row of industrial beneficiaries of this project. Manufacturers of high value, sophisticated products, such as the aerospace sector, benefit from increased reliability and lower cost due to reduced levels of non-conformity in their high value products. This becomes possible through better knowledge of the underlying mechanisms leading to microstructure formation, and improved modelling tools for process optimisation. This will enhance the competitiveness of the UK manufacture industry.
Society and people in general benefit from having competitive manufacture industry, as this will provide jobs and social security. A knowledge driven society like the UK relies heavily on people developing expertise and skills. This is achieved through involving people at various stages of their career in advanced research projects like this. These are given the chance developing their professional identity through the interaction with industrial and academic partners and peers. The chance for industry to recruit well trained professionals closes this loop.

Impact will be achieved by establishing close working relationship with beneficiaries at a very early stage of the project. Representatives from industry will provide guidance through regular meetings, and will monitor the progress of the project. Outcomes will be made accessible at a very early stage. This involves knowledge as well as computer models and methods. Academic impact will be achieved in a similar way, by seeking close contact and exchange with leading academics in the field. Finally, a symposium organised at the end of the project will provide opportunity to invite other parties to join the discussion of outcomes and comparison with achievements made elsewhere in the world. The aim for the symposium is having an equal mix of academics and industrial representatives.