Recent advances in grain refinement of light metals and alloys

• The interdependence of nucleation and growth determines grain size.
• Epitaxial growth and the formation of metastable clusters in the melt may facilitate nucleation.
• Ultrasonic treatment and pulsed magneto-oscillation are effective grain refiners.
• In-situ X-ray radiography is being used to understand nucleation and growth events.
• Additive manufacturing provides the next challenge for grain refinement.

Grain refinement leads, in general, to a decreased tendency to hot tearing, a more dispersed and refined porosity distribution, and improved directional feeding characteristics during solidification. Reduced as-cast grain size can also lead to improved mechanical properties and wrought processing by reducing the recrystallized grain size and achieving a fully recrystallized microstructure. It is now well established that the two key factors controlling grain refinement are the nucleant particles including their potency, size distribution and particle number density, and the rate of development of growth restriction, Q, generated by the alloy chemistry which establishes the undercooling needed to trigger nucleation events and facilitates their survival. The theories underpinning our current understanding of nucleation and grain formation are presented. The application of the latest theories to the light alloys of Al, Mg and Ti is explored as well as their applicability to a range of casting and solidification environments. In addition, processing by the application of physical processes such as external fields and additive manufacturing is discussed. To conclude, the current challenges for the development of reliable grain refining technologies for difficult to refine alloy systems are presented.