Strip Casting of Sm2TM17-type Alloys for Production of the Metastable SmTM7 Phase

Conventional book casting of Sm2TM17-type alloys (where TM=Co,Fe,Cu,Zr) leads to a coarse, highly segregated microstructure, predominantly due to slow, variable cooling rate from the mould surface towards the centre of the ingot. These cast alloys require a long homogenisation treatment to remove this segregation and develop a super-saturated, metastable SmTM7-type hexagonal phase. This SmTM7 phase is phase is a vital precursor phase for magnet production in order to precipitate the Sm2TM17 rhombohedral and SmTM5 hexagonal phases required to develop the cellular structure responsible for high magnetic properties.
In this work, strip casting was employed to facilitate rapid solidification to develop thin flakes (<0.5 mm thick) with a columnar grain structure. Rapid cooling has the potential to produce a homogenous microstructure consisting predominantly of the metastable SmTM7 phase. This could remove or significantly reduce the need for the energy-intensive homogenisation treatment.
This paper investigates the effect of wheel speed (and hence cooling rate) on flake thickness, microstructure and phase balance of the cast alloys. It was shown that for wheel speeds between 1.1-3.0 m/s the microstructure showed large variation, however, in all cases evidence of the columnar SmTM7 phase was presented. The adhesion between the melt and the wheel was critical for nucleation of SmTM7 grains and the wheel speed controlled the thickness of the flake. It was determined that in order to achieve a homogenous columnar SmTM7 structure, the maximum flake thickness should be limited to 280 m to avoid formation of equiaxed Sm2TM17 grains through insufficient cooling.