• Sheridan, Richard
• Farthing, Joseph Gresle
• Appleby, Alice
• Brown, Mangaliso

Abstract

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.<br/><br/>In this work, strip casting was employed to facilitate rapid solidification to develop thin flakes (&lt;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.<br/><br/>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 μm to avoid formation of equiaxed Sm₂TM₁₇ grains through insufficient cooling. <br/>

Topics

• impedance spectroscopy
• surface
• grain
• melt
• precipitate
• casting
• rapid solidification