Presentation Information
[P2-41]Strip Casting of SmCo Alloys to Reduce Manufacture Costs of Sm2(Co,Fe,Zr,Cu)17 Permanent Magnets
*Charlie Lyle Gormly Gardner1, Oliver Peter Brooks 1, Zakareya Nashwan1, Richard Stuart Sheridan1 (1. University of Birmingham (UK))
Keywords:
Strip Casting,Samarium Cobalt 2:17,Dendritic Microstructures
Alloy casting is a vital stage of sintered magnet manufacture, and the microstructure produced can have a substantial effect on the properties that can be achieved. During the solidification process, different microstructures form dependent on their position in the ingot. On the edge, where cooling rate is the greatest, dendritic grains precipitate from the mould wall and grow towards the centre via directional solidification. As the solidification continues inwards, the cooling rate decreases and large equiaxed grains begin to precipitate and grow. This means that careful control of the cooling rate allows for tailoring of the microstructure to achieve the desired grain size and phase balance.
The process of strip casting prioritises cooling rate, producing a microstructure which is comprised mainly of dendrites with little-to-no equiaxed grain growth. During the strip casting process, the molten alloy is poured onto a spinning cooled wheel, imposing a high cooling rate on the solidifying alloy which forms into thin flakes.
This process has been beneficial for NdFeB magnet production, as the dendrites produced are susceptible to hydrogen decrepitation (HD) due to the Nd rich phases at their grain boundaries which expand rapidly under hydrogen absorption. These dendrites fracture transgranularly under the expansion of the boundary phase and break down into highly friable particles that are easily milled for sintering. The high cooling rates also limits precipitation of free iron, improving the sintered magnets final properties without the need for heat treatment.
Casting of Samarium Cobalt (SmCo) magnets is done through conventional book mould casting, causing the formation of an inhomogeneous microstructure. The cast alloy is then mechanically crushed and milled to yield powder suitable for sintering. After sintering into a fully dense body, a solution heat treatment is required to produce a SmCo5 hexagonal (1:5H) and SmCo7 hexagonal (1:7H) microstructure. This is then quenched and aged to precipitate a nanostructure of a Sm2Co17 rhomboidal (2:17R) cells with 1:5H type cell walls.
The microstructure produced from the strip casting of SmCo alloys has shown to consistent of a similar phase composition to that of solution heat treated Sm2Co17 under specific processing conditions. This could make this solution heat treatment stage redundant, reducing production costs. In addition, the 1:5H phase has been seen to be the most susceptible to HD. The 1:5H phase is present as a grain boundary phase to the 1:7H dendrites in strip cast SmCo and could act similarly to the Nd rich phase seen in the NdFeB strip cast product, removing the energy intensive mechanical crushing and utilising low energy HD.
This work investigates three key processing parameters and their effects on strip cast microstructure. Varying the wheel speed was shown to influence the average thickness of the flake as well as its consistency of both thickness and microstructure. Fluid flow showed a similar effect to wheel speed and pairing these two parameters could lead to consistent flakes thicknesses and microstructure.
Wheel texture affected the nucleation behaviour in the flake leading to changes to the microstructure as well as the deviation of flake thicknesses. Influencing the nucleation behaviour of dendrites is key to forming an optimal strip cast microstructure as dendrite widths produced from strip casting of Sm2Co17 are far smaller than that of sinter powder particle sizes. Slight increases in the roughness of the wheel lead to thicker dendrite structures and a reduced nucleation zone thickness, emphasising that slight changes in parameters could have a greater effect on the final product.
Future work will be centred around the production of Sm2Co17 magnets from strip cast material via HD and milling routes. Batches produced via different parameters, and hence, different microstructures will be analysed to determine their effect on the final magnet’s properties.
The process of strip casting prioritises cooling rate, producing a microstructure which is comprised mainly of dendrites with little-to-no equiaxed grain growth. During the strip casting process, the molten alloy is poured onto a spinning cooled wheel, imposing a high cooling rate on the solidifying alloy which forms into thin flakes.
This process has been beneficial for NdFeB magnet production, as the dendrites produced are susceptible to hydrogen decrepitation (HD) due to the Nd rich phases at their grain boundaries which expand rapidly under hydrogen absorption. These dendrites fracture transgranularly under the expansion of the boundary phase and break down into highly friable particles that are easily milled for sintering. The high cooling rates also limits precipitation of free iron, improving the sintered magnets final properties without the need for heat treatment.
Casting of Samarium Cobalt (SmCo) magnets is done through conventional book mould casting, causing the formation of an inhomogeneous microstructure. The cast alloy is then mechanically crushed and milled to yield powder suitable for sintering. After sintering into a fully dense body, a solution heat treatment is required to produce a SmCo5 hexagonal (1:5H) and SmCo7 hexagonal (1:7H) microstructure. This is then quenched and aged to precipitate a nanostructure of a Sm2Co17 rhomboidal (2:17R) cells with 1:5H type cell walls.
The microstructure produced from the strip casting of SmCo alloys has shown to consistent of a similar phase composition to that of solution heat treated Sm2Co17 under specific processing conditions. This could make this solution heat treatment stage redundant, reducing production costs. In addition, the 1:5H phase has been seen to be the most susceptible to HD. The 1:5H phase is present as a grain boundary phase to the 1:7H dendrites in strip cast SmCo and could act similarly to the Nd rich phase seen in the NdFeB strip cast product, removing the energy intensive mechanical crushing and utilising low energy HD.
This work investigates three key processing parameters and their effects on strip cast microstructure. Varying the wheel speed was shown to influence the average thickness of the flake as well as its consistency of both thickness and microstructure. Fluid flow showed a similar effect to wheel speed and pairing these two parameters could lead to consistent flakes thicknesses and microstructure.
Wheel texture affected the nucleation behaviour in the flake leading to changes to the microstructure as well as the deviation of flake thicknesses. Influencing the nucleation behaviour of dendrites is key to forming an optimal strip cast microstructure as dendrite widths produced from strip casting of Sm2Co17 are far smaller than that of sinter powder particle sizes. Slight increases in the roughness of the wheel lead to thicker dendrite structures and a reduced nucleation zone thickness, emphasising that slight changes in parameters could have a greater effect on the final product.
Future work will be centred around the production of Sm2Co17 magnets from strip cast material via HD and milling routes. Batches produced via different parameters, and hence, different microstructures will be analysed to determine their effect on the final magnet’s properties.