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Investigation of the effect of equal channel angular pressing on the microstructural and magnetic properties of Nd-Fe-B alloys
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posted on 23.01.2017by Onal, Ezgi
Thermo-mechanical processing is used to produce anisotropic Nd2Fe14B based permanent magnets having high energy products. Among thermo-mechanical processing methods, equal channel angular pressing has a potential to be an alternative method to die-upsetting which is most commonly used owing to its effectiveness in grain alignment. Moreover, ECAP allows virtually unlimited strain and manipulation of texture by processing route since the cross-section of the sample does not change during pressing.
For the first time, a detailed investigation was done on the effect of ECAP-BP process on the magnetic properties and macrotexture of the Nd2Fe14B based permanent magnets. The relationship between texture formation and alignment of the easy axis is established by studying the texture and magnetic properties of the sample after pressed by ECAP-BP.
ECAP-BP process was performed on MQP 13-9, MQP 15-7 and MQU-F1 (Nd13.5Fe73.8Co6.7B5.6Ga0.4) commercial melt-spun isotropic alloys. The effectiveness of ECAP-BP was more dominant in MQU-F1 product since it`s almost stoichiometric Nd2Fe14B whereas MQP products are sub-stoichiometric. The best magnetic properties and texture formation was observed when the MQU-F1 powders were pressed at 773 K with a fixed back pressure of 0.5 GPa. The (001) pole figure for this sample shows a high intensity of 5.5 located at about 30° from PD towards the pole which corresponds to the exit direction of ECAP. Hence, the c axis of the Nd2Fe14B grains appears to align about 60° from ED towards PD, this direction of the c axis alignment is approximately right angles away from the shear band associated with the ECAP process. Following the identification of the easy axis direction by the macrotexture analysis, a cube specimen was cut from the exit channel section with one of the cube edge directions parallel to the c axis texture direction. The highest remanent magnetization (107.3 Am2/kg) is obtained when the field is applied along the c axis texture direction and the maximum energy product [(BH)max] measured along this axis is 156.7 kJ/m3 while the (BH)max value is 99.6 kJ/m3 for the isotropic powder sample, demonstrating the benefit of the texture development upon equal-channel angular pressing.
After confirming the effectiveness of ECAP-BP on the Nd13.5Fe73.8Ga0.4 Co6.7B5.6 powders, different processing parameters effect on the magnetic properties and macrotexture were investigated. Temperature and back pressure were changed systematically. Temperature effect was studied with pressings at 723 K, 773 K and 823 K with a fixed back pressure of 0.5 GPa. The die used in this project was made of tool steel, so the upper limit for the maximum processing temperature was 823 K. Back pressure effect was tested at 773 K and 823 K by changing it from 0.3 GPa to 1.3 GPa. The upper limit was determined by the upper punch used as the punch cannot carry pressures higher than 5.0 GPa. It is observed that rather than back pressure, the difference between forward and back pressures is more effective on the magnetic properties and texture of the MQU-F1 sample pressed by ECAP. Moreover, the change in processing temperature is more influential to the magnetic properties when the effect of these two processing parameters is compared.
Multiple passes were studied by following Route A and Route C at 823 K with a back pressure of 0.5 GPa. After the second passage of the sample following Route A, due to no rotation of the sample between each pass, the shear band was elongated and the 001 pole figure of this sample shows an extended texture formation about 60°-900 from ED towards PD. The coercivity measured along the PD was smaller than that of single pressed sample, indicating the grain refinement after each pass with ECAP process. Sample pressed twice with Route C, 1800 rotation of the sample after each pass around its axis, lost the extent of the texture imposed after the first pass as shear band restores itself at every even numbered passage with Route C. This yields to a lower remanence magnetization and maximum energy product of the sample. Route A has a potential to increase the grain alignment if tried more than twice.
Copper tube casing was studied by sealing powders in the copper tubes prior to press. As a result of decreased friction when using copper tube casing, much higher back pressures, 2.3 GPa was tried. Still, a compaction problem was observed with copper tube casing as the pressures were not enough and copper was filling the dead zone which is the channels intersection part. As a result, the intersection angle was not anymore 900 for the actual powder. The effect of ECAP-BP was lost when using copper tube casing, the remanence measured along PD was around 82 Am2/kg whereas it is around 104 Am2/kg for the sample pressed by conventional ECAP-BP.
More studies should be done with increasing temperature, or doing multiple passes and increasing the strain imposed into the sample. To apply suggested work, firstly computer controlled ECAP die in which it is possible to control not only the forward and back pressures but also the speed of the upper and back punches should be used.