Magnetic property effects in highly grain-refined Dy-free Nd-Fe-B sintered magnets

Abstract

Strong permanent magnets (PMs) are critical for electric vehicle (EV) traction motors to reach high power density levels. Although Nd-Fe-B magnets have superior ambient magnetic strength, Dy additions currently provide thermal stability to enable motor operating temperatures of 150-180C. For sustainable PMs for EV motors, replacement of Dy in Nd-Fe-B magnets is needed since Dy is a scarce heavy rare earth (HRE) and is from (essentially) one source. Fortunately, ultrafine (<4µm) grain size also can improve high temperature magnetic properties and increase ambient temperature coercivity in Dy-free Nd-Fe-B magnets, although not without reduced mechanical properties and only in lab-scale samples [1,2]. Our initial work used commercial Dy-free Nd-Fe-B alloy powders which we ball milled conventionally up to 11h, causing average particle size to decrease to 2.1 µm, but average grain size of resulting sintered magnets grew to 3.8 µm, due to grain growth from high temperature sintering (1080C). Intrinsic coercivity Hcj and maximum energy product (BH)max of the ultrafine grained magnets increased up to 13.7 kOe and 43.6 MGOe. While our new work with high energy milling has further refined feedstock powder size, reduction of sintering temperature also is needed to suppress grain growth in magnets and fully exploit ultrafine grain benefits on magnetics. Thus, an extrinsic transient liquid phase (TLP) sintering aid, Pr-30.2at.%Cu (442C eutectic), was studied for consolidation at reduced sintering temperature with suppression of grain growth. A reduced sintering temperature with full heat treatment (FHT) raised coercivity (Fig 1), presumably from suppression of grain growth. When the 1045C sintered sample with FHT (from Fig 1) was annealed at 480C, 5h (Fig. 2), coercivity also rose, presumably from increased grain defect “smoothing” by penetration of residual liquid TLP further into the magnet microstructure. These improved magnetic properties will be correlated with microstructural analysis. Supported by USDOE-EERE-VTO-EDT through Ames Lab contract no. DE-AC02-07CH11358.  Fig 1. Effect of TLP and sintering temperatures on coercivity.  Fig 2. Effect of 480C anneal with TLP on coercivity (sample sintered at 1045C, 5h).