A novel technique for manufacturing metal-bonded Nd-Fe-B magnets by squeeze casting
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I. INTRODUCTION
SINTERED rare-earth permanent magnets (REPMs) such as SmCo5, Sm2 TM17, and Nd-Fe-B offer high-energy density and resistance to demagnetization but they are expensive, especially if their full range of properties is not required (as is often the case in electrical machines).[1–4] There are many industrial applications of REPMs due to previously mentioned properties. However, REPMs have some practical shortcomings: they are hard and brittle, difficult to produce in thin or intricate shapes, and large section sizes are nonuniform in their properties. Many of these disadvantages can be eliminated by bonded magnets, which are mainly composed of magnet powders and binder materials. There are many variations of bonded magnets according to the combination of powder and binder.[5–12] Rubbers, elastomers, polymers, and low melting point metal and alloys can be used as the binder. Bonded magnets are less expensive and easier to produce, especially in the case of complex-shape parts. Polymer bonded magnets are very useful near room temperature but deteriorate in air at elevated temperatures. Even at the specified maximum use limits, between about ⫹60 ⬚C and ⫹125 ⬚C, the rate of loss of coercivity and remanence is often intolerable. Thermal cycling can also have disastrous consequences.[6,12] Compared to thermal conductivity of polymer-bonded magnet, the high thermal conductivity of metal-matrix permanent magnets allows for easy heat removal (important for many electrical machines and microwave tubes) and would facilitate temperature uniformity in critical applications such as nuclear magnetic resonance (NMR) CAT-scanner magnets. The combination of high thermal conductivity and the ability to dilute the magnetic flux to specified values has already been used to successfully make two prototype traveling wave tube stacks for the United States army.[13] Suzuki[5] reported that zinc-bonded magnets were prepared from the powder by a compression molding process. Strnat et al.[6] used lead-tin solder as a binder. Rodewald et al.[10] reported that the remanent polarization
and the coercivity of the magnet with 5 wt pct Zn additions were 0.66 T and 880 kA/m, respectively. Rowlinson et al.[14] used a rotary forging technique to fabricate soft metal (Al, Zn, Sn, and Cu) bonded Nd-Fe-B magnets. However, most of the reports used powder metallurgical techniques and rotary forging to produce metal-bonded magnets, which were relatively high cost components due to the variety of processing steps employed. Several studies employed squeeze casting to produce metal matrix composites.[15,16] Squeeze casting is a method that applies pressure to force molten alloys into preforms made of ceramic particles. Preforms can be manufactured by a number of known ceramic-processing routes, including injection molding, dry pressing, and slip casting. The discovery of Nd-Fe-B permanent magnet materials in 1983 had kindled technological and scientific interest.[17,18,19] Unfortunately, Nd-Fe-B is highly vulnerable to corrosive attack and their
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