Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Powder pretreatment
Reexamination Certificate
2002-04-26
2004-08-03
Jenkins, Daniel (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Powder pretreatment
C419S036000, C148S105000, C148S108000, C148S513000
Reexamination Certificate
active
06770242
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
REFERENCE TO A MICROFICHE APPENDIX
Not Applicable.
BACKGROUND
1. Field of Invention
The present invention relates to rare earth permanent magnets. More specifically, the present invention relates to rare earth permanent magnets for voice coil actuator motors used to actuate head-arm assemblies in small form factor disk drives.
2. Description of Prior Art
The rapidly expanding use of personal computers and the ensuing demand for increased functionality, performance and portability, places tremendous demands on the disk drive systems of such computers.
The major improvement in this direction has been the development of Winchester type disk drive systems in replacement of the earlier used floppy disk drive systems. The former provide much higher capacity and faster speed of operation, factors which are of great importance for the effectiveness of personal computers running advanced software packages.
Winchester disk drives typically utilize a plurality of rotating storage disks and data tranducers to interact with each storage disk. An E-block having a plurality of spaced apart actuator arms maintains the data transducers proximate to each storage disk. Typically, a voice coil motor (VCM) is used for actuating the E-block and the data transducers relative to the storage disks.
The portability requirement of notebook computers places a premium on the ‘form factor’, the overall dimensions of the hermetic enclosure housing the disk drive. As a result, space available to lodge the VCM is limited. This situation inherently requires the use of smaller VCM magnets. Unfortunately, this in turn reduces the strength of the magnets and increases data retrieval time as the efficiency of the actuator motor is directly proportional to the strength of the VCM magnets.
The need to rapidly access information has further led to disk drives having storage disks which are rotated at ever increasing speeds and requiring an actuator motor which moves the E-block at ever increasing rates. Unfortunately, this typically results in increased heat, noise and power consumption of the disk drive.
VCM magnets must also hold their strength under adverse conditions, lest they will change in their performance. Hence, high coercivity and temperature stability are essential. Finally, in order to reduce the magnet volume and weight, it is important that the magnets have the highest possible energy product (BHmax).
Only the sintered rare earth permanent magnet materials samarium-cobalt, Sm
2
Co
17
, and neodymium-iron-boron, Nd
2
Fe
14
B—usually shortened to just NdFeB,—have energy products greater than 22MGOe (170 kJ/m3) and are used to machine VCM magnets. The amount of machining depends on the shape—usually arch-shaped flats or vertical sections of a cylinder—and on the dimensional tolerances of the magnets.
Sintered rare earth magnets are anisotropic and must be magnetized in the orientation direction. Hence, the provision of a prealloyed powder is a prerequisite. The process starts by vacuum induction melting a carefully optimized blend of alloying ingredients and casting an ingot. The ingot is then crushed under protective atmosphere to a coarse, typically minus 50 mesh (297 micron) prealloyed powder. The resulting powder is further coarse ground, and finally jet milled under high pressure (about 120 psi) N
2
to a critical size depending on the size of individual crystallites. Following screening to remove undesirable undersize and oversize particles the highly pyrophoric powder is stored under argon atmosphere until ready for pressing.
Pressing starts by blending a powder mixture based on chromatography results. Depending on the type of magnet being produced, the powder is either isostatically pressed into a block or die pressed into a particular component shape. In either case the operation is conducted in a pulsed magnetic field (typically 10 kOe). The effectiveness of the pulses in magnetically aligning the crystallites diminishes as the powder is being compacted. During the latter part of the pressing step, stresses introduced as a result of plastic deformation as well as density gradients may lead to a less than perfect grain alignment. In the VCM this non-linearity translates in difficulty in precisely moving the coil. Inaccurate positioning of the coil leads to data transfer errors between the data transducers and the storage disks.
After pressing, the block or shapes are demagnetized, followed by sintering in high vacuum. The partial formation of liquid phase during sintering affects the angularity tolerance of the magnetization. This fact, combined with the aforementioned imperfect grain alignment results in difficulties to achieve the theoretical maximum energy product.
Sintering is followed by quenching and aging. The intrinsic coercivity is defined during the quenching step. The presence of large amounts of non-magnetic secondary phase adversely affects the energy product. The sintered ingots are diamond-sawed to the required dimensions and ground to the required tolerance. The blocks are normally pulse magnetized at 40-50 kOe before being shipped. In the case of VCM magnets, magnetization of the final machined product usually takes place at the disk drive assembly plant.
As can be inferred from above, the prior art process of making sintered rare earth VCM magnets is labor-intensive and wasteful in terms of material and energy utilization.
BRIEF SUMMARY OF THE INVENTION
In accordance with the present invention the labor-intensive and inefficient techniques of the prior art are substantially overcome by forming VCM magnets from a mixture of prealloyed rare earth magnetic particles and a thermoplastic binder. Green VCM magnets are produced by either casting the mixture into a tape in a magnetic field, followed by blanking of the rotors, by extrusion or by injection molding the mixture in a mold cavity placed in a magnetic field. Following extraction of the binder the green parts are sintered to net shape. Improved magnetic properties, smaller dimensions, better tolerances and 100% material utilization are thus achieved.
OBJECTS AND ADVANTAGES
It is a primary object of this invention to provide an economic, simple, energy and material efficient process to mass-produce VCM magnets
An additional object of this invention is to provide a method to fabricate VCM magnets that are smaller in size and to a greater dimensional accuracy than in the prior art.
Fine prealloyed rare earth magnet powder being extremely reactive and pyrophoric, sintering of the prior art magnets is necessarily performed as soon as possible after the cast ingot has been crushed and milled into a powder. As a result, the operations of vacuum induction melting, casting, comminution, pressing and sintering of the rare earth alloy are of necessity carried out in the same facilities. This situation has lead to a virtual monopoly over rare earth magnet manufacturing by only a handful of manufacturers. It is an object of this invention to break this monopoly and to reduce the end user's dependence on suppliers of sintered permanent magnet ingots and shapes. Raw rare earth materials in solid or powder form can be easily obtained from suppliers who are not magnet producers themselves. Likewise raw non-rare earth alloy constituents like iron or cobalt in granules, flakes or powder can be easily procured. The ability to source the raw materials from non-magnet producers inherently results in substantial cost reductions. The rare earth alloy can be cast by any foundry equipped with a vacuum induction furnace. Likewise crushing and rough grinding of the cast ingot does not represent any problem since it is only the fine jet milled powder that becomes pyrophoric. Hence this invention has the potential to dramatically reduce the raw material cost for rare earth magnets.
Conventional isostatic pressing of rare earth ingots or shapes in a magnetic field is done with bulky presses which represent a sizeable investment. This invention does not require such investme
Billiet Romain L.
Nguyen Hanh T.
Foley and Lardner
Jenkins Daniel
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