APPARATUS AND METHOD FOR BULK STRUCTURAL MODIFICATION OF METALLIC MATERIALS AT REDUCED TEMPERATURES

Abstract

An apparatus and method of mechanical milling and grinding of various materials at temperatures ranging from sub-ambient conditions to well-below their ductile-brittle transition temperature (DBTT) are presented. In one embodiment the present invention entails the design of a cryogenic milling chamber compatible with horizontal high-energy mills. The new design and configuration of the milling vessel provides robust and efficient cryomilling of various materials with no contact between the cryogen and the powders. Some embodiments of the invention improve the heat removal rate from the non-uniform heat load generated by the impact energy deposited into the chamber wall by the milling media.

Claims

1. A method for the fabrication of an efficient milling vessel with improved cooling capacity: From a monolithic billet, Wherein there are no welded sections that could permit the egress of the refrigerant or coolant into the interior of the vessel; and Wherein the surface pattern is specifically designed for improved heat exchange between the coolant and vessel.

2. The method of claim 1, wherein the operational temperature of the vessel is at least the boiling point of liquid nitrogen, −196° C.

3. The method of claim 1, wherein the refrigerant flow pattern is improved and thermal signature is more uniform.

4. The method of claim 1, wherein heating modules are strategically placed as part of the milling chamber to control temperature gradients, ensure proper function of mechanical components and seals.

5. The method of claim 1, wherein the refrigerant flow is controlled by the readings of thermocouples.

6. The method of claim 1, wherein using thermocouples to regulate the refrigerant flow allows for control of temperatures between −196° C. and −50° C. during milling operations.

7. The method of claim 2, wherein the precursor metal powders used are selected from the elemental metals consisting of aluminum, magnesium, iron, zinc, boron, beryllium, silver, titanium, zirconium, zinc and their alloys.

8. The method of claim 2, wherein process control agents are not needed during the milling process.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG. 1 presents the schematic drawing of the Zoz GmbH horizontal attritor milling system.

[0033] FIG. 2 presents a schematic drawing of the milling chamber and cooling jacket featuring a wide flow path where the cryogen travels through the entire jacket before exiting.

[0034] FIG. 3 presents an exemplary flow pattern for the cryogenic fluid flow with divergent channels guiding the cryogen from the lateral entry points to the exit port on the face plate side.

[0035] FIG. 4 presents an alternative flow pattern for cryogenic fluid flow featuring parallel radial channels.

[0036] FIG. 5 presents a flow pattern for the cryogen featuring a single flow path with multiple redirections to provide turbulent mixing.

DETAILED DESCRIPTION

[0037] The present invention described herein entails the design of a cryogenic milling chamber compatible with the Zoz GmbH mills. The intent of this modification is to enable cryomilling operations with this series of equipment, where the new design and configuration of the milling vessel provides robust and efficient cryomilling of various materials with no contact between the cryogen and the powders. As secondary benefit, some embodiments of the invention improve the heat removal rate from the non-uniform heat load generated by the impact energy deposited into the chamber wall by the milling media.

[0038] The unique aspect of this novel design addresses the mechanical loads and stresses induced by the thermal gradients within the milling chamber and between the milling chamber and the motor housing mounting plate. This allows the milling chamber to operate at cryogenic temperatures with mechanical integrity over multiple repeated runs. A second aspect of the invention relates to the ability to design the flow of the cryogenic fluid in the chamber jacket to establish uniform temperature conditions in the entire chamber wall quickly and efficiently. In turn, the uniformity of the temperature leads to the elimination of cold and hot spots, resulting in constant and unvarying conditions and thus the consistent treatment of the powders undergoing the milling process.

[0039] A schematic diagram of the Zoz Simoloyer® milling apparatus is shown in FIG. 1. As depicted, the direct drive motor (1), through a series of couplings and gear shaft (2) is attached to the horizontal impeller (3) consisting of roughly equally spaced knife-like blades (4). The base flange (5) of the hat-shaped milling jar (6) is bolted to the back of the drive shaft and motor mount (7) that holds the drive shaft assembly and gear couplings. There is a withdrawal tube (8) attached to a square-shaped flange (9) located on the top of the milling jar; the chamber can be rotated to invert the tube and remove the as-milled powders from the chamber.

[0040] As shown in FIG. 2, the present construction of the Zoz milling jar is such that it comes with an integral outer jacket that enables the circulation of the refrigerant (e.g., water or ethylene glycol-based liquids). If not cooled, during milling the chamber reaches temperatures that are detrimental to the effective processing of the materials. Thus, the refrigerant provides active cooling that facilitates the extraction of heat from the powder mass and milling media that allows for extended and uninterrupted operations.

[0041] There are several challenges or considerations in the current Zoz apparatus that the invention addresses. These are related to the thermal and mechanical loads and stresses created by the extreme cold operating temperatures.

[0042] A first challenge is related to the construction of the stainless-steel milling jar that consists of welded sections that have been machined to create a smooth interior wall surface. In particular, both the base flange 5 and the powder removal pipe flange 9 have been attached to the cylindrical vessel and welded into place. As such, repeated operation with a cryogenic coolant will likely cause the weld seams to embrittle, fail, and crack, and hence allow the entry of the cryogen into the milling chamber. Similarly, if there are any incomplete welds, such gaps, cracks, and bull defects will also expand and leak cryogen once cooled to low temperatures.

[0043] A second challenge is related to the temperature differential that exists under extreme operational conditions. Unlike under normal operating conditions and temperatures, extreme cooling results in significant and differential shrinkage leading to thermal gradients induced internal stresses that will cause warping and incomplete sealing between the chamber flange and motor housing base plate, again leading to possible leakage of the cryogen into the milling volume.

[0044] Another challenge is related to the powdered material undergoing milling. Ultra-fine and nano-scale particulates can flow through any of the aforementioned open cracks, warped seals, and fittings into the environment, cooling jacket, or accumulate in the gear shaft housing and other places within the milling assembly.

[0045] A fourth challenge is related to the limitations of the chamber design itself. The cooling jacket in the present milling chamber has a fixed flow pattern and thus it has a fixed cooling efficiency that is optimized for a water-based coolant. The temperature and specific heat of the coolant is such that for the available flow rates it can remove the heat at sufficiently high rates to ensure continuous milling operations.

[0046] Optimization of the flow pattern and cooling jacket volume area can eliminate the presence of hot and cold spots that otherwise would lead to non-uniform chamber wall temperatures. Further, more uniform wall temperatures will reduce adhesion or cold welding of the milled powders to the chamber wall, and thus extend the effectiveness of the mechanical alloying process, and increase the quality and quantity of powder production and powder recovery.

[0047] The invention described herein overcomes problems of the prior art by providing a more efficient and reliable method for controlling both the temperature at the chamber wall as well as the internal temperature inside the chamber. In one of the embodiments, the flow of the refrigerant is altered to facilitate a uniform and rapid spread and covering the entire surface of the chamber, instead of the stepwise laminar flow from sector to sector as was done in the prior art. The schematic diagram of exemplary flow channel patterns is illustrated in FIG. 3. It will be obvious to those skilled in the art that the inner division of the jacket into sections by axial or longitudinal ribs and patterns will be instrumental in ensuring optimal heat removal ability by the cryogen. In one embodiment of the invention, there are a series of orifices in the jacket at the base of the chamber that allows for the passage of the refrigerant through the back flange and the motor housing plate to provide cooling to the gear shaft and couplings.

[0048] Another aspect of the preferred embodiment is its unibody construction. This non-obvious aspect of the invention relates to the inner chamber being constructed and machined out of a single block of stainless steel or related metal. This is unlike that of the prior art designs, where the chambers consist of a multitude of individual parts and components welded together, including a uniform diameter cylinder, with a square hole cut into it for the placement and insertion of a flange base that would permit the attachment of the powder withdrawal tube. In one embodiment of the invention, the integration of the flange base as an integral part of the chamber will minimize thermal fatigue and cryogen leak, in opposition to standard chamber designs.

[0049] In one embodiment, the inner chamber is made of Type 316 steel. In another embodiment, Type 310 steel is used. Those skilled in the art will recognize that other types of steel can be used.

[0050] One embodiment of the invention features strategically placed heating units embedded in the cryogenic milling chamber. The function of heating units allows for minimizing the thermal gradients between connecting components of the milling assembly, such as withdrawal tube 8 and back flange connection to the motor mount 7. In one embodiment, controlled heating of these components and others minimizes thermal shrinkage of metal parts, gaskets and seals and thus minimize leaking potential of cryogen, powder and/or contamination with air. In another embodiment, thermal modules can ensure the proper function of mechanical components by enabling their function even at cryogenic temperature.

[0051] The milling of powders under cryogenic conditions can significantly benefit from controlled milling parameters, including temperature. As such, the introduction of thermocouples within the milling chamber walls to monitor temperature points of the assembly. In a preferred embodiment, thermocouples are incorporated at multiple points of the chamber walls to monitor the temperature of the cryogenic fluid. In another embodiment, the emplacement of thermocouples at the inner wall of the milling chamber allows for the measurement of temperature in close proximity to the powder being milled.

[0052] Temperature regulation during cryomilling operations can be performed by manually adjusting the flow of cryogen within the milling chamber jacket. Alternatively, a preferred embodiment of the invention features the automated flow of cryogen controlled in real time by selected thermocouple input.

[0053] These features of the invention provide a significant advantage and enable the cryomilling of materials where the powder and cryogenic fluid are not in contact. Those skilled in the art of milling will understand that the design and added controls enable cryomilling operations to be transitioned to other vessel sizes and geometries.

REFERENCES

[0054] 1. C. Suryanarayana, “Mechanical alloying and milling,” Progress in Materials Science, 46, 1-2, pp. 1-184, 2001.