Abstract:
A method of fabricating a porous metal structure of a molten liquid metal within a casting chamber to form a porous solid structure upon controlled chamber cooling and depressurization. The method includes provision of a pressurizable stationary mold casting chamber having a gas pressure release valve, a gas pressure measurement sensor, and a plurality of sites with respective surface-temperature or heat flux sensors and respective independently operable temperature controllers for regulating each respective site temperature. A data base driven microprocessor receives pressure and temperature data and selectively and independently adjusts pressure and temperature in accord with algorithmic commands relative required pressure reduction for pore formation and cooling for solidification to chosen extents of porosity and of solidification over a time period terminating upon porous solid-structure fabrication.

Description:
STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT 
     (Not Applicable) 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     (Not Applicable) 
     BACKGROUND OF THE INVENTION 
     The present invention relates in general to the production of mold-cast structures, and in particular to a method for controlling solidification rate and pore formation of a molten liquid metal within a mold casting chamber by measuring and regulating soluble-gas pressure within the chamber and temperature and/or heat flow change at a plurality of chamber sites to thereby fabricate a solid porous metal structure having known characteristics produced as a result of such chosen pressure and temperature regulation. 
     Production of numerous products is accomplished through employment of mold fabrication technology whereby hot liquid material constituting the substance of a finished product is placed within a mold chamber shaped in the form of the desired final product and thereafter cooled to solidify and yield the finished product. Eligible materials for moldable products generally must be able to withstand heating to a flowable liquid state without untoward breakdown of components and to ultimately cool after formation into an acceptable product. Two typical families of such materials are found in plastics and metals, thereby resulting in various plastic polymers and feasibly-meltable metals being mold-formed into a myriad of products. 
     While the generalized steps of heating a material to melt, introducing the molten material to a mold cavity, and cooling the material to form a finished product are well known, specific procedures and methodology during these steps can significantly contribute to end product results. Thus, for example, the rate of cooling and thus solidification of particular molten metals can affect the microstructure of the finished metal structure. One prior art attempt to regulate cooling includes actual movement of a mold cavity having therein the metal through a series of decreasing temperature zones to thereby produce a general, and obviously non-precise, cooling effect over a period of time. Another prior art attempt to regulate cooling is a simple reduction of heat to the mold cavity in a non-precise manner. While solid structure formation of a molded product readily occurs through these prior art methods, the actual microstructure of the product is not standardized because consistency of cooling and therefore consistency of the solidification rate is not achieved. 
     In addition to forming solid structures in general, it many times is desirous to form solid structures, such as metal structures for example, that have internal porosities to thereby provide weight and structural characteristics congruent with particular product applications. One known procedure for providing pores within a mold-fabricated metal structure is to force a soluble gas such as hydrogen under pressure into molten metal, as shown for example in U.S. Pat. No. 5,181,549 to Shapovalov. Dissolved-gas behavior is such that its solubility decreases with decreasing temperature and decreasing pressure, thereby simultaneously responding to two separate parameters that influence activity. During cooling and/or depressurization, the dissolved gas precipitates and goes to bubbles that do not leave, but, instead, form pores. While the prior art recognizes such gas behavior in porosity formation, the prior art does not teach methodology employing precision parameter measurement followed by precision parameter adjustment for controlled structural formation. 
     In view of the short comings noted above, it is apparent that a need is present for a method of providing significant control over solidification rates along with internal pore formation of structures formed within a mold casting chamber. Accordingly, a primary object of the present invention is to provide a method of controlling a solidification rate of a molten liquid metal within a casting chamber of a mold while additionally controlling pore formation within the metal by continuously monitoring and adjusting pressure within the chamber and continuously monitoring and adjusting temperature values at a plurality of sites relative the casting chamber. 
     Another object of the present invention is to provide a method of controlling such porosity and rate of solidification wherein a microprocessor determines and accordingly regulates pressure within the chamber and temperature values at each such site in concordance with stored pressure and temperature measurements relating to respective extents of pore formation and solidification. 
     These and other objects of the present invention will become apparent throughout the description thereof which now follows. 
     SUMMARY OF THE INVENTION 
     The present invention is a method of fabricating a porous metal structure from a molten liquid metal within a casting chamber of a mold upon controlled cooling thereof. The method first comprises providing a stationary mold comprising a gas-pressurizable casting chamber with a heat-transferable wall having a plurality of sites each having in communication therewith a respective surface-temperature sensor for determining a respective temperature at each such site. Each site additionally includes an independently operable temperature controller for regulating each respective site temperature. The mold is provided with a gas pressure release valve for releasing gas from the casting chamber and an internal gas pressure measurement sensor for measuring chamber pressure. The method next includes providing a microprocessor comprising first a plurality of stored temperature measurements relating to respective extents of solidification of molten liquid metal at each of the plurality of stored temperature measurements, and second a plurality of stored gas pressure measurements relating to respective extents of solubilized gas molecules within the molten liquid metal for determining porosity thereof. The microprocessor is in communication with each respective surface-temperature sensor for receiving each respective temperature at each site, in communication with each respective temperature controller for selective operation thereof, in communication with said gas pressure measurement sensor for receiving pressure magnitude within the casting chamber, and in communication with the gas pressure release valve for selective operation thereof. The casting chamber is heated to a temperature sufficient to maintain the liquid metal in a molten state, and the molten liquid metal is situated within the casting chamber. A gas at least partially soluble in the molten metal is introduced thereto under pressure of a magnitude sufficient to force a sufficient quantity of solubilized gas molecules into the molten metal for forming pores upon cooling thereof to a porous metal structure. Finally, the microprocessor is activated for receiving each respective temperature at each site and pressure magnitude within the chamber, comparing each respective temperature and pressure magnitude to the stored temperature and pressure measurements, and regulating in response thereto the gas pressure release valve and each respective temperature controller for continuously maintaining a magnitude of pressure and rate of cooling within the casting chamber equal to chosen extents of porosity and solidification over a time period terminating upon fabrication of the porous metal structure. 
     In a second preferred embodiment, pressure control is identical to that of the first embodiment while the surface-temperature sensors are replaced with or provided in conjunction with heat flux sensors for determining a respective heat removal rate at each site. In addition to stored depressurization rates, the microprocessor includes a plurality of stored heat removal rates relating to respective extents of solidification of liquid metal at each of these stored heat removal rates. The activated microprocessor receives each respective heat removal rate at each site, compares each heat removal rate to the stored heat removal rates, compares and correlates depressurization rates, and regulates in response thereto the pressure relief valve and each respective temperature controller for continuously maintaining pore formation and cooling rate again equal to chosen extents of porosity and solidification over a time period terminating upon fabrication of the solid structure. 
     The methodology here defined permits precision temperature and pressure management in accord with historical parameters as reflected in algorithmic analyses and regulation via the microprocessor to achieve structure development in accord with specified product production. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An illustrative and presently preferred embodiment of the invention is shown in the accompanying drawings in which: 
     FIG. 1 is a schematic view of a first embodiment of a mold system for regulating formation of a solid structure from a molten metal; and 
     FIG. 2 is a schematic view of a second embodiment of a mold system for regulating formation of a solid structure from a molten metal. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a mold system  10  having a stationary mold  12  with a casting chamber  14  therein is illustrated. The.casting chamber  14  is defined by a heat-transferable wall  16  having a plurality of standard surface-temperature sensors  18  in contact with the wall  16  at a plurality of wall sites  20  for determining respective temperatures at each such site  20 . Because the wall  16  of the casting chamber  14  is heat transferable, temperatures at each site  20  directly reflect site-associated temperatures within the casting chamber  14 . Each sensor  18  is in communication with a standard computer microprocessor  22  for receiving each respective temperature as ascertained by the surface-temperature sensors  18 . Also situated in juxtaposed association with each wall site  20  at the location of each sensor  18  are respective heaters non-limitedly exemplified as standard electric heaters  24  functioning as individual temperature controllers at each such site  20 . Each heater  24  is in communication with, and operable by, the data base driven microprocessor  22 . A temperature-adjustable cooler  26 , controlled by the microprocessor  22 , distributes cooling fluid air around the wall  16  within encircling ducting  28 . A pressurization conduit  30  leads into the chamber  14  for introduction of gas under pressure, while a pressure release valve  32  for releasing gas from the casting chamber and an internal gas pressure measurement sensor  34  for measuring chamber pressure each lead from the chamber  14 . The measurement sensor  34  is in communication with the microprocessor  22  for receiving chamber pressure magnitude, while the pressure release valve  32  is in communication with, and operable bye, the microprocessor  22 . 
     FIG. 2 illustrates a second embodiment of a mold system  40  substantially identical to the embodiment of FIG. 1 except for substitution of respective heat flux sensors  42  in place of surface-temperature sensors  18 . Thus, the system  40  has a stationary mold  12  with a casting chamber  14  therein defined by a heat-transferable wall  16 . The wall  16  has a plurality of heat flux sensors  42  in contact with the wall  16  at a plurality of wall sites  20  for determining respective heat removal rates at each such site  20 . Each sensor  42  is in communication with the computer microprocessor  22  for receiving each respective heat removal rate as ascertained by the heat flux sensors  42 . Also situated, as in the embodiment of FIG. 1, in juxtaposed association with each wall site  20  at the location of each sensor  42  are respective heaters  24  functioning as individual temperature controllers at each such site  20 . Each heater  24  is in communication with, and operable by, the microprocessor  22 . Once again, a cooler  26 , powerable by the microprocessor  22 , distributes cooling fluid around the wall  16  within encircling ducting  28 . As in the embodiment of FIG. 1, a pressurization conduit  30  leads into the chamber  14 , while a pressure release valve  32  and internal gas pressure measurement sensor  34  each lead from the chamber  14 . In the same manner as above described, the measurement sensor  34  is in communication with the microprocessor  22  while the pressure release valve  32  is in communication with, and operable by, the microprocessor  22 . 
     In operation of the embodiment of FIG. 1, the data base of the microprocessor  22  is programmed with an algorithm embodying a plurality of stored temperature measurements each relating to respective extents of solidification of liquid metal at each of such stored temperature measurements, and an algorithm embodying a plurality of stored gas pressure measurements relating to respective extents of solubilized gas molecules within the molten liquid metal for determining porosity thereof. Product fabrication begins by first heating the casting chamber  14  to a temperature sufficient to maintain the liquid metal in a molten state and thereafter providing the molten metal within the chamber  14 . As is apparent, the temperature for a molten state is determined by the metal to be molded. The metal can be heated to the molten state either in the casting chamber  14  or within a separate vessel from which it is transferred to the chamber  14 . When the molding process is begun, the microprocessor  22  receives respective temperatures from the surface-temperature sensors  18  at each respective wall site  20  and pressurization value within the chamber  14  from the gas pressure measurement sensor  34 , and compares these temperatures and pressurization to stored temperature and pressure measurements for the metal. As required to meet proper solidification rates and pore formation, the microprocessor  22  continuously individually monitors, activates, and deactivates the heaters  24  while also continuously monitoring pressure and opening and closing the pressure release valve  32  to uniformly regulate temperature reduction within the casting chamber  14  as correlated to pressure reduction in achieving desired porosity presence. While the cooler  26  is optional, and without it the ambient temperature in conjunction with activation control of the heaters  24  would function to cool the casting chamber  14 , inclusion of the cooler  26  with a constant cooling output enhances standardized ambient conditions to thereby allow greater operating precision of the respective heaters  24  in the control of metal solidification through cooling. Ultimately, the liquid metal within the casting chamber  14  cools to a solid porous structure shaped identically to the casting chamber  14 , and is thereafter removed from the chamber  14 . 
     Operation of the embodiment exemplified in FIG. 2 is substantially identical to that of FIG. 1 except for modifications relating to heat flux measurement as opposed to temperature measurement. Thus, the microprocessor  22  is programmed with an algorithm embodying a plurality of stored heat removal rates each relating to respective extents of solidification of liquid metal at each of such stored heat removal rates. Algorithmic programming for pressurization is as described above for the embodiment of FIG.  1 . When the molding process is begun, the microprocessor  22  receives respective heat removal rates from the heat flux sensors  42  at each respective wall site  20  and compares these heat removal rates to stored rates for the metal. As required to meet proper solidification rates, the microprocessor  22  continuously individually monitors, activates, and deactivates the heaters  24  to uniformly regulate temperature reduction within the casting chamber  14 . Pressurization control again continues identically as earlier described for the first embodiment. Ultimately, in like manner to the embodiment of FIG. 1, the liquid metal within the casting chamber  14  cools to a solid porous structure in accord with chosen parameters. 
     EXAMPLE 
     In accord with the above described methodology, a mold system  10  is employable in the fabrication of a porous metal structure such as an aluminum structure. Specifically, the metal is heated to a molten liquid state in a standard heating vessel while the mold system  10  becomes operational and the casting chamber  14  thereof likewise is heated to the temperature of the molten liquid. Thereafter, the molten liquid is ladled into the casting chamber  14 , and the chamber is pressurized with hydrogen gas. Hydrogen gas quantity and pressure is chosen as being known to introduce a sufficient amount of solubilized gas into the molten metal such that precipitation thereof yields desired porosity quantity and distribution. The microprocessor  22  continuously receives and responds first to the respective temperature measurements from all sites  20  as reported by the respective surface-temperature sensors  18 , and second to pressurization magnitude as reported from the pressure measurement sensor  34 . Algorithmic control of the cooling rate within the casting chamber  14 , and thus of the solidification rate of the metal therein, is immediately initiated through the microprocessor  22 . In like manner, algorithmic control of the depressurization rate proceeds in correlation to the cooling rate to thereby interrelate structure solidification and attendant pore formation occurring from both temperature and pressure reduction as earlier described. Specifically, the required rate of cooling of the metal from its molten state to its solid state calls for a uniform temperature reduction of per unit of time throughout the entire liquid mass in order to achieve a desired microstructure strength within the finished structure, while the correlated pressure reduction likewise is uniform per unit of time. The microprocessor  22  continuously individually monitors, activates, and deactivates all heaters  24  to uniformly regulate this required temperature reduction within the casting chamber  14  while uniformly opening and closing the pressure relief valve  32  until solidification contemporaneous with pore formation within the metal is complete. Thereafter, the finished porous solid structure is removed from the casting chamber  14 . In like manner, in the embodiment employing heat flux sensors, heat removal rate data replaces temperature data, and the microprocessor functions identically to continuously individually monitor, activate, and deactivate all heaters  24  and the pressure relief valve  32  to uniformly regulate the algorithmic-required heat removal and pressure reduction rates within the casting chamber until the porous solid structure is formed. 
     The methodology here illustrated accomplishes precision temperature and pressure management, and therefore precision solidification and pore-formation management, in accord with historical parameters as reflected in algorithmic analyses and regulation to thereby fabricate molded porous structures exhibiting chosen specific structural development. While illustrative and presently preferred embodiments of the invention have been described in detail herein, it is to be understood that the inventive concepts may be otherwise variously embodied and employed and that the appended claims are intended to be construed to include such variations except insofar as limited by the prior art.