Abstract:
A method of directly fabricating metal parts with surface features only requires first preparing a mold of the desired metal part. A powder blend is poured into the mold, which includes a base metal, a lower melting temperature alloy of the base metal, and a polymer binder. The mold containing the powder blend is heated until the polymer binder melts and adheres the metal particles to form a green part. The green part is removed from the mold and placed in a crucible, and loose ceramic powder is packed around the part to support it. The supported green part is then heated as needed to vaporize the binder and consolidate the part via liquid phase sintering. Once cool, the consolidated part can be machined to meet precise dimensional tolerances, if necessary.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of metal part fabrication, and particularly to the direct metal fabrication of parts with surface features only. 
     2. Description of the Related Art 
     Many techniques have been developed for fabricating metal parts. Some parts, such as metal stamping dies, have surface features only; i.e., with no re-entrant angles at the part&#39;s side faces. Historically, such parts have been machined by removing material from a block to form a final part. However, machining is both time-consuming and expensive, and is rarely cost efficient when done on a production scale. More typically, large scale production uses a casting process that is fairly time and cost efficient and produces cast quality final parts. However, the cost of retooling and machining a new part can be very high, both in dollar and man hour investment, and in the delay in getting a new design into production. This can be a significant deterrent to updating and improving the design of the part. 
     Alternatives to machining and casting have been developed in recent years. U.S. Pat. No. 5,745,834 to Bampton et al. and assigned to Rockwell International, the Assignee of the present invention, uses a powder blend of a parent metal alloy X such as Haynes 230, a metal alloy Y that is identical to alloy X except that it is doped with another alloying element such as boron to lower its melting point, and a polymer binder. A thin layer of the powder blend is spread on a table, and a green form part is built up layer-by-layer by localized laser melting of the polymer binder; computer aided design (CAD) data is typically used to control the laser. The polymer binder is eliminated from the green part by heating in either a vacuum furnace or a furnace with an inert environment. Densification is performed at a temperature above the melting point of the lower temperature alloy, but below the melting point of the base metal alloy, to produce transient liquid sintering to near full density. 
     Though effective for the fabrication of three-dimensional metal parts, the method described in Bampton requires a considerable amount of complex equipment, which may be prohibitively expensive for the manufacture of simpler metal parts with surface features only. 
     SUMMARY OF THE INVENTION 
     A method of directly fabricating metal parts with surface features only is presented, which is simpler, quicker and less expensive than previously-known methods such as those described above. 
     A mold is made of a metal part having surface features only. A powder blend is poured into the mold, which includes a base metal, a lower melting temperature alloy of the base metal, and a polymer binder. The mold containing the powder blend is heated until the polymer binder melts and adheres the metal particles to form a green part. The green part is removed from the mold and placed in a crucible, and loose ceramic powder is packed around the part to support it. The supported green part is then heated as needed to vaporize the binder and consolidate the part via liquid phase sintering. The mold is scaled to account for the consolidation of the part. Once cool, the consolidated part can be machined to meet precise dimensional tolerances, if necessary. 
     The described method enables a surface-feature-only metal part to be directly fabricated in hours, rather than days or weeks. The scaled mold can be repeatedly re-used to produce additional parts if needed. No complex laser equipment is required, nor must each part be individually machined. Despite the expediency of the novel process, the finished part is tough enough to serve as, for example, a metal stamping die (after appropriate heat treatment to enhance hardness) or an electrical discharge machining (EDM) electrode. 
     Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow chart of a direct metal fabrication process per the present invention. 
     FIG. 2 is a pictorial view of a direct metal fabrication process per the present invention. 
     FIG. 3 is a flow chart of the process steps required to produce a negative part cavity per the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Metal parts having surface features only are easily and quickly directly fabricated in accordance with the process steps shown in FIG. 1, and illustrated pictorially in FIG.  2 . In step  10 , a part negative cavity  12  is provided. The cavity is a negative version of the metal part to be fabricated; i.e., depressions  14  in the part negative cavity are used to produce corresponding raised surface features  16  on the finished part  18 . 
     A powder blend is prepared in step  20 , which is made from three components, each of which, is provided in powdered form: a base metal “A”, a lower melting temperature alloy of the base metal “B”, and a polymer binder “C”. Illustrative lists of acceptable base metals, lower melting temperature alloys and polymer binders, and the preferred size and distribution of powders in the blend, are given in U.S. Pat. No. 5,745,834 to Bampton et al., between column 3, line 51 and column 4, line 18, the disclosure of which is hereby incorporated by reference. A powder made from 17-4 ph stainless steel may also be employed. Typically, the powder blend is comprised of about 75-85% metal “A”, 5-15% alloy “B”, and 5-15% of polymer binder. Powders A, B and C are blended in the correct size ranges and in the correct volume fractions in a container  22 , and then poured into part negative cavity  12  until full. 
     In step  24 , the filled part negative cavity is placed into a furnace  26 . The furnace atmosphere is preferably either vacuum or inert, to prevent oxidation of the part being fabricated. The temperature of the furnace is increased until the polymer binder melts and adheres the metal particles, forming a “green part”  28 . 
     The green part  28  is removed from the part negative cavity in step  30 , and is placed into a heat-resistant container  32 , typically a crucible, and packed with loose ceramic powder “D”, typically boron nitride, in step  34 . The ceramic powder D, which should cover the green part  28 , provides support for the part during the subsequent consolidation step. 
     In step  36 , the crucible  32  and supported green part are placed in a furnace  38 , the atmosphere of which is preferably vacuum or inert. The temperature of the furnace is increased to 1)vaporize the polymer binder, and 2)bring the resulting phases into a liquid/solid proportion of about 15%/85% to facilitate transient liquid phase sintering and consolidation. The furnace temperature will be on the order of 1000° C., depending on the alloy blend, which must be held for a time on the order of 1 hour, depending on the part mass. Liquid phase sintering is well known in the art of power metallurgy, and is discussed, for example, in R. M. German,  Powder Metallurgy Science , 2nd edition, Metal Powder Industries Federation (1994), pp. 274-275. 
     The resulting component  18  is a near net shape metal part. Consolidation shrinks the size of the green part by a predicable amount, but in the event that precision tolerances must be met, a finish machining step (step  40 ) can be performed to bring the part into conformance with the tolerances. 
     An optional step (step  42 ) can be performed after the powder-filled part negative cavity is heated and the green part formed (step  24 ). In step  42 , the furnace temperature is increased to at least 500° C. and held until all of the binder evolves (time and temperature required are material and part-size dependent). The higher temperature acts to “pre-sinter” the green part; i.e., some sintering and thus some shrinkage (&lt;0.5%) occurs, making removal of the green part from the part negative cavity much easier. The binder, though burned out, leaves a residue that acts as a sintering aid. 
     It is possible to remove the green part from the cavity without it being pre-sintered, and thus it is not essential that step  42  be performed. However, without pre-sintering, the strength of the green part is low, and the probability of damaging the part is higher. 
     The time and temperature profile to use for the polymer binder melting and consolidation steps (steps  24  and  36 , respectively), and the pre-sintering step (step  42 ) vary with the type of materials used; general profile guidelines are given in U.S. Pat. No. 5,745,834 to Bampton et al. 
     A specific example for a powder blend consisting of (90 wt % 17-4 stainless steel+10 wt % borided stainless)+10 vol % binder, is as follows: 
     1. Place powder-filled part negative cavity in furnace (with vacuum or inert atmosphere), raise temperature to about 200° C., and hold for about 1 hour. This melts the polymer binder and produces the green part. 
     2. Raise furnace temperature to about 800° C. and hold until all binder evolves (time required is part-size dependent). This step also pre-sinters the part, which improves its strength and thereby helps the part to remain intact as it is removed from the mold. 
     3. The part negative cavity is removed from the furnace. The green part is removed from the cavity, placed in a crucible with supporting powder, and placed back in the furnace (with vacuum or inert atmosphere). The furnace temperature is raised to sintering temperature. The rate and temperature required are the same as would be needed for a part of similar size and composition being fabricated using powder metallurgy. 
     The novel process enables metal parts with surface features only to be directly fabricated from a wide variety of base metals, without the use of complex laser equipment or time-consuming machining operations. The method provides a way of creating custom-designed metal parts with a quickness and economy that has heretofore been impossible. These characteristics make the described process an ideal choice for making parts such as metal stamping dies (after appropriate heat treatment to enhance hardness) and electrical discharge machining (EDM) electrodes. 
     As noted above, the green part will shrink somewhat during the consolidation step. The amount by which the part will shrink is preferably determined in advance, by subjecting cube-shaped test parts having precisely known dimensions to the same time and temperature profile that the green part will be subjected to. After the test parts have cooled, measurements are made of their x, y and z-axis dimensions. A range of expected shrinkage percentages is established by comparing the test parts&#39; dimensions before and after their exposure to the consolidation profile. Consolidation shrinkage of about 14-22% is typical. 
     The expected shrinkage percentage is preferably taken into account when preparing the part negative cavity that will serve as the mold for the green part, with the part negative cavity being scaled up in accordance with the pre-determined percentage. An illustrative set of process steps that may be followed to produce a part negative cavity is shown in FIG.  3 . In step  50 , any one of a number of free form fabrication techniques, such as stereolithography (SLA), laminated object manufacturing (LOM), or selective laser sintering, for example, is used to construct a properly scaled negative model cavity of the metal part to be fabricated, using a 3-D CAD file or similar technique. A number of different materials can be used for the negative model cavity, including plastic, metal, polymer, or ceramic powders. 
     An RTV-type rubber is poured into the negative model cavity in step  52 . The rubber is allowed to solidify (step  54 ), forming a silicone positive. In step  56 , the silicone positive is removed from the negative model cavity. A permanent, reusable ceramic compound or equivalent is poured around the silicone positive (step  58 ) and allowed to solidify (step  60 ). The silicone positive is carefully withdrawn from the solidified ceramic mold (step  62 ), which can then serve as a re-usable part negative cavity for the metal part to be fabricated. The material from which the part negative cavity is made must be able to withstand the heat to which it is subjected during the polymer binder melting and consolidation steps (steps  24  and  36 ). In order to properly scale the part negative cavity, the range of expected shrinkage percentages is preferably determined as described above (step  64 ), prior to the negative model cavity&#39;s fabrication. 
     While particular embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.