In high-volume forging operations, one of the highest per-piece cost items is replacement tooling. In such applications, tool life can be as low as 50 pieces and is seldom over 50,000 pieces. For this reason, significant effort has been devoted to increasing tool life. Many approaches are used either individually or in combination, including flood welding, re-sinking, exotic alloys and various coatings. All of these methods add cost to the tool and reduce profits.

In addition to the basic tooling cost there are also the costs of purchasing, sampling, dimensional checks and set up for each replacement tool. These added costs can often be more than the tool itself. Consequently, any improvement in tool life can mean a significant advantage to the forger.

Figure 1. RSP tooling machine in production (inset shows actual metal-spraying process).

NEW METHOD

Now, however, there is a new method of tooling production that can double or triple tool life of comparable materials. This method, developed by the U.S. Department of Energy, is called the Rapid Solidification Process (RSP) and is being commercialized by RSP Tooling, LLC, Solon, Ohio.

The process is an indirect, additive spray-forming process. “Indirect” means it is made from an easily produced soft model of the tool. “Additive” means the metal is added to create the shape instead of removing or subtracting it from a block as in standard machining. “Spray-forming” refers to the process through which metal is melted, atomized by an inert gas and projected onto the negative substrate (ceramic) of the tool.

RSP Tooling was formed in 2002 and has been developing the process to make forging dies as well as metal tooling for most other forming processes. It has recently achieved the established targets for uptime, cycle time and quality and has gone into a full production mode starting in January 2007. The equipment necessary to perform the process is shown in Figure 1. RSP Tooling’s efforts – with respect to the development of forging-die applications – were supported by the Defense Logistics Agency and the Forging Industry Association Department of Defense Manufacturing Program known as PRO-FAST.

Figure 2. Forging die in production

PROCESS ADVANTAGES

These test tools have established that RSP technology offers several advantages to the forger. The first and most important of these is the ability to increase tool life, particularly in applications for which the failure mode is die wear or heat checking. The first two dies put into full production increased tool life by 200% and 250%, respectively (Figure 2). Several other dies are now in production and are showing similar levels of improvement.

The improvement occurs because of the rapid solidification of the very small atomized metal droplets, which result in a fully heat-treated structure directly from the machine. H13 tool steel leaves the machine at 50Rc and can be easily aged to 56Rc. The tool is 99.7% dense. Also, the heat-treated structure is continuous and non-directional. As a result, RSP tools can be re-machined and die life increased with each process iteration.

Figure 3. The RSP process is capable of replicating details as small as 0.003-inch.

The second advantage comes from the fact that the cost of the process is independent of die complexity. For the simplest of die designs, the process may be more expensive than machining. But as the standard tool cost increases, RSP technology can significantly reduce tool costs. The advantage also applies to delivery time, which is not affected by tool complexity. This means that engraving, tight radii or any additional features can be added to the tool design with no increase in cost (Figure 3).

The third advantage derives from the use of melted metal in the process. Because of this, any form of the metal can be used. This is especially valuable for very expensive alloys. Scrap or old tools can be placed in the crucible and remelted, thus significantly reducing the price of the tool.

Figure 4. Flow chart illustrates the sequence of turning a CAD file into a finished tool.

RAPID SOLIDIFICATION PROCESS

RSP is a spray-forming technology developed by Dr. Kevin McHugh, a scientist at DOE’s Idaho National Laboratory, for the production of molds and dies. The general concept involves converting a mold design described by a CAD file to a tooling master using a suitable rapid prototyping (RP) technology such as stereolithography (SLA) or rapid machining of a wax form. A pattern transfer is made to a castable ceramic – typically alumina or fused silica. This is followed by spray-forming a thick deposit of tool steel (or other alloy) on the ceramic pattern to capture the desired shape, surface texture and detail. The deposit is built up to the desired thickness at a rate of about 500 lbs/hour. Thus the spray time for an 8-inch x 8-inch x 4-inch steel insert is only nine minutes. The resultant metal block is cooled to room temperature and separated from the pattern. Typically, the deposit’s exterior walls are machined using a wire EDM and boltholes are added (Figure 4).

Ceramic patterns are made by slip casting or freeze casting a ceramic slurry – typically made of alumina or fused silica – onto the tool master. Ease of casting, material cost, surface finish, strength, thermal shock resistance, maximum-use temperature, flatness and dimensional accuracy are parameters assessed at this stage. With the right equipment and procedures, accurate and reproducible ceramics are easily made.

Figure 5. Schematic of RSP tooling machine operation

The next step is spray-forming, which is the heart of the rapid solidification process. Spray-forming involves atomizing, or breaking up, a molten-metal stream into small droplets using a high-velocity gas jet. Aerodynamic forces overcome the surface tension of the liquid metal, producing an array of droplet sizes that are entrained by the jet and deposited onto the pattern as shown in Figure 5.

The processing of tools by spray-forming can be divided into two distinct but closely related stages – flight and deposition – in sequential order. During flight, the thermal energy of the atomized droplets is extracted via convection heat transfer between the droplets and the atomization gas via radiation heat transfer. At this stage, the temperatures and the solid fractions of individual droplets can be calculated using an equation of energy conservation. As a result, a combination of liquid, solid and semi-solid (slushy) droplets impact upon the ceramic and “weld” together to form a coherent deposit.

During deposition, heat conduction within the spray-formed material can be assumed to be along the thickness of the spray-formed material because the thickness of molds/dies is usually much smaller than their width/length. The buildup of the deposit occurs via discrete deposition of individual droplets. This means there exists a time interval between two groups of droplets that successively arrive at the previously deposited material’s surface. At the end of this time interval, a new group of droplets is incorporated to the previously deposited material to generate a new deposit.

Figure 6. Micrograph of spray-formed H-13 tool steel at 500X magnification.

The high cooling rate of the deposit greatly impedes atomic diffusion. Therefore, segregation is very limited compared to cast metal. It also minimizes the erosive interaction of the metal and ceramic tool pattern, allowing the deposited metal to accurately capture surface details of the ceramic, which would not be possible if the metal was cast onto the ceramic. The rapid solidification rate also results in non-equilibrium solidification – extended solid solubility – and very limited segregation as can be seen in Figure 6.

The turnaround time for a cavity or insert is unaffected by its complexity. From receipt of a CAD solid model to shipment of the cavity is eight days. In addition to use in forging operations, molds and dies produced in this way have been used for prototype and production runs in metal diecasting and plastic injection-molding operations.

Figure 7. a) Simulation of re-designed die insert; b) spray-formed die insert; and c) finished forging made from the die insert.

LIMITATIONS

The biggest limitation to the existing machine is tool size. The largest tool that can be produced is 8 inches in diameter by 4 inches thick or about 50 pounds. To reduce the effect of this size constraint, we have been working with Scientific Forming Technologies Corporation in Columbus, Ohio, – also a FDMC member company – to design insert tooling. Inserting has several advantages to the forger. First and foremost, they can take advantage of the RSP technology to improve tool life by reducing the effect of wear and heat checking. Secondly, by using the simulation software and adding compressive stress to the insert, cracking issues can be reduced or eliminated. Thirdly, the new tools are significantly smaller and easier to handle in the tool room (Figures 7a, 7b and 7c).

The accuracy of the process is about 0.002 inches per inch and the finish is 35 micro-inches. These are usually adequate for most forging dies. In fact, several forgers have indicated the forged material seems to flow better across the surface of the RSP tool. If more accuracy or a better finish is required, the tool can be skim-machined or polished without adversely affecting tool-life benefits.

CONCLUSION

RSP technology has the ability to substantially reduce cost for any forged part having significant replacement tool issues. The process can also reduce delivery time and tool cost for any complex tool design or when specialty alloys are required.

Author James R. Knirsch is president and CEO of RSP Tooling, Solon, Ohio, which is the commercial developer of the Rapid Solidification Process under license to the U.S. Department of Energy. Knirsch may be reached at (440) 349-5262; or knirsch@rsptooling.com. For additional information visit www.rsptooling.com