Machining Refractory Metals
You can machine refractories, but it's no walk in the park
By James D. Petrush
Lockheed Martin Naval Electronics & Surveillance Systems -- Undersea Systems
Refractory metals such as Wah Chang's (Albany, OR) C-103 (Niobium + Hafnium + Titanium) provide design engineers with a suite of unique and wonderful properties. These alloys have high strength in continuous operation to 2700ºF (1480ºC) or higher, making them attractive for a variety of rocket-propulsion components and industrial-process assemblies. They are relatively impervious to exposure to high-temperature propellant gases (also important in propulsion applications). They have low ductile-to-brittle transition temperatures for withstanding high-frequency vibration; can be made into forgings, plates, sheets, and tubes; and are readily welded into complex assemblies.
These materials present special challenges to machinists, however. They impose new constraints on cutting tool design, coolants, and speeds and feeds, compared to more conventional materials. If you are searching for a manufacturer of critical refractory-metal components or assemblies, evaluating a candidate supplier's capability to perform such work, or planning for an in-house fabrication activity using refractory metals, there are several critical areas to evaluate.
These fall in the general areas of development of machining processes, tool definition, and process-control discipline. Our comments on these subjects are based on more than 20 years of experience in the design, process development, and manufacture of rocket propulsion components (hot gas control valves) made from C-103 and other refractory alloys that must meet the most exacting quality and reliability requirements. Typical machining operations on C-103 bar stock and forgings in our plant consist of turning, drilling, boring, milling, and threading fairly thick-walled body sections. Because machined parts will later be welded into higher-level assemblies, we require close-tolerance weld interfaces and fine surface finishes with no voids, tears, or inclusions. Typical tolerances for diametral features are ±0.001" (0.03 mm), with positional tolerances running as close as 0.002" (0.05 mm). Surface-finish callouts of 16 RMS are typical, as are closely controlled filet radii of 0.005" (0.13 mm).
Development of machining processes is not straightforward. The unique properties of refractory metals present special challenges to the machine shop that require the development of special machining methods, tools, and coolants.
For example, C-103 is a highly ductile, soft, and stringy material. (One of our machinists has likened cutting it to "trying to machine an old shingle.") It has a high abrasive-wear characteristic that breaks down tool edges, and creates high heat buildup. This alloy is prone to tearing, galling, and chip welding to the tool face. Because of the high cutting forces and the tough, stringy chips produced when machining C-103, chipbreakers are completely ineffective.
During our development of turning processes, standard HSS tool profiles with zero or negative top rakes produced ragged, torn, poorly sized diameters that were unacceptable. Tool wear was tremendous as well, with extensive cratering of the cutting edge, and in some cases tools were broken off at the shank. Tungsten carbide inserts with similar profiles also produced bad results.
Tools with high positive top rake angles of 5, 10, 15, and 20º were tried until a favorable result was established. Turning tools of HSS and carbide with a positive top rake of 15 - 20º are able to machine C-103, providing that proper feeds, depths-of-cut, and speeds are used. Due to the abrasive nature of C-103, cutting speeds must be reduced to approximately 250 fpm (76 m/min) for carbide and 75 fpm (23 m/min) for HSS and cobalt-alloy tools. Typical feed rates for turning C-103 are approximately 0.005 ipr (0.131 mm/rev). Due to the unavailability of off-the-shelf carbide inserts with the required high positive top rakes, special toolholders and modified inserts are used throughout the shop.
One of the most important things that we learned about the machining of C-103 relates to the size of cuts that can be taken to produce the desired size and finish. In that it tears and does not abrade, C-103 is similar to copper; therefore, it's very difficult to form a chip less than 0.001" (0.03-mm) thick. Rough and finish cuts must be adjusted to the proper ratio in order to produce a satisfactory size and finish.
Boring of C-103 poses some interesting problems. Standard boring bars of both HSS and carbide have geometries that will not allow regrinds of 15 - 20º of positive top rake without severely weakening the tool. The unsupported lengths of these boring bars create excessive deflection and chatter. Many different shapes were tried without success. One machinist suggested that we try a two-flute end mill, with one flute ground away for clearance. The results were miraculous. We got excellent chip control, with a beautiful surface finish, and we were able to hold size. Today we use both HSS and carbide single-flute router bits for almost all of our boring requirements.
Milling of C-103 is accomplished with standard HSS end mills with chip loads of no more than 0.004 ipt (0.10 mm/tooth) and speeds of 75 fpm (23 m/min). Drilling is done using HSS and cobalt drills with standard relief angles and radiused corners to prevent hole-size shrinkage and subsequent seizure of the drill in its hole. Tapping C-103 successfully requires sizing minor diameters to their maximum limits, along with the use of high-strength taps and special tapping fluids.
Single-point thread generation is possible, but achieving consistent, high-quality results requires careful experimentation during process development. Because of the tendency of C-103 to gall and tear, single-point threading must be accomplished with high positive-top-rake tools. Allowances must be made for "spring passes" of 0.001" (0.03 mm) or smaller when generating the thread form to achieve acceptable surface finishes. With a properly ground threading tool, these spring passes burnish the surface of the work. Our experience has been that, whenever possible, thread milling produces a superior thread, both in size and finish.
Selecting the proper coolant for machining C-103 was crucial to the success of production machining. Coolant testing went on for about six months. During that time, experiments were conducted with many different coolants and water-soluble oils. The ultimate selection of the proper coolant would not only provide good lubrication, but also minimize the effects of material contamination and corrosion on C-103.
The fluid selected was a water-soluble coolant with high lubricity and good wetting characteristics that cleans up using water-based cleaning processes. Periodic replacement of all coolants used in the shop was instituted to ensure that alloy cross-contamination is prevented. After the selection process, the coolant and the machined parts were qualified according to customer requirements.
All of the preliminary work involved in machining C-103 was performed initially on manual lathes and milling machines. As we progressed from the development stages and into production, the need to transition from manual to CNC machining was evident. In our shop, the need was fulfilled by using a combination of small Mazak CNC lathes, larger Mazak CNC lathes with milling capabilities, and some Fadal VMCs. Much of the tooling developed on the manual machines required modification to adapt to the production environment. Tooling for CNC machines needed to be more robust, readily duplicated, and quickly changeable. Accurate indexing and maintenance of corner radii are critical elements in production machining. Standard "off-the-shelf" insert tooling required remachining of holders and regrinding of the inserts to meet our requirements.
We determined that tool control drawings were needed to ensure that the required modifications were performed in the same manner each and every time. Cutting tools that required modification were assigned part numbers to correspond with a specific drawing. Tool control provides a means to purchase, stock, modify, and inspect cutting tools to ensure consistency.
During both the early development phase of the program and during production, it's critical to have in-house tool-grinding expertise to work with machinists and manufacturing engineers to develop the proper tool geometries. It was through much trial and error that we determined the final tool configurations that became part of the tool-control drawings.
When we considered outsourcing the bulk of our grinding requirements, it became apparent that there were significant differences in the quality of the product we were getting from the outside versus the consistency of quality we were receiving from our own grinder personnel. The value of an experienced tool and cutter grinder cannot be overstated. Subtle differences in edge preparation derived through years of training are the key to our continuing success.
Process control discipline is vital. Controls on the manufacturing process are critical in the propulsion components business. Reaction control valves are "one mission" devices. Operational use, with high-flame-temperature gas flowing through the valves at high pressure, is usually a destructive event. It is not unusual for a valve to perform flawlessly during a hot firing, then crack, deform, or suffer metallurgical degradation during post-firing cooldown. Therefore, it's not permissible to subject each valve to operational conditions as part of the acceptance-testing process.
These rocket propulsion components present QA issues analogous to those found in drug manufacturing. The pill you take has never been taken by anyone else, so you are relying on the manufacturer's process controls to ensure that "your" pill matches the effectiveness and safety of the pills that were tested during development. In the same way, in-flight reliability of a hot-gas valve depends on its manufacturing process being exactly replicated, unit after unit. Once the product has been thoroughly designed and tested, each unit produced must possess the same pedigree as all the others. The required flight reliability is, of course, essentially 100%, and the only assurance that the unit meets this demand arises from the adoption of strict process controls. The controls define and monitor every operation that is performed in transforming raw stock into completed assemblies.
Process controls are employed not only to guarantee the pedigree and functionality of hardware, but also to ensure that the manufacturing process is not corrupted. Process control reaches into every step of our manufacturing process from raw-stock sawing to machining, assembly, cleaning, welding, and test.
Because refractory metals are soft, they are highly susceptible to contamination caused by contact with other alloys. You must consider machine cleanliness, cutting-tool cross-contamination, and tooling design. Abrasive wheels, files, and sandpaper must be segregated for use only with refractory metals, and not with any other alloys. This practice prevents cross-contamination of the refractory-metal weld interfaces. Because of the high melting point of C-103 and its susceptibility to intergranular corrosion, foreign-alloy contamination must be avoided at the weld interface, and at any other locations that will be exposed to high temperatures during operation. Care must be taken in the design of tooling, and controls must be rigorous to ensure that unapproved tooling is effectively prohibited. We've seen situations where merely pressing (not scraping) an Inconel rod against a refractory metal part transferred a minute amount of contaminating material, which led to unacceptable weld joints later in the process.
In shops where many materials are machined, a valuable contamination-control discipline is shop cleanliness. Frequent regular cycles of machine cleaning, floor sweeping, and wet mopping go a long way toward preventing accidental contamination. Beware of "community baths," where parts may be pre-cleaned after machining. We've seen evidence of cross-contamination in shared ultrasonic tanks.
With any new project, there are many lessons learned along the path to success. The two most important ingredients in successful refractory metal parts fabrication are the absolute necessity of strictly adhered-to process controls, and the talent of a well-trained, highly disciplined workforce. Development of the tools and processes required to avoid the pitfalls associated with refractory metals manufacturing relies on careful planning, complete documentation of the steps that lead to success, and the skill and creativity of your manufacturing team.
This article was first published in the March 2004 edition of Manufacturing Engineering magazine.