Tooling Matters When Cutting Exotic Alloys
Materials that are being characterized with their resistance to heat, corrosive environment, acid, and alkaline environment as well as their ability to retain their material properties under these unfavorable situations are also referred to as heat resistant super alloys or exotic materials. They used to be a little challenging and frightening topic for machinists back in the days due to being extremely difficult to machine and have been related to their unpredictable and abrupt end of tool life.
Therefore, although many of these materials have been developed for 100 years, their usage has been more or less restricted despite their great advantages in many applications. In fact, even at this very moment they still are (to some extent) a difficult task for machinists, programmers, toolmakers, etc. because of the tooling requirements, machine rigidity, and special programming approaches that need to be employed, amongst many other things. However, the unsurpassed advantages this material group has, makes them ideal for industries like aerospace, medical, oil, gas, and subsea applications, etc. Typical utilization of HRSA can be found in the designs of impellers, blisks, gas turbines, shafts, medical implants, and so on.
Which are the exotic materials?
The group of materials referred by tool manufacturers under the ISO group of S is relatively small, and typical examples among the list are the heat resistant super alloys Inconel, Monel, Hastelloy, Waspaloy, Udimet, Haynes, Stellite, and the titanium alloy grades. According to their chemical composition, the stainless steel group ones can be split into a few main groups - nickel based, iron based and cobalt based. They are relatively expensive materials due to the high content of expensive alloying elements like nickel, cobalt, and copper, as well as the titanium from the other group.
The physical properties of these materials vary significantly one from another which is caused not only by their chemical composition but also by the metallurgical processes involved in the processing during production.
Nickel based alloys are the most widely spread ones, and typical examples are the Inconel and Wasp alloy. Iron based ones are derived on the base of stainless steels and applications that require low thermal expansion are suitable for them. The cobalt based ones are most difficult to machine, and at the same time, most expensive and characterizing at the same time with the greatest hot corrosion resistance. Therefore, you can see them in hot combustion chambers and surgical implants.
The medical industry, and especially the implants, take advantage of the titanium alloys as well. They are extremely resistant to rust and oxidation, and at the same time, possess the great advantage of having big strength to weight ratio with a density of about half of one of the most steels making them ideal for where such solutions are sought.
Why are exotic materials difficult to machine?
The usage of heat resistant superalloys and titanium alloys have greatly improved over the recent decades with the development and introduction to very advanced technologies in CNC tooling, tool holding, machines, coolant supply, and emulsion, etc. They are of course not impossible to machine but one should have an agenda to many of the points listed below in order to provide optimal cutting conditions and approach to do so.
The answer to why exotic materials are difficult to machine lay at a couple of main reasons like CNC machine stability, work holding, and tool holding stability, appropriate programming methods, and of course the major one - the cutting tools. But before we put our main focus on the tool we have to look at the other sources of possible problems. Overall, the whole setup to machine a part must be stronger in comparison to the other not so demanding materials.
Reasons for poor machinability of exotic materials
The major issues come from a few of the specifics of the exotic materials that basically have to do with their high physical and chemical properties. The first thing to mention is their generally high toughness and mechanical wear resistance, especially at high temperatures. The behavior of the material is highly abrasive to the cutting edge and increases significantly the notching and flank and crater wear of the tool. Moreover, after cutting has been performed especially with higher surface speed imposing higher temperatures in the cutting zone, processes like oxidation and work hardening are becoming contributors to the reduced tool life. This means that the very shallow cutting passes in finishing operations, for example, may turn out to be even more aggressive and highly deteriorate or even destroy the cutting edge of the tool. For the same reason while performing roughing due to the proneness of notching at the depth of the cut line, variable chip thickness can be employed so as the wear can be slightly distributed at the length of the cutting edge. Therefore a must to be observed is to have the tooltip cut below the oxide layer.
Another point to make is regarding the low thermal conductivity. The exotic materials group remains strong at the chip formation during machining making the chip control difficult. What happens while cutting is that the heat does not get released through the chip as the scenario with other common materials like stainless steel for example. Instead, the temperature stays much higher, and therefore for most of the tools an abundant quantity of coolant must be applied. Especially useful is the use of internal coolant and jet coolant with high pressure and direction straight to the cutting zone is applied. The emulsion percentage depends on the manufacturer but relatively higher values of about 9-10% are recommended.
Having already exposed the reasons leads to answers related to the proper tooling selection necessary for a reliable and optimized process. In order to reduce heat in the cutting zone and minimize the work hardening effect tools with a sharp cutting edge and positive rake angle are recommended.
The cutting speeds should be about 2-4 times smaller if compared to the AISI 304 stainless steel, for example, a limitation also due to attempting to control the temperature in the cutting zone. For the same reason, except for the ceramic tools feed rates are limited and cannot be increased significantly.
Usage of HSS tools for example is almost impossible, resulting in extreme wear of the tool without almost any work to have been done. Therefore they are not a choice for exotic materials.
What type of tools gives good results? The answer is carbide tools and ceramic and cermet tools. Although each should be employed completely differently from the other, in terms of cutting parameters, tool life, and overall performance they give the best results.
Turning - From the carbide inserts CVD and PVD coatings of TiAlN are preferred choices and depending on the cutting operation various geometries from sharper to more reinforced ones are used while programming remains more standard.
Having a ceramic tool on the other hand must employ a different programming strategy utilizing much higher cutting speeds and feeds and lower depths of a cut as insert cracking may occur. They take advantage of the chip thinning effect as the ceramic tools should preferably have about 45 degrees entry angle with maximum chip thickness between 0.003-0.006in. Carbide inserts also dislike interrupted cuts so the tool should be kept in cutting contact as much as possible.
Milling - as far as it concerns milling a few recommendations that are also valid for the drilling especially when employing solid carbide tools are valid. These are maximum stability of the part for vibration control and minimum runout. They are of extreme importance because of the material toughness which for small sized tools will lead to their breakage. So this means that compromise with the tool holding cannot be made either.
Using high flute count end mills in combination with trochoidal and high efficiency milling toolpaths definitely facilitates a longer and more predictable tool life by controlling this way and keeping down the temperature in the cutting zone.
Drilling - for hole making a recommendation is to use replaceable tip carbide drills. They can provide good drill times due to higher feeds and leave a good surface finish. Solid carbide drills are also a solution for the smaller diameters, but need to have the cutting edge maintained sharp. A through coolant is an absolute must for hole making operations in exotic alloys. Overall roughing the HRSA could be a slow operation making large and even mid size cutting take a lot of time. However challenging and difficult machining of heat resistant super alloys could be it is not a mission impossible to machine them especially if the time and effort to prepare enough in terms of tooling, work holding and knowledge of the processes. They are unique materials so should be treated differently while working with them as well!