Turning Exotic Materials
Heat-resistant superalloys and titanium alloys are difficult to machine. This overview looks at machining demands and some recent developments that have elevated performance and process security in turning these materials.
Exotic materials are chosen for numerous applications because they have superb properties, such as strength-to-weight ratio, strength and hardness retention at high temperatures and excellent corrosion resistance. But they are also demanding to machine; they need “exotic” solutions that are well planned with dedicated tools, carefully chosen methods and tool paths to achieve efficiency, security and good results.
Unique Properties are Demanding to Machine
The ability to machine a component material is determined by several factors that influence and determine requirements and outcomes in metalcutting operations. In a broad sense, it is the ability of the component material to be cut in relation to the tendency for tool wear to be generated and how chips can be formed, with the difference being that on most scales, these exotic materials have poor machinability. They are seen as demanding to cut—but not impossible—if approached in the right way.
The more exotic of relatively common component materials are classified under the ISO group of S: heat-resistant superalloys (HRSA) and titanium alloys. For machining, these can all be split into several sub-groups, depending upon composition, condition and properties.
The chemical nature and metallurgical composition of an S-classified alloy will determine the physical properties and consequently, machinability. Chip control is generally demanding because of chip segmentation, and it is not unusual for the specific cutting force to be twice that of steel (that is the direct measure of how hard it is to cut a material and which determines the cutting force and power needed).
The main reason that HRSA materials are demanding to cut is because they retain high strength at high temperatures. They do not soften and flow in the way most other materials do, and they also work-harden readily.
High mechanical load and considerable heat are concentrated on the cutting edge. Nickel-, iron- or cobalt-based alloys are sub-groups of HRSA, having unique capabilities for component use mainly in aerospace, energy and medical industries, as their advantageous properties do not change much until close to their melting point and are very anti-corrosive.
But from a machinability point of view, they need a capable machine, rigid setup conditions, dedicated insert grades and geometries, optimized coolant application, and last, but not least, the right machining method and tool approach. Certainly more planning is critical as well, requiring more work at the front end of the manufacturing.
Titanium alloys are also divided into sub-groups with varying machinability grading. Generally, machinability is rated as various degrees of poor (toughness), which then makes special demands on tools and methods. Demands are set by the low thermal conductivity, high strength at high temperatures, highly sheared thin chips, with a tendency for galling, creating a narrow contact area on the insert rake-face and high cutting forces concentrated close to the cutting edge.
Chips can have cyclic formation, leading to variable cutting forces, and some alloys have a relatively high level of carbides, making the material extra abrasive. Excessive cutting speeds can give rise to a chemical reaction between chip and tool material, resulting in sudden chipping/fracture of the cutting edge and also material smearing/welding on cutting edges. Some alloys also work-harden readily, giving rise to diffusion-type wear, which leads to excessive burr formation and making subsequent operations difficult.
The window for successful machining of many HRSAs and titanium alloys is relatively small.
Planning for Best Results
When turning these materials, successful outcome hinges on balancing the combined effect of the material and the application factors. There are basic rules of thumb, which if followed, contribute hugely to positive results:
Pre-plan a sound machining strategy, as detailed as possible
Establish the best tool approach
Establish the best tool paths and use very stable toolholding
Use the best of new, dedicated cutting tool technology
Apply qualified cutting data to establish process security and productivity
Use spiral cutting length calculation for predicting cuts
Apply coolant correctly using modern, high pressure solutions
Make use of specialist recommendations and support
The machining process should always be carefully planned because of the critical, decisive factors involved, such as consideration to the condition of the workpiece material. Cast, forged, barstock, heat treatment, solution treatment and aging considerably affect the component in ways that should influence the selection of tools and methods.
Workpiece surface conditions vary and affect machining, as does the hardness. The strategy for turning should also include the demands made from the design features on the component that are to be machined, as well as the various stages of machining, in regards to roughing (first stage), semi-finishing (intermediate stage) and finishing (last stage). Complex features and surface integrity are common issues.
Successful Cutting Action
In these materials, successful cutting action is to an extent affected by the approach of the cutting edge to the workpiece. The lead/entering angle of the cutting edge, in combination with the insert geometry, dominates performance, tool life, security and results. Insert shapes often have to be chosen in relation to the cut that needs to be taken, but the fact that using a small entering angle contributes to performance and tool life should always be a key consideration for the application.
The choice of insert grade needs to be made partly in relation to the lead/entering angle. Among other things, this angle influences the type of wear that forms a notch on the cutting edge, the size of which affects results and can cause premature tool failure. Getting the approach part of the application right also means that an insert grade capable of higher productivity can be chosen, while also providing long tool life and security.
Making the tool material (insert grade) choice should be influenced by the stage of turning operation involved—roughing, semi-finishing or finishing—as well as the workpiece condition and the type of cut. Because of the hardness of these materials, plastic deformation of the cutting edge should always be considered as the primary risk factor when selecting the insert grade.
Insert grade selection should also be combined with the insert shape, as this is a strength issue to be assessed with the approach, chip load and whether the cut is continuous or interrupted. Dedicated grades are vital for HRSA machining.
A high degree of insert hot hardness, the right level of insert toughness and sufficient adhesion of the insert coating are the primary requirements. A positive cutting geometry, a sharp cutting edge, a strong edge and a comparatively open chipbreaker should characterize the indexable insert for these materials.
Establishing the most suitable cutting data is as vital to success in these materials. Cutting speed is limited to the combination of speed, feed and depth of cut, which can be optimized to provide high levels of productivity, security and quality.
The cutting speed is related to heat generation and how this affects the insert; it has to be high enough for the chip to have sufficient plasticity, but not too high to unbalance the tool material. Speeds are usually in the region of 130 to 260 sfm (40 to 80 m/min), with dedicated cemented carbide inserts and 490 to 1310 sfm (150 to 400 m/min) with ceramic inserts.
The feed rate is the main factor that affects the cutting time and the chip thickness. In exotics, this has to be more carefully balanced as limits are relatively tight: In roughing, the chip has to be maximized, but not so as to overload the edge, while in finishing, the chip has to be thick enough to prevent excessive heat and work hardening.
The depth of cut often affects the approach of the edge and therefore has to be below a certain value. When using round inserts in HRSA, the depth of cut should not exceed 15 percent of the insert diameter. The depth of cut also has to be programmed carefully when profiling, recesses or shoulders are involved so it does not exceed the suitable arc of cutting edge engagement.
With regards tool life, we work extensively with the spiral cutting length (SCL). Establishing this parameter correctly means that machine stoppages for insert indexing can be predicted and programmed, and that passes with a tool used at the right speed can be completed without the cutting edge becoming incapable of maintaining the required surface quality.
The high cutting forces, in combination with higher cutting edge temperatures, greatly affect tool wear with a tendency for certain types of cutting edge wear to develop. The main parameters are notch wear (the mechanical wear type where the depth of cut sets the workpiece material line); plastic deformation of the cutting edge (a consequence of the combined high temperature and pressure); and third, abrasive wear caused mainly by the harder materials.
Another indicator is top-slice wear, which develops on ceramic inserts, where layers at the top of the cutting edge are sliced off. The approach of the cutting edge plays a major role.
The most suitable entering/lead angle for turning exotics is when the angle is small, equal to or less than 45 degrees. The worst condition is when the lead/entering angle is 90/0 degrees or when the depth of cut is larger than the nose radius of the insert. A large/small angle means a thin chip and higher feeds.
For HRSA turning, the entering/lead angle of the tool will determine whether a PVD- or CVD-coated insert is the most suitable. PVD is best for a 90/0-degree entering/lead angle and CVD is best for a 45-degree entering/lead angle.
Titanium turning, on the other hand, does not benefit from insert coating. For Ti, a more important factor is to ensure the most suitable cutting edge condition, such as round inserts when finishing. New families of PVD inserts provide high hardness, good resistance to edge deformation and thermal shocks and can combine with sharp edges to excel in smearing materials for roughing to finishing operations.
The shape of the insert is an important application factor when machining these materials. The round (R-shape) has become one of the main recommendations for these materials. The round insert provides strength for a sharp, positive cutting edge; a chip thickness that varies along a long cutting edge, allowing higher feed rates; a large insert radius, which does not restrict the feed rate because of the surface finish it creates. The round insert also gives the programming flexibility to perform profiling and pocketing operations required by many component shapes.
A square insert (S-shape) is in some cases the most suitable for first stage machining, with its capacity for roughing cuts in various directions with a 45-degree angle. The rhomboid insert (C shape) has built in flexibility in regards to tool paths, and when extended to be an Xcel-type insert, it provides even more tool accessibility into corners, shoulders and recesses. This combination of insert shape and 45-degree lead/entering angle also reduces radial cutting forces, gives a constant chip thickness and reduces notch wear. The result is higher productivity, longer tool life and better security.
Getting the programming right is important. Here are some recommendations toward improved performance, especially when using round inserts in exotic materials:
Avoid plunging into cuts, and soften impacts; if these cuts are necessary, halve the feed rate
When turning to a shoulder, the feed should also be reduced by half or the tool should roll up to the shoulder where the programmed radius is the same as the insert diameter. (Guidelines are for the minimum programmed radius to be about 25 percemt of the insert diameter and the component radius 75 percent of insert diameter. The tool center feed is for the programmed radius)
For roughing with round inserts, allow the programmed radius to equal the insert diameter, and for finishing, make sure the programmed radius is larger than the insert diameter
Consider alternative tool paths, multiple passes and machining in both directions to fully utilize inserts
Protect ceramic inserts by pre-chamfering the workpiece and feed into the chamfer
Maintain a satisfactory lead/entering angle balance to the arc of insert engagement throughout machining. Limiting the angle is essential to good performance, and the use of round inserts is an important booster to realizing available potentials with modern insert grades
Avoid any wrap-around effects when profiling or plunging so the insert isn’t overloaded or use alternative tool paths or smaller insert diameters
Consider trochoidal turning, breaking the cut up into suitable smaller cuts, especially when pocketing
High pressure coolant application should always be on the agenda, as major developments have taken place in this area. With precision jets acting behind and at the cutting edge, supplied by through-coolant tooling, for turning, milling and drilling operations, there are many benefits that can be claimed. Nozzle technology, available as standard and engineered solutions with various ranges of pressure can be applied to all types of machinery with adequate coolant supply. Coolant pressure for new machine investments should always include a coolant pressure possibility of 70 bar for titanium turning to facilitate improved chipbreaking. However, pressure of up to 200 bar is advantageous for HRSA turning, because of hard-to-break chips. Broad modularity and advanced nozzle technology should be looked for.
Accessibility is often an issue when turning exotics. Complex features and tool overhangs make tooling and methods critical to get right. In modular systems where a reliable concept with tool blades presenting the cutting edges at different angles and overhangs is available, the range of adaptor and blade alternatives gives the flexibility to build almost any tool from a limited tool inventory to suit configurations and give accessibility in external or internal confined spaces using standard tooling. Blades should include the required radial and axial clearances for reaching deep into angled grooves with high-pressure coolant supplied through the tool to the cutting edge.
Tool material is at the heart of any machining operation, and for exotics they are a critical factor. This area requires dedicated insert grades, in combination with the right insert geometry, mainly in the form of the latest in cemented carbide and ceramics. Uncoated carbide grades still have an important position, but the latest developments in insert coating technology has provided especially coated cemented carbide inserts that have moved cutting edge capability on by shortening cutting time and extending tool life.
Built-in tool dampening features in boring bars, blades and even milling cutters should be included to minimize vibration tendencies. Anti-vibration technology has taken huge leaps and should be a natural option when tool overhangs tend to generate instability effects. Productivity, process security and component quality are factors directly related to the quality and availability of anti-vibration tooling. Numerous operations are impossible to perform without this facility: today, internal turning, involving overhangs of up to 14 × diameter for 250 mm or less, can be performed efficiently with a high quality finish.
Turning exotics is doable and is increasingly in demand. Following the experience of adapters that are machining these exotics and consulting with the companies that are researching better tools, methods and processing developments is a key to a quick success when processing these unique materials.
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