Heat resistant super alloys are among the demanding materials to machine. And when greater efficiency, predictability and quality levels concerning manufacturing in these materials are increasing, as they are today, the right approach to machining is crucial.
Chris Mills is senior applications manager at Sandvik Coromant and is deeply involved in the development of tools and methods for heat resistant super alloys (HRSA). In this article, Chris Mills answers a number of important questions concerning the present approach to optimising machining in this challenging area.
Why are HRSA demanding to machine?
HRSA are metallurgical composed to have high strength at high temperatures and the consequence of this is that the stresses involved when machining the HRSA are also high. The nickel-, iron- or cobalt-based super alloys’ unique capability of component-use so close to the melting point of the basic metal gives the HRSA varied but generally poor to very poor machinability. (About twice as much power is needed to machine HRSA as is needed for low-alloy steel, with the specific cutting force being around 4000 for HRSA compared to 2500 for steel, expressed in Newton per square mm).
Although being ductile, a consequence of HRSA fatigue resistance, hardness and toughness at high temperatures is that a number of wear mechanisms develop rapidly, as the cutting edge is exposed to considerable mechanical stress, strain and heat. High compressive and shearing forces attack the cutting edge, while HRSA is in its susceptible condition, as the chip-flow temperature approaches that of the limit, which the cutting tool material needs to maintain sufficient hardness.
Many of the HRSA also work harden readily, give rise to diffusion wear, which usually leads to heavy burr formation and can also make subsequent operations more difficult. This means that the cutting speed (the factor largely responsible for the amount of heat generated) is limited and has to be kept well below that of more common work piece materials. What is all the more important is the combination of cutting data, speed, feed and depth of cut?
Why should the machining process for HRSA be carefully planned?
There are a number of decisive factors. First, consideration should be given to the state of the work piece material, as the various types of machining (roughing, semi-finishing and finishing) are more suited to certain conditions. Heat treatment, solution treatment and aging, all affect the work piece in ways that should influence machining and, consequently, the different machining operations need to be carried out at a suitable stage of manufacturing.
Whether the material is cast, forged or bar stock affects the machinability and applications as well. Then follows the review of a number of crucial tool and method factors, which will be discussed individually. (For example, regarding the state of HRSA, the specific cutting force varies from about 3500, in the annealed/solution treated condition, to about 4150 in an aged condition and hardness varies from 30 to 48 HRC.)
Machining HRSA has, since the introduction of the first super alloys during the mid-1900s, always been seen as difficult. But with developments in machinery, equipment and above all in tool materials, and now also in new machining processes, HRSAs can be machined in a way that makes the component manufacturing more efficient and economical. This is also a priority for aerospace, energy and medical industries, as competitive and environmental issues are increasingly important. Planning a new HRSA machining strategy, which includes optimisation with the recent cutting tools and new findings in machining methods lead to proven manufacturing benefits.
What should the HRSA machining strategy include?
Adopting a suitable machining strategy for the type, condition and design of super alloy work piece in question will bring improvements, if the right tools are applied in the correct way and good methods are employed. It is not so much the individual levels of each characteristic of HRSAs that create the machining demands; it is the combination of several high qualities that sets the machinability levels. In this context, Sandvik Coromant has recognised the importance of being a solution provider through dedicated cutting tools, capable holding tools, application know-how and pioneering solutions.
To start with, one of the crucial factors that affect the cutting action in HRSAs is the approach of the cutting edge. The size of the entering angle dominates the performance, tool-life and results that the cutting tool provides. Sandvik Coromant has built a lot of its solutions around this issue.
As important is the capability of the cutting edge to cope with cutting HRSAs. The tool material and tool geometry have to be dedicated to these work piece materials because of their demands. 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 chip breaker should characterise the indexable insert.
Establishing a suitable cutting data is vital to the success of machining HRSAs. Cutting speed is limited in these materials, but the combination of speed, feed and depth of cut can be optimised 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 40 to 80 m/min with dedicated cemented carbide inserts and 150 to 400 m/min with ceramic inserts).
The feed is the main factor that affects the cutting time and the chip thickness. In HRSAs, this has to be more carefully balanced as limits are relatively tighter: in roughing, the chip has to be maximised, 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 in HRSA machining and consequently has to be below a certain limit. For example, when using round inserts in HRSA, the depth of cut should not exceed 15% of the insert diameter. The depth of cut also has to be programmed carefully when profiling, recesses or shoulders are involved, so as not to exceed the suitable arc of cutting edge engagement.
Regarding tool-life, Sandvik Coromant works extensively with the spiral cutting length (SCL). Establishing this 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.
Why are tool-lifes so much shorter in HRSA machining and what can be done to optimise performance?
High cutting forces, in combination with the higher cutting edge temperatures means a tendency for certain types of cutting edge wear to develop. The main ones are notch wear (the mechanical wear type, where the depth of cut sets the work piece material line); plastic deformation of the cutting edge, a product of the combined high temperature and pressure, and thirdly, abrasive wear caused by the high hardness of the material. Another is top slice wear, which develops on ceramic inserts, where layers of the top of the cutting edge is sliced off.
These destructive wear patterns have to be contained mainly by a combination of suitable cutting tools and correct machining methods. Cemented carbide inserts, particularly modern fine-grained ones, are suitable and broadly applicable, along with modern Sialon ceramics and to some extent whiskered ceramics.
The insert grade selection is not as directly related to roughing and finishing as it is in other work piece materials. Instead, grade selection is more an optimisation factor dependant upon the shape of the insert and the approach of the cutting edge, and the type of operation (turning, profiling or groove and recess machining).
Why is the cutting edge approach so critical in HRSA machining?
The entering angle of the tool affects the chip thickness, the feed rate, cutting forces as well as the type and cuts that can be taken with the tool. The choice of entering angle has direct consequences for the productivity and process reliability. The entering angle will influence the insert shape and the nose radius, and how well the insert grade can be utilised.
Suitable for HRSA turning is when the angle is equal to or less than 45 degrees and the worse condition is when the entering angle is 90 degrees or when the depth of cut is larger than the nose radius of the insert. A small entering angle means a thin chip and higher feeds.
This has led to round (R-shape) being the main recommendation for this area. The round insert provides strength for a sharp, positive cutting edge; a chip thickness that varies along a long cutting edge allowing high feed rates; a large insert radius, which does not restrict the feed rate because of the surface finish 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 suitable for first stage machining, with its capacity for roughing cuts in various directions with 45 degree entering angle. The rhomboid insert (C-shape) has built in flexibility regarding tool paths and when extended to be an Xcel-type insert provides even more tool accessibility into corners, shoulders and recesses. This combination of insert shape and 45-degree 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.
What precautions should be taken when programming HRSA machining operations?
Programming technique is as important as any of the other HRSA machining factors and the following provides a few recommendations towards improved performance, especially when using round inserts.
- Avoid plunging into cuts, and if necessary, reduce feed by half.
- 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 some 25% of the insert diameter and component radius 75% of insert diameter. The tool centre feed is for the programmed radius.)
- Generally, employ rolling in and out of cuts to soften sudden impacts and to reduce wear. 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, such as ramping, multiple passes and machining in both directions to utilise more of the insert.
- Protect ceramic inserts by pre-chamfering the work piece and feed into chamfer.
- Maintain a satisfactory entering angle as well as arc of insert-engagement throughout machining, but especially when roughing HRSAs in corners, etc. A maximum of 45 degrees is often suitable as regards both values. Balance the demands of these values to the strength of the cutting edge, smaller entering angle needs a stronger insert shape and thickness.
- Avoid wrap-around effects when profiling or plunging, so as not to overload the insert, use alternative tool paths or smaller insert diameter.
- Consider trochoidal turning, breaking the cut up into suitable smaller cuts, for pocketing.
Finally, how does one go about securing a high productivity level when machining HRSAs?
Good HRSA machining is about optimising the combined effect of the factors involved. There are a few main rules of thumb for HRSA turning:
- plan the machining strategy
- use the right insert approach
- select good insert/tool characteristics
- programme good tool paths and cutting data
- use spiral cutting length calculation
- apply coolant correctly
In planning the HRSA machining strategy, analyse the component design and the material. Then, if applicable, establish how first-, intermediate- and last-stage machining should be carried out in relation to the material condition and to the quality demands.
Ensuring the right insert approach is essential to improving other HRSA machining factors and the overall performance. Limiting the entering angle is essential to good performance and the use of round inserts is an important booster to realising available potentials with modern insert grades.
Modern indexable insert technology has made available capable cemented carbide grades and ceramic grades. Both PVD and CVD coated, fine grained cemented carbide grades will optimise machining, largely in relation to the approach of the cutting edge and material hardness. Ceramic Sialon and whisker grades will optimise machining of different work pieces condition and type of cuts at high cutting speed levels and suitable conditions.
Planning tool paths and employing feed reduction when programming, will determine the extent of wear, type of tool, cycle times and security. Consequently, the way the tool machines HRSAs has been found to be decisive as to results and become a focused development area at Sandvik Coromant.
Using the spiral cutting length calculations will help in predicting the time or cutting length, so as to reduce the machining time and improve the surface finish in demanding materials such as HRSA. The insert changing stops are planned to better suit the operation and cutting data to optimise tool-life.
The use of coolant is important for many HRSA machining. The high temperatures generated make it necessary to provide some cooling effect. Coolant should be plentiful, at high pressure and well directed. Methods and equipment such as with Jet Break provide good results, where even chip breaking is improved.