New tools and strategies take on ISO S materials

By PATRICK DE VOS

Corporate Technical Education Manager, Seco Tools

THE ISO S classification of workpiece materials includes heat resistant superalloys (HRSA) and titanium alloys. The hot hardness and strength of these materials prompt their use in a wide range of critical aerospace, energy and other applications. The alloys’ beneficial properties, however, also produce machining characteristics different than those of traditional irons and steels.

In response, makers of cutting tools have developed products and application strategies that address material machinability and enable reliable, consistent and relatively economical processing of ISO S group alloys. These toolmakers also now seek to educate manufacturers on the new tools and strategies as well as convince machinists to rethink any outdated machining techniques that, most likely, will not apply to today’s advanced materials.

Machinability factors

The term machinability describes a metal’s responses to the machining process. Machinability includes four basic factors: the mechanical forces produced in machining, chip formation and evacuation, heat generation and transfer, and cutting tool wear and failure. Excessive effects of any or all of these factors can cause a material to be deemed “difficult to machine.”

Machinability issues arise with regard to tool life, process time and reliability and part quality when HRSA and titanium alloy machining is attempted with the same tools and techniques used over many decades on, for instance, steels and irons. Only in the last few years have tools been developed with nickel-based and titanium-based alloys in mind. Machining these relatively new materials is not necessarily more difficult than machining traditional metals; it simply is different.

For example, the usual approach to machining a “difficult” material is to proceed cautiously and use less-aggressive cutting parameters – including reduced feed rates, depths-of-cut and speeds. However, with cutting tools developed specifically for these high-performance workpiece materials, a basic rule is to, instead, increase depths-of-cut and feed rates. Tools engineered to handle these more aggressive parameters include fine-grained carbide grades that provide good high-temperature edge strength and coating adhesion, with particular attention to resistance to notching caused by work-hardened workpieces. Ceramic and PCBN tools have also been developed for roughing and finishing of these high-performance alloys (Read more about Machining superalloys more productively).

Machinability of super alloys

Regarding specific machinability factors, HRSA present mechanical or force-related issues that are not vastly different to tough irons or steels. There is a major difference, however, in the generation and dissipation of heat. Heat is generated when metal cutting deforms the workpiece material, and chips generated in cutting processes can carry heat away. However, the segmented chips produced by these materials often fail to do the job well. In addition, the heat-resistant materials themselves are poor conductors of heat. Temperatures in cutting zones can be 1100? - 1300? C., and when heat cannot be dissipated, it builds up in the tool and the workpiece. The result is reduced tool life and even deformation of the workpiece and changes in its metallurgical characteristics.
To help solve this problem, a change in perception about cutting tool strength is necessary. Sharp-edged cutting tools are generally considered to be weak, but one way to control build-up of tool temperatures is to use sharp cutting tools that cut the material more than deforming it, thereby generating less heat. Executing this strategy requires tools engineered for edge strength, applied on machine tools with sufficient power, stability, and vibration resistance.

Tendencies toward strain and precipitation hardening also complicate the machining of HRSA. In strain hardening, material in the cutting zone becomes harder when subjected to the stress and high temperatures of the cutting process. Nickel- and titanium-based alloys exhibit greater strain hardening tendencies than steel. In precipitation hardening, hard spots form in a workpiece material when high temperatures activate an alloying element that was otherwise at rest. With either tendency, the structure of the material may change significantly after only one pass of a cutting tool, and a second pass will have to cut through a much harder surface. A solution is to minimise the number of passes. Instead of removing 10 mm of material with two 5-mm-deep cutting passes, for example, it would be better to use one pass at 10 mm depth-of-cut. In many situations single-pass machining is not possible, but it is the theoretical goal.

This approach also requires rethinking the finishing process, which traditionally involves multiple passes at small depths-of-cut and light feed rates. Instead, machinists should look for possibilities to increase the parameters as much as possible. Doing so can improve tool life as well as surface finish.

A slightly deeper depth-of-cut for a finishing pass also positions the sharpest part of the cutting edge below any strain- or precipitation-hardened areas of the part. However, too deep a finishing pass may generate vibration and negatively affect surface finish. Finding the optimum balance between aggressiveness and caution is the key. 
Reliability and economics

With today’s tools and strategies developed specifically for nickel- and titanium-based alloys, machining can be accomplished essentially without technological problems. The ongoing challenge is not simply machining the workpiece, it is machining the workpiece correctly in a given time at a given cost. The goal is to improve process reliability and production economics. 
Considering the high cost of advanced workpiece materials and the components made from them, machining processes must be totally reliable. Manufacturers cannot afford to produce scrap parts while seeking a reliable machining process. Using appropriate tools and machining parameters help ensure consistent machining results.
Regarding machining parameters, increasing depths-of-cut and feed rates contributes to productivity. Higher cutting speeds also can expedite part processing, but that opportunity has yet to be fully exploited. The speeds employed today in nickel- and titanium-based alloys are still lower than those used with steels. But current research is focused on developing cutting tool properties that will allow even higher cutting speeds while still maintaining reasonable tool life. 

Increasing productivity
In addition to cutting tools, other components of the metal cutting process such as use of a high pressure direct coolant (HPDC) system can also help increase productivity. If cutting speeds for an ISO S material is 50 m/min., HPDC can permit cutting speeds as high as 200 m/min and thereby quadruple output. 
Tool life is another element of productivity that can be viewed from a new perspective when machining HRSA. The traditional measure of tool life counts minutes of cutting before required replacement. Another measure is cost.
If, for example, producing a certain workpiece takes 2 hours and tools must be changed every 20 minutes, then 6 tools must be purchased to complete the part. Along those lines of thinking, the goal would be to reduce tool cost and get 30 minutes of tool life instead of 20.
Tool cost, however, is a very small portion of the overall value of the parts when processing costly components made from HRSAs or titanium alloys. A more relevant measure is tool utilisation, also called a tool’s utilisation index. When comparing two sample tools, if one lasts 10 minutes and produces one workpiece, the tool cost is one tool per workpiece. Another tool, applied in a different way, might last only 5 minutes, but produce two parts. Even though the second tool’s life in minutes is half that of the first tool, the output of parts is doubled. The goal is to create the maximum number of correct workpieces in the shortest time at an acceptable price. Considering the high cost of parts made of HRSAs, the tool utilisation index is a better gauge of true productivity. 
Conclusion
As is always the case, the key factor in maximising the benefits of newly developed metal cutting technology is the knowledge of the best way to apply it in a particular operation. As progress in high-performance workpiece materials such as HRSA and titanium-based alloys continues, toolmakers will also continue to engineer new ways to maximise productivity in machining processes for the new alloys. Manufacturers will benefit from awareness of the availability of the new tools as well as the toolmakers’ comprehensive knowledge of the best ways to apply them.womens nike shoes