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Industry tips from Seco, machining HRSA and titanium alloys
Industry tips from Seco, machining HRSA and titanium alloys

Industry tips from Seco, machining HRSA and titanium alloys

Added to MTDCNC by Seco Tools (UK) Ltd on 06 May 2014

The ISO S classification of work-piece materials includes heat resistant superalloys (HRSA) and titanium alloys. The hardness and strength of these materials mean they are widely used and increasingly specified for many performance-critical aerospace, power generation and motorsport parts and components.

However, the very properties that make these alloys ideal for such applications also affect their machinability resulting in cutting tool manufacturers developing a range of new and different products and strategies to ensure reliable, effective and economical machining of them.

Machinability factors

The term machinability describes a metal’s response and reaction 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, reliability and part quality when HRSA and titanium alloy machining is attempted with the same tools and techniques used for machining more conventional steels and irons.

Machining these relatively new materials is not necessarily more difficult than machining traditional metals; it’s just different and requires a different approach.
 
For example, conventional wisdom for machining a 'difficult' material is to proceed cautiously and use less-aggressive cutting data e.g. reduced feed rates, depths-of-cut and speeds.

However, with the advent of new cutting tools developed specifically for these high-performance alloys, the basic rule of thumb is to 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, and resistance to notching caused by work-hardening. Ceramic and PCBN tools have also been developed for roughing and finishing of these high-performance alloys.

With regard to specific machinability issues, although HRSA, from a mechanical or force-related perspective, are not vastly different from tough irons or steels - there is one major difference - most notably with regard to the generation and dissipation of heat.

Heat is generated when metal cutting deforms the work-piece material, and the chips created during the cutting process (in an ideal world) carry the heat away. However, the segmented chips produced when machining HRSA and titanium alloys do not do this that well.

In addition, the heat-resistant materials themselves are poor conductors of heat. Temperatures in cutting zones can often reach 1100Ëš - 1300Ëš C., if not dissipated quickly or effectively builds up in the tool and the work-piece causing reduced tool life and even deformation of the work-piece.

To help solve this problem, a change in perception about cutting tool strength is needed. Sharp-edged cutting tools are generally considered to be weak, but one way to control the build up of tool temperatures is to use sharp cutting tools that cut rather than deforming the material.  This approach generates less heat.

Executing this strategy requires tools with high edge strength combined with rigidly-built machine tools with good power, stability, and vibration resistance characteristics.

Tendencies toward strain and precipitation hardening also complicate the machining of HRSA.

With 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 work-piece 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. The solution is to minimise the number of passes.

Instead of removing say 10mm 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 and in doing so improve tool life as well as surface finishes.

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 without many technological problems.

However, the challenge is not simply machining the work-piece - it is machining the work-piece efficiently and economically.

Considering the high cost of advanced work-piece materials and the components made from them, machining processes must be secure and reliable. Manufacturers cannot afford to produce scrap or have to re-work parts.

Regarding machining parameters, increasing depths-of-cut and feed rates contributes to productivity.

The speeds employed today in machining 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 to be used while still maintaining reasonable tool life.

In addition to cutting tools, other elements involved in 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 issue 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 work-piece 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, represents a small proportion 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 work-piece, the tool cost is one tool per work-piece. Another tool, applied in a different way, might last only 5 minutes, but produce two parts.

The goal is to create the maximum number of correct work-pieces in the shortest possible 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.

Summary

As is always the case, the key factor in maximising the benefits of newly developed metal cutting technology is knowing how and when it should be applied.

Seco’s expertise and know-how makes us a good place to start.
 

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