Mastering difficult-to-cut aerospace materials

Difficult-to-cut materials are defined as engineering materials with significantly lower machinability than typical. These materials are often referred to in shoptalk as ‘hard-to-machine,’ ‘tough-to-cut,’ or even ‘nasty.’ It is important to note that high hardness is not the only characteristic that makes these materials challenging to machine; several other factors contribute to their machining difficulties.

Every industrial branch, in one way or another, must deal with such materials. However, the leading consumer of these materials is the aerospace industry. It is in this sector—whether in the production of turbine engines, landing gear components, or primary airframe elements—that difficult-to-cut materials have found their most significant application. In the aerospace industry, the key characteristics of required materials are strength-to-weight ratio, robustness, fatigue life, and resistance to corrosion and hot temperatures. It turns out that materials meeting these characteristics are difficult to machine, posing a serious challenge for production. When speaking about such materials, the following information should be considered:
l High-alloy high-strength steel,
l Titanium alloys,
l Hot temperature superalloys,
l Composites.
High-strength steels are used in elements subjected to extreme mechanical load, such as landing gear. They are also the primary material for various fasteners.
Titanium alloys, with their excellent strength-to-density ratio, high corrosion-proof, and creep resistance, have become the preferred material for producing the main parts of a jet engine’s cold section. These alloys also provide a suitable alternative to steel and stainless steel for weight savings.
Hot-temperature superalloys (HTSA), which can maintain their strength in high-temperature environments, are common in the components of a jet engine’s hot section.
Composites offer impressive specific strength, helping to decrease an aircraft’s weight and opening new prospects for aerodynamic fairing. In military aviation, composites contribute to improved aircraft’s stealth characteristics.
However, machining these materials presents numerous challenges for several reasons. The main challenges are as follows:
l High material strength results in significant cutting forces that increase the mechanical load on the cutting tool.

l Intensive heat generation and low thermal conductivity of the material (for example, when machining titanium alloys) cause poor heat transfer. This leads to a growing thermal load on the tool and increases the risk of build-up on a cutting edge.
l Work hardening of materials (specifically HTSA) increases the strength and hardness of the workpiece surface during cutting, making machining difficult.
l The high abrasion factor (when machining composites) intensifies the galling of the tool’s active area.
Additionally, other specific material features affect machinability. For example, the ‘springiness’ of titanium alloys contributes to vibration, worsening machining accuracy and surface finish. The inner structure of composites is associated with material delamination during machining.
All the mentioned aspects accelerate tool wear and diminish tool life.
The trends in the aerospace industry distinctly point to the increased usage of difficult-to-cut materials. This is manifested by introducing new alloys with higher strength and heat-resistance characteristics, the broad application of composites, and the utilisation of hybrid-structure (metal-composite) materials.
To meet these challenges, the metalworking industry introduces new cutting strategies based on the impressive capabilities of modern CNC machines and CAM systems. However, the cutting tool is the final link in the entire machining chain, which directly contacts the material and removes its layer during operation. This link is the most ‘conservative’ part of the chain, and its development is slow, often hindering the full utilisation of advanced machines’ capabilities. To overcome the existing difficulties, it is necessary to reduce machining data, which leads to low productivity and increased machining costs. Understandably, every step forward, even a small one, in the tool field can significantly improve machining ‘nasty’ materials. Therefore, the metalworking industry welcomes the latest advances in cutting tools, hoping they will radically change the situation.

What are the main requirements for tools intended for efficient machining of tough-to-cut aerospace materials? In brief, they need to be hard, durable, and precise to enable productive machining while ensuring predictable and stable prolonged tool life and the required accuracy level. However, meeting these understandable and reasonable demands requires significant efforts, and even small progress in this direction faces challenges. Every new advance involves serious R&D work and extensive testing.
Hence, tool development focuses on the following areas:

Cutting material grades.

Tool design.

A tool’s digital component.
The emphasis in cutting materials is on improving hardness, wear resistance, and high-temperature strength. This is achieved by introducing new coatings, particularly those based on nanotechnology, and expanding the use of extra-hard cutting materials like cubic boron nitride (CBN) and ceramics.
In tool design, enhancements optimise macro and micro-geometries, such as the cutting edge design and cutting edge preparation, to improve cutting capabilities. Computer modelling, combined with computational dynamics and the impressive possibilities of additive manufacturing (AM), provides new opportunities for shaping flutes, chip gullets, and internal coolant channels. This contributes to better chip evacuation and coolant supply, improving tool performance. A key area is increasing the vibration resistance of the tool through advanced cutting geometry, enhancing the rigidity of the tool structure, and using vibration-damping toolholders.
The tool’s digital component includes the digital twin and various software products, which allow for virtual assembly, machining simulation, cutting data selection, tool life estimation, and necessary calculations such as machining power and cutting forces. ISCAR expects that this component will be compatible with data exchange standards to meet the requirements of Smart manufacturing. The noted trends are evident in the new developments from ISCAR, one of the leaders in tool manufacturing, presented within the framework of the latest LOGIQUICK campaign.
The company has introduced two new carbide grades with PVD coating: IC1017, which ISCAR created for turning Ni-based HTSA, and IC716, which is intended for high-performance indexable milling titanium alloys. The range of chatter-dampening solid carbide endmills has been expanded with contemporary designs made from the bronze-coloured IC608 carbide grade, which is the first choice for the ISO S group of applications (cutting superalloys and titanium). Another carbide grade, IC5600, has been developed by ISCAR’s prolific R&D Materials Sciences engineers for milling steel (ISO P group). The combination of the grade’s submicron substrate, CVD coating, and post-coating treatment aims to enhance resistance to abrasive wear and thermal loading, increasing cutting speed, particularly when machining high-strength steel.

CERAMILL, a new family of endmills with indexable single-sided round inserts (Fig. 1), is interesting from two points of view. First, the family utilises an ingenious mechanism for high-rigidity insert clamping. This mechanism allows for an increased number of teeth compared to a typical design of similar tools in the same diameter. Additionally, it enables simple indexing and insert replacement without removing the endmill from the toolholder. Second, the inserts are produced from ceramic grades specially engineered for machining HTSA.
ISCAR has expanded its range of anti-vibration products based on the innovative built-in damping mechanism, which features a heavy mass supported by a rubber spring element containing oil for enhanced dampening. The turning line now includes vibration-absorbing boring bars for machining depths of 12 and 14 times the shank diameter (Fig. 2). Additionally, the milling line has incorporated anti-vibration shanks with MULTI-MASTER adaptation.
The QUICK-X-FLUTE is a family of extended flute shell mills developed to achieve high metal removal rates (MRR) in rough milling of challenging materials such as HTSA and titanium (ISO S), austenitic and duplex stainless steel (ISO M), and steel (ISO P). These ‘porcupines’ utilise cost-effective, double-sided square inserts with 8 indexable cutting edges. A key aspect of the mill design is the optimised flute geometry (Fig. 3), which balances mill rigidity and the necessity for adequate chip flow when cutting at high MRR. The QUICK-X-FLUTE mills also feature an inner coolant supply option. Using high-pressure pinpointed coolant with replaceable nozzles and face frontal outlets ensures a direct supply of coolant to the cutting zone. This enhances the cooling and lubrication action, contributing to optimal chip control.
Effective coolant supply has become a primary emphasis in contemporary tool designs. In the PICCO line of miniature turning tools, ISCAR has introduced new inserts with internal coolant channels (Fig. 4). In hole making; the company has unveiled new 3-flute solid carbide drills for machining flat-bottom holes featuring three coolant outlets. The latest additions to tool holding include holders with coolant channels along the shank bore. Additionally, in ISCAR’s classical HELIMILL indexable milling family, the company’s R&D engineers have upgraded the design to maximise flow rate with minimal pressure drop using computerised fluid dynamic (CFD) software.
Improvements have also been made to NEOITA, an integral part of the company’s digital tool component. NEOITA, an expert system that recommends optimal tooling solutions and forecasts tool life for a specific application, now includes new features. One of these is the AI-powered information search, specifically designed to retrieve detailed data on workpiece materials, including their metallurgical composition and typical uses.
Enhanced performance in machining difficult-to-cut aerospace materials is a complex challenge. Tool manufacturers continually strive to provide more advanced and productive solutions to address this. While no radical breakthroughs have been observed, recent developments indicate steady evolutionary progress in this area.