As a result of healthy aerospace, energy and medical industry segments, the consumption of machined components from the ISO S category (superalloys and titanium-based alloys) is increasing.
Nickel and cobalt-based superalloys have outstanding hot hardness and strength properties, especially when compared to more traditional structural alloys such as steel and cast iron. Titanium alloys have excellent strength-to-weight ratios, making them particularly attractive when weight and fuel savings are important.
Unfortunately, the very properties that make these alloys attractive for critical applications also make them more difficult to machine. Consequently, it’s important to understand the machinability factors of these alloys to ensure reliable, consistent and relatively economical part processing.
Machinability, which describes a material’s response to the machining process, includes four basic factors: mechanical force, chip formation and evacuation, heat generation and transfer, and cutting tool wear and failure. Difficulties in any or all of these factors can cause a material to be deemed “difficult to machine.”
Manufacturers using the same tools and machining techniques with ISO S materials as they were with iron and steel will find themselves battling issues with tool life, process time, reliability and overall part quality. Therefore, it’s important to rethink outdated machining techniques and make use of new cutting tools and strategies.
Below are three strategies to consider when working with ISO S materials:
#1 – The traditional approach to machining difficult materials is to proceed cautiously and use less-aggressive cutting parameters, such as reduced feed rates, depths-of-cut and speeds. However, many cutting tools made specifically for ISO S materials are meant to run at increased depths-of-cut and feed rates. The tools engineered to handle these more aggressive parameters include fine-grained carbide grades with good high-temperature edge strength, deformation resistance and wear resistance. The most common failure mode encountered when machining superalloys is notching at the depth-of-cut and depth-of-feed areas. This is usually caused by a work-hardened surface. Utilizing heavier depths-of-cut and feed rates will minimize the amount of contact time between the work piece and the cutting edge.
#2 – Compared to machining steel or cast iron, there’s a major difference when it comes to heat generation and dissipation. Heat-resistant materials are poor conductors of heat, meaning heat builds up in the tool and workpiece and, in turn, causes shorter tool life and part deformation. Thereby, manufacturers should use sharp-edged cutting tools. While generally considered weak, these sharp tools – when used on machines with sufficient power, stability and vibration resistant – cut the material more than deform it, reducing heat and temperature buildup.
#3 – Nickel- and titanium-based alloys exhibit greater strength hardening tendencies than steel. Thereby, it becomes important to minimize the number of cutting passes, when possible. For instance, instead of removing 0.4″ of material with two 0.2″-deep cutting passes, it’s better to use one pass at 0.4″ depth-of-cut. With single-pass machining, however, machinists need to rethink the finishing process, which traditionally involves multiple passes at small depths-of-cut and light feed rates. They should look for possibilities to increase the machining parameters as much as possible because doing so can improve tool life and surface finish. Finding the optimum balance between aggressiveness and caution is key.