By Don Graham, Manager of Education and
Technical Services
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 Therefore, it’s important to rethink outdated machining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.
At Seco, we’ve developed several advanced
products and strategies that address ISO S machinability – and will continue to
do so as progress with these high-performance workpiece materials continues. If
you have any questions or would like more information on how our tools and
machining strategies for ISO S materials can bring you increased success,
please don’t hesitate to contact me.
About the Author
Don
is the manager of education and technical services for Seco, responsible for
all educational activities for the NAFTA market, new product testing and
various other technical functions. Outside of work, he enjoys making maple
syrup, restoring antique tractors and farming.
