The role of surface engineering in increasing machining performance with nickel-based alloys
Nickel-based alloys offer a variety of benefits thanks to the high strength at increased temperatures, chemical resistance and wear resistance properties of the material.
Characteristics that have led to the material being widely used across industry sectors.
However, nickel-based alloys are very challenging to machine, particularly if milling is required; due to the hardness of the material only low cutting speeds can be used and tool life is typically short, resulting in a difficult and expensive machining process.
However, surface engineering has a significant role to play in helping machining companies overcome the challenges of machining nickel-based alloys, as outlined in research conducted by F. Barthelmä et al in their 2016 research paper; Hard coatings to improve the machining of nickel-based alloys.
The challenge
There are several challenges when looking to machine nickel-based alloys:
– High thermal stress at the cutting edges as a result of low thermal conductivity through the chips and workpiece
– The low E modulus, which can lead to deflection and cause vibration or chatter
– The high strength of the material at high temperatures
Numerous research papers have taken place to develop tools and technology that will improve the efficiency of machining tough to machine materials like nickel-based alloys, with reducing wear and improving speed being the two central aims of the research.
Finding a solution
In the research conducted by F. Barthelmä et al the objective of the study was to develop cutting edge preparation, coating and finishing to identify significant improvements for reducing tool wear and increasing cutting speed for hard to machine nickel-based alloys.
To do this a series of coatings were developed to test cutting tools against different parameters; primarily cutting tool material, geometry, forces and temperature.
A range of hard coatings with high thermal and mechanical stability, low friction coefficient and good adhesion were tested. Oxynitridic (high thermal stability), Nanocomposite (high mechanical strength), and Multilayer (high crack resistance) coatings were of particular interest to researchers.
The milling of a Inconel 718 material using a DMC 64V machine was used to test the different coatings on cutting tools. Process forces and temperatures were first simulated, along with cutting edge radius ranging from 8 to 15 degrees, to test the impact on process temperature and forces.
The results
Researchers found that the thermal and mechanical stability of the substrates and coating systems is very important for improving the machining of nickel-based alloys.
It was found that AlTiCrN coatings show a reduced tool wear versus uncoated tools.
A significant increase in the mean cutting length can be achieved when using the coating system TiXCo – a combination of gradient and multilayer structures with a TiSiN top layer.
Multilayered structures based on AlTiCrN coating systems with a nanocomposite top layer proved to be very resistant to mechanical and thermal load during the milling of the Inconel 718 test piece. Particularly at high cutting speeds of 125 m/min – tool life was increased by 40% compared with the tools originally used for the cutting process.
As the research in question shows, surface engineering can have a significant impact on improving the efficiency, cost and quality of machining.
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