High Speed Finish Turning of Inconel 718 Using PCBN Tools under Dry Conditions

Abstract

Inconel 718 is a superalloy, considered one of the least machinable materials. Tools must withstand a high level of temperatures and pressures in a very localized area, the abrasiveness of the hard carbides contained in the Inconel 718 microstructure and the adhesion tendency during its machining. Mechanical properties along with the low thermal conductivity become an important issue for the tool wear. The finishing operations for Inconel 718 are usually performed after solution heat treatment and age hardening of the material to give the superalloy a higher level of hardness. Carbide tools, cutting fluid (at normal or high pressures) and low cutting speed are the main recommendations for finish turning of Inconel 718. However, dry machining is preferable to the use of cutting fluids, because of its lower environmental impact and cost. Previous research has concluded that the elimination of cutting fluid in these processes is feasible when using hard carbide tools. Recent development of new PCBN (Polycrystalline Cubic Boron Nitride) grades for cutting tools with higher tenacity has allowed the application of these tool grades in the finishing operations of Inconel 718. This work studies the performance of commercial PCBN tools from four different tool manufacturers as well as an additional grade with equivalent performance during finish turning of Inconel 718 under dry conditions. Wear tests were carried out with different cutting conditions, determining the evolution of machining forces, surface roughness and tool wear. It is concluded that it is not industrially viable the high-speed finishing of Inconel 718 in a dry environment.

PCBN Performance in High Speed Finishing Turning of Inconel 718

“…This zone causes a sudden increment of undeformed chip cross section at the end of the following passes. Since cutting depth is small in finishing operations, significant increments of cutting forces were observed at the end of the pass affecting tool wear [22]. A secondary tool was provided in the lathe in order to remove this zone at the end of each cutting pass (see Figure 1).…”

“…The workpiece was a cylinder with diameter 100 mm of superalloy Inconel 718, hardened by solution heat treatment and aged, obtaining an homogeneous hardness throughout the thickness of the bar of 44-45.5 HRc [22].…”

“…The tool KB5625 was coated with TiAlN while the tool 7015 was coated with TiN, in both cases the coating was applied in a thin layer with reduced influence during machining process (these coatings got removed from the tool-workpiece interface on the first instants of the tests). Although these PCBN grades are not specifically recommended for finishing turning of Inconel 718 (mainly recommended for hardened alloys), they are similar to CBN170 grade (sharp cutting edge and low CBN content) and in some cases they are currently being used for this machining operation in industry [22,26].…”

“…In previous works developed by the authors, a systematic and broader analysis of tool wear can be found for this type of cutting process both with coolant and in dry conditions [22,23].…”

“…In a previous work of the authors [22] the viability of machining Inconel 718 with PCBN tools under dry conditions was analyzed. Elevated levels of chipping and fragile cutting edge breakages resulted from the instability produced by machining without coolant.…”