Long Sump Life Effects of a Naturally Aged Bio-Ester Oil Emulsion on Tool Wear in Finish Turning a Ni-Based Superalloy

Abstract

This paper discusses a method of finish turning Inconel 718 alloy to compare machining performance of a naturally aged and used metalworking fluid (MWF), which had been conventionally managed through its life cycle, with the same new unaged product. The MWF concentrate was a new-to-market bio-ester oil, diluted with water to produce an emulsion. In the experiments, 50 mm diameter bars were turned down with multiple passes at a 250 μm depth of cut to reach a tool flank wear of 200 μm. The machining was interrupted at several stages to measure the flank wear and compare the chip forms for the aged and unaged MWF. The method of finish turning used a small tool nose radius and a small depth of cut that was found to be sensitive in detecting a difference in the flank wear and chip forms for the aged and unaged MWF. On the chemistry, the findings suggest that higher total hardness of the aged MWF was the cause of reduced lubricity and accelerated flank wear. This paper discusses the state of the art with the insights that underpin the finish turning method for the machinability assessment of MWFs. The findings point to stabilization of the MWF chemistry to maintain machining process capability over an extended sump life.

Comparing Bio-Ester and Mineral-Oil Emulsions on Tool Wear and Surface Integrity in Finish Turning a Ni-Based Superalloy

“…Whilst some key performance measures in these published articles were studied, such as tool wear or surface integrity, none considered the broader range of performance measures covering, for example, tool wear rate, cutting forces, chip forms, surface and subsurface integrity in the performance evaluation of the CF to finish turn this alloy. This paper complements the recent work [24] that studied the effect of a long sump life of a bio-ester emulsion on tool wear in finish turning a Ni-based superalloy. Furthermore, the number of performance measures was increased to compare the bio-ester and mineral-oil-based emulsions with the aim of increasing confidence in the findings.…”

“…The cause of notch wear was attributed to high surface work hardening of the workpiece in the chip-forming region that developed a high adiabatic temperature. Wood et al [24] identified significant notching when flank wear exceeded 0.3 mm in finish turning Inconel 718. Darshan et al [44] found different CFs, and their application methods influenced the chip forms that were attributed to the prevailing friction conditions.…”

“…The four CFs were modern, commercially available products that complied with the latest H&S requirements. To compare their performance in finish turning an Inconel 718 alloy bar, the approach used an adaptation of the setup described in [24]. In this study, the approach used a coolant with a lean concentrate diluted at 6.5% in water with stabilised hardness, to turn down a bar of 52.5 mm diameter in multiple passes to reach a maximum tool flank wear limit of 200 µm.…”

“…But due to their high relative cost, the use of MQL was essential to their commercial viability. More recently, Wood et al [24] showed that the stabilisation of the water hardness in an aged bio-ester emulsion coolant could maintain as-new service performance over a very long sump life. Overall, it was concluded that whilst there is a growing number of published articles that studied clean coolants to machine Ni-based superalloys that were derived from either vegetable oils, bio-based esters, or nanoparticle doped formulations, there were no published works that directly compared their performance with modern commercially available mineral-oil-based coolants that comply with the latest health and safety requirements [25].…”

“…Most of the published works on turning Inconel 718 used a tool nose radius (r n ) of 0.8 [33][34][35] or 1.2 mm [4,35] or larger [5]. Fewer articles [36][37][38] have studied the finish turning of Inconel 718, using a smaller r n of 0.4 mm, and only one article [24] used a bio-ester emulsion. Whilst some key performance measures in these published articles were studied, such as tool wear or surface integrity, none considered the broader range of performance measures covering, for example, tool wear rate, cutting forces, chip forms, surface and subsurface integrity in the performance evaluation of the CF to finish turn this alloy.…”