Evaluation of the cutting insert condition influence on the microstructure of surface layer after turning of Inconel 718 alloy

* Dr inż. Piotr Szablewski, piotr.szablewski@pwk.com.pl, https://orcid.org/0000-0003-2435-2126 – Pratt & Whitney Kalisz, Polska, Państwowa Wyższa Szkoła Zawodowa, Kalisz, Polska Mgr inż. Tadeusz Chwalczuk, tadeusz.chwalczuk@put.poznan.pl, https://orcid.org/0000-0002-4009-0552 – Politechnika Poznańska, Poznań, Polska Mgr inż. Tomasz Dobrowolski, tomasz.dobrowolski@pwk.com.pl – Pratt & Whitney Kalisz, Polska


Introduction
Heat resistant alloys -such as Inconel or Waspaloy -are widely used in aircraft engine constructions. Unfortunately, the properties, due to which they are used so widely, also hamper their mechanical machining [1,2]. Poor machinability of these alloys is manifested, among others, by short durability of cutting blades, which directly translates into the quality of the surface layer (roughness of the treated surface, microstructure) [3].
The notion of a white layer (WL) occupies a special place in the assessment of the surface condition of Inconel 718 alloy. Due to its properties, WL is often a critical element of technological surface layer (TSL), which affects the operational properties of critical parts of the structure.
White layer formed in the machining process of the Inconel 718 alloy after precipitation hardening is primarily nanocrystalline grains with a size of 50÷100 nm [4], the size of which before machining was about 23 µm. Areas of surface unevenness with numerous dislocations with grains of approximately 200 nm can also be observed. In the white layer, there is mainly the strengthened phase γ, phase γ' Ni3 (Al, Ti) and δ of the Ni3Nb type [4].
Mechanism of the formation of a white layer in the cutting process is associated with the appearance of high temperatures, plastic deformation of the machined layer and chemical reactions [5], which is undoubtedly due to the state of the blade edge. Furthermore, carrying out machining with a worn tool causes significant stresses in the surface layer (SL) [6,7].
It has been shown that during conventional turning of the Inconel 718 alloy, tensile stresses are observed at the surface of the machined material, followed by -to a certain depth -compressive stresses. Values of these stresses depend on the cutting parameters and geometry of the blade, and above all -on the value of the rounding radius rn.
The authors of the article [8] showed that the use of coolant reduces the depth of occurrence of SL stress changes compared to dry machining. During the finishing turning of the Inconel 718 alloy, it was observed that the depth of the blade impact on the stress values may differ by 0.15 mm in the cutting speed range vc = 40÷80 m/min. This effect is associated with less deformation of the machined layer. For many grades of Inconel 718 alloy, formation of a white layer during machining is inevitable. Direction of a modern research and issue for optimization should be the selection of cutting parameters and conditions to obtain a white layer of assumed thickness and known character. The state of the art regarding the formation of this characteristic zone in nickel-based alloys is insufficient. This is especially true for turning Inconel 718 after various heat treatments.

Scope, conditions and technique of research
The tests concerned longitudinal turning of an Inconel 718 shaft in the supersaturated state with a hardness of 20 HRC. The state of the core microstructure is shown in fig. 1.
WNMG 080408-PP multi-edge inserts with corner radius rε = 0.8 mm and TiCN/TiN coating (applied by PVD method), DWLNR 2525 M08 holder and main entering angle κr = 95° were used. Turning tests were carried out on a universal TUR560 lathe; no cooling lubricant was used.
Photographs of the microstructures were taken with an EPIPHOT 200 microscope from Nikon. Samples were incorporated in duracryl and digested with Kallings reagent after polishing.

Analysis of test results
Figs. 2-6 show the microstructure of Inconel 718 with a hardness of 20 HRC after turning with a new blade and blades with a wear of VBC = 0.12 mm and 0.25 mm. In each of the analyzed cases, an austenitic alloy microstructure with clearly defined grain boundaries can be observed.
A characteristic feature of the Inconel 718 alloy is the presence of hard niobium and titanium carbides in the structure. This clearly affects the quality of the machined surface [9]. In fig. 2, after turning with a new blade, one can see clear gaps related to the kinematic and geometrical nature of the blade mapping in the material. In fig. 3, after turning with a worn blade (VBC = 0.12 mm), the intervals between the blade passes disappear. There are material losses on the machined surface due to the phenomenon of adhesion between the blade and the chip as well as the effect of lateral flow of the material.  fig. 6 shows a microcrack and a very thin (approximately 1.5 μm) WL, mentioned by the authors of the work [10]. This layer is a load in the presented research. It is not associated with the presence of additional chemical agents that may affect the microstructure of WW. Based on the analysis of figs. 2-5, it can be clearly stated that the occurrence of WL is primarily associated with the state of the cutting edge. As a result of the progressive wear of the cutting edge, it is impossible to avoid its generation, while the technological goal should be to avoid discontinuities manifested in machined surface anomalies. Under the WL, the deformation zone of austenite grains can be seen. This is a typical image associated with the load resulting from the contact of blade with material. This deformation strengthens TSL. The increase in surface hardness is an important factor affecting the machinability of the Inconel 718 alloy. The effect of strengthening the TSL has been described in detail, among others in works [3,8].
Values of roughness parameters for individual blade states are summarized in the table, while in fig. 7, the roughness profiles for subsequent transitions are collected. Progressive wear of the blade did not contribute to the increase in the value of Ra and Rz roughness parameters. This effect can be explained by the phenomenon of plasticizing and indenting of uncut material (resulting from the appearance of build-up and lateral flow of the chip). This may be the result of TSL discontinuities described previously.

TABLE. Roughness parameters of the machined surface of the Inconel 718 alloy with varying degrees of wear
The highest regularity of the profile shape was noted for the surface machined with a blade with the highest degree of wear analyzed. The nature of the profile for the initial blade state does not have the typical blade representation associated with longitudinal turning kinematics. This effect is not reflected in the images of the surface layer microstructure. It can be assumed that the accumulation on the machined surface was removed as a result of the preparation of metallographic specimens.
Research on the impact of the feedrate value on the cutting edge wear during turning of Inconel 718 material confirmed that the increase in feedrate promotes the build-up of the rake surface at the cutting edge ( fig. 8). The change in the contact surface of the blade with the workpiece favored the intensification of the phenomenon of adhesion. The problem of the build-up impact on the quality of the machined surface was raised, among others at work [9].
Described WW discontinuities or torn off parts of the machined surface, presented on metallographic specimens for a worn blade, are associated with stratification of individual surface zones.