Chemical Interactions Between Cemented Carbide and Difficult-to-Machine Materials by Diffusion Couple Method and Simulations
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A simple and efficient diffusion couple method is utilized to study the chemical interactions between cemented carbide cutting tools and difficult-to-machine materials (Ti, Ti-6Al-4V, Ni, Inconel 718, Fe, and AISI 316L). The experimental results and simulations probe different chemical interactions between the cemented carbide and work materials. In particular, the formation of a thick TiC layer is observed at the cemented carbide/Ti and Ti-6Al-4V interface while η-phase is formed at the interface between the cemented carbide and work materials Inconel 718, Fe and AISI 316L. Pure titanium and Ti-6Al-4V both interact strongly with the tool causing formation of TiC and dissolution of WC-grains. Experiments and diffusion simulations confirm bcc-W formation and progressive diffusion of W into bcc-Ti. For both Ti and Fe work materials a dense band of carbides (WC + η or WC + TiC) forms at the interface, effectively inhibiting further diffusion. Ni does not form any stable carbide and lowers the η-phase limit in terms of carbon content, wherefore diffusion can occur freely. The diffusion couple method used in this work, corroborated by DICTRA simulations should serve as a useful tool in the detailed analysis of worn tools where chemical wear is dominant.
Keywordscemented carbide DICTRA modeling diffusion couples machining
Titanium and titanium based alloys are increasingly used for aerospace, medical, marine and chemical processing applications. Thus, the need for high productivity when machining those materials grows stronger. Similarly, nickel based alloys are used due to their excellent properties for demanding applications, where high temperature stability and corrosion resistance are important properties. These alloys are classified as difficult to cut materials, mainly due to high cutting temperatures and adhesion of work material on the cutting tool edge. Given the great need of products, and the difficulty to produce them, it is of interest to study interactions between cemented carbide and these materials.
By far, the most common work material used in any cutting operation is steel. Cemented carbide interacts with steels in diverse ways depending on the alloying elements. Since steel finds so many important applications, machining this work material with high productivity is vital.
An increased understanding of the mechanisms responsible for tool degradation and failure can provide means to design better cutting tool materials and achieve more profitable machining. In many preceding studies, it is stated that the dominating wear mechanism at high temperature is chemical wear. Therefore it is of interest to study high temperature interactions between cutting tools and work materials on a controlled laboratory scale.
A few different approaches for diffusion couple testing have been used in the past.[4, 5, 6, 7, 8] Common for most of these methods is either complex sample preparation or the limitation in studying only one diffusion couple at a time. Recently, Hatt et al. utilized a diffusion couple method similar to the one presented in this work to study the interaction between cemented carbide and titanium alloys. Hatt et al. could thereby explain phase formation due to diffusion and interface reactions. In Hatt’s method, the interface is created by pressing the workpiece and tool together. The present authors have developed a relatively simple and flexible diffusion couple method which utilizes the difference in thermal expansion in the work material and the cutting tool to achieve a good contact at the interface between the materials during annealing. In this work, the capability of the suggested diffusion-couple method is demonstrated by a detailed experimental investigation along with diffusion simulations of difficult to machine materials (Ti, Ti-6Al-4V, Ni, Inconel 718, Fe and AISI 316L).
2.1 Sample Preparation
Work materials investigated in this study
Commercially pure titanium: Ti (99.7%)
Titanium alloy: Ti-6Al-4V
Pure Nickel: Ni (99.5%)
Nickel-based alloy: Inconel 718
Pure iron: Fe 99.95%
Austenitic stainless steel: AISI 316L
The samples were annealed in a sintering furnace at 1100 °C for 2 h under Ar atmosphere at 5.5 kPa pressure, with a heating rate of ~ 5.5 °C/min and a subsequent cooling rate of ~ 10 °C/min. The higher thermal expansion of the work materials will ensure a well-defined tool-work material interface. After annealing, the samples were ground on the rake face of the insert and subsequently polished using 1 µm diamond suspension.
The characterisation of the diffusion couple specimens was carried out using a variety of experimental methods.
Light optical microscopy (LOM) was employed using a Zeiss microscope in combination with Murakami’s etchant for the samples where η-phase was suspected (Fe, 316L and Inconel718). Scanning electron microscopy (SEM) was performed using a LEO ultra 55 FEG-SEM equipped with a Noran-Energy dispersive spectroscopy (EDS) detector. The Inconel 718, nickel and titanium based samples were analyzed using SEM. EDS analysis was employed to analyze the Titanium sample. Wavelength dispersive spectroscopy (WDS) analysis was carried out for the Ni-based sample using a JEOL JXA-8530F instrument. Electron back scattered diffraction (EBSD) data was obtained for the Ti-based sample using a Zeiss Sigma VP SEM equipped with an Oxford Nordlys EBSD detector.
2.3 Thermodynamic and Diffusion Simulations
Phase diagram calculations and diffusion simulations were performed using Thermo-Calc and DICTRA respectively, in combination with the TCFE7 and MOBFE2 thermodynamic and kinetic databases. The TCFE7 database has been developed with steels in mind and will thus yield most reliable results for Fe-based materials. However, initial calculations showed that the known phase equilibria of Co-based cemented carbides, along with Ti, Ni or Fe, could be reproduced satisfactorily. The set-up of the diffusion calculations was performed in the same manner as described by Odelros et al.
3 Results and Discussion
3.1 Ti and Ti-6Al-4V
3.2 Ni and Inconel718
To obtain absolute values from WDS measurements suitable reference materials with known composition and which closely resemble the sample are required. The situation becomes even more complex when materials with widely different compositions are studied in the same sample, as is the case in a diffusion couple. The limited accuracy of WDS-measurements of such a system is evident in Fig. 6, where the Ni content should approach 100 wt.% to the right of the interface and zero close to the left of the interface, as is seen in the simulated result. Similarly, the Co and W contents obtained from WDS in the Ni-rich side of the diffusion couple are higher and lower, respectively, as compared to the simulated result.
Interactions between cemented carbide and Inconel 718 proved to be of a different nature, as can be seen in Fig. 9(a) and (b). This couple was analyzed with LOM combined with Murakami’s etchant, which revealed an etched zone at the interface between the tool and the work material (Fig. 9b). Since Murakami’s etchant is designed to erode η-phase only, it was concluded that this zone indeed did consist of (M6C or M12C). The reason to why η-phase was formed in this region is most probably carbon depletion of the tool. This is consistent with the fact that Inconel 718 contains > 20 wt.% Fe and that Fe is known to shift the η-phase limit to higher carbon contents.
3.3 Pure Fe/316L
Pure Ti and Ti-6Al-4V interact strongly with the tool causing TiC-formation and dissolution of WC-grains. In agreement with previous studies, it is shown by diffusion simulations and EBSD analysis that bcc-W will form at the tool/Ti interface and progressively diffuse into bcc-Ti.
The tool-Ni interface does not contain any carbide phase, which can be explained as follows. Ni does not form any stable carbide on its own (contrary to Ti) and will in addition shift the η-phase limit to lower carbon contents, as compared to Co or Fe containing binder phases. Thus no stable η-phase will form. Diffusion of Co and W into the Ni work material was observed in both simulated and measured data.
At the tool-Inconel 718 interface, η-phase was formed. The η-phase formation in this Ni-based alloy can be explained by that Inconel 718 contains > 20 wt.% Fe, where Fe is known to shift the η-phase limit to higher carbon contents.
As for Inconel 718, both low carbon austenitic steel and pure iron exhibit η-phase formation. This suggests that the chemical interactions are not caused by Ni in the Inconel but rather by the alloying elements.
Agreement between the diffusion simulations, measured composition and observed phases at the tool-work piece interfaces is very good. Therefore the diffusion couple method used in this work, corroborated by simulations may prove useful in the detailed analysis of worn tools where chemical wear is dominant.
The authors would like to dedicate this paper to the memory of their late colleague Bo Jansson who suggested this simple but yet very efficient method for performing diffusion couple experiments. Christer Fahlgren at Sandvik Coromant is also acknowledged for performing the WDS-analysis.
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