Procedia Engineering 19 (2011) 138 – 143

1877-7058 © 2011 Published by Elsevier Ltd.doi:10.1016/j.proeng.2011.11.092

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Procedia

Engineering Procedia Engineering 00 (2012) 000–000

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1st CIRP Conference on Surface Integrity (CSI)

Investigation into the characteristics of white layers produced in a nickel-based superalloy from drilling operations

C.R.J. Herbertaa*, D. A. Axintea, M.C. Hardyb, P. D. Brownc

a, Rolls-Royce University Technology Centre in Manufacturing Technology, School of Engineering,

The University of Nottingham, NG7 2RD, UK bRolls-Royce plc. Derby, DE24 8BJ, UK

c,School of Mechanical, Materials and Manufacturing Engineering, University of Nottingham, NG7 2RD, UK

Abstract

The metallurgical nature/structure of white layers generated during metal cutting operations has for sometime been the object of scientific controversy. This research studies the structure of the white layer generated from non-production abusive drilling parameter in a nickel-based superalloy at macro and micro scales. This was achieved using (1) a Focus Ion Beam (FIB) to mill a sample for Transmission Electron Microscopy (TEM), (2) Scanning Electron Microscopy (SEM) and (3) Nano-indentation to evaluate material within and under this layer. In-depth analysis of this layer from non-standard drilling parameters in alloy RR1000 showed the layer is Face Centre Cubic (FCC) in structure and is the first to reveal it is polycrystalline with an average grain size of 50 nm compared to the bulk material of 22-63 µm (ASTM 8-5). Nano indentation testing has shown the white layer has a 45% higher hardness than the bulk material.

© 2012 Published by Elsevier Ltd. Selection and peer-review under responsibility of Prof. E. Brinksmeier

Keywords: White layer, Nickel-based superalloy, Drilling

1. INTRODUCTION

Nickel-based superalloys are widely employed in the manufacture of gas turbine engines due to their ability to withstand high temperatures and mechanical loads as described by Warburton [1]. However as Kitagawa et al. [2] and Sharman et al. [3] explain their outstanding performance comes at a cost regarding machining difficulties and high tool wear rates. Further, it is essential that the surface integrity complies with the tight quality standards imposed by aero-engine manufacturers.

During cutting operations of nickel-based superalloys Wardany et al. [4] showed significant temperatures (600-900°C) can be reached. Kwong et al. [5] has shown that poor thermal and indeed mechanical management can cause changes on/under the machine surface, in particular leading to a

* Corresponding author. Tel.: 441159514063; E-mail address: emxch1@nottingham.ac.uk

Open access under CC BY-NC-ND license.

Open access under CC BY-NC-ND license.

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distinct featureless white band forming at the surface described by Bosheh et al.[6]. These layers have been reported in a wide range of materials, from mild steels to nickel-based superalloys and have been referred in various ways (e.g. white/recast/amorphous layers). These layers can form, in abusive cutting operations (e.g. drilling) up to 20 μm depths as reported by Kwong et al. [5].

White layers from machining operations have been stated by Griffiths [7] to form under three possible mechanisms: i) phase transformation from rapid heating and quenching, ii) severe plastic deformation creating fine grain structure and iii) chemical reaction of the material with the environment forming an oxide layer. Akcan et al. [8] has suggested white layers are in fact not resistant to etching, but possess a very small grain structure causing incoming light to scatter forming a white layer at the macro scale. This has been show by Garofano et al. [9] in laser machining nickel-based superalloys revealing the layer to consist of nano-particles arranged in a crystalline pattern, while orientated randomly with respect to the surrounding metallic phases they were extracted from. However no clear evidence (e.g. high magnification microscopy) has been provided for this regarding mechanical drilling operations in nickel based superalloys, something this report hopes to address.

2. METHODOLOGY

In order to generate surfaces with white layers, a workpiece made of nickel-based superalloy (RR1000, which was manufactured through powder metallurgy and defined by Hardy et al.[10] to consist of 15% Cr, 18.5% Co, 5% Mo, 3% Al, 3.6%Ti, 2% Ta, 0.5% Hf, 0.06% Zr, 0.027% C, 0.015% B, Ni balance.) was drilled on a 5-axis Makino A55 machining centre with a self centring two fluted TiN/TiAlN coated 6.0 mm diameter drill with 30° helix angle. Extremely abusive non-production cutting parameters were employed to ensure repetitive generation of white layers: cutting speed Vc=35 m/min; feed rate, f=0.12 mm/rev; dry cutting; tool flank wear VB=0.1 mm. These were sectioned using electro discharge machining (EDM) and cut in both the axial and hoop direction (Figure 1) with a SiC cutting disk at low feed speeds and abundant coolant preventing further thermal damage. These were mounted in conductive Bakelite and polished to 1µm and electronically etched in 10% Orthophosphoric acid to enable SEM analysis using a Phillips XL30 ESEM with a 20 kV operating voltage to inspect the white layer.

Figure 1: Schematic of sectioned hole and FIB lift out the sample for metallurgical /crystallographic analysis

A FEI Quanta 200 3D FIB operating at 20kV was used to section samples of 30 μm x 5 μm in both the axial and hoop direction (Figure 1). A tungsten layer of 2-3 μm was deposited on the surface and attached to a needle enabling the pull out. These samples were then analysed by TEM using a JEM-2000Fx II, with 20 kV operating voltage to give both light and dark field imagery as well as electron diffraction patterns to state the structure of the white layer and bulk material.

A Micro-materials LTD nano-indenter compared alterations in the hardness of the white layer with that of the bulk material. An inbuilt microscope ensured indents located in the white layer and bulk material. A small maximum load of 5 mN was applied to keep the influence of the lattice edge effect to a

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minimum. The raw data was analysed using the method by Oliver and Pharr [11] to determine hardness and reduce modulus.

3. RESULTS AND DISCUSSIONS

The SEM images revealed that a white layer of 3 μm thickness had been produced (Figure 2a and 2b), with both the axial and hoop direction showing material drag of around 7 μm accompanying the white layer formation. This suggests large plastic deformation and heating effects had taken place during the operation, also confirmed by a change in colour (indicating material oxidation caused at high temperatures) on the surface of the sample. The image of the hoop direction (Figure 2a) shows there is also severe cracking in the material around the white layer zone. It is of interest to note white layer veins formed at the surface of the component; this could be due to a combination of material drag and high thermal influences drawing the material out in elongated laps. These results suggest that during abusive mechanical drilling the white layer formation is likely to be caused due to the combination of intensive thermo-mechanical loading of the surface in the close vicinity of the cutting edge.

    

Figure 2: White layer in hoop (a) and axial (b) directions generated from abusive drilling parameters (Vc=35 m/min, f=0.12mm/rev, VB=0.2 mm).

The FIB lift-out produced a 0.3 µm thick section of the white layer to be analysed. TEM bright field imaging revealed a very fine grain size of approximately 50 nm had formed (Figure 3) compared with the 22-63µm of the bulk material, giving strong evidence the white layer produced during abusive machining is in fact not amorphous as believed in some literature Krawczyk and Pacyna [12] and it is of scientific interest to understand further its crystallinity.

Figure 3: A bright field TEM image of the white layer (axial direction) showing 50nm grain size

To address the above, diffraction patterns obtained from TEM analysis were indexed. This showed the bulk material (Figure 4a-b) had a FCC crystalline pattern with standard 111, 200 and 220 structure. The

a) b)

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white layer however (Figure 4c & d) had formed rings similar to those of polycrystalline structures. Indexing these found the samples were in fact a FCC structure with the radii of the first three rings corresponding to those expected for 111, 200 and 220 FCC structures, revealing the rings to be formed due to the reduced grain size, leading to diffractions of the electrons from the TEM. These rings were more obvious in the axial direction than that in the hoop direction, the reasoning for this could be that during the FIB removal of the sections the hoop direction sample produced higher fringing than that of axial direction. Due to this fringing the sample is subjected to more beam broadening and as such the intensity of the diffraction pattern is reduced. This could also be due to material falling off during the hoop extraction due to the layers brittle nature resulting in inspection of the layer/ bulk material interface.

   

    

Figure 4: (a) Diffraction pattern bulk material (hoop sample), (b) Diffraction pattern grain boundary (hoop sample), (c) Diffraction pattern white layer (hoop sample), (d) Diffraction pattern white layer (axial sample)

The reason for the reduction of grain size is believed to be from two mechanisms: i) mechanical reduction of the grains due to crushing from the mechanical and thermal effects of the drill and ii) dissolution of grains during the machining process and the re-precipitation afterwards due to cooling, as stated by Barry and Byrne [13] in relation to white layers in stainless steel.

The results from the TEM work shows that even with the high temperature and stresses induced in the material during the drilling process there is no alteration to the final phase of the material in the white layer. Instead it is a re-cast structure, which due to the low thermal conductivity of the material makes it hard for heat to diffuse into the material.

Analysis from the nano-indenter has shown that the white layer is 45% harder than the bulk material Table 1. This is due to the smaller grains within the sample preventing the material from deforming and hence reducing dislocation motion.

Table 1: Nano-indentation results

Hardness (GPa) Young’s Modulus (GPa) Nickel-based Superalloy 7.8±0.20 255.5±11.25 White Layer 11.3±0.39 291.6±13.44

a) b)

c) d)

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Hence it can be concluded that the formation of a white layer does not cause an alteration in the end phase of the material, but it does result in a reduced grain size which causes a variation in the hardness and hence strength of the material.

4. CONCLUSION

The main findings investigating white layer to the sub-micron level were: 1. The white layer produced in these trials was found to be similar to the original bulk material FCC

crystalline structure; however had a very fine grain structure of approximately 50 nm. 2. The fine grain structure was the result from a high thermal and mechanical effect from the drilling

operation. This linked with the low thermal coefficient of the material resulting in large heating effects being generated at localised positions of the material and the surface cooling quickly, explaining the very small grain size from the material.

3. The hardness of the white layer was found to increase by 45% compared with the bulk material. This could be caused by the reduction in the grain size within the white layer. The results presented highlight to the machining community white layers obtained in abusive cutting

conditions have similar crystalline structure to that of the base material but their grain size is vastly reduced and randomly oriented. With these clarifications made, the paper opens avenues for understanding the mechanism of the formation of these white layers and how they can be eliminated or properties altered depending of the required part applications.

ACKNOWLEDGEMENTS

The authors would like to thank the EPSRC and Rolls-Royce Plc for their assistance throughout this research. The authors would also like to thank M. Fay, M. Davies, M. Daine and N. Neate for their invaluable technical help with the regards to the work. REFERENCES

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