INVESTIGATION INTO THE CHARACTERISTICS OF WHITE LAYERS PRODUCED IN A NICKEL-BASED SUPERALLOY FROM DRILLING OPERATIONS

The nature of white layers generated during metal cutting operations have for some time been the object of scientific controversy in reference to their metallurgical nature/structure. This research aims to study the structure of the white layer from an abusive cutting operation (i.e., drilling) in a nickel-based superalloy at both macro and micro scale levels. This has been achieved by using (1) a Focus Ion Beam to mill a sample for Transmission Electron Microscopy to analyse the grain size within the white layer, (2) Scanning Electron Microscopy to see shape characteristics of the white layer generated, (3) X-ray diffraction to see any alterations to the crystallinity of the structure, and (4) nano-indentation within this layer to compare its hardness with that of the bulk material. The in-depth analysis of white layers generated from non-standard drilling parameters of alloy RR1000 has shown that the layer is of the same structure as the bulk material: face centre cubic (FCC). The analysis has also shown the layer possesses a greatly reduced grain size of approximately 50 nm as opposed to the bulk material which has an average grain size of 22-63 mu m (ASTM 8-5). Although nano-indentation has also shown that the analysed layer has a 45% higher hardness than that of the bulk material. This analysis indicates that the white layer created from machining is a combination of both mechanical and thermal effects and not only thermal as previously shown.