Chip Rebonding in Dry Turning of Nickel Based Alloys

At some cutting conditions chips formed during high-speed face turning of nickel based alloys are re-bonded to the machined workpiece surface, even when coolant is applied. Unfortunately, chip-rebonding reduces surface quality, which leads to a shorter fatigue lives for machined parts. Although several researchers have documented this phenomenon and its effects, the root causes of this phenomenon is currently unknown. In order to determine the root causes of chip rebonding, past test samples exhibiting chip rebonding were first analyzed. Metallographic analysis revealed that the chip rebonding material is the same as the workpiece material and that the bonding is mechanically driven. Next, screening design of experiments (DOE) were completed to reliably reproduce chip rebonding in dry cutting cases. Chip rebonding detection and severity were measured using multiple equally spaced surface roughness measurements (R_t parameter). In addition, in-process cutting forces and tool wear measurements were recorded and compared. Finally Taguchi methods were applied to identify the key variables their influence on chip-rebonding. In dry cutting tests it was found that decreasing feed-rate while cutting at a constant cutting speed is the most influential factor in obtaining chip rebonding. High-speed video revealed that at lower feed-rates the chip curls back to the surface of workpiece, while at higher feed-rates the chip flows away from the cutting region with minimal curl. Additional testing performed verifies this theory.