Experimental Investigation and Analysis of Chip Rebonding Phenomenon in Turning Superalloys.

Abstract

Inconel 718 is widely used in the aerospace industries due to its high temperature endurance properties and good corrosion resistance. However, the machining of Inconel 718 generates more heat in the cutting zone than that of most other materials, which makes this material prone to surface defects. Among several possible surface defects, the chip rebonding phenomenon is investigated in this study. To avoid this defect and achieve a high surface quality, it is necessary to understand the mechanism of chip rebonding. In order to identify the root causes of chip rebonding in turning Inconel 718, chip rebonding samples are observed. Overall observations of the machined surface show that chip rebonding occurs periodically along a feed mark, and that the chip’s side edge is periodically torn and curled. On the basis of the observations, the torn chips at the side edge of the chip are assumed to be smeared periodically by the tool along the feed mark. A chip curl blocking device is designed to test this hypothesis and the test results with the chip curl blocking device are shown. Residual stress distribution, which is important for material life, is affected by chip rebonding. To evaluate the chip rebonding effect on residual stresses, a FE model is developed to simulate the chip rebonding phenomenon. The changes of residual stress distribution in subsurface material are simulated while chip rebonding occurs and after the material cools down. Subsurface residual stresses are measured by the nanoindentation method and compared with simulation results. In order to identify the key variables that influence the occurrence of chip rebonding, a parametric study is performed in dry and wet cutting conditions. DOE is carried out to study the effect of each cutting parameter on chip rebonding in dry cutting. In wet cutting, various MWFs including pure water and two kinds of commercial lubricants in varying concentrations and viscosity are used to investigate their effect on chip rebonding occurrence. This dissertation research provides an in-depth understanding of the chip rebonding phenomenon in turning of Inconel 718 and a method to control the machining conditions to avoid chip rebonding.