Tools with built-in thin film thermocouple sensors for monitoring cutting temperature

Abstract

The ability to monitor in real time, the thermal activation and thermal impact to cutting tools has been very appealing to the manufacturing industries. Such responses can be measured with appropriate sensors such as thin film thermocouples (TFTs) built in cutting tools. The challenges have been to instrument the tool, equipment and sensors, which can withstand high stress and temperature in machining process. In this work, the sequence of fabricating the built-in TFTs and experimental setups are proposed and demonstrated. The cutting experiments are carried out under different cutting conditions for A6061-T6 aluminum alloy and finally cutting temperature is measured at very high cutting speeds up to 16 m/s.

Introduction

In manufacturing process, especially in machining, there has been a surge of interest to know more about the tool temperature. Temperature has critical influences on machining: it accelerates tool wear shortening tool life; it causes the thermal deformation of the workpiece, cutting tool and machine tool, which degrades the machining accuracy; it affects the subsurface layers of machined parts through phase transformation, thermally induced residual stresses and increase in thermally activated defects. However, it is hard to predict the intensity and distribution of the heat sources in a machining operation due to the complexity of the mechanics and heat transfer of machining. Especially, because the properties of work materials are dependent upon strain, strain rate and temperature, the mechanics and heat transfer are tightly coupled together in machining. Although lots of efforts in theoretical analyses and experiments [1], [2] have been made to understand this phenomenon, many problems are still remaining unsolved.

Many experimental methods have been devised to measure the temperature at the tool–chip contact surface: tool–chip thermocouple technique [3], embedded thermocouple technique [4], [5], infra red radiation measurement [6], measurement of metal microstructure and microhardness variation [7], utilization of melting points of thermo-sensitive materials [8], and thin film sensor technique, which is based on the property of pure platinum film that its ohmic resistance changes with temperature [9], [10]. Each of them has its own advantages and disadvantages. Thin film sensors have a great potential to industry applications [9], but measurement of temperature at the tool–chip interface was limited to a free machining aluminum alloy among the engineering alloys and measured temperature was approximately 200 K higher than solidus temperatures of aluminum alloys [10]. By contrast, the numerical methods such as finite difference method [4], [5], finite element method [11], [12], hybrid analytical-FEM method [13], boundary element method [14] and inverse method [15] have developed to solve the problems related to cutting temperature. However, rather simple assumptions are implemented into analysis codes due to the complexity of thermal problems in cutting processes.

Sensors in a form of thin film thermocouples (TFTs) have been developed by the authors for measuring temperature at the tool–chip contact zone under high temperature and pressure. A prototype tool with a Pt—(Pt-13%Rh) TFT and a hard protective coating of Al₂O₃ and AlN had been applied to machining of 0.45%C carbon steel S45C at cutting speeds 1.67–5.0 m/s and feed rates 0.08–0.25 mm/rev [16], but the width of TFT 1.8 mm was too wide to measure the temperature distribution and cutting experiment had to be finished in a few seconds to avoid the injury of TFT. In this paper, fine TFTs were made of Ni and Ni–Cr (80:20 in mass%) layers deposited on Al₂O₃ substrate tool. The TFT layers approximately 0.5 μm thick were insulated by a PVD layer of hafnium oxide (HfO₂). Then, a TiN hard layer was deposited on the HfO₂ insulator for protecting the built-in TFTs against the wear and high stresses in high-speed cutting. Calibration of the TFTs was followed by machining experiment and temperature measurement with only an insert with TFTs at different cutting speeds.