Analysis of thermal fields in orthogonal machining with infrared imaging

https://doi.org/10.1016/j.jmatprotec.2007.07.002Get rights and content

Abstract

The validation of a previously developed finite difference temperature prediction model is carried out for orthogonal machining process with a high precision infrared camera set-up, considering the temperature distribution in the tool. The thermal experiments are conducted with two different materials; Al 7075, AISI 1050, with two different tool geometries; inserts having a rake angle of 6° and 18°, for different cutting velocities and feedrates. An infrared camera set-up is utilized for the thermal experiments. The results of the high precision infrared thermal measurements are compared with the outputs of the finite difference temperature model, considering the maximum and the mean temperatures in the tool–chip interface zone and the temperature distributions on the tool take face. The maximum tool–chip interface temperature increases with increasing cutting velocity and feedrate. The relationship between the maximum tool–chip interface temperature and the rake angle of the tool is not distinctive. The experimental results show good agreement with the simulations.

Introduction

Temperature prediction is one of the most complex subjects in the metal cutting literature. It is extremely difficult to develop a precise temperature prediction model in machining due to the complicated contact phenomenon; metal cutting is very highly localized and non-linear, occurs at high temperatures, high pressures and strains. Therefore, accurate and repeatable temperature prediction still remains challenging due to the complexity of the contact phenomena in the metal cutting process (Abukhshim et al., 2005). Apart from the temperature prediction, temperature measurement is even more challenging in this research area, because it is very difficult to make temperature measurement very close to the cutting edge. Due to the lack of experimental data verifying the proposed mathematical models, most published articles rely on the few published experimental data (Lazoglu and Altintas, 2002).

The primary effect of temperature is on tool wear. Although there are various tool wear mechanisms, it is generally known that the progressive tool wear is produced by temperature dependent mechanisms (Wanigarathne et al., 2005). Moreover, the life of a tool will be determined by the maximum temperature on the tool rake face or the clearance face of a cutting tool. Apart from the tool, the maximum temperature and the temperature gradient influences subsurface deformation, metallurgical structural alterations in the machined surface, and residual stresses in the finished part (Komanduri and Hou, 2001). Another well-known problem occurs in the cutting of low thermal conductivity materials; such as titanium, where the heat generated during the cutting process flows much more in the tool than the chip due to the low conductivity of the workpiece material and this causes thermal stresses to occur in the tool. As a result of the thermal stresses, tool fatigue, failures due to fracture, and wear occur more frequently. Sometimes the temperatures can also exceed the crystal binding limits of the tool, this causes rapid wearing of the tool because the loss of bindings between the crystals in the tool material accelerates (Lazoglu and Altintas, 2002). In the hard turning process, due to high hardness values of the workpiece materials, high temperatures and high mechanical stresses are created, which lead to early tool wear. Tool wear not only reduces tool life, but also increases the forces and tensile residual stresses, affects surface finish and causes white layer surface damages (Lazoglu et al., 2006).

Numerous attempts have been made to measure the temperature in the machining operations (Abukhshim et al., 2006). One of the most extensively used experimental techniques to measure the temperature in machining is the use of thermocouples. Their basic working principle is that thermocouples produce an output voltage which depends on the temperature difference between the junctions of two dissimilar metal wires. There are mainly two types of thermocouples; the embedded thermocouples and the tool-work thermocouple, also known as the dynamic thermocouple. Kitagawa et al. (1997) constructed a micro thermocouple, between an alumina-coated tungsten wire and the carbide inserts. Hot junctions were created on the rake face and within the tool to measure local temperatures. For Inconel 718 material, which is cut with a ceramic tool, they measured temperatures close to 1200 °C at 150 m/min. Grzesik (1999) utilized a standard K-type thermocouple embedded in the workpiece to convert measured efms to the interfacial temperature. For high speed machining of medium carbon steel and an austenitic stainless steel, some optimal coating structures were selected corresponding to the minimum interface temperature. Moreover, it was reported that by an appropriate selection of tool coating and workpiece materials, the effect of a thermal barrier in the top layer of the coating can occur. O'Sullivan and Cotterell (2001) measured the temperature in the turning of aluminum 6082-T6 by using two thermocouples in the workpiece. They indicated that increased cutting speeds resulted in decreased machined surface temperatures due to the higher metal removal rate, which results in more heat being carried away by the chip and thus less heat being conducted into the workpiece. It was also reported that tool wear resulted in increased machined surface temperature. Ay et al. (1998) positioned nine K-type fine thermocouples at the edges of three surfaces of a tool insert. It is proposed that the progression of wear is accompanied by a consistent increase in the tool temperature which in turn accelerates the wearing process. Moreover, chip geometry is found to affect local steady-state temperatures in the cutting tool. Even though thermocouples are inexpensive and easy to operate transducers, they have several disadvantages such as; they can interfere with the flow of heat, they have limited transient response, and it is hard to estimate the gradient of temperature with thermocouples.

In addition to thermocouples, the infrared (IR) radiation techniques are probably the second most used method for the temperature measurement in machining. In the IR technique, the surface temperature of the body is measured based on its emitted thermal energy. The IR technique is applied for the temperature field measurements with the use of cameras with films or chips sensitive to IR radiation. Dewes et al. (1999) used an IR camera system and a thermocouple system to measure the temperature in the high speed machining of hardened steel. It was reported that the IR technique indicated lower temperatures than the thermocouple method. Muller and Renz (2003) and Lazoglu et al. (2006) employed a fast fibre–optic two-colour pyrometer with high local resolution, which enables measurements of surfaces with unknown emissivities. This pyrometer has been applied in a turning process, where the fibre–optic enables different measurement positions to show the influences of cutting speed and tool wear on the temperatures. Ming et al. (2003) used an IR pyrometer as a remote sensor to measure the temperature at the tool–chip interface in high-speed milling. The IR radiation technique has many advantages over the thermocouple technique. First of all, the IR radiation technique is a non-intrusive technique, i.e., no physical contact with the heat source is attained so there is no adverse effect on the temperatures and materials; in other words the IR radiation technique does not interfere with the flow of heat like thermocouples do. Second, the IR radiation technique has a very fast response, making it a very useful instrumentation in temperature measurements because the high cutting speeds used in machining experiments today made the response of the experimental system a very important criterion. The disadvantage of the radiation technique is that the exact surface emissivity should be known in order to make an accurate measurement. Knowing the exact surface emissivity during cutting is extremely difficult because the surface emissivity is a function of surface temperature, surface roughness, and possible phase transitions (Sutter et al., 2003). Chip obstruction is another disadvantage in this technique. There is a limited optical access to interesting measurement positions (Muller and Renz, 2003). The temperature measured is usually that of the top face of the chip and not the interface temperature because the tool–chip interface is obscured. Other attempts that exist in the literature in order to measure the temperature in metal cutting can be mentioned as metallographic methods (Wright and Trent, 1973) and the estimation of temperature by chip colour approach (Yeo and Ong, 2000).

It is obvious that temperature prediction is very important and is one of the most complex subjects in the metal cutting literature. Apart from the prediction, temperature measurement is an even more challenging part in this research area because it is very difficult to make temperature measurement very close to the cutting edge, and due to this fact there is a lack of experimental data verifying the proposed mathematical models in the literature. The aim of this research is to present IR experimental temperature measurements to validate a previously developed temperature prediction model.

Section snippets

Thermal modeling of chip, tool and workpiece

The shear energy created in the primary shear zone, the friction energy produced at the rake face—chip contact zone, and the heat balance between the chip, tool and workpiece are considered for the temperature distribution model. The temperature distribution is solved using a finite difference method. The meshes for the chip, tool and the workpiece can be seen in Fig. 1. In this model, average values of mechanical properties, such as average shear angle, are used. Furthermore, thermal material

The experimental set-up

The infrared (IR) camera system used in the experiments is shown in Fig. 3, where every part of the whole system is illustrated individually. The main part of this system is the IR camera Electrophysics PV-320, which has an uncooled focal plane array (FPA) detector, made of pyroelectric elements. The PV-320's detector takes advantage of a ferroelectric phase transition in certain dielectric materials. The FPA is a ceramic material consisting of barium, strontium and titanium salts. The nominal

Simulations and validations

In order to validate the theoretical temperature model, simulations and experiments with the IR camera are performed at conditions as stated in Table 1. A total number of 40 different experiments are performed with the infrared camera set-up for orthogonal cutting of two different workpiece materials: Al 7075 and AISI 1050, by using two different set of tools with varying rake angle. Moreover, the cutting velocity and the feedrate are varied in order to see the effect of cutting parameters as

Conclusions

In this paper, the validation of a previously developed finite difference time domain temperature prediction model is carried out for orthogonal machining process with an IR camera set-up, considering the temperature distribution in the tool. In the test conditions for the orthogonal cutting of Al 7075 and AISI 1050:

  • (i)

    the maximum tool–chip interface temperature increases with increasing cutting velocity,

  • (ii)

    the maximum tool–chip interface temperature increases with increasing feedrate,

  • (iii)

    the

Acknowledgements

The authors acknowledge the Scientific and Technical Research Council of Turkey (TUBITAK) for its support for the Project No. MAG-101M043 and the technical discussions with Adnan Kurt on the IR camera interface.

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