Environmentally conscious machining of difficult-to-machine materials with regard to cutting fluids

Abstract

Machining difficult-to-machine materials such as alloys used in aerospace, nuclear and medical industries are usually accompanied with low productivity, poor surface quality and short tool life. Despite the broad use of the term difficult-to-machine or hard-to-cut materials, the area of these types of materials and their properties are not clear yet. On the other hand, using cutting fluids is a common technique for improving machinability and has been acknowledged since early 20th. However, the environmental and health hazards associated with the use of conventional cutting fluids together with developing governmental regulations have resulted in increasing machining costs. The aim of this paper is to review and identify the materials known as difficult-to-machine and their properties. In addition, different cutting fluids are reviewed and major health and environmental concerns about their usage in material cutting industries are defined. Finally, advances in reducing and/or eliminating the use of conventional cutting fluids are reviewed and discussed.

Introduction

Material cutting also known as machining is one of the most used techniques for producing different components. In the machining processes a cutting tool removes material from a workpiece of a less resistant material. The removed material called chip or swarf slides on the tool face and leaves the workpiece material. As a result of this process the cutting tool would be subjected to high normal and shear stresses [1]. Fig. 1 shows a schematic view of a typical cutting operation also known as the single shear plane model. Although from the modelling point of view this model is a matter of debate and suffers from a lack of accuracy [2] it provides sufficient information required for this paper. As shown the chip formation is categorised into two zones, namely primary and secondary shear zones. In the primary shear zone, the material is being cut by elasto-plastic deformation. The majority of the energy used for cutting in this section is transformed into heat. At the secondary shear zone the produced chip slides on the rake face of the cutting tool resulting in high frictional force and heat. In addition to these zones, sliding of the tool flank face on the machined surface at the tertiary deformation zone generates friction and heat and could cause flank wear on the flank face [3], [4]. Furthermore friction and heat on the rake face could result in chipping and crater wear leading to tool failure.

Machining advanced engineering materials is usually associated with high machining costs and low productivity. This is due to the excessive generation of heat at the cutting zone and difficulties in heat dissipation due to relatively low heat conductivity of these materials. High material hardness and strength together with high temperatures at the cutting zone could result in excessive tool wear and thus short tool life and poor surface quality. Most components produced from advanced materials such as titanium and nickel based alloys have high buy-to-fly ratio as most of them are made to be used in aerospace, engine and gas turbine industries [5].