A review of developments towards dry and high speed machining of Inconel 718 alloy

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Abstract

The increasing attention to the environmental and health impacts of industry activities by governmental regulation and by the growing awareness in society is forcing manufacturers to reduce the use of lubricants.

In the machining of aeronautical materials, classified as difficult-to-machine materials, the consumption of cooling lubricant during the machining operations is very important. The associated costs of coolant acquisition, use, disposal and washing the machined components are significant, up to four times the cost of consumable tooling used in the cutting operations. To reduce the costs of production and to make the processes environmentally safe, the goal of the aeronautical manufacturers is to move toward dry cutting by eliminating or minimising cutting fluids. This goal can be achieved by a clear understanding of the cutting fluid function in machining operations, in particular in high speed cutting, and by the development and the use of new materials for tools and coatings. High speed cutting is another important aspect of advanced manufacturing technology introduced to achieve high productivity and to save machining cost. The combination of high speed cutting and dry cutting for difficult-to-cut aerospace materials is the growing challenge to deal with the economic, environmental and health aspects of machining.

In this paper, attention is focussed on Inconel 718 and recent work and advances concerning machining of this material are presented. In addition, some solutions to reduce the use of coolants are explored, and different coating techniques to enable a move towards dry machining are examined.

Introduction

The development of governmental pollution-preventing initiatives and increasing consumer focus on environmentally conscious products has placed increased pressure on industries to minimise their waste streams. In this way, the ISO 14000 international environmental management system standards have been developed to help industries to manage better the impact of their activities on the environment. Particularly concerned is the metal-working sector which includes automotive and aerospace industries. Attention is being directed to the role of cutting fluids in machining, machine tool energy efficiency and the impact of process wastes on the environment.

The ADEME, French Agency for Environment and Energy Management, supports a project with the goal of improving the machining processes of difficult-to-cut materials for the aerospace industry, in order to move towards dry cutting operations that are more friendly for environment and health, and in the same way, to reduce energy consumption.

The advantages of dry machining are:

  • non-pollution of atmosphere or of water which reduces the danger to health, in particular, skin and respiratory damage,

  • no residue of lubricant on machined components which reduces or eliminates cleaning costs and associated energy consumption,

  • no residue of lubricant on evacuated chips which reduces disposal costs and the associated energy consumption.

At high cutting speeds, it is well known that the lubrication in the cutting zone is not evident and not really effective. That is why high speed machining and dry machining are often associated. High speed machining leads to lower cutting forces, higher removal rates and therefore to lower energy consumption. The project is concerned with these two aspects of machining to realise the ecological importance and high performance machining of hard-to-cut aerospace materials. In the first step, the dry machining of the Inconel 718 alloy used by Snecma-Moteurs will be studied.

Before introducing dry machining, it is important to summarise the functions of cutting fluids and to search how the effects of cutting fluids may be substituted. Generally, the use of cutting fluid leads to an increase of tool life by the reduction of cutting forces (lubrication effect) and temperatures in the tool (cooling effect). However, these effects are not evident in high speed machining, in particular, when ceramic inserts are employed [1]. The energy consumed in performing a machining operation is mainly converted into heat. Cutting fluids are employed to remove heat from the workpiece, the tool, the fixtures and the machine tool (cooling effect). The heat generated is mainly dissipated in the chip and in the workpiece, a rather small part of heat flows to the tool. However, the highest temperature is obtained at the tool–chip interface which leads to diffusion wear and cutting edge degradation. The other important functions of the cutting fluids are to flush away the chips from the cutting zone (flushing effect) and to provide corrosive resistance to the machined component.

In addition, it is necessary to understand well the mechanisms that contribute to tool wear and to workpiece surface integrity when working with Inconel 718. Hence, this paper is a general review of the recent developments in the machining of this material and an exploration of the possible ways to dry cutting. In the first part, the characteristics of Inconel 718 that are responsible for its poor machinability are reviewed and the associated problems are listed. Then, the latest research carried out on the use of uncoated and coated carbide tools under wet and dry conditions is summarised. The constant demand to increase productivity and quality has led to the development of ceramic tools. They are used for machining nickel-based alloys at higher cutting speeds and some of their results are given. When surfaces are produced, they need to meet functional service requirements, in particular for the aerospace components. As a consequence, attention is focussed on the parameters influencing the surface quality during machining Inconel 718. Finally, to examine the move towards dry cutting of Inconel 718, interesting alternatives to conventional flooding coolant supply that are minimum quantity lubrication technologies, are reported and recent innovations of tool coatings for dry machining are discussed.

Section snippets

Machinability of Inconel 718

Nickel-based superalloys are widely employed in the aerospace industry, in particular in the hot sections of gas turbine engines, due to their high-temperature strength and high corrosion resistance. They are known to be among the most difficult-to-cut materials. Attention is focussed on the Inconel 718 family in the following paragraphs.

The properties responsible for the poor machinability of the nickel-based superalloys, especially of Inconel 718, are [2], [3], [4], [5], [6]:

  • a major part of

Cutting tools for machining Inconel 718

The requirements for any cutting tool material used for machining nickel-based alloys should include [3]:

  • good wear resistance,

  • high hot hardness,

  • high strength and toughness,

  • good thermal shock properties,

  • adequate chemical stability at elevated temperature.

Turning, milling and drilling are common operations carried out in the manufacture of jet engine mounts and blades, while turning and drilling are the predominant machining operations in the manufacture of disks for gas turbines. Most published

Surface integrity when machining Inconel 718

For safety critical industries such as aerospace, surface integrity is important for the components submitted to high thermal and mechanical loads during their use, Axinte and Dewes, [32]. Structures in aerospace applications are subjected to severe conditions of stress, temperature and hostile environments. Section size is continually reduced in order to minimise weight so that surface condition has an ever-increasing influence on its performances.

Service histories and failure analyses of

The way to dry machining

The use of coolants, in addition to being undesirable to the environment and for the human health, entails high costs in production and disposal. Depending on the machined workpiece, cost savings up to 17% of the total workpiece cost can be made by introducing dry machining. This is mainly due to the elimination of coolant supply, cleaning, maintenance and disposal costs [41], [42]. Reducing costs in the cutting process together with reduced environmental pollution by the use of dry machining

Conclusions

Inconel 718 is a high strength thermal resistant material alloy. It is a highly strain rate sensitive material which work hardens readily, and contains hard particles making it a very difficult-to-cut material. The difficulty of machining Inconel 718 resolves into short tool life and poor surface integrity. The main wear mechanism is abrasion observed for all the tested tools. Welding and adhesion on the cutting tool frequently occur to form a BUE. The BUE is repeatedly removed leading to

Acknowledgements

The research work published in this paper was carried out with the financial aid of ADEME, French Agency for Environment and Energy Management. It corresponds to the first stage of the study concerning high speed and dry cutting of Inconel 718.

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