High Speed Machining of Aero-Engine Alloys
11Pages

High Speed Machining of Aero-Engine Alloys London South Bank University, School of Engineering Machining Research Centre, 103 Borough Road London, SE1 0AA, England. ezugwueo@lsbu.ac.uk Materials used in the manufacture of aero-engine components generally comprise of nickel and titanium base alloys. Advanced materials such as aero-engine alloys, structural ceramic and hardened steels provide serious challenges for cutting tool materials during machining due to their unique combinations of properties such as high temperature strength, hardness and chemical wear resistance. These materials are referred to as difficult-to-cut since they pose a greater challenge to manufacturing engineers due to the high temperatures and stresses generated during machining. The poor thermal conductivity of these alloys result in the concentration of high temperatures at the tool- workpiece and tool-chip interfaces, consequently accelerating tool wear and increasing manufacturing cost. The past decade has witnessed a radical approach to product manufacture, particularly in the developed economy, in order to remain competitive. Modern manufacturing philosophies, principles and techniques geared primarily towards reducing non value added activities and achieving step increase in product manufacture have been widely adopted. Recent advances in the machining of aero-engine alloys include dry machining at high speed conditions, the use of high pressure and/or ultra high pressure coolant supplies, minimum quantity lubrication, cryogenic machining and rotary (self-propelled) machining technique. Tool materials with improved hardness like cemented carbides (including coated carbides), ceramics, polycrystalline diamond and polycrystalline cubic boron nitride are the most frequently used for high speed machining of aero-engine alloys. These developments have resulted to significant improvement in the machining of aero- engine alloys without compromising the integrity of the machined surfaces. This paper will provide an overview on these recent developments and their application in the aerospace industry. Keywords : Nickel alloys; titanium alloys; self-propelled rotary tooling, high pressure coolant supply, minimum quantity lubrication, cryogenic cooling, tool wear; and temperature reduction development of super-stainlessӔ alloys, or superalloys, Figure 1 (1). There was a steady increase in typical engine temperature from 1910 till the 1980s while the 1990s till date witnessed almost a two-fold increase in engine temperature. New materials are becoming available that allow the engine temperature to increase at a rate of almost 10 > Machinability is the term used to describe how easily a material can be cut to the desired shape (surface finish and tolerance) with respect to the tooling and machining processes involved. In a machining operation tool life achieved, metal removal rate, component forces and power consumption, surface finish generated and surface integrity of the machined component as well as the shape of the chips can all be used to measure machinability. The machinability index can be significantly affected by the properties of the material being machined, properties and geometry of the cutting tool, cutting conditions employed and other miscellaneous factors such as rigidity of the machine tool, cutting environment, etc. Machining productivity can be significantly improved by employing the right combination of cutting tools, cutting conditions and machine tool that will promote high speed machining without compromising the integrity and tolerance of the machined components. This is particularly essential for the economic machining of difficult-to-cut aero-engine alloys whose peculiar characteristics generally impair machinability. C per annum since the 1950s. Engine efficiency increases and fuel consumption decreases with each increase in temperature. > 1 The driving force for the continual development of many materials over the years is the need for harder, stronger, tougher, stiffer, more corrosion resistant or oxidation resistant material that can also exhibit high strength to weight ratio, in the case of aero-engine alloys. The wide spread use of jet engine has increased demand for materials that have excellent high temperature mechanical and chemical properties relative to steels and stainless steel alloys originally employed in jet engine applications. Demand for hotter, more powerful and more efficient engines led to the > Figure 1. Evaluation of the high temperature strength of material usage in jet engines over the passed century. (After Seco Technical guide, Turning Difficult-To-Machine Alloys). Heat resistant alloys with high melting temperatures are major materials used in the manufacture of aero-engine components. These exotic superalloys can be grouped into four major categories: Nickel base alloys; cobalt base alloys; iron base alloys (e.g. high chromium stainless steel); and titanium alloys. Figure 2 shows that two-thirds of superalloy production is consumed by the aerospace industry for the manufacture of jet engines and associated components, mainly > Presented at COBEF 2003 II Brazilian Manufacturing Congress, 18-21 May 2003, Uberl֢ndia, MG. Brazil. Paper accepted October, 2003. Technical Editor: Alisson Rocha Machado. J. of the Braz. Soc. of Mech. Sci. & Eng. Copyright 2004 by ABCM January-March 2004, Vol. XXVI, No. 1 / 1 >

E. O. Ezugwu in the hot end of aircraft engines and land based turbines (1). Ability to retain high mechanical and chemical properties at elevated temperatures make superalloys ideal materials for use in both rotating and stationary components in the hot end of jet engines. Components produced with superalloys are smaller and lighter than if they were made of conventional steel. This results in significant fuel savings and reduction in pollution. Each kilogram weight reduction typically results in a US$150,000 savings in fuel cost over the life of the engine. The remaining third of...