Machining low-machinability aerospace materials

A wide variety of materials are considered to have low machinability, including hard steels, certain types of cast iron, high-temperature alloys, titanium alloys, ceramics, certain composite materials, etc. This article deals only with materials used in aeronautics, whose applications require the removal of a large quantity of material by machining in order to go from a rough forged or cast part to a finished part, with the volume of material to be removed by machining reaching 80%. These materials are mainly superalloys and titanium alloys, used in particular for the construction of turbojet engines.

The main characteristics of turbojet engines are thrust-to-weight ratio and specific fuel consumption. The most favorable figures are achieved by using nickel and cobalt alloys (superalloys) for their high-temperature resistance, and titanium alloys for their specific properties (property to density ratio). Superalloys account for around 55% of the materials used in jet engines, and titanium alloys for around 25%. These materials present major machining difficulties, and developments such as the new powder-metallurgy alloys (PM) and single-crystal alloys generally amplify these difficulties.

However, the imperatives of competitiveness demand that parts be produced with enhanced productivity and quality performance. To this end, all potential improvements to conventional and non-conventional machining processes must be considered and evaluated.

The subject of this article is to analyze the specific machining characteristics of superalloys and titanium alloys, then to consider and comment on developments in advanced machining processes such as high-speed and cutting assistance, and finally to describe new strategies for optimizing machining parameters. Older machining processes, which are proving highly productive with new materials, are also considered.