Titanium and nickel alloys represent a significant metal portion of the aircraft structural and engine components. When these critical structural components in aerospace industry are manufactured with the objective to reach high reliability levels, surface integrity is one of the most relevant parameters used for evaluating the quality of finish machined surfaces. The residual stresses and surface alteration (white etch layer and depth of work hardening) induced by machining of titanium alloys and nickel-based alloys are very critical due to safety and sustainability concerns. This review paper provides an overview of machining induced surface integrity in titanium and nickel alloys. There are many different types of surface integrity problems reported in literature, and among these, residual stresses, white layer and work hardening layers, as well as microstructural alterations can be studied in order to improve surface qualities of end products. Many parameters affect the surface quality of workpieces, and cutting speed, feed rate, depth of cut, tool geometry and preparation, tool wear, and workpiece properties are among the most important ones worth to investigate. Experimental and empirical studies as well as analytical and Finite Element modeling based approaches are offered in order to better understand machining induced surface integrity. In the current state-of-the-art however, a comprehensive and systematic modeling approach based on the process physics and applicable to the industrial processes is still missing. It is concluded that further modeling studies are needed to create predictive physics-based models that is in good agreement with reliable experiments, while explaining the effects of many parameters, for machining of titanium alloys and nickel-based alloys.

Machining induced surface integrity in titanium and nickel alloys: A review

An overview of the machinability of aeroengine alloys

Significant advances have been made in understanding the behaviour of engineering materials when machining at higher cutting conditions from practical and theoretical standpoints. This approach has enabled the aerospace industry to cope with constant introduction of new materials that allow the engine temperature to increase at a rate of 10 °C per annum since the 1950s. Improvements achieved from research and development activities in this area have particularly enhanced the machining of difficult-to-cut nickel base and titanium alloys that have traditionally exhibited low machinability due to their peculiar characteristics such as poor thermal conductivity, high strength at elevated temperature, resistance to wear and chemical degradation, etc. A good understanding of the cutting tool materials, cutting conditions, processing time and functionality of the machined component will lead to efficient and economic machining of nickel and titanium base superalloys. This paper presents an overview of major advances in machining techniques that have resulted to step increase in productivity, hence lower manufacturing cost, without adverse effect on the surface finish, surface integrity, circularity and hardness variation of the machined component.

Key improvements in the machining of difficult-to-cut aerospace superalloys

Dry Machining and Minimum Quantity Lubrication

Surface integrity in material removal processes: Recent advances

Titanium alloys and their machinability—a review

The machinability of nickel-based alloys: a review

Cubic Nitride Boron (CBN) tools are generally used for machining harder alloys such as hardened high Cr steels, titanium and nickel alloys. The tools are expected to withstand the heat and pressure developed when machining at higher cutting conditions because of their high hardness and melting point. This paper evaluates the performance of different CBN tool grades in finish turning Ti–6Al–4V (IMI 318) alloy at high cutting conditions, up to 250 m min−1, with various coolant supplies. Tool wear, failure modes, cutting and feed forces and surface roughness of machined surfaces were monitored and used to access the performance of the cutting tools. Comparative trials were carried out with uncoated carbide tools when machining at a speed of 150 m min−1. Test results show that the performance of CBN tools, in terms of tool life, at the cutting conditions investigated is poor relative to uncoated carbide tools, as expected and often, reported due probably to rapid notching and excessive chipping of the cutting edge associated with a relatively high diffusion wear rate that tends to weaken the bond strength of the tool substrate. An increase in the CBN content of the cutting tool also led to a reduction in tool life when machining at the cutting conditions investigated.

Emmanuel O. Ezugwu

Papers

Titanium alloys and their machinability—a review

An overview of the machinability of aeroengine alloys

Key improvements in the machining of difficult-to-cut aerospace superalloys

The machinability of nickel-based alloys: a review

Evaluation of the performance of CBN tools when turning Ti-6Al-4V alloy with high pressure coolant supplies