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

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.

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

As human improve their ability to fabricate materials, alloys have evolved from simple to complex compositions, accordingly improving functions and performances, promoting the advancements of human civilization. In recent years, high-entropy alloys (HEAs) have attracted tremendous attention in various fields. With multiple principal components, they inherently possess unique microstructures and many impressive properties, such as high strength and hardness, excellent corrosion resistance, thermal stability, fatigue, fracture, and irradiation resistance, in terms of which they overwhelm the traditional alloys. All these properties have endowed HEAs with many promising potential applications. An in-depth understanding of the essence of HEAs is important to further developing numerous HEAs with better properties and performance in the future. In this paper, we review the recent development of HEAs, and summarize their preparation methods, composition design, phase formation and microstructures, various properties, and modeling and simulation calculations. In addition, the future trends and prospects of HEAs are put forward.

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Science and technology in high-entropy alloys

Titanium alloys and their machinability—a review

The machinability of nickel-based alloys: a review

Test results show that when machining nickel-based Inconel 718 alloy under the cutting conditions investigated, the multilayer PVD-coated carbide grades gave the best performance in terms of tool life. This was due to their higher hardness, toughness, abrasion resistance, good heat transmission behavior of the multiple (TiN/TiCN/TiN) coatings as well as thicker coating layer (relative to the single-coated grade). The smaller grain size and higher density of their substrates also enhance the strength and hardness of the multi-coated PVD tools, thus providing higher resistance to attrition wear. Machining with the single-PVD TiN-coated grade generally produced a better surface finish due to the polishing action of the honed cutting edge and the generation of a uniform finishing edge with coating bottom on the trailing edge during machining. The absence of any significant tearing on the machined surfaces after cutting with the PVD- and CVD-coated tools at the cutting conditions investigated can be attributed...

Tool Life and Surface Integrity When Machining Inconel 718 With PVD- and CVD-Coated Tools

Nickel base, Inconel 718, and titanium base, IMI 318 (Ti-6A1-4V) alloys were machined with single (TiN) and multiple (TiN/TiCN/TiN) PVD coated carbide inserts at high cutting conditions in order to evaluate their performance and also to analyze their failure modes. The machining results show that coating material(s), tool geometry, machining parameters as well as material properties can singly or jointly affect tool performance and failure modes. Flank wear was the dominant failure mode at cutting speeds up to 42 m/min for the Inconel 718 alloy and up to 100 m/min for the IMI 318 alloy when machining at feed rates up to 0.25 mm/rev and depth of cut up to 2.00 mm. Excessive chipping/flaking off of tool particles and premature edge fracture also contributed to tool rejection, particularly when machining with the multi-coated inserts due to their sharp edges and instability of the machining system at higher speed conditions. The multi-layer coated tools generally produced lower flank wear rates, thus the bes...

Wear of Coated Carbide Tools When Machining Nickel (Inconel 718) and Titanium Base (Ti-6A1-4V) Alloys

This work investigates the performance of TiN and TiN/TiCN/TiN physical vapor deposition (PVD)-coated carbide tools in continuous turning of Ti-6Al-4V. Significant improvement in tool life was achieved when machining Ti-6Al-4V with both PVD-coated tools under the optimum cutting conditions. Notching at the depth of cut (DOC) was suppressed and flank wear, chipping and flaking of tool material on the rake face and/or at the nose of tools were the dominant failure modes. Tool life equations of the coated tools (when machining Ti-6Al-4V) were achieved by using a statistical regression analysis of the experimental data.

Z. M. Wang

Papers

Titanium alloys and their machinability—a review

The machinability of nickel-based alloys: a review

Tool Life and Surface Integrity When Machining Inconel 718 With PVD- and CVD-Coated Tools

Wear of Coated Carbide Tools When Machining Nickel (Inconel 718) and Titanium Base (Ti-6A1-4V) Alloys

Performance of PVD-Coated Carbide Tools When Machining Ti-6Al-4V©