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

Nickel-based super alloys are gaining more significance, now-a-days, with extensive applications in aerospace, marine, nuclear reactor and chemical industries. Several characteristics including superior mechanical and chemical properties at elevated temperature, high toughness and ductility, high melting point, excellent resistance to corrosion, thermal shocks, thermal fatigue and erosion are primarily responsible for wide domain of application. Nevertheless, machined surface integrity of nickel-based super alloys is a critical aspect which influences functional performance including fatigue life of the component. This review paper presents state-of-the-art on various surface integrity characteristics during machining of nickel-based super alloys. Influence of various cutting parameters, cutting environment, coating, wear and edge geometry of cutting tools on different features of surface integrity has been critically explained. These characteristics encompass surface roughness, defects (surface cavities, metal debris, plucking, smeared material, redeposited material, cracked carbide particles, feed marks, grooves and laps), metallurgical aspects in the form of surface and sub-surface microstructure phase transformation, dynamic recrystallisation and grain refinement and mechanical characteristics such as work hardening and residual stress. Microstructural modification of deformed layer, profile of residual stresses and their influence on fatigue durability have been given significant emphasis. Future research endeavour might focus on development of new grades, advanced processing techniques of the same to ensure their superior stability of microstructure and thermo-mechanical properties along with advanced manufacturing processes like additive manufacturing to achieve highest level of fatigue durability of safety critical components while maintaining acceptable surface integrity and productivity.

State-of-the-art in surface integrity in machining of nickel-based super alloys

This paper presents an overview of the recent advances in high performance cutting of aerospace alloys and composite currently used in aeroengine and aerostructure applications. Progress in cutting tool development and its effect on tool wear and surface integrity characteristics of difficult to machine materials such as nickel based alloys, titanium and composites is presented. Further, advances in cutting technologies are discussed, focusing on the role of hybrid machining processes and cooling strategies (MQL, high pressure coolant, cryogenic) on machining performance. Finally, industrial perspectives are provided in the context of machining specific components where future challenges are discussed.

High performance cutting of advanced aerospace alloys and composite materials

Surface integrity in cryogenic machining of nickel based alloy—Inconel 718

Titanium and nickel alloys are the most commonly used in the demanding industries like aerospace, energy, petrochemical, and biomedical. These highly engineered alloys offer unique combination of heat resistance, corrosion resistance, toughness, high operating temperature, and strength-to-weight ratio. These alloys are termed as “Difficult to cut materials” because of their low machinability rating. They are difficult to machine because of properties like low thermal conductivity, high strength at elevated temperatures, and high chemical reactivity. Machining of titanium- and nickel-based alloys causes problems of surface integrity and selection of cutting tool materials that is always a challenge for manufacturers. In this work, machinability studies for titanium and nickel alloys are reviewed with reference to cutting tool materials, associated wear mechanisms, failure modes, and novel tooling techniques. It also discusses major surface integrity defects like carbide cracking, white layer formation, wor...

Influence of Tool Materials on Machinability of Titanium- and Nickel-Based Alloys: A Review

A high strength high γ ′ fraction nickel-based superalloy powder RR 1000 has been hot isotatically pressed (HIPed) at different temperatures. Microstructural analysis and assessment of the tensile properties were performed on these samples. It was found that HIP led to the formation of (Hf,Zr)O 2 particles on prior particle boundaries (PPBs) which were not present in the as-received powder. It is suggested that the oxides are formed by the diffusion of Hf and Zr from the interior of powder particles to the particle surfaces where oxygen level is usually high. When different HIP temperatures were used, no obvious effect on oxide size and distribution was observed but there was an effect on the microstructure and tensile properties. Thus, HIPing at super-solvus temperatures reduced the density of PPBs over the density observed in samples HIPed at sub-solvus temperatures by making grains within the original powder particles grow beyond the precipitates on PPBs, resulting in larger grains with serrated boundaries. Slow cooling from HIPing temperatures also led to the formation of irregular-shaped γ ′. The 0.2% yield strengths at room temperature and at 700 °C were found to decrease with increase of HIP temperature but the high temperature ultimate tensile strengths and elongation increased considerably. Increasing HIPing temperature from sub-solvus to super-solvus temperatures also led to the transition of fracture mode from interparticle debonding to transgranular fracture mode.

Influence of hot isostatic pressing temperature on microstructure and tensile properties of a nickel-based superalloy powder

Turning of advanced Ni based alloys obtained via powder metallurgy route

Influence of heat treatment on microstructure and tensile behavior of a hot isostatically pressed nickel-based superalloy

Residual stresses in face finish turning of high strength nickel-based superalloy

Higher-strength heat-resistant superalloys are continually being developed to meet the engineering requirements for increased efficiency of gas turbine engines In addition, work on machinability assessment is needed to fill the gap between the material development and manufacturing capabilities before considering the implementation of such materials in production lines The present paper presents selected results on optimization of cutting conditions for hole-making processes of a new aerospace nickel (RR1000)-based superalloy manufactured via a powder route The mechanical properties and the chemistry of RR1000 alloy indicate lower machinability when compared with the current disc alloys (eg Inconel, Waspaloy, Udimet) The hole-making processes, involving a succession of roughing (drilling) and finishing (normal/special reaming or plunge milling) operations, were evaluated through a multicriteria procedure consisting of the following output measures: tool life, hole accuracy, surface roughness

Paul Andrews

Papers

Influence of hot isostatic pressing temperature on microstructure and tensile properties of a nickel-based superalloy powder

Turning of advanced Ni based alloys obtained via powder metallurgy route

Influence of heat treatment on microstructure and tensile behavior of a hot isostatically pressed nickel-based superalloy

Residual stresses in face finish turning of high strength nickel-based superalloy

Some considerations on tool wear and workpiece surface quality of holes finished by reaming or milling in a nickel base superalloy