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

Background: Titanium alloy, as a high-strength alloy material, has been widely used in aerospace and other fields due to its high specific strength, good corrosion resistance, super fracture toughness, and fatigue resistance. However, its small elastic modulus, low thermal conductivity, and strong chemical affinity result in high cutting forces and cutting temperatures during the cutting process. Different thermo-mechanical coupling effects can change the surface structure, composition, and mechanical properties of the workpiece and show different surface integrity. This paper summarizes the recent progress in the experimental research on surface integrity in the cutting of titanium alloy from four aspects, including surface roughness, microhardness, white layer, and residual stress. It focuses on analyzing the experimental results about the influence of process parameters, tool wear, and lubrication methods on surface integrity. The problems existing in current research and the future development direction are pointed out in this article. It is helpful to provide theoretical guidance for the machining of titanium alloy that meets the requirement of service performance. Introduction: Surface integrity encompasses both geometric features and physical and mechanical properties. Geometric features include surface roughness, surface morphology, and blade direction. Extensive research has been conducted on surface roughness, microhardness, the white layer, and residual stress. This article reviews recent advancements in experimental research on surface integrity during titanium alloy cutting, focusing on the impact of cutting parameters, tool wear, and lubrication methods. It also addresses current research challenges and suggests directions for future development. Methods: Scholars have conducted extensive research on surface roughness, microhardness, the white layer, and residual stress using both experimental methods and finite element simulations. Experiments are the primary method due to existing issues with finite element simulations, which often suffer from low accuracy and poor reliability in studying surface integrity during titanium alloy machining. Results: At present, a large amount of experiments have been conducted. However, due to the numerous influencing factors and insufficient consideration of materials microstructure, the conclusions of current research are not consistent. Therefore, it is necessary to conduct more in-depth and systematic experimental research to clarify the influencing factors and the influence of cutting parameters on surface integrity. Conclusion: There is a need for a deeper understanding of the physical and mechanical fundamentals of surface integrity. Integrating experimental research with finite element simulations can enhance the investigation of these mechanisms and improve predictions related to phase transformation, microhardness, white layer, and residual stress. It is helpful to provide theoretical guidance for the machining of titanium alloy that meets the requirement of service performance.

Manufacturing processes, such as machining, can produce residual stresses in products. Residual stress and its distribution can be the main factor influencing the fatigue life of machined components and has already been the subject of many experimental and numerical studies. The high-temperature condition, as a result of machining, makes a change in the microstructural properties of the material and consequently affect the mechanical properties of the workpiece. A major metal component of aircraft structure and engine components is nickel-based alloys due to their resistance to heat, corrosion, thermal fatigue, thermal shock, creep, and erosion. When these critical structural components in the 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 alterations including white layer, depth of work hardening, micro-cracks, and oxidation induced by machining of nickel-based alloys are extremely critical due to safety and sustainability concerns. Integrated Computational Materials Engineering (ICME) links physics-based models to predict the performance of materials based on their processing history. The Johnson–Mehl–Avrami-Kolmogrov (JMAK) model is used to develop a microstructure-based modeling approach that takes into account dynamic recrystallization (DRX) that causes grain size changes. Allied with that, a grain size parameter on the flow stress behavior of the material is considered by adding a grain size-dependent term to the traditional Johnson–Cook (JC) model as a novel framework. The impact of the simulation of the orthogonal cutting process is implemented in a finite element method (FEM) model–based commercial software, ABAQUS-explicit, with a coupled Euler-Lagrangian (CEL) approach. By relying on the VUHARD user subroutine capabilities with Fortran language, ABAQUS-explicit can be steered to model the material behavior considering the term of DRX. The forecast capability of the developed model is assessed by comparison of the results by changing the depth of cut and cutting edge radius effect on the residual stress. Then, the correlation between the grain size evolution and temperature distribution by changing the cutting velocity is investigated.

State-of-the-art of surface integrity induced by tool wear effects in machining process of titanium and nickel alloys: A review

GH4169 is comparatively a new superalloy mainly used as turbine components because of its outstanding combination properties such as high-temperature strength, thermal stability and wear resistance. But these also make it hard to cut, and its machined surface quality and integrity are particularly sensitive to the manufacturing process employed. The existing researches on machining-induced surface integrity and machinability of hard-to-cut materials are briefly reviewed; the effects of processing parameters on surface integrity for GH4169 components are studied in detail via orthogonal-designed external grinding experiment. The single-factorial plain grinding experiment was designed to further investigate the influence of depth of cut on the surface integrity characteristics. The surface roughness, residual stress distribution, microhardness profile and microstructural alteration within the subsurface were obtained and analyzed. It was shown that the surface integrity is susceptible to the magnitude of depth of cut, and the components ground with low depth of cut are of more acceptable surface quality with less variation in residual stress and microhardness within the machining-affected layer than those obtained with high depth of cut. No severe microstructural alteration or adverse surface cracking was discerned when the depth of cut is reasonably set.

The rise in need for aerospace production is directly impacted by the machinability of materials used in aerospace parts. The Ultrasonic Assisted Turning is steadily becoming a research focus as an eco-friendly and cost-efficient unconventional machining method. It has been shown to enhance the machinability of various materials by creating a separation between the tool and the workpiece. The increase in demand for aerospace production created a need for a better understanding of this method’s effect on the machinability of aerospace materials. This research investigates the effect of the Ultrasonic Assisted Turning method on aerospace titanium alloy Ti-6Al-4V and low carbon steel A36. The effect of adding a cryogenic coolant to the ultrasonic machining of titanium alloys is also investigated. The investigation focuses on the variation of the cutting parameters, where the cutting speed and the depth of cut are varied to determine their effect on the machinability of the aerospace materials in Ultrasonic Assisted Turning. The study begins with experimental investigation of the cutting forces and surface roughness for different cutting parameters. Tool wear analysis is then conducted for the optimal cutting parameters. The separation is then investigated theoretically. A Finite Element Model (FEM) is then developed to simulate the cutting conditions and investigate the processes involved in Ultrasonic Assisted Turning for a better understanding of its effect on the machinability of titanium and steel alloys. It is shown that the smallest cutting speed and depth of cut lead to the largest reduction in the cutting forces and enhancement in tool wear whereas the largest cutting speed resulted in the largest reduction in surface roughness. The Ultrasonic Assisted Turning was found to decrease adhesion and microchipping. The theoretical investigation proves the presence of separation. The theoretical and the FEM results show that the separation duration decreases as the cutting speed and depth of cut increase. The temperature, strain rate and stress profiles are extracted from the FEM and are used to provide a better understanding of the ultrasonic effect on the machinability of these alloys. The combined effect of the strain rates and temperature is dominated by the softening caused by the increase in temperature. The addition of a coolant is shown to limit the benefits of this method. This research provides a better understanding of the beneficial effect of the Ultrasonic Assisted Turning on the machinability of titanium and steel alloys.’

A Multiscale Study on Machining Induced Surface Integrity in Ti-6Al-4V Alloy

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, work hardening layer formation, residual stresses, and microstructural alterations. Major aim of this work is to evaluate the challenges involved in improving machinability of the titanium- and nickel-based alloys, and determine the future research direction for productivity improvements in machining these alloys.

Ti-alloy represents a significant metal portion of aircraft structural and engine components for high reliability. Surface integrity is one of the most relevant parameters used for evaluating the quality of finish machined surfaces. The residual stress and surface alteration with each layer and depth of work hardening by machining Ti-alloy are critical due to safety and sustainability concerns. Residual stresses, white layers well microstructural alterations can be figured out to improve surface qualities of end products. Many parameters such as cutting speed, feed rate, depth of cut affect the machined surface quality particularly surface fisnish. This article provides details of lathe turning for investigation of surface roughness for varying cutting parameters. An attempt has been made to search for best ranges of cutting regimes that could produce best surface roughness for machining Ti-6Al-4V alloy using uncoated cutting tool. Taylor-Hobson device is used to measure the surface roughness on the machined workpieces. In this project three series of experimentaions were carried out and a total of 16 steps of operations in each series are performed for determining the surface roughness. Real life experimental investigation has allowed to express the results in graphical form (using tabulated data) that has suggested best ranges of cutting regimes (parameters) for obtaining the best ranges of surface roughness for machining Ti-6Al-4V using uncoated carbide tool. The work has indicated to investigate the science of bulk flow, particularly the plastic deformation, for difficult to machine materials, at a much higher temperature. Behaviour of cutting tool materials for high speed cutting is another isuue to develop.

The main objective of this paper is to develop the relationship between microstructure and machinability of titanium alloy Ti-6Al-4V which is given heat treatment beyond β transus temperature, with the help of milling experiments using polycrystalline diamond (PCD) tools. Two of the specimens were solution treated at 1050 °C/1 h. After the treatment, one of the specimen was cooled in water while the other specimen was air-cooled, followed by aging treatment at 550 °C/4 h. The third specimen was used in as-received condition (casted). Cutting speed, feed rate, and depth of cut were varied, and their effects have been analyzed on cutting forces, vibration amplitude, temperature, and tool wear for all the three specimens. The results have shown improved and better machinability for the specimen which was cooled in air after the solution treatment with PCD tools. This behavior of the alloy is mainly attributed to formation of homogeneous lamellar α + β Ti-6Al-4V microstructure. Although, the air-cooled specimen has higher hardness than the as-received specimen. But, the former alloy’s fine homogeneous and lamellar α + β Ti-6Al-4V microstructure as compared to the microstructure of the latter specimen (irregular α + β Ti-6Al-4V structure) and absence of α’ secondary precipitate (found in water-cooled specimen) has helped in giving improved performance. Thus, from this study, it can be concluded that machinability of titanium alloy Ti-6Al-4V can be further improved with controlled heat treatment process using PCD tools.

Surface integrity in ultrasonic-assisted turning of Ti6Al4V using sustainable cutting fluid

The titanium alloy Ti-6Al-4V represents a significant metal portion of state-of-the-art aircraft structural and engine components. When critical structural components in the 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 machined surfaces. The residual stresses and the surface alteration induced by machining titanium alloys are critical due to safety and sustainability issues. In this paper, a series of end milling experiments was conducted to comprehensively characterize the surface integrity at various milling conditions. The experimental results have shown that the surface roughness value increases with the feed and the cutting velocity. However, the residual stress state in the surface layer zone is influenced by the variation of the process control variables. Here, compressive residual stresses occur both in cutting and in feed direction. In addition, a new type of sensory tool holder is presented, which should enable the indirect measurement of residual stresses during the milling process.

Processing of Titanium by Machining: A Closer Look Into Performance Metrics in Bio-Fabrications

Deformation-phase transformation coupling mechanism of white layer formation in high speed machining of FGH95 Ni-based superalloy

Effects of cutting conditions on the microstructure and residual stress of white and dark layers in cutting hardened steel

In recent years, with the continuous development of my country's industrial level, people's research and exploration in the machining and application of porous titanium alloys has become more and more in-depth. In order to achieve greater breakthroughs and development in the field of optimization test research on surface integrity parameters of porous titanium alloy machining. In this paper, experiments and finite element simulation methods are used to adjust the process of titanium alloy machining by multi-step cutting and prestressed cutting, in order to control the surface quality of titanium alloy machining. Titanium alloys have superior performance and have the advantages of high strength and high corrosion resistance, but the small thermal conductivity and elastic modulus make titanium alloys difficult to cut. In this paper, a simulation model of multi-step cutting and pre-stressed cutting of titanium alloys is established, and the specific laws of multi-step cutting and pre-stressed cutting process regulation affecting chip shape, cutting force and residual stress of machined surface layer are studied. The results show that both multi-step cutting and prestress cutting can increase the residual compressive stress on the finished surface. The final results of the study showed that when the titanium alloy was cut at a distance of 4.63 mm, the corresponding machine wear was 5.41%. In the application process of porous titanium alloy cutting, the wear degree caused by titanium alloy cutting is not affected by its cutting distance, and the wear degree caused by titanium alloy cutting has always maintained an average level of about 5.5%, which is relatively stable.