Cryogenic Processing of \u3cem\u3eAl 7050-T7451\u3c/em\u3e Alloy for Improved Surface Integrity

Al 7050-T7451 alloy with good combinations of strength, stress corrosion cracking resistance and toughness, is used broadly in the aerospace/aviation industry for fatigue-critical airframe structural components. However, it is also considered as a highly anisotropic alloy as the crack growth behavior along the short transverse direction is very different from the one in the long transverse direction, due to the inhomogeneous microstructure with the elongated grains distributed in the work material used in the sheet/plate applications. Further processes on these materials are needed to improve its mechanical and material properties and broaden its applications. The material with ultra-fine or nano grains exhibits improved wear and corrosion resistance, higher hardness and better fatigue life, compared to the one with coarse grains. In recent times, the development of novel processing technologies has gained great attention in the research community to enhance the properties of the materials employed in the aerospace, biomedical, precision instrument, automotive, nuclear/power industries. These novel processing technologies modify the microstructure of this alloy and improve the properties. The aim of this dissertation is to investigate the effects of cryogenic processes, including friction stir processing (FSP), machining and burnishing, on Al 7050-T7451 alloy to solve the inhomogeneity issue and improve its surface integrity. FSP is applied to modify the microstructure of Al 7050-T7451 alloy for achieving more homogeneous structure with near ultra-fine grains (UFG) which were less than 2 µm, particularly in cryogenic FSP with liquid nitrogen as the coolant. Approximately 10% increase could be observed from the hardness measurement from the samples processed by cryogenic FSP, in contrast to dry FSP. Also, the texture change from Al (200) to Al (111) could be achieved in all the samples processed by dry and cryogenic FSP. Cryogenic machining and burnishing processes were also applied to enhance the surface integrity of the manufactured components with near-UFG structure. The highest cutting temperature was reduced by up to 44.7% due to the rapid cooling effect of liquid nitrogen in cryogenic machining, compared with dry machining. Nano grains were produced in the refined layers induced by cryogenic burnishing. And, up to 35.4% hardness increase was obtained within the layer depth of 200 µm in the cryogenically-burnished surface. A numerical finite element method (FEM) model was developed for predicting the process performance in burnishing. Less than 10% difference between the experimental and predicted burnishing forces was achieved in the simulation of cryogenic burnishing, and reasonable predictions were also achieved for temperatures, severe plastic deformation (SPD) layers

Recent advancements in nano-lubrication strategies for machining processes considering their health and environmental impacts

Industries have been seeking an efficient lubrication system that meets the requirement of sustainability without compromising manufacturing efficiency or final part quality. Conventional cutting fluids have been recognized as hazardous to the environment, health and economy of industries. The nano lubrication strategy has emerged as a sustainable and power-efficient lubrication system with encouraging performance in machining processes. This paper encapsulates an overview of the impact regarding usage of nanofluid as a cutting fluid in different machining processes. The recent innovations in the past decade, altered nano lubrication systems have been briefly summarized. A state of art review commences with a short synopsis of the historic perspective followed by a summary of the impact of nanofluid on different machining processes. The discussion section has been bifurcated according to the characterization of machining performance metrics. The environmental and health issues that emerged with the use of nanofluid are then discoursed thoroughly. Finally, the major findings are summarized and the future scope of research is identified. It can be quantified that the implementation of a nano lubrication system can significantly improve the heat transfer characteristic of base fluid which ultimately leads to the functionally tremendous product. However, there are major unknowns related to the health and environmental impact of nanoparticles

Temperature in Machining of Aluminum Alloys

The objective this work is to study the effect of the mechanical property of the workpiece (tensile strength) and cutting conditions (cutting speed, feed rate, depth of cut and lubri-cooling system) over the cutting temperature in turning of aluminum alloys. A 2k factorial planning was used to determine the machining test conditions. ANOVA and Regression Analysis of the results were performed. The main contribution of this work lies on its efficiency of describing the behavior of the cutting temperature as a function of the input variables. The results found in the present work have considered the interactions of the input variables, describing the cutting temperature in a complete way, not seen previously in the literature

THE INFLUENCE OF CRYOGENIC MACHINING ON SURFACE INTEGRITY AND FUNCTIONAL PERFORMANCE OF TITANIUM ALLOYS FOR BIOMEDICAL AND AEROSPACE APPLICATIONS

The excellent properties of titanium alloys such as high strength, as well as good corrosion and fatigue resistance are desirable for the biomedical and aerospace industry. However, the same properties that make titanium alloys desirable in high-performance applications also make these space-age materials “difficult-to-machine” materials, as the titanium alloys exhibit high cutting temperatures because of their high strength and low thermal conductivity. Cryogenic machining is a severe plastic deformation (SPD) processes which uses liquid nitrogen as the coolant to take away the heat generated during machining in a relatively short time. Cryogenic machining can significantly reduce the cutting temperatures at the tool-workpiece interface, thereby improving the surface integrity of the manufactured components. This dissertation presents the results of experimental and numerical investigations of the effects of different cooling conditions on the machining performance and machining-induced surface integrity of Ti-6Al-7Nb and Ti-5553 alloys. Surface integrity and residual stresses induced by cryogenic machining are studied and compared with dry machining. Corrosion tests were also conducted to study the influence of machining parameters on the corrosion resistance of machined Ti-6Al-7Nb alloy. The results of the numerical and experimental studies show that compared with dry machining, cryogenic machining generates superior surface finish, along with higher surface layer hardening. The sub-surface residual stress profile is more compressive after cryogenic machining, and evidence of nanostructured grains is also observed in the influenced surface layer under both cooling conditions. Also, cryogenically-cooled machined sample showed better corrosion resistance compared with dry machined sample

Surface composites and functionalisation : enhancement of aluminium alloy 7075-T651 via friction stir processing

Abstract: This research work is aimed at modifying and enhancing the properties of aluminium alloy 7075- T651 through the friction stir processing (FSP) technique, in order to improve the mechanical, electrochemical, structural, tribological as well as the metallurgical properties which include micro- and macro- structural analysis through XRD and Image processing of grain size and grain flow patterns determination, by reinforcing the parent metal. The surface modification of the parent metal has been made possible in the past via different techniques,such as laser surfacing, electronbeam welding and thermal spraying; but in recent years, the friction stir processing (FSP) technology has been adopted to cater for the complex methods of surface enhancement. FSP is well-renowned for its short route of fabrication, densification, grain refinement, homogenization of the precipitates of composite substances, nugget zone homogeneity. These have led to the efficient surface enhancement, significant and remarkable improvement in hardness, ductility, strength, increased fatigue life, as well as formability within which the bulk properties are still intact. The use of FSP in the fabrication of metal matrix composites (MMCs), especially aluminium matrix composites (AMCs) and aluminium hybrid composites (AHCs) were dealt with in this study...Ph.D. (Mechanical Engineering

ENHANCED SURFACE INTEGRITY WITH THERMALLY STABLE RESIDUAL STRESS FIELDS AND NANOSTRUCTURES IN CRYOGENIC PROCESSING OF TITANIUM ALLOY TI-6AL-4V

Burnishing is a chipless finishing process used to improve surface integrity by severe plastic deformation (SPD) of surface asperities. As surface integrity in large measure defines the functional performance and fatigue life of aerospace alloys, burnishing is thus a means of increasing the fatigue life of critical components, such as turbine and compressor blades in gas turbine engines. Therefore, the primary objective of this dissertation is to characterize the burnishing-induced surface integrity of Ti-6Al-4V alloy in terms of the implemented processing parameters. As the impact of cooling mechanisms on surface integrity from SPD processing is largely unexplored, a particular emphasis was placed upon evaluating the influence of cryogenic cooling with liquid nitrogen in comparison to more conventional methodologies. Analysis of numerical and experimental results reveals that burnishing facilitates grain refinement via continuous dynamic recrystallization. Application of LN2 during SPD processing of Ti-6Al-4V alloy suppresses the growth of new grains, leading to the formation of near-surface nanostructures which exhibit increased microhardness and compressive residual stress fields. This is particularly true in cryogenic multipass burnishing, where successive tool passes utilizing lower working pressures generate thermally stable work hardened surface layers, uniform nano-level surface finishes, and significantly deeper layers of compressive residual stresses

Light Weight Alloys: Processing, Properties and Their Applications

There is growing interest in light metallic alloys for a wide number of applications owing to their processing efficiency, processability, long service life, and environmental sustainability. Aluminum, magnesium, and titanium alloys are addressed in this Special Issue, however, the predominant role played by aluminum. The collection of papers published here covers a wide range of topics that generally characterize the performance of the alloys after manufacturing by conventional and innovative processing routes

Machining strategies for distortion control during high speed machining

Airframe structural components that are machined from aluminium forgings or plate stock represent a significant contribution to the cost of both military and commercial aircraft. These components tend to distort due to heat treating induced bulk stresses and machining. Correcting these distortions increase costs and manufacturing lead times, especially for a high-volume, high-quality production company. In addition to this, variation in the residual stress profile from component to component is common due to variation in the condition of supply state. There is therefore a need to understand and model the effects of heat-treating and machining strategies on distortion and to predict, minimize, and control these distortions. This thesis addresses the modeling, data acquisition, and validation of residual stress and distortion models using different aluminium test cases. The project is divided into different technical studies to build the modelling capability: In the first study, aluminium 7050 material data and heat transfer coefficients were experimentally acquired. This data was to be used as an input to demonstrate the capability of Finite Element (FE) modelling as the main tool to predict and design robust strategies in the presence of residual stress variation due to processing or geometric differences. In the second study, the simulation study was performed to improve the machining distortion by using finite element (FE) modelling on varying residual stress profiles of aluminium coupons. Other studies included the influence of tool paths, the pocketing sequence, billet orientation and part location on machining distortion. Finally, utilizing the knowledge acquired, a machining process strategy for distortion control was proposed

Characterization Studies on Graphene-Aluminium Nano Composites for Aerospace Launch Vehicle External Fuel Tank Structural Application

From the aspect of exploring the alternative lightweight composite material for the aerospace launch vehicle external fuel tank structural components, the current research work studies three different grades of Aluminium alloy reinforced with varying graphene weight percentages that are processed through powder metallurgy (P/M) route. The prepared green compacts composite ingots are subjected to microwave processing (Sintering), hot extruded, and solution treated (T6). The developed Nano-graphene reinforced composite is studied further for the strength–microstructural integrity. The nature of the graphene reinforcement and its chemical existence within the composite is further studied, and it is found that hot extruded solution treated (HEST) composite exhibited low levels of carbide (Al4C3) formations, as composites processed by microwaves. Further, the samples of different grades reinforced with varying graphene percentages are subjected to mechanical characterisation tests such as the tensile test and hardness. It is found that 2 wt% graphene reinforced composites exhibited enhanced yield strength and ultimate tensile strength. Microstructural studies and fracture morphology are studied, and it is proven that composite processed via the microwave method has exhibited good ductile behaviour and promising failure mechanisms at higher load levels

Chapter 2: Aerospace Materials Characteristics

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