Experimental Study And Modeling Of Mechanical Micro-machining Of Particle Reinforced Heterogeneous Materials

This study focuses on developing explicit analytical and numerical process models for mechanical micro-machining of heterogeneous materials. These models are used to select suitable process parameters for preparing and micro-machining of these advanced materials. The material system studied in this research is Magnesium Metal Matrix Composites (Mg-MMCs) reinforced with nano-sized and micro-sized silicon carbide (SiC) particles. This research is motivated by increasing demands of miniaturized components with high mechanical performance in various industries. Mg-MMCs become one of the best candidates due to its light weight, high strength, and high creep/wear resistance. However, the improved strength and abrasive nature of the reinforcements bring great challenges for the subsequent micro-machining process. Systematic experimental investigations on the machinability of Mg-MMCs reinforced with SiC nano-particles have been conducted. The nanocomposites containing 5 Vol.%, 10 Vol.% and 15 Vol.% reinforcements, as well as pure magnesium, are studied by using the Design of Experiment (DOE) method. Cutting forces, surface morphology and surface roughness are characterized to understand the machinability of the four materials. Based on response surface methodology (RSM) design, experimental models and related contour plots have been developed to build a connection between different materials properties and cutting parameters. Those models can be used to predict the cutting force, the surface roughness, and then optimize the machining process. An analytical cutting force model has been developed to predict cutting forces of MgMMCs reinforced with nano-sized SiC particles in the micro-milling process. This model is iv different from previous ones by encompassing the behaviors of reinforcement nanoparticles in three cutting scenarios, i.e., shearing, ploughing and elastic recovery. By using the enhanced yield strength in the cutting force model, three major strengthening factors are incorporated, including load-bearing effect, enhanced dislocation density strengthening effect and Orowan strengthening effect. In this way, the particle size and volume fraction, as significant factors affecting the cutting forces, are explicitly considered. In order to validate the model, various cutting conditions using different size end mills (100 µm and 1 mm dia.) have been conducted on Mg-MMCs with volume fraction from 0 (pure magnesium) to 15 Vol.%. The simulated cutting forces show a good agreement with the experimental data. The proposed model can predict the major force amplitude variations and force profile changes as functions of the nanoparticles’ volume fraction. Next, a systematic evaluation of six ductile fracture models has been conducted to identify the most suitable fracture criterion for micro-scale cutting simulations. The evaluated fracture models include constant fracture strain, Johnson-Cook, Johnson-Cook coupling criterion, Wilkins, modified Cockcroft-Latham, and Bao-Wierzbicki fracture criterion. By means of a user material subroutine (VUMAT), these fracture models are implemented into a Finite Element (FE) orthogonal cutting model in ABAQUS/Explicit platform. The local parameters (stress, strain, fracture factor, velocity fields) and global variables (chip morphology, cutting forces, temperature, shear angle, and machined surface integrity) are evaluated. Results indicate that by coupling with the damage evolution, the capability of Johnson-Cook and Bao-Wierzbicki can be further extended to predict accurate chip morphology. Bao-Wierzbiki-based coupling model provides the best simulation results in this study. v The micro-cutting performance of MMCs materials has also been studied by using FE modeling method. A 2-D FE micro-cutting model has been constructed. Firstly, homogenized material properties are employed to evaluate the effect of particles’ volume fraction. Secondly, micro-structures of the two-phase material are modeled in FE cutting models. The effects of the existing micro-sized and nano-sized ceramic particles on micro-cutting performance are carefully evaluated in two case studies. Results show that by using the homogenized material properties based on Johnson-Cook plasticity and fracture model with damage evolution, the micro-cutting performance of nano-reinforced Mg-MMCs can be predicted. Crack generation for SiC particle reinforced MMCs is different from their homogeneous counterparts; the effect of micro-sized particles is different from the one of nano-sized particles. In summary, through this research, a better understanding of the unique cutting mechanism for particle reinforced heterogeneous materials has been obtained. The effect of reinforcements on micro-cutting performance is obtained, which will help material engineers tailor suitable material properties for special mechanical design, associated manufacturing method and application needs. Moreover, the proposed analytical and numerical models provide a guideline to optimize process parameters for preparing and micro-machining of heterogeneous MMCs materials. This will eventually facilitate the automation of MMCs’ machining process and realize high-efficiency, high-quality, and low-cost manufacturing of composite materials

Investigation into micro machinability of Mg based metal matrix compostites (MMCs) reinforced with nanoparticles

PhD ThesisAs composite materials with combination of low weight and high engineering strength, traditional metal matrix composites (MMCs) with micro-sized reinforcement (micro-MMCs) have been utilized in numerous area such as aerospace, automobile, medical and advanced weapon systems in the past two decades. With the development of composite materials, metal matrix composites reinforced with small volume fraction of nano-sized reinforcements (nanoMMCs) exhibits an equivalent and even better properties than that reinforced with large volume of micro-sized reinforcement and thus receive increasing attention from academia and industries. MMCs components are typically fabricated in near net shape process such as casting. But micro machining processes are indispensable in order to meet the increasing demands on the component with high dimensional accuracy and complex shapes. However, the enhanced mechanical properties of MMCs and tool-like hardness of reinforced particles bring challenges to machining process. The deteriorative machined surface finish and excessive tool wear have been recognised as the main obstacles during machining of MMCs due to their heterogeneous and abrasive nature. In this research, the detailed material removal mechanism of nano-MMCs in terms of micro machinability, micro tool wear and simulated material removal process with finite element analysis (FEA) is investigated. The systematic experimental studies on micro machining mechanism of magnesium-based MMCs reinforced with nanoparticles (Ti, TiB2, BN, ZnO) are conducted. The cutting force, burr formation, surface roughness and morphology are characterised to investigate the micro machinability under the effect of various machining parameters, particle volume fraction and matrix/reinforcement materials using design of experiment (DoE) and analysis of variance (ANVOA) methods. The micro structure changes of Mg-MMCs by addition of nanoparticles were taken into account. In addition, surface morphology and the minimum chip thickness is obtained and characterised with the aim of examining the specific cutting energy. A comprehensive investigation of tool wear mechanisms in the micro milling of Mg-MMCs is conducted. The tool wear is characterised both quantitatively and qualitatively by observing tool wear patterns and analysing the effect of cutting parameters and tool coating on average flank wear, reduction in tool diameter, cutting forces, surface roughness, and burr formation. The main wear mechanisms at different machining conditions are determined. Finally, the tool wear phenomenon observed from experiments is explained by simulating the tool-particles interaction using finite element modelling, and hence new wear mechanisms are proposed for machining nano-MMCs. iv The two dimensional micromechanical finite element (FE) models are established to study the material removal mechanism of MMCs reinforced with micro-sized and nanoparticles in micro machining process with consideration of size effect. Two phases, namely particle and matrix are modelled in FE cutting models. Particle fracture properties are involved in micro-sized particles to study the fracture behaviours. The cutting force, tool-particles interaction, particle fracture behaviours, stress/strain distribution, chip formation process and surface morphology are investigated in the FE models. The surface defect generation mechanism is studied in details by developing the additional three dimensional (3D) FE models in machining micro-MMCs. Moreover, the cutting mechanism comparison between machining nano-MMCs and microMMCs is conducted to investigate the effect of significant particle size reduction from micro to nano-scale. The model validation is carried out by studying the chip morphology, cutting force, surface morphology obtained from machining experiments and good agreements are found with the simulation results

The Effects of Nanoparticle Reinforcement on the Micromilling Process of A356/Al2O3 Nanocomposites

Abstract Improving scientific knowledge around the manufacturing of nanocomposites is key since their performance spreads across many applications, including those in meso/micro products. Powder metallurgy is a reliable process for producing these materials, but usually, machining postprocessing is required to achieve tight tolerances and quality requirements. When processing these materials, cutting force evolution determines the ability to control the microcutting operation toward the successful surface and part quality generation. This paper investigates cutting force and part quality generation during the micromilling of A356/Al2O3 aluminum nanocomposites produced via powder metallurgy. A set of micromilling experiments were carried out under various process parameters on nanocomposites with different nano-Al2O3 reinforcements (0–12.5 vol.%). The material’s ductility, internal porosity, and lack of interparticle bonding cause the cutting force generation to be irregular when nanoparticle reinforcements were absent or small. Reinforcement ratios higher than 2.5 vol.% strongly affect the cutting process by regularizing the milling force generation but lead to a proportionally increasing average force magnitudes. Hardening due to nano-reinforcement positively affects cutting mechanisms by reducing the plowing tendency of the cutting process, resulting in better surface quality. Therefore, a threshold on the nano-Al2O3 particles’ volumetric loadings enables an optimal design of these composite materials to support their micromachinability

A new semi-solid casting technique for fabricating SiC-reinforced Mg alloys matrix composites

The capability of the newly developed rheocasting (RC) technique in combination with the RheoMetal process for producing SiC particulate-reinforced AM50 and AZ91D matrix composites (Mg-based MMCs) was investigated. The quality of the MMCs was studied by analyzing the fraction of casting pores, number density of SiC clusters and the uniformity of SiC particles. Solid fraction, particle size and oxidation of SiC particles had strong impacts on the overall quality of the MMCs. The MMCs produced by 40% solid fraction and oxidized micron-sized SiC particles exhibited an excellent casting quality. A low-quality MMC was obtained when non-oxidized sub-micron sized SiC particles were employed. The results showed the formation of various types intermetallic particles and carbides such as MgO, Mg2Si, Al2MgC2, Mg2C3, Al4C3 as the interfacial reaction products of SiC/Mg alloy's melts. Mg hydride (alpha-MgH2) was also identified in inter-dendritic regions of the MMCs for the first time

Synthesis, Microstructural Characterization, Mechanical, Fractographic and Wear Behavior of Micro B4C Particles Reinforced Al2618 Alloy Aerospace Composites

In the current studies an investigations were made to know the effect of 63 micron sized B4C particles addition on the mechanical and wear behavior of aerospace alloy Al2618 metal composites. Al2618 alloy with different weight percentages (2, 4, 6 and 8 wt. %) of 63 micron sized B4C particles reinforced composites were produced by stir cast process. These synthesized composites were tested for various mechanical properties like hardness, compression strength and tensile behavior along with density measurements. Further, microstructural characterization was carried by SEM/EDS and XRD analysis to know the micron sized particles distribution and phases. Wear behavior of Al2618 alloy with 2 to 8 wt. % of B4C composites were studied as per ASTM G99 standards with varying loads and sliding speeds. By adding 63 micron sized B4C particles hardness, compression and tensile strength of Al2618 alloy was enriched with slight decrease in elongation. Further, wear resistance of Al2618 alloy was enriched with the accumulation of B4C particles. As load and speed on the specimen increased, there was increase in wear of Al2618 alloy and its composites. Various tensile fracture surface morphology and worn surface behavior was observed by SEM analysis

In-Situ Synthesis of Aluminum- Titanium Diboride Metal Matrix Hybrid Nanocomposite

Metal matrix nanocomposites (MMNC’s) are reported to have improved mechanical, thermal and electrical properties as compared to their respective base alloys. To date, these materials have been synthesized mainly by powder metallurgy or deformation processing. Solidification synthesis of MMNCs is a promising method, capable of economically producing large and complex shapes, however technical challenges including nanoparticle agglomeration, and poor interfacial strength have hindered the adoption of this technology. In-situ processing methods, in which the reinforcements are synthesized in liquid metals, typically via exothermic reactions offer the potential for improved dispersion and interfacial bonding between the reinforcement and the matrix, however this technique has been largely unexplored in the literature for metal matrix nanocomposites. The objectives of this research were to examine the feasibility of synthesizing nano or sub-micron size particulates in liquid aluminum using in-situ stir mixing and squeeze casting. An exothermic reaction was designed to synthesize Al2O3 and TiB2 from TiO2 particles and elemental boron in an aluminum melt. This dissertation investigates (i) the mechanism of aluminothermic and borothermic reduction of titanium oxide in the presence of molten aluminum and boron, (ii) in situ synthesis of micron and nano sized particles via solidification processing, and (iii) the effects of processing variables on the physical, microstructural, mechanical and tribological properties of in-situ MMNCs. Microstructural examination and theoretical analysis indicates that the reaction to form TiB2 and Al2O3 proceeds through several complex non-equilibrium reactions. A multi-stage reaction model is proposed to describe the process by which the TiO2 surface is reduced to form Al2O3 and TiB2. The effects of the powder particle size on the formation of reinforcing phases and microstructural evolution have been investigated and it was found that nanosized TiO2 powder promoted the formation of smaller size reinforcing phases. Furthermore, a solidification route has been designed to fabricate in-situ aluminum composites reinforced with submicron Al2O3 and TiB2 particulates. Experimental and theoretical analysis is presented that shows that the particle size and refining power of nanoparticles is controlled by the viscosity of the melt, rather than precipitation and growth. In addition, it was found that increasing the weight percentage of nanoparticles of TiO2 resulted in an increase in elastic modulus with good agreement to analytical models. Increasing the weight percentage of reinforcement up to 4 wt% resulted in an increase in the hardness greater than that predicted by the rule of mixtures or the Hall Petch relationship

DEVELOPMENT AND CHARACTERIZATION OF SUSTAINABLE NOVEL ALUMINUM METAL MATRIX COMPOSITES

Low crude oil prices have impacted the economy of the Gulf Cooperation Council (GCC) member countries especially the United Arab Emirates (UAE). Hence it is vital to accelerate the diversification of the economy. Among the many potential diversification avenues, manufacturing is a promising area that could add to the GDP. This work brings out a sustainable and cost-effective method for manufacturing AMCs and expanding their applications. This work deals with the processing and characterization of recycled AMCs manufactured using a novel approach. With the emphasis on sustainable manufacturing, this work aims to use Scrap Aluminum Alloy Wheels (SAAW) of cars as the matrix. SAAW was easily obtained from the scrap wheels of cars. The reinforcement material, Spent Alumina Catalyst (SAC), was sourced from the local oil refineries which is a waste material from crude oil refining. To achieve the objectives of this work, the following steps were followed. Firstly, four AMCs were developed using stir-gravity casting. Four composites were made with different combinations such as LM25+Al₂O₃, SAAW+Al₂O₃, LM25+SAC, and SAAW+SAC. The microstructure analysis showed a nonhomogeneous distribution of reinforcements with a high amount of porosity. Therefore, this method was not used for the optimization and casting of AMCs. Secondly, AMCs were produced using the stir-squeeze casting method. Similar to the previous casts, four composites of LM25+Al₂O₃, SAAW+Al₂O₃, LM25+SAC, and SAAW+SAC were made. The samples from this method exhibited better strength when compared to gravity cast samples. SAAW+Al₂O₃ exhibited an almost uniform distribution of reinforcement particles and superior mechanical properties with the lowest porosity (7.3%), highest hardness (69 VHN), and minimum abrasive wear loss (0.001g), second highest tensile (129 MPa) and compressive (320 MPa) strengths among the four composites. The results also revealed that optimizing the stir squeeze casting process parameters can further contribute to the performance of the recycled AMCs. Thirdly, optimization of casting parameters using the Taguchi method was carried out. Taguchi-Grey Relational Analysis (GRA) was successfully utilized to handle the multi-response objective system for optimizing process parameters in the squeeze casting of AMCs. This method was used to determine the optimized condition with a minimal set of experiments, which is relevant in the stir–squeeze casting process. Taguchi method developed 9 samples (L1-L9) and out of that L5 and L6 exhibited the best mechanical properties. Thus, the optimum levels of process parameters are squeeze pressure of 100MPa, squeeze time of 30 s, die preheat temperature of 250°C and stirrer speed of 525 rpm. Fourthly, the optimized sample (M2) was produced. Taguchi’s confirmation test was run based on the obtained mechanical properties and the L6 method showed an improvement in the GRG value by 12.5%. Based on the confirmation test, the optimized sample M2 was produced using a squeeze pressure of 100 MPa, a squeezed time of 45s, a die preheating temperature of 250°C, and a stirrer speed of 525 rpm. The M2 sample showed the lowest porosity (5.29%) and significantly higher ultimate compression strength (433 MPa) although it exhibited slightly lower hardness and ultimate tensile strength when compared with the L6 and L5 samples, respectively. Fifthly, a hybrid AMC was produced to further enhance the performance. Five casts (1% graphite+ Al₂O₃, 3% graphite+ Al₂O₃, 4% graphite+ Al₂O₃, 3% SiC+Al₂O₃, 6% SiC+ Al₂O₃) were prepared with SAAW as matrix and alumina, graphite and SiC as fillers with different percentage. AMC with 4% graphite along with alumina showed the highest tensile and compressive strength of 250 MPa and 508 MPa respectively, followed by a sample with 3% SiC and alumina. Lastly, Friction Stir Welding (FSW) was carried out to check the weldability. L5, L6, M2, and hybrid AMC samples were successfully welded using a cylindrical tool pin with 4 mm pin depth, tool rotation of 1100 rpm and feed rate of 50 mm/min. Tensile results from the welded zone showed that sample M2 and AMC with 4% graphite exhibited high strength of 185 and 210 MPa respectively. From these results, it can be seen that this approach can easily be scaled up for production in large volumes as well as open avenues for developing AMCs reinforced with other waste materials

Graphene-Based Lubrication for Tribological Applications: Nanolubricants and Self-lubricating Nanocomposites

In this work, the effects of graphene nanoplatelets (GNPs) additives on tribological properties of aluminum are investigated. The objective of this research is to investigate and explain the enhancement mechanisms of GNPs at the contact surface during tribological testing. The graphene nanoplatelets are studied both as an oil additive (Chapter I) and as a reinforcement (Chapter II) experimentally. The coefficient of friction (COF) and wear rate were identified using a pin-on-disk test setup. Mineral, organic, and synthetic oils are not always efficient enough to satisfy the demands of a high-performance lubricant; therefore, mixing additives with base fluids is an approach to improve the lubrication ability and to reduce friction and wear. In chapter I, GNPs are used as lubricant additives to make nanolubricants. Then, the combined effect of the material’s variables (GNPs loading, size, and dispersion stability) and tribo test’s variable (applied normal load) are investigated on COF and wear rate of aluminum. Tribological studies are all carried out in the boundary lubrication regime. Three-dimensional surface metrology is performed using an optical profilometer. Various surface analyses, including Scanning Electron Microscope (SEM), Energy-Dispersive X-ray Spectroscopy (EDX), and Raman Spectroscopy are performed to assess the chemical elements on the tested surfaces. The experimental and theoretical analyses show that GNPs are effective in reducing friction and wear, although, this positive effect is more influential at higher loads. Also, it is demonstrated that there is a critical concentration of GNPs, below which a reduced wear rate is not sustained. The proposed mechanism to describe the effect of GNPs in boundary lubrication condition is “reduced direct metal-metal contact area” at the contact surface. In other words, a material which has low shear strength layers sits between two contacting surfaces and separates the two sliding metal surfaces with no actual contact between them. This means that there is less formation of asperity junctions between the two surfaces. Although liquid-based lubricants are efficient enough in most tribological applications, there are circumstances, such as extreme environmental conditions such as high or low temperatures, vacuum, radiation, and high contact pressure in some aerospace applications, where no liquid lubricants can be present. In addition, interminable providing of lubricant at the contact surface is another challenge ahead. In order to respond to these challenges of using liquid oil at extreme environmental conditions, in chapter II of this dissertation, the synthesis and performance of self-lubricating aluminum matrix nanocomposite are evaluated (Chapter II). Aluminum powder is mixed with varying concentrations of GNPs and alumina nanoparticles to form a hybrid metal matrix nanocomposite. High-energy ball milling is conducted at room temperature while powders are immersed and protected by benzene bath. Degassing is accomplished by heating to 135oC. Consolidation of the powders is conducted by single action cold compaction and single action hot compaction. Pin-on-disk experiments are conducted to investigate the tribological behavior of aluminum matrix composites reinforced by GNPs and compare them with unreinforced aluminum. Then, the combined effect of material’s variables (reinforcement type and loading) and tribo test’s variable (applied normal load) were investigated on COF and wear rate of aluminum. SEM and EDX were performed to assess the stoichiometry of the elements on the tribo surfaces. In addition, Raman Spectroscopy and Transmission Electron Microscopy (TEM) were also performed to identify the bonding/interactions between the phases on the surface. Results imply that the COF and wear rate of composites decrease by embedding graphene nanoparticles due to reduction the real contact area between the mating surfaces by forming the lubricant. Besides, the addition of alumina particles in Aluminum/GNPs composites can further improve COF and wear rate because of rolling effect of alumina nanoparticles. Increasing the loading of GNPs reduces the COF, while there is an optimum concentration of GNPs, above and below which the wear rate is increased. In addition, the COF and wear of all composites decreases by increasing normal load. Based on the observations, multiple mechanisms are proposed to describe the improved tribological behavior of the synthesized self-lubricating nanocomposites. In addition to the reduced direct metal-metal contact area at the contact surface, the fact that the layered GNPs structure is exposed to at the contact surface keeps the surface lubricated. In other words, under sliding conditions, the transfer layer formation of the GNPs on the tribo surfaces acts as a solid lubricant film, which prevents direct contact between the mating surfaces. Additionally, it is experimentally confirmed that GNPs prevent the surface from oxygen diffusion, thereby reducing the amount of oxides which are harder and more abrasive at the contact surface. “Load bearing” of added alumina nanoparticles, in addition to the increased hardness of the matrix, is another proposed mechanism of wear resistance enhancement. It has been shown that an effective lubricant layer forms when the solid lubricant has a strong adhesion to the bearing surface; otherwise, this lubricant layer can be easily rubbed away and tends to have a very short service life. Raman data confirms the formation of Al4C3 bonds on the tribo layer under certain test conditions

Effect of Graphene addition on the mechanical and tribological behavior of nanostructured AA2124 /Graphene self-lubricating metal matrix composite

In the current research, the mechanical and tribological behavior, and structural evolution of AA2124-3 and 5-wt.% graphene (G) composites prepared by a combination of ball milling and hot extrusion were investigated. Mixing followed by energy ball milling of the powders was conducted under argon atmosphere. Hot extrusion of the green compacts was carried out at 0.46 and 0.68 of the alloy melting temperature. Properties such as macro and micro-hardness, nanohardness, tensile and lattice strain were characterized. Wear rates, coefficient of friction (COF) were characterized using dry pin-on-disc test under loads of 50, 100N and 150N. Nanoscratch testing were employed to investigate the self-lubricating tribological behavior. X-ray diffraction, optical and scanning electron microscopy were used to determine the influence of the G-content on the crystallite size variation and the lattice strain for the ball milled powders compared to the hot extruded rods. Density measurements and optical microscopy (OM) were employed to investigate the consolidation degree and porosity variation as a function of increasing G- of the G and Al-matrices for the variable conditions. Bulk texture variation was analyzed to evaluate the influence of the extrusion temperature. AA 2124-3 wt.% G composites displayed the highest tensile properties, highest hardness and lowest wear rates and COF, as well as lowest scratch width and depth compared to the 5 wt.%G and the plain alloy. The uniform distribution of the G-particles within the Al-matrices for the 3wt.% containing composites hindered grain coarsening by the induced lattice strain compared to that of 5 wt% G ones. Moreover, addition of 3 wt.% G smeared thin uniform tribofilm on the surfaces of the worn composite rods. The formed layer reduced friction and wear. Increasing the G content up to 5 wt.% resulted in segregation and clustering of the G-particles within the Al-matrices, which caused microplouging and sever plastic deformation wear mechanism and excessive delamination. IV Lower consolidation temperatures of 300oC produced composites with lower wear rates due to the excessive strain hardening effect. Extrusion at 300oC produced a continuous G-encapsulating layer around the Al-matrix compared to an interrupted G-layer for the 450oC extrusions. The G-layer morphology influenced the dominating mechanism of the composite during deformation. Texture analysis of the AA2124-3 wt.%G extruded at low and high temperatures proved that both the Cu-and Shear are the dominating texture components, while increased texture intensities from 1.2-to-1.76 occurred with increasing the extrusion temperature