IN-SITU CHARACTERIZATION OF SURFACE QUALITY IN γ-TiAl AEROSPACE ALLOY MACHINING

The functional performance of critical aerospace components such as low-pressure turbine blades is highly dependent on both the material property and machining induced surface integrity. Many resources have been invested in developing novel metallic, ceramic, and composite materials, such as gamma-titanium aluminide (γ-TiAl), capable of improved product and process performance. However, while γ-TiAl is known for its excellent performance in high-temperature operating environments, it lacks the manufacturing science necessary to process them efficiently under manufacturing-specific thermomechanical regimes. Current finish machining efforts have resulted in poor surface integrity of the machined component with defects such as surface cracks, deformed lamellae, and strain hardening. This study adopted a novel in-situ high-speed characterization testbed to investigate the finish machining of titanium aluminide alloys under a dry cutting condition to address these challenges. The research findings provided insight into material response, good cutting parameter boundaries, process physics, crack initiation, and crack propagation mechanism. The workpiece sub-surface deformations were observed using a high-speed camera and optical microscope setup, providing insights into chip formation and surface morphology. Post-mortem analysis of the surface cracking modes and fracture depths estimation were recorded with the use of an upright microscope and scanning white light interferometry, In addition, a non-destructive evaluation (NDE) quality monitoring technique based on acoustic emission (AE) signals, wavelet transform, and deep neural networks (DNN) was developed to achieve a real-time total volume crack monitoring capability. This approach showed good classification accuracy of 80.83% using scalogram images, in-situ experimental data, and a VGG-19 pre-trained neural network, thereby establishing the significant potential for real-time quality monitoring in manufacturing processes. The findings from this present study set the tone for creating a digital process twin (DPT) framework capable of obtaining more aggressive yet reliable manufacturing parameters and monitoring techniques for processing turbine alloys and improving industry manufacturing performance and energy efficiency

Development of a digital manufacturing process chain for ceramic composites

The development of ceramic matrix composites, with their increasing use in high temperature and corrosive environment applications, is still restricted to ‘trial and error’ approach in comparison to other conventional materials like metals. The main reason behind that is the lack of experimental data due to high manufacturing costs of CMCs which generally includes a chain of several complex processes. This adds to the complexity of this material class and thus, makes it a difficult task to establish a relationship between a component with desired properties and the manufacturing parameters required to realise it. In the current work, the digital aspects are investigated from two point of views to use numerical methods to support the material design process: ‘material’ and ‘manufacturing process’. The case ‘material’ is the focus of this work where, ‘process-structure-property-performance’ (PSPP) relationship is established to study the entire life cycle of a CMC component, starting from the intermediate products, such as fibre preforms or green bodies prior to siliconization process, used in the processing to the mechanical performance of the final machined component under operating conditions. Each aspect of the PSPP relationship is discussed in detail and its implementation is demonstrated with the help of a numerical example. Cohesive zone elements at micro-level and homogenous damage development at macro-level were used to define the non-linear behaviour of the material under mechanical loading. Experimental results obtained for different CMCs such as C/C-SiC, C/SiCN, SiC/SiCN and Al2O3/ Al2O3 were used to validate the results obtained for the finite element models at different scales ranging from micro to macro. With the help of data analysis techniques like image segmentation and machine learning algorithm, computationally inexpensive data-based surrogate models were generated from accurate but computationally expensive physics-based models. A detailed review of the available numerical methods to model the manufacturing process and the process monitoring techniques is given. Based on the data and information obtained from the modelling of the material and the manufacturing process, a concept is proposed for optimized development of a CMC part. The concept combines the generated data with quantified expertise in the fields of material science to realise a manufacturing process chain to facilitate the material design process for CMCs. With the implementation of such an approach, the production cost of CMCs can be reduced by knowledge-based selection of the CMC constituents and manufacturing parameters. This will open the door for new applications of CMCs which would enable the material community to extend their use to other cost-efficient high temperature applications.Die Entwicklung von Verbundwerkstoffen mit keramischer Matrix, die zunehmend bei hohen Temperaturen und in korrosiven Umgebungen zum Einsatz kommen, ist im Vergleich zu anderen herkömmlichen Werkstoffen wie Metallen noch immer auf ein "Versuch-und-Irrtum"-Konzept beschränkt. Der Hauptgrund dafür ist der Mangel an experimentellen Daten aufgrund der hohen Herstellungskosten von CMCs, die im Allgemeinen eine Prozesskette aus mehreren komplexen Verfahrensschritten umfassen. Dies trägt zur Komplexität dieser Werkstoffklasse bei und macht es somit schwierig, eine Beziehung zwischen einem Bauteil mit gewünschten Eigenschaften und den Herstellungsparametern herzustellen. In der vorliegenden Arbeit werden die digitalen Aspekte aus zwei unterschiedlichen Blickwinkeln untersucht, um numerische Methoden zur Unterstützung der Werkstoffauslegung einzusetzen: 'Werkstoff' und 'Herstellungsprozess'. Im Mittelpunkt dieser Arbeit steht der "Werkstoff", bei dem die "Process-Structure-Property-Performance"-Beziehung (PSPP) hergestellt wird, um den gesamten Lebenszyklus eines CMC-Bauteils zu untersuchen. Angefangen bei den Zwischenprodukten, wie z. B. den Faser-Vorkörpern (Preform)vor dem Silizierverfahren, die die Basis der Verarbeitung bilden, bis hin zur mechanischen Belastungsgrenze des fertig bearbeiteten Bauteils unter Betriebsbedingungen. Jeder Aspekt der PSPP-Beziehung wird im Detail untersucht und ihre Umsetzung anhand eines numerischen Beispiels demonstriert. Kohäsive Zonenelemente auf der Mikroebene und homogene Schädigungsentwicklung auf der Makroebene wurden verwendet, um das nichtlineare Verhalten des Werkstoffs unter mechanischer Belastung zu definieren. Experimentelle Ergebnisse, die für verschiedene CMCs wie C/C-SiC, C/SiCN, SiC/SiCN und Al2O3/ Al2O3 erzielt wurden, dienten zur Validierung der Ergebnisse der Finite-Elemente-Modelle auf verschiedenen Skalen von Mikro bis Makro. Mit Hilfe von Datenanalysemethoden wie Bildsegmentierung und ‚Machine-Learning-Algorithmen‘ wurden aus genauen, aber rechenintensiven physikalischen Modellen zeiteffiziente datenbasierte Ersatzmodelle erstellt. Es wird ein detaillierter Überblick über die verfügbaren numerischen Methoden zur Modellierung des Fertigungsprozesses und der Prozessüberwachungstechniken gegeben. Auf der Grundlage der Daten und Informationen, die aus der Modellierung des Materials und der Herstellungsprozesse gewonnen wurden, wird ein Konzept für die optimierte Entwicklung eines CMC-Bauteils vorgeschlagen. Das Konzept kombiniert die generierten Daten mit quantifiziertem Fachwissen in den Bereichen der Materialwissenschaft, um eine Fertigungsprozesskette zu realisieren, die die Werkstoffauslegung für CMCs erleichtert. Mit der Umsetzung eines solchen Ansatzes können die Produktionskosten von CMCs durch eine wissensbasierte Auswahl der CMC-Bestandteile und Herstellungsparameter gesenkt werden. Dies wird die Tür für neue Anwendungen von CMCs öffnen, die es der Materialgemeinschaft ermöglichen wird, ihre Verwendung auf andere kosteneffiziente Hochtemperaturanwendungen auszuweiten

Exploring the possibilities of extending the application of PcBN cutting tools to the machining of ADI through the understanding of wear mechanisms

Experimental studies of wear, cutting forces and chip characteristics when dry turning ASTM Grade 2 ADI with cBN-TiC cutting tools under finishing conditions were carried out. A depth of cut of 0.2 mm, a feed of 0.05 mm/rev and cutting speeds ranging from 50 to 800 m/min were used. Oxidation experiments of the cBN-TiC cutting tools were carried out in the temperature range 500-1110 °C for a constant dwell time of 60 min. Static interaction experiments were done with ADI/cBN-TiC/ADI and Si/cBNTiC/ Si sandwiches in the temperature range 1000-1100 °C under argon for a constant dwell time of 60 min and a pressure of 200 MPa. An X-ray diffractometer, optical, scanning and transmission electron microscopes as well as an energy dispersive spectroscope (EDS) were appropriately used for characterization of ADI workpieces, PcBN cutting tools, chips, wear scars on PcBN cutting tools, scale on PcBN cutting tools after oxidation experiments and interaction interfaces after static interaction experiments. Flank wear and crater wear were the main wear modes within the range of cutting speeds investigated. At cutting speeds greater than 150 m/min, shear localization within the primary and secondary shear zones of chips was the key-process that controlled the wear rate indirectly, the static cutting forces and the dynamic cutting forces. Cutting speeds between 150 and 500 m/min were found to be optimum for the production of workpieces with acceptable cutting tool life, flank wear rate and lower dynamic cutting forces. Adhesion and adhesion induced abrasion were the main wear mechanisms at cutting speeds less than 150 m/min. Abrasion and wear by thermally activated diffusion and oxidation / chemical reaction wear were the main wear mechanisms at cutting speeds greater than 150 m/min. At cutting speeds greater than 150 m/min, the superficial melting of the BUL on the chip underside produced a lubricating film that maintained more or less constant tribological conditions at the toolchip interface, thus reducing shear localization in the secondary shear zone of chips. This lubricating film also played a role in the reduction of the tool-chip contact length, the increase of the shear angle and, consequently in the reduction in average chip thickness. The BUL on the crater wear scar was a sandwich of approximately 3 layers: a very thin C rich layer, an intermediate layer containing mainly Si, Mg, and O, slight amounts of B, C and N and very low amounts of Al and Ti as well as a Fe layer in preferential contact with the TiC binder. It became apparent that clues of diffusion wear should be sought in the BUL on the wear scars rather than in the secondary shear zone of chips. Constituents in the BUL on contact zones and non-contact zones of the tools were the products of thermally activated chemical reactions between constituents of the cutting tool, constituents of ADI (particularly Mg and Si) and atmospheric O2. Accordingly, the superficial melting of the BUL increased the wear rate of cBN-TiC cutting tools for cutting speeds greater than 150 m/min. IV Although a minor phase in cBN-TiC cutting tools, TiB2 emerged as a critical phase with regard to their wear behaviour. Its thermally activated dissolution in Fe and its thermally activated reactions with O2 or O2 and Mg as well as its thermally induced cracks might be expected to have synergistic effects on cBN grain pull-out and thus on the wear rate. Blistering and cracking of the non-contact zones of cBN-TiC cutting tools characterized the oxidation behaviour of cBN-TiC cutting tools. The cBN-TiC cutting tool material is not oxidation-resistant in air above 550 °C and the scale that formed on cBN-TiC cutting tools is not able to provide effective protection against oxidation. The TiC binder of the cutting tool material showed extensive oxidation, producing brittle titanium oxide crystallites. Oxidation experiments on cBN-TiC cutting tools showed that oxidation involved intense outward diffusion of elements such as Ti and Al. In addition to the grain coarsening of TiO2 and increased segregation of Al2O3 in the outer scale layer, it was clear that the formation and evaporation of B2O3 affected the morphology of the inner oxygen-affected zone significantly in terms of porosity. Mutual dissolution or reaction between B2O3 and TiO2 which could be expected to reduce such porosity was not evident. The same was true for TiO2 and Al2O3. From the static interaction experiments, strong indications of diffusion of Fe in the TiC binder as well as reprecipitation of Fe in TiC were evident. Strong indications of dissolution of TiC and diffusion of Ti and C as well as reprecipitation of TiC in Fe were also evident. The reprecipitation in Fe of this C in the form of Fe3C or Ti(C,N) is evidence of depletion in Si of the ADI close to the interface and thus evidence of diffusion of Si towards the cBN-TiC side. The evidence of diffusion of N in Fe and its reprecipitation in the form of Ti(C,N) is a strong indication of the dissolution of N-bearing phases such as Ti(C,N), AlN and BN in the cBN-TiC cutting tool material. Strong indications of diffusion of Si in the TiC binder as well as its reprecipitation as SixC or TiSi4-x were evident. All this correlates with the penetration of Si and Fe observed approximately 0.5 μm below the crater scar of the cBN-TiC cutting tools, at the boundaries between cBN grains and the TiC binder. The diffusion of Si in the TiC binder and its eventual reprecipitation as SixC or TiSi4-x give an indication of how the degradation of the wear resistance of the cBN-TiC cutting tool can occur during the machining of ADI. The same applies to the diffusion of Fe in the TiC binder. The strong evidence of interaction of Si, Fe and TiC obtained from the static interaction couples clearly indicates that during the machining of ADI with cBN-TiC cutting tools, bonding of ADI to the tool occurs preferentially through the TiC binder. Indication is also given of how the depletion in Si of the ADI can occur at the tool-chip and tool-workpiece interfaces. The knowledge generated in this work should contribute towards improved design and processing of PcBN cutting tools for use in the machining of ADI

Current Challenges and Opportunities in Microstructure-Related Properties of Advanced High-Strength Steels

This is a viewpoint paper on recent progress in the understanding of the microstructure–property relations of advanced high-strength steels (AHSS). These alloys constitute a class of high-strength, formable steels that are designed mainly as sheet products for the transportation sector. AHSS have often very complex and hierarchical microstructures consisting of ferrite, austenite, bainite, or martensite matrix or of duplex or even multiphase mixtures of these constituents, sometimes enriched with precipitates. This complexity makes it challenging to establish reliable and mechanism-based microstructure–property relationships. A number of excellent studies already exist about the different types of AHSS (such as dual-phase steels, complex phase steels, transformation-induced plasticity steels, twinning-induced plasticity steels, bainitic steels, quenching and partitioning steels, press hardening steels, etc.) and several overviews appeared in which their engineering features related to mechanical properties and forming were discussed. This article reviews recent progress in the understanding of microstructures and alloy design in this field, placing particular attention on the deformation and strain hardening mechanisms of Mn-containing steels that utilize complex dislocation substructures, nanoscale precipitation patterns, deformation-driven transformation, and twinning effects. Recent developments on microalloyed nanoprecipitation hardened and press hardening steels are also reviewed. Besides providing a critical discussion of their microstructures and properties, vital features such as their resistance to hydrogen embrittlement and damage formation are also evaluated. We also present latest progress in advanced characterization and modeling techniques applied to AHSS. Finally, emerging topics such as machine learning, through-process simulation, and additive manufacturing of AHSS are discussed. The aim of this viewpoint is to identify similarities in the deformation and damage mechanisms among these various types of advanced steels and to use these observations for their further development and maturation.</p

Current Challenges and Opportunities in Microstructure-Related Properties of Advanced High-Strength Steels

This is a viewpoint paper on recent progress in the understanding of the microstructure–property relations of advanced high-strength steels (AHSS). These alloys constitute a class of high-strength, formable steels that are designed mainly as sheet products for the transportation sector. AHSS have often very complex and hierarchical microstructures consisting of ferrite, austenite, bainite, or martensite matrix or of duplex or even multiphase mixtures of these constituents, sometimes enriched with precipitates. This complexity makes it challenging to establish reliable and mechanism-based microstructure–property relationships. A number of excellent studies already exist about the different types of AHSS (such as dual-phase steels, complex phase steels, transformation-induced plasticity steels, twinning-induced plasticity steels, bainitic steels, quenching and partitioning steels, press hardening steels, etc.) and several overviews appeared in which their engineering features related to mechanical properties and forming were discussed. This article reviews recent progress in the understanding of microstructures and alloy design in this field, placing particular attention on the deformation and strain hardening mechanisms of Mn-containing steels that utilize complex dislocation substructures, nanoscale precipitation patterns, deformation-driven transformation, and twinning effects. Recent developments on microalloyed nanoprecipitation hardened and press hardening steels are also reviewed. Besides providing a critical discussion of their microstructures and properties, vital features such as their resistance to hydrogen embrittlement and damage formation are also evaluated. We also present latest progress in advanced characterization and modeling techniques applied to AHSS. Finally, emerging topics such as machine learning, through-process simulation, and additive manufacturing of AHSS are discussed. The aim of this viewpoint is to identify similarities in the deformation and damage mechanisms among these various types of advanced steels and to use these observations for their further development and maturation

Development of a digital manufacturing process chain for ceramic composites

Die Entwicklung von Verbundwerkstoffen mit keramischer Matrix, die zunehmend bei hohen Temperaturen und in korrosiven Umgebungen zum Einsatz kommen, ist im Vergleich zu anderen herkömmlichen Werkstoffen wie Metallen noch immer auf ein "Versuch-und-Irrtum"-Konzept beschränkt. Der Hauptgrund dafür ist der Mangel an experimentellen Daten aufgrund der hohen Herstellungskosten von CMCs, die im Allgemeinen eine Prozesskette aus mehreren komplexen Verfahrensschritten umfassen. Dies trägt zur Komplexität dieser Werkstoffklasse bei und macht es somit schwierig, eine Beziehung zwischen einem Bauteil mit gewünschten Eigenschaften und den Herstellungsparametern herzustellen. In der vorliegenden Arbeit werden die digitalen Aspekte aus zwei unterschiedlichen Blickwinkeln untersucht, um numerische Methoden zur Unterstützung der Werkstoffauslegung einzusetzen: 'Werkstoff' und 'Herstellungsprozess'. Im Mittelpunkt dieser Arbeit steht der "Werkstoff", bei dem die "Process-Structure-Property-Performance"-Beziehung (PSPP) hergestellt wird, um den gesamten Lebenszyklus eines CMC-Bauteils zu untersuchen. Angefangen bei den Zwischenprodukten, wie z. B. den Faser-Vorkörpern (Preform)vor dem Silizierverfahren, die die Basis der Verarbeitung bilden, bis hin zur mechanischen Belastungsgrenze des fertig bearbeiteten Bauteils unter Betriebsbedingungen. Jeder Aspekt der PSPP-Beziehung wird im Detail untersucht und ihre Umsetzung anhand eines numerischen Beispiels demonstriert. Kohäsive Zonenelemente auf der Mikroebene und homogene Schädigungsentwicklung auf der Makroebene wurden verwendet, um das nichtlineare Verhalten des Werkstoffs unter mechanischer Belastung zu definieren. Experimentelle Ergebnisse, die für verschiedene CMCs wie C/C-SiC, C/SiCN, SiC/SiCN und Al2O3/ Al2O3 erzielt wurden, dienten zur Validierung der Ergebnisse der Finite-Elemente-Modelle auf verschiedenen Skalen von Mikro bis Makro. Mit Hilfe von Datenanalysemethoden wie Bildsegmentierung und ‚Machine-Learning-Algorithmen‘ wurden aus genauen, aber rechenintensiven physikalischen Modellen zeiteffiziente datenbasierte Ersatzmodelle erstellt. Es wird ein detaillierter Überblick über die verfügbaren numerischen Methoden zur Modellierung des Fertigungsprozesses und der Prozessüberwachungstechniken gegeben. Auf der Grundlage der Daten und Informationen, die aus der Modellierung des Materials und der Herstellungsprozesse gewonnen wurden, wird ein Konzept für die optimierte Entwicklung eines CMC-Bauteils vorgeschlagen. Das Konzept kombiniert die generierten Daten mit quantifiziertem Fachwissen in den Bereichen der Materialwissenschaft, um eine Fertigungsprozesskette zu realisieren, die die Werkstoffauslegung für CMCs erleichtert. Mit der Umsetzung eines solchen Ansatzes können die Produktionskosten von CMCs durch eine wissensbasierte Auswahl der CMC-Bestandteile und Herstellungsparameter gesenkt werden. XIV. Dies wird die Tür für neue Anwendungen von CMCs öffnen, die es der Materialgemeinschaft ermöglichen wird, ihre Verwendung auf andere kosteneffiziente Hochtemperaturanwendungen auszuweite

Subsurface deformation micromechanisms induced during machining of titanium alloys at low temperatures, and a novel testing methodology to examine their machining behaviour

The aerospace industry’s drive towards higher productivity has led manufacturers to strive for higher surface speeds in metal cutting. Machining of titanium alloys leads to high temperatures attributed to their low thermal properties, resulting in high tool wear rates. To counter this, large amounts of coolants are used. These contain toxic chemicals, which are harmful to both people and the environment. To reduce these hazards, near-dry strategies such as cryogenic cooling and minimum quantity lubrication (MQL) are investigated in this thesis. A fundamental knowledge gap in the literature was identified, which is the characterisation of the subsurface microstructural evolution during plastic deformation in the machining of titanium alloys. Besides, its impact on surface integrity needs to be investigated in detail. The aims of this PhD research were (1) To determine the “machinability” of titanium alloys by designing a novel and straightforward cutting test. (2) To determine the effect of low temperatures (LTs) on the underlying deformation mechanisms during plastic deformation in the machining of aero-structural Ti-6Al-4V. In particular, during the application of a cryogen media such as LN2 and CO2. (3) To build a constitutive model to predict the experimental flow behaviour. (4) To analyse the imparted subsurface deformation and relate to its subsurface integrity. A material's inherent mechanical, physical and thermal properties strongly influence its machining behaviour. In the uniaxial compression test, it was determined that β annealed Ti-6Al-4V ELI: undergoes shear localisation even at quasi-static strain rates, has a high sensitivity to temperature and a lower sensitivity to strain rate. The higher the temperature, the higher the strain rate sensitivity. Plastic deformation at LTs exhibited higher flow stresses vs ambient temperature. The true strain at the onset of thermal instability (softening) and at fracture was identified, it was c30% smaller at LTs vs room temperature, leading to a reduction in c20% energy for cutting at LTs. The strain-hardening rate during plastic deformation decreases linearly with further imparted strain and decreases faster at LTs. Machining generates a graded subsurface microstructure. Four different regions were identified in this investigation. (1) Severe plastic deformation region (SPD), where a nanocrystalline grain structure was observed through electron microscopy from cryogenic machining under CO2, resulting in a significant increase in strength. (2) Gross plastic deformation region. (3) Twinned region. (4) Undeformed bulk. In conclusion, machining of titanium alloys at cryogenic temperatures is easier as a lower strain is required for shearing, leading to lower energy spent for chip generation. Nevertheless, a larger microstructural damage depth is introduced into the subsurface, leading to more potential sites for crack nucleation. The main challenges that lie ahead are: (1) to determine the extent of the effect the microstructural damage has on the fatigue life during dynamic loading, and (2) to determine whether easy diffusion is allowed to occur under thermal exposure, which would negatively affect their mechanical properties