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Titanium alloys have outstanding mechanical properties such as high hardness, a good strength-to-weight ratio and high corrosion resistance. However, their low thermal conductivity and high chemical affinity to tool materials severely impairs their machinability with conventional techniques. Conventional machining of Ti-based alloys is typically characterized by low depth of cuts and relatively low feed rates, thus adversely affecting the material removal rates (MRR) during the machining process. Ultrasonically assisted turning (UAT) is an advanced machining technique, in which ultrasonic vibration is superimposed on a cutting tool. UAT was shown to improve machinability of difficult-to-machine materials, such as ceramics, glass or hard metals. UAT employment in the industry is, however, currently lacking due to imperfect comprehensive knowledge on materials' response and difficulties in obtaining consistent results. In this work, significant improvements in the design of a UAT system were performed to increase dynamic and static stiffness of the cutting head. Concurrent improvements on depth-of-cut controls allowed precise and accurate machining operations that were not possible before. Effects of depth of cut and cutting speed were investigated and their influence on the ultrasonic cutting process evaluated. Different cutting conditions -from low turning speeds to higher recommended levelwere analysed. Thermal evolution of cutting process was assessed, and the obtained results compared with FE simulations to gain knowledge on the temperatures reached in the cutting zone. The developed process appeared to improve dry turning of Ti-15-3-3-3 with significant reduction of average cutting forces. Improved surface quality of the finished work-piece was also observed. Comparative analyses with a conventional turning (CT) process at a cutting speed of 10 m/min showed that UAT reduced the average cutting forces by 60-65% for all levels of ap considered. Temperature profiles were obtained for CT and UAT of the studied alloy. A comparative study of surface and sub-surface layers was performed for CT- and UAT-processed work-pieces with notable improvements for the UAT-machined ones. Two- to three-fold reductions of surface roughness and improvements of other surface parameters were observed for the UAT- machined surfaces. Surface hardness for both the CT- and UAT-machined surfaces was investigated by microindentation. The intermittent cutting of the UAT-process resulted in reduction of hardening of the sub-surface layers. Optical and electronic metallographic analyses of cross-sectioned work-pieces investigated the effect of UAT on the grain structure in material's sub-surface layers. Backscatter electron microscopy was also used to evaluate the formation of α-Ti during the UAT cutting process. No grain changes or α-precipitation were observed in both the CT- and UAT-machined work-pieces.

Titanium alloys have outstanding mechanical properties such as high hardness, a good strength-to-weight ratio, excellent fatigue properties and high corrosion resistance. However, several inherent properties including their low thermal conductivity and high chemical affinity to tool materials impairs severely their machinability with conventional machining techniques. Conventional machining of Ti-based alloys is typically characterized by low depths of cuts and relatively low feed rates, thus adversely affecting the material removal rates during the machining process. Recently, a non-conventional machining technique known as ultrasonically assisted turning (UAT) was introduced to machine modern alloys, in which low-energy, high-frequency vibration is superimposed on the movement of a cutting tool during a conventional cutting process. This novel machining technique results in a multi-fold decrease in the level of cutting forces with a concomitant improvement in surface finish of machined modern alloys. Also, since the late 20th century, machining of wear resistant materials that soften when heated has been carried out with hot machining techniques. In this work, a new hybrid machining technique called Hot Ultrasonically Assisted Turning (HUAT) is introduced for processing of a Ti-based alloy Ti-15V3Al3Cr3Sn. In this technique, UAT is combined with a traditional hot machining technique to gain combined advantages of both schemes for machining of intractable alloys. HUAT of the studied alloy was analysed experimentally and numerically to demonstrate its benefits in terms of reduction in cutting forces over a wide range of industrially relevant speed-feed combinations. Thermal evolution in the cutting process was assessed, and the obtained results were compared with FE simulations to gain knowledge of temperatures reached in the cutting zone. The developed novel turning process appeared to improve dry turning of the Ti alloy with significant reduction of average cutting forces without any substantial metallurgical changes in the workpiece material. Nano-indentation, light microscopy and SEM studies were performed to get an insight into the development of hardness in a zone near the machined surface in the workpiece. Backscatter electron microscopy was also used to evaluate the formation of α-Ti during the novel HUAT. No grain changes or α-precipitation were observed in machined workpieces in conventional and hybrid turning processes. 3D elasto-plastic thermomechanically coupled finite-element models for the orthogonal turning process were developed for conventional turning (CT), hot conventional turning (HCT), UAT and HUAT, followed by a more realistic novel 3D finite-element model for the oblique turning process. These 3D models were used to study the effects of cutting parameters (cutting speed, feed rate and depth of cut, ultrasonic vibration, ultrasonic frequency, rake angle and tool nose radius) on cutting forces, temperature in the process zone and stresses. The later model was used to analyse the effect of vibration and heat on the radial and axial components of cutting forces in HUAT, which was not possible with the developed 3D orthogonal-turning model. Comparative studies were performed with the developed CT, HCT, UAT and HUAT finite-element models and were validated by results from experiments conducted on the in-house prototype and in literature. The HUAT for the Ti-15333 was analysed experimentally and numerically to demonstrate the benefits in terms of a significant reduction in the cutting forces and improvement in surface roughness over a wide range of industrially relevant speed-feed combinations.

This thesis presents a newly developed manufacturing method, based on cyber-physically enhanced hybrid machining, regarding an optical bench (OB) made of Ti6Al4V alloy for the Advanced Telescope for High-ENergy Astrophysics (ATHENA). The method includes sophisticated hybrid laser metal deposition equipment and state-of-the-art cryogenic machining hardware. The derived strategy combines localized energy input, preheating, heat treatment, intermediate stress relief and machining. This results in a complex thermal history and remaining residual stresses, representing a considerable challenge for final precision machining. The method targets first time right machining based on iterative machining, process data-based tool path correction and spatially resolved root cause research based on process data modeling.:II. Table of Contents I. Acknowledgement ............................................................ III II. Table of Contents ................................................................. I 1. Introduction ........................................................................ 1 1.1 Foreword .................................................................................... 1 1.2 Research Subject Lot Size One ....................................................... 2 1.2.1 Historical Perspective ................................................................. 2 1.2.2 Going Full Cycle ......................................................................... 3 2. State of the Art in Titanium Processing ............................... 4 2.1 Conventional Processing................................................................ 4 2.2 Additive Manufacturing ................................................................. 5 2.2.1 Introduction .............................................................................. 5 2.2.2 Powder Bed Fusion ..................................................................... 6 2.2.3 Direct Energy Deposition ............................................................. 8 3. Derivation of a Flexible Hybrid Manufacturing System ...... 11 3.1 The ATHENA OB – a Large Structural Space Component ..................11 3.2 Material Constraints ....................................................................12 3.3 Solidification and Microstructural Content .......................................17 3.4 Residual Stresses and Intrinsic Heat Treatment ..............................22 3.4.1 Transient Temperature Gradients ................................................22 3.4.2 Residual Stresses and Degree of Fixity ........................................24 3.4.3 In-situ Stress Relief and Plastic Deformation ................................28 3.4.4 In-situ Martensite Decomposition and Thermal Trade-off ...............30 3.5 Melt Pool Considerations in Laser Metal Deposition ..........................36 3.6 Concept of Flexible Hybrid Manufacturing Cell .................................43 3.7 Process and Equipment Review by ESA ..........................................45 4. Realization of a Flexible Manufacturing Cell ...................... 45 4.1 Additive Processing with Hybrid Laser Metal Deposition ....................45 4.1.1 Principle Hardware ....................................................................45 4.2 Novel Local Shielding Solution ......................................................47 4.2.1 Melt Pool Observation towards Process Data Model ........................51 4.2.2 Energy Source Coupling .............................................................57 4.3 Subtractive Processing with Cryogenic Milling .................................57 4.3.1 General Considerations for Subtractive Processing ........................57 4.3.2 Cryogenic Machining Approach ...................................................58 4.3.3 Cryogenic Machining from the Materials Viewpoint ........................60 4.3.4 Cryogenic Machining of Additively Manufactured Ti-6Al-4V .............62 4.3.5 Principle Hardware for Cryogenic Milling with CO2..........................66 4.3.6 Intelligent Tool Spindle Future Part of the Process Data Model ........69 4.3.7 Carbon Dioxide Weighing Equipment and Switching Station ............70 4.3.8 Protective Measures for Safe Use of Cryogenic CO2 .......................72 4.4 Handling System .........................................................................74 4.4.1 Framework Considerations .........................................................74 4.4.2 Twin Robot System in the Initial State .........................................76 4.4.3 Integration of the ATHENA Turntable ...........................................79 4.4.4 Robot Calibration ......................................................................81 4.5 Lighting for Visual Inspection ........................................................84 4.6 Critical Design Review by ESA .......................................................84 5. Implementation and Validation ......................................... 85 5.1 Powdery Filler Material Selection ...................................................85 5.2 Basic Parameter Set for Additive Manufacturing ..............................87 5.2.1 Operating Point Selection ...........................................................87 5.2.2 Characterization and evaluation ..................................................89 5.2.3 Substrate to Structure Transition ................................................95 5.3 Energy Source Coupling ...............................................................99 5.3.1 Process Development ................................................................99 5.3.2 As-built Surface Treatment ...................................................... 103 5.3.3 Heat Treatment ...................................................................... 104 5.3.4 Mechanical Testing .................................................................. 106 5.3.5 Fractured Surfaces .................................................................. 108 5.3.6 Microstructure ........................................................................ 110 5.3.7 Linear Expansion Coefficient ..................................................... 113 5.4 Cryogenic Milling ....................................................................... 114 5.4.1 Strategy Approach .................................................................. 114 5.4.2 Milling Implementation ............................................................ 116 5.4.3 Technical Cleanliness ............................................................... 120 5.4.4 Accuracy and Duration ............................................................. 122 5.4.5 Surface Roughness.................................................................. 122 5.5 Process Data Model ................................................................... 123 6. Final Discussion and Conclusions..................................... 130 6.1 Summary ................................................................................. 130 6.2 Conclusions .............................................................................. 131 6.3 Outlook .................................................................................... 132 III. List of Figures ...................................................................... I IV. List of Tables .................................................................. VIII V. References ......................................................................... IX VI. Symbols and Units ....................................................... XXXVI VII. Abbreviations .............................................................. XXXIX VIII. Annex I ............................................................................ XLI IX. Annex II ....................................................................... XLIII X. Annex III ....................................................................... XLIV XI. Annex IV.......................................................................... XLV XII. Annex V ......................................................................... XLVI XIII. Annex VI....................................................................... XLVII XIV. Annex VII ................................................................... XLVIII