Cette étude s’est focalisée sur l’influence d’une opération de fraisage sur l’intégrité de surface et la durée de vie en fatigue des pièces usinées en titane (Ti-6Al-4V). L’influence des paramètres géométriques tels que les angles de coupe et les rayons de bec sur la performance de l’outil à savoir la durée de vie de l’outil est analysée. Une deuxième partie est consacrée à l’influence des paramètres géométriques sur l’intégrité de surface définie par des facteurs tels que la topographie, les contraintes résiduelles, la dureté et la microstructure. Afin de déterminer la nature des contraintes résiduelles, une approche thermomécanique a été utilisée. Une relation entre les contraintes résiduelles et la dureté sous la surface usinée est établie. Une dernière partie est dédiée à la l’étude de la fatigue des pièces en titane usinées selon les différents paramètres géométriques. Une comparaison des limites d’endurance expérimentales et analytiques issue de la littérature a permis de déterminer les paramètres influents de l’intégrité de surface sur la performance en fatigue. Une analyse fractographique a révélé les phénomènes liés à l’usinage influençant la rupture du matériau. / This study is carried out on the influence of milling process on surface integrity and fatigue life of a titanium alloy (Ti-6Al-4V). Geometric parameters like cutting angles and nose radius are investigated. In the first part, the observation is focused on tool life. The second part is dedicated to the influence of geometric parameters on surface integrity which is defined by topography, residual stress, hardness and microstructure. To determine the nature of the residual stresses a thermomechanical approach is used. A relation between residual stress and hardness is established. The last part deals with the study of the fatigue of milled titanium samples. A comparison is made between experimental fatigue limit and analytical fatigue limit of a model from literature. With this comparison, the most influential factor of surface integrity on fatigue limit is determined. A fractography analysis reveals the phenomenon related to milling process which can influence the material rupture.

The PhD thesis deals with the influence of temperature and strain rate on the mechanical behaviour of the Ti-6Al-4V titanium alloy. After verification tests under static loading conditions, the samples were deformed at high strain rates and elevated temperatures, using device for Hopkinson pressure bar test. The result is dependence of stress and strain rate on strain in the temperature range of 20 to 500°C. The deformed shape of specimen from the Taylor anvil test is compared with the results of the simulation in the Ansys – LS Dyna software. The parameters of Johnson-Cook equation were determined from these experiments. Also, the influence of loading conditions on the microstructure was studied. Both optical and scanning electron microscopes were used for the observations. During the research, some adjustments to the experimental devices were made in order to suppress the high-frequency components and noise in the recorded pulses. A functional tensile test adapter for the Hopkinson test was developed; it is registered under No. 2007/008 at the Technology Transfer Office of BUT.

Millimeterstora och svårupptäckta defekter kan uppstå internt i stora och komplexa gjutgods av Ti-6Al-4V, ibland går dessa oupptäckta tills detaljen genomgått mekanisk bearbetning och en stor kostnad redan har gått in i den. Dessa defekter och andra industriella olyckshändelser leder till ett behov av additiva reparationsmetoder där den för tillfället rådande metoden är TIG-svetsning. Denna metod reparerar defekterna men leder till oacceptabla restspänningar vilka kan åtgärdas med värmebehandling som i sin tur kan orsaka ytdefekten alpha case. Därav finns ett industriellt behov av reparationsmetoder som leder till mindre eller negligerbara restspänningsnivåer i reparerad detalj. Detta arbete utfört hos GKN Aerospace – Engine Products Sweden i Trollhättan analyserar eventuella förhållanden mellan parametrarna Effekt, Spot size, och Svetshastighet och de resulterande restspänningarna i ett lågparameterområde på materialet Ti-6Al-4V. En parameterrymd uppspänd av 17 parameteruppsättningar etablerades, svetsades och analyserades med mikrografi. Ur denna rymd simulerades de 8 yttre parametrarna med hjälpa av Finita Elementmetoden i svetssimuleringsmjukvaran MARC och ett förhållande mellan ingående parametrar och resulterande restspänningar undersöktes. En statistiskt säkerställd trend erhölls för att en minskad Svetshastighet leder till minskade tvärspänningar i mitten på en 20mm lång svetssträng. Detta är applicerbart för svetsar nyttjande start och stopplåtar. Det noterades även att en ökning i Effekt eller Spot size, eller en minskning utav svetshastigheten leder till att det av restspänningar utsatta området ökar i storlek. Detta är har implikationer för efterföljande värmebehandling i avgörandet av form och storlek på området som skall värmebehandlas. / Minute defects may occur in large complex Ti-6Al-4V castings, sometimes these are unnoticed until after machining and a high cost has been sunk into the part. These defect and other potential manufacturing mishaps render a need for additive repair methods. The state of the art method TIG welding can repair the parts but may leave unacceptable residual stresses, where the state of the art solution of Post Weld Heat Treatment might create a surface defect known as alpha case. Therefore there is a need for a repair weld method that results in lesser or negligible residual stresses. This thesis, carried out at GKN Aerospace – Engine Products Sweden, Trollhättan analyses the potential relationships between the laser welding parameters Power, Spot size, and Weld speed and the resulting residual stresses in a low energy parameter area on the material Ti-6Al-4V. A parameter box of 17 parameter sets was established, laid down and analyzed under micrograph, of this box the outer 8 parameter sets were simulated via the Finite Element Analysis welding simulation software MARC and a relationship between the input parameters and their resulting residual stresses was analyzed. A statistically significant trend was found supporting the claim that a decrease in transversal stresses in the center of a 20mm weld line is caused by an increase in Weld speed. This has implications for welds using run-on & run-of plates. It was also noted that an increase in Power or Spot size, or a decrease in Weld speed increases the area under residual stress; both as individual parameters and in synergy. This has implications for Post Weld Heat Treatment in determining the size and shape of the area in need of treatment.

This thesis focuses on the topic of PBF-LB applied to titanium alloys. Of allalloys, an α + β is chosen, named Ti-6Al-4V. The selection of this particular alloy is driven by its current widespread use in many industrial applications where high strength coupled with low density are both desirable properties. For the last 50 years, parts made with this alloy have been cast or forged and then machined to achieve the final geometry. There is now an opportunity totransform this process chain by additive manufacturing, hence reducing material waste and achieving near net shape from powder feedstock. The process is summarised as follows: a laser selectively melts areas on a build plate where powder is pre-placed. Then a successive powder layer is spread and the process is repeated until completion. Upon removal of the part from the build plate, loose powder in the chamber is collected and recycled whenever possible. The design freedom provided by powder bed fusion methods enables production of intricate geometries and added functionality, despite the need for post-build consolidation and/or microstructural adjustments. Today’s fine and narrow powder cuts (e.g., 15-50μm) are designed to be coupled with low layer thicknesses (i.e., 30μm) to achieve smooth surfaces and high resolutions of small features e.g., internal cooling channels. However, costs associated with production of fine and narrow powder cuts are substantial as refinement of batches requires multiple sieving steps. In addition, resulting building times are considerably long (i.e., days), therefore a beneficial alternative could be that of exploring higher layer thicknesses together with wider and coarser powder cuts. The main idea of this work is to investigate the effects of employing a powder with a wider size distribution 15-90μm. The aim is to reduce the sievingrequired and consequently decrease the costs of developing and building parts made by PBF-LB. An extensive microstructural investigation is conducted on single tracks and cubes built with 27 different process parameter combinations, which also attempts to establish correlations between characteristics of tracks and responses measured in cubes. As a second step, the amount of residual porosity of asbuilt cubes is chosen as the discriminant for further mechanical testing of sub and super-β transus high-pressure heat treated material. / Den här avhandlingen fokuserar på additiv tillverkning av titanlegeringar med laser pulverbädd metoden. Den legering som främst är i fokus är Ti-6Al-4Vsom är en α+β legering. Anledningen till valet av denna titanlegering är att det är den vanligast förekommande titanlegeringen och att den används i ett antal olika industriella tillämpningar där hög styrka i kombination med låg vikt är önskvärda egenskaper. Under de senaste 50 åren har komponenter utav denna legering tillverkats med gjutning eller smide, följt av bearbetning till slutlig geometri. Med hjälp av additiv tillverkning finns nu en möjlighet att förändra tillverkningskedjan i vilket minskat materialspill och en mer nära-slutgeometri kan erhållas direkt genom användning av metallpulver som utgångsmaterial. Processen kan summeras enligt följande: en laser smälter ett förbestämt område på en byggplatta som täckts mer pulver. Därefter adderas ytterligare ett lager med metallpulver ovanpå, på vilket samma process sker igen, och igen osv, tills hela detaljen är färdigtillverkad. När detaljen ska tas loss ifrån byggplattan samlas det kvarvarande icke-smälta pulvret upp och återanvänds i så stor utsträckning som möjligt. Frihetsgraderna vid design i processen möjliggör tillverkning av komplexa geometrier och adderade funktionaliteter, även fast efterbehandling och/eller justeringar av mikrostrukturen kan behövas. Dagens smala pulverstorleksfördelning (tex 15-50μm) är avsedd att ge tunna lagertjocklekar (tex 30μm) för att åstadkomma en fin yta och hög upplösning av små geometrier, såsom exempelvis interna kylkanaler. Men kostnaderna som det innebär att framställa och sortera ut fina och smala kornstorleksfördelningarär avsevärd eftersom det innebär flera steg med silning. Vidare leder de tunnalagertjocklekarna till långa byggtider (typiskt dagar). Ett alternativ, som därför vore fördelaktigt, är att undersöka möjligheten med att bygga tjockare lager med en bredare och större pulverstorleksfördelning. Huvudfokuset i detta arbete fokuserar på att undersöka effekterna av att använda en bredare pulverpartikelstorleksfördelning 15-90μm, med syfte at minska silningsbehovet och därmed reducera kostnaden för att utveckla och tillverka detaljer med laser pulverbädd additiv tillverkning. En omfattande mikrostrukturundersökning har gjorts på enkelsträngar och kuber byggda med 27 olika processparameter-kombinationer, vilket samtidigt försöker identifiera korrelationer mellan enkelsträngarnas karaktäristik med resultaten uppmätta hos kuberna. I ett nästa steg har material, som tillverkats med processparametrar som renderade i minst/mest porer hos kuberna, mekaniskt provats efter att det högtrycksvärmebehandlats över- respektive under β-transus. / <p>Paper A is not included due to the copyright.</p><p>Paper B and C are to be submitted.</p>

This thesis discusses on results of experiments carried outwith austenitic, duplex and super duplex stainless steels andTi-6Al-4V. In these it has been seen that the heat cycle duringwelding or weld simulation has a great effect on the propertiesof thematerials. The properties are related to the resultedgrain size after processing, as well as, to the chemicalcomposition of the material. The thesis discusses on the grainsize resulted after hot processing and how the properties areinfluenced by it. Also, it has been seen how large grain sizeinfluences the fracture toughness of the duplex stainlesssteel. The CTOD values become so low that an alternativeproduction method should be used instead of casting. The thesisdescribes the behavior of grain size in the hot processing ofTi-6Al-4V and impact toughness resulted by various weldingprocesses. The thesis shows correlation between grain growthbehavior of metallurgically different materials, i.e. duplexstainless steel, Ti-6Al-4V and quenced and tempered low alloysteel. Keywords:Stainless steel, Duplex Stainless Steel,Titanium Grade 5, Ti-6Al-4V microstructure, mechanicalproperties, welding, welding simulation / <p>QCR 20161026</p>

The use of composites materials in aerospace applications can provide significant weight reduction. However, in airframe designs composites are frequently required to be joined to metal components, which is a challenging issue owing to the very different thermo-physical properties of the two classes of materials. In many cases adhesive bonding has insufficient durability and the requirement for large lapped areas and mechanical fasteners adds considerable weight, reducing the mass savings associated with the introduction of composite structures. A promising approach for improving joint performance is to surface engineer locking features on to the metal part which then integrate with the composite laminate to increase shear load transfer, both via better adhesion and also by mechanical "fit" throughout the thickness of the composite. Such hybrid joints are known as "hyper-joints". For this work, three main techniques are currently being investigated for generating the required surface features. These are; (i) sculpting surfaces using power beam local surface melting techniques (Surfi-Sculpt), (ii) building surface protrusions by additive layer manufacturing (AM) and (iii) arc percussive welding process. The present work aims to provide understanding of the critical metallurgical interactions during the growth/production of the surface engineered features and how some build parameters might affect the eventual joint integrity, durability and performance. To enable the use of the arc-percussive welding process as a viable manufacturing route for these hyper-joints, optimisation of its process parameters were studied in the course of this work. Further work on the effect of process parameters such as voltage, travel velocity, gap setting and time delay on the quality of the weld were also investigated in this study. Of the above mentioned process parameters, the voltage across the capacitor and travel velocity of the actuator have been found to have far greater effect on the weld quality. More importantly, the travel velocity of the actuator was also found to help determine the shape, size and distribution of the melt pool. Based on the systematic study and analysis of all of the process parameters involved, an optimised process window has now been proposed. Microstructural characterisations of hyper-joint samples made via the three manufacturing routes were performed in the course of the work. The observed microstructures were related to the process history and the process parameters. Most importantly it was for found that in common with most welding and AM processes, columnar prior β grains developed in all the samples studied by epitaxial-regrowth either along the build or weld directions. It was also found that the fusion zone of the similar titanium alloy (Ti-6Al-4V to Ti-6Al-4V) arc-percussive welded samples were characterised by extremely fine acicular alpha' martensitic platelets formed as a result of the high cooling rate associated with the process. On the other hand, the fusion zone of the dissimilar titanium alloy (Ti-6Al-4V to β21S) arc-percussive welded samples was characterised by an extremely fine cellular structure prompted by constitutional supercooling. Finally the microstructures observed in the Surfi-Sculpt samples were found to be dependant on the swipe pattern and duration. In order to assess the performance of hyper-joints made via two of the candidate manufacturing routes, tensile testing of standard and modified tensile samples was performed. The result indicated that on average the strength of these hyper-joint pins were within nominal values expected of the Ti-6Al-4V and β21S alloys. It was also found that with optimum weld conditions, failure occurred only along the gauge length for the arc-percussive welded samples whilst the strength of the AM samples degrade as the gauge diameter decreases as the effect of surface defects became prominent. Results are also reported on a novel method for testing the shear strength and properties of individual hyper-pins manufactured via this process. Interpretation of the shear test was assisted by finite element modelling. The shear test results indicated that the arc-percussive welded samples outperformed the SLM-AM samples thereby giving credence to the process as a viable means for making the pins.

The thermal oxidation of titanium alloys is a technique studied to improve the tribological performance, despite its evidences of loss of performance in fatigue. Thermal oxidation treatment of Ti-6Al-4V alloy was performed in air at 600 °C for 2, 5 and 10 h. The fatigue behavior were evaluated and compared with those of the untreated one. The characterization was performed through optical and electronic microscopy, x-ray diffraction, microhardness and roughness measurements. The results of the XRD showed a growth of the rutile peaks when increasing the treatment time from 2 to 10 h. Microhardness and roughness were also elevated by increasing treatment time up to 10 h. There was an increase in mean thickness from 0.12 μm to 1.09 μm, increasing the treatment duration from 2 to 5 h. The results indicated a reduction in the fatigue limit of samples subjected to oxidation. The results indicated a reduction of 35% for oxidized samples for 2 h, 27% for oxidized samples for 5 h and 29% for samples treated for 10 h, in relation to the test with untreated specimens. The detrimental effect caused by the layer formed after oxidation for 2 h was as damaging as that caused by the layer formed after 10 h, even if presented with a higher thickness. The morphology found in the fractured samples indicated embrittlement on the surface of the oxidized samples, contributing to the nucleation of initial cracks in the layer that favored the propagation of fatigue, independent of the layer thickness. / A oxidação térmica de ligas de titânio é uma técnica estudada para melhorar o desempenho tribológico, apesar das evidências de perda de desempenho em fadiga. O tratamento de oxidação térmica da liga Ti-6Al-4V foi realizado ao ar a 600 °C durante 2, 5 e 10 h. O comportamento à fadiga foi analisado e comparado com a condição não tratada. Foi feita caracterização através de análise por microscopia ótica e eletrônica, difração de raios-x, microdureza e medição da rugosidade. Os resultados do DRX demonstraram crescimento dos picos de rutilo ao elevar o tempo de tratamento até 10 h. Observou-se, também, elevação da microdureza e da rugosidade ao elevar o tempo de tratamento até 10 h. Verificou-se um aumento na espessura média da camada formada de 0,12 μm para 1,09 μm ao elevar a duração do tratamento de 2 para 10 h. Os resultados apontaram redução de resistência a fadiga de 35 % para amostras oxidadas durante 2 h, 27 % para amostras oxidadas por 5 h e 29 % para as amostras tratadas durante 10 h, em relação ao ensaio com corpos de prova não submetidos ao tratamento. O efeito prejudicial causado pela camada formada após oxidação de 2 h foi tão danoso quanto o causado pela camada formada após 10 h, apesar da diferença entre as espessuras da camada. A morfologia encontrada nas amostras fraturadas indicou fragilização na superfície das amostras oxidadas, contribuindo para a nucleação de trincas iniciais na camada que favoreceram a propagação de fadiga, independendo da espessura de camada. / São Cristóvão, SE

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.

<p>This thesis discusses on results of experiments carried outwith austenitic, duplex and super duplex stainless steels andTi-6Al-4V. In these it has been seen that the heat cycle duringwelding or weld simulation has a great effect on the propertiesof thematerials. The properties are related to the resultedgrain size after processing, as well as, to the chemicalcomposition of the material. The thesis discusses on the grainsize resulted after hot processing and how the properties areinfluenced by it. Also, it has been seen how large grain sizeinfluences the fracture toughness of the duplex stainlesssteel. The CTOD values become so low that an alternativeproduction method should be used instead of casting. The thesisdescribes the behavior of grain size in the hot processing ofTi-6Al-4V and impact toughness resulted by various weldingprocesses. The thesis shows correlation between grain growthbehavior of metallurgically different materials, i.e. duplexstainless steel, Ti-6Al-4V and quenced and tempered low alloysteel.</p><p><b>Keywords:</b>Stainless steel, Duplex Stainless Steel,Titanium Grade 5, Ti-6Al-4V microstructure, mechanicalproperties, welding, welding simulation</p>

Το ερευνητικό πεδίο της παρούσας διδακτορικής διατριβής ήταν η οστεοενσωμάτωση οστικών εμφυτευμάτων. Οι βασικοί στόχοι της έρευνας αφορούσαν την κατανόηση του ρόλου τριών βασικών ρυθμιστικών παραγόντων της διαδικασίας οστεοενσωμάτωσης, δηλαδή της μηχανικής φόρτισης, της επιφανειακής τοπογραφίας και της επιφανειακής (βιο)χημείας, καθώς και τη μελέτη του βασικότερου λόγου αστοχίας των ορθοπεδικών εμφυτευμάτων, της παραγωγής μικροσωματιδίων προϊόντων φθοράς τους. / -