[en] Nowadays, the requirements demanded to many of the parts used in such industries as the aeronautic or the automotive industry are increasingly higher: more complex shapes, previously unimaginable, lower and lower radii, impossible geometries, etc., all of this combined with high values of mechanical strength, corrosion resistance, impact absorption, fatigue resistance, etc. In addition, the increasingly more severe environmental regulations regarding the minimization of the CO2 emissions as well as the increase in energetic efficiency claim for lightweight materials able to lighten the currently known structures without penalizing their mechanical properties, i.e., materials of high specific mechanical properties. High specific strength steels, some aluminium alloys and some magnesium alloys are within this category. All of these materials answer properly the requirements of high mechanical properties and low weight; however, all of them exhibit a low formability at room temperature, so they are not able to answer the current geometric requirements. To answer both requirements simultaneously (high mechanical properties and complex geometries), new advanced forming process to manufacture materials of high mechanical properties have been researched; that is the case of the hydroforming processes. Strategies of increasing forming temperature are used in order to increase the formability of the materials of high specific strength; in the case of the previously mentioned processes, this leads to the so-called Warm Hydroforming (WH). It is a process currently under research so the development and improvement possibilities are many: identification of optimal process parameters, development of working methodologies, optimization of process times, identification of process control strategies, etc. Thereby, this thesis answers some of those development and improvement possibilities of Warm Tube Hydroforming processes (WTHF); the research is focused on the application of those kinds of processes to 6xxx series aluminium alloys (6082 alloy to be precise) in a tubular format; these alloys have high specific mechanical properties and they are currently used in the automotive industry. The correct characterization of the material to be formed is a key factor for the development and improvement of WTHF processes. Therefore, in order to understand better the material behaviour, in the first stage of this research the manufacturing process of the raw material is analysed and its dimensional tolerances are verified; besides, the material is also characterized by means of conventional testing techniques: hardness, uniaxial tensile test and microstructural analysis. In a parallel way, that alloy is characterized by means of a specific test for hydroforming processes: the so-called bulge test, hydraulic bulge test or biaxial free expansion test, where an incremental analytical model is applied to calculate the thickness of the tube. The differences observed between the results achieved by means of the uniaxial and the biaxial way show the need to develop specific hydroforming tests for material characterization. On the other hand, that characterization is carried out for different material conditions of thermal treatment, thickness, strain rate and temperature. This helps to the selection of the optimal forming conditions for this alloy: annealed condition, 2 mm thickness and 250°C temperature. In a second stage, in order to predict the maximum formability of the material in the process (without failure), different failure criteria representative of the material and the process are applied to the finite element analysis (FEA) of the thermo-mechanical problem; this analysis allows to identify the values of the input variables of the process as well as the optimal control strategies. Besides, by means of the application of different analysis procedures to simple problems (bulge test) and complex problems (WTHF process) a simulation methodology is developed for these kinds of processes. In a parallel way, thanks to the acquired knowledge of material and simulation, different numerical models and analysis strategies are developed for the FEA of WTHF processes. Once the inputs of the process are obtained through FEA, they are applied to the hydroforming of a generic part of aluminium alloy. By means of an iterative trial-and-error process, a working methodology is developed for these kinds of processes as well as the optimal values of the process variables and the control strategies are identified. Finally, once the desired part is achieved, through the comparison of the FEA prediction and the experimental results, the validity and accuracy of the numerical models developed for WTHF processes is discussed.

[eu] Gaur egun, aeronautika eta automobilgintza sektoreetan erabiltzen diren pieza askori eskatzen zaizkien eskakizunak geroz eta zorrotzagoak dira: geroz eta forma konplexuagoak, lehen imajinaezinak, geroz eta erradio nabarmenagoak, ezinezko geometriak… Horri guztiari beste hauek ere gehitu behar zaizkio: erresistentzia mekaniko altua, korrosioarekiko erresistentzia, inpaktu xurgatzea, nekearekiko erresistentzia… Halaber, CO2 isurketak gutxitzera eta energi eraginkortasuna hobetzera bideratutako ingurumen arauek pisu gutxiko materialak eskatzen dituzte, gaur egun ezagutzen ditugun egiturak arintzeko gai izango direnak, baina propietate mekanikoak galdu gabe: erresistentzia espezifiko handiko materialak. Hain zuzen ere, azken multzo horren barruan erresistentzia handiko altzairuak, hainbat aluminio aleazio eta hainbat magnesio aleazio daude. Material horiek guztiek egoki erantzuten dizkiete propietate mekaniko handiak eta pisu gutxi izateko eskakizunei; aitzitik, giro tenperaturan konformagarritasun txikia dute eta ondorioz, ez dira gai gaur egungo eskakizun geometrikoei erantzuteko. Bi eskakizun hauei batera erantzun ahal izateko (propietate mekaniko handiak eta geometria konplexuak), propietate mekaniko handiko materialen transformaziorako konformazio prozesu aurreratuak ikertu izan dira (eta ikertzen ari dira); hidrokonformazio prozesuaren kasua adibidez. Erresistentzia espezifiko handiko materialen eta konformagarritasuna handitzeko erabiltzen den estrategia, konformazio tenperaturaren igotzea dira. Aurretik aipatutako prozesuen kasuan honek, tenperatura moderatuetan hidrokonformazio izena hartzen du. Gaur egun, ikerketa fasean dagoen prozesu bat denez, garapen eta hobekuntza aukerak asko dira, hala nola: prozesuetako parametro hoberenak identifikatzea, lan metodologien garapena, teknologia honen bidez konformatu daitezkeen material hoberenak identifikatzea, prozesu zikloen optimizazioa, prozesua kontrolatzeko estrategiak identifikatzea,… Zentzu honetan, tesi honek Warm Tube Hydroforming (WTHF) prozesuen garapen eta hobekuntza aukerei erantzuten die; azterketa tutu-formako 6xxx serieko (zehazki 6082) aluminio aleazioei aplikatutako prozesu motetan zentratzen da, propietate mekaniko espezifiko handiko aleazio motak direlako, eta baita, automobil-industrian erabiltzen direlako ere. WTHF prozesuak garatu eta hobetzeko gakoetako bat konformatu beharreko materialaren karakterizazio egokia egitea da. Hortaz, bere portaera hobeto ulertzeko, hasiera batean materiala lortzeko prozesua ikertzen da eta bere perdoi dimentsionalak egiaztatzen dira; halaber, ohiko entseguen bidez karakterizatzen dira: gogortasuna, trakzio uniaxiala eta mikroegituren azterketa. Modu paraleloan, aleazio hau hidrokonformazio prozesuen entsegu espezifiko bidez karakterizatzen da: hedapen libreko entsegu biaxiala edo bulge test, zeinetan metodo inkremental bat aplikatu behar den hodiaren lodiera kalkulatzeko. Modu uniaxialan eta biaxialan lortutako emaitzen artean antzematen diren ezberdintasunek, prozesu hauetarako materialaren karakterizazio entseguak garatzeko beharra erakusten dute. Beste alde batetik, karakterizazio hau egiten da materialaren tratamendu termiko, gogortasunerako, deformazio abiadura eta tenperatura ezberdinetan egiten da. Honek aleazio hau konformatzeko baldintza hoberenak aukeratzen laguntzen du: suberaketa egoera, 2 mm-ko lodiera eta 250°C-ko tenperatura. Hurrengo etapan, materialak prozesuan gehienez ere izango duen deformazio ahalmena aurreikusteko xedearekin (akatsik gabe), materialaren eta prozesuaren erakusleak izango diren akatsari buruzko hainbat irizpide inplementatzen dira arazo termomekanikoa den analisi horretan, elementu finituen bidez; analisi horrek aukera ematen du prozesuaren sarrerako aldagaien balioak eta kontrolerako estrategia ezin hobeak identifikatzeko. Halaber, azterketak egiteko hainbat prozedura aplikatuz, bai problema errazei (bulge test) bai problema konplexuei (WTHF prozesua), simulazio metodologia bat garatzen da prozesu mota hauetarako. Paraleloki, materialaren eta simulazioetatik jasotzako ezagutzarekin, WTHF prozesuen azterketarako eredu numerikoak eta simulazio estrategiak garatzen dira. Behin prozesuaren sarrerak simulazio bidez jaso ondoren, aluminio aleaziozko pieza generiko bati hidrokonformazioa aplikatzen zaio. Akats-proba prozesua errepikatuz, prozesu mota honetarako lan metodologia bat garatzen da, eta prozesu hau gauzatzeko aldagaien balio eta estrategia ezin hobeak identifikatzen dira. Azkenik, nahi dugun pieza lortutakoan, simulazioaren bidez lortu ditugun aurreikuspenak eta emaitza esperimentalak konparatuz, WTHF prozesuetarako garatutako eredu numerikoen baliagarritasuna eta zehaztasuna aztertzen dira.