[en] Due to the evolution of the last decades actual society is not focused exclusively in production anymore and people, safety and the environment preservation have become necessary issues in any aspect of the everyday living. Thus, worldwide governments are taking actions and have adopted new rules regarding those aspects, being remarkable the efforts to minimise the gas emissions responsible of global warming. As a consequence, new rules have direct repercussion in automotive and aerospace industries where weigh reduction has become one of the main goals to reduce vehicle fuel consumption and consequently minimise CO2 emissions. Thus, Magnesium alloys present high potential for automotive applications due to their lower density in comparison with other widely used structural materials. This property allows not only reducing vehicle weight but also improving dent resistance and shell resistance behaviour by increasing thickness in structural sheet applications. However, even magnesium alloys are already used to manufacture autoparts by die casting processes, some limitations, reported by many researchers, emerge when forming magnesium sheet parts. It is well known that due to their hexagonal close-packed (HCP) microstructure, magnesium alloys present low formability at room temperature. Although this property can be improved by increasing the forming temperature, this is not always achievable in conventional manufacturing processes. Consequently, the development of innovative forming processes, such as warm tube hydroforming (WTHF), is needed to extend the use of magnesium alloys in automotive and aerospace industry. WTHF processes allow combining the benefit of increasing forming temperature to overcome formability limitations in magnesium alloys with the widely known advantages that conventional Tube Hydroforming (THF) processes offer, such as the possibility to obtain complex hollow shapes, minimise the need of welding or simplify the assembly of components. In the present research, AZ31 and ZM21 magnesium alloys (the two mainly used alloys for sheet metal forming applications) have been uniaxially tested at a temperature range up to 250°C at quasi-static strain rates and microstructure evolution of the tested specimens has been analysed. This way, the mechanical behaviour of each alloy has been characterised and the influence of the different alloying elements in the forming mechanism has been studied. As a result of the study, the optimum forming conditions for each alloy have been determined and ZM21 alloy has been selected to develop the current research. Once the material and the optimum forming temperature have been determined numerical models to predict the WTHF process of a high elongation level generic part have been developed. These numeric models allow numerically characterising WTHF processes and supporting all the process design and the tooling design. It has been observed that due to material softening, very common in magnesium alloys, it is necessary to limit the maximum strain rate when being formed so that premature bursting does not occur. For this purpose, flow rate control strategy has been selected. This has been a challenge from the numeric point of view as the behaviour of the fluid has to be modelled minimising the computational cost. Then, based also in the numeric models, a novel WTHF facility has been developed. This facility is provided with a warm temperature hydraulic circuit to heat up the dies and the tube, under a controlled temperature pattern, up to a maximum nominal forming temperature of 260°C. A second high pressure circuit provides pressurized tempered warm fluid up to a maximum internal pressure of 700bar. A third parallel water circuit refrigerates feeding areas and axial punches to provoke controlled gradients in the edge of the tube. These temperature gradients provide the material a higher stiffness and help tube feeding to the expansion area during the hydroforming process. The facility can perform flow rate control strategies, more appropriate than conventional pressure control strategies to indirectly control maximum strain rate in the part. This new facility concept integrates technical solutions for two of the main limitations of this Warm Hydroforming (WHF) facilities: the high cost of the use of hydraulic presses to provide the necessary closing force at a static working position and the high cost of high temperature pressure intensifiers. Both solutions are detailed in the two patents P200800326 "Dispositivo de cierre para compensar deformaciones" and P200800327 "Dispositivo hidráulico y procedimiento para un aparato de hidroconformado". The first solution replaces conventional hydraulic presses for a mechanic device which ensures the correct closing of the dies for a constant closing position. The second solution consists in a new high temperatures fluid pressurizing concept based on conventional hydraulic equipments. Once the facility has been set up, the warm hydroforming process of the generic part has been experimentally characterized. Furthermore, geometry, strain pattern, thickness distribution, mechanical properties and the microstructure of the obtained parts have been verified. To end up the current research, numeric results and experimental results have been compared in order to validate the developed numeric models.

[eu] Gaur egungo gizarteak atzean utzi du ekoizpen soilean oinarritu zen garai hura gizartearekiko eta ingurumenarekiko kontzientzia garatuz. Eboluzio hori mundu-mailan gobernuek hartu dituzten neurri eta arauetan islatzen da eta bereziki negutegi efektuan eragin zuzena duten gasen emisio murrizketarako neurrietan. Guzti honek eragin zuzena du teknologikoki aurrerakoi diren automozio eta hegazkingintza sektoreetan, non pisu murrizketa ezinbesteko helburu bilakatu den erregai kontsumoa murrizteko eta ondorioz CO2 emisioak. Magnesio aleazioak oso aproposak dira sektore mota hauetan erabiltzeko oso dentsitate baxua baitute osagai estrukturaletan erabili ohi diren materialekin alderatuz. Propietate honek material honekin ekoiztutako pieza eta osagaien pisua murrizten laguntzeaz gain indentaziorako erresistentzia edo zurruntasun laminarra hobetzen laguntzen du lodiera handiagoak erabiltzea baimenduz. Halere, magnesio aleazioak injekzioko piezetan erabiltzen dira batik bat, izan ere ingurune tenperaturan duten konformagarritasun baxuak xafla konformaketa prozesuetarako erabilgarritasuna mugatzen baitu. Mundu-mailan egindako ikerketek erakutsi dute ingurune baldintza kontrolatuan konformaketa tenperatura igoaz magnesio aleazioen konformagarritasuna asko hobetu daitekeela, naiz eta baldintza horiek prozesu konbentzionaletan erreproduzitzea ez den hain samurra. Honen ondorioz, ezinbestekoa da konformaketa prozesu berritzaileak garatzea muga hauek gainditu ahal izateko. Tutu hidrokonformaketa beroan (aurrerantzean WTHF) teknologia berritzaile hauetariko bat da non magnesio aleazioak konformatzeko baldintza aproposak tutu hidrokonformaketa prozesu konbentzionalek (THF) eskaintzen duten abantailekin konbinatzen diren. Prozesu mota hauen abantaila nagusienetarikoak dira lodiera estuko pieza itxi konplexuak konformatzeko gaitasuna, lotura soldatuen beharra murriztea eta osagaien muntaia erraztea besteak beste. Tesi honetan garatu den ikerkuntzan xafla konformaketa prozesuetan gehien erabiltzen diren bi magnesio aleazioak karakterizatu dira: AZ31 aleazioa eta ZM21 aleazioa. Propietate mekanikoak eta deformatzerakoan mikroegiturak jasaten duen eboluzioan elementu aleanteek duten eragina aztertu ondoren, aleazio bakoitzarentzako konformazio baldintza egokienak finkatu dira, konformagarritasuna eta errekristalizazio dinamikoaren (DRX) ondorioz gertatu daitekeen propietateen galerarekiko konpromisoa bilatuaz. Ondoren, ZM21 aleazioa aukeratu da magnesio aleazioen WTHF prozesuak bai numerikoki bai esperimentalki garatzeko eskaintzen dituen abantailek eta aleazio berri honen ezagutza faltak bultzatuta. Behin materiala aukeratuta eta konformaketarako tenperatura baldintza egokiak zehaztu ondoren konformaketa maila handiko pieza generiko baten WTHF prozesuaren modelo numerikoak garatu dira. Modelo hauek prozesu mota hauen karakterizazio numerikoa baimentzeaz gain ezinbesteko erreminta dira prozesuaren zein utilajeen diseinurako. Material hauetan ohizkoa den softening efektuaren ondorioz, materialaren deformazio abiadura prozesuan zehar mugatzea beharrezkoa dela ikusi da garai aurretik pieza apurtzea ekiditeko. Horretarako, emari kontrol estrategiak erabiltzea hautatu da ohizkoa den presio kontrol estrategia ordez, eta horrek erronka berri bat suposatu du fluidoaren eragina modelizatzeko berez altua izan ohi den koste konputazionala handitu gabe. Ondoren, herraminta numeriko hauetaz baliatuz WTHF-rako instalazio berri bat diseinatu eta garatu da. Instalazio hau gauzatzeko, instalazio mota hauen bi arazo handienetakoei, itxiera indarra lortzeko erabiltzen diren prentsa hidraulikoen koste altuari eta fluido presurizatu beroa lortzeko erabiltzen diren berorako presio intensifikagailuek suposatzen duten koste altuari, aurre egiten dien bi soluzio tekniko garatu dira. Bi soluzio teknikoak patentatu egin dira "Dispositivo de cierre para compensar deformaciones" eta P200800327 "Dispositivo hidráulico y procedimiento para un aparato de hidroconformado" izenekin. Behin instalazioa muntatu eta ajustatu ondore piezaren fabrikazio prozesua prestatu eta esperimentalki karakterizatu da. Ondoren lortutako piezen geometria, deformazio egoera, lodiera distribuzioa, propietate mekanikoak eta mikroegiturak jasandako eraldatzea aztertu dira. Amaitzeko, emaitza esperimentalak emaitza numerikoekin alderatu dira garatutako modeloak baieztatzeko.