[en] Hot forging processes are highly influenced by the contact conditions between the work piece and the dies. An inaccurate definition of the contact conditions may lead to high deviation predictions of the final component geometry, the quantity of material necessary to fill in the cavity, the wear of the tools and the force necessary to manufacture the component. Furthermore, when dealing with high added value materials such as titanium alloys, the wrong prediction of the aforementioned variables also could lead to inaccurate microstructure predictions. The main goal of the present dissertation was to understand the contact condition between Ti-6Al-4V work piece and heated tool steel with the purpose of calculating the coefficient of friction (COF) and the heat transfer coefficient (HTC) at the interface of the work piece and the tools. Both coefficients of Ti-6Al-4V titanium alloy under hot forging situation were determined by the combined approach of experimental tests and finite element (FE) simulations using FORGE3® finite element software. At the same time, in order to improve the flow of the material within the tools and reduce the alpha-case layer when forging titanium alloys at high temperature, three types of surfaces were analyzed in the present study: billets without coating, 40-45 μm and 80-90 μm of CONDAERO 228 glass coating. The objectives of this dissertation were four-fold. First, a Finite element (FE) parametric study has been performed in order to obtain the sensitivity of ring and T-Shape test when addressing the issue of varying input parameters. The objective of this study was to determine and interpret the factors that affect friction behaviour. Special care must be put in some specific inputs, to obtain accurate calibration curves and avoid errors when finding the real COF based on experimental observations. It is noticed that, HTC is the most influencing factor among all and has high impact on calibration curves of the friction test, thereby affects the measurement of interfacial friction factor. Then, tribological contact conditions in both the tests were compared. It concluded that, T-shape test is best suitable to evaluate the friction condition as this test induces large contact pressure and large surface expansion similar to what is occurred during a real forming operation. Second, columnar upsetting test was conducted for the determination of the HTC at the workpiece-die interface at two different contact pressures. An efficient numerical 3D model has been developed where, simulations have been carried out defining different values of the HTC. That gives as a result of different temperature evolutions which were used to estimate the HTC value which best fits the temperature read by the thermocouple in the real experiments. The validation of final coefficients determined by the inverse algorithm was made by comparison between the upsetting process and the FE analysis results. Variations in HTC have been obtained under different pressure with different surface condition during upsetting. Therefore, HTCs in function of pressure should be employed to generate the calibration curves for determining the interfacial COFs in friction tests. Third, a comprehensive numerical and experimental study on the ring and T-Shape friction test were carried out to calculate the COF where, geometrical shape of specimen was chosen to simplify this methodology. The aim of this study is to analyze the same tribo-system but having different contact pressures and surface enlargement factors, which could affect the coating behavior and to estimate the friction factor taking into account the HTC as pressure dependent. Calibration curves for the tests were obtained numerically by using finite element simulations. Then, the COFs are calculated by the comparison of the experimental data and numerical simulation results using inverse analysis. The results obtained that the glass coating clearly improves the forgeability of titanium alloys reducing the COF value. Finally, an experimentation was performed to observe the effects of alpha-case formation, cracks on the deformed T-Shape specimen and to investigate effect of coatings to reduce alpha-case formation. Using a microscope, photographs were taken and visible alpha-case region was measured and also identified the cracks on each sample. Microhardness testing was then performed to determine exact depth range of the visible alpha-case region profile of the sample. It was found that alpha-case thickness and crack depth is more in uncoated specimen than the coated specimen.

[eu] Tresneria eta landu beharreko piezaren arteko kontaktu baldintzek eragin nabarmena dute beroan egindako forjaketa prozesuetan. Kontaktu baldintzak era desegokian zehazten badira, aldez aurretik zenbakizko simulazioetan iragarritako azken piezaren geometrian, tresneria betetzeko beharrezkoa den material kantitatean, tresneriaren higaduran eta pieza egiteko beharrezkoa den indarrean desbiderapen handiak eman daitezke. Gainera, titanioa bezalako balio erantsi handiko aleazioetan, aurrez aipaturiko aldagaien iragarpen ez zuzenak, materialaren mikroestrukturaren aurreikuspen okerra ekar dezake. Tesiaren helburu nagusia, landu beharreko Ti-6Al-4V piezaren eta berotutako altzairuzko tresneriaren arteko kontaktu baldintza ulertzea da, beraien arteko marruskadura koefizientea (COF) eta bero-transferentzia koefizientea (HTC) zehazteko helburuarekin. Beroko forjaketa egoeran Ti-6Al-4V titanio-aleazioaren aipaturiko bi koefizienteak zehazteko, bi teknika ezberdinen emaitzak hartu dira kontutan: entsegu esperimentalen lorturiko emaitzak, alde batetik, eta FORGE3® simulazio softwareko emaitzak, bestetik. Aldi berean, pieza eta tresneriaren arteko fluxua hobetzeko eta titanio-aleazioen tenperatura altuko forjaketan alpha case geruza murrizteko, hiru gainazal mota aztertu dira: estaldura gabeko aleazio totxoak, 40-45 μm eta 80-90 μm CONDAERO 228 estalduradun aleazio totxoak Lan honen helburua, lau zatitan banatzen da. Lehenik eta behin, azterketa parametriko bat burutu zen elementu finituekin (FE) hainbat parametroen sentsibilitatea Ring test eta T-Shape-n lortzeko. Ikerketa honen helburua marruskaduraren portaeran eragina duten faktoreak zehaztea eta interpretatzea. Arreta berezia jarri behar zaio sarrerako datu zehatz batzuei, kalibrazio kurba egokiak lortzeko eta akatsak saihesteko benetako COF bilatzen denean entsegu. Faktore guztien artean eragin handiena eta marruskadura entseguko kalibrazio kurbetan inpaktu handien izan duen faktorea HTC koefizientea izan da, aurpegien arteko marruskadura faktorean eragina duelako. Azkenik, entseguen arteko kontaktu tribologikoen baldintzak alderatu ziren. Marruskadura egoera ebaluatzeko T-Shape entsegua egokiena dela ondorioztatu zen proba honetan kontaktu presioak eta azalera handiko hedapenak, benetako prozesuko egoera baldintzak ondo irudikatzen direlako. Bigarren atalean, bi presio maila desberdinetan konpresio entseguak egin ziren landu beharreko piezaren aurpegien arteko HTC koefizientea zehazteko. Zenbakizko 3D modelo eraginkor bat garatu da, eta horrekin HTC balio desberdinak erabiliz, entseguaren simulaziak egin dira. Simulazio horiekin tenperatura eboluzio ezberdinak lortu dira, eta eboluzio hauek, entsegu esperimentaletan termopareen bidez lortutako tenperaturei gehien gerturatzen zaien HTC koefizientea zehazteko balio izan dute. Alderantzizko simulazioen bitartez zehaztutako koefizienteak, konpresio entseguaren emaitzak eta FEM modeloaren emaitzen arteko konparaketaren bitartez balioztatu dira. Konpresio ematen den bitartean HTC koefizientearen aldakuntzak lortu dira, kontaktu presio eta gainazal egoera desberdinetan. Horregatik, marruskadura entseguetan aurpegien arteko COF zehazteko sortu beharreko kalibrazio kurbak lortzeko, presioaren menpekoak diren HTC koefizienteak erabili beharko lirateke. Hirugarren, Ring test eta T-Shape entseguen zenbakizko eta esperimental ikasketak egin ziren COF kalkulatzeko, azkeneko forma geometrikoa aztertuz. Ikerketa honen helburua Tribo-system berbera aztertzea da, baina kontaktu presio eta hedapen faktore azalera desberdinetan, estalduraren jarreran eragina izan dezakelako eta COF balioa kalkulatzea presioaren menpeko HTC-a kontutan hartuz. Entsegu esperimentalentzako kalibrazio kurbak lortu ziren elementu finituen simulazioak erabiliz. Geroago, datu esperimentalak zenbakizko simulazioko emaitzekin alderatuz eta alderantzizko simulazio teknika erabiliz COF balioak kalkulatzen dira. Emaitzek, estaldurak titaniozko aleazioak forjatzeko ahalmena nabarmen hobetzen dituela erakusten dute, COF balioa murrizten delako. Azken atalean, entsegu esperimentalak egin dira alpha case geruzaren osaketaren ondorioz sortzen diren efektuak ikusteko, deformaturiko T-Shape probetaren pitzadurak aztertzeko eta estalduren ondorioz eragindako alpha case geruzaren murrizketa efektua ikusteko. Mikroskopio baten laguntzaz, alpha case geruzaren lodiera neurtzeaz gain, probeta bakoitzean agertutako pitzaduren argazkiak atera dira. Probetetan aurki daitekeen alpha case sakonera zehazteko mikro-gogortasun entseguak egin ziren. Estaldura gabeko aleazio totxoen alpha case geruza eta pitzaduren sakonera, estalduradun totxoetan baino handiagoa dela ondorioztatu da.