Constitutive laws commonly used to model friction stir welding have been evaluated, both qualitatively and quantitatively, and a new application of a constitutive law which can be extended to materials commonly used in FSW is presented. Existing constitutive laws have been classified as path-dependent or path-independent. Path-independent laws have been further classified according to the physical phenomena they capture: strain hardening, strain rate hardening, and/or thermal softening. Path-dependent laws can track gradients in temperature and strain rate characteristic to friction stir welding; however, path-independent laws cannot. None of the path-independent constitutive laws evaluated has been validated over the full range of strain, strain rate, and temperature in friction stir welding. Holding all parameters other than constitutive law constant in a friction stir weld model resulted in temperature differences of up to 21%. Varying locations for maximum temperature difference indicate that the constitutive laws resulted in different temperature profiles. The Sheppard and Wright law is capable of capturing saturation but incapable of capturing strain hardening with errors as large as 57% near yield. The Johnson-Cook law is capable of capturing strain hardening; however, its inability to capture saturation causes over-predictions of stress at large strains with errors as large as 37% near saturation. The Kocks and Mecking model is capable of capturing strain hardening and saturation with errors less than 5% over the entire range of plastic strain. The Sheppard and Wright and Johnson-Cook laws are incapable of capturing transients characteristic of material behavior under interrupted temperature or strain rate. The use of a state variable in the Kocks and Mecking law allows it to predict such transients. Constants for the Kocks and Mecking model for AA 5083, AA 3004, and Inconel 600 were determined from Atlas of Formability data. Constants for AA 5083 and AA 3004 were determined with the traditional Kocks and Mecking model; however, constants for Inconel 600 could not be determined without modification to the model. The temperature and strain rate combinations for Inconel 600 fell into two hardening domains: low temperatures and high strain rates exhibited twinning while high temperatures and low strain rates exhibited slip. An additional master curve was added to the Kocks and Mecking model to account for two hardening mechanisms. The errors for the Kocks and Mecking model predictions are generally within 10% for all materials analyzed.

Modeling of friction stir welding (FSW) is challenging, as there are large gradients in both strain rate and temperature that must be accounted for in the constitutive law of the material being joined. Constitutive laws are most often calibrated using flow stresses from hot compression or hot torsion testing, where strain rates are much lower than those of the FSW process. As such, the current work employed a recently developed method to measure flow stresses in AA 2219-T67 at the high strain rates typical of FSW. These data were used in the development of a finite element simulation of FSW to study the effect of the new flow stress data on temperature, torque, and load predictions, compared to standard material models calibrated with hot compression or hot torsion data. It was found that load predictions were significantly better with the new material law, reducing the error with respect to experimental measurements by approximately 79%. Because heat generation during FSW is primarily a function of friction between the rapidly spinning tool and the workpiece, the choice of friction law, and associated parameters, were also studied with respect to FE model predictions. It was found that the Norton (viscoplastic) friction law was the most appropriate for modeling FSW, because its predictions were more accurate for both the transient and steady-state phases of the FSW plunge experiment. The postulated reason for the superior performance of the Norton law was its ability to account for temperature and rate sensitivity of the workpiece material sheared by the tool, while the Tresca limited Coulomb law favored contact pressure, with essentially no temperature or rate dependence of local material properties. The combination of the new flow stress data and the optimized Norton friction law resulted in a 63% overall reduction in model error, compared to the use of a standard material law and boilerplate friction parameters. The overall error was calculated as an equally weighted comparison of temperatures, torques, and forces with experimentally measured values.

No description available.

As lajes mistas de aço e concreto consistem de um sistema estrutural formado por fôrmas de aço (chapas perfiladas) permanentes sobre a qual é lançado o concreto armado. Após a cura do concreto, esses elementos trabalham conjuntamente, garantindo o comportamento misto. Esse tipo de sistema é amplamente utilizado na construção civil brasileira, por propiciar diversas vantagens, como a não utilização de fôrmas de madeiras e escoras. Com a finalidade de avaliar o comportamento térmico, estrutural e termoestrutural acoplado desse tipo de sistema, foram desenvolvidos modelos numéricos em elementos finitos no pacote computacional TNO DIANA em duas etapas distintas. Na primeira etapa é feita uma análise térmica bidimensional, cujos campos térmicos na seção transversal da laje são calibrados por meio de resultados experimentais disponíveis na literatura, avaliando os principais parâmetros que influenciam na distribuição de temperatura na laje, como a emissividade resultante e a resistência térmica na interface aço/concreto. Nessa etapa também se desenvolve um pós-processador em Visual Basic 11.0 capaz de ler os campos térmicos gerados pelo DIANA e calcular o momento fletor plástico resistente em função do tempo de exposição ao fogo. Esses resultados são comparados com aqueles calculados de acordo o modelo de cálculo da ABNT NBR 14323:2013. A segunda etapa consiste no desenvolvimento de modelos numéricos tridimensionais para se avaliar o comportamento termoestrutural acoplado de lajes mistas simplesmente apoiadas. Nessa etapa, primeiramente se faz a validação e calibração dos modelos numéricos à temperatura ambiente, avaliando fatores como a simetria dos modelos e a resistência ao cisalhamento longitudinal na interface entre a fôrma de aço e o concreto. Posteriormente, se realiza a validação do modelo numérico em temperatura elevada por meio de resultados experimentais disponíveis na literatura, considerando os efeitos dos gradientes térmicos na degradação das propriedades mecânicas e na expansão térmica do aço e concreto. Após a calibração do modelo numérico tridimensional em temperatura ambiente e elevada, se realizam análises paramétricas considerando a influência da armadura positiva, da espessura fôrma de aço, da resistência à compressão do concreto, da altura da laje e do vão entre apoios, por meio da curva de incêndio padrão. Os resultados obtidos dos modelos numéricos tridimensionais são comparados com aqueles obtidos pelo método do momento fletor plástico, calculados por meio dos resultados dos modelos térmicos, e de acordo o modelo de cálculo da ABNT NBR 14323:2013. / Steel-concrete composite slabs consist on a structural system comprising of permanent steel sheeting on which reinforced concrete is cast. After the cure of concrete, these elements work in conjunction, ensuring the composite behavior. This structural system is widely used in Brazilian civil construction, especially for providing several advantages, such as no use of formwork and post shores. In order to assess thermal, structural and staggered flow-stress behavior of this structural system, finite elements numerical models were developed in software TNO DIANA in two distinct steps. In the first step a two-dimensional thermal analysis is carried out, whose thermal fields in the slab cross section are calibrated by experimental results available in the literature, evaluating the main parameters that affect the temperature distribution in the slab, as the resulting emissivity and thermal resistance in the steel/concrete interaction. In this stage is also developed a post-processor in Visual Basic 11.0, which is able to read the thermal fields generated by DIANA and calculate the plastic bending moment capacity versus fire exposure time. These results are compared with those calculated according to the ABNT NBR 14323:2013 calculation model. The second step consists in the development of three-dimensional numerical models to evaluate the staggered heat-flow behavior of simply supported composite slabs. In this step, first is done the calibration and validation of the numerical models at ambient temperature, assessing factors such as the symmetry of the models and the longitudinal shear resistance between steel sheeting and the concrete. Later, numerical models are validated in fire through experimental results available in the literature, considering the effects of thermal gradients in the degradation of the mechanical properties and thermal expansion of steel and concrete. After validation of the three-dimensional numerical models at ambient and elevated temperature, parametric analysis are performed considering the influence of sagging reinforcement, the thickness of steel sheeting, compressive strength of the concrete, the slab height and the span between supports, through standard fire curve. The bending resistance of tri-dimensional numerical models is compared with that obtained by the plastic theory, calculated through the results of two-dimensional thermal models, and according to ABNT NBR 14323:2013 calculation model.

Hexagonal closed packed (hcp) materials, in contrast to cubic materials, possess several processing challenges due to their anisotropic structural response, the wide variety of deformation textures they exhibit, and limited ductility at room temperature. The aim of this work is to investigate, both experimentally and theoretically, the effect os severe plastic deformation, ultrafine grain sizes, crystallographic textures and number of phases on the flow stress anisotropy and tension compression asymmetry, and the mechanisms responsible for these phenomena in two hcp materials: AZ31B Mg alloy consisting of one phase and Zn-8wt.% Al that has an hcp matrix with a secondary facecentered cubic (fcc) phase. Mg and its alloys have high specific strength that can potentially meet the high demand for light weight structural materials and low fuelconsumption in transportation. Zn-Al alloys, on the other hand, can be potential substitutes for several ferrous and non-ferrous materials because of their good mechanical and tribological properties. Both alloys have been successfully processed using equal channel angular extrusion (ECAE) following different processing routes in order to produce samples with a wide variety of microstructures and crystallographic textures for revealing the relationship between microstructural parameters, crystallographic texture and resulting flow stress anisotropy at room temperature. For AZ31B Mg alloy, the texture evolution during ECAE following conventional and hybrid ECAE routes was successfully predicted using visco-plastic self-consistent (VPSC) crystal plasticity model. The flow stress anisotropy and tension-compression (T/C) asymmetry of the as received and processed samples at room temperature were measured and predicted using the same VPSC model coupled with a dislocation-based hardening scheme. The governing mechanisms behind these phenomena are revealed as functions of grains size and crystallographic texture. It was found that the variation in flow stress anisotropy and T/C asymmetry among samples can be explained based on the texture that is generated after each processing path. Therefore, it is possible to control the flow anisotropy and T/C asymmetry in this alloy and similar Mg alloys by controlling the processing route and number of passes, and the selection of processing conditions can be optimized using VPSC simulations. In Zn-8wt.% Al alloy, the hard phase size, morphology, and distribution were found to control the anisotropy in the flow strength and elongation to failure of the ECAE processed samples.

As lajes mistas de aço e concreto consistem de um sistema estrutural formado por fôrmas de aço (chapas perfiladas) permanentes sobre a qual é lançado o concreto armado. Após a cura do concreto, esses elementos trabalham conjuntamente, garantindo o comportamento misto. Esse tipo de sistema é amplamente utilizado na construção civil brasileira, por propiciar diversas vantagens, como a não utilização de fôrmas de madeiras e escoras. Com a finalidade de avaliar o comportamento térmico, estrutural e termoestrutural acoplado desse tipo de sistema, foram desenvolvidos modelos numéricos em elementos finitos no pacote computacional TNO DIANA em duas etapas distintas. Na primeira etapa é feita uma análise térmica bidimensional, cujos campos térmicos na seção transversal da laje são calibrados por meio de resultados experimentais disponíveis na literatura, avaliando os principais parâmetros que influenciam na distribuição de temperatura na laje, como a emissividade resultante e a resistência térmica na interface aço/concreto. Nessa etapa também se desenvolve um pós-processador em Visual Basic 11.0 capaz de ler os campos térmicos gerados pelo DIANA e calcular o momento fletor plástico resistente em função do tempo de exposição ao fogo. Esses resultados são comparados com aqueles calculados de acordo o modelo de cálculo da ABNT NBR 14323:2013. A segunda etapa consiste no desenvolvimento de modelos numéricos tridimensionais para se avaliar o comportamento termoestrutural acoplado de lajes mistas simplesmente apoiadas. Nessa etapa, primeiramente se faz a validação e calibração dos modelos numéricos à temperatura ambiente, avaliando fatores como a simetria dos modelos e a resistência ao cisalhamento longitudinal na interface entre a fôrma de aço e o concreto. Posteriormente, se realiza a validação do modelo numérico em temperatura elevada por meio de resultados experimentais disponíveis na literatura, considerando os efeitos dos gradientes térmicos na degradação das propriedades mecânicas e na expansão térmica do aço e concreto. Após a calibração do modelo numérico tridimensional em temperatura ambiente e elevada, se realizam análises paramétricas considerando a influência da armadura positiva, da espessura fôrma de aço, da resistência à compressão do concreto, da altura da laje e do vão entre apoios, por meio da curva de incêndio padrão. Os resultados obtidos dos modelos numéricos tridimensionais são comparados com aqueles obtidos pelo método do momento fletor plástico, calculados por meio dos resultados dos modelos térmicos, e de acordo o modelo de cálculo da ABNT NBR 14323:2013. / Steel-concrete composite slabs consist on a structural system comprising of permanent steel sheeting on which reinforced concrete is cast. After the cure of concrete, these elements work in conjunction, ensuring the composite behavior. This structural system is widely used in Brazilian civil construction, especially for providing several advantages, such as no use of formwork and post shores. In order to assess thermal, structural and staggered flow-stress behavior of this structural system, finite elements numerical models were developed in software TNO DIANA in two distinct steps. In the first step a two-dimensional thermal analysis is carried out, whose thermal fields in the slab cross section are calibrated by experimental results available in the literature, evaluating the main parameters that affect the temperature distribution in the slab, as the resulting emissivity and thermal resistance in the steel/concrete interaction. In this stage is also developed a post-processor in Visual Basic 11.0, which is able to read the thermal fields generated by DIANA and calculate the plastic bending moment capacity versus fire exposure time. These results are compared with those calculated according to the ABNT NBR 14323:2013 calculation model. The second step consists in the development of three-dimensional numerical models to evaluate the staggered heat-flow behavior of simply supported composite slabs. In this step, first is done the calibration and validation of the numerical models at ambient temperature, assessing factors such as the symmetry of the models and the longitudinal shear resistance between steel sheeting and the concrete. Later, numerical models are validated in fire through experimental results available in the literature, considering the effects of thermal gradients in the degradation of the mechanical properties and thermal expansion of steel and concrete. After validation of the three-dimensional numerical models at ambient and elevated temperature, parametric analysis are performed considering the influence of sagging reinforcement, the thickness of steel sheeting, compressive strength of the concrete, the slab height and the span between supports, through standard fire curve. The bending resistance of tri-dimensional numerical models is compared with that obtained by the plastic theory, calculated through the results of two-dimensional thermal models, and according to ABNT NBR 14323:2013 calculation model.

Abstract In this thesis, the hot deformation behavior of stabilized 12–27% Cr ferritic stainless steels was investigated in order to find ways to improve the current hot rolling schedules for enhancing texture structures and deep drawability of the end product. Hot deformation was studied using axial and plane strain compression in two thermomechanical simulators: a Gleeble and a TMC machine. In addition to flow stress measurements, the resultant microstructures and textures were investigated using electron backscatter diffraction (EBSD), and the dislocation structures using transmission electron microscopy (TEM). In the case of 21% Cr steel, industrial multi-pass hot rolling, including low finish rolling temperatures, was simulated in order to investigate the microstructure and texture development under varying deformation conditions. Flow behavior of high-Cr ferritic stainless steels during hot deformation was mainly controlled by intense dynamic recovery. However, the deformation conditions greatly affected the extent of dynamic recovery. Cr increased the flow stress through solid solution hardening, although increasing the Cr content reduced the activation energy for hot deformation. Two modeling approaches for flow stress were successfully applied: an empirical constitutive equation and a dislocation density-based flow stress model. Continuous dynamic recrystallization was identified regardless of the Zener-Hollomon parameter, whereas discontinuous dynamic recrystallization was not observed. Static recrystallization slowed down towards the completion of the process, and especially the α fiber grains were difficult to recrystallize. Static recrystallization was enhanced by lowering the deformation temperature to 800 °C or below due to the accelerating effect of in-grain shear bands on the static recrystallization kinetics. However, an intensifying effect on γ fiber texture development was achieved after deformation at 600 °C or below. Two different improved process routes for hot rolling were proposed based on the results: 1) sufficiently long inter-pass times together with lowering the finish rolling temperature in order to promote static recrystallization during inter-pass times and hot band annealing, and 2) hot band annealing preceded by a warm rolling procedure, in which thin gauge hot band is produced by multiple heavy warm rolling deformation passes. / Tiivistelmä Tässä väitöstyössä tutkittiin stabiloitujen 12–27 % kromia sisältävien ferriittisten ruostumattomien terästen käyttäytymistä kuumamuokkauksessa tavoitteena kehittää nykyisin käytössä olevia kuumamuokkauskäytäntöjä lopputuotteen tekstuurirakenteen ja siten sen syvävedettävyyden parantamiseksi. Kuumamuokkausta simuloitiin sylinteri- ja tasomuodonmuutospuristuskokeilla Gleeble- ja TMC-laitteistoissa. Kokeista saatuja jännitys–venymä-käyriä analysoitiin ja syntyneet mikrorakenteet ja tekstuuri tutkittiin EBSD-menetelmällä pyyhkäisyelektronimikroskoopissa sekä dislokaatio- ja erkaumarakenteet läpäisyelektronimikroskoopilla. Lisäksi 21 % kromia sisältävälle teräkselle tehtiin monipistoista kuumavalssausta simuloivia puristuskokeita, joissa varioitiin myös valssauksen lopetuslämpötilaa ja jäähtymisnopeutta. Jännitys–venymä-käyriä mallinnettiin käyttäen sekä empiirisiä yhtälöitä että dislokaatiotiheyteen perustuvaa fysikaalista mallia. Kromipitoisuus kasvatti muodonmuutosvastusta mutta pienensi deformaation aktivaatioenergiaa. Dynaaminen toipuminen oli erittäin voimakasta kuumamuokkauslämpötiloissa, joskin lämpötila ja muodonmuutosnopeus vaikuttivat merkittävästi sen määrään. Jatkuvan dynaamisen rekristallisaation todettiin tapahtuvan riippumatta Zener-Hollomon -parametrin arvosta, mutta epäjatkuvaa dynaamista rekristallisaatiota ei havaittu. Staattinen rekristallisaatio hidastui, kun rekristallisaatioaste saavutti 90 %, ja erityisesti α-rungon rakeet pyrkivät vain toipumaan. Staattista rekristallisaatiota pystyttiin voimistamaan laskemalla muokkauslämpötila 800 °C:een tai sen alle, jolloin rakeiden sisälle syntyi staattisen rekristallisaation ydintymistä nopeuttavia leikkausnauhoja. γ-rungon intensiteetti voimistui rekristallisaatiossa kuitenkin vasta, kun muokkauslämpötila oli 600 °C tai tätä matalampi. Koetulosten perusteella ehdotettiin kahta erilaista kuumavalssauspraktiikkaa, joiden avulla kuumanauhan ominaisuuksia voidaan parantaa: 1) staattisen rekristallisaation edistäminen sekä pistojen välillä että kuumanauhahehkutuksessa käyttämällä pitkiä pistojen välisiä aikoja sekä laskemalla valssauksen lopetuslämpötilaa, tai 2) kuumanauhahehkutus yhdistettynä edeltävään voimakkaaseen lämminvalssaukseen, jolloin on mahdollista valmistaa ohutta kuumanauhaa.

No description available.

Abstract Austenitic high-Mn (15–30 wt.%) based twinning-induced plasticity (TWIP) steels provide great potential in applications for structural components in the automotive industry, owing to their excellent tensile strength-ductility property combination. In certain cases, these steels might also substitute austenitic Cr-Ni stainless steels. The aim of this present work is to investigate the high-temperature flow resistance, recrystallisation and the evolution of microstructure of high-Mn steels by compression testing on a Gleeble simulator. The influence of Al alloying (0–8 wt.%) in the hot rolling temperature range (800°C–1100°C) is studied in particular, but also some observations are made regarding the influence of Cr alloying. Microstructures are examined in optical and electron microscopes. The results are compared with corresponding properties of carbon and austenitic stainless steels. In addition, the mechanical properties are studied briefly, using tension tests over the temperature range from -80°C to 200°C. Finally, a preliminary study is conducted on the corrosion behaviour of TWIP steels in two media, using the potentiodynamic polarization technique. The results show that the flow stress level of high-Mn TWIP steels is considerably higher than that of low-carbon steels and depends on the Al concentration up to 6 wt.%, while the structure is fully austenitic at hot rolling temperatures. At higher Al contents, the flow stress level is reduced, due to the presence of ferrite. The static recrystallisation kinetics is slower compared to that of carbon steels, but it is faster than is typical of Nb-microalloyed or austenitic stainless steels. The high Mn content is one reason for high flow stress as well as for slow softening. Al plays a minor role only; but in the case of austenitic-ferritic structure, the softening of the ferrite phase occurs very rapidly, contributing to overall faster softening. The high Mn content also retards considerably the onset of dynamic recrystallisation, but the influence of Al is minor. Similarly, the contribution of Cr to the hot deformation resistance and static and dynamic recrystallisation, is insignificant. The grain size effectively becomes refined by the dynamic and static recrystallisation processes. The tensile testing of TWIP steels revealed that the Al alloying and temperature have drastic effects on the yield strength, tensile strength and elongation. The higher Al raises the yield strength because of the solid solution strengthening. However, Al tends to increase the stacking fault energy that affects strongly the deformation mechanism. In small concentrations, Al suppresses martensite formation and enhances deformation twinning, leading to high tensile strength and good ductility. However, with an increasing temperature, SFE increases, and consequently, the density of deformation twins decreases and mechanical properties are impaired. Corrosion testing indicated that Al alloying improves the corrosion resistance of high-Mn TWIP steels. The addition of Cr is a further benefit for the passivation of these steels. The passive film that formed on 8wt.% Al-6wt.%Cr steel was found to be even more stable than that on Type 304 steel in 5–50% HNO3 solutions. A prolonged pre-treatment of the steel in the anodic passive regime created a thick, protective and stable passive film that enhanced the corrosion resistance also in 3.5% NaCl solution.

Although the austenitic stainless steels 304 and 321 are often treated nominally as equivalents in their hot rolling characteristics, the question remains whether any subtle differences between the two allow further optimisation of their respective hot rolling schedules. The hot workability of these two types of austenitic stainless steels was compared through single-hit Gleeble simulated thermomechanical processing between 800℃ and 􀀄􀀅00℃ while the strain rate was varied between 0.00􀀄s􀀈􀀉 and 5s􀀈􀀉. It was found that the constants for the hyperbolic sinh equation for hot working of AISI 321 steel are Q = 465 kJ/mol, 􀀖􀀗 = 􀀘.􀀙6 􀀚 􀀄0􀀉􀀛 􀀜􀀝􀀞􀀈􀀉􀀟􀀈􀀉, 􀀠 = 0.00􀀘 􀀜􀀝􀀞􀀈􀀉 and 􀀡 = 6.􀀄 while for 304 steel the constants are Q = 446 kJ/mol, 􀀖􀀗 = 􀀅.􀀄4 􀀚 􀀄0􀀉􀀛 􀀜􀀝􀀞􀀈􀀉􀀟􀀈􀀉, 􀀠 = 0.008 􀀜􀀝􀀞􀀈􀀉and 􀀡 = 6.􀀄. It is shown that the occurrence of dynamic recrystallisation starts when the Zener-Hollomon parameter 􀀢 􀀣 6.4 􀀚 􀀄0􀀉􀀛s􀀈􀀉 for both steels but that the differences in the values of Q and A3 (the structure factor) between the two steels does lead to consistently lower steady state stresses for the steel 321 than is found in the steel 304 at the same Z values. This may, therefore, offer some scope for further optimisation of the hot rolling schedules and in particular in the mill loads of these two respective steels. A modelled constitutive equation derived from hot working tests to predict hot rolling mill loads is proposed and validated against industrial hot rolling data for AISI 321 stainless steel. Good correlation is found between the predicted Mean Flow Stress, the Zener-Hollomon Z parameter and actual industrial mill load values from mill logs if allowances are made for differences in Von Mises plane strain conversion, friction and front or back end tension. The multipass hot working behaviour of this steel was simulated through Gleeble thermomechanical compression testing with the deformation temperature varying between 1200℃ down to 800℃ and the strain rate between 0.001s-1 and 5s-1. At strain rates greater than 0.05s-1, dynamic recovery as a softening mechanism was dominant, increasing the dynamic recrystallisation to dynamic recovery transition temperature DRTT to higher temperatures. This implies that through extrapolation to typical industrial strain rates of about 60s-1,most likely no dynamic recrystallisation in plant hot rolling occurs in this steel but only dynamic recovery. Grain refinement by DRX is, therefore, unlikely in this steel under plant hot rolling conditions. Finally, mill load modelling using the hot working constitutive constants of the near-equivalent 304 instead of those specifically determined for 321, introduces measurable differences in the predicted mill loads. The use of alloy-specific hot working constants even for near-equivalent steels is, therefore, recommended. / Thesis (PhD)--University of Pretoria, 2014. / lk2014 / Materials Science and Metallurgical Engineering / PhD / unrestricted