Magnesium is the lightest structural metal and magnesium alloys are therefore obvious candidates in weight critical applications. The environmental imperative to reduce vehicle emissions has recently led to intensified research interest in magnesium, since weight reduction is one of the most effective ways of improving fuel efficiency. The hexagonal close-packed structure of magnesium results in poor room temperature formability. However, on heating, several magnesium alloys show superplastic properties, with the ability to deform to very high strains (up to 3000%). This opens up the possibility of forming complex components directly by superplastic forming (SPF). As a result, SPF of magnesium is a highly active research topic. The most widely used class of magnesium alloys contain aluminium as the major alloying addition, which has a relatively high solubility in magnesium, and manganese, which has a less solubility. The effect of these elements on the deformation behaviour and failure mechanisms operating in the superplastic regime is not yet well understood. The objective of this work was to gain fundamental insights into the role of these elements. To do this, alloys with different aluminium content (AZ31 and AZ61) and manganese levels have been studied in-depth.After casting, all alloys were subject to a hot rolling procedure that produced a similar fine grain size and texture in each material. Hot uniaxial testing was performed at temperatures between 300 to 450 degC and at two strain rates to investigate the material flow behaviour, elongation to failure and failure mechanism. All of the alloys exhibited flow curves characterised by an initial hardening and extensive flow softening region. Dynamic recrystallization did not occur, and the flow softening was attributed to grain growth and cavity formation. Increasing the level of aluminium in solution was observed to increase the grain growth rate, and also reduce the strain rate sensitivity. The elongation to failure, however, depended strongly on the manganese level but not on the aluminium content. This attributed to the role of manganese in forming coarse particles that act as sites for cavitation.To study cavity formation and growth, and its effect on failure, a series of tests were conducted to different strain levels followed by investigation of cavitation in 3-dimensions using X-ray tomography. New methods were developed to quantify the correlation between cavities and coarse particles using X-ray tomography data and it was shown that over 90% of cavities are associated with particles. Cavity nucleation occurred continuously during straining, with progressively smaller particles forming cavities as strain increased. The mechanism of cavity formation and growth was identified, and it has been demonstrated that particle agglomerates are effective sites for cavity formation even when the individual particles in the agglomerates are below the critical size predicted by theory for cavity nucleation sites. These results suggest that to improve the ductility of magnesium alloys in the superplasticity regime, it is most critical to minimise the occurrence of particle agglomerates in the microstructure.

No description available.

The hot working behavior of AISI 1018 steel was studied by hot-compression deformation tests on the Gleeble 1500 thermo-mechanical simulator at true strain values of -0.143 and -0.405, true strain rate values of 0.01 and 0.1, and working temperatures of 900°C and 1000°C. The tests show that a lower working temperature and lower true strain value results in a greater maximum compressive force. The apparent activation energy Qapp was calculated by using the Zener-Hollomon parameter combined with the low stress law. Qapp ­was calculated to be 311 kJ mol-1 K-1.

A soldagem por atrito com pino não consumível (SAPNC), processo de união no estado sólido, tornou-se conhecido devido à alta resistência das juntas produzidas em comparação ao metal de base e aos métodos convencionais de união. No entanto, o próprio processo apresenta seus desafios, relacionados principalmente à combinação do efeito de geometrias de ferramenta com os parâmetros utilizados. O desenvolvimento de métodos de simulação numérica tem criado a possibilidade de otimização destes efeitos, prevendo as interações entre os materiais, parâmetros e geometrias de ferramenta com menor custo e tempo. O presente trabalho teve como objetivo simular o processo de soldagem por atrito utilizando método dos elementos finitos (MEF) no software DEFORM 3D e avaliar a capacidade do modelo em representar fenômenos presentes do processo, tais como esforços da ferramenta, ciclo termomecânico do material e microestrutura. O estudo foi realizado em amostras de uma chapa de liga de magnésio AZ31B. Para inclusão da evolução mecânica e microestrutural do material de estudo no modelo, dados foram obtidos pelo estudo da compressão isotérmica da liga AZ31 simulador termomecânico Gleeble, em condições típicas às encontradas no processo de soldagem por atrito em. Os dados de tensão, deformação e microestrutura obtidos nos ensaios de compressão foram tratados analiticamente afim de se obter os parâmetros para as equações de deformação à quente e evolução microestrutural do material de estudo. Ademais, na próxima etapa, utilizou-se um pino roscado e não roscado em soldagens dissimilares afim de analisar o efeito da geometria da ferramenta no fluxo de material. Para auxiliar a análise foram utilizadas técnicas de microscopia óptica, microscopia eletrônica de varredura (MEV) e difração de raios X. Por fim, um modelo numérico de soldagem por atrito elaborado no software DEFORM-3D é apresentado. Os resultados do modelo foram comparados com os resultados experimentais com auxílio de técnicas de caraterização microestrutural, EBSD, ciclos térmicos capturados por termopares e torque das ferramentas. Os resultados e conclusões obtidos no projeto permitiram a identificação do ciclo térmico, mecânico e microestrutural do material durante a soldam por atrito, além da demostração do efeito da geometria para distintos parâmetros de processo, indicando um método alternativo eficiente na otimização de geometrias de ferramenta e busca de parâmetro ótimos do processo de soldagem por atrito com tempo e custo reduzidos. / Friction stir welding (FSW), a solid-state process, has become known due to the high strength of the produced joints compared to the base metal and the conventional welding methods. However, the friction stir welding has its challenges, related with tools geometry and the process parameters. The development of numerical simulation methods has been able to aid in the process optimization, including the study of different materials, parameters and tool geometries with lower cost and time. The obejective of the present work is propose a friction stir welding numerical model through finite element analysis (FEA) in DEFORM 3D software and evaluated its capacity to represent the process features such as the tool chacteristics, the thermomechanical cycle of the material and its microstructure. The material chosen for the study was a magnesium alloy AZ31B. In order to include the mechanical and microstructural evolution of the study material in the model, data were obtained through isothermal compression of the AZ31 in Gleeble thermomechanical simulator, under typical conditions found in the friction welding process. Stress, strain and microstructure data obtained in the compression tests were analyzed analytically to obtain the parameters for the hot deformation equations and microstructural evolution of the material. Moreover, a threaded and non-threaded pin in dissimilar welds was used to verify the effect of tool geometry in the material flow during the friction stir welding process. The evaluation was made with the support of optical microscopy and scanning electron microscopy (SEM). Then, a numerical model of friction welding developed in the DEFORM-3D software is presented. The results of the model were compared with experimental results supported by microstructural characterization techniques, EBSD, temperature profiles captured by thermocouples and tool torque response. The results and conclusions obtained in the project allowed the identification of the thermal, mechanical and microstructural cycle of the material during the friction stir welding process, besides the demonstration of the effect of the tool geometry for different process parameters, indicating an efficient alternative method in the optimization of tool geometries and optimal parameter for the friction welding process with reduced time and cost.

This thesis contains the development of a physically-based model describing the microstructural evolution during hot deformation of microalloyed steels. The work is mainly focused on the recrystallization kinetics. During hot rolling, the repeated deformation and recrystallization provides progressively refined recrystallized grains. Also, recrystallization enables the material to be deformed more easily and knowledge of the recrystallization kinetics is important in order to predict the required roll forces. Hot strip rolling is generally conducted in a reversing roughing mill followed by a continuous finishing mill. During rolling in the roughing mill the temperature is high and complete recrystallization should occur between passes. In the finishing mill the temperature is lower which means slower recrystallization kinetics and partial or no recrystallization often occurs. If microalloying elements such as Nb, Ti or V are present, the recrystallization can be further retarded by either solute drag or particle pinning. When recrystallization is completely retarded and strain is accumulated between passes, the austenite grains will be severely deformed, i.e. pancaking occurs. Pancaking of the grains provides larger amount of nucleation sites for ferrite grains upon transformation and hence a finer ferrite grain size is achieved. In this work a physically-based model has been used to describe the microstructural evolution of austenite. The model is built-up by several sub-models describing dislocation density evolution, recrystallization, grain growth and precipitation. It is based on dislocation density theory where the generated dislocations during deformation provide the driving force for recrystallization. In the model, subgrains act as nuclei for recrystallization and the condition for recrystallization to start is that the subgrains reach a critical size and configuration. The retarding effect due to elements in solution and as precipitated particles is accounted for in the model. To verify and validate the model axisymmetric compression tests combined with relaxation were modeled and the results were compared with experimental data. The precipitation sub-model was verified by the use of literature data. In addition, rolling in the hot strip mill was modeled using process data from the hot strip mill at SSAB Strip Products Division. The materials investigated were plain C-Mn steels and Nb microalloyed steels. The results from the model show good agreement with measured data. / QC 20100706

Microstructural and mechanical characterizations of heat treatable Al-Mg-Si-Cu based wrought aluminum alloys have been studied. The aim of this work was to produce fine grained, high strength alloy by adjusting processing conditions: deformation, solutionizing and aging. First, primary characterization was carried out via SEM-EDS analyses and tensile tests. Then an extensive experimental study has been carried out on two sets of samples. The first set has been studied to determine the ideal conditions for solutionizing and aging processes by analyzing the variation of hardness with different solutionizing and aging time and temperature. The second set, have first been mechanically deformed by swaging at four different deformations and four different temperatures, then heat treated. The hardness measurements have been carried out before and after solutionizing and also after aging. Finally, recrystallization behavior has been investigated by measuring grain size before and after solutionizing treatment using image analyzer software. The initial characterizations showed that Mg2Si and complex iron, manganese bearing intermetallics were the primary particles observed in the &amp / #945 / -Al matrix. Nearly 140HB hardness could be obtained with solutionizing at 530&deg / C and aging at 175&deg / C for 8 hours which was determined as the optimum treatment for obtaining peak hardness. When shaping (deformation) was concerned / strength loss was the overall outcome of any hot or cold deformation before solutionizing / which was most probably due to the destruction of the initial microstructure. Improvement in the percent elongation was the promising aspect of this application. Strength loss was increased for samples deformed at higher temperatures and higher reductions.

<p>Recrystallization kinetics, during and after hot deformation, has been investigated for decades. From these investigations several equations have been derived for describing it. The equations are often empirical or semi-empirical, i.e. they are derived for certain steel grades and are consequently only applicable to steel grades similar to these. To be able to describe the recrystallization kinetics for a variety of steel grades, more physically based models are necessary.</p><p>During rolling in hot strip mills, recrystallization enables the material to be deformed more easily and knowledge of the recrystallization kinetics is important in order to predict the required roll forces. SSAB Tunnplåt in Borlänge is a producer of low-carbon steel strips. In SSAB’s hot strip mill, rolling is conducted in a reversing roughing mill followed by a continuous finishing mill. In the reversing roughing mill the temperature is high and the inter-pass times are long. This allows for full recrystallization to occur during the inter-pass times. Due to the high temperature, the rather low strain rates and the large strains there is also a possibility for dynamic recrystallization to occur during deformation, which in turn leads to metadynamic recrystallization after deformation. In the finishing mill the temperature is lower and the inter-pass times are shorter. The lower temperature means slower recrystallization kinetics and the shorter inter-pass times could mean that there is not enough time for full recrystallization to occur. Hence, partial or no recrystallization occurs in the finishing mill, but the accumulated strain from pass to pass could lead to dynamic recrystallization and subsequently to metadynamic recrystallization.</p><p>In this work a newly developed physically based model has been used to describe the microstructural evolution of austenite. The model is based on dislocation theory where the generated dislocations during deformation provide the driving force for recrystallization. The model is built up by several submodels where the recrystallization model is one of them. The recrystallization model is based on the unified theory of continuous and discontinuous recovery, recrystallization and grain growth by Humphreys.</p><p>To verify and validate the model, rolling in the hot strip mill was modeled using process data from SSAB’s hot strip mill. In addition axisymmetric compression tests combined with relaxation was modeled using experimental results from tests conducted on a Gleeble 1500 thermomechanical simulator at Oulu University, Finland. The results show good agreement with measured data.</p>

Out of the known refractory metals and alloys, molybdenum (Mo) and its alloys are very important due to their unique combination of properties which render them suitable for various applications. Owing to their good creep properties, minimum damage from neutron irradiation and good compatibility with the liquid alkali metals, molybdenum and its alloys are well suited candidates for structural components in the newly developed Compact High Temperature Reactor (CHTR). However, to fabricate components for structural application from molybdenum and its alloys, the processing response needs to be established. The present thesis is an attempt to address this issue in a more generic manner. The study have been specifically aimed to examine the hot deformation behaviour of molybdenum and two of its alloys (Mo-TZM and Mo-TZC) over a high temperature range, for obtaining stable microstructure with good mechanical properties. The thesis basically addresses the following (i) the thermos-mechanical response of the material with change in deformation conditions, and (ii) the evolution of microstructure during hot deformation, and identification of associated mechanisms. Chapter 1 of the thesis includes an introduction of the material system and alloys with a detailed survey of the literature on the deformation behaviour of refractory metals and alloys that are used as structural materials in nuclear reactors. More emphasis is given to molybdenum and two of its alloy Mo-TZM and Mo-TZC. Chapter 2 includes the detail of the experimental techniques and analysis procedures that are followed in the course of investigation. The hot deformation behaviour of molybdenum in temperature range 1400 - 1700°C and strain rate range 0.001 - 10.0s-1 is discussed in chapter 3. The stress - strain behaviour has been further analysed to obtain strain rate sensitivity maps. The micro-mechanisms operative in different deformation domain has been analysed extensively by Electron Back Scatter Diffraction (EBSD) technique. Different restoration processes which include dynamic recrystallization, recovery and grain growth have been identified in different domains of deformation conditions. Chapter 4 of this thesis is dedicated to the hot deformation behaviour of Mo-TZM alloy. Deformation behaviour was studied under identical conditions as molybdenum. Mo-TZM showed higher strain rate sensitivity and high temperature strength than molybdenum. Dynamic recovery is the most predominant mechanism in Mo-TZM alloy as revealed through the analysis of stress strain curve as well as EBSD based investigation. At higher temperature and strain rates dynamic recrystallization has also been observed. The effect of excess carbon which results in Mo-TZC alloy, deformation behaviour has been investigated in chapter 5. The analysis of stress – strain curves in this case indicates the predominance of dynamic recrystallization over a range of deformation conditions. The mechanism has been identified as particle simulated nucleation (PSN). The significant growth of the deformed grains is observed at the highest temperature of deformation. A comparison of deformation behaviour of alloying addition in molybdenum alloys has been discussed in chapter 6. The results of deformation behaviour of molybdenum and its alloys has been compared vis-a-vis with another similar class of alloys based on Niobium (Nb) and apparent similarities and differences in the deformation behaviour has also been discussed in chapter 6. Finally, the overall summary of the thesis has been presented in chapter 7.

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 &#945; 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 &#947; 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 &#945;-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. &#947;-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.

Recrystallization kinetics, during and after hot deformation, has been investigated for decades. From these investigations several equations have been derived for describing it. The equations are often empirical or semi-empirical, i.e. they are derived for certain steel grades and are consequently only applicable to steel grades similar to these. To be able to describe the recrystallization kinetics for a variety of steel grades, more physically based models are necessary. During rolling in hot strip mills, recrystallization enables the material to be deformed more easily and knowledge of the recrystallization kinetics is important in order to predict the required roll forces. SSAB Tunnplåt in Borlänge is a producer of low-carbon steel strips. In SSAB’s hot strip mill, rolling is conducted in a reversing roughing mill followed by a continuous finishing mill. In the reversing roughing mill the temperature is high and the inter-pass times are long. This allows for full recrystallization to occur during the inter-pass times. Due to the high temperature, the rather low strain rates and the large strains there is also a possibility for dynamic recrystallization to occur during deformation, which in turn leads to metadynamic recrystallization after deformation. In the finishing mill the temperature is lower and the inter-pass times are shorter. The lower temperature means slower recrystallization kinetics and the shorter inter-pass times could mean that there is not enough time for full recrystallization to occur. Hence, partial or no recrystallization occurs in the finishing mill, but the accumulated strain from pass to pass could lead to dynamic recrystallization and subsequently to metadynamic recrystallization. In this work a newly developed physically based model has been used to describe the microstructural evolution of austenite. The model is based on dislocation theory where the generated dislocations during deformation provide the driving force for recrystallization. The model is built up by several submodels where the recrystallization model is one of them. The recrystallization model is based on the unified theory of continuous and discontinuous recovery, recrystallization and grain growth by Humphreys. To verify and validate the model, rolling in the hot strip mill was modeled using process data from SSAB’s hot strip mill. In addition axisymmetric compression tests combined with relaxation was modeled using experimental results from tests conducted on a Gleeble 1500 thermomechanical simulator at Oulu University, Finland. The results show good agreement with measured data. / QC 20101118