Rozprawy doktorskie na temat „Metallurgical Techniques”

In recent years there have been significant new developments in welding processes for joining stainless steel and dissimilar metals. This is associated with the rise in interest of using stainless steel in the automotive industry from both car manufacturers and stainless steel producers. The main reason for using stainless steel within the automotive industry is the combination of formability and high strength but also the improved corrosion resistance when compared to zinc coated mild steel. This research explores the mechanical and metallurgical properties of dissimilar metal joining and determines a relationship between the fatigue properties and weld geometry. The research focuses on the relatively unexplored joining techniques of Laser Hybrid Welding and Cold Metal Transfer applied to joining stainless steel grades Hy-Tens 1000 and LDX 2101 to Dogal 260RP-X mild steel. The joints are assessed in terms of tensile, fatigue and metallurgical properties. Experimental results and analysis show that the fatigue properties of both laser hybrid welding and cold metal transfer joints are a linear relationship with a negative gradient to value of the root angle on the mild steel side of the joints, as the angle at the root decreases the fatigue life increases. It was found that when joining the material combinations outlined in this research with Laser Hybrid Welding the resulting solidified weld pool was chemically inhomogeneous. However, welds produced using Cold Metal Transfer resulted in a chemically homogenous weld pool and consistent microhardness. Comparisons with laser welding show that laser hybrid welding and cold metal transfer can produce joints with mechanical properties comparable to welding methods currently being used in the automotive industry, for example, laser welding.

Thesis (MEng)--Stellenbosch University, 2000.
ENGLISH ABSTRACT: A desire to increase the efficiency of the comminution process in mineral process systems has led to the need of determining the size distribution of ore particles at various stages in the system. The objective of this research is to investigate the feasibility of the use of an acoustic sensor for measuring particle size distribution. The acoustic signal generated when the particles impact on a cantilever bar is analysed using digital signal processing techniques. As rocks fall onto a metal bar, the bar vibrates. The vibrations contain information th a t is extracted to determine the size of particles tha t impacted on the bar. The bar is modelled as a linear system which is excited by impulses (impact of particles). The response of the bar is deconvolved from the acoustic signal to obtain an impulse whose amplitude is proportional to the energy of the impact. In order to improve size estimates, deconvolution is performed using a statistical model of the impulse sequence (Bernoulli-Gaussian) and then estimated using MAP estimation. Size estimates are not only a function of the mass of particles, but also on the exact position of impact on the bar. Since there is always a variation in the position of impact, size estimates are erroneous. It was found that the position of impact can be determined as to reduce variances dramatically. Due to physical sampling in space, the sensor has a bias towards larger particles. We show how this can be represented mathematically and removed. This project is mainly concerned with rocks in the +8-25mm (+0,7-22 gram) size range.
AFRIKAANSE OPSOMMING: Vergruising van erts in die mineraalbedryf verg groot hoeveelhede energie. Daar is ’n behoefte gei'dentifiseer orn hierdie proses meer effektief te maak. Aangesien die effektiwiteit van ’n meul ’n funksie is van die ertsgroottes wat gemaal word, kan partikel grootte inligting aangewend word om effektiwiteit te bevorder. Die doel van hierdie tesis is om die lewensvatbaarheid van ’n akoestiese sensor vir die doel van partikelgrootte estimasie, te ondersoek. Erts partikels wat val vanaf ’n vervoerband op ’n kantelbalk, veroorsaak dat die balk vibreer. Deur hierdie vibrasies te meet en verwerk, kan inligting aangaande partikel grootte verkry word. Die stelsel word gemodelleer as ’n lineere sisteem met impulse as intree. Die geobserveerde sein is die konvolusie tussen die intree impulse en die impulsweergawe van die sisteem. Deur gebruik te maak van ’n statistiese model en MAP-estimasie, word die effek van die sisteem gedekonvuleer vanaf die geobserveerde sein om ’n benadering van die intree impuls sein te verkry. Die amplitudes van die impulse word gebruik as ’n aanduiding van partikel massa. Partikelgroottes soos benader deur die stelsel, is ’n funksie van die die posisie waar die partikel die balk tref. Deur van patroonherkenning tegnieke gebruik te maak, word die posisie van impak bepaal om sodoende grootte benaderings aan te pas en die variansie van grootte verspreidings te verminder. As gevolg van die feit dat partikels gemonster word deurdat slegs ’n klein persentasie van die hele omvang van partikels ondersoek word, onstaan daar ’n oorhelling ( “bias” ) na groter partikels. Die kans dat groter partikels die balk tref is groter as vir klein partikels. ’n Wiskundige model vir hierdie verskynsel word voorgestel en gewys hoe die die oorhelling geneutraliseer kan word. Hierdie projek het te doen met ertsgroottes +8-25mm (+0,7-22 gram).

Rapid Manufacturing (RM) technologies as Electron Beam Melting (EBM) and Selective Laser Melting (SLM) are able to produce fully dense parts from pre-alloyed powders in a layer-wise way. Moreover, they are able to create tailored surfaces with interconnected porosity. Applied to biomedical prostheses, such porosity can favour cell adhesion and osteointegration. The most important intrinsic characteristic of RM techniques is the large undercooling the parts undergo during the process. This undercooling results in peculiar, very fine, metastable microstructures, associated to peculiar mechanical behaviour. Metastable microstructures can change on post-melting operations, making the materials match the standard requirements and gain interesting properties.

Rapid Manufacturing (RM) technologies as Electron Beam Melting (EBM) and Selective Laser Melting (SLM) are able to produce fully dense parts from pre-alloyed powders in a layer-wise way. Moreover, they are able to create tailored surfaces with interconnected porosity. Applied to biomedical prostheses, such porosity can favour cell adhesion and osteointegration. The most important intrinsic characteristic of RM techniques is the large undercooling the parts undergo during the process. This undercooling results in peculiar, very fine, metastable microstructures, associated to peculiar mechanical behaviour. Metastable microstructures can change on post-melting operations, making the materials match the standard requirements and gain interesting properties.

In the present work, the thermodynamic activities of chromium and vanadium oxide in CaO-SiO2-MgO-Al2O3 slags were measured using gas-slag equilibration technique. The slag was equilibrated with a gas mixture of CO, CO2 and Ar gases enabling well-defined oxygen partial pressures in the gas mixture (PO2=10-3,10-4,10-5 Pa) at temperatures 1803, 1823K, 1873, 1923 K. The slags were kept in Pt crucibles during the equilibration and the duration of which was 20 h. From a knowledge of the thermodynamic activity of chromium and vanadium in Cr or V in Pt alloy, obtained from literature, and the oxygen partial pressure in the gas stream calculated by Thermo Calc software, the thermodynamic activity of chromium, vanadium oxide in the slags could be observed.An assessment of the experimental studies in earlier works reveal that, the activities of chromium at low chromium contents and vanadium in their respective alloys in platinum exhibits a strong negative deviation from ideality, the logarithms of activity coefficient of these elements were found to increase with increasing mole fractions of these metals in the Pt-alloys.Regarding the slag phase, all the chromium in the slags was assumed to be present in the divalent state in view of the low Cr contents and the low oxygen potentials employed in the present studies. Analogously, vanadium in the slag was assumed to be in the trivalent state in view of the low vanadium contents in the slag and the low oxygen partial pressure in the gas phase. Activity of chromium oxide, CrO decreases with increasing temperature and decreasing content of chromium oxide in slag and oxygen partial pressure in the gas phase. Activity of vanadium oxide, VO1.5 in slag phase shows a negative deviation from ideality. Activity coefficient of vanadium oxide shows a decrease with basicity of slag and the “break point” occurs at about slag basicity of 1 under the oxygen partial pressure of 10-3 Pa and temperature of 1873 K.A relationship for estimating the actual content of chromium, vanadium in slag as a function of activities of chromium or vanadium, temperature, oxygen partial pressure and slag basicity were developed from the present results, the agreement between the estimated and experimental values is satisfactory, especially at lower oxygen partial pressure.

An experimental investigation of the binary system Co-Cr and the ternary system C-Co-Cr has been carried out in the present thesis. The experimental strategy adopted for the binary system was to measure the thermodynamic activities of Cr, the molar heat capacity as function of time, the phase transformation temperatures with corresponding enthalpies, the Curie transition temperature as well as melting temperatures with corresponding enthalpies. In the ternary system the strategy was to determine the solubility of Co in the Cr7C3 phase as well as the C and Cr contents in the Co rich (fcc) binder phase. The experimental results were compared with atomistic simulations of the solubility of Co in the Cr7C3 phase. Solid state galvanic cell measurements were conducted with both ZrO2-7.5 mol % CaO and CaF2 as the solid electrolyte. In view of possible errors in the measurements with ZrO2-7.5 mol % CaO, as a result of electronic contributions to the conduction of the solid electrolyte, new measurements were conducted with CaF2 as the solid electrolyte. The results indicated that the measured EMF values showed trends which were contrary to the thermodynamic behaviour expected from phase diagram considerations. It was concluded that further detailed experimentation was necessary in order to throw more light on the thermodynamic behaviour of the Co-Cr system. Two different series of DSC measurements were conducted, i.e. one in an atmosphere of pure hydrogen and another in pure argon. In the first investigation, conducted in an atmosphere of pure hydrogen in the temperature interval 318-1660 K, evidence was obtained for the existence of a phase transformation around 900 K in the compositional range 20.7-67.1 wt.% Cr. No indications of such a phase transformation had earlier been seen. In the second investigation, conducted in an atmosphere of pure argon in the temperature interval 298-1823 K, special attention was given to alloys in the Co rich corner of the phase diagram, i.e. 0-10 wt.% Cr. This investigation verified earlier findings of a phase transformations around 900 K in the compositional range 20.7-67.4 wt.% Cr. The magnetic transition temperatures for alloys low in Cr content were also obtained. With the use of the DTA technique the melting temperatures with corresponding enthalpy values for alloys in the compositional range 0.9-7.7 wt.% Cr were obtained. The three-phase triangle fcc+Cr7C3+graphite was investigated at 1373 K, 1423 K and 1473 K. The obtained results showed that the solubility of cobalt in the Cr7C3 phase was significantly higher than previously predicted by thermodynamic calculations.
QC 20100930

The objective of this research is to increase the understanding of the solidification behaviour of some industrially important wrought aluminium alloys. The investigation methods range from direct investigations of as-cast ingots to laboratory-scale techniques in which ingot casting is simulated. The methods span from directional solidification at different cooling rates to more fundamental and controlled techniques such as DTA and DSC. The microstructure characteristics of the castings have been investigated by optical and Scanning Electron microscopy. Hardness tests were used to evaluate the mechanical properties. The effects of adding alloying elements to 3XXX and 6XXX aluminium alloys have been studied with special focus on the effects of Zn, Cu, Si and Ti. These elements influence the strength and corrosion properties, which are important for the performance of final components of these alloys. Solidification studies of 0-5wt% Zn additions to 3003 alloys showed that the most important effect on the microstructure was noticed at 2.5 wt% Zn, where the structure was fine, and the hardness had a maximum. Si addition to a level of about 2% gave a finer structure, having a relatively large fraction of eutectic structure, however, it also gave a long solidification interval. The addition of small amounts of Cu, 0.35 and 1.0 wt%, showed a beneficial effect on the hardness. Differences have been observed in the ingot surface microstructures of 6xxx billets with different Mg and Si ratios. Excess Si compositions showed a coarser grain structure and more precipitations with possible negative implications for surface defect formation during DC casting. The comparison of alloys of different Ti content showed that the addition of titanium to a level of about 0.15 wt% gave a coarser grain structure than alloys with a normal Ti content for grain refinement, i.e. < 0.02 wt%, although a better corrosion resistance can be obtained at higher Ti contents. The larger grain size results in crack sensitivity during DC casting. A macroscopic etching technique was developed, based on a NaOH solution, and used in inclusion assessment along DC cast billets. Good quantitative data with respect to the size and spatial distribution of inclusions were obtained. The results from studied billets reveal a decreasing number of inclusions going from bottom to top, and the presence of a ring-shaped distribution of a large number of small defects in the beginning of the casting. The present study shows how composition modifications, i.e. additions of certain amounts of alloying elements to the 3xxx and 6xxx Al alloys, significantly change the microstructures of the materials, its castability, and consequently its mechanical properties
QC 20100901

L'objectif de ce travail de thèse a été de développer l'utilisation des isotopes du fer pour le traçage des métaux anciens, principalement ferreux. Notre approche méthodologique s'est articulée autour de trois axes majeurs. Le premier axe a consisté à étudier l'influence potentielle des processus métallurgiques sur la composition isotopique du fer des métaux produits. Pour cela, nous avons mesuré la composition isotopique du fer d'échantillons issus d'expérimentations de réduction de minerai de fer en bas fourneau, réalisées sur un site sidérurgique majeur de la période romaine (la Montagne Noire, Sud-Ouest de la France). Le second axe a eu pour objectif de valider ce nouveau traceur en mesurant la composition des isotopes du fer de minerais, scories et objets en fer issus d'un contexte archéologique connu et dont la provenance avait déjà été identifiée par des analyses élémentaires. Nous avons alors mesuré la composition des isotopes du fer de barres de fer principalement retrouvées dans des épaves romaines au large des Saintes-Maries-de-la-Mer (Bouches-du-Rhône, France). Enfin, le troisième axe a eu pour vocation d'estimer la pertinence, mais aussi les limites de ce nouvel outil en l'appliquant à deux terrains archéologiques très différents, où aucune étude de traçage classique n'avait été utilisée précédemment. Ces deux terrains concernaient la sidérurgie ancienne au Togo et la production de plomb argentifère médiévale au Maroc. Les résultats obtenus montrent que la composition isotopique du fer de la scorie et du métal produit est similaire à celle du minerai correspondant. Il n'y a donc pas de fractionnement des isotopes du fer tout au long de la chaîne opératoire de production de fer. De plus, et contrairement à certains traceurs élémentaires, les isotopes du fer ne sont pas contaminés par la paroi du four très pauvre en cet élément durant la réduction. Ceci permet ainsi d'établir des liens de provenance directs entre un objet en fer et un minerai. L'application de cette méthode de traçage à un contexte archéologique déjà largement étudié a permis de valider les hypothèses de provenance d'objets archéologiques. En outre, les isotopes du fer peuvent être plus discriminants que les éléments en trace car ils permettent notamment de différencier des productions de fer temporellement et géographiquement très proches. Le traçage est ainsi affiné. Enfin, nos résultats préliminaires suggèrent que les analyses des isotopes du fer pourraient également être appliquées à l'étude de la production de métaux non ferreux. Cette étude offre ainsi de nombreuses perspectives, telles que l'étude de la provenance de pièces de musée étant donné la faible quantité de matière nécessaire, l'établissement d'une base de données de composition isotopique du fer de minerais archéologiques et l'élaboration d'une méthode de traçage commune aux métaux ferreux et non ferreux
The objective of this work was to develop the use of iron isotopes for ancient, essentially ferrous metal tracing. Our methodological approach was based on three major directions. The first one consisted in the assessment of the potential influence of metallurgical processes on iron isotope compositions of the reduction products. For this purpose, we measured the iron isotope composition of materials from experiments of iron ore reduction in bloomery furnace performed in a major site of iron production during the Roman period (Montagne Noire, SW France). The second direction aimed at validating this new tracer through the iron isotope measurement of ores, slags and iron artefact samples from a well-defined archaeological context, and whose provenance was previously investigated by elemental analyses. We thus measured the isotopic composition of iron bars discovered in Roman shipwrecks found offshore Les-Saintes-maries-de-la-Mer (Bouches-du-Rhône, France). Thirdly, we estimated the relevance and limitations of our new tracing approach by applying it to the study of two different archaeological fields, on which no previous provenance study had been performed. These were the ancient iron production from North East Togo and the Medieval lead-silver production in Morocco (Anti Atlas Massif). The results demonstrate that the isotopic composition of slags and metals produced reflect that of their corresponding ores because no iron isotope fractionation occurs along the entire chaîne opératoire of iron production. Moreover, and in contrast to several elemental tracers, iron isotopes are not impacted by iron contribution from the smelting device during the reduction process, which allows to establish provenance links directly between an iron artifact and a specific ore. The application of this tracing method in a well-studied archaeological context has allowed to validate the provenance assumption of several archaeological artifacts. Furthermore, iron isotopes may provide a more discriminative tracer than trace elements because a temporal and geographical distinction is possible between close iron production sites. The tracing is thus more precise. Finally, our results suggest that iron isotope analyses could also be used in the study of non-ferrous metal production. This work offers many perspectives in provenance studies of museum pieces given the very small amount of material needed, in the setup of a database of iron isotope compositions of archaeological iron ores and the establishment of a common tracing approach for both ferrous and non-ferrous metals

Two methods for refining metallurgical grade silicon to solar grade silicon have been investigated. The first method involved the reduction of impurities from metallurgical grade silicon by high temperature vacuum refining. The concentrations of analyzed elements were reduced several times. The main steps in the second refining method include alloying with copper, solidification, grinding and heavy media separation. A metallographic study of the Si-Cu alloy showed the presence of only two microconstituents, mainly pure silicon dendrites and the Cu3Si intermetallic. SEM analysis showed a distinct boundary between the silicon and the Cu3Si phases, with a large concentration of microcracks along the boundary, which allowed for efficient separation. After alloying and grinding, a heavy media liquid was used to separate the light silicon phase from the heavier Cu3Si phase. Cu3Si residues together with the remaining impurities were found to be located at the surface of the pure silicon particles, and should be efficiently removed by acid leaching. Thirty elements were analyzed by the Inductively Coupled Plasma Mass Spectrometry (ICP) chemical analysis technique. ICP revealed a several times higher impurity level in the Cu3Si intermetallic than in the pure silicon; furthermore, the amounts of 22 elements in the refined silicon were reduced below the detection limit where the concentrations of 7 elements were below 1ppmw and 6 elements were below 2ppmw. The results showed that the suggested method is efficient in removing impurities from metallurgical grade silicon with great potential for further development.

碩士
中原大學
電子工程研究所
97
In this study, we use upgraded metallurgical silicon (UMG-Si) by directional solidification casting as the starting materal. UMG-Si is a relatively low cost substrate; it avoids the expensive Siemens process during fabrication of electronic grade poly-silicon material from which mono- and multicrystalline silicon materials are manufactured. The use of UMG-Si substrate is possible to reduce the fabrication cost of Si-based solar cells. However, this material has a localized region of high density of dislocations and metal impurity precipitates, which in turn act as carrier recombination sites and deteriorate the performance of solar cell fabricated thereon. It may present a solution to the silicon feedstock shortage by purification of UMG-Si. By porous silicon gettering (PSG) technique, undesired impurities can be driven away from UMG-Si wafer by combining the formation of porous silicon (PS) and the following heat treatments. The latter was conducted in an infrared furnace at different temperatures for a constant time duration of 30 min. The gettering technique used in this work is effected by the formation of a PS film at the front side of the UMG-Si wafer. Experimental results showed that when the base material underwent such a gettering process, the short-circuit current and efficiency of the corresponding solar cells fabricated can be improved. So far, an increase of 1.6 mA in the short-circuit current and therefore a relatively high efficiency of 13.1% has been achieved for the specimen performed with the PSG treatment.

碩士
中原大學
電子工程研究所
97
This study investigated for upgraded metallurgical grade silicon (UMG-Si) substrates by gettering process and expected to improve the device’s electronic properties. After gettering process, we develops polycrystalline silicon solar cell by this UMG-Si substrates. In recent years, the demand of solar cells was increasing in the world, which causes the deficiency of silicon feedstock and therefore makes the cost of silicon raw material remain high. So, the aim of this experiment is to reduce cost of material, low cost upgraded metallurgical grade silicon (UMG-Si) wafers will be used as the substrates and an epitaxial amorphous Si (a-Si：H) thin film will be grown thereon by Plasma-enhanced chemical vapor deposition(PECVD) . Loose structure of amorphous Si thin film has many cavities or dislocations that will trap the metal impurities by thermal annealing treatment. The gettering process could improve the electrical properties of UMG-Si substrate. In analytical part, using Hall Measurement, we discuss the improvement of moility after gettering process. In order to understand the metallic impurities characteristic of the substrate which are trapped, we exhibited the metal impurities distribution of substrate by secondary ion mass spectroscopy(SIMS). Then we made solar cell by this substrate. Forming P-N Junction by high temperature diffusion furnace chamber、fabricating front side electrode by E-gun evaporator、rear side electrode by Al paste screen printing、making anti-reflection coating(ARC) by PECVD. We finish solar cell and measure the conversion efficiency.

碩士
中原大學
電子工程研究所
98
The cost of Si wafers has been the bottleneck for a widespread use of Si solar cells. The upgraded metallurgical grade silicon ( UMG-Si ) is inexpensive compared to the single crystal or solar grade polysilicon material. In this study, the UMG-Si substrates are used to fabricate solar cells with a view to reducing the production cost of Si solar cells. However, these substrate materials contain a large number of harmful internal metal impurities. These impurities tend to be the recombination centers for carriers and cause a serious degradation in the efficiency of solar cells. To solve this problem, we use an extrinsic gettering technique to remove those unwanted impurities. The extrinsic gettering process is denoted as polysilicon film gettering (PSFG) and was conducted as follows. First, a polysilicon film was deposited using atmosphere pressure halide chemical vapor deposition ( APHCVD ) at different temperatures: 800, 900, and 1000˚C. Then this film was annealed at various temperatures ranging from 600 to 850˚C. The as-deposited polysilicon film (what is called, a sink layer) contains a large number of grain boundaries and cavities, which can be used as the dumps or places for absorbing and storing metal impurities present in UMG-Si substrate. The annealing treatment was performed to promote the interdiffusion behavior of metal impurities. The minority carrier lifetime of the UMG-Si substrate treated with PSFG was obtained from microwave photocurrent decay (μW-PCD) measurements, which was used to evaluate the effect of PSFG. Finally, the electrical properties of solar cells fabricated on the UMG-Si substrates with and without PSFG treatment were compared. It was found that depositing a polysilicon film at at 800˚C and annealing it at 700˚C for 15 minutes would have the lifetime of UMG-Si substrate increase to 33.2 µs. Also, the higher the minority carrier lifetime of UMG-Si substrate, the higher the efficiency of solar cell fabricated thereon. Ultimately, an efficiency high to 13.66 % has been achieved for the cell fabricated on the PSFG-treated substrate, which is elevated by 1.6 % compared to that of the solar cell without PSFG treatment.