Various metallurgical microstructures and their formation are studied in this thesis by using a laser beam to melt a variety of materials with different chemical compositions over a range of thermal cycles. The laser beam was used conventionally in a narrow gap multi-layer weld, used for welding large depths with filler wire additions, as well as a non-traditional simulated welding approach labelled here as the Snapshot method. In laser beam welding, materials go through rapid heating and cooling cycles that are difficult to mimic by other techniques. In welding, any microstructural development depends on complex combinations of chemistry and thermal cycles but is also influenced by melt flow behavior. In turn, microstructural morphologies influence the mechanical behavior which can suffer due to inappropriate microstructural constituents. The Snapshot method, through control of thermal cycling and material composition, can achieve the same rates while guiding microstructural development to form tailored properties. The tunable laser beam properties can be exploited to develop an experimental welding simulation (Snapshot method), which enables the complex interlinked chemical and thermal events which take place during welding to be studied in a controlled manner. Exploring the microstructural relationships to their thermal history provides a greater knowledge into tailoring microstructural compositions to obtain various required mechanical properties for laser welding, additive manufacturing and also non-laser welding techniques. The feasibility of the Snapshot method is investigated in the three appended journal publications. High speed imaging and thermal recording have proved to be essential tools in this work, with analysis from optical microscopy and EDX/EDS to provide additional support. The Snapshot method is introduced as a concept in Papers I and II, demonstrating successfully guided thermal histories after obtaining molten material. Application of a second and third heating cycle, reheating the structure without melting, yielding altered microstructures. Reaching the austenitisation temperature range allowed for the simulation of complex multi-layer welding thermal histories. Geometrically non-uniform material additions are utilized in Paper III, which investigated the formation of microstructures through the chemical composition route. New chemical compositions were obtained by different degrees of dilution of the weld filler wire by the base material.

Additive manufacturing (AM), also known as 3D printing, has gained significant interest in aerospace, energy, automotive and medical industries due to its capabilities of manufacturing components that are either prohibitively costly or impossible to manufacture by conventional processes. Among the various additive manufacturing processes for metallic components, electron beam melting (EBM) and selective laser melting (SLM) are two of the most widely used powder bed based processes, and have shown great potential for manufacturing high-end critical components, such as turbine blades and customized medical implants. The futures of the EBM and SLM are doubtlessly promising, but to fully realize their potentials there are still many challenges to overcome. Inconel 718 (IN718) is a nickel-base superalloy and has impressive combination of good mechanical properties and low cost. Though IN718 is being mostly used as a turbine disk material now, the initial introduction of IN718 was to overcome the poor weldability of superalloys in 1960s, since sluggish precipitation of strengthening phases λ’/λ’’ enables good resistance to strain-age cracking during welding or post weld heat treatment. Given the similarity between AM and welding processes, IN718 has been widely applied to the metallic AM field to facilitate the understandings of process-microstructure-property relationships. The work presented in this licentiate thesis aims to better understand microstructures and mechanical properties EBM and SLM IN718, which have not been systematically investigated. Microstructures of EBM and SLM IN718 have been characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM) and correlated with the process conditions. Monotonic mechanical properties (e.g., Vickers microhardness and tensile properties) have also been measured and rationalized with regards to the microstructure evolutions before and after heat treatments. For EBM IN718, the results show the microstructure is not homogeneous but dependant on the location in the components, and the anisotropic mechanical properties are probably attributed to alignment of porosities rather than texture. Post heat treatment can slightly increase the mechanical strength compared to the as-manufactured condition but does not alter the anisotropy. SLM IN718 shows significantly different microstructure and mechanical properties to EBM IN718. The as-manufactured SLM IN718 has very fine dendritic microstructure and Laves phases in the interdendrites, and is “work-hardened” by the residual strains and dislocations present in the material. Mechanical properties are different between horizontally and vertically built samples, and heat treatment can minimize this difference. Results from this licentiate thesis provide the basis for the further research on the cyclic mechanical properties of EBM and SLM IN718, which would be the focus of following phase of the Ph.D. research. / <p>Information about opponent and seminar are missing.</p>

MCrAlY coatings (M=Ni and/or Co) have been widely used for the protection of superalloy components against oxidation and hot corrosion in the hot sections of gas turbines. The drive to improve engine combustion efficiency while reducing emissions by increasing the operation temperature brings a big challenge for coating design. As a result, the need for improvement of MCrAlY coatings for better oxidation resistance is essential. Formation of a stable, dense, continuous, and slow-growing α-Al2O3 layer, on the MCrAlY coating surface, is the key to oxidation protection, since the protective α-Al2O3 scale offers superior oxidation resistance due to its lower oxygen-diffusion rate as compared with other oxides. The ability of a MCrAlY coating to form and maintain such a protective scale depends on the coating composition and microstructure, and can be improved through optimization of deposition parameters, modification of coating surface conditions, and so on. Part of this thesis work focuses on studying the effect of post-deposition surface treatments on the oxidation behavior of MCrAlX coatings (X can be yttrium and/or other minor alloying elements). The aim is to gain fundamental understanding of alumina scale evolution during oxidation which is important for achieving improved oxidation resistance of MCrAlX coatings. Oxide scale formed on coatings at initial oxidation stage and the effect of surface treatment were investigated by a multi-approach study combining photo-stimulated luminescence, microstructural observation and weight gain. Results showed that both mechanically polished and shot-peened coatings exhibited superior performance due to rapid formation of α-Al2O3 fully covering the coating and suppressing growth of transient alumina, assisted by the high density of α-Al2O3 nuclei on surface treatment induced defects. The early development of a two-layer alumina scale, consisting of an inward-grown inner α-Al2O3 layer and an outer layer transformed from outward-grown transient alumina, resulted in a higher oxide growth rate of the as-sprayed coating. The positive effect of the surface treatments on retarding oxide scale growth and suppressing formation of spinel was also observed in oxidation test up to 1000 hrs. As the oxidation proceeds to the close-to-end stage, a reliable criterion to estimate the capability of coating to form α-Al2O3 is of great importance to accurately evaluate coating lifetime, which is the aim of the other part of the thesis work. Survey of published results on a number of binary Ni-Al and ternary Ni-Cr-Al, Ni-Al-Si systems shows that the empirical Al-concentration based criterion is inadequate to properly predict the formation of a continuous α-Al2O3 scale. On the other hand, correlating the corresponding Al-activity data, calculated from measured chemical compositions using the Thermo-Calc software, to the experimental oxidation results has revealed a temperature dependent, critical Al-activity value for forming continuous α-Al2O3 scale. To validate the criterion, long-term oxidation tests were performed on five MCrAlX coatings with varying compositions and the implementation of the Al-activity based criterion on these coatings successfully predicted α-Al2O3 formation, showing a good agreement with experiment results.

In order to evaluate the feasibility of forming Pure Mg – Pure Mg and Mg Alloy – Mg Alloy joints using soldering and brazing, four filler metals (Sn, Sn-9Zn, Sn-9Zn+X wt%Mg and Mg:Zn:X(Sn-9Zn)) were used at peak heating temperatures of ~250°C, 350°C and 450°C. Differential Scanning Calorimetry (DSC) and microstructure characterization were used to evaluate the successful joints. When using Sn as a filler metal, a joint forms, at temperatures as low as 235°C, which consists of a Mg-Sn eutectic filler layer and a continuous Mg2Sn intermetallic layer at the filler metal/Mg interface. This joint microstructure persists up to peak temperatures of 350°C and 450°C. The main effect of increasing the peak temperature causes an increase in the thickness of the Mg2Sn layer. Joints formed using Sn-9Zn prealloyed filler metal, developed a filler metal composition according to a Sn rich Mg-Sn-Zn ternary microstructure. A continuous Mg2Sn layer still formed in the joint. Ternary phase diagram predictions support the development of a number of MgxZny intermetallic compounds. However, only isolated experimental evidence of Mg-Zn liquid formation was present at peak heating temperature of 350°C and no evidence of MgxZny intermetallic layers was observed for both base metal systems using Sn-9Zn filler metal at 250°C and 350°C. Upon further heating above 350°C, a Mg7Zn3 layer forms which eventually leads to a Mg-Zn eutectic liquid formation as 450°C is reached. Wide gap Transient Liquid Phase Bonding (TLPB) was carried out by adding Mg to Sn-9Zn filler metal. Decrease in liquid volume fraction during solidification was noted with increase in Mg wt% as well as an increase in heating temperatures. Complete consumption of the filler metal was observed at 30wt% Mg and 450°C. Microstructural events were noted to be similar to Mg-Mg couples using Sn-9Zn filler metal. Mg-Zn, 50:50 wt% filler metal was thus used to evaluate joints without Mg2Sn intermetallic formation. Mg-Zn binary liquid phase formation was found to correspond to 342°C upon initial heating and presence of solid Mg7Zn3 in microstructure was found in samples heated to 350°C. Increase in diffusional solidification of Zn into the unreacted Mg powder and Mg base metal was noticed upon further heating to 450°C. Significant influence on the microstructure was noticed with addition of prealloyed Sn-9Zn powder to Mg-Zn filler metal mixture. Sn-9Zn was observed to be consumed in solid state during initial heating while alloying and dissolution of Pure Mg particles was seen to be accelerated. All Pure Mg powder was consumed indicating a large volume fraction present at 450°C in the case of filler containing 10% Sn-9Zn. The primary cellular grains of (Mg) grow from the Mg base metal substrate and Mg7Zn3 solidifies in the intercellular region. In the original filler metal portion of the diffusion couple, a mixture of (Mg), Mg7Zn3 and eutectic appears with a more dispersed character indicative of an equiaxed growth process. Promising results from 45Mg:45Zn:10(Sn-9Zn) wt% and peak heating temperature of 450°C could form basis for future work which can eventually lead to a commercial application.

Investigation into city image business cluster and citizen' joinning.

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A central focus of computational biology is to organize and make use of vast stores of molecular sequence data. Two of the most studied and fundamental problems in the field are sequence alignment and phylogeny inference. The problem of multiple sequence alignment is to take a set of DNA, RNA, or protein sequences and identify related segments of these sequences. Perhaps the most common use of alignments of multiple sequences is as input for methods designed to infer a phylogeny, or tree describing the evolutionary history of the sequences. The two problems are circularly related: standard phylogeny inference methods take a multiple sequence alignment as input, while computation of a rudimentary phylogeny is a step in the standard multiple sequence alignment method.Efficient computation of high-quality alignments, and of high-quality phylogenies based on those alignments, are both open problems in the field of computational biology. The first part of the dissertation gives details of my efforts to identify a best-of-breed method for each stage of the standard form-and-polish heuristic for aligning multiple sequences; the result of these efforts is a tool, called Opal, that achieves state-of-the-art 84.7% accuracy on the BAliBASE alignment benchmark. The second part of the dissertation describes a new algorithm that dramatically increases the speed and scalability of a common method for phylogeny inference called neighbor-joining; this algorithm is implemented in a new tool, called NINJA, which is more than an order of magnitude faster than a very fast implementation of the canonical algorithm, for example building a tree on 218,000 sequences in under 6 days using a single processor computer.

Silver nanowires (Ag NWs) have a wide range of applications in the electronic industry and are attracting growing world-wide interest because of their unique thermal, chemical, electrical and mechanical properties. Understanding of mechanical properties of Ag NWs and joining processes for them at a nano scale is urgently needed to support exploitation of their applications. Particularly, study of processing-structure-property relationships is of much significance. In the present thesis, the following research works were conducted. Ag NWs were synthesized using the polyol method. Joining of individual Ag NWs in an end-to-end orientation at room-temperature without assistance of external pressure was investigated. Selective surface activation of Ag NWs provided surface free of protective organic layers for metallurgical joining. A similar crystal orientation was maintained between the NWs, and diffusion along the boundary contributed to the nanojunction formation. Monocrystalline V-shaped or zig-zag silver prisms were formed after nanojoining, terminated by twin boundaries and free surfaces. The feasibility of room-temperature pressure-free joining of copper (Cu) substrates using Ag NW paste was conducted and demonstrated for flexible electronic packaging applications. The organic content in water-based Ag NW pastes was largely reduced by a repeated washing process to decrease the joining temperature. The formation of end-to-side or side-to-side joints between Ag NWs was observed concurrently with those joined end-to-end. The mechanical and electrical properties of Ag NW joints were examined. It was found that self-generated local heating within the Ag NW paste and Cu substrate system promoted the joining of Ag-to-Ag and Ag-to-Cu without any external energy input. The localized heat energy could be delivered in-situ to the interfaces and promoted atomic diffusion and metallic bond formation while the bulk component temperature maintaining near room-temperature. The organic layer on the side surfaces of the Ag NWs could be broken down through consumption of the residual PVP by a CuO-PVP reaction and which produced localized heating, increasing activated surface sites dramatically and making three-dimensional networks feasible. Ag NWs were introduced into Ag nanoparticle (NP) matrices joined at low-temperature. Joining was facilitated by solid state sintering of the Ag nanomaterials and metallic bonding at Cu-Ag interfaces. It was found that Ag NWs in a Ag NP matrix acted as a second reinforcement phase. In addition to improving the fracture toughness of joints, the introduction of Ag NWs affected the path of fracture propagation, where necking, breakage and pullout of Ag NWs occurred during loading.

In order to evaluate the feasibility of forming Pure Mg – Pure Mg and Mg Alloy – Mg Alloy joints using soldering and brazing, four filler metals (Sn, Sn-9Zn, Sn-9Zn+X wt%Mg and Mg:Zn:X(Sn-9Zn)) were used at peak heating temperatures of ~250°C, 350°C and 450°C. Differential Scanning Calorimetry (DSC) and microstructure characterization were used to evaluate the successful joints. When using Sn as a filler metal, a joint forms, at temperatures as low as 235°C, which consists of a Mg-Sn eutectic filler layer and a continuous Mg2Sn intermetallic layer at the filler metal/Mg interface. This joint microstructure persists up to peak temperatures of 350°C and 450°C. The main effect of increasing the peak temperature causes an increase in the thickness of the Mg2Sn layer. Joints formed using Sn-9Zn prealloyed filler metal, developed a filler metal composition according to a Sn rich Mg-Sn-Zn ternary microstructure. A continuous Mg2Sn layer still formed in the joint. Ternary phase diagram predictions support the development of a number of MgxZny intermetallic compounds. However, only isolated experimental evidence of Mg-Zn liquid formation was present at peak heating temperature of 350°C and no evidence of MgxZny intermetallic layers was observed for both base metal systems using Sn-9Zn filler metal at 250°C and 350°C. Upon further heating above 350°C, a Mg7Zn3 layer forms which eventually leads to a Mg-Zn eutectic liquid formation as 450°C is reached. Wide gap Transient Liquid Phase Bonding (TLPB) was carried out by adding Mg to Sn-9Zn filler metal. Decrease in liquid volume fraction during solidification was noted with increase in Mg wt% as well as an increase in heating temperatures. Complete consumption of the filler metal was observed at 30wt% Mg and 450°C. Microstructural events were noted to be similar to Mg-Mg couples using Sn-9Zn filler metal. Mg-Zn, 50:50 wt% filler metal was thus used to evaluate joints without Mg2Sn intermetallic formation. Mg-Zn binary liquid phase formation was found to correspond to 342°C upon initial heating and presence of solid Mg7Zn3 in microstructure was found in samples heated to 350°C. Increase in diffusional solidification of Zn into the unreacted Mg powder and Mg base metal was noticed upon further heating to 450°C. Significant influence on the microstructure was noticed with addition of prealloyed Sn-9Zn powder to Mg-Zn filler metal mixture. Sn-9Zn was observed to be consumed in solid state during initial heating while alloying and dissolution of Pure Mg particles was seen to be accelerated. All Pure Mg powder was consumed indicating a large volume fraction present at 450°C in the case of filler containing 10% Sn-9Zn. The primary cellular grains of (Mg) grow from the Mg base metal substrate and Mg7Zn3 solidifies in the intercellular region. In the original filler metal portion of the diffusion couple, a mixture of (Mg), Mg7Zn3 and eutectic appears with a more dispersed character indicative of an equiaxed growth process. Promising results from 45Mg:45Zn:10(Sn-9Zn) wt% and peak heating temperature of 450°C could form basis for future work which can eventually lead to a commercial application.

<p>Godkänd; 1991; 20080410 (ysko)</p>