Thèse de doctorat : Sciences et ingénierie des matériaux : Vandoeuvre-les-Nancy, INPL : 2005. / Titre provenant de l'écran-titre. Bibliogr.

Thèse de doctorat : sciences. Mécanique des fluides : Ecully, Ecole centrale de Lyon : 2006. / Titre provenant de l'écran-titre. 81 réf.

Since 1987 Uddcomb Engineering has repaired pulps by their own developed overlay welding method even called Uddcomb method. Currently each welding machine is operated by two persons. To increase Uddcomb Engineering competitiveness the reduced number of operators is desired. An installation of a monitoring system which can aid humans in the welding quality control also helps to improve company’s position. A future goal would be to make this monitoring system automatic without a human operator in the loop. In this thesis, arc voltage, weld current and audio signals were collected and analyzed with aim on finding algorithms to monitor the quality of the welding process. The use of statistics tools is the basis for detecting variations in the voltage and current data, associated with welding process. It has been shown that voltage signal can be used as a part of the welding quality control. The audio signal from welding at low frequencies varies with the speed of the process. The signal can also be incorporated in the monitoring of the process. The use of filters, growing sums and statistics are key elements in the algorithms presented in this report.

Vacuum arc remelting is a secondary melting process used to produce a variety of segregation sensitive and reactive metal alloys. The present day VAR practice for superalloys involves, typically, melting electrodes of 17'' into ingots of 20'' in diameter. Even larger diameter forging stock is desirable. However, beyond 20'' ingots of superalloys are increasingly prone to segregation defects if solidification is not adequately controlled. In the past years a new generation of model-based controllers was developed to prevent segregation in VAR by controlling melt rate, or the total amount of power flowing into the liquid pool. These controllers were seen as significant improvements in the industry of remelting processes, but these controllers were still focusing on the melting sub-process and ignoring ingot solidification. Accurate control of the liquid pool profile is expected to result in segregation-free ingots, but unfortunately a controller capable of stabilizing the solidification front in VAR is currently not available. The goal of the proposed research is to develop a cyber-enabled controller for VAR pool depth control that will enhance the capabilities of current technologies. More specifically, the objectives of this research are threefold. Firstly, a control-friendly model is proposed based on a high-fidelity ingot solidification model and is coupled to a thermal model of electrode melting. Secondly, sequential Monte Carlo estimators are proposed to replace the traditional Kalman filter, used in the previous VAR controllers. And finally, a model predictive controller (MPC) is designed based on the proposed reduced-order model. The time-critical characteristics of these methods are studied, and the feasibility of their real-time implementation is reported. / text

This thesis presents a study of a pulsed distributed arc plasma deposition method that has been developed to produce highly ionized pulsed plasma plumes of metallic species in the presence of a low-pressure inert or reactive gas glow discharge. A pulse-forming network (PFN) is used to form a transient electrical discharge in a hollow electrode which is triggered by two different methods; a pulsed CO2 laser or a pulsed high voltage glow discharge. With the PFN charged to a voltage of 70 - 100 VDC, current pulses with peak currents up to 3 kA and pulse widths as long 3.7 milliseconds have been reached. A detailed treatment of the influence of process parameters, such as the PFN discharge energy and ambient gas pressure and type, on the plasma properties is presented. These experiments also demonstrated a higher on-axis growth rate of carbon in an ambient of nitrogen than in argon. The higher argon mass leads to broader plasma expansion producing broader deposition profiles which results in lower on-axis growth rates. Deposition rates of 3.5 angstrom/pulse for carbon and 2.1 angstrom/pulse for titanium have been achieved. Thickness profiles and the morphology of carbon films and titanium films deposited by this method, which utilize the energetic advantage of ions in film formation allowing reduced substrate temperatures and good adhesion, are presented.

Vacuum Arc Remelting (VAR) is a secondary process used for homogenization of high-melting-point and oxygen-sensitive materials such as superalloys and titanium alloys. The VAR process is carried out with the aim of melting a large consumable electrode in such a way that the resulting ingot has improved homogeneity. The Specialty Metals Processing Consortium (SMPC) has spent the past 20 years developing technology to improve control over the final ingot remelting and solidification processes to alleviate conditions that lead to the formation of inclusions and segregation. Channel segregates are concentration defects arising during the solidification of large-diameter solute-rich alloys. As manufacturers for turbine engines and generators call for larger ingots, it becomes more difficult to produce them without these defects. If, however, liquid pool depth can be controlled precisely to stabilize the solidification zone in the ingot, we could, in principle, produce larger ingots that are defect free. A problem arises because measurements obtained from the VAR furnace do not give enough information to accurately estimate the liquid pool shape in dynamic melting situations. Also, the solidification process in VAR is extremely complex due to the multiple physical domains present and a high-fidelity model is required to give an accurate description of the dynamic process. The Basic Axisymmetric Remelting (BAR) code was initially developed by Lee Bertram at Sandia National Laboratories as a high-fidelity multi-energy model to describe ingot casting in this system. In this work we present a new strategy to improve the accuracy of the estimates used in the control system. This strategy consists of implementing BAR as a new set of measurements to be used by the estimator. This new strategy was used in tests jointly sponsored by SMPC and Los Alamos National Laboratory (LANL) in February 2011 using a laboratory-scale furnace and alloy 718 electrodes. / text

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