Análise e modelagem empírica do processo de soldagem a plasma com Keyhole em aço inoxidável

Abstract

The plasma welding process has been receiving a significant acceptance in the last few years for its use in automated applications, mainly in developed countries. It has stimulated scientific researches, which intend a better understanding of the physical phenomena involved in the process. Nowadays, there is a huge number of technical and scientific work available in the current literature, but there are some effects, which have still not been studied fully or present divergent information. For example, applications with chamfered V joints and welding of carbon steels are recommended in the literature, but these applications are not easy to be reproduced in production lines. There is also a lack of information related with those variables considered as secondary. Such variables can present a significant effect on the weld quality and process productivity, though. In recent work, empirical and theoretical models have been developed, which are often used to the prediction of the process behavior under certain welding conditions. As a matter of fact, model developments have been one of the main activities in the modern engineering. However, an adequate model depends on a wide and systematic study of the process variables effects. In order to do this, this work aims the verification of the effect of the process variables on the weld bead geometry in plasma keyhole welding of stainless steel and, considering these effects, to obtain mathematical models, which allow predicting the weld profile as a function of the welding conditions. These models can be also used to optimize the process based on its productivity. It was used a statistical technique, the Response Surface Methodology and the Similitude Theory to obtain the models. The results show the possibility of the development of mathematical models to predict the process behavior inside the operational ranges studied. Actually, it was verified that the Response Surface Methodology is more indicated when a few number of variables are studied. On the other hand, the Similitude Theory presented a easier means to obtain mathematical models, even when the number of variables is large. Besides, the equations allow a better understanding of the physical phenomena involved. Another interesting topic studied in this work was the verification of the use of the active flux layer technique, which was developed for the GTA welding, also in the keyhole plasma process. The results obtained indicate that the use of the traditional way of flux application tend to produce slag hard to be removed. In order to avoid this problem, it was xvi proposed a new technique to apply the flux on the pieces to be welded. The results with this new methodology were satisfactory, allowing to increase the productivity (depth of penetration)and ensuring a good weld bead surface finish (smooth and free of slag). This work represents a general study of the keyhole plasma welding of stainless steel AISI 304L and it can be used as support for further studies. The effect of the process variables on the weld bead geometry was assessed, since it represents the process robustness for this kind of application. Moreover, it is possible to obtain mathematic models for weld profile prediction under certain conditions. The techniques employed here to obtain the mathematic models, Response Surface and Similitude, obtained satisfactory results and they can be used for additional studies or modelling. The study carried out on the active-flux welding and on the new methodology proposed to apply the flux on the surface makes this kind of welding feasible and allows rising the process productivity without spoiling the weld surface finishing, which are observed in the traditional welding with active flux. All these results represent an advance in the analysis and the understanding of the fenomena involved in the keyhole plasma process.