Paulo Villani Marques

Statistical modelling of narrow-gap GTA welding with magnetic arc oscillation

Abstract Statistical experimental design and linear-regression modelling were used to study the effect of the welding parameters on the bead shape in a narrow gap-GTAW process with magnetic are oscillation. It was shown that, for the conditions studied, are oscillation has little influence on the lateral fusion of the joint, although it favourably influences the shape of the bead. The undercutting level tended to increase rapidly when the gap width was reduced. Optimized welding conditions are presented, based on models calculated from the experimental data.

An SEM-Based Method for the Evaluation of the Cavitation Erosion Behavior of Materials

The ultrasonically induced vibratory cavitation test is used to compare the performance of materials against the cavitation erosion. This article describes a new method to evaluate the results of such tests, using scanning electron microscopy.

Estudo comparativo da resistência ao desgaste abrasivo do revestimento de três ligas metálicas utilizadas na indústria, aplicadas por soldagem com arames tubulares

As ligas metalicas aplicadas por soldagem em superficies, objetivando a protecao contra o desgaste e o consequente aumento da vida util de pecas e equipamentos, tem sido utilizadas em larga escala nas industrias de bens de consumo e nos setores de mineracao e sucroalcooleiro. O desgaste abrasivo em pecas e equipamentos representa, nestas industriais, um dos principais fatores de depreciacao de capital e uma importante fonte de despesas com manutencao. Para a aplicacao do revestimento por soldagem, os arames tubulares tem sido uma alternativa cada vez mais viavel, devido a sua alta produtividade e qualidade de solda, substituindo, em parte, o uso do eletrodo revestido. O objetivo deste trabalho e fazer um estudo comparativo da resistencia ao desgaste abrasivo do revestimento aplicado por soldagem com arames tubulares autoprotegidos de tres ligas metalicas utilizadas na industria, uma do tipo Fe-Cr-C, outra do tipo Fe-Cr-C com adicao de niobio e boro e a terceira, do tipo Fe-Cr-C com adicao de niobio. Os revestimentos anti-desgaste, conhecidos como revestimento duro, foram aplicados em chapas de aco carbono, com os mesmos parâmetros e procedimentos de soldagem. Os corpos de prova foram obtidos por corte e retificacao e foram submetidos a ensaios de desgaste abrasivo, em um abrasomero Roda de Borracha, conforme procedimento estabelecido pela norma ASTM G65-91. Os resultados obtidos demonstraram que a liga Fe-Cr-C com adicao de Niobio e Boro apresentou desempenho superior em relacao ao desgaste abrasivo.

Algumas equações úteis em soldagem

Most welding processes are characterized by a large number of variables, many of them related to each other by complex and mostly unknown relationships. This makes it rather difficult to develop mathematical models, to simulate different aspects of welding, and to preview the results of a welding procedure beforehand. As a result, a great number of empirical, or semi-empirical, equations have been developed by many authors since welding has been of some industrial importance. Many of those models can be useful to welding professionals but they are generally scattered in many different papers and in other publications. In this paper, an attempt is make to put together some equations related with different aspects of the welding technology. It is expected that this could assist welding professionals to resolve some problems in their daily work. The meaning of the different terms in those equations, their units, and values of some of the constants present are shown. Equations are grouped according with their application and, when it was possible, an attempt was made to unify the terminology and units used by the different authors.

Comparative study of the wear resistance of three metal-fluxed core wire-welded coatings used in industry

Metal alloys deposited by welding on the surface of components with the objective of protection against wear and resulting in an increase in the lifetime of parts and equipment have been used extensively in the consumer products industry and sectors of mining and sugar–ethanol. The abrasive wear on parts and equipment represents one of the main depreciation factors of capital and the major cause of expenditure of maintenance in industries. To apply coating by welding, fluxed core wire has been a viable alternative because of its high productivity and high-weld quality, replacing in part the use of the stick electrode. The objective of this paper is to make a comparative study of the abrasive wear-resistant coating deposited by welding with self-shielded fluxed core wires of three metal alloys used in industry, firstly Fe–Cr–Co alloy, secondly Fe–Cr–C alloy with niobium and boron addition[s], and thirdly Fe–Cr–C with niobium addition. Wear-resistant coatings, known as hardfacing, were deposited on carbon steel plates with the same welding parameters and procedures. Samples were obtained by cutting and grinding, and were subjected to abrasive wear tests in a Rubber Wheel apparatus according to the procedure established by ASTM G65-91. The results showed that Fe–Cr–C alloy with niobium and boron addition[s] (presented superiority in terms of wear resistance) was superior in terms of resistance to wear.

Tig arc characteristics in different atmospheres (Part 1)

Summary The physical properties of gases such as density, thermal and electrical conductivity, as well as ionisation potential, determine to a great extent the operational characteristics of welding arcs. These properties may vary widely for different gases. As a result, arc voltage, heat output, mean radius, thermal profile, performance and other parameters which influence bead geometry (shape, width and penetration) depend on the chemical composition of the shielding gas. Rather complex physical models have been proposed to explain and quantify the effects of shielding gas chemical composition on welding arc performance. The influence of arc atmosphere may also be easily predicted by empirical methods. However, only changes in arc operational parameters and weld bead geometry due to variations in gas composition appear in the literature. This article presents an optical study of the TIG welding arc, based on a digital system of image capture and welding parameter recording, during arc operation. Variations in arc aspect, dimensions and electrical parameters were observed, and the results related to the physical properties of gases and mixtures used, as well as forecasts by theoretical models.

Arc fusion of self-fluxed nickel alloys

Self-fluxed nickel alloys are usually flame fused after thermal spraying. However, due to the practical aspects of high temperatures reached during flame fusing, large structures such as the hydraulic turbines for power generation, can not be efficiently coated. An alternative is to fuse the sprayed coating with a gas tungsten electric arc. In this case, heating is much more intensive and substrate temperature during and after the fusing operation is much lower, thus reducing the possibility that any problem will occur. In this work, coatings of self-fluxed nickel alloy fused by flame and gas tungsten arc were evaluated as protection of hydraulic turbines against cavitational damage. Several tests were performed, including the ASTM ultrasonically vibration-induced cavitation, optical and scanning electronic microscopic metallography, and hardness tests. The results showed that the arc-fused coating presented better cavitation damage resistance, probably due to its finer microstructure. A field application of this new technique is also described. A self-fluxed Ni alloy was flame sprayed in critical regions of Francis-type hydraulic turbine blades and fused by a gas tungsten arc after spraying. The blades will be inspected during the next two years.