Desenvolvimento de metodologia experimental e modelagem matemática para a goivagem a plasma de aço estrutural naval

Abstract

Plasma gouging is a thermal process that removes metal by fusion and it is widely used in welding repairs, when the objective is to remove weld beads, cracks, porosity, localized corrosion or when some discontinuity is detected. During the fabrication of structures, construction and maintenance, repair demands may occur as a result of fabrication or maintenance, especially involving welding techniques. These demands can directly impact on time and costs of construction, assembly, and maintenance. Thus, in this work we developed an experimental methodology of the plasma gouging process in search of a survey of parameters to determine a mathematical model, in which it is possible to determine the output parameters, namely width and depth of removal, from the input parameters, which are: current, gouging speed (VG), contact tip to workpiece distance (CTWD), compressed air pressure and torch angle of attack, necessary for a purposeful removal of material in regions of interest in a welded joint. From this, in the first stage of the work, gouging was performed without oscillation of the torch, varying the current in three levels (60, 80 and 100 A) and gouging speed (with several values, the minimum is 5,1 and maximum is 37,3 mm/s), keeping fixed the pressure of compressed air (55 psi), CTWD of 1,5 mm and angle of attack of 45°. However, the average results obtained for width (5,64 mm) and depth (3,87 mm), were relatively small. Due to the variation in width and depth of the welded joint profiles, a search for gouging parameters that meet these variations was necessary. Thus, to obtain larger widths, it was decided to apply torch oscillation during the process. For this second approach, a Central Face Central Composite Planning was adopted, raised through statistical software, totaling 46 tests in which the variables were considered in three levels: current (60, 80 and 100 A), CTWD (5, 10 and 15 mm), width (10, 20 and 30 mm), air pressure (50, 55 and 60 psi), angle of attack (20, 32,5 and 45°) and VG (3, 4,5 and 6 mm/s). After the gouging, the widths and depths of each removal were measured, as well as their respective macrographs. The analysis of the results obtained was performed using statistical software, making it possible to define a mathematical model for estimating the width and depth of the gouging as a function of the parameters considered significant. Finally, a software code was developed, in which the desired width and depth values are inserted in the program, so that it delivers a suggestion of values for the parameters inherent to the process (current, VG, CTWD, amplitude, attack angle and compressed air pressure).