EFFECT OF WELDING PARAMETERS ON CHARACTERISTICS OF PULSE CURRENT GAS TUNGSTEN ARC WELDED STEEL SHEETS

Abstract

Welding of thin sheets is more difficult than thick sections as control over the heat input is most necessary task in thin sheets. Conventional GTAW provides higher heat input which in turn leads to various weld defects. Pulsed GTAW process in which welding current fluctuates in between two levels- Peak current and base current gives better quality of weld in all aspects when welding thin sheets by reducing the overall heat input thus diminishes the possibilities of the defects in the welds than conventional GTAW process. . The maximum current is selected to give adequate penetration and bead contour, while the minimum is set at a level sufficient to maintain a stable arc and to allow the cooling to the weld pool. This permits arc energy to be used efficiently to fuse a spot of controlled dimensions in a short time producing the weld as a series of overlapping nuggets. By contrast, in P-GTAW welding, the heat required to melt the base material is supplied only during the peak current pulses allowing the heat to dissipate into the base material leading to a narrower heat affected zone (HAZ). Advantages include improved bead contours, greater tolerance to heat sink variations, lower heat input requirements, reduced residual stresses and distortion, refinement of fusion zone microstructure, and reduced width of HAZ. So during High speed and high current PGTA welding the main task is to select the proper pulse and other welding parameters to avoid the defects in the weld. As Arc pressure and gas shear can be controlled to avoid humping in the weld. Due to appearance of weld defects in high speed PGTA welding, parameter selection becomes extremely tidy process while manually selecting the pulse parameters in Pulsed GTAW welding by screening experimentation technique. Effect of Mean Current and Welding Speed on mechanical properties, Micro-structure and weld Bead Geometry of autogenously welded square butt joints in 2-mm thin sheets of Low Carbon Steel AISI 1008 and 304 austenitic stainless steel considering pulsing effect to be constant using hypothetically derived dimensionless factor 0 = [(I&'Ip)ftb] [28] which can be used to summarize the influence of pulse parameters, in Pulse current Gas tungsten arc welding process. To make the P-GTAW welding more economic, welding speed can be increased which can lead to enormous amount of monetary savings in welding industries per year. Increment in welding speed has to overcome by increasing the mean current to maintain the proper weld geometry. But at high mean current and speed, various defects like Humping, parallel humping, Split bead, tunneling and segregation of weld metal intermittently in the weld pool are found to be common that can limits the use of higher welding speed. . Increase iii in welding speed has to be matched by increase in heat input in order to maintain the proper weld bead geometry. The most practical way to increase the Heat input is to increase the welding current because the Arc voltage and the Arc efficiency vary moderately. Mechanism of humping in thin sheets is broadly discussed and various methods to improve weld quality at higher speed and to avoid humping are suggested in this work. Premature solidification of gouging region, rims of molten metal and trailing region is to be considered the mechanism of humping [29]. Arc pressure which is the main driving force for defect generation at high speed and high mean current as it varies exponentially with mean current is suppressed by various methods in the P-GTAW process. Transmission electron microscopy has also been used to study the dislocation density in austenitic stainless steel 304 at low speed-low mean current and high speed-high mean current. Weld bead geometry like area of fusion was also studied with variation in welding speed. Mechanical testing like macro hardness, tensile test were also performed to study the quality of weld at high speed and high mean current by putting the heat input, arc length, flow rate and type of shielding gas and geometry of the weld to be constant. In each condition Pulsing effect is kept constant separately using hypothetically derived dimensionless factor = [(liulp)tii,] [28]. Quality of the weld varies even after putting many variables constant. So for comparing the different welds, some samples are chosen from all experimental data which also shows geometric similarity like equality of fusion zone area, dipping depth(at high Im due to high arc pressure) size, reinforcement height etc and similar porosity content calculated by volume fraction method. Increasing the welding speed and Mean current beyond a certain limit, undercut defects are found to be fairly unavoidable. So only those samples are grouped together for comparison which either having no undercut or same depth of undercut. Selection of samples is done for making comparison realistic and meaningful. Optical emission spectrometer is used for verification of composition of the steel sheets. Microstructure analysis were also performed using optical microscopy and variation in solidification is observed together with the change in grain type and their orientation as per the change in welding speed and rate of solidification. Width of heat affected zone and their microstructural change is also explained with change in welding speed. Calculation of porosity is also done by using standard volume fraction method on polished samples without etching.