Masters Theses

Date of Award

5-1995

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

Carl D. Lundin

Committee Members

Charlie R. Brooks, Raymond A. Buchanan

Abstract

The work presented investigated the effects of shield gas purity and gas purification on the behavior of gas metal arc welds and gas tungsten arc welds of important industrial alloys. A "Nanochem" resin-based purification system was used to purify the inert argon and helium shielding gases used . The effect of moisture (up to 300 ppm) and oxygen (up to 100 ppm) contamination deliberately introduced into the shield gas was investigated, and the subsequent benefit derivable from the use of "Nanochem" purification was assessed. The feasibility and effectiveness of using a newly patented design concept (Gas Injection) to prevent shield gas contamination in gas delivery systems of GMA and GTA torches was also evaluated. Nanochem gas purification when GMA welding aluminum 1100, 5556, and 6061 with shielding gas originally contaminated with moisture and oxygen resulted in significant improvements in weld surface appearance. In contrast, the surface appearance of GMA welds in 5086, 6061, and 1100 aluminum was significantly deteriorated when welding with contaminated gas. Similar findings were observed in the GTA welding of aluminum 1100. Nanochem purification also significantly reduced porosity in GMA welds of 5086 and 6061 aluminum, resulting in significant improvements in bend ductility. Corrosion properties were also significantly improved in Nanochem purified welds as determined by electrochemical polarization tests. However, in aggressive corrosive mediums, the beneficial effect of purification was lost. In autogenous GTA welds on AL-6XN alloy, Nanochem purification consistently and significantly offset the deleterious oxidation effects created by moisture and oxygen contaminated argon shield gas. Heat tinting was also totally eliminated. Corrosion resistance properties, as determined in various acidic and caustic media, although not quantitatively significant, were qualitatively better in Nanochem purified welds. In GTA weld deposits of 100-S high strength steel filler wire, Nanochem purification effectively reduced the diffusible hydrogen level by up to 20 ppm, when welding with moisture saturated gas lines. When the oxygen and moisture impurities were added to the shield stream, Nanochem purification reduced the diffusible hydrogen levels by 2 ml/100g. In autogenous GTA welds made on aluminum 1100, 316 and 304 stainless steel, titanium, and 255 duplex stainless steel, Nanochem purification was effective in preventing gross weld pool contamination and deleterious oxidation of as-welded surfaces caused by shield gas impurities. In the GMA welding of alloys such as HY-80 steel and 304 stainless steel, the deployment of Nanochem was ruled out because of its incompatibility with the necessary active shielding gas component (2% oxygen). Attempts to weld with inert gas devoid of any active components was deemed inappropriate due to degradation of arc characteristics and bead appearances. Similar, but not so serious effects were found in the GMA welding of duplex 2205 stainless steel using argon, which lacked the 2% recommended oxygen additive. GMA welded 2205 weldments qualitatively exhibited a higher corrosion resistance when shielded with argon-2% oxygen without Nanochem. Toughness properties, however, were significantly degraded in the welds shielded with the recommended argon-2%oxygen shield. In contrast, Nanochem-purified, originally contaminated gas was found to exhibit the highest toughness properties. The effect of moisture up to 300 ppm in the argon shield gas did not seem to degrade the toughness, bend ductility, and the corrosion behavior of the 2205 duplex stainless steel weldments. No problems with the purity of the bottled gas cylinders were experienced throughout the entire work. Welds made with bottled gas were comparable to welds made with Nanochem purified gas. In field situations however, gas purity may not always be satisfactory due to cylinder-to-cylinder variations, manifold cross-contamination problems, mositure permeation into gas plumbing, and/or minute piping leaks. On the basis of the test results, the use of Nanochem is highly recommended in such situations to eliminate such problems that may cause weld contamination. "Gas-injection" modification to prevent air entrainment in the automatic GTA weld torch resulted in regular shield stream disruption. Hence, it is not recommended for torches of this general configuration. In the GMA torch used in this study, a new concept called "dual-injection" was implemented in conjunction with "gas-injection" modifications to prevent air entrainment in the wire carrying conduit of the welding system.

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