Effect of Repair Welding on Electrochemical Corrosion and Stress Corrosion Cracking Behavior of TIG Welded AA2219 Alumin
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I.
INTRODUCTION
AA2219 is a precipitation-hardenable alloy widely used in aerospace industries for the fabrication of cryogenic propellant tanks due to its high strength, good weldability, and stress corrosion resistance.[1–4] The gas tungsten arc welding process is used successfully for joining aluminum alloy plates and sheets during the fabrication process. It is well known that heat-treatable aluminum alloys exhibit variations in microstructure over the heat-affected zone (HAZ) as a result of heating and cooling cycles involved in the welding process.[5,6] This may affect the quality of the weld joint and the integrity of the propellant tank as a whole. Furthermore, weld defects (if any) that are identified by radiography are again repaired manually after removing the defective portions. This may further deteriorate the microstructure due to the reheating effect of repair welding.
Although this alloy in wrought form is known to have good stress corrosion cracking (SCC) resistance especially in T87 temper condition, the stress corrosion resistance of this alloy in the welded condition has not been studied in detail. Several studies have addressed the issue of partially melted zone (PMZ) cracking of this alloy due to eutectic melting along the grain boundaries in a very narrow region adjacent to the fusion zone (FZ).[7–11] The formation of such a zone was reported to result in a decrease in mechanical properties and premature cracking at the weld interface.[11] All of these factors can influence the SCC resistance depending on the microstructural variations and environmental severity. This study therefore is carried out to understand the effect of weld repair on the microstructural variations and SCC behavior of the weld joint.
II.
EXPERIMENTAL PROCEDURES
A. Material A. VENUGOPAL, Scientist, and K. SREEKUMAR, Group Director, are with the Material Characterisation Division, Vikram Sarabhai Space Centre, Thiruvananthapuram-695 022, India. V.S. RAJA, Professor, is with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai-400 076, India. Contact e-mail: [email protected] Manuscript submitted November 11, 2009. Article published online August 10, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
AA2219 aluminum alloy plates of 7.5-mm thickess in T87 temper condition (solution treated at 807 K (534 °C) for 1 hour) + 7 pct stretched followed by aging at 436 K (163 °C) for 24 hours) were welded by the ac-TIG auto welding method using AA2319 filler wire. The chemical compositions of the base metal (BM) and VOLUME 41A, DECEMBER 2010—3151
Table I.
Chemical Compositions of AA2219-T87 Aluminum Alloy and the Filler Wire Used in the Study Elements (Weight Percent)
Alloy
Cu Mn
Zr
V
Ti
Fe
Si
Zn
Al
AA2219-T87 5.95 0.27 0.1 0.09 0.06 0.12 0.05 0.02 bal AA2319 6.1 0.29 0.15 0.1 0.15 0.1 0.04 0.02 bal
Table II.
Process Parameters of Ac-TIG for the As Weld and the Repair Weld
Current Voltage Arc voltage Electrode diameter Configuration Torch speed Filler wir
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