Effect of Postweld Aging Treatment on Fatigue Behavior of Pulsed Current Welded AA7075 Aluminum Alloy Joints V. Balasubramanian, V. Ravisankar, and G. Madhusudhan Reddy (Submitted August 21, 2006; in revised form March 1, 2007) This article reports the effect of postweld aging treatment on fatigue behavior of pulsed current welded AA 7075 aluminum alloy joints. AA7075 aluminum alloy (Al-Zn-Mg-Cu alloy) has gathered wide acceptance in the fabrication of light weight structures requiring high strength-to weight ratio, such as transportable bridge girders, military vehicles, road tankers, and railway transport systems. The preferred welding processes of AA7075 aluminum alloy are frequently gas tungsten arc welding (GTAW) process and gas metal arc welding (GMAW) process due to their comparatively easier applicability and better economy. Weld fusion zones typically exhibit coarse columnar grains because of the prevailing thermal conditions during weld metal solidification. This often results inferior weld mechanical properties and poor resistance to hot cracking. In this investigation, an attempt has been made to refine the fusion zone grains by applying pulsed current welding technique. Rolled plates of 10 mm thickness have been used as the base material for preparing multipass welded joints. Single V butt joint configuration has been prepared for joining the plates. The filler metal used for joining the plates is AA 5356 (Al-5Mg (wt.%)) grade aluminum alloy. Four different welding techniques have been used to fabricate the joints and they are: (i) continuous current GTAW (CCGTAW), (ii) pulsed current GTAW (PCGTAW), (iii) continuous current GMAW (CCGMAW), and (iv) pulsed current GMAW (PCGMAW) processes. Argon (99.99% pure) has been used as the shielding gas. Rotary bending fatigue testing machine has been used to evaluate fatigue behavior of the welded joints. Current pulsing leads to relatively finer and more equi-axed grain structure in GTA and GMA welds. Grain refinement is accompanied by an increase in fatigue life and endurance limit. Simple postweld aging treatment applied to the joints is found to be beneficial to enhance the fatigue performance of the welded joints. Keywords AA7075 aluminum alloy, artificial aging, fatigue behavior, gas metal arc welding, gas tungsten arc welding, grain refinement, pulsed current welding 1. Introduction Many of the structural components in machines, pressure vessels, transport vehicles, earthmoving equipments, space- crafts, etc. are made of welded joints. The butt welds are the most common ones in the fabrication and construction of many structures. The wide application of butt welds in various structures including offshore and nuclear, gives large scope for the researchers to analyze the behavior under different types of loading conditions (Ref 1). Failure analysis of the weldments indicated that fatigue alone is to be considered to account for most of the disruptive failures. Even though the fatigue properties of the weld metal is good, problems can be caused when there is an abrupt change in section caused by excess weld reinforcement, undercut, slag inclusion, and lack of penetration, and nearly 70% of fatigue cracking occurs in the welded joints (Ref 2). There is growing interest in the structural use of aluminum alloys, for such applications as automotive and railway vehicles, bridges, offshore structure topsides, and high-speed ships. In all cases, welding is the primary joining method and fatigue is a major design criterion. However, as is well known, welded joints can exhibit poor fatigue properties. Thus, clear design guidelines are needed to ensure that fatigue failures are avoided in welded aluminum alloy structures. Apart from basic design of new structures, there is also increasing interest in methods for assessing the remaining lives of existing structures. Prompted by difficulties experienced in reaching a consensus on fatigue design rules, extensive testing and analysis of the fatigue performance of welded aluminum alloys have been undertaken over the past 10 years (Ref 3). Almost all the heat treatable aluminum alloys are unfortu- nately prone to hot cracking. The main problems in welding these alloys are: (i) hot cracking (solidification cracking) in the weld and (ii) excessive micro-fissuring due to hot tearing in the partially melted zone (PMZ) of the heat affected zone (HAZ). The heat treatable alloys, especially 2xxx and 7xxx series possess a substantial amount of copper and have a wide melting range with a low solidus temperature and is extremely sensitive to weld cracking when fusion welded. During solidification, there are two distinctive regions are formed and they are: (i) copper free zone and (ii) copper rich zone. The copper rich zone solidifies first, but copper free zone solidifies little later due to low melting temperature. Due to the difference in V. Balasubramanian and V. Ravisankar, Department of Manufacturing Engineering, Annamalai University, Annamalai Nagar 608 002 Tamil Nadu, India; and G. Madhusudhan Reddy, Metal Joining Section, Defence Metallurgical Research Laboratory (DMRL), Kanchanbag (PO), Hyderabad 560 058, India. Contact e-mail: visvabalu@yahoo.com. JMEPEG (2008) 17:224–233 ÓASM International DOI: 10.1007/s11665-007-9129-9 1059-9495/$19.00 224—Volume 17(2) April 2008 Journal of Materials Engineering and Performance