ABSTRACT. The effects of enhanced con- vection induced by a high-gravity envi- ronment on the resulting weld mi- crostructure of a 2195-T8 (Al-Cu-Li) alloy have been investigated. Stationary (spot) bead-on-plate gas tungsten arc welds were performed at 1, 5 and 10 g (1 g = 9.8 m/s 2 ) using the multigravity research welding system (MGRWS). Of particular interest was the gradual disappearance of a narrow band of fine equiaxed grains (EQ) located along the fusion boundary of the weld as g level increased. The presence of this equiaxed zone (EQZ) may affect weld mechanical properties and therefore compromise structures in- corporating welds of Al-Cu-Li alloys. The qualitative verification of a pro- posed mechanism for equiaxed grain for- mation along the fusion boundary of Al- Cu-Li alloy welds by Gutierrez and Lippold is also presented. This mecha- nism proposes that EQZ formation occurs by heterogeneous nucleation aided by Al 3 Zr and Al 3 (Li, Zr) precipitates in a stag- nant boundary layer located in the un- mixed zone of the fusion boundary layer. Here, thermal and fluid flow conditions are believed to be insufficient to sweep the precipitates into the weld pool, hence causing the formation of the EQZ. The high-g environment causing en- hanced convection is believed to alter the thermal and fluid flow conditions within the weld pool, thereby creating an environment in which there is neither a stagnant boundary layer nor an unmixed zone. Furthermore, the precipitates aid- ing in the precipitation of the fine, equiaxed grains are believed to be swept into the weld pool at high-g and com- pletely dissolved. As a result, the envi- ronment for equiaxed grain formation has been eliminated. The analysis of the microstructural evolution from 1 to 5 to 10 g qualitatively verifies this proposed mechanism. At 1 g, a prominent EQZ formed; at 5 g, the EQZ was scattered in location along the fusion boundary and of reduced width; at 10 g, the EQZ had completely disappeared leaving a near perfect line separating the large grains of the heat-affected zone from the fine den- drites of the fusion zone. Introduction Aluminum-lithium alloys represent an advanced development in high-perfor- mance, weight-saving aluminum alloys designed for aerospace, including, most recently, cryogenic applications for liq- uid hydrogen and liquid oxygen fuel tanks for launch vehicles. Promising fea- tures of aluminum-lithium alloys include advantages in strength and stiffness over conventional 2XXX- and 7XXX- series aluminum alloys. Major development of aluminum-lithium alloys began in the 1970s in an effort to introduce lower- density and higher-performance alu- minum alloys into aircraft structural com- ponents. This development led to the introduction of 8090, 2090 and 2091 commercial alloys in the 1980s, with the Weldalite 049 family the most recent de- velopment in aluminum-lithium technol- ogy (Refs. 1, 2). To take advantage of these promising features in structural applications, meth- ods of joining aluminum-lithium, partic- ularly welding, must be thoroughly in- vestigated and understood to maximize the structural capabilities of this light alloy. The Weldalite 049 family repre- sents a favorable alternative to both con- ventional aluminum alloys and other alu- minum-lithium alloys used in welded structures because of its good weldabil- ity, greater yield strengths and improved fracture toughness (Ref. 2). However, rel- atively little research has been performed on microstructural characterization and mechanical properties of welded alu- minum-lithium alloys, including the Weldalite 049 family, when compared to the level of research conducted on as- quenched and various heat-treated Al-Li and Al-Li-X alloys. This is particularly true with regards to novel welding processes, namely multigravity gas tung- sten arc (GTA) welds, which attempts to eliminate weld defects through en- hanced convection flow by means of in- ducing a high-gravity environment on weld geometry and solidification struc- ture. The development and implementa- tion of novel welding processes, such as a multigravity welding process, may lead to the use of Weldalite 049 and other alu- minum-lithium alloys in light armored vehicles, marine hardware and extensive space applications for small-size struc- tures and components (Refs. 1, 3). This paper discusses the qualitative verification of a proposed mechanism for equiaxed grain (EQ) formation along the fusion boundary of Al-Cu-Li welds pro- posed by Gutierrez and Lippold (Ref. 4). The findings of a microstructural charac- terization of multigravity spot GTA welds of 2195-T8 alloy will be discussed. The effects of enhanced buoyancy force on weld geometry, varying microstructure and orientation within the fusion and heat-affected zones and the gradual dis- appearance of an equiaxed band of fine grains located along the fusion boundary with increasing g-level will be addressed. Background Since the 1970s, a growing and signif- icant interest in aluminum-lithium alloys has occurred. This is primarily due to lithium’s unique ability to decrease the WELDING RESEARCH SUPPLEMENT | 349-s RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT Effect of Enhanced Convection on the Microstructure of Al-Cu-Li Welds BY D. K. AIDUN AND J. P. DEAN Microstructural analysis of welds made at different gravity levels reveal changes in the narrow band of fine equiaxed grains along the fusion zone KEY W O RD S Al-Cu-Li Alloys Weldalite GTAW Gravity Aerospace Cryogenic Gas Tungsten Arc HAZ D. K. AIDUN and J. P. DEAN are with Clark- son University, Potsdam, N.Y.