.2195 Aluminum-lithium Alloy Welding Hot Cracking Susceptibility And Welding Materials

The aluminum-lithium (Al-Li) alloy 2195 is a candidate material for the next generation of launch vehicles and aero-engineering structures because of its low density and high strength. While considerable research efforts have been devoted to composition and properties of the base metal, little attention has been paid to the study of its weldability. The purpose of this thesis was to study the welding hot cracking susceptibility and welding materials of 2195 Al-Li alloy.Domestic 2195 Al-Li alloy plate, 2mm in thickness, was welded by argon tungsten-arc welding (GTAW). By using the transvarestraint test, the welding hot cracking susceptibility of 2195 alloy has been evaluated; and by houldcroft test, the possibility for improving the welding-cracking -resistance has been discussed. The fractography of cracking and microstructure of welded joint were systemically investigated by means of optical microscope, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and welding simulation technology.The results have shown that the critical strain is below 0.19%. As a result, the welding hot cracking susceptibility of 2195 alloy was larger than other aluminum alloys such as LC9CS andд16qчT . The welding hot cracking of 2195 alloy took place among the following locations: weld metal, fusion boundary equiaxed zone and weld pool crater, in which the weld metal and fusion boundary equiaxed zone (EQZ) were the most susceptible to the welding hot cracking. All of the cracks originated and propagated along the grain boundary or boundary between dendrites. This is associated with the eutectic constituent. The mechanism of welding hot cracking susceptibility of 2195 alloy can be described as the follows: during the weld metal solidifying, due to the solute element segregation between grains or dendrites, liquid film formed and wetted the boundaries, but the amount of the eutectic liquid was insufficient to heal those formed cracks. For this reason, 2195 alloy exhibited great weld cracking susceptibility.Microscopic examination of the joint microstructure has revealed that fine equiaxed grains zone (EQZ) existed adjacent to the fusion boundary within the fusion zone. Furthermore, the grain boundaries have an eutectic constituent, resulting in the severe welding hot cracking. The EQZ could not be eliminated by selecting any welding filler.Based on the composition of ER2319,two kinds of welding fillers were developed, that is Al-Cu-Ag-Mg and Al-Cu-Sc. The properties of joints welded by these developed welding fillers were contrasted to those of ER4043 welding material. The results have shown that the effects of different microalloying elements such as Sc or Ag+Mg on the mechanical properties were different. The Sc element could eliminate the dendrites of the as-welded weld metal, and cause grain refinement. As a consequence, the properties of welded joints with Sc as alloying element were found to be:σb > 330MPa. The optimum content of Sc element should be within the range from 0.14wt% to 0.32wt% for the mechanical properties of the joints.Taking account of the welding hot cracking susceptibility and the properties of welded joint, the developed Sc-containing welding material could be selected as filler metal for the welding of 2195 aluminum-lithium alloy. In addition, ER4043 can also be used as welding filler for 2195 alloy ,if the requirement for the joint properties is not very high.