| 2195 Al-Li alloy apparently offers significantly higher strength to weight ratio than the 2219 aluminum alloy. It was discovered that 2195 Al-Li has a greater tendency to crack, generates peculiar kind of porosity, and is vulnerable to deleterious microparticulate emission during welding than its 2219 predecessor. An experimental investigation has been carried to characterize these weld imperfections in 2195 Al-Li alloy.; This work presents a scientific account of an analytical study and of the clues it has provided towards an understanding of the weld imperfections in 2195 Al-Li welds.; The study begins with the observation of peculiar pore formation in 2195 welds, which occurs not as in the case of 2219 welds upon solidification, but in a thermal ageing process subsequent to solidification. An apparent reaction (DTA) between the fusion zone dendritic surface and nitrogen gas implies a porous fusion zone. Tiny surface melting sites, designated as Blisters, due to its resemblance to skin blisters, testify to the conjunction of outgassing and melting effects and suggest that porosity formation in the solid phase depends upon local melting as well as outgassing. The absence of a dark magnesium rich substance, designated as smut in the immediate vicinity of a crack opening next to a weld repair bead implies either an umbrella of gas emission keeping off a condensate evaporated under the welding arc or, possibly an expulsion of atomized, liquified metal from the crack itself in the form of microparticulate emission. These microparticulate emission from VPPA welds takes various forms herein labeled as smut, snow, and Lava. It is attributed to a gas generating reaction taking place at molten grain boundaries or crack surfaces. The reaction could only be release of hydrogen displaced from lithium hydrides by a coming influx of dissolved nitrogen. There appears to be a close link between porosity, cracking and microparticulate emission. Observations of melting on the surface of repair and E-stop cracks suggest interdendritic melting as the main factor responsible for cracking during welding. Heating in nitrogen reveals a weight loss (TGA) characteristic of an outgassing process before the weight gain thought to be associated with a nitrogen reaction takes over; hence the outgassing process, whatever it is, is thought to be independent of the nitrogen reaction. If the nitrogen contribution to porosity generation is then assigned to the promotion of local melting, the fusion zone fracture of laser weld beads subject to residual stress and heated under nitrogen atmosphere, but not under vacuum nor helium, is explicable. If this has been accomplished, cracking during welding is understood, and control procedures should be implicit in the understanding. (Abstract shortened by UMI.)... |
