Electron beam welding characteristics of high strength aluminium alloys for express train applications

Abstract

The basic characteristics of electron beam (EB) welding of high strength aluminium alloys for express train applications were evaluated. The aluminium alloys tested were non-heat treatable A5083–O, heat treatable A6N01–T6, and A7N01–T6. Principal welding process parameters, such as accelerating voltage, beam current, welding speed, and chamber pressure were investigated. The dimensions and microstructures of welds were evaluated using optical light microscopy and SEM (EDAX). In addition, variation in weldability (in terms of formation of cracking and porosity) owing to process parameters was evaluated. Electron beam welds showed discontinuities such as cracks, cold shuts, porosities, and spikes; the tendency to form weld discontinuities was strongly dependent on the EB process parameters and chemistry. Although the three aluminium alloys were welded using the same conditions, alloying elements had an important effect on the dimensions of the weld and thus on weldability. Alloy A6N01 showed a lower depth than the other alloys and variation in the weld depth was found to be sensitive to the vaporisation tendency of the alloying elements. Silicon, which is a major element of A6N01, is more difficult to vaporise than other elements (such as aluminium, magnesium, and zinc). The degree of cracking in the EB fusion zone appeared to be affected mainly by aspect ratio (depth/width), such that as aspect ratio increased the cracking tendency also increased. The alloying element itself may also affect the hot cracking resistance, but its role is considered to be an indirect effect, such that the relatively higher vaporisation pressures for zinc and magnesium give deeper weld penetration and thus result in a greater tendency towards cracking.