This article is Part 2 of Welding Digest's series on aluminum welding. Read Part 1 here.

Fluidity, the reciprocal of viscosity, is the ability of a substance to flow easily. At the melting point, molten aluminum is five times more fluid than molten iron. As viscosity and its sensitivity to temperature (viscosity index) increase, fluidity decreases. High fluidity means molten aluminum will more readily run out of the weld pool during welding in a vertical or overhead position. Ideally, aluminum weld should be made in the horizontal position to avoid drips or oddly shaped beads. Welding technologies, filler alloys and processes designed to minimize heat input can reduce these problems and enable vertical position, or all-position, welding of aluminum.

Aluminum has different melting and solidification characteristics compared to steel. The melting point of pure aluminum, 660° C, is much lower than the melting point of steel or pure iron, 1538° C resulting in an aluminum’s increased burn-through propensity compared to steel. The fusion welding heat sources should be traverse quickly during welding to avoid burn-through during aluminum welding. Aluminum melts at such a low temperature, welders familiar with steel welding must grow accustomed to the lack of color changes.

Aluminum’s solidification shrinkage of 6% is twice that of steel, which increases tendency for crater cracking especially at the end of convex or lean weld beads. Crater cracking defects can be eliminated by laying down concave or plump weld beads and by back stepping during gas metal arc welding (GMAW). In GTAW of aluminum, the crater crack can be filled in by feeding a few additional drops into the weld bead.

While pure aluminum has a specific melting point, an aluminum alloy melts during heating or solidifies during cooling over a temperature range between the liquidus and solidus on the alloy’s phase diagram. An aluminum alloy with a wide melting range is highly prone to hot cracking (hot tearing or fissuring), liquation cracking (hot shortness) and solidification cracking (crater or center line cracking). When an alloy starts to solidify in the weld bead, the higher melting constituents solidify first and the remaining melt is enriched in lower melting point alloying elements.

Applied tensile thermal strains combined with localized melting at grain boundaries in the heat-affected zone (HAZ) in solidified weld bead and parent metal cause liquation cracking or hot shortness. An aluminum alloy’s sensitivity varies with the percentage of alloy element, with alloy exhibiting a peak sensitivity at certain alloy element percentages. Hot cracking in aluminum welds can be avoided by selecting a filler alloy to produce a weld bead composition with a low hot shortness sensitivity. Filler alloys with melting points near to the aluminum workpiece alloy’s melting temperature can reduce hot cracking. High welding speeds and concentrated heat sources (laser and GTAW) can reduce hot shortness problems by minimizing the length of time the weld is in the hot short temperature range and by decreasing the HAZ size.

Magnetic arc oscillations can also reduce hot cracking. Nucleating agent additions such as titanium, zirconium and scandium added to the filler alloy form weld beads with finer crystal sizes, which are inherently more resistant to hot tearing. Welding fixtures and clamping should be designed to encourage applied compressive stresses and avoid inducing tensile stresses in the weld bead and HAZ. Welds between different classes of aluminum alloys should use a filler alloy selection that avoids creating a weld bead composition prone to hot shortness. Arc current levels and joint geometry can be adjusted to provide dilution or mixing between the parent and filler alloy producing a composition with low hot cracking sensitivity.

Read the third and final part of this series, on thermal stress, here.