Investigation of trace elements in an Al-4.8 wt.% Mg-0.3 wt.% Mn alloy

Sheets of alloy 5182 are commonly made by direct chilled (DC) casting and subsequent homogenisation and hot rolling. One major problem is cracking of the edges during hot rolling. This is a microstructure investigation of sodium and other trace elements, which have been found to contribute to edge cracking, by means of microprobe analysis and SIMS. Microprobe analysis revealed that the DC-castings had an outer zone ∼0.3 mm wide that was rich in alloying elements. The main intermetallic phases in this zone are Al6(Mn,Fe), Al3(Mn,Fe,Cu), Al8(Mg,Cu)5, a smaller amount of copper-rich phases and Mg2Si. The bulk material consists of small grains with mainly Al6(Mn,Fe) particles on the grain boundaries. Microsegregations of Si, Ga, Mg and Cu towards dendrite- and grain boundaries were detected. The SIMS analyses revealed that the DC-castings had a surface layer 3–7 µm thick that was rich in salts. The latter were particles, mainly (Na,K)F polluted with Cl, Ca, Li and possibly H. The content of salts in the surface was of the order of 1 wt.%. Small amounts of salts, mainly as fluorides, were present down to depths of 10–30 µm. Small numbers of Na- and (Na,K)-particles were found as separate particles and on the surface of other intermetallic particles. The homogenised and hot-rolled material had much less microsegregation of copper and virtually no segregation in gallium, magnesium and silicon. A large number of Mg2Si and a smaller number of (Na,K)-particles were detected. The latter seemed to be heterogeneously nucleated on intermetallic phases. A possible model is that the Na- and (Na,K)-particles form thin layers on other phases and melt above 100 °C, resulting in a larger tendency of edge cracking of alloy 5182. Copyright © 2000 John Wiley & Sons, Ltd.