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 s ubsequent homogenisation and hot rolling. One major problem is cracking of the edges during hot rolling. This is a microstructure investigation of sod ium and other trace elements, which have been found to contribute to edge c racking, by means of microprobe analysis and SIMS, Microprobe analysis revealed that the DC-castings had an outer zone similar to 0.3 mm wide that was rich in alloying elements. The main intermetallic phases in this zone are Al-6(Mn,Fe), Als(Mn,Fe,Cu), Al-8(Mg,Cu)(5), a small er amount of copper-rich phases and Mg2Si, The bulk material consists of sm all grains with mainly Al-6(Mn,Fe) particles on the grain boundaries. Micro segregations of Si, Ga, Mg and Cu towards dendrite- and grain boundaries we re detected. The SIMS analyses revealed that the DC-castings had a surface layer 3-7 mu m thick that was rich in salts. The latter were particles, mainly (Na,K)F p olluted 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, w ere present down to depths of 10-30 mu m. Small numbers of Na- and (Na,K)-p articles were found as separate particles and on the surface of other inter metallic particles. The homogenised and hot-rolled material had much less microsegregation of c opper and virtually no segregation in gallium, magnesium and silicon. A lar ge number of Mg2Si and a smaller number of (Na,K)particles were detected. T he latter seemed to be heterogeneously nucleated on intermetallic phases. A possible model is that the Na- and (Na,K)-particles form thin layers on ot her phases and melt above 100 degrees C, resulting in a larger tendency of edge cracking of alloy 5182, Copyright (C) 2000 John Wiley & Sons, Ltd.