Mid Sweden University

Surface defects found during air-slip direct chill casting are today a major quality factor for extruded aluminum, as they can cause increased segregation, pores and unwanted precipitations. The surface zone in billets of the aluminium alloys 6063, 6005 and 6082 have been analysed by metallographic methods and by chemical analysis. Surface defects, of the type vertical drags, were investigated and compared to defect free surfaces. Inverse segregation to the surface was quantitatively analysed. The concentration profiles were coupled to the appearance of the defects and to microstructures from corresponding areas. It was shown, for vertical drags on 6005 billet surfaces, that either the segregation depth or the precipitated particles were different from surface areas without defects. For the 6063 alloys on the other hand the vertical drag zones contained different particles than surfaces without defects and the segregation had noticeably increased.

Using the Bridgman technique, the solidification of aluminum alloys of types 6005, 6063, and 6082, was studied. The solidification process differs both between the alloys and between billet surface and bulk, due to higher concentrations of Fe, Si, Mn and Mg near the surface. Previously determined surface concentrations were used to calculate the Fe, Si, Mn and Mg additions needed for Bridgman experiments that simulate surface region solidification. Microstructures were studied and grain size, secondary dendrite arm spacing, intermetallic particles, segregation, and coarsening were evaluated. The increased alloy element concentrations at the surface were found to influence both the structure coarseness and the kind of intermetallic precipitation. In addition, clear differences could be determined between the alloy types, depending on their pull rate in the furnace.