Al-Mg-Si billets with high extrudability – state of the art and beyond

Formation of intermetallic particles during solidification and homogenisation of two Al–Mg–Si alloys

“…Finally, after homogenisation the large constituents were slightly more rounded than before, pointing at a spheroidisation of the constituents during high-temperature soaking (e.g. [20][21][22][23]28]). Analysis of the chemical composition of the constituent phases by means of EDS showed that after homogenisation several of the species observed in the as-cast state, including Mg 2 Si, Si, the Cu-bearing eutectics as well as π -Al 8 FeMg 3 Si 6 , largely disappeared.…”

“…These two consecutive reactions of precipitation and re-dissolution are reflected in the strong drop in specific electrical resistivity displayed in Figure 12(a). At further increasing temperatures above 500 °C the volume of the β-AlFeSi phase begins to shrink and is continuously replaced by the α-Al(Fe,Mn)Si phase, again pointing at the β-AlFeSi to α-Al(Fe,Mn)Si phase transformation of AA 6xxx alloys [20][21][22][23][24][25][26][27][28]. At variance with alloy AA 6005C, this phase transformation seems to be finished already at the beginning of the soaking period, however.…”

“…The experimental investigations have shown that during homogenisation annealing the fraction of the Mn-bearing α-Al(Fe,Mn)Si phase increased at the expenses of both α-AlFeSi and β-AlFeSi (Figure 9(a)) which is clearly visible in the ClaNG simulations (Figure 13). To a large part, this effect is attributed to the β-AlFeSi to α-Al(Fe,Mn)Si phase transformation of the constituent particles which is well-documented in AA 6xxx series alloys [20][21][22][23][24][25][26][27][28]. Furthermore, at high homogenisation temperatures and, especially, during cooling back to the sub-homogenisation temperature there is the formation of a certain fraction of α-Al(Fe,Mn)Si phase dispersoids (section "Changes in microstructure and microchemistry during homogenisation").…”

“…These unfavourable effects can be leveraged by suitable homogenisation practices before hot rolling, which typically involve soaking for several hours at high temperatures in the range of 540-580 °C (e.g. [20][21][22][23][24][25]). During high-temperature annealing microsegregations of Mg and Si are equalised and soluble Mg 2 Si precipitates are dissolved which facilitates agehardening at the final gauge.…”

“…A very important metallurgical phenomenon occurring during homogenisation of AA 6xxx series alloys is the transformation of primary, platelike β-AlFeSi constituents into several more rounded α-AlFeSi and α-Al(Fe,Mn)Si particles which are less detrimental to the properties of AA 6xxx series semi-finished products. This phase transformation of the constituent particles is alleviated by small additions of Mn and by higher soaking temperatures [20][21][22]25]. Finally, homogenisation annealing is accompanied by the formation of dispersoids of the type α-Al(Fe,Mn)Si or, when the alloy contains Cr, α-Al(Fe,Mn,Cr)Si with sizes of 0.1-0.2 μm.…”