The effect of thermal exposure in the 500 – 600 C range is dissolve the precipitate formed during casting, as can be seen by microscopic, electron probe and electrical resistivity data. The hold temperature in critical factor in determining extrudability and surface appearance of extrudate by converting B(Al,Fe,Si) to the less detrimental a-phase. The B to a transformation involves loss of Si from B to the matrix which renders the a-iron phase less brittle and also provides a greater amount of Mg2Si for subsequent precipitation. The iron phase transformation is complete afther 6 h at 565 C. The introduction of small amounts of Mn moves the a(Al,Fe,Si) phase field in direction of lower Fe:Si ratio thus accelerating the B to a transformation. The initial transformation occurs with no shape change but continued exposure produces a gradual spheroidization of the dispersoid which is