The influence of the various elements normally found in 6xxx alloys are:
1. Magnesium lowers extrudability by significantly increasing maximum extrusion pressure regardless of silicon content, increases quench sensitivity for strength, increases strength and lowers both ductility and toughness.
2. Silicon lower extrudability if in excess although it is not as influential as magnesium. It has a minor influence on quench sensitivity for strength. It is a good strengthener, especially up to about 0.4%. With a high ratio of Si/Mg2Si, the alloy extrudes well, has lower quench sensitivity and higher strength compared to low excess silicon combinations. Silicon will diminish both ductility and toughness, by precipitating elements silicon on grain boundaries.
3. Iron has negligible effect on extrudability, quench sensitivity for strength and strength itself. Ductility and toughness will improve at low concentrations; at higher concentrations. Toughness diminishes. It is in general detrimental to surface finish and care must be taken during homogenization to minimize this factor,
4. Copper is normally kept in concentrations less than 0.1% and treated as a minor (tramp) element. As such, it has very little effect on extrudability or other properties including quench sensitivity for strength, inherent strength ductility or toughness.