Effect of Si content on surface quality of extruded Al-Mg₂ Si system alloys

The effect of Si content on surface quality of extruded Al-Mg₂Si system alloys has been investigated in terms of surface roughness, specular glossiness, and lightness difference of the as extruded and surface treated products. The mechanism forming surface roughness has been discussed on the basis of results obtained from a new extrusion technique using a split die. The bearing surfaces were polished to a mirrorlike finish so that the effect of chemical composition of the billet on the behaviour of the bearing surface coating might be clearly observed. A mixture of bright and non-bright surfaces was observed in the lower Si content alloys containing 0·23-0·43 wt-%Si. Only the non-bright surface was observed in higher Si content alloys containing 0·53-0·73 wt-%Si. An acceptable uniform surface appearance and roughness across the extrusion could be obtained by Si additions greater than about 0·5 wt-%. A high quality surface with uniform surface roughness could be obtained on extrusion when the continuous coating film was formed uniformly on the bearing surface during the extrusion of a billet with the higher excess Si content and the β-AlFeSi compound.