EFFECT OF SI CONTENT ON SURFACE QUALITY OF EXTRUDED AL-MG2SI SYSTEM ALLOYS

The effect of Si content on surface quality of extruded Al-Mg2Si syste m alloys has been investigated in terms of surface roughness, specular glossiness, and lightness difference of the as extruded and surface t reated products. The mechanism forming surface roughness has been disc ussed 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 S i content alloys containing 0.23-0.43 wt-%Si. Only the non-bright surf ace 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 obta ined on extrusion when the continuous coating film was formed uniforml y on the bearing surface during the extrusion of a billet with the hig her excess Si content and the beta-AlFeSi compound.