Effect of Si Addition on the Mechanical Properties and Material Structure of Al-Zn-Mg Alloys

Among Al-Zn-Mg alloys, A7003 alloy is considered to be an excellent alloy from the viewpoint of weldability because it is an alloy with relatively low Zn and low Mg composition. In this paper, changes in mechanical properties and microstructure during aging treatment are investigated by the addition of Si in the Al-5.6 mass%Zn-0.75 mass % Mg alloy containing Cu, Mn, Zr and Fe. Cast billets of alloys with different Si contents (Si: 0.05 mass%, 0.15 mass%, and 0.30 mass%) were prepared, and these cast billets were homogenized and extruded. After extrusion, four aging treatments were performed: one step aging at 423 K for 8 hours, and two step aging at 373 K for 3 hours, 6 hours, and 9 hours, followed by 423 K for 8 hours. The higher the amount of Si, the smaller the thickness of recrystallization layer near the inner surface and near the outer surface, and the smaller the existence rate of recrystallized grains in the cross section. After one step aging at 423 K for 8 hours, Si: 0.05 mass% showed lower strength than Si: 0.15 mass% and Si: 0.30 mass%. On the other hand, two-step aging resulted in Si: 0.05 mass%, Si: 0.15 mass%, and Si: 0.30 mass% with similar strength. The low strength of Si: 0.05 mass% after one step aging at 423 K for 8 hours is thought to be due to the coarse eta( )phase, which was precipitated in the grain boundaries and grains. In Si: 0.15 mass% and Si: 0.30 mass%, Al(Mn, Fe)Si which formed during homogenizaiton process is present in the grain boundaries and grains after extrusion. It is thought to suppress the generation of the precipitate of the coarse eta phase in the aging at 423 K for 8 hours, which is a condition that the coarse eta phase is easy to generate.