Effects of preheating on directional properties and stress corrosion cracking of Al-Zn-Mg alloy thick plates

Abstract

Directional properties in polycrystalline metals are resulted from crystallographic factors which are also called preferred orientations, or mechanical factors such as elongated inclusions, segregation, cavities, primary crystals, elongated crystal grain form, etc.This paper describes the effects of preheating on directional properties concerning mechanical properties and stress corrosion cracking of Al-Zn-Mg alloy thick plates, and the factors affecting these directional properties were also discussed.The results obtained were as follows:(1) The directional properties concerning tensile strength in 3 main directions (L, LT, and ST) were little in variation. The tensile strengths in the directions at an angle of 45°C to the main directions were lower than those in the main directions when the material was preheated at lower temperatures. However, the variation of these directional properties was less with the rise of preheatting temperature. The variation of directional properties was explained by crystallographic factors.(2) The elongation was largest in the direction at the angle of 45°C to L and LT, and was gradually decreased with the change of direction toward L, LT or ST; it was smallest in ST direction. The elongations in these directions were increased with the rise of preheating temperature. These directional properties were explained by the combination of crystallographic and mechanical factors.(3) The notch tensile strength was greatest in L direction and was gradually decreased with the change of direction from L to LT or ST; it was least in ST direction. The strength was decreased in L or LT direction, but was increased in ST direction with the rise of preheating temperature. These directional properties were explained by crystal grain forms in mechanical factors.(4) The stress corrosion cracking resistance was lowest in ST direction and was gradually improved with the change of direction from ST to LT or L; it was highest in L direction. These directional properties were explained by crystal grain forms in mechanical factors.(5) High stress corrosion cracking resistance was obtained in the material having subgrains and small, disperesed, and insolutble particles. The resistance was decreased with the rise of preheting temperature, because these structures were destroyed by preheating.(6) The shear crack resistance was decreased with the rise of preheating temperature.