Abstract:The hardening behavior and microstructure evolution of AlMgSiCu alloy after pre-aging(PA) treatment, natural aging(NA) treatment and artificial aging(AA) treatment were investigated in this paper. The micro-hardness was tested. The TEM microstructure was characterized. The results indicate that PA treatment increases the hardness of the initial state, while decreases the NA effect. The NA hardening value arrives 34HV of T4 state, while T4P has a value of 25HV. The micro-hardness increases first and then decreases with increasing aging time during aging at 180℃. The hardening curve could be divided into three parts: higher hardening rate zone(Ⅰ, 20~60min), slow hardening rate zone(Ⅱ, 60~480min) and decreasing zone(Ⅲ). The peak value of hardness(127HV) arrives at 480min. The coherent GP zones are the main phase after NA of T4P state, whereas semi-coherent η’’ phase are the dominant phase after AA treatment. The discontinuous precipitation with a size of 15~40nm and PFZ zone with a width of 45nm exist at grain boundaries.