| Al-Mg-Si-Cu alloys are the most promising materials used in atuomobile for weight reduction due to their medium-high strength,good formability,high corrosion resistance,excellent surface quality and high precipitation hardening response after paint baking.These alloys have been widely used in body-in-white of automotive body,such as hoods,doors,wings,bumpers and interiors.The main feature of these alloys is a significant increase in strength due to the formation of a large number of nano-sized semi-coherent metastable precipitates during heat treatment.Understanding the precipitate structure and the relationship between precipitates and alloy properties is essential for improving mechanical properties of these alloys.Besides,for wide use in auto body panel,Al-Mg-Si-Cu alloys should have good formability and high bake hardening response.It requires to fully understand the precipitates evolution mechanism and thus design and develop the new alloys and heat treatment processes.In this work,a series of Al-Mg-Si-Cu alloys with different Mg/Si ratios and Cu additions were designed based on the commercial AA6022 alloy.Combined techniques and methods,such as transmission electron microscopy(TEM),high resolution transmission electron microscopy(HRTEM),atomic resolution high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM),three dimensional atom probe(3DAP),Vickers hardness measurement,tensile test,differential scanning calorimetry(DSC),first principle calculation and Monte Carlo simulation were used to systematically study:(1)the structural and compositional evolution mechanism of precipitates in Al-Mg-Si-Cu alloys,(2)the influence of composition on the clustering and precipitation behavior of these alloys during natural aging and artificial aging,and(3)the influence of pre-treatment processes on natural aging and bake hardening of these alloys.The main conclusions include:(1)The precipitates formed during the early and peak stages of aging at 180 oC in Al-Mg-Si-Cu alloys are mostly structural disordered with a nonperiodic atomic arrangement.The disordered precipitates contain the structural unit of ??,Q? or C phase could be termed as ??,Q? and L precipitates,respectively.The Cu cluster-like zones are the building block of all the Cu-containing precipitates in Al-Mg-Si-Cu alloys,which plays an important role in providing structural stability of the precipitates in these alloys.The sluggish diffusion of Cu cluster-like zones is the main reason for forming the disordered precipitates formed during the early and peak aging conditions.(2)During continuous aging,the disordered precipitates gradually transform to well-ordered precipitates through a continuous ordering process.The ?? phase can transform to the disordered Q? precipitate by the incorporation of Cu atoms and the formation of Cu cluster-like zones.The ordering of Q? precipitate mainly contains the formation of Cu cluster-like zone,the ordering of Si-network and the ordering of Cu-network.In contrast,ordering and transformation of the L/C precipitate are rather sluggish.Compared with Q? phase,the L phase is more preferential to have a disordered structure.The high strengthening response and excellent thermal stability of the L phase could significantly improve the strength and thermal stability of these alloys.All the metastable phases(including the Q?,L/C precipitates)transform to the Q equilibrium phase after long-time aging.The new precipitation sequence of Al-Mg-Si-Cu alloys is proposed as: SSSS ? atomic clusters ? GP zones ? ??,Q?,L/C ? Q?,L/C ? Q,Si.Accompanying with the structural transformation of these precipitates,a continuous incorporation of Mg,Si and Cu atoms and release of Al atoms are involved.(3)The properties of Al-Mg-Si-Cu alloys could be controlled by simultaneously regulating the Mg/Si ratio and Cu addition.During the natural aging,both excess Si and Cu could increase the natural aging hardening of these alloys.As the Si atoms play a dominant role in the formation of Cluster(1)during natural aging,excess Si is more effective for increasing natural aging kinetics of these alloys.During artificial aging at 180 oC,in the low Cu alloys,the Si-rich alloy has a higher hardness than the Mg-rich alloy and the alloy with Mg/Si ratio = 1 has the highest peak aging hardness.While in the high Cu alloys,the Cu addition increases the aging hardening of these alloys.The Mg-rich high Cu alloy has higher hardness in under-and over-aged stage of aging,while slightly higher hardness in the peak-aged condition.The incorporation of Cu atoms could significantly refine the clusters formed during artificial aging due to the strong interaction energy between Cu atom and Mg atom.Besides,the combination of excess Mg and high Cu promotes the precipitation of L phase,which could increase the thermal stability of these alloys.The negative effect of natural aging on subsequent artificial aging behavior is less dependent on Mg/Si ratio in the high-Cu alloys,but is closely related to Mg/Si ratio in low-Cu alloys.(4)The new designed Mg-rich high Cu alloy exhibits significant advantages than the commercial AA6022 alloy.This alloy not only has a lower T4 temper yield strength,but also has higher precipitation kinetics,bake hardening response and thermal stability.Besides,the alloy maintains acceptable corrosion resistance.Therefore,the Mg-rich high Cu alloy has good potential for use in auto body panel applications.(5)Pre-aging in a wide temperature(80oC-170oC)is effective in suppressing the natural aging and improving the bake hardening response of Al-Mg-Si alloys.The formation of Cluster(2)can suppress the formation of Cluster(1)during natural aging,and during artificial aging,Cluster(2)can transform readily into ?? phase and increase the bake hardening response of these alloys.The optimum pre-aging process in this study is proposed as: 100 oC for 3h,which gives a lower T4 P temper hardness and good bake hardening response.Compared with pre-aging,interrupted quenching process can not only suppress the natural aging and improve the bake hardening response of these alloys,but also increase the efficiency of pre-treatment.The optimum interrupted quenching process is proposed as: 100 oC for 45 min,which has similar effect with pre-aging at 100 oC for 3h,but the pre-treatment is significantly reduced.However,the interrupted quenching produce coarse quench-induced precipitates and wide precipitate-free zones(PFZ)along the grain boundaries,which could cause fracture toughness problems in these alloys.(6)Pre-straining can suppress the natural aging and promote the heterogeneous nucleation of ?? phase during paint bake cycle through the introduced dislocations.But a rather high T4 temper hardness due to deformation hardening is generated,which can risk the formability in the stamping process.The new designed high temperature pre-straining process can not only effectively resolves the rather high T4 temper hardness induced by conventional pre-straining,but also give a higher bake hardening response of these alloys. |
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