The Effects Of Pre-deformation And Pre-aging On The Natural Aging And The Subsequent Artificial Aging In Al-Mg-Si Alloys

Al-Mg-Si alloys are widely used in automobile,aviation and high-speed railway,owing to their many advantages such as high strength,good formability and so on.In practical manufacture,before they are given an artificial aging treatment,these alloys are stored and shaped at room temperature,so they would inevitably go through a process of natural aging.For most of the commercial 6000 series alloys(with a solute content of Mg+Si>1 wt.%),natural aging can depress the hardening kinetics and the maximum hardness obtained in subsequent artificial aging,which is called negative natural aging effect.It is extensively reported that pre-treatment before natural aging,such as pre-aging,pre-deformation and combined pre-aging and pre-deformation can effectively suppress the natural aging and improving the subsequent age hardening response in Al-Mg-Si alloys.Therefore,the effect of pre-treatment on the natural aging and the subsequent artificial aging in Al-Mg-Si alloys has attracted a lot of interest recently,but so far it has not been clearly understood.In the present study,microhardness test and tensile test were used to investigate the influence of pre-aging(150 ℃/30 min),pre-deformation(5%)and pre-deformation(5%)+pre-aging(150 ℃/30 min)on the mechanical properties of three alloys with different Si contents.Differential scanning calorimetry(DSC)and transmission electron microscopy(TEM)were further used to reveal the relationship between the microstructure and the mechanical properties of the alloys.The main conclusions are as follows:(1)Pre-aging treatment can effectively inhibit natural aging in the three alloys.Moreover,the pre-aging clusters formed during pre-aging can act as the heterogeneous nucleation sites for the(3”phases,so as to promote the formation of the β" phases and effectively improve the age hardening response in the subsequent artificial aging at 180℃.(2)Pre-deformation treatment can suppress natural aging in the three alloys to a certain extent,but the contribution of pre-deformation is not as large as that achieved with pre-aging.The same case for their contributions to the improvement of the age hardening response in the subsequent artificial aging at 180℃.Moreover,the higher Si content in the alloy,the larger contribution of pre-deformation will be.The dislocations introduced by pre-deformation treatment can provide heterogeneous nucleation sites for the β’ phases and thus the formation of the β’ phases is promoted.(3)Pre-deformation + pre-aging treatment can also effectively inhibit natural aging in the three alloys,and its contribution to the improvement of the age hardening response in the subsequent artificial aging at 180℃ lies between that achieved with pre-aging and pre-deformation.The main conclusions of pre-deformation effect on the subsequent high temperature(250℃)aging in Al-Mg-Si alloys are as follows:(1)Pre-deformation has a significant effect on the age-hardening behavior upon high temperature aging in the three alloys with different Si content.The negative natural aging effect in Si-rich alloy is reversed,while the natural aging effect in Mg-rich alloy is changed from a positive effect to a negligible effect.(2)The introduction of pre-deformation changes the precipitation behavior upon high temperature aging in the three alloys.For the Si-rich alloy without pre-deformation,whether there is natural aging before artificial aging or not,the precipitates are ℃" phases.Whereas when pre-deformation is added,the β" phases and β’ phases both exist.For the Mg-rich alloy,when directly artificially aged,pre-deformation leads to a precipitate type change from mainly β’ phases to mainly β" phases.When the alloy is subjected to "natural aging +artificial aging" treatment,pre-deformation leads to a precipitate type change from mainlyβ" phases to coexistence of β" phases and β’ phases.