The Precipitates Evolution And Heat TREATMENT Process Of Microalloying Al-Mg-Si Alloys

Heat-treatable 6000 series Al-Mg-Si alloys have been widely used in automobile body due to their low density,medium-high strength,good formability and high corrosion resistance.The main feature of these alloys is a significant increase in strength due to the formation of a large number of nano-sized metastable precipitates during heat treatment.Microalloying is one of the most effective methods to improve the mechanical property of Al-Mg-Si alloys.Understanding the effect of microalloying elements on the precipitation evolution and mechanical property for Al-Mg-Si alloys is very important.Solute segregation could significantly influence the morphology and strengthening ability of precipitates.Understanding the interaction between solute segregation,precipitate morphology and strengthening ability is essential for improving the property of Al-Mg-Si alloys.Besides,in order to obtain the excellent combination of formability and bake hardening ability,it requires to fully understand the combined effect of heat treatment and microalloying on the precipitation hardening for Al-Mg-Si alloys.In this work,four Al-Mg-Si alloys with different Ag and Cu additions were designed.Combined techniques and methods,such as atomic resolution high-angle annular dark-field scanning transmission electron microscopy?HAADF-STEM?,transmission electron microscopy?TEM?,three dimensional atom probe?3DAP?,differential scanning calorimetry?DSC?,Vickers hardness measurement,tensile test and first principle calculation were used to systematically study:?1?the effect of Ag and Cu addition on the natural aging and precipitation hardening for Al-Mg-Si alloys,?2?the effect of Ag addition on the precipitation evolution for Al-Mg-Si alloys,?3?the Cu segregation at the precipitate/matrix interfaces,?4?the combined effect of pre-aging and microalloying on the precipitation hardening for Al-Mg-Si alloys.The main conclusions include:?1?Ag and Cu additions could influence the natural aging and precipitation hardening for Al-Mg-Si alloys.During natural aging,Ag and Cu additions could improve the T4 temper hardness responses for Al-Mg-Si alloys.Compared with the Cu-added alloy,the Ag-added alloy has lower T4 temper hardness and better formability.During artificial aging,Ag and Cu additions facilitate the precilitation kinetics and precipitation hardening for Al-Mg-Si alloys,especially Ag addition.?2?Ag atoms could change the atomic strcucture of precipitates for Al-Mg-Si alloys.At the early aging stage,Ag atoms could enter the interior of clusters and change their chemical,facilitate the formation and growth of clusters.At the peak aging stage,Ag atoms preferentially segregated at the GP zone/?-Al and??/?-Al interfaces by the replacement of Al atoms in FCC matrix.At the over-aged stage,Ag atoms generally incorporate into the interior of?"precipitates,facilitating the formation of QP lattice?a hexagonal network of Si atomic columns?and the local symmetry substructures,Ag sub-unit?1?and Ag sub-unit?2?.With the prolonging over aging time,the Ag sub-unit?1?transforms to??_Ag1 and??_Ag2 unit cells and the Ag sub-unit?2?evolves into Q?_Ag.The coexistence of??_Ag1,??_Ag2,Q?_Agg and??unit cells within one single precipitate was universal at the over-aged stage.In the equilibrium stage,the Ag atoms incorporated in precipitates release into?-Al matrix or form the Ag particles.In general,Ag atoms undergo a process of“segregate at the precipitate/matrix interfaces—incorporate into the interior of precipitates—release to the?-Al matrix”in the precipitation for Al-Mg-Si-Ag alloys.?3?For all metastable phases in Al-Mg-Si-Ag alloys,many Ag atoms segregate at the precipitate/matrix interfaces,which is energetically favored.Ag segregation could increase the length/diameter ratio of??phase and thus improve its strengthening ability.While for the??phases formed in peak aging,Ag segregation has nearly no influence on their morphology.?4?Six different ORs and two morphologies,i.e.rod-and lath-like are revealed for??precipitates formed in the over-aged stage for the Al-Mg-Si0.01Cu alloy,and Cu segregation at the??/?-Al interface is observed in all these precipitates.The rod-like??precipitate has multiple??-angles ranging from 6.1^o to 14.1^o and non-uniform Cu segregation at the??/?-Al interface,while the lath-like??precipitate has a constant??-angle of 0^o and a periodic Cu segregation at the coherent??/?-Al interface.?5?An even number spacing of Cu segregation(d_Cu=4d_Al,6d_Al,8d_Al)was found in the habit plane of the lath-like??,C and QP2 precipitates,which is caused by the arrangement of??and C unit cells and the strong interaction between the segregated Cu atoms and the Si atoms within the precipitates.The Cu segregation at habit plane of these precipitates can accommodate the lattice misfit that exists between??and?-Al matrix.?6?Ag and/or Cu addition could further improve the positive effect of pre-aging on bake hardening for Al-Mg-Si alloys.The pre-aged alloys with Ag or Cu individual addition have a relatively high T4P temper and paint baked hardness due to effectively forming of Cluster?2?during pre-aging.While the pre-aged Ag-Cu-added alloy has low T4P temper hardness and higher bake hardening response to satisfy the required formability and in-service dent resistance.The strong interaction energies between Ag/Cu atoms and Mg atoms are responsible for the improved clustering and precipitation processes.