| In order to speed up the process of energy saving and emission reduction in the automotive industry,passenger cars are extremely urgent for the lightweighting of vehicles.The 6xxx series aluminum alloy has become the material of choice for automotive parts due to its excellent molding properties,workability and corrosion resistance.In China,the current product stability is significantly lower than similar foreign alloy products.In particular,the fatigue property is poor,and the main reason is the formation of coarse recrystallized grains.This thesis is to develop a new type of Al-Mg-Si-Cu alloy.Through the exploration and research of alloy heat treatment,aluminum alloy with good yield strength,plasticity and fatigue strength is developed to meet the requirements of automotive chassis components.In this paper,the main crystal phase types,morphology and alloying elements distribution,single-stage high-temperature homogenization and two-stage homogenization are studied.The influence of uniformity on ?-Al(Mn,Fe)disperse phase precipitation was explored,and the influence of T5 and T6 heat treatment on the properties of the alloy after homogenization and extrusion was explored.The main work and research results of the paper are as follows:(1)The as-cast microstructure analysis of the new Al-Mg-Si-Cu alloy shows that the gray slats Al5FeSi phase or Als(FeMn)Si and the black elongated Mg2Si phase are distributed at the grain boundary.And a small amount of yellowish spherical Q phase is distributed in the matrix,and the dendrite arm spacing is 26.67 ?m.(2)The results of single-stage homogenization heat treatment of the alloy show that the Q phase has been dissolved at 510? for 2 h.With the increase of homogenization temperature and time,the number of second phase at the grain boundary is gradually reduced.The percentage of Mg2Si phase area decreased from 1.06%in the as-cast state to 0.2%,the percentage of Al5(FeMn)Si phase area decreased to 0.45%,and the area of a-Al(Mn,Fe)Si dispersed phase decreased to 7.05%.At 560?,there is a phenomenon of over-sintering such as remelting balls;the best single-stage homogenization system is"550?/10 h".(3)The results of alloy two-stage homogenization heat treatment show that ?' is most favorable for the precipitation of ?-Al(Mn,Fe)Si dispersed phase,and the optimal low temperature homogenization system is determined as "300?/8h",the best double The level homogenization system is "300?/8h+550?/10h".Compared with other two-stage homogenization systems,the ?-Al(Mn,Fe)Si diffuse phase has the largest number,and the coarse diffuse phase region has a minimum percentage of 0.33%.The alloys subjected to two-stage homogenization heat treatment have more dispersed phases than single-stage homogenization,higher density,and more uniform distribution of dispersion regions.(4)After the alloy is extruded,it is subjected to T5 heat treatment,and the maximum hardness is 134.48 HV after being kept at 150? for 8 h.The T6 heat treatment was carried out.The T6 heat treatment was at 550?/40 min after water quenching and then 180?/6 h.The yield strength and tensile strength of the alloy were the highest,326.1 MPa and 380.7 MPa,respectively,and the plasticity was 16.4%. |
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