预时效对汽车用新型Al-Mg-Si-Cu-Zn合金烘烤硬化性的影响

Bake-Hardening Response in a Novel Al-Mg-Si-Cu-Zn Alloy with Pre-Aging

采用显微硬度和拉伸性能测试,结合透射电镜(TEM)和三维原子探针(3DAP)观察,研究了预时效对汽车车身板用新型Al-Mg-Si-Cu-Zn合金烘烤硬化性的影响。研究结果表明,合金经高温短时(150℃,20 min)和低温长时(50℃,6 h)预时效处理后,随即进行185℃,25 min的模拟烤漆处理,合金的显微硬度和屈服强度显著提高;而经高温短时预时效处理样品模拟烤漆后的硬度和强度更高,表现出更显著的烘烤硬化性。两种预时效过程中均形成了一定数量的原子团簇,该团簇的主要溶质原子为Mg和Si,同时发现含有少量的Cu和Zn,两种预时效制度下原子团簇中Cu/Mg,Zn/Mg原子比相当。与低温长时预时效相比,高温短时预时效后原子团簇的尺寸和体积分数更大,而数量更少。但高温短时预时效过程促进了大尺寸且Mg/Si原子比接近于1的原子团簇的形成,该类原子团簇在后续模拟烤漆过程中更易转变为β″相。预时效态合金经后续185℃,25 min模拟烤漆处理后,形成了分布均匀、致密的β″相,从而显著提高了合金强度。经统计分析,发现高温短时预时效合金在烤漆过程中形成的β″相数量更多,体积分数更大,尺寸更小,说明150℃,20 min高温短时预时效有益于促进β″相的形成,从而使合金表现出更好的烘烤硬化性。

6xxx series(Al-Mg-Si)aluminum alloy has excellent properties such as medium strength,good formability and corrosion resistance,facilitating their wide use as automotive skin panels.The addition of Zn can enhance the age hardening response of 6xxx series aluminum alloy,but the related aging mechanism is still controversial and needs further research.The composition,size and quantity of atomic clusters under two pre-aging conditions,namely high-temperature pre-aging with short-time and low-temperature pre-aging with long-time,were studied in an Al-Mg-Si-Cu-Zn alloy.The precipitated microstructure and mechanical properties in the subsequent paint baking process were evaluated in order to explore the effects of pre-aging on the atomic cluster behavior and bake hardening response of the new Al-Mg-Si-Cu-Zn alloy.Ingots were homogenized,hot rolled and intermediate annealed,and then cold rolled to a thickness of 1 mm.The cold-rolled sheet was water quenched after solution treatment at 550℃for 20 min.The solution treatment samples were immediately pre-aged at 150℃for 20 min or 50℃for 6 h and then conducted simulated paint-baking treatment at 185℃for25 min.The microhardness and tensile properties of the pre-aged and baking-hardening samples were tested.The microstructure of these samples was observed by transmission electron microscope(TEM)and three-dimensional atom probe(3DAP).The composition,size and quantity of atomic clusters and precipitates were carefully analyzed based on TEM and 3DAP results.Al-Mg-Si-Cu-Zn alloy exhibited high increases in values of hardness and strength during pre-aging at 150℃for 20 min or 50℃for 6 h followed by paintbaking treatment at 185℃for 25 min.Compared to pre-aging at 50℃for 6 h,the yield strength and tensile strength were higher and the elongation was lower after pre-aging at 150℃for 20 min.After paint baking treatment,the strength of the two pre-aged samples increased and the elongation decreased.It should be noted that the pre-aging 150℃for 20 min resulted in a higher yield strength increment compared with pre-aging at 50℃for 6 h.Thus,the sample pre-aged at 150℃for 20 min exhibited an enhanced bake-hardening response.After pre-aging at 150℃for 20 min followed by paint baking treatment,the size ofβ″phase was smaller,the volume fraction was larger and the number was more.This indicated that pre-aging at 150℃for 20 min could better promoteβ″phase precipitation during paint-baking process.β″phase was the main strengthening precipitates of 6xxx aluminum alloy,which could significantly enhance the strength.Thus,the sample pre-aged at 150℃for 20 min had a higher strength after paint-baking and thus exhibited an enhanced bake-hardening response.The Mg-Si clusters could form after pre-aging at 150℃for 20 min or 50℃for 6 h.Cu and Zn atoms could be also incorporated into these Mg-Si clusters,the Cu/Mg ratio and Zn/Mg ratio were similar in the clusters formed under the two different pre-aging conditions.The difference between the two pre-aging treatments was that pre-aging at 150℃for 20 min prompted the formation of larger clusters with Mg/Si ratio close to 1,which could easily transform toβ″phase.During subsequent paint baking treatment at 185℃for 25 min,β″phases were formed and had a refine and dense distribution,resulting in the high bake-hardening response.Compared with the pre-aging at 50℃for 6 h,the sample pre-aged at 150℃for 20 min followed by paint-baking treatment at 185℃for 25 min had higher microhardness and strength,presenting more significant bake hardening response.β″phases were formed during paint-baking treatment at 185℃for 25 min after pre-aging at 50℃for 6 h or 150℃for 20 min.Compared with pre-aging at 50℃for 6 h,pre-aging at 150℃for 20 min promoted precipitation ofβ″phase during paint-baking process.The number ofβ″phases was more and the size was smaller,so the sample pre-aged at 150℃for 20 min had a better bake-hardening response.The atomic clusters were formed after pre-aging at 50℃for 6 h or 150℃for 20 min.Mg and Si were the main solute atoms for the atomic clusters,and there was also a small amount of Cu and Zn in the atomic clusters.The size of atomic clusters pre-aged at 150℃for 20min was larger,which improved the strength.In addition,pre-aging at 150℃for 20 min promoted the formation of atomic clusters with large size and Mg/Si close to 1.This type of atomic cluster was easier to transform intoβ″phase.