【作者】 王敏；

【导师】 刘春明； 田妮；

【作者基本信息】 东北大学 ， 材料工程， 2014， 硕士

【摘要】 6xxx系铝合金板材由于具有良好的冲压成形性及耐腐蚀性,还可以利用烤漆工艺实现烘烤硬化,因而是理想的轻质汽车车身板材料。为了使6xxx系铝板在短时(一般不超过30min)低温(一般不超过175.℃)的白车身涂装烤漆过程中表现出良好的烘烤硬化性能,同时还不会损害其冲压成形性,必须对其采取合理的预时效工艺。虽然各国研究人员先后开发出许多适用于6xxx系铝合金车身板的预时效工艺,然而有关预时效工艺能同时改善6xxx系铝合金车身板冲压成形性和烤漆硬化性的机理尚不明确。本文针对目前在欧洲广泛应用的是强度较低的AA6016铝合金车身板及在北美广泛应用的强度较高的AA6111铝合金车身板(以后的表述中省略AA),将其先进行540℃×10min的固溶处理后立即进行70℃～400℃×1s-24h的预时效处理,随后于室温停放两周,研究了其烤漆前后的电导率、硬度及析出相的变化规律。得到的主要研究结论如下：1.6016和6111合金烤漆前后的电导率均随预时效时间延长先缓慢增加后迅速增大,预时效温度越高,合金电导率开始迅速增大的预时效时间越短。6111合金烤漆前后的电导率均大于其T4态的电导率,而6016合金经70℃-150℃预处理不超过12min,其电导率均低于其T4态的电导率,但再经烤漆后的电导率均高于其T4态的电导率。2.6111合金在70-90℃预时效时,其烤漆前的硬度随预时效时间延长缓慢增大,当预时效时间较短时,烤漆前后的硬度低于其T4态的硬度。当预时效温度在110-180℃时,烤漆前后的硬度随预时效时间延长先缓慢后较快增大。当预时效温度在190-250℃时,烤漆前后的硬度随预时效时间延长先缓慢后迅速增大至峰值,然后再下降；预时效温度越高,烤漆前后硬度出现峰值的时间越短。当预时效温度在270-400℃时,烤漆前后的硬度均随预时效时间延长呈现出单调递减的趋势。低温(小于130℃)短时、高温(大于170℃)长时以及预时效温度超过250℃,合金均出现烤漆软化现象。3.6016合金在70-170℃预时效时,烤漆前后的硬度均随预时效时间延长呈现出先缓慢后迅速增大的趋势；当预时效时间较短时,烤漆前后的硬度低于其T4态的硬度；在180-250℃预时效时,烤漆前后的硬度随预时效时间延长先缓慢后迅速增大至峰值,然后再下降；预时效温度升高,烤漆前后硬度出现峰值的时间缩短,对应的硬度最大值逐渐降低。经190℃×6min预时效后的烘烤硬化量达到17HV,预时效后合金基体中出现少量直径约为2nm的点状析出相；经170℃×30min模拟烤漆后,基体出现大量直径约5nm长约20nm且弥散分布的针状析出相；经230℃×8s预时效后,烘烤硬化量达到12.8HV,预时效后基体中出现许多细密的直径约1-2nm的点状析出相,模拟烤漆后析出相直径增大至2-5nm,且密度略有增加；经250℃×1min预时效后,烘烤硬化量减小至6.1HV,预时效后合金基体中出现大量直径约2nm、长约8nm-20nm、尺寸及分布极其不均匀的针状析出相,再经模拟烤漆后针状析出相尺寸略有增大,尺寸及分布均趋于均匀；在270-400℃预时效时,合金烤漆前后的硬度均随预时效时间延长呈现出单调递减的趋势；低温(小于150℃)短时、高温(大于190℃)长时以及超过270℃预时效,该合金均出现烤漆软化现象。更多还原

【Abstract】 Since6xxx series aluminum alloy sheet has advantages of good formability and excellent corrosion resistance as well as high bake-hardening properties by using painting bake process, it is an ideal light-weight material for automotive body sheet. In order to make the white body sheet made of6xxx series aluminum alloy have a good bake-hardening properties and a favorable formability during coating and baking finishing for a short time (usually no more than30min) and at a lower temperature (usually no more than175℃), a reasonable pre-aging process must be taken. Although the researchers around the world have developed many pre-aging processes appropriate for6xxx series aluminum alloy sheet, the mechanism that the pre-aging process can improve the formability but also the the bake-hardening properties is unclear.The present paper focuses on two kinds of6xxx series aluminum alloy sheets, one is AA6016aluminum alloy sheet which has a lower strength and is widely used in Europe, the other is AA6111aluminum alloy sheet which has a higher strength and is widely used in the North America (afterwards the AA statement will be omitted). At first the aluminum alloy sheets were subjected to a solution treatment at540℃for10min, and subjected to pre-aging treatment at a temperature of70℃～400℃for1s～24h, then parked at room temperature for two weeks. After that, the conductivity and the hardness of the alloy sheets as well as the variation rule of the precipitated phase before and after simulating the baking finish. The main conclusions obtained in the research are as follows:1. The conductivities of6016and6111alloys before and after baking finishing increase slowly at first and then rapidly increase with extending the pre-aging time. Higher the pre-aging temperature is, shorter the pre-aging time for which the conductivity begins to rapid increase. The conductivities of6111alloy before and after baking finishing were greater than that of the T4state alloy. However, the conductivity of6016alloy after pre-aged at70℃-150℃for no more than12min is lower than that of the T4state alloy, but the conductivity of the6016after baking finished was higher than that of the T4state alloy.2. When6111alloy is pre-aged at70-90℃, the hardness of the alloy before baking finished increases slowly with extending the pre-aging time. When the pre-aging time is shorter, the hardness of alloy is lower than that of its T4state. When pre-aged at a temperature of110-180℃, the hardness of the alloy before and after baking finishing slowly at first and then faster increases with extending the pre-aging time. When pre-aged at a temperature of190-250℃, the hardness of the alloy before and after baking finishing slowly at first and then rapidly increases to a peak, then decreases with extending the pre-aging time. Higher the pre-aging temperature is, shorter the time at that the hardness before and after baking finishing reaches to a peak. When pre-aged at a temperature of270-400℃, the hardnesses of the alloy both before and after baking finishing monotonically decrease with extending the pre-aging time. The softening phenomenon of the alloy due to baking finishing appears if pre-aged at the low-temperature (less than130℃) for short-term or at high temperature (more than170℃) for long term or at a temperatures at or higher than250℃.3. When6016alloy is pre-aged at70-170℃, the hardnesses of the alloy before and after baking finishing slowly at first and then rapidly increase with extending the pre-aging time. When the pre-aging time is shorter, the hardness of the alloy before and after baking finishing are lower than that of the T4state alloy. When6016alloy is pre-aged at180-250℃, the hardness of the alloy before and after baking finishing slowly at first and then rapidly increases to a peak and further decrease with extending the pre-aging time. Higher the pre-aging temperature is, shorter the time at that the hardness before and after baking finishing reaches to a peak, and the maximum of the hardness gradually reduce. When pre-aged at190℃for6min, the bake-hardening quantity after pre-aged reaches to17HV, there appears a small amount of dot precipitates with about2nm diameter in the matrix of the alloy. After simulating baking finishing at170℃for30min, there appears a large number of needle precipitates with about5nm diameter and20nm length in the matrix of the alloy. When pre-aged at230℃for8s, the bake-hardening quantity reaches to12.8HV; and there appears many dot precipitates with about1-2nm diameter. After simulating the baking finishing at170℃for30min, the diameter of the precipitates is increased to2-5nm, and the density increases slightly. When pre-aged at250℃for lmin, the bake-hardening quantity is decreased to6.1HV; and there appears a large number of needle precipitates with about2nm diameter and8-20nm length in the alloy matrix, but their size and distribution is extremely uneven. After simulating the baking finishing, the size of the needle precipitates increase slightly and their size and distribution tend to become more uniform. When pre-aged at270-400℃, the hardnesses of the alloy before and after baking finishing monotonically decrease with prolonging the pre-aging time. The alloy shows baking-finishing softening when pre-aged at lower temperature (less than150℃) for short time or at higher temperature (more than190℃) for long time or at temperatures at and over270℃.更多还原