| 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℃. |
