溫度效應對冷軋5083鋁－鎂－錳合金超塑性變形及其變形活化能之探究

Abstract

Aluminum and its alloys are expected to become the main structural material in the global trends in lightweight and energy saving. Aluminum alloy is considered as an important material for the production of lightweight automotive, especially in terms of car body panels and frame. Superplastic forming of aluminum alloy (superplastic forming, SPF) has become one of popular attention to the shaping and forming industry. SPF process is expected to be among the less steps and even a single step to manufacture complex workpiece. The SPF process required under low strain rate and high temperature, which makes the whole process (cycle time) rather lengthy. To reduce the cycle time of the SPF process, many studies put a lot of effort in the production of ultra-fine grain materials. On the other hand, the SPF temperature could be lowered when the material has ultra-fine grains. However, it is very difficult to produce Al alloy (conventional Al alloy, such as AA5083 alloy) with ultra-fine grains by a typical rolling process. In the present study, we proposed a new concept for SPF process of an aluminum alloy. One of them is the idea of ＂high heating rate treatment,＂ ＂and the other is ＂off-optimal condition.＂ As-cold-rolled 5083 aluminum alloy sheet has superplastic characteristic after recrystallization treatment was revealed in most literature. The high-temperature superplastic property of as-cold-rolled 5083 aluminum alloy sheet was investigated. The total amount of rolling reduction is about 80%. Recrystallization treatment for the as-cold-rolled 5083 alloy was not necessary. This study found that the as-cold-rolled Al alloy could exhibit superplasticity, simply heating the sample to test temperature (i.e., the processing temperature). Specimen was heated by rapid heating rates (it were used about 150 sec. to arrive each testing temperature after putting the sample into stove) to reach each testing temperature. Experimental results show that high heating rate will improve the total ductility of the as-cold-rolled 5083 superplastic alloy. The heating rate will directly retard the formation of cavities, reducing the size of cavity during superplastic deformation. As a result, the sampled being heated in a higher rate will exhibit better elongation. The optimal elongation (600%) was obtained from strain rate of 1 × 10^(-3) s^(-1) at 500℃. The reason was discussed from this investigation. For off-optimal condition, the elongation up to 280% at strain rate 10^(-1) s^(-1). In fact, 280% elongation is enough for any kinds of sheet forming products. By using the off-optimal condition, the total cycling time could be reduced almost only 1% time of the original cycle time. Moreover, after deforming at testing temperature 500℃ at strain rate 10^(-1) s^(-1), the distribution and size of cavities was unobvious. Therefore, the off-optimal condition not only can reduce the cycle time of the SPF process but also the defects and cavities in the post-deformation were reduced. By analyzing the activation energy of 5083 aluminum alloy, the true activation energy are 90 kJ/mole in the temperature ranges between 450℃ and 500℃, and 128 kJ/mole in the temperature ranges between 500℃ and 550℃.