5xxx系铝合金的快速时效析出强化及锯齿变形行为研究

Among all the alloys, Al-Mg alloy has been one of the most suitable materials for automotive applications due mainly to its reasonable mechanical properties, good corrosion resistance, high formability, and good weldability. Basic problems related to the limited consumption of Al-Mg alloy sheets in automotive applications are concentrated on the appearance of stretcher strain markings on the alloy during the forming operations. Another obstacle for its wider application is related to the softening behavior during the paint baking process in the deformed panels. Adding a small amount of Cu to the Al-Mg alloy could additionally harden the alloy by precipitation during paint baking process. But it is still not enough for the use on advanced automotive panels. Both solutions for simultaneously weakening the surface relief or the paint baking induced softening are rather complicate for routine industrial practice. .In this proposal, we deliberately add a large amount of Cu and Zn elements to the traditional Al-Mg alloy. By introducing a pre-aging process before the forming operations, we are able to dramatically increase the paint baking strength of the alloy because of the rapid precipitation hardening of Zn-containing and Cu-containing phases. At the same time, we are able to relieve the stretcher strain markings on the surface of the alloy by optimizing the chemical composition and pre-aging process. The newly developed alloy is the promising solution for overcoming both the undesired surface appearance as well as the paint baking softening problems. In this proposal, we systematically investigate the mechanism of rapid precipitation hardening during paint baking and the mechanism of serrated deformation behavior. The final goal is to establish the corresponding relationship among the chemical composition, pre-aging treatment, microstructures and properties during paint baking process, and serrated deformation behavior of the newly developed Al-Mg alloy.

5xxx系铝合金具有良好的成形性、可焊性以及抗腐蚀性和中等强度，是汽车减重用板材的最佳选择之一。但5xxx系铝合金冲压后的表面质量较差，并且合金在烤漆过程中发生软化，使该系合金的发展受到了极大的限制。在合金中添加少量的Cu可在一定程度上提高合金冲压后的烤漆强化性能，但对未来汽车板材所用5xxx系合金仍然不足。本项目拟突破传统5xxx系合金的成分设计思路，通过增加并调节合金中Zn/Cu的含量，结合适当的预时效工艺，在合金中形成均匀分布的含Cu以及含Zn强化相，使合金具有较高的快速时效响应能力，显著提高合金的烤漆硬化性能，同时降低合金的锯齿屈服效应，提高合金的表面质量。课题将针对快速时效响应的控制及机理进行深入分析研究，同时深入研究新合金的锯齿屈服机理。建立新型合金成分、预时效工艺、烤漆组织和性能以及锯齿变形规律之间的关联模型，为研究开发具有自主知识产权的新型汽车用5xxx系铝合金奠定基础。

退火态的5xxx系铝合金具有良好的成形性、可焊性以及抗腐蚀性和中等强度，是汽车减重用板材的最佳选择之一。但5xxx系铝合金汽车板冲压成形后在160-180℃进行20-30min的烤漆过程中容易发生烤漆软化，另外5xxx系铝合金在冲压变形过程中试件表面会形成吕德斯带，并产生锯齿形屈服效应，烤漆后残留在板材表面，严重影响成品的表面质量及外观。因此5xxx系铝合金目前只能用于对强度及表面质量要求不高的汽车内板,大大制约了其在国民经济中的应用。如何在保证冲压时合金表面质量的同时,又能够显著提高后续烤漆过程中的硬化强度,满足现代汽车设计制造对铝合金材料提出的更高要求是亟须解决的关键问题。.本项目突破传统5xxx系合金的成分设计思路，通过增加并调节合金中Zn/Cu的含量，开发出了(Zn+Cu)/Mg<1.0的新型合金，区别于传统的以Mg为主合金元素的5xxx系合金以及Zn/Mg>2.0的7xxx系合金。结合适当的预时效处理工艺，使合金中形成了大量细小弥散分布的GP区，作为后期高温时效的析出形核点。预时效处理后的峰时效组织由单级峰时效的S-Al2CuMg相和T-Mg32(AlZn)49相共存转变为单一的T-Mg32(AlZn)49相，使合金具有较高的快速时效响应能力。合金的烤漆硬化强度提高到120MPa，和传统的烤漆软化型合金相比显著提高了合金的烤漆硬化能力。同时预时效还能提高板材的室温稳定性，避免了由于自然时效引起的烤漆软化。新型铝合金在具有较高烤漆硬化能力的同时，还可以在较小的晶粒尺寸下抑制吕德斯带。降低合金的锯齿屈服敏感性，较传统退火态5xxx系铝合金有明显的改善。预时效及Zn和Cu元素的添加，能够产生大量GP区，从而消耗基体中大量的Mg原子和空位，降低了冲压过程中固溶原子与位错之间的相互作用，从而抑制了合金的吕德斯现象以及锯齿屈服行为，可显著改善合金的表面质量。.本研究成功开发出的新型合金和传统合金相比同时具有高烤漆强度以及高表面质量，为研究开发具有自主知识产权的新型汽车用铝合金奠定基础。