大型复杂镁合金汽车集成件材料制备与成形相关基础研究

.　To meet increasingly strict requirement of vehicle emission, vehicle weight loss technique is urgent to be updated. Application of large-scale, complex and integrated auto parts made of magnesium alloys could optimally reduce vehicle weight, and become an important development direction of vehicle magnesium alloys. The collaborative research team manufactured magnesium centre console sample composed by 42 parts, which is highly integrated and optimally reduces weight. However, there are still some key problems including unstable processing properties, insufficient strength and ductility and corrosion in joints of dissimilar metals. Therefore, to figure out these key scientific problems in manufacture of large-scale, complex and integrated auto parts made of magnesium alloys, the present project will reveal the influence of rare earth elements and rheo-forming technical parameters on AZ91 alloy melt flow, the strengthening and toughening mechanism of rheo die-cast and heat treated AZ91-RE alloy, the corrosion protection mechanism of double coating layers for joints of dissimilar metals. Large-scale, complex and integrated auto part products will be manufactured, and corrosion protection method of double coating layers for joints of dissimilar metals will be developed, which are both crucial problems existing in application of magnesium alloy auto parts. Many academic disciplines including solidification theory, rheology, physical metallurgy and interface corrosion science are involved in the present project, which present its significance of theoretical direction and model application prospect.

日益严苛的汽车排放要求迫使汽车轻量化向更高水平发展，大型复杂镁合金汽车集成件可最大限度地降低车身重量，成为车用镁合金重要发展方向。项目组前期探索了由42个零件组成的中通道镁合金框架的高度集成，最大限度降低了重量，但仍存在成形性能不稳定、制品强度和塑性不足、异质金属连接处腐蚀的问题。为此，以大型复杂镁合金汽车集成件制备与成形中关键科学问题为着力点，研究稀土元素与流变成形工艺参数对AZ91合金流动机制的影响，流变压铸与热处理AZ91-RE合金的强韧化机制，以及双重覆层条件下异质金属界面腐蚀防护机制，以实现大型复杂镁合金汽车集成件的制备与成形，获得异质金属界面双重覆层腐蚀防护方法，解决镁合金汽车部件应用中的难题。研究涉及到凝固理论、流变学、物理冶金、界面腐蚀科学等学科理论知识，具有重要理论指导价值和示范应用前景。

获得了Ce-La稀土元素（RE）与流变压铸工艺参数对AZ91合金熔体粘度、流动性、充型性及其凝固过程组织演化和力学性能的影响与机制，建立了不同RE和流变压铸工艺参数条件下AZ91合金熔体的流体类型判据与本构方程，获得了充型热流耦合场数值模拟模型。进一步调控合金成分与制备工艺，优化了流变压铸工艺参数与热处理工艺参数，明确了RE与流变成形工艺参数对AZ91合金流动机制的影响机理，揭示了流变压铸及热处理AZ91-RE合金的强韧化机制。采用优化成分AZ91-RE合金，通过压铸和流变成形技术制备了红旗HS7的汽车中通道框架样品。掌握了AZ91合金表面化学钝化和高耐蚀、耐磨涂层制备方法，NaMgF3钝化膜显著抑制了Mg基体阳极反应过程中的电子转移，腐蚀电流密度比裸合金小一个数量级，腐蚀速率仅为0.15 mm/y；微弧氧化（MAO）电解液中添加15g/L ZrSiO4后获得了ZrO2封孔并增韧的MAO涂层，硬度高达HV167.16，而杨氏模量（652MPa）则成半降低。探索了含B低碳钢表面微弧氧化(MAO)处理技术，发明的新型MAO电解液配方，通过适量Na2CO3和Na2B4O7的添加和相互协同作用，获得了绝缘性良好的α-Al2O3涂层；而电解液中添加6 g/L PTFE和2 ml/L甘油制备的MAO-PTFE复合涂层可形成致密结构，腐蚀速率仅为1.71 g•m-2•h-1，电阻率比裸合金增加1个数量级，解决了包括汽车中通道在内的AZ91合金汽车部件之间及其与异类金属的绝缘连接问题。制备的铸态AZ91-0.7CaO合金比AZ91合金抗压强度和屈服强度分别提高82%和67%，压缩率从12%提升到17.5%，72h盐雾腐蚀面积减少1/3，CaO颗粒强韧化和提高AZ91合金耐蚀性的机理主要是细化晶粒，降低第二相数量和尺寸，其作用与添加适量RE元素相当，已试用于汽车轮辐和仪表盘管梁。.综上，项目成果解决了大型镁合金汽车复杂结构铸件成形过程中的关键问题，为此类产品的生产提供了理论和技术指导，研究成果将应用于后续车型的各类大型镁合金铸件的制造，对汽车轻量化和节能减排具有一定的指导意义。