高强铝合金的选区激光熔化成形技术基础研究

The demand for fabrication of high strength aluminium components with complex thin wall grows rapidly. Selective laser melting (SLM) is a promising method to provide a rapid integrated manufacturing method for them. Because high strength aluminium alloy is easy to oxidize, has the characteristics of high thermal conductivity, low ionization energy and containing lots of alloying elements which are easy to ablation, fabrication of high strength aluminum alloy parts is very difficult and the properties of the fabricated parts are hard to control. This project is proposed to reveal the key technologies, including the interaction mechanism of laser & aluminium alloy powder, characteritics of the molten pool flowing, solidification microstructure and formation mechanism of metallurgical defects, influencing factors and control method of dimension accuracy and surface roughness, heat treatment technology by investigating the unsteady interaction between laser and Al alloy powder. By using the technolgies, such as, taking the mixture of N2 and Cl2 as process atmosphere, taking the diluted plasma to protect the molten pool and the high temperature metal to eliminate the hazard of oxidization generated in the process, homogenizing the laser energy distribution in the laser spot to reduce the nonuniform burning loss of the alloying elements and control the produce of high density plasma, dimension compensating to realize the high accuracy, the fabrication of Al alloy using SLM is expected to be successful and the properties of fabiration parts is expected to be controlled.

高强Al合金复杂精密构件的需求日益旺盛。选区激光熔化成形技术（Selective Laser Melting，SLM）有望为其提供一种快速整体高精度制造方法。高强铝合金对激光吸收率低、热导率高、易氧化且含大量易烧损合金元素，成形难度大，性能难以控制。本项目通过研究激光与高强铝合金粉末的非定常相互作用过程，揭示高功率密度激光与高强合金粉末相互作用机理；高强铝合金SLM成形的熔池流动特性、非平衡凝固组织特点及冶金缺陷形成机理；成形件的尺寸精度、表面粗糙度的影响因素及控制方法；后续热处理技术及其综合性能评价等关键技术基础，得到SLM加工成形高强铝合金机理和质量控制规律。拟采用N2和Cl2混合气体作为成形气氛、稀薄等离子体的"保护"作用等方法来消除氧化的危害，利用均匀光束技术控制合金元素的非均匀烧损和浓密等离子体产生、尺寸补偿实现高精度等技术方案，实现高强铝合金的SLM良好成形和性能控制。

激光选区熔化成形技术有望实现复杂结构功能一体化的高强铝合金构件的整体成形。本项目对激光选区熔化成形2XXX和7XXX高强铝合金的技术基础进行了研究，揭示了激光与铝合金粉末相互作用过程，明晰了非平衡凝固组织演变过程、冶金缺陷形成机制及控制方法，得到了无缺陷高性能成形的工艺窗口及其热处理工艺，实现了显微组织和性能调控方法。发表论文23篇，专利3项，培养学生8名。研究结果表明：铝合金粉末对激光的吸收率随着成形层数的增加是先增加后趋于稳定；成形气氛中的氧含量为200ppm时，氧化物颗粒由于碰撞概率增加而聚集长大，引发提前断裂；7050高强铝合金的Zn元素的烧损影响强化相MgZn2的形成，ZL205A中的Cd元素存在烧损问题影响其后续时效过程中强化相θ’的粗化速率；传导熔池中流动稳定形成粗大的柱状晶，深熔熔池中流动剧烈形成柱状晶与等轴晶夹杂的非平衡凝固结晶组织；低熔点共晶相在柱状晶边界处聚集，SLM成形过程中存在较高的应力，裂纹易在柱状晶边界处产生；采用提高熔池存在时间、降低温度梯度和应力、减少或者增大低熔点共晶相含量的方法可以抑制、甚至消除裂纹。采用足够的能量输入，提高液相金属量和熔池存在时间可以消除未熔合的工艺孔；实现了2XXX系和7XXX系两种高强铝合金无裂纹和高致密成形，其中Al-Cu-Mg和ZL205A铝合金可以在无添加下实现无缺陷高性能成形，7050则需要添加变质剂Si才能实现无缺陷成形；采用Zr元素对Al-Cu-Mg合金进行微合金化，实现了柱状晶到等轴晶转变，降低热裂敏感性，扩大工艺窗口，Zr含量为2 wt.%时，极限抗拉强度达到500 MPa；Al-Cu-Mg合金经过530℃固溶1h的热处理之后，得到了优良的综合静态拉伸性能，极限抗拉强度532±8.5 MPa，屈服强度338±10 MPa，断后伸长率13.6±1.5%；ZL205A合金，固溶处理温度530℃，保温时间6 h后，采用153℃时效48h，可得到优良的综合静态拉伸性能：极限抗拉强度476.2 MPa，屈服强度383 MPa，断后伸长率7.85%；对于7050合金，其优化的热处理制度为：160℃单级时效12h，维氏硬度从115HV升至139HV。