选区激光熔化成形7050高强铝合金用纳米复合细化剂及协同机制研究

To refine grains, avoid hot tears and improve the comprehensive properties via a synergetic effect, we, for the first time, propose to develop a kind of nano composite refiner which integrate nano-sized ceramic particles and miacro-sized alloying elements particles for 7050 high-strength aluminum alloy produced by selective laser melting (SLM). Our preliminary study results indicate that the composite refiner not noly decreases the grain sizes, and reduces the pores/tears a lot but also enhances the mechanical properties like strength and hardness of the 7050 aluminum alloy produced by SLM, dramatically. However, the match law and synergetic refinement/strengthen mechanism of the composite refiner are still not clear, till now. In this project, we will ascertain the matching law via integration the structural similarity analysis, and the phase diagram, thermodynamics, kinetics calculations, together with the experimental verification; achieve the SLM production of the 7050 high-strength aluminum alloy with equiaxed grains, no crack, high density, and excellent mechanical properties by combination of the component choices and corresponding addition content of the composite refiner together with the selective laser melting technological conditions; clarify the effect of the composite refiner on the microstructures, and properties as well as the synergetic refinnment/strengthen mechanism of the 7050 aluminum alloy produced by SLM through observing the distributions of the composite refiner components and the interface between the nano-sized ceramic particles assisted with alloying elements and Al matrix as well as first principle calculation; further strengthen the alloy by solution-aging treatment. The project implementation can provide theoretical guidance and technical reference for the composite refiner application in the aluminum alloy additive manufacturing.

本项目首次提出将纳米陶瓷颗粒与微米合金元素颗粒复合成一种适用于选区激光熔化（SLM）成形7050高强铝合金的纳米复合细化剂，以协同作用来细化晶粒、避免热裂并提升合金的综合性能。申请人近期研究发现，该复合细化剂不仅能细化晶粒，显著减少孔洞和热裂纹，还能提高合金强度、硬度，但是复合细化剂匹配规律及协同细化、强化机制等尚不清楚。本项目将通过结构相似性分析及相图、热力学、动力学仿真计算，结合实验，揭示匹配规律；通过调整细化剂组分、添加量及SLM工艺参数，实现晶粒细小均匀、无明显裂纹、致密度高、力学性能优良的7050铝合金成形；通过对复合细化剂各组分在合金中的分布、合金元素协同下纳米陶瓷颗粒与Al界面的观察及第一性原理计算，阐明复合细化剂对7050铝合金组织、性能的影响及协同细化、强化机制，并结合固溶时效处理进一步强化合金，为其在铝合金增材制造中的应用提供理论指导和技术借鉴。

由于7XXX系高强铝合金合金化程度高，凝固范围大，热裂敏感性高，在选区激光熔化（SLM）成形过程中易因凝固收缩产生孔洞及贯穿整个晶粒的热裂纹缺陷并沿晶界扩展，严重影响合金的成形性和力学性能。细小均匀的等轴晶结构能够容纳热应变并防止产生裂纹，提升合金的力学性能。添加合金元素或纳米颗粒可细化晶粒，然而，经合金元素或纳米颗粒单独细化后的7XXX铝合金仍存在力学性能差的问题。因此,我们将纳米陶瓷颗粒与微米合金元素颗粒复合成一类适用于选区激光熔化成形7XXX系高强铝合金的纳米复合细化剂，以协同作用来细化晶粒、避免热裂并提升合金的综合性能。通过结构相似性分析及相图、热力学、动力学仿真计算，结合实验，揭示了选区激光熔化成形高强铝合金用复合细化剂中纳米陶瓷颗粒、合金元素与Al三者相互之间的匹配规律及机制；通过调整细化剂组分、添加量及SLM工艺参数，实现了晶粒细小均匀（平均晶粒尺寸仅为775 nm的等轴晶）、无裂纹、致密度高（99.7%）、力学性能优良（极限抗拉强度408~618 MPa，延伸率13.2~8.8%）的7050铝合金成形；通过对复合细化剂各组分在合金中的分布、合金元素协同作用下纳米陶瓷颗粒与Al界面的观察及第一性原理计算，发现复合细化剂中的合金元素和纳米陶瓷颗粒不仅能各自发挥其对铝基体的细化、强化作用，合金元素还促进纳米陶瓷颗粒的均匀分散、增强纳米陶瓷颗粒与铝基体之间的界面结合强度，协同细化、强化铝合金。此外，这类复合细化剂还能协同细化、强化Al-Si系合金，使其具备平均晶粒尺寸为3~5 μm的等轴晶，极限抗拉强度高达562±7 MPa，延伸率达 8.8 %±1.3 %。这些力学性能均达到或超过传统锻造铝合金的力学性能。对细化后的选区激光熔化成形7050铝合金进行热处理实验结果表明，该类合金可直接进行一步时效处理，使其强度进一步提升。这些结果和理论为增材制造高强铝合金的进一步应用提供了理论指导和技术借鉴。