Fracture Characteristics and Heat Treatment of Laser Powder Bed Fusion Additively Manufactured GRCop-84 Copper

Laser Powder Bed Fusion (LPBF) of Glenn Research Copper 84 (GRCop-84), a Cr2Nb (8 at. % Cr, 4 at. % Nb) precipitation hardened alloy, produces a fully dense high conductivity alloy with a yield strength of 500 MPa and ultimate tensile strength (UTS) of 740 MPa, superior to other competing copper alloys, and 20% elongation at fracture for material stressed perpendicular to the build direction. The high thermal stability of the Cr2Nb precipitate in the copper matrix reduces coarsening and maintains a 300 MPa yield, 520 MPa UTS and 26% elongation after a 900°C, 5-hour heat treatment, while a 3h 450°C heat treatment increases yield to 810 MPa, UTS to 970 MPa with 9% elongation at fracture, for samples stressed perpendicular to the build direction. Tensile strength anisotropy based on print direction was attributed to internal stress and columnar grain formation. Void nucleation during tensile fracture was initiated by brittle fracture of precipitate particles within the copper matrix. Fracture cusps contain matching precipitate fragment geometry on opposing sides located near the cusp center in at least 80% of fracture cusps. An optimal precipitate size of 100 nm is predicted for maximum tensile strength from precipitates on fracture surfaces, while tensile testing with varying heat treatments shows maximum strength with 100 nm and smaller precipitates. Cr2Nb precipitates are shown to transition between polycrystalline and monocrystalline structures at high temperature.

激光粉末床熔融（LPBF）技术对Glenn研究铜84（GRCop-84）合金进行加工，该合金为Cr2Nb（8原子百分比的铬，4原子百分比的铌）沉淀硬化合金，能够制造出全致密的具有高导电性的合金，其屈服强度为500 MPa，抗拉强度（UTS）为740 MPa，优于其他竞争性的铜合金，且在垂直于构建方向的应力作用下，材料断裂延伸率达到20%。铜基体中Cr2Nb沉淀物的高热稳定性降低了粗化现象，并在900°C、5小时的热处理后，保持了300 MPa的屈服强度、520 MPa的抗拉强度和26%的断裂延伸率。而3小时450°C的热处理则将屈服强度提升至810 MPa，抗拉强度提升至970 MPa，断裂延伸率为9%，对于垂直于构建方向的样品。基于打印方向的拉伸强度各向异性归因于内部应力和柱状晶粒的形成。拉伸断裂过程中的空穴成核是由铜基体中沉淀颗粒的脆性断裂引起的。在至少80%的断裂尖端，可见与尖端中心位置相对应的沉淀碎片几何形状。预测最佳的沉淀物尺寸为100纳米，以实现断裂表面沉淀物最大的抗拉强度。而不同热处理条件下的拉伸测试表明，具有100纳米及以下尺寸的沉淀物具有最大的强度。研究表明，Cr2Nb沉淀物在高温下会在多晶和单晶结构之间发生转变。