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.

采用激光粉末床熔融（Laser Powder Bed Fusion，LPBF）工艺加工格伦研究中心铜84合金（GRCop-84）——一种成分为8at.%铬、4at.%铌的Cr₂Nb沉淀强化合金，可制备出全致密高导电合金，其屈服强度达500 MPa、抗拉强度极限（ultimate tensile strength，UTS）740 MPa，性能优于其他同类竞争铜合金；当试样沿垂直于打印方向加载时，断裂延伸率可达20%。铜基体中的Cr₂Nb析出相具有优异的热稳定性，可抑制粗化现象：经900℃保温5小时的热处理后，仍可维持300 MPa的屈服强度、520 MPa的抗拉强度极限以及26%的断裂延伸率；而经450℃保温3小时的热处理后，垂直于打印方向加载的试样屈服强度提升至810 MPa、抗拉强度极限达970 MPa，断裂延伸率为9%。拉伸强度的各向异性由内部残余应力与柱状晶组织的形成共同导致。拉伸断裂过程中的空穴形核，起源于铜基体中析出相颗粒的脆性断裂。在至少80%的断裂尖点中，其两侧靠近尖点中心位置的析出相碎片几何形貌相互匹配。通过对断裂表面析出相的分析预测，当析出相尺寸为100 nm时可实现抗拉强度峰值；而不同热处理工艺下的拉伸测试结果表明，尺寸为100 nm及更小的析出相可带来最高的拉伸强度。研究证实，Cr₂Nb析出相在高温条件下会在多晶与单晶结构之间发生转变。