Design of high strength Fe–C–Cu alloys for laser powder bed fusion

Abstract

Two Co- and Ni-free high strength steel alloys from the Fe–C–Cu system, Fe-0.2C–6Cu wt.% (FeCu) and Fe-0.2C-3.5Cu-0.2Si–1Mo–1Cr-0.2 V wt. % (FeCu+), were designed and evaluated for use in laser powder bed fusion (LPBF) printing for non-stainless automotive applications. A computational materials design approach combined with melt-spinning was employed to design Co- and Ni-free printable compositions. Custom gas atomized powders were acquired and characterized for chemistry, contaminants, defects, and particle size distribution. Parameter development was performed on a Renishaw AM250 to obtain specimens with consolidation greater than 99.8% density. Single step heat treatment studies determined peak hardnesses of 426 VHN for the FeCu alloy and 435 VHN for the FeCu+ alloy. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) revealed the heat-treated microstructure to be lath martensite with 2.6–2.8 nm diameter Cu precipitates, within the same diameter range as previously reported for BCC Cu strengthening in other steels. Tensile tests demonstrated yield strengths of 1208 MPa for the FeCu alloy and 1274 MPa for the FeCu+ alloy. Process-induced porosity and lack-of-fusion defects were observed on the fracture surfaces of the printed tensile specimens. With further parameter optimization, these alloys have the potential to be a low-cost, high-strength steel option for use in metal 3D printing for automotive applications.