Additive manufacturing of carbon steel with site-specific compositions and properties using binder jetting and spark plasma sintering

The rational variation of properties with location in a unitized component can confer highly desirable traits such as site-specific mechanical properties. To achieve voxel level control throughout the 3D printed part, we employed binder jet 3D printing to deposit carbon in selected regions of each layer of the steel powder bed. The printed green part was then de-binded and densified using Spark Plasma Sintering (SPS). With the use of SPS process, the sintering time was reduced dramatically, restricting the diffusion of carbon in steel and allowing a sharp change in carbon concentration over strips as narrow as ∼ 0.5 mm. Our results show that the porosity was reduced from ∼ 10% to ∼ 1% and the grain sizes from ∼ 100 µm to ∼ 10 µm, when compared to conventionally sintered parts, which in turn improved the yield strength (up to 3.2×), ultimate tensile strength (up to 2.5×) and ductility of the samples (up to 1.9×). Isostrain (Voight composite) and isostress (Reuss composite) samples, composed of high carbon steel and low carbon steel materials, exhibited modulus and strength values in between those of the constituents. However, the ductility and toughness of the site-specific patterned parts were lower than either high carbon or low carbon steel for the selected design parameters, which is in good agreement with predictions from analytical modelling and finite element simulations. This result indicates that the ductility and toughness of a composite are not mathematically bounded by that of its constituent materials, unlike modulus, density and strength.