Elucidating laser directed energy deposition based additive manufacturing of copper-stainless steel functionally graded material: Processing and material behavior

Copper (Cu)-stainless steel (SS 304L) multi-material is enduring research topic for various engineering appli-cations such as tooling industry, aerospace, power-generation and cryogenic applications. Laser directed energy deposition (LDED) based additive manufacturing is a one of the promising technologies for fabricating Cu-SS multi-material component. However, LDED of Cu-SS is difficult due to the significant difference in thermo-physical properties and poor solubility of Cu and Fe. One of the methods to overcome this issue is adopting a functionally graded material (FGM) approach. In this pretext, the fabrication of Cu-SS FGM using the LDED process is taken up in the present study with three different Cu compositional grading approaches, such as direct (GD100Cu), 50 % (GD50Cu) and 20 % (GD20Cu). The LDED process windows for single tracks and bulk deposition of Cu-SS FGM are identified by varying the laser power, scan speed and composition of Cu and SS304L. Subsequently, the identified process window is used to deposit bulk structures of Cu-SS FGM. The LDED-built Cu-SS FGMs are characterized in terms of build quality, chemical composition, microstructural evolution and mechanical behavior. Microscopic analysis reveals, the defect-free deposition of GD100Cu and GD50Cu, at identified process parameters. For GD20Cu, the micro-crack formation in the graded region and inter-layer porosity in the Cu-rich region are two competing phenomenon, and it is observed that layer-wise increase in laser power and interlayer delay leads to minimum crack density and lower porosity. Further, increase in Cu content leads to significant change in microstructural morphology from columnar dendrites to fine cellular morphology. In Cu-SS FG structures, the chemical composition and mechanical properties are found to be changing gradually. Further, obtained ultimate tensile strength (UTS) of GD100Cu and GD50Cu is found higher than GD20Cu and the mode of failure is found to be ductile. The improved UTS in GD100Cu and GD50Cu in comparison with pure Cu arises due to formation of nano precipitate of Cu in the inter-dendritic zone of Fe rich phase. This establishes the suitability of LDED for building FG Cu-SS components at GD50Cu over GD20Cu.