Microstructural engineering in carbon steel walls printed by directed energy deposition to enhance mechanical properties through heat-input control

Abstract

The main object of the present study is focused on the systematic processing heat-input design based on directed energy deposition (DED) methodology, enrolling carbon steel phase transformation to control the mechanical properties. Three different 4-layer low-carbon alloy steel (ER70S-6) walls are fabricated to investigate this matter with varying heat-inputs. To justify heat-inputs, single layers by metal-inert gas (MIG) welding with different heat-inputs were carried out, and the results were used as input data to the 4-layers fabrication. In addition, to get a more homogenous structure, some walls' heat-inputs have descended from the first layer to the top layer. The microstructural studies show that reducing heat-input causes a higher solidification rate and the formation of finer columnar grains, which significantly affects the mechanical properties. Electron microscopy analysis reveals that a higher cooling rate in the wall with descending heat-input from bottom to top (496 J/mm to 382 J/mm), motivates formation of the acicular ferrite and bainite, which significantly enhances the mechanical properties. The exceptional tensile toughness of the wall, ∼28.86 GPa%, with an ultimate tensile strength (UTS) of ∼740 MPa, yield stress of ∼560 MPa, and an elongation to failure of ∼38 % depicted the impact of fine and multi-constituents' microstructure by heat-input control.