Evaluation of defect density, microstructure, residual stress, elastic modulus, hardness and strength of laser-deposited AISI 4340 steel

Laser-aided direct metal deposition (DMD) was used to form a AISI 4340 steel coating on an AISI 4140 steel substrate. Stress in the coating was relieved before mechanical testing. The defect density and microstructural property of the DMD coating were analyzed. The thermal analysis, continuous cooling transformation (CCT), residual stress, micro-hardness, nano-hardness and elastic modulus of the DMD coating before and after stress relief were studied. Tensile testing, lap shear testing and bend testing were done on the stress-relieved DMD coatings. Finally, evaluation of the effect of porosity on fracture behavior was characterized. A porosity and residual stress-based fracture mechanism was proposed. A better deposition strategy is needed to improve the microstructure (decrease porosity), and the mechanical properties of DMD could be tailored through appropriate heat treatments.