Hybrid additively manufactured tool steel bimetals produced using laser powder bed fusion
University of New Brunswick
This study attempts to broaden our knowledge on the fabrication of tool steel bimetals using powder-based additive manufacturing techniques. It mainly involves the fabrication of bimetals for tool and dies applications in the automotive industry along with broad microstructural and mechanical characterizations, and heat treatment procedures. Two conventionally made alloys, including hot work tool steel H13 and cold work tool steel P20, were selected as the conventionally made substrates. Maraging steel (MS1) powder was used as the feedstock material and deposited on the substrates via the laser powder bed fusion (LPBF) technique. Two bimetallic systems were fabricated and designated as MS1-H13 and MS1-P20 hybrid steels in the as-built condition. Comprehensive investigations were conducted on characterizing the morphological and microstructural aspects of these two hybrid steels as well as their mechanical properties. Scanning electron microscope (SEM), X-ray diffraction (XRD), energy dispersive spectrometer (EDS), electron probe microanalysis (EPMA), electron backscatter diffraction (EBSD), focus ion milling (FIB), and transmission electron microscope (TEM) were utilized as advanced diagnosis techniques to identify the features of hybrid steels at different microscopic scales. Various mechanical tests were conducted at either micro- or macro-scales to determine the mechanical performance of the hybrid steels in service conditions. Microhardness tests, uniaxial tensile measurements, fractography, digital image correlation (DIC) along with various bending tests were carried out for this purpose. Outstanding results were achieved for both hybrid steels. The PBF technique led to the production of a defect-free interface between the substrates and deposited MS1 powder. Cracks, delamination, and porosities were not observed, and a very sound joint was built between both sides of the hybrid steels. The additive side of hybrid steels showed finer morphology, higher hardness, and superior strength in comparison with the substrate side. Both hybrid steels exhibited high mechanical strength and did not fail at the interface, making the hybrid steels suitable for practical use. However, heterogeneity was found in the microstructural and mechanical properties of the as-built hybrid steels since two dissimilar alloys were involved. The heterogeneity across the interface area might impair the efficiency of hybrid steels in working conditions. Therefore, I tried to mitigate this effect on the additively manufactured hybrid steels via post-processing heat treatment. Various heat treatment cycles were, designed, performed, and evaluated on the as-built samples. For MS1-H13, a homogenized martensitic microstructure was generated across the interface using a solution treatment. For MS1-P20, a heat treatment sequence including hardening with subsequent aging was found to be effective in this regard. The mechanical properties were greatly enhanced for both hybrid steels in terms of hardness, yield strength, and fracture strength.