Abstract Scope |
Metal additive manufacturing is of tremendous interest has been rapidly evolving with potential towards the improvement of key characteristics. However, some steel products such as as-rolled high-strength low-alloy steel and as-printed stainless steel that fabricated via traditional formative and some AM methods are generally possessing a strong texture and mechanical anisotropy. Although some powder-based AM methods enable a rapid melting-solidifying mode and favors the formation of finer, nearly-equiaxial grains, they are not capable of building completely full-dense, super-large components. To resolve this issue, this study aims at investigating the effectiveness of a novel wire-arc additive manufacturing method in which the heat source is substituted by high-efficiency submerged arc plasma, i.e. submerged arc additive manufacturing (SAAM). An isotropic microstructure with high toughness (greater than 300J at -60℃) can be obtained in large-scale low-carbon steel components by ensuring a complete columnar to equiaxed transition (CET). Our approach allows for adjusting the microstructure in-situ and avoiding post-processing. Additionally, the flexible track and complex geometry accessibility are significant advantages of SAAM that ensure the overall characteristics (the combination of mechanical properties and geometry complexity) of components can compete with those fabricated via conventional approaches and therefore holds tremendous promise in industry manufacturing.
The SAAM component was fabricated using a submerged arc power source guided by 3D programmable controlled modules. EM12K (AWS A5.17, Φ=3.2 mm) and OK Flux 10.62 (AWS A5.17) were used as the consumable alloy and shrouding media, respectively. Previous extensive trials have optimized the parameters for the production of single-pass, multi-layer depositions: current: 490A, voltage: 27V, travel speed: 0.4m/min and 28mm of contact tip-substrate. Thermocouples were used to record the thermal history of the build instead of infrared cameras due to their continuous flux coverage. Their final positions were recorded, and the information was subsequently used to simulate the transformation temperature and phase content across the dual-phase region via a dilatometer using JMatPro software. During fabrication, an alternating scan strategy were adopted and resulting in a straight wall with a uniform deposited bead profile (length ~255 mm and width ~20.5 mm). Note that the interlayer temperatures were maintained at ~300 °C instead of room temperature to ensure a high heat input and low heat dissipation, thereby enabling deeper penetration depth for in-situ intrinsic heat treatment (IHT) and better wettability. After the fabrication, the entire cross-section (parallel to the depositing direction) of a part was selected for characterization. Optical images of the samples were obtained. High magnifications of the metallography, fracture surfaces and morphology of dislocations in different regions of the SAAM part were observed via scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. Electron backscattered diffraction (EBSD) and X-ray diffraction was conducted to study the homogeneity and phase composition of the build. Tensile, Charpy V-notch specimens from different locations and orientations were extracted and tested using an Instron machine and a Charpy impact tester equipped with oscilloscope, respectively. Vickers hardness on the cross-section was also measured.
The top zone of the as-deposited component exhibited columnar α-Fe grains with a typical preferential <001>α orientation while multiple allotropic transformations were triggered by an in-situ IHT in the intermediate zone. When the net bead height was no greater than the width of fine grain zone affected by the previous layer of deposition, the microstructures in each new layer could be progressively refined and thoroughly homogenized. The pearlite revealed by TEM showed a change in morphology from lamellar (~4μm in size) to globular (~2μm in size) and a decrease in area fraction (from ~5% to ~1%), thereby mitigating cracking susceptibility. Moreover, not only were intracrystalline dislocations significantly reduced but their shapes evolved from tangled to movable lines. These changes and finer grains along with the formation of high-angle grain boundaries, led to lower and more dispersed internal strain in the α-Fe matrix. Therefore, the intermediate zone was predominantly comprised fully equiaxed α-Fe and exhibited homogeneous characteristics along the build direction. Excellent Charpy toughness (greater than 300J at -60°C) with minimal deterioration in strength was achieved. The strength versus ductile-brittle transition temperature (DBTT) relations demonstrates our SAAMed steel exhibits a much lower DBTT (-102°C) than most of the existing rolled, additive manufactured and even advanced marine steels. The exceptional combination of strength and DBTT clearly proves that the in-situ toughening mechanism can be highly effective in maximizing the low temperature properties while minimizing production cost. This economical SAAMed component obtained in situ exhibits an irreplaceable role and application prospects in the extremely low temperature environment with moderate strength requirements but extremely high in toughness and DBTT. Hence, this novel method is of value to further advance the welding science and applications.
The novel, economical and high-efficient SAAM method can realize large-scale fabrications with isotropic microstructure and properties which exhibits an irreplaceable role and application prospects in the extremely low temperature environment with moderate strength requirements but extremely high in toughness and DBTT. The establishment of this method will provide a new idea in AM for the fabrication of heavy and complex structures in low-temperature environments. |