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With the aim of modifying the microstructure and improving the corrosion performance of a wire arc additive manufactured 420 martensitic stainless steel, heat treatment cycles consisting of austenitizing at 1,150°C followed by air cooling and subsequent tempering at different temperatures (300°C, 400°C, 500°C, and 600°C) were applied to the as-printed alloy. Microstructural analysis revealed that the austenitization and subsequent air-cooling treatment led to the removal of retained austenite and delta ferrite from the as-printed structure, while the tempering process resulted in the precipitation of a variety of carbide particles at different tempering temperatures. Electrochemical tests performed in an aerated 3.5 wt% NaCl solution showed that tempering at 400°C led to the highest corrosion resistance, while tempering at 500°C deteriorated the alloy’s resistance against localized corrosion. The most stable passive layer was found to form on the 400°C tempered sample due to the uniformity of Cr-concentration in the formed carbide precipitates and their surrounding matrix. However, Cr-rich carbide precipitates formed in the 500°C tempered sample were found to deteriorate the passive film stability throughout the immersion time in the electrolyte.

In this study, wire arc additive manufacturing process is employed to fabricate a low-carbon low-alloy steel block, using an ER70S-6 solid wire. Three sets of samples with different orientations, including perpendicular (Vertical), parallel (Horizontal), and 45° (45-degree) relative to the deposition plane, were prepared in order to investigate the anisotropy in mechanical properties and microstructure of the fabricated part. Both Horizontal and 45-degree samples showed a uniform microstructure containing mostly ferritic grains with a small volume fraction of pearlite at their grain boundaries. Differently, a periodic microstructure was detected in the Vertical sample, consisting of a combination of acicular ferrite, bainite, and allotriomorphic ferrite formed in the interlayer regions in addition to polygonal ferrite within the melt pools’ center. Moreover, the uniaxial tensile and Charpy impact results exhibited isotropic tensile, yield, elongation, and impact properties for both Horizontal and 45-degree samples; however, the Vertical sample showed a lower mechanical performance. The improved mechanical properties of the Horizontal and 45-degree samples were correlated to their uniform ferritic microstructure.

The emerging technology of wire arc additive manufacturing (WAAM) has been enthusiastically embraced in recent years mainly by the welding community to fabricate various grades of structural materials. In this study, ER70S-6 low-carbon low-alloy steel wall was manufactured by WAAM method, utilizing a gas metal arc welding (GMAW) torch translated by a six-axis robotic arm, and employing advanced surface tension transfer (STT) mode. The dominant microstructure of the fabricated part contained randomly oriented fine polygonal ferrite and a low-volume fraction of lamellar pearlite as the primary micro-constituents. Additionally, a small content of bainite and acicular ferrite were also detected along the melt-pool boundaries, where the material undergoes a faster cooling rate during solidification in comparison with the center of the melt pool. Mechanical properties of the part, studied at different orientations relative to the building direction, revealed a comparable tensile strength along the deposition (horizontal) direction and the building (vertical) direction of the fabricated part (~ 400 MPa and ~ 500 MPa for the yield and ultimate tensile strengths, respectively). However, the obtained plastic tensile strain at failure along the horizontal direction was nearly three times higher than that of the vertical direction, implying some extent of anisotropy in ductility. The reduced ductility of the part along the building direction was associated with the higher density of the interpass regions and the melt-pool boundaries in the vertical direction, containing heat-affected zones with coarser grain structure, brittle martensite–austenite constituent, and possibly a higher density of discontinuities.

Wire arc additive manufacturing (WAAM) is a pioneer additive-based technology for fabrication of large-scale engineering components. Despite the many advances in the field of additive manufacturing, formation of solidification defects, including discontinuities and microstructural imperfections, in the fabricated components is still inevitable, regardless of the feedstock material or fabrication process applied. In this study, the effects of solidification defects on the anisotropic mechanical properties of a low-carbon low-alloy steel (ER70S-6) wall produced by WAAM have been investigated. Analysis of the microstructure and mechanical properties of the fabricated part confirmed the formation of various solidification defects, i.e., interpass lack of fusions, localized brittle zones, and grain coarsening in the heat-affected zones, leading to anisotropic behavior in the ductility along the deposition versus building directions of the component. The effect of discontinuities on the anisotropic mechanical properties was minimized through microstructural modifications of the fabricated part using postprinting normalizing heat treatment.

This study aims to investigate the effects of annealing, quenching, and tempering (Q&T) heat treatments on the microstructure, crystallographic orientation, and electrochemical performance of a wall shaped 420 martensitic stainless steel part fabricated by wire arc additive manufacturing technology. The formation of a martensitic matrix with delta ferrite in the as-printed sample, islands of spherical chromium carbides embedded in a ferritic matrix in annealed sample, and intergranular chromium-rich carbides along the primary austenite grain boundaries in addition to intra-lath Fe-rich carbides in the quenching and tempering heat treated sample were detected. To characterize the corrosion performance of the fabricated samples, open circuit potential, potentiodynamic polarization, and electrochemical impedance spectroscopy tests were performed on all samples in aerated 3.5 wt.% NaCl electrolyte at room temperature. The corrosion morphology of the as-printed sample was characterized by localized corrosion attacks adjacent to the delta ferrite phase, while severe pitting occurred in the annealed sample due to the high susceptibility of ferritic matrix-carbide interface to pitting. In contrast to the as-printed and annealed sample, the electrochemical performance of the quenched and subsequently tempered samples was found to be significantly improved, ascribed to elimination of the chromium depleted regions adjacent to the delta ferrite phase, and enhanced protectiveness of the passive film on the alloy’s surface.

Wire arc additive manufacturing (WAAM) is known to induce a considerable microstructural inhomogeneity and anisotropy in mechanical properties, which can potentially be minimized by adopting appropriate post-printing heat treatment. In this paper, the effects of two heat treatment cycles, including hardening and normalizing on the microstructure and mechanical properties of a WAAM-fabricated low-carbon low-alloy steel (ER70S-6) are studied. The microstructure in the melt pools of the as-printed sample was found to contain a low volume fraction of lamellar pearlite formed along the grain boundaries of polygonal ferrite as the predominant micro-constituents. The grain coarsening in the heat affected zone (HAZ) was also detected at the periphery of each melt pool boundary, leading to a noticeable microstructural inhomogeneity in the as-fabricated sample. In order to modify the nonuniformity of the microstructure, a normalizing treatment was employed to promote a homogenous microstructure with uniform grain size throughout the melt pools and HAZs. Differently, the hardening treatment contributed to the formation of two non-equilibrium micro-constituents, i.e., acicular ferrite and bainite, primarily adjacent to the lamellar pearlite phase. The results of microhardness testing revealed that the normalizing treatment slightly decreases the microhardness of the sample, however, the formation of non-equilibrium phases during hardening process significantly increased the microhardness of the component. Tensile testing of the as-printed part in the building and deposition directions revealed an anisotropic ductility. Although normalizing treatment did not contribute to the tensile strength improvement of the component, it suppressed the observed anisotropy in ductility. On the contrary, the hardening treatment raised the tensile strength, but further intensified the anisotropic behavior of the component.

Wire arc additive manufacturing (WAAM) was applied to fabricate precipitation hardened (PH) 13–8Mo martensitic stainless steel parts for applications in injection molding equipment, aerospace components, and marine. The microstructural features, microhardness evaluation, and room temperature tensile properties of the as-printed parts have been comprehensively investigated at different locations and orientations. The experimental results showed that the predominant microstructure of the WAAM fabricated PH 13–8Mo stainless steel part mainly consists of vermicular and lathy remnant δ-ferrite embedded in a low-carbon fine martensitic matrix in addition to a small fraction of retained austenite. The content of both retained austenite and remnant δ-ferrite reduced through the building direction of the WAAM fabricated wall, causing a gradual increase of microhardness and ultimate tensile strength values from the bottom to the top of the component. Moreover, the measured tensile strength and ductility in horizontal and vertical directions of the wall revealed anisotropic mechanical properties associated with the columnar growth of primary δ-ferrite grains structure formed during the solidification of the material, creating a strong cubic texture along the building direction. The present study, for the first time, revealed successful implementation of WAAM technology to fabricate PH 13–8Mo stainless steel parts with comparable hardness and tensile strength with respect to other fabrication methods.

In this study, the effects of inter-layer temperature variation on the microstructure, crystallographic orientation, and corrosion performance of a multilayer single-pass wall-shaped 420 stainless steel part fabricated using wire arc additive manufacturing (WAAM) were investigated. The microstructural evolution and texture formation after fabrication were analyzed using scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD) analysis. The microstructural characterization revealed the formation of a martensitic matrix with delta ferrite and retained austenite as secondary phases as a result of complex thermal history experienced by deposited layers through subsequent reheating and cooling during the fabrication process. The samples deposited at the inter-layer temperature of 200 °C exhibited anisotropic crystallographic behavior, while a relatively isotropic texture was observed for the samples fabricated at the inter-layer temperature of 25 °C. To characterize the electrochemical response of the fabricated samples, open circuit potential (OCP), potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS) tests were conducted on the prepared samples in naturally aerated 3.5 wt% NaCl electrolyte at ambient temperature. The volume fraction of the retained austenite phase was found to increase by increasing the inter-layer temperature from 25 °C to 200 °C, contributed to the enhanced corrosion performance of the fabricated wall. Localized corrosion attacks were primarily detected adjacent to the delta ferrite phase, regardless of the implemented interlayer temperature during the fabrication process.

In this study, thin-wall components of ER420 martensitic stainless steel were fabricated through wire arc additive manufacturing (WAAM) at two different interpass temperatures of 25 °C and 200 °C. The effects of thermal history on the formation, volume fraction, and distribution of retained austenite were characterized by scanning electron microscopy, electron backscattered diffraction, and X-ray diffraction analyses. It was concluded that the higher interpass temperature of 200 °C leads to the formation of a higher content of retained austenite resulted from in-situ quenching and partitioning heat treatment during the fabrication process.

In this study, microstructural features and mechanical properties of a wire arc additively manufactured 420 martensitic stainless steel were investigated in as-printed and heat-treated conditions. Initial microstructural investigations on the as-printed part revealed the formation of residual δ-ferrite during the solidification process, which is known as a deleterious phase to both mechanical and corrosion performance of stainless steels. To remove the residual δ-ferrite and obtain a fully martensitic microstructure, the as-printed samples were subjected to different austenitizing temperatures of 950, 1050, 1150, and 1300 °C. Austenitizing at 1150 °C was selected as the optimum cycle due to removal of undesirable phases, such as δ-ferrite and carbides, resulting in a fully martensitic microstructure. Following the austenitizing heat treatment, the samples were tempered at different temperatures including 200, 300, 400, 500, and 600 °C. Increasing the tempering temperature was found to vary the size, morphology, and distribution of chromium carbides precipitated during the tempering process. Although, tempering at lower temperatures (200 and 300 °C) decreased the hardness due to the formation of tempered martensite and stress relieving of the structure, the intermediate temperature of 400 °C increased the hardness value by virtue of the formation of carbides at optimum size and distribution. However, tempering at 500 and 600 °C decreased the hardness as compared to 400 °C due to intergranular segregation and coarsening of carbides. The results of uniaxial tensile testing were consistent with the hardness measurements and confirmed that the tempering temperature of 400 °C led to the optimal combination of strength and ductility ascribed to the formation of fine and homogenously distributed chromium carbides embedded in a moderately tempered martensitic matrix.

In this study, liquid phase surface nitriding of Ti-6Al-4V was carried out by pre-placing of chromium powder on the substrate and subsequent Tungsten Inert Gas (TIG) surface melting. The effect of the application of low and high heat inputs on the microstructure, microhardness and wear resistance of the treated layers were studied. Surface alloying with chromium in a nitrogen containing atmosphere resulted in the formation of hard intermetallic compounds such as TiN, Cr 2 N and TiCr 2 . Moreover, the presence of beta stabilizer chromium together with the application of high heat input during surface treatment resulted in the presence of beta phase at room temperature. However, applying low heat input could not prevent transformation of beta to martensite. The hardness of the layers fabricated at high and low heat inputs were respectively 1050 and 1200 HV 0.3 compared to average 280 HV 0.3 for the as-received material. Liquid phase surface treatment of titanium at the aforementioned conditions improved the wear resistance. The lowest weight loss belonged to the specimen with the beta phase matrix. The formation of the fairly ductile bcc-β phase hindered crack nucleation during wear. The weight loss in this condition was 7 times lower than that of the base material.

In this study, liquid-phase surface alloying of AZ31 magnesium alloy was conducted by tungsten inert gas (TIG) melting of preplaced ferrosilicon powder. The effects of TIG processing parameters on the formation of a defect-free surface-alloyed layer were investigated. Microstructures of alloyed layers were studied by optical and energy dispersive spectroscopy-enabled scanning electron microscopy. The phases in the resolidified layer were identified by X-ray diffraction analysis coupled with microhardness. The surface hardness increased from 65 HV0.3 for AZ31 magnesium alloy to as high as 330 HV0.3 for alloyed sample due to the formation of Mg2Si and FeSi intermetallic compounds in the alloyed region and structural refinement.

In this paper, liquid-phase surface melting of Ti-6Al-V in atmospheres of argon and a mixture of argon and nitrogen was carried out using tungsten inert gas process. The microstructures of treated layers were investigated by optical microscope, scanning electron microscope, and phases formed were studied by X-ray diffraction pattern. The hardness of these layers was also evaluated using microhardness machine. The results showed that the surface hardness was enhanced from 280 HV for the untreated substrate to 600 HV for the surface-treated layer achieved by surface melting in the argon atmosphere which was due to the formation of martensite microstructures. The surface layer fabricated by surface melting in the mixed atmosphere of argon and nitrogen also resulted in formation of TiN in a martensitic matrix with a hardness of about 1000 HV.

This study investigates the interfacial microstructure and mechanical properties of the joint between a wire arc additive manufactured 420 stainless steel part and its AISI420 wrought base plate. The results revealed a defect-free metallurgical bonding possessing a desired microstructure with better mechanical performance as compared to the substrate.

This study aims to investigate the fabrication feasibility of a conventionally rolled low-carbon low-alloy shipbuilding steel plate (EH36) by emerging wire arc additive manufacturing (WAAM) technology using ER70S feedstock wire. Following the fabrication process, different heat treatment cycles, including air-cooling and water-quenching from the intercritical austenitizing temperature of 800 °C, were applied to both conventionally rolled and WAAM samples. Microstructural features and mechanical properties of both rolled and WAAM fabricated ship plates were comprehensively characterized and compared before and after different heat treatment cycles. Both air-cooling and water-quenching heat treatments resulted in the formation of hard martensite-austenite (MA) constituents in the microstructure of the rolled ship plate, leading to the increased hardness and tensile strength and reduced ductility of the component. On the other hand, air-cooling heat treatment was found to homogenize the microstructure of the WAAM ship plate, causing a slight decrease in the hardness and tensile strength, while the water-quenching cycle led to the formation of acicular ferrite and intergranular pearlite, contributing to the improved mechanical properties of the part. Therefore, the enhanced mechanical integrity of the water-quenched WAAM component as compared to its rolled counterpart verified the fabrication feasibility of the ship plates by the WAAM.