Wire Arc Additive Manufacturing (WAAM), one of the Additive Manufacturing (AM) deposition methods which employs the basic principles of Gas Metal Arc Welding (GMAW) welding technique was used to show that it is possible to build moulds for ceramic products using AISI 420 stainless steel due to its characteristics such as corrosion resistance, machinability, hardness and dimensional stability. The WAAM process also is efficient in terms of its deposition rate, reduced material wastage and high surface quality. To investigate the feasibility of AM of AISI 420 stainless steel using WAAM, Response Surface Methodology (RSM - a predictive technique) was used to navigate within the input parameter range for process optimization. Bead-on-plate welding experiments were performed with a MIG welding robot on a structural steel (S355J2) as the substrate. In the tested range according to RSM analyses, the optimum weld condition was 261 A (Current), 29 V (Volts) and 0.59 m/min S (Travel Speed) with preheating at 200°C. However, this condition was found to be unsuitable for AM due to its low deposition speed, non - uniform building surface morphology and inter-run porosities when overlapping welds were deposited. Further analyses on the metallurgy of the WAAM AISI 420 stainless steel through Scanning Electron Microscope (SEM) revealed that the weld metal consisted of a martensite matrix and delta-ferrite at the grain boundaries. The Vickers Hardness of the weld metal was 514HV. To achieve WAAM feasibility using AISI 420 stainless steel, modified RSM method was performed by expanding the input parameters and visually inspecting the bead for its shape, size and quality. The experiment adopted was called the Ramping Procedure wherein a single resulting weld bead can represent many input parameter combinations. The optimum condition for the input parameters were identified to be 200 A (Current), 18.5 V (Volts) and 1.00 m/min S (Travel Speed). With the optimal processing conditions, rectangular blocks or walls were modelled and designed in the Autodesk Powermill software and built to evaluate the feasibility of WAAM AISI 420 stainless steel. Samples were built without preheating and no surface defects and cracking were observed. Microstructural and hardness studies were then performed. Results show that the as-welded weld metal consisted of delta-ferrite present in a martensite matrix. The hardness of the weld metal was 623 HV. The WAAM optimization procedure for AISI 420 stainless steel that has not been explored for AM processes due to its high sensitivity to welding thermal cycles that can lead to cracking. In this study, it has been successfully demonstrated that crack free AISI 420 stainless steel can be deposited with WAAM.

Purpose: Additive manufacturing has enabled innovative, cost and resource efficient mass customization of products over a wide range of manufacturing industries. Under the broad spectrum of options for manufacturing of metallic components, Wire + Arc Additive Manufacturing, WAAM, offers the combination of high design freedom and productivity at a comparably low cost. As a maturing technology, the application in the building industry of steel and stainless-steel products becomes increasingly interesting. Research is nevertheless necessary for the identification of the material’s performance under dynamic loading. It has been demonstrated that the manufacturing procedure inherently produces a heterogenous material matrix and wavy surface, which may be susceptible to fatigue. Methodology: Based on a broad literature review concentrated on the material performance of stainless steels as welding consumable, stainless-steel SAE316L is chosen as the material of interest for characterization. Test coupons are extracted from heat-treated wire+arc-additive-manufactured single-bead walls to perform monotonic and fully reversed strain-controlled fatigue tests in both longitudinal and build directions. The material’s microstructural characteristics are documented. Test methods include Wavelength Dispersive X-Ray Fluorescence for chemical characterization; X-Ray Diffraction for phase identification and quantification; and Scatter Electron Microscopy and Optical Microscopy for qualitative sub-grain and grain microstructural characterization. The latter methods are subsequently used to characterize the monotonic and fatigue fracture surface morphology of post-mortem test samples. A constitutive model for the material is described based on the results of the monotonic and cyclic behavior in both orthogonal directions. Inspection of the fracture surface of the tested coupons is realized to validate the assumptions made for the application of the fatigue models at hand. Addressing a commonly cited problem, the macroscopic surface waviness of the printed material is quantified, and its effect as a stress concentrator is numerically modelled and verified through a smaller group of tested coupons retaining the original surface waviness. Finally, suggestions for the extrapolation of these results are done for the design of structural components produced with the studied material and designed for Fatigue Limit State. Findings: Microstructural characteristics include a strong metallurgical texture in the build orientation, limiting the validity of X-Ray diffraction results. Ferrite is quantified at approximate 13% volume ratio through image analysis of SEM micrographs. Possible traces of σ-phase are identified through optical microscopy, specifically in inter-grain boundaries, although further work is necessary to confirm their constitution. Fracture surface analysis indicates mostly ductile material deformation, confirmed by the elongation capacity of tensile test results, and a predominantly smooth fatigue fracture surface populated by ridges with isolated striation locations, indicating crack propagation at a low stress intensity amplitude. The material shows marginally lower performance in terms of quasi-static characteristics when compared to the one cited by the consumable manufacturer for all-weld material. This remains nevertheless consistently higher than the allowed design values published by commercially available standards. A notoriously low elastic modulus is observed, source of which remains unidentified; the elastic modulus is measured between 54% and 58% of that of commercially available hot-rolled stainless-steel alloys. Cyclic material performance corresponds with all-weld predicted performance for ferritic-austenitic stainless-steels, where the first few cycles are characterized by a slight material hardening, followed by a consistent softening throughout the cyclic life of the material. Average fatigue performance is undistinguishable from mean design values proposed for structural stainless steels under standardized testing conditions. At a 95% reliability and 75% confidence interval, the material performance is superior to the design curves proposed by ASME. The theoretical stress concentration obtained through numerical analysis of the surface geometry is modeled as a function of maxima between 3.150 and 3.927 in the build orientation with a 75% reliability for component lengths of 20 mm for the former value and 100 mm for the later value. Similarly, the values 1.944 and 2.413 in the longitudinal direction describe the stress concentration effect for 20 mm and 100 mm component lengths correspondingly. Originality / Value: The research contributes to the characterization efforts made for the improvement of Wire+Arc additive Manufacturing technologies. This work is carried out both at a microstructural and a bulk level, allowing to establish a correlation between the manufacturing parameters, the microstructural morphology and the mechanical behavior of a given material. Design aids are presented at a typically cited reliability level for engineering practice, accompanied by a thorough description of design considerations based on the material’s inherit characteristics as a product of WAAM process.