Volker Uhlenwinkel

Duplex stainless steel powders for laser additive manufacturing have not been developed extensively. In this study, the melts of a super duplex stainless steel X2CrNiMoCuWN25-7-4 (AISI F55, 1.4501) were atomized with different process gases (Ar or N₂) at different atomization gas temperatures. The process gas N₂ in the melting chamber leads to a higher nitrogen dissolution in the steel and a higher nitrogen content of the atomized powders. The argon-atomized powders have more gas porosity inside the particles than the nitrogen-atomized powders. In addition, the higher the atomization gas temperature, the finer the powder particles. The duplex stainless steel powders showed good processability during PBF-LB/M (Laser powder bed fusion). The gas entrapment in the powder particles, regardless of the gas chemistry and the gas content, appears to have a negligible effect on the porosity of the as-built parts.

This work aims to show the impact of the allowed chemical composition range of AISI 316L stainless steel on its processability in additive manufacturing and on the resulting part properties. ASTM A276 allows the chromium and nickel contents in 316L stainless steel to be set between 16 and 18 mass%, respectively, 10 and 14 mass%. Nevertheless, the allowed compositional range impacts the microstructure formation in additive manufacturing and thus the properties of the manufactured components. Therefore, this influence is analyzed using three different starting powders. Two starting powders are laboratory alloys, one containing the maximum allowed chromium content and the other one containing the maximum nickel content. The third material is a commercial powder with the chemical composition set in the middle ground of the allowed compositional range. The materials were processed by laser-based powder bed fusion (PBF-LB/M). The powder characteristics, the microstructure and defect formation, the corrosion resistance, and the mechanical properties were investigated as a function of the chemical composition of the powders used. As a main result, solid-state cracking could be observed in samples additively manufactured from the starting powder containing the maximum nickel content. This is related to a fully austenitic solidification, which occurs because of the low chromium to nickel equivalent ratio. These cracks reduce the corrosion resistance as well as the elongation at fracture of the additively manufactured material that possesses a low chromium to nickel equivalent ratio of 1.0. A limitation of the nickel equivalent of the 316L type steel is suggested for PBF-LB/M production. Based on the knowledge obtained, a more detailed specification of the chemical composition of the type 316L stainless steel is recommended so that this steel can be PBF-LB/M processed to defect-free components with the desired mechanical and chemical properties.

Porosity is one of the most important quality criteria of spray‐formed materials in the as‐sprayed condition. Typically, spray‐formed sheets have a porous rim close to the substrate and depending on the spray conditions cold or hot porosity may also be present in the core of the deposit. This porosity has to be removed or minimized to make further processing steps such as rolling, forging or extrusion possible.In this paper, the influence of both substrate temperature and deposit surface temperature on porosity in spray‐formed sheets is studied.For this purpose spray forming experiments (sheet size 1000 mm × 250 mm) were carried out using three different materials: aluminium‐bronze, tin‐bronze and a nitriding steel. For the copper‐base alloys preheated steel‐substrates with different temperatures were moved through a scanning spray cone. In the case of steel a ceramic substrate at room temperature was used. In addition to the variation of the substrate temperature, the gas to metal mass flow ratio (GMR) was varied to achieve different deposit surface temperatures. During the run the surface temperature in the deposition zone was measured using a scanning, multi‐wavelength pyrometer. Samples of the deposits were polished and rasterized by light microscopy. The local porosity was characterized by digital image analysis.The influence of the substrate temperature and the GMR on the porosity in the vicinity of the substrate is evaluated and discussed in detail. The impact of the deposit surface temperature on the porosity was analyzed and is discussed as well. It was found that the deposit surface temperature has a strong impact on porosity for spray‐formed sheets.Finally, experimental results were used to develop a new approach to predict the porosity in spray‐formed sheets. The results clearly show the dependence on material properties. This approach can be used to identify process parameters to generate high density sheets in the future.