Chengsong Cui

Nitrogen (N) in steels can improve their mechanical strength by solid solution strengthening. Processing N‐alloyed steels with additive manufacturing, here laser powder bed fusion (PBF‐LB), is challenging as the N‐solubility in the melt can be exceeded. This degassing of N counteracts its intended positive effects. Herein, the PBF‐LB processed 316L stainless steel with increased N‐content is investigated and compared to PBF‐LB 316L with conventional N‐content. The N is introduced into the steel by nitriding the powder and mixing it with the starting powder to achieve an N‐content of approximately 0.16 mass%. Thermodynamic calculations for maximum solubility to avoid N outgassing and pore formation under PBF‐LB conditions are performed beforehand. Based on the results, a higher defect tolerance under fatigue characterized by Murakami model can be achieved without negatively influencing the PBF‐LB processability of the 316L steel. The increased N‐content leads to higher hardness (+14%), yield strength (+16%), tensile strength (+9%), and higher failure stress in short time fatigue test (+16%).

Laser additively manufactured duplex stainless steels contain mostly ferrite in the as-built parts due to rapid solidification of the printed layers. To achieve duplex microstructures (ferrite and austenite in roughly equal proportions) and, thus, a good combination of mechanical properties and corrosion resistance, an austenitic stainless steel powder (X2CrNiMo17-12-2) and a super duplex stainless steel powder (X2CrNiMoN25-7-4) were mixed in different proportions and the powder mixtures were processed via PBF-LB/M (Laser Powder Bed Fusion) under various processing conditions by varying the laser power and the laser scanning speed. The optimal process parameters for dense as-built parts were determined by means of light optical microscopy and density measurements. The austenitic and ferritic phase formation of the mixed alloys was significantly influenced by the chemical composition adjusted by powder mixing and the laser energy input during PBF-LB/M. The austenite content increases, on the one hand, with an increasing proportion of X2CrNiMo17-12-2 in the powder mixtures and on the other hand with increasing laser energy input. The latter phenomenon could be attributed to a slower solidification and a higher melt pool homogeneity with increasing energy input influencing the phase formation during solidification and cooling. The desired duplex microstructures could be achieved by mixing the X2CrNiMo17-12-2 powder and the X2CrNiMoN25-7-4 powder at a specific mixing ratio and building with the optimal PBF-LB/M parameters.

Deep cryogenic treatment of tool steels, incorporated in conventional hardening and tempering, has been a topic of intensive research in recent years. Yet, the governing microstructural mechanisms involved in the deep cryogenic treatment of high‐alloyed tool steels are still controversial. Thus, an in situ X‐ray diffraction study is conducted on three tool steels X38CrMoV5‐3, X153CrMoV12, and ~X190CrVMo20‐4 to shed light on microstructural evolution during cryogenic treatment and subsequent tempering. For these high‐alloyed tool steels, the transformation of retained austenite into martensite is detected during the cooling phase of the cryogenic treatment. A change in tetragonality of martensite occurs mainly in the heating phase of the subsequent tempering process, which indicates the diffusion of carbon and carbide precipitation from the martensite. The microstructure evolution of the tool steels after hardening, cryogenic treatment, and tempering is further examined by scanning electron microscopy.

In order to meet the requirements of micro rotary swaging, the local properties of the tools should be adjusted properly with respect to abrasive and adhesive wear, compressive strength, and toughness. These properties can be optimally combined by using different materials in specific regions of the tools, with a gradual transition in between to reduce critical stresses at the interface during heat treatment and in the rotary swaging process. In this study, a newly developed co-spray forming process was used to produce graded tool material in the form of a flat product. The graded deposit was subsequently hot rolled and heat treated to achieve an optimal microstructure and advanced properties. Micro plunge rotary swaging tools with fine geometrical structures were machined from the hot rolled material. The new forming tools were successfully applied in the micro plunge rotary swaging of wires of stainless steel.

In order to meet the requirements of micro rotary swaging, the local properties of the tools should be adjusted properly with respect to abrasive and adhesive wear, compressive strength, and toughness. These properties can be optimally combined by using different materials in specific regions of the tools, with a gradual transition in between to reduce critical stresses at the interface during heat treatment and in the rotary swaging process. In this study, a newly developed co-spray forming process was used to produce graded tool materials in the form of a flat product. The graded deposits were subsequently hot rolled and heat treated to achieve an optimal microstructure and advanced properties. Micro plunge rotary swaging tools with fine geometrical structures were machined from the hot rolled materials. The new forming tools were successfully applied in the micro plunge rotary swaging of wires of stainless steel.

In order to develop low‐melting‐point filler materials for laser beam brazing of aluminum alloys, Al–Si–Cu based alloys and Al–Si–Zn based alloys were produced via spray forming and associated deformation processes. The selected alloys were spray formed in the form of billet with a diameter around 160 mm and hot extruded into wire rods with a diameter of 8 mm. The extrusions were further cold worked into thin wires with a diameter of 1.2 mm via rotary swaging. Finally, the filler wires were applied in the laser beam brazing of AA6082 structures. It showed that the newly developed filler materials could meet the requirements of the laser beam brazing of aluminum alloys. Particularly, the laser beam brazing process using the spray‐formed filler materials required a significantly lower energy input and allowed for higher brazing speed as compared to the conventional AlSi12 filler material.