ABSTRACT
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.
ABSTRACT
To achieve a core strength that meets the requirements during service life, components to be nitrided are subjected to a pre-heat treatment. Since a higher strength prior to nitriding also has a positive effect on the achievable strength in the nitrided layer, an optimization of the pre-heat treatment may lead to better service characteristics of nitrided components. For this purpose, different optimizations of pre-heat treatment were investigated on the nitriding and quenching and tempering steels EN31CrMoV9 and EN42CrMo4 (AISI4140). One strategy was a change of the austenitization temperature for EN31CrMoV9 from 870 °C to 950 °C in order to solve the coarse carbides of the as-delivered state and realize a finer distribution of the carbides in the quenched and tempered structure. This special treatment lead to a higher hardness compared to the conventional treatment. The second investigated pre-heat treatment variant was a bainitic treatment instead of quenching and tempering. The bainitic initial microstructure increased the diffusion depth compared to conventionally quenched and tempered specimens. In addition the maximum hardness of the nitrided layer, the core hardness was significantly higher on the specimens with the bainitic microstructure. During subsequent nitriding, however, the bainite is tempered and loses some of its hardness.
ABSTRACT
Metal foam inserts are known for their high potential for weight and vibration reduction in composite gear wheels. However, most metal foams do not meet the strength requirements mandatory for the transfer of sufficiently high levels of torque by the gears. Syntactic iron and steel foams offer higher strength levels than conventional two-phase metal foams, thus making them optimum candidates for such inserts. The present study investigates to what extent surface hardening treatments commonly applied to gear wheels can improve the mechanical properties of iron-based syntactic foams. Experiments performed thus focus on case hardening treatments based on carburizing and carbonitriding, with subsequent quenching and tempering to achieve surface hardening effects. Production of samples relied on the powder metallurgical metal injection molding (MIM) process. Syntactic iron foams containing 10 wt.% of S60HS hollow glass microspheres were compared to reference materials without such filler. Following heat treatments, the samples' microstructure was evaluated metallographically; mechanical properties were determined via hardness measurements on reference samples and 4-point bending tests, on both reference and syntactic foam materials. The data obtained show that case hardening can indeed improve the mechanical performance of syntactic iron foams by inducing the formation of a hardened surface layer. Moreover, the investigation indicates that the respective thermo-chemical treatments can be applied to composite gear wheels in exactly the same way as to monolithic ones. In the surface region modified by the treatment, martensitic microstructures were observed, and as consequence, the bending limits of syntactic foam samples were increased by a factor of three.
ABSTRACT
Medium manganese steels can exhibit both high strength and ductility due to transformation-induced plasticity (TRIP), caused by metastable retained austenite, which in turn can be adjusted by intercritical annealing. This study addresses the laser additive processability and mechanical properties of the third-generation advanced high strength steels (AHSS) on the basis of medium manganese steel using Laser Powder Bed Fusion (LPBF). For the investigations, an alloy with a manganese concentration of 5 wt.% was gas atomized and processed by LPBF. Intercritical annealing was subsequently performed at different temperatures (630 and 770 °C) and three annealing times (3, 10 and 60 min) to adjust the stability of the retained austenite. Higher annealing temperatures lead to lower yield strength but an increase in tensile strength due to a stronger work-hardening. The maximum elongation at fracture was approximately in the middle of the examined temperature field. The microstructure and properties of the alloy were further investigated by scanning electron microscopy (SEM), hardness measurements, X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and element mapping.
ABSTRACT
A change in component design could help achieve objectives in lightweight construction. However, lightweight component design can incur serious distortion problems after the final heat treatment due to reduced stiffness or asymmetries in the mass distribution. The analysis of design modification through geometrical variations and their consequences on the distortion behavior through experiments can be costly and time consuming. In this paper, using 3D simulation models, different modified lightweight geometries are simulated. Using these simulation results, the authors try to understand the complex distortion behavior and correlate it with the effects of design modification.
ABSTRACT
If conventional methods are used, the development of new structural materials is experience-based, but still intensive in terms of materials, time and cost. As part of the development of a new method for material development and characterization, particle-oriented peening is used in this work. By means of samples of different sizes-but matching microstructures (100Cr6 (AISI 52100), five different material states)-it is examined whether the quantities determined on microscopic samples can be transferred to macroscopic samples. Therefore, peening processes with matching peening parameters but different deformation related aims are compared. While the particle-oriented peening is used to deform the microscopic samples (d = 0.8 mm), the new method of single-impact peening is used to deform the macroscopic samples. For the cross-scale comparison, values characterizing the plastic material deformation (∆l and rf, rc) are used as well as the particle velocities after the impact influenced by the elasto-plastic material properties. It could be shown that the highly dynamic (material) behavior is comparable in both dimensions. For the future examination of new (unknown) material states it is therefore conceivable to make predictions regarding their material behavior and later on regarding their material properties on the basis of particle-oriented peening of quickly generated microscopic samples e.g., from drop-on-demand processes.
ABSTRACT
In recent years, magnetocaloric materials have been extensively studied as materials for use in alternative cooling systems. Shaping the magnetocaloric material to thin-walled heat exchanger structures is an important step to achieve efficient magnetocaloric cooling systems. In the present work, experimental investigations were carried out on the heat treatment of LaFe11.4Si1.2Co0.4 alloy processed by Laser Beam Melting (LBM) technology. Due to the rapid solidification after melting, LBM results in a refined micro structure, which requires much shorter heat treatment to achieve a high percentage of magnetocaloric 1:13 phase compared to conventional cast material. The influence of the heat treatment parameters (temperature, time, and cooling rate) on the resulting microstructure has been extensively studied. In addition to the conventional heat treatment process, induction technology was investigated and the results were very promising in terms of achieving good magnetocaloric properties after short-time annealing. After only 15 min holding time at 1373 K, the magnetic entropy change (∆S) of -7.9 J/kg/K (0-2 T) was achieved.
ABSTRACT
The development of novel structural materials with increasing mechanical requirements is a very resource-intense process if conventional methods are used. While there are high-throughput methods for the development of functional materials, this is not the case for structural materials. Their mechanical properties are determined by their microstructure, so that increased sample volumes are needed. Furthermore, new short-time characterization techniques are required for individual samples which do not necessarily measure the desired material properties, but descriptors which can later be mapped on material properties. While universal micro-hardness testing is being commonly used, it is limited in its capability to measure sample volumes which contain a characteristic microstructure. We propose to use alternative and fast deformation techniques for spherical micro-samples in combination with classical characterization techniques such as XRD, DSC or micro magnetic methods, which deliver descriptors for the microstructural state.
