Dilatometric studies in 18-Ni steel components fabricated by selective laser melting technique were carried out to determine the influence of heating rate on transitions occurring during the heating cycle. SLM components have been examined in controlled heating and cooling cycles. For analysis, heating of the analysed materials was carried out at heating rates of 10, 15, 20, 30 and 60 °C min −1 . During the heating process, two solid-state reactions were identified-i.e. precipitation of intermetallic phases and the reversion of martensite to austenite. A simplified procedure based on the Kissinger equation was used to determine the activation energy of individual reactions. For precipitation of intermetallic phases, the activation energy was estimated 301 kJ mol −1 , while the martensite to austenite reversion was determined at the activation energy 478 kJ mol −1 .Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
This research aims to characterize and examine the microstructure and mechanical properties of the newly developed M789 steel, applied in additive manufacturing. The data presented herein will bring about a broader understanding of the processing–microstructure–property–performance relationships in this material based on its chemical composition and heat treatment. Samples were printed using the laser powder bed fusion (LPBF) process and then the solution was annealed at 1000 °C for 1 h, followed by aging at 500 °C for soaking times of 3, 6 and 9 h. The AM components showed a relative density of 99.1%, which arose from processing with the following parameters: laser power of 200 W, laser speed of 340 mm/s, and hatch distance of 120 µm. Optical and electron microscopy observations revealed microstructural defects, typical for LPBF processes, like voids appearing between the melted pools of different sizes with round or creviced geometries, nonmelted powder particle formation inside such cavities, and small spherical porosity that was preferentially located between the molten pools. In addition, in heat-treated conditions, AM maraging steel has combined oxide inclusions of Ti and Al (TiO2:Al2O3) that reside along the grain boundaries and secondary porosities; these may act as preferential zones for crack initiation and may increase the brittleness of the AM steel under aged conditions. Consequently, the elongation of the AM alloy was low (<3%) for both annealed and aged solution conditions. The tensile strength of AM M789 increased from 968 MPa (solution annealed) to 1500–1600 MPa after the aging process due to precipitation within the intermetallic η-phase. A tensile strength and yield point of 1607 ± 26 and 1617 ± 45 MPa were obtained, respectively, after a full heat treatment at 500 °C/6 h. The results show that 3 h aging of solution annealed AM M789 steel achieves satisfactory material properties in industrial practice. Extending the aging time of printed parts to 6 h yields slightly improved properties but may not be worth the effort, while long-term aging (9 h) was shown to even reduce quality.
Production-related preliminary damage and residual stresses have significant effects on the functions and the damage development in fiber composite components. For this reason, it is important, especially for the safety-relevant components, to check each item. This task becomes a challenge in the context of serial production, with its growing importance in the field of lightweight components. The demand for continuous-reinforced thermoplastic composites increases in various industrial areas. According to this, an innovative Continuous Orbital Winding (COW) process was carried out within the framework of the Federal Cluster of Excellence EXC 1075 “MERGE Technologies for Multifunctional Lightweight Structures”. COW is aiming for mass-production-suited processing of special semi-finished fiber reinforced thermoplastic materials. This resource-efficient and function-integrated manufacturing process contains a combination of thermoplastic tape-winding with automated thermoplastic tape-laying technology. The process has a modular concept, which allows implementing other special applications and technologies, e.g. integration of different sensor types and high-speed automated quality inspection. The results show how to control quality and improve the stability of the COW process for large-scale production. This was realized by developing concepts of a fully integrated quality-testing unit for automatic damage assessment of composite structures. For this purpose, the components produced in the COW method have been examined for imperfections. This was performed based on obtained results of non-destructive or destructive materials testing.
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