Oxide Evolution During the Solidification of 316L Stainless Steel from Additive Manufacturing Powders with Different Oxygen Contents

The oxide evolution during the solidification of 316L stainless steel from additive manufacturing powders with different oxygen contents is studied by in situ observation of the melting and solidification of the powder materials, advanced characterization of the solidified materials, and non-equilibrium thermodynamic analysis. An oxide evolution map is established for the 316L powders with different oxygen contents. It reveals the relationship between the surface oxidation in the reused powder and its expected oxide species and morphology in the as-solidified component. For the 316L powder with oxygen content higher than ~ 0.039 pct, the liquid oxide formed first from the steel melt and then crystallized to certain oxide phases during solidification, while for the powder with lower oxygen, oxide phases are suggested to directly form from the steel melt. The oxide species in the as-solidified sample was predicted by the Scheil–Gulliver cooling calculation and verified by the TEM-based phase identification. The oxides formed in the melt of low O 316L alloy (0.0355 pct O) are predicted to be (Mn, Cr)Cr2O4 spinel and SiO2 oxide. In the high O (0.4814 pct O) 316L melt solidification, the final oxides formed are (Mn, Cr)Cr2O4 spinel, SiO2 oxide, and Cr2O3 corundum. As an important characteristic of powder materials, the oxygen pick-up due to the powder surface oxidation significantly influences the inclusion evolution in the powder fusion process.

Scanning Strategies in the PBF Process: A Critical Review

The powder-bed fusion (PBF) process is capable of producing near-fully dense metallic parts; however, various defects — particularly thermal abnormalities — can still be observed during the process. Some of these thermal defects — cracks, distortion, delamination of layers, and microporosity — cannot be removed by post-processing operations. The majority of these abnormalities are the result of residual stress, heat accumulation, lack of inter-track /inter-layer bonding, lack of powder fusion, or a combination of these factors. Modifying the scanning strategy (the topology of scanning tracks) can efficiently mitigate these abnormalities by adjusting the process parameters and adopting proper scanning patterns. The implementation of different scanning strategies significantly changes the ultimate quality of printed parts and manufacturing process lead time. Choosing a proper scanning strategy minimizes the residual stress and internal porosity, generates homogeneous microstructure, and avoids heat accumulation throughout the part during the printing process. In this work, we conducted a critical review of different scanning strategies, their pros and cons, limitations, and influence on the resulting properties of fabricated parts. Furthermore, we report the latest efforts for improvement of the current scanning strategies and introduce the-state-of-the-art strategies in the multi-laser PBF (ML-PBF) process. The insights provided here can assist scholars in evaluating existing scanning strategies and scanning patterns, and in identifying ways both to overcome scanning limitations and to modify them. On the other hand, it can assist manufacturers in selecting the best scanning strategies for their products based on their designs, demands, and resources.

Numerical Model and Experimental Validation for Laser Sinterable Semi-Crystalline Polymer: Shrinkage and Warping

Shrinkage and warping of additive manufacturing (AM) parts are two critical issues that adversely influence the dimensional accuracy especially in powder bed fusion processes such as selective laser sintering (SLS). Powder fusion, material solidification, and recrystallization are the key stages causing volumetric changes of polymeric materials during the abrupt heating–cooling process. In this work, the mechanisms of shrinkage and warping of semi-crystalline polyamide (PA) 12 in SLS are well investigated. Heat-transfer and thermo-mechanical models are established to predict the process-dependent shrinkage and warping. The influence of raw material- and laser-related parameters are considered in the heat-transfer and thermo-mechanical models. Such models are established considering the natural thermal gradient and dynamic recrystallization, which induce internal strain and volumetric change. Moreover, an experimental design via orthogonal approach is introduced to validate the feasibility and accuracy of the proposed models. Finally, the quantitative relationships of process parameters with product shrinkage and warping are established; the dimensional accuracy in part-scale can be well predicted and validated with printed parts in a real experiment.

Optimization of Microstructural Evolution during Laser Cladding of Ni Based Powder on GCI Glass Molds

In glass industry, laser cladding is an innovative surfacing technique allowing to deposit a layer of nickel to protect glass mold against corrosion, abrasion and thermal fatigue. This method (powder fusion by projection), well known in additive manufacturing represents a real technological leap for the glass industry. But during laser cladding of Ni-based powder on gray cast iron, cracks can be observed for some process conditions. These cracks are often due to the Heat Affected Zone that creates structural stresses linked to the development of a martensitic structure in the ferritic matrix of the lamellar graphite cast iron. The aim of this work is to observe the impact of laser cladding (without substrate pre-heating usually employed to limit cracking) on the coating behavior but also on the flake-graphite cast iron substrates. The microstructure and the mechanical properties were studied (SEM and microanalysis, microhardness) around the interface cladding/substrate. Also, the impact of the processing parameters (power P (1500-2300 W), scanning speed v (2.5-10 mm/s) and powder feeding rate PFR (24.5-32.5 g/min) was studied by using the ANOVA (ANalysis Of VAriance) technique. It has been observed that laser cladding on graphite cast iron is possible without cracks by limiting the linear energy induced by the process. Also, an optimization of the processing parameters (P, v, PFR) in order to obtain the industrial expected geometry of the coating has been proposed.