Effect of the process atmosphere composition on alloy 718 produced by laser powder bed fusion

The detrimental effect of nitrogen and oxygen when it comes to the precipitation of the strengthening γ’’ and γ’ phases in Alloy 718 is well-known from traditional manufacturing. Hence, the influence of the two processing atmospheres, namely argon and nitrogen, during the laser powder bed fusion (L-PBF) of Alloy 718 parts was studied. Regardless of the gas type, considerable losses of both oxygen of about 150 ppm O2 (≈30%) and nitrogen on the level of around 400 ppm N2 (≈25%) were measured in comparison to the feedstock powder. The utilization of nitrogen as processing atmosphere led to a slightly higher nitrogen content in the as-built material—about 50 ppm—compared to the argon atmosphere. The presence of the stable nitrides and Al-rich oxides observed in the as-built material was related to the transfer of these inclusions from the nitrogen atomized powder feedstock to the components. This was confirmed by dedicated analysis of the powder feedstock and supported by thermodynamic and kinetic calculations. Rapid cooling rates were held responsible for the limited nitrogen pick-up. Oxide dissociation during laser–powder interaction, metal vaporization followed by oxidation and spatter generation, and their removal by processing atmosphere are the factors describing an important oxygen loss during L-PBF. In addi-tion, the reduction of the oxygen level in the process atmosphere from 500 to 50 ppm resulted in the reduction in the oxygen level in as-built component by about 5%

Powder degradation during powder bed fusion processing: impact of processing conditions and alloy composition

In the recent decade, powder bed fusion (PBF) metal additive manufacturing (AM) has attracted huge attention both from the industry and the research community. This effort has helped mature PBF technology as a potential alternative to conventional manufacturing processes as casting, machining, forging, etc. However, there remain various challenges hindering the path of large-scale adoption of these techniques in the manufacturing industry. One such challenge affecting the cost, reproducibility of the products and sustainability of the process, is the reusability of unconsumed powder after each build job. The issue during powder reusability is the likelihood of degraded quality of the reused powder compared to virgin powder either by oxidation during exposure to the atmosphere, or accumulation of process byproducts, referred to as spatters, during processing. The quality degradation of the feedstock powder can lead to an increased number of defects in the produced products and affect the robustness and reproducibility of the PBF process. This thesis is focused on determining the dominant powder degradation mechanisms in powder bed fusion laser-beam (PBF-LB) and powder bed fusion electron-beam (PBF-EB) processes. Here, the approach to investigate the degradation of reused powder is based on the dedicated analysis of changes in powder surface chemistry, analysis of oxygen pick-up, and variation in surface morphology. During the analysis of the powders during the PBF-LB process, three different alloy systems were studied, namely aluminum alloys (AlSi10Mg), nickel-based superalloys (Alloy 718 and Hastelloy X (HX)), and titanium alloys (TiAl6V4). The assessment of powder degradation was initiated with the investigation of AlSi10Mg powder reused for over 30 months. The analysis showed that the powder degradation is mainly triggered by the accumulation of highly oxidized spatter particles in the powder, characterized by the overall greater oxide layer thickness (~75-125 nm) on the surface of powder. These oxidized spatter particles are contributing towards increasing the oxygen content and number of defects in the as-printed components. Analysis of the surface oxide state of spatter particles, generated during the processing of Alloy 718, HX alloy, and TiAl6V4 revealed that the extent of oxidation of spatters from different alloy systems is dependent on the content of oxidation-sensitive elements e.g., Al. Ti, Cr, etc. The impact of the part design in terms of surface to volume ratio of the part on the spatter generation and accumulation was also shown. Results also show an increasing amount of spatter formation with increasing layer thickness per layer deposited. However, the total amount of spatter generated per build job is lower when a higher layer thickness was applied. The results have shown that by employing appropriate processing gas composition containing He the generation of spatter can be reduced. Furthermore, by reducing residual oxygen content in the build chamber, the extent of spatter oxidation can be reduced. Finally, the effect of powder degradation on the quality of fabricated parts was analyzed where the accumulation and redeposition of spatters on the powder bed resulted in a lack of fusion defects, higher porosity, and a decrease in the strength of fabricated parts. In the PBF-EB process, powder oxidation and sublimation of volatile elements during the processing of Alloy 718 have been investigated. The results have identified powder oxidation during PBF-EB processing, due to the long-term powder exposure to high temperature, as the dominant powder degradation mechanism. Furthermore, the sublimation of the alloying elements such as Al and Cr in the case of PBF-EB processing of Alloy 718 was detected

Fatigue response of as built DMLS processed Maraging Steel and effects of machining and heat and surface treatments

The main motivations for this study arise from the need for an assessment of the fatigue performance of DMLS produced Maraging Steel MS1, when it is used in the \u201cas fabricated\u201d state. The literature indicates a lack of knowledge from this point of view, moreover the great potentials of the additive process may be more and more incremented, if an easier and cheaper procedure could be used after the building stage. The topic has been tackled experimentally, investigating the impact of heat treatment, machining and micro-shot-peening on the fatigue strength with respect to the \u201cas built state\u201d. The results indicate that heat treatment significantly enhances the fatigue response, probably due to the relaxation of the post-process tensile residual stresses. Machining can also be effective, but it must be followed (not preceded) by micro-shot-peening, to benefit from the compressive residual stress state generated by the latter

Impact of Powder Variability on the Microstructure and Mechanical Behavior of Selective Laser Melted Alloy 718

Powder-bed additive manufacturing processes use fine powders to build parts layer-by-layer. Alloy 718 powder feedstocks for selective laser melting (SLM) additive manufacturing are produced commercially by both gas and rotary atomization and are available typically in the 10-45 or 15-45 microns size ranges. A comprehensive investigation was conducted to understand the impact of powder variability on the microstructure and mechanical behavior of SLM 718 heat treated to Aerospace Material Specification (AMS) 5664. This study included sixteen virgin powders and three once-recycled powders within the 10-45 and 15-45 microns size ranges that were obtained from seven direct source suppliers and one reseller. Although alike as highly regular spheroids, these powders showed distinct differences in composition (especially Al, C and N contents), particle size distributions, and powder features such as degree of agglomeration, fusion and surface roughness. Compositional differences expectedly had the strongest impact on microstructure. High N and C contents formed TiN-nitrides and/or (Nb,Ti,Mo)-C carbides on the grain boundaries, prevented recrystallization during heat treatment, and resulted in retained (001)-scalloped shaped grains that ranged from 19 to 41 microns in average size. In the absence of this particle pinning, the average grain size of the heat treated SLM 718 ranged from 51 microns to 90 microns. Room temperature tensile and high cycle fatigue (HCF) testing compared as-fabricated (AF) and low stress ground (LSG) surface conditions. Tensile testing revealed consistent behavior between the two surface conditions and amongst the powder lots. The finer grained SLM 718 builds displayed the lowest tensile properties. A SLM 718 build fabricated from a powder with eight times lower C content showed statistically better tensile properties presumably due to enhanced coarsening of (delta)-Ni3Nb precipitates. The specimens from once-recycled powders had slightly higher tensile strengths and slightly higher ductility compared to their virgin equivalents; once-recycling also did not substantially degrade the mean HCF life. The LSG fatigue lives agreed with conventionally manufactured 718 data, while AF lives exhibited a knock-down due to surface roughness. The fatigue lives of AF specimens were statistically equivalent across powder lots except for one and failures typically initiated at stress concentrators associated with SLM surface asperities. Fatigue testing of low stress ground specimens result in both transgranular and within facet crack initiations. More than half of the cracks initiated from these facets for the machined condition; however, these facets appeared to be within grains that were larger-than-average in size. A nitrogen-atomized powder with fine prior particles of TiN-nitrides and M(Ti,Nb,Mo)C carbides from atomization on powder surfaces resulted in the best fatigue performance with segregation of these particles to the SLM 718 grain boundaries leading to higher resistance to early-stage crack propagation. Typically the fine-grained builds with minor phases along the grain boundaries did not perform well in fatigue, whereas a larger-grain build with lower carbon content and coarser delta-Ni3Nb precipitates showed the next best HCF response. Further details of the build microstructure and its impact on tensile and fatigue behavior was considered

Index to NASA Tech Briefs, January - June 1967

Technological innovations for January-June 1967, abstracts and subject inde

Thermophysical Phenomena in Metal Additive Manufacturing by Selective Laser Melting: Fundamentals, Modeling, Simulation and Experimentation

Among the many additive manufacturing (AM) processes for metallic materials, selective laser melting (SLM) is arguably the most versatile in terms of its potential to realize complex geometries along with tailored microstructure. However, the complexity of the SLM process, and the need for predictive relation of powder and process parameters to the part properties, demands further development of computational and experimental methods. This review addresses the fundamental physical phenomena of SLM, with a special emphasis on the associated thermal behavior. Simulation and experimental methods are discussed according to three primary categories. First, macroscopic approaches aim to answer questions at the component level and consider for example the determination of residual stresses or dimensional distortion effects prevalent in SLM. Second, mesoscopic approaches focus on the detection of defects such as excessive surface roughness, residual porosity or inclusions that occur at the mesoscopic length scale of individual powder particles. Third, microscopic approaches investigate the metallurgical microstructure evolution resulting from the high temperature gradients and extreme heating and cooling rates induced by the SLM process. Consideration of physical phenomena on all of these three length scales is mandatory to establish the understanding needed to realize high part quality in many applications, and to fully exploit the potential of SLM and related metal AM processes

Latest Developments in Industrial Hybrid Machine Tools that Combine Additive and Subtractive Operations

Hybrid machine tools combining additive and subtractive processes have arisen as a solution to increasing manufacture requirements, boosting the potentials of both technologies, while compensating and minimizing their limitations. Nevertheless, the idea of hybrid machines is relatively new and there is a notable lack of knowledge about the implications arisen from their in-practice use. Therefore, the main goal of the present paper is to fill the existing gap, giving an insight into the current advancements and pending tasks of hybrid machines both from an academic and industrial perspective. To that end, the technical-economical potentials and challenges emerging from their use are identified and critically discussed. In addition, the current situation and future perspectives of hybrid machines from the point of view of process planning, monitoring, and inspection are analyzed. On the one hand, it is found that hybrid machines enable a more efficient use of the resources available, as well as the production of previously unattainable complex parts. On the other hand, it is concluded that there are still some technological challenges derived from the interaction of additive and subtractive processes to be overcome (e.g., process planning, decision planning, use of cutting fluids, and need for a post-processing) before a full implantation of hybrid machines is fulfilledSpecial thanks are addressed to the Industry and Competitiveness Spanish Ministry for the support on the DPI2016-79889-R INTEGRADDI project and to the PARADDISE project H2020-IND-CE-2016-17/H2020-FOF-2016 of the European Union's Horizon 2020 research and innovation program

Hardness, grainsize and porosity formation prediction on the Laser Metal Deposition of AISI 304 stainless steel

The presented numerical model solves the heat and mass transfer equations in the Laser Metal Deposition process and based on the evolution of the thermal field predicts the grainsize, the resulting hardness and evaluates the pores formation probability in an AISI 304 stainless steel. For this purpose, in a first step, the model calculates the shape of the deposited material and the variations of the temperature field. In a second step, and based on the evolution of the thermal field, the model calculates the resulting hardness of the deposited material, the grainsize and the porosity formation probability after the deposition process. Numerical results are experimentally validated, and good agreement is obtained. Consequently, besides predicting the geometry of the resulting part and the evolution of the thermal field, the developed model enables to evaluate the quality of the deposited material. Therefore, the optimum process conditions and strategy when depositing AISI 304 stainless steel can be determined without initial trial-and-error tests.“LaCaixa” foundation . In addition, this work has been founded by the H2020- FoF13-2016 PARADDISE project (contract No.: 723440). This work has been also carried out in the framework of the DPI2016-79889-R – INTEGRADDI project, funded by the Spanish Ministry of Industry and Competitiveness