In situ measurements and thermo-mechanical simulation of Ti–6Al–4V laser solid forming processes

Residual stresses and distortions are two technical obstacles for popularizing the additive manufacturing (AM) technology. The evolution of the stresses in AM components during the thermal cycles of the metal depositing process is not yet clear, and more accurate in situ measurements are necessary to calibrate and validate the numerical tools developed for its simulation. In this work a fully coupled thermo-mechanical analysis to simulate the laser solid forming (LSF) process is carried out. At the same time, an exhaustive experimental campaign is launched to measure the temperature evolution at different locations, as well as the distortions and both the stress and strain fields. The thermal and mechanical responses of single-wall coupons under different process parameters are recorded and compared with the numerical models. Good agreement between the numerical results and the experimental measurements is obtained. Sensitivity analysis demonstrates that the AM process is significantly affected by the laser power and the feeding rate, while poorly influenced by the scanning speed.Peer ReviewedPostprint (author's final draft

Simulation-assisted investigation on the formation of layer bands and the microstructural evolution in directed energy deposition of Ti6Al4V blocks

This is an Accepted Manuscript of an article published by Taylor & Francis Group in Virtual and Physical Prototyping on 2021, available online at: http://www.tandfonline.com/10.1080/17452759.2021.1942077Additive manufacturing (AM) of titanium alloy entails severe microstructural heterogeneity and layer bands due to diverse thermal histories. While the thermal-microstructure relationship in AM has been reported, the details on how complex thermal histories influence the microstructural evolution have not been so addressed, and the formation of layer bands in multi-layer multi-pass builds is still unclear. To undertake such investigation, a thermal model is firstly calibrated using two part-scale blocks fabricated on differently sized substrates, and then used to study the relationship between key microstructural characteristics and the thermal cycling involved. Results show that the different evolutions of the temperature ranges just underneath the ß-transus temperature (Tß) controlled by the printing path are responsible for the different band distributions at the centre and corner of the blocks. Also, the a sizes in the normal region are closely linked to the integral area obtained from the thermal curve as temperature fluctuates between Tß and a dissolution temperature, which helps linking AM variables to metallurgy. This further demonstrates that the a coarsening during thermal cycles is primarily driven by multi dissolution and precipation transformations instead of Ostwald ripening. Finally, the quantitative thermal-microstructure-microhardness relationship is established, helpful for the microstructural design.This work was funded by the National Key Technologies R & D Program (No. 2016YFB1100100), the National Natural Science Foundation of China (Grants No. 51874245), the European KYKLOS 4.0 project (No. 872570), the Severo Ochoa Programme for Centres of Excellence in R&D (CEX2018-000797-S) and the China Scholarship Council (No. 201906290011).Peer ReviewedPostprint (author's final draft

Finite element analysis and experimental validation of the thermomechanical behavior in laser solid forming of Ti-6Al-4V

A three-dimensional (3D) thermomechanical coupled model for Laser Solid Forming (LSF) of Ti-6Al-4V alloy has been calibrated through experiments of 40-layers metal deposition using different scanning strategies. The sensitivity analysis of the mechanical parameters shows that the thermal expansion coefficient as well as the elastic limit of Ti-6Al-4V have a great impact on the mechanical behavior. Using the validated model and optimal mechanical parameters, the evolution of thermo-mechanical fields in LSF has been analyzed. It has been found that the stresses and distortions develop in two stages, after the deposition of the first layer and during the cooling phase after the manufacturing of the component. The cooling phase is the responsible of 70% of the residual stresses and 60% of the total distortions. The analyses indicate that by controlling the initial substrate temperature (pre-heating phase) and the final cooling phase it is possible to mitigate both distortion and residual stresses. Hence, the influence of different pre-heating procedures on the mechanical fields has been analyzed. The results show that increasing the pre-heating temperature of the substrate is the most effective way to reduce the distortions and residual stresses in Additive Manufacturing

In situ measurements and thermo-mechanical simulation of Ti–6Al–4V laser solid forming processes

Residual stresses and distortions are two technical obstacles for popularizing the additive manufacturing (AM) technology. The evolution of the stresses in AM components during the thermal cycles of the metal depositing process is not yet clear, and more accurate in situ measurements are necessary to calibrate and validate the numerical tools developed for its simulation. In this work a fully coupled thermo-mechanical analysis to simulate the laser solid forming (LSF) process is carried out. At the same time, an exhaustive experimental campaign is launched to measure the temperature evolution at different locations, as well as the distortions and both the stress and strain fields. The thermal and mechanical responses of single-wall coupons under different process parameters are recorded and compared with the numerical models. Good agreement between the numerical results and the experimental measurements is obtained. Sensitivity analysis demonstrates that the AM process is significantly affected by the laser power and the feeding rate, while poorly influenced by the scanning speed

The gut microbiome dysbiosis and regulation by fecal microbiota transplantation: umbrella review

BackgroundGut microbiome dysbiosis has been implicated in various gastrointestinal and extra-gastrointestinal diseases, but evidence on the efficacy and safety of fecal microbiota transplantation (FMT) for therapeutic indications remains unclear.MethodsThe gutMDisorder database was used to summarize the associations between gut microbiome dysbiosis and diseases. We performed an umbrella review of published meta-analyses to determine the evidence synthesis on the efficacy and safety of FMT in treating various diseases. Our study was registered in PROSPERO (CRD42022301226).ResultsGut microbiome dysbiosis was associated with 117 gastrointestinal and extra-gastrointestinal. Colorectal cancer was associated with 92 dysbiosis. Dysbiosis involving Firmicutes (phylum) was associated with 34 diseases. We identified 62 published meta-analyses of FMT. FMT was found to be effective for 13 diseases, with a 95.56% cure rate (95% CI: 93.88–97.05%) for recurrent Chloridoids difficile infection (rCDI). Evidence was high quality for rCDI and moderate to high quality for ulcerative colitis and Crohn’s disease but low to very low quality for other diseases.ConclusionGut microbiome dysbiosis may be implicated in numerous diseases. Substantial evidence suggests FMT improves clinical outcomes for certain indications, but evidence quality varies greatly depending on the specific indication, route of administration, frequency of instillation, fecal preparation, and donor type. This variability should inform clinical, policy, and implementation decisions regarding FMT

Morphology evolution of TiO2 nanotubes with additional reducing agent: Evidence of oxygen release

In order to explore wider application of porous anodic alumina and anodic TiO2 nanotubes (ATNTs), the formation mechanism research of porous anodic materials plays a more significant role. Traditional field-assisted dissolution theory has been put into question and oxygen bubble mold has been accepted gradually. However, it is difficult to prove oxygen release. In this work, we present a method to demonstrate the presence of oxygen in the pores of nanotubes. A kind of water soluble reducing agent (NH4H2PO2) was added into electrolyte containing NH4F. Cavities exists not only between the double walls of nanotubes but also in the inner walls, which are different from normal nanotubes. The morphology evolution of nanotubes results from the reaction between NH4H2PO2 and oxygen. Therefore, the release of oxygen during the formation of ATNTs was further proved. Keywords: Anodic TiO2 nanotubes, Reducing agent, Morphology evolution, Anodizatio

The large deviation for the least squares estimator of nonlinear regression model based on WOD errors

Abstract As a kind of dependent random variables, the widely orthant dependent random variables, or WOD for short, have a very important place in dependence structures for the intricate properties. And so its behavior and properties in different statistical models will be a major part in our research interest. Based on WOD errors, the large deviation results of the least squares estimator in the nonlinear regression model are established, which extend the corresponding ones for independent errors and some dependent errors

Graphene-Assisted Thermal Interface Materials with a Satisfied Interface Contact Level Between the Matrix and Fillers

Abstract Reduced graphene oxide (RGO) and three-dimensional graphene networks (3DGNs) are adopted to improve the performance of thermal interface materials (TIMs). Therein, the 3DGNs provide a fast transport network for phonons, while the RGO plays as a bridge to enhance the phonon transport ability at the interface between the filler and matrix. The types of surface functional groups of the RGO are found to exert a remarkable influence on the resulting thermal performance; the carboxyl groups are found in the optimal selection to promote the transport process at the interface area because a strong chemical bond will form between the graphene basal plane and epoxy resin (ER) through this kind of group. The resulting thermal conductivity reaches 6.7 Wm−1 K−1 after optimizing the mass fraction and morphology of the filler, which is 3250% higher than that of the pristine ER. Moreover, the mechanical properties of these as-prepared TIMs are also detected, and the specimens by using the RGO(OOH) filler display the better performances