Fatigue life and microstructure of additive manufactured Ti6Al4V after different finishing processes

Finishing methods of additive manufactured metal parts are becoming a key driver of industrial viability, increasingly with additive processes being challenged in demanding end-product applications. The same scenario stresses the requirements as to fatigue life of parts built by Additive Manufacturing (AsM). The paper addresses fatigue life of Ti6Al4V produced by Powder Bed Fusion in four finishing conditions: as-built, tool machined, after tumbling and after tumbling and subsequent shot-peening. Failure mechanisms at the micro-scale are observed in order to reinforce the mechanical results by identifying the role of different surface morphologies in crack initiation. X-ray diffraction (XRD), scanning electron microscopy (SEM) techniques and microanalysis (EDX) are used to investigate microstructural modifications generated by the different finishing methods. Results show that tumbling alone does not improve fatigue life against the as built condition, whereas tumbling and subsequent shot peening allow matching the fatigue endurance of tool machined specimens. The shot peening process causes surface amorphization and implantation of the peening media turning into subsurface inclusions. Based on the results, an optimized finishing process can be envisaged, consisting in prolonged tumbling up to the removal of a stock allowance at least equal to the powder size, before shot peening

Tailoring the Microstructure of an AA5754 Aluminum Alloy by Tuning the Combination of Heat Treatment, Friction Stir Welding, and Cold Rolling

Friction stir welding (FSW) has now reached a technological impact and diffusion that makes it a common joining practice for several classes of metallic materials. These include light alloys (aluminum, titanium, magnesium), steels, and other metallic alloys. In addition, the combination of FSW with pre- or post-welding heat treatments or plastic deformation, such as cold rolling (CR), can favor minimal necessary plate thicknesses and induce effective alloy strengthening mechanisms that make the FSW joint lines as plate reinforcing zones. Process parameters, such as pin rotation and transverse speed, can be tuned to optimize the mechanical properties of the resulting joint. This work presents a microstructural study of the mechanical response of different sequences of heat treatment, FSW, and CR in a non-age hardened Al-Mg AA5754 alloy. By using polarized optical microscopy and microhardness tests, two FSW conditions were used to fabricate a joint; and were than subjected to different sequences of heat treatment and cold rolling. The results suggest that FSW conditions have a limited effect on the microstructure, while microhardness profiles show a higher variability of the different datasets related to the low welding speed investigated

Phagocyte Escape of <i>Leptospira</i>: The Role of TLRs and NLRs

The spirochetal bacteria Leptospira spp. are causative agents of leptospirosis, a globally neglected and reemerging zoonotic disease. Infection with these pathogens may lead to an acute and potentially fatal disease but also to chronic asymptomatic renal colonization. Both forms of disease demonstrate the ability of leptospires to evade the immune response of their hosts. In this review, we aim first to recapitulate the knowledge and explore the controversial data about the opsonization, recognition, intracellular survival, and killing of leptospires by scavenger cells, including platelets, neutrophils, macrophages, and dendritic cells. Second, we will summarize the known specificities of the recognition or escape of leptospire components (the so-called microbial-associated molecular patterns; MAMPs) by the pattern recognition receptors (PRRs) of the Toll-like and NOD-like families. These PRRs are expressed by phagocytes, and their stimulation by MAMPs triggers pro-inflammatory cytokine and chemokine production and bactericidal responses, such as antimicrobial peptide secretion and reactive oxygen species production. Finally, we will highlight recent studies suggesting that boosting or restoring phagocytic functions by treatments using agonists of the Toll-like or NOD receptors represents a novel prophylactic strategy and describe other potential therapeutic or vaccine strategies to combat leptospirosis.Facultad de Ciencias ExactasInstituto de Biotecnologia y Biologia Molecula

An Italian Validation of ABILHAND-Kids for Children With Cerebral Palsy

Limitations in hand function are common among children with cerebral palsy (CP), with almost 50% presenting an arm–hand dysfunction. However, there is no standardized assessment tool available in Italian for evaluating bimanual performance for this population. Our objective in this study was to evaluate the psychometric properties of an Italian translation of the ABILHAND-Kids (ABILHAND-Kids-IT) among children with CP. We examined internal consistency using Cronbach’s Alpha and Omega coefficients, and we investigated test-retest reliability with intraclass correlation coefficients (ICC). We performed explorative factor analysis (EFA) to investigate structural validity. We calculated Pearson’s correlation coefficients between the ABILAND-Kids IT and the Manual Ability Classification System (MACS) to assess criterion validity; and, to demonstrate the score variability of the ABILHAND-Kids-IT, we used analyses of variance (ANOVAs) to compare the 181 children with CP in this sample with their levels on the MACS. We enrolled 181 children with CP in the study. EFA confirmed a uni-dimensional scale. We obtained internal consistency on both Cronbach’s Alpha and Omega coefficient of 0.98, and a one-week test-retest reliability analysis revealed an ICC with 95% of confidence interval of.992. The ANOVA revealed significant score variability (p &lt; 0.01) and the Pearson correlation coefficient comparing the ABILHAND-Kids-It score with the MACS was –0.929 (p &lt; 0.01). We conclude that the ABILHAND-Kids-IT is valid and reliable for use with Italian children with CP

Enhancing the quality of metal powder feedstock for laser PBF through cross-contamination removal

The presence of impurities in metal powder feedstock for laser powder bed fusion (L-PBF) can strongly affect the mechanical properties of the sintered part. As a matter of fact, the contamination particles trapped in the uniform metal matrix of the raw material, act as a discontinuity and, therefore, as a site for fatigue crack nucleation and growth, leading to a dramatic reduction of the expected fatigue life. While cross-contamination detection is a key parameter to establish the quality level of the metal powder feedstock, its removal is crucial to ensure that the sintered parts will fulfil the service requirements. In this study, metal powder feedstock for laser PBF were intentionally contaminated, submitted to a removal process based on their magnetic properties, through suitably developed equipment. The contaminated and processed powders were characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) techniques

Laser powder bed fusion: Tailoring the microstructure of alloys for biomedical applications

Additive manufacturing (AM) is particularly attractive for biomedical applications, where complex geometries and a high degree of individualization are required. Laser powder bed fusion (LPBF) is an AM technology exploiting the action of a solid-state laser to locally melt a metal powder according to a computer aided design (CAD) model. In the present study, the EOS Cobalt Chrome SP2 (Co-Cr-Mo-W) and Ti64 (Ti6Al4V) powders were sintered by the system equipped with a Yb fiber laser. During LPBF, the Co-Cr-Mo-W metal powder undergoes total melting followed by rapid cooling, giving rise to athermal martensitic phase transformation from the high-temperature \u3b3 (fcc) phase to the low-temperature \u3b5 (hcp) phase. This produces an intricate network of thin \u3b5-lamellae inside the \u3b3 phase matrix. After the firing cycle this structure is maintained, and a massive presence of coarse precipitates is also observed. Owing to the rapid cooling taking place during LPBF, in Ti6Al4V sintered samples only the acicular martensitic \u3b1' phase is present. The firing cycle induces the \u3b2 phase formation at the \u3b1 plate boundaries and this microstructure leads to reduced values of strength, with respect to those of the as-sintered samples. The highlighted behaviors show that by tuning the post-production heat treatments it is possible to tailor the microstructure and the mechanical properties

Precipitates formation and evolution in a Co-based alloy produced by powder bed fusion

Metal additive manufacturing is strongly employed in aerospace and biomedical applications, whose high degree of customization and low production volumes are the main characteristics. Cobalt-based alloys have been widely used for dental prosthesis and can be produced via metal additive manufacturing, or rather powder bed fusion, in a more convenient way compared to traditional manufacturing techniques. In the present paper, a comprehensive study of the tight correlation between the microstructure and the static mechanical properties of Co-Cr-Mo-W samples produced by powder bed fusion, is reported. In particular, the formation of precipitates during the sintering process is observed, and the evolution of their size, shape and frequency due to post-processing and to a heat-treatment typical of dental applications, is exhaustively studied. By coupling different characterization techniques such as scanning transmission electron microscopy and small-angle neutron scattering, it was possible to correlate the ductility degradation of the samples with the formation and growth of coarse and elongated precipitates

Enhancing the quality of metal powder feedstock for laser PBF through cross-contamination removal

The presence of impurities in metal powder feedstock for laser powder bed fusion (L-PBF) can strongly affect the mechanical properties of the sintered part. As a matter of fact, the contamination particles trapped in the uniform metal matrix of the raw material, act as a discontinuity and, therefore, as a site for fatigue crack nucleation and growth, leading to a dramatic reduction of the expected fatigue life. While cross-contamination detection is a key parameter to establish the quality level of the metal powder feedstock, its removal is crucial to ensure that the sintered parts will fulfil the service requirements. In this study, metal powder feedstock for laser PBF were intentionally contaminated, submitted to a removal process based on their magnetic properties, through suitably developed equipment. The contaminated and processed powders were characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) techniques