8 research outputs found
The use of a ns-pulsed, high repetition rate green laser for SLM of 99.9% pure Cu
No full textThe most common laser type used in selective laser melting (SLM) machines is continuous fiber laser emitting at 1 m. The low optical absorptivity of Cu to ~1 m wavelength renders pure Cu a highly demanding material for SLM . The low optical absorptivity along with high thermal conductivity causes unstable processing conditions when standard SLM machine are used with pure Cu. Conversely, Cu has a much higher optical absorptivity at the green wavelength. Until recently, high power green lasers have not been available for material processing. This work investigates the use of a novel ns-pulsed fiber laser operating at the second harmonic (532 nm) for SLM of 99.9% pure Cu powder. In particular, the laser source operates at 30 MHz repetition rate providing ns regime and up to 110 W average power. The green laser is implemented to a bespoke open SLM platform. Results show that cubic specimens with densities >99.5% could be achieved
Selective laser melting of pure Cu with a 1 kW single mode fiber laser
No full textComplex geometries and fine critical features enabled by the powder bed fusion technologies are highly appealing for heat exchanger manufacturing. Copper is the main metal of choice for such applications. However, its processability using Selective Laser Melting (SLM) systems is limited by its high reflectivity at 1 μm, the emission wavelength of the laser sources commonly employed. Furthermore, the high conductivity of the pure copper, desirable for the final use of the products, causes instability and difficulties in the powder bed solidification. Resultantly, high density components are difficult to obtain. In this work, a 1 kW single mode fiber laser is used to process pure Cu powder with 99.9% purity. The high power is required to overcome the low efficiency of the process due to the high reflectivity. A prototype SLM machine is employed allowing for a flexible manipulation of the process parameters. The densification phenomenon is discussed as well as the causes that lead to porosity
High speed videography of gap bridging with beam oscillation and wire feeding during the laser welding of stainless steel and aluminum alloys
No full textLaser beam welding is known for its quality and speed. Given its susceptibility to gaps, the technology is applied in the industrial field with hard automation and dedicated fixtures rather than small-batch production. The latter cannot always guarantee the strict conditions on the fit-up of joints, especially with complex geometries. Gap-bridging techniques may be exploited to overcome these inaccuracies. The present work investigates the simultaneous use of low frequency circular beam oscillation and wire feeding as means to produce a continuous weld seam in the presence of constant air gaps. Lap joint welding of 2 mm-thick AISI301LN and butt joint welding of 3 mm-thick AW6005A-T6 alloy were conducted with gaps up to 1mm. High-speed imaging at 10kHz provided an insight in the dynamics of the oscillating weld pool and spatter formation. Optical inspection and metallographic analyses were used to verify the gap-bridging capability as well as the resulting seam quality
Long-Term Outcome of Acute Coronary Syndromes in Young Patients
No full textIntroduction: Acute coronary syndromes (ACS) in young patients are uncommon and their influence on morbidity and mortality in this population is still debated. Aim: We investigated clinical and angiographic characteristics, risk factors and outcome in young patients diagnosed with ACS, compared with those of older patients, evaluating survival free from death and/or nonfatal myocardial infarction (MI) and/or coronary revascularization (primary endpoint), and then with respect to each component of the primary endpoint. Methods: We retrospectively analyzed 1696 patients diagnosed with ACS between 2007 and 2013. 116 were aged ≤45 years (young adults), 1116 were >45 and <75 years (older adults) and 464 were ≥75 years. Results: Young adults were mostly male, with a prevalent diagnosis of STEMI, had less frequently typical cardiovascular risk factors and lower prevalence of extensive coronary artery disease. Over a median 3 years follow up, survival free from composite endpoint was better in young than in older adult patients (11.2 vs. 24.2%; p = 0.001), mainly due to a lower rate of death while the occurrence of non fatal MI and of coronary revascularization was similar (7.8 vs. 8.7%, p = 0.86; 8.7 vs. 12.9%, p = 0.23 respectively). Diabetes was the strongest independent risk factor of worse prognosis in the young cohort (OR 3.47; 95% CI 1.01–11.9; p = 0.04). Conclusions: Young adults showed peculiar clinical features and lower mortality compared with older adults. Morbidity was not different between the two populations, with diabetes independently associated with a worse prognosis
Processability of 99.9% pure Cu by SLM using a 1kW single mode fiber laser
No full textLAUREA MAGISTRALERecentemente il Selective laser melting (SLM) si è dimostrato un processo chiave per settori industriali come l’elettronica, l’automotive e l’aerospace nei quali, la sua capacità di creare forme complesse, può garantire la possibilità di progettare componenti appositamente ottimizzati per lo scambio di calore e di corrente elettrica. In questo tipo di applicazioni, il rame puro è il materiale di riferimento grazie alla sua elevata conducibilità sia termica che elettrica. Benché la tecnologia SLM potrebbe migliorare fortemente l’efficienza nello scambio termico ed elettrico per componenti in Cu puro, la lavorabilità di questo materiale non è stata ampiamente studiata nell’ambito dell’attuale stato dell’arte e risulta più complessa rispetto ad altre polveri metalliche. Il rame puro è caratterizzato da un’elevata riflettività per emissioni laser tipicamente impiegate nei sistemi SLM (1 μm circa), la quale combinata con l’elevata conducibilità termica di questo metallo, ostacola la concentrazione locale di energia necessaria ad ottenere una corretta densificazione del letto di polvere, causando, quindi, instabilità nel processo.
Il presente lavoro di tesi esplora la lavorabilità di polvere di Cu pura al 99.9% in un processo SLM utilizzando un laser in fibra singolo modo da 1kW. Il livello di potenza del laser impiegato è superiore rispetto a comuni sistemi SLM (approssimativamente 500 W), inoltre questa sorgente laser garantisce la possibilità di monitorare l’emissione laser riflessa durante il processo. Dalla letteratura emerge che il substrato sul quale avviene la deposizione dei primi layers gioca un ruolo fondamentale nella stabilità del processo nel caso di rame puro. Attraverso un modello analitico è stata simulata la propagazione del campo termico all’interno del substrato confrontando gli effetti di un substrato in rame, uno in AISI316L e di una maschera di rame da 1 mm sovrapposta ad un substrato di acciaio inossidabile. L’analisi sperimentale condotta su queste tre alternative conferma le previsioni teoriche in merito all’influenza del materiale di cui è composto il substrato e la sua geometria sulla densificazione della polvere di Cu puro. Ulteriori studi sull’effetto di strategie di scansione multi-passata sono stati condotti sul substrato di acciaio inossidabile e sulla maschera di rame. L’acquisizione on-line dell’emissione laser riflessa durante il processo ha garantito un’osservazione degli effetti dovuti alla variazione di assorbimento ottico nella polvere tre due scansioni consecutive.
Dal presente lavoro emerge che il rame puro può essere lavorato attraverso Selective Laser Melting, in particolare è possibile raggiungere densità relative superiori al 99% su un substrato di acciaio inossidabile attraverso l’impiego di strategie multi-passata del fascio laser (con una ripartizione 50%+50% della densità ottimale di energia per il processo). Da un punto di vista tecnologico il processo necessita di un laser fibra ad alta potenza in grado di resistere alla riflessione e di un substrato in AISI316L che permetta una deposizione stabile sin dai primi layers.Selective Laser Melting (SLM) is configuring day by day as a key process for industrial fields such as electronics, automotive and aerospace sectors where its capability to fabricate complex shapes can allow the design of components optimized for the exchange of heat and electricity. In these types of applications, pure copper is the reference material due to its high electrical and thermal conductivities. Although the SLM technology could strongly improve the efficiency in thermal and electrical transfer of pure Cu components, the processability of this material has not been widely investigated at the actual state of the art and results more difficult in comparison to other metallic powders. Pure copper is characterised by a high reflectivity to laser emission typically employed for SLM systems (i.e. 1 μm), which combined to its high thermal conductivity, hinders the local concentration of energy required to obtain a proper densification of the powder bed, thus causing process instability.
The present thesis work explores the processability of 99.9% pure Cu powder by means of SLM using a 1 kW single mode fiber laser. The power level of the employed laser is higher than the common SLM systems (approximately 500 W) and the laser source provides the capability to monitor the laser back-reflection during the process. In literature there is evidence that the substrate upon which the deposition of the initial layers occurs plays a crucial role on the process stability of pure Cu. Employing an analytical model, the thermal field propagation inside substrate was simulated comparing the effect of a substrate made of copper, AISI 316L and a 1 mm thick copper mask on top of the stainless-steel substrate. Experimental analysis on these three alternatives supported the theoretical predictions regarding the influence of the substrate material and geometry on the densification of pure Cu. Further investigations regarding the effect of multi-scan strategies have been performed on the stainless-steel substrate and copper mask solutions. The on-line acquisition of back-reflected laser emission during the process allowed further insight on the effect of powder absorptivity variations in between two successive scans.
The results of this work lead to conclude that selective laser melting is capable of processing pure Cu, depositing material with a relative density higher than 99% on a stainless-steel substrate through the use of a multi-scan strategy of the laser beam (with a 50%+50% delivery of the optimal process energy density). The technological requirements are a high power single mode fiber laser capable of withstanding back-reflections and an AISI 316L substrate which allows stable deposition since the first layers
Dietary Exposure to Pesticide and Veterinary Drug Residues and Their Effects on Human Fertility and Embryo Development: A Global Overview
No full textDrug residues that contaminate food and water represent a serious concern for human health. The major concerns regard the possible irrational use of these contaminants, since this might increase the amplitude of exposure. Multiple sources contribute to the overall exposure to contaminants, including agriculture, domestic use, personal, public and veterinary healthcare, increasing the possible origin of contamination. In this review, we focus on crop pesticides and veterinary drug residues because of their extensive use in modern agriculture and farming, which ensures food production and security for the ever-growing population around the world. We discuss crop pesticides and veterinary drug residues with respect to their worldwide distribution and impacts, with special attention on their harmful effects on human reproduction and embryo development, as well as their link to epigenetic alterations, leading to intergenerational and transgenerational diseases. Among the contaminants, the most commonly implicated in causing such disorders are organophosphates, glyphosate and antibiotics, with tetracyclines being the most frequently reported. This review highlights the importance of finding new management strategies for pesticides and veterinary drugs. Moreover, due to the still limited knowledge on inter- and transgenerational effects of these contaminants, we underlie the need to strengthen research in this field, so as to better clarify the specific effects of each contaminant and their long-term impact
Correction to: Human behavior and Homo-mammal interactions at the first European peopling: new evidence from the Pirro Nord site (Apricena, Southern Italy) (The Science of Nature, (2019), 106, 5-6, (16), 10.1007/s00114-019-1610-4)
No full textIn the original publication of this article, one of the author names was incorrectly captured. The first name should be Razika, then family name should be Chelli–Cheheb
Under-dilated TIPS Associate With Efficacy and Reduced Encephalopathy in a Prospective, Non-randomized Study of Patients With Cirrhosis
No full textPortosystemic encephalopathy (PSE) is a major complication of trans-jugular intrahepatic porto-systemic shunt (TIPS) placement. Most devices are self-expandable polytetrafluoroethylene-covered stent grafts (PTFE-SGs) that are dilated to their nominal diameter (8 or 10 mm). We investigated whether PTFE-SGs dilated to a smaller caliber (under-dilated TIPS) reduce PSE yet maintain clinical and hemodynamic efficacy. We also studied whether under-dilated TIPS self-expand to nominal diameter over time.
METHODS:
We performed a prospective, non-randomized study of 42 unselected patients with cirrhosis who received under-dilated TIPS (7 and 6 mm) and 53 patients who received PTFE-SGs of 8 mm or more (controls) at referral centers in Italy. After completion of this study, dilation to 6 mm became the standard and 47 patients were included in a validation study. All patients were followed for 6 months; Doppler ultrasonography was performed 2 weeks and 3 months after TIPS placement and every 6 months thereafter. Stability of PTFE-SG diameter was evaluated by computed tomography analysis of 226 patients with cirrhosis whose stent grafts increased to 6, 7, 8, 9, or 10 mm. The primary outcomes were incidence of at least 1 episode of PSE grade 2 or higher during follow up, incidence of recurrent variceal hemorrhage or ascites (based on need for at least 1 large-volume paracentesis by 4 weeks after TIPS placement), incidence of shunt dysfunction requiring TIPS recanalization, and reduction in porto-caval pressure gradient.
RESULTS:
PSE developed in a significantly lower proportion of patients with under-dilated TIPS (46%) than controls (73%) during the first year after the procedure (P=.015), but the proportions of patients with recurrent variceal hemorrhage or ascites did not differ significantly between groups. No TIPS occlusions were observed. These results were confirmed in the validation cohort. In an analysis of self-expansion of stent grafts, during a mean follow-up period of 252 days after placement, none of the PTFE-SGs self-expanded to the nominal diameter in hemodynamically relevant sites (such as portal and hepatic vein vascular walls).
CONCLUSION:
In prospective, non-randomized study of patients with cirrhosis, we found under-dilation of PTFE-SGs during TIPS placement to be feasible, associated with lower rates of PSE, and effectiv
