Microstructure evolution and mechanical properties of selective laser melted Ti-6Al-4V

Selective laser melting (SLM) has been shown to be an attractive manufacturing route for the production of α/β titanium alloys, and in particular Ti-6Al-4V. A thorough understanding of the relationship between the process, microstructure and mechanical properties of the components produced by this technology is however crucial for the establishment of SLM as an alternative manufacturing route. The purpose of the present study is thus to determine the microstructure evolution, crystallographic texture and the mechanical properties of SLM Ti-6Al-4V. The effect of several processing parameters on the density and the microstructure of the SLM samples were initially investigated. It was found that different sets of process parameters can be used to fabricate near fully dense components. It was found that the samples built using the optimised process window consist exclusively of α′ martensitic phase precipitated from prior β columnar grains. It was observed that the β grain solidification is influenced by the laser scan strategy and that the β phase has a strong texture along its grain growth direction. The α′ martensitic laths that originate from the parent β grains precipitate according to the Burgers orientation relationship. It was found that α′ laths clusters from the same β grain have a specific misorientation that minimise the local shape strain. Texture inheritance across successive deposited layers was also observed and discussed in relation to various variant selection mechanisms. The mechanical properties of as-built and stress relieved SLM Ti-6Al-4V built using the same optimised process parameters were then investigated. It was found that the build orientation affects the tensile properties, and in particular the ductility of the samples. Samples built perpendicularly to the building direction showed higher ductility than those built in the vertical orientation. It was also observed that a stress relief heat treatment was beneficial to the mechanical properties of SLM Ti-6Al-4V. The ductility of the stress relieved samples was indeed higher than those found in the as-built condition. It was found that the predominant fracture mode during tensile testing is inter-granular. In terms of high-cycle fatigue, it was found that SLM Ti-6Al-4V is comparable to HIPed cast Ti-6Al-4V but it has a significantly lower fatigue resistance than that of wrought and annealed alloys. It was observed that porosity and the elongated prior β grain boundaries decrease substantially the fatigue life of the components. Cracks propagate either by fatigue striation or ductile tearing mechanisms. Using alternative laser scan strategies it was possible to control the microstructure of the as-built samples. It was observed that the laser scan vector length influences several microstructural features, such as the width of the prior β grains and the thickness of the α′ laths. It was found that re-melting the same layer has instead little effect on the microstructure. A novel laser scan strategy characterised by much lower laser power and scan speed than those typically used in SLM enabled finally to fabricate SLM Ti-6Al-4V with a microstructure close to that of conventionally manufactured Ti-6Al-4V. This study investigates for the first time the crystallographic texture evolution in Ti-6Al-4V manufactured by SLM. Further, this research presents for the first time the effect of the characteristic microstructure and crystallographic texture on the mechanical properties and fracture of SLM Ti-6Al-4V. Lastly, for the first time this research shows examples of microstructural control during the SLM fabrication of the same alloy using long laser dwell times

Bank Exposures and Sovereign Stress Transmission

Using novel monthly data for 226 euro-area banks from 2007 to 2015, we investigate the determinants of banks' sovereign exposures and their effects on lending during and after the crisis. Public, bailed-out and poorly capitalized banks responded to sovereign stress by purchasing domestic public debt more than other banks, consistent with both the "moral suasion" and the "carry trade" hypothesis. Public banks' purchases grew especially in coincidence with the largest ECB liquidity injections, which therefore reinforced the "moral suasion" mechanism. Bank exposures significantly amplified the impact of sovereign stress on bank lending to domestic firms, as well as on lending by foreign subsidiaries of stressed-country banks to firms in non-stressed countries. Altogether, our evidence connects this amplification effect and its cross-border tramission to the moral suasion exerted by domestic governments on banks during the crisis

Short-Selling Bans and Bank Stability

In both the subprime crisis and the eurozone crisis, regulators imposed bans on short sales mainly aimed at preventing stock price turbulence from destabilizing financial institutions. Contrary to the regulators’ intentions, financial institutions whose stocks were banned experienced greater increases in the probability of default and volatility than unbanned ones. Increases were larger for more vulnerable financial institutions. To take into account the endogeneity of short sales bans, we match banned financial institutions with unbanned ones with similar sizes and levels of riskiness and instrument the 2011 ban decisions with regulators’ propensity to impose a ban in the 2008 crisis

Extraversion and neuroticism in sexually dysfunctional men suffering from erectile dysfunction and premature ejaculation: a cross-sectional study

The role of personality traits in sexual complaints and dysfunctions is ever more evident. However, the literature is very scarce as to the possible relationships between such personality traits and specific sexual dysfunctions like ED, PE or both, their subtypes as to time of onset and severity levels. The main aim of the present study was to investigate if Neuroticism and Extraversion have different roles and trends in men suffering from PE and/or ED, both lifelong and acquired. Moreover, we verified if, by adopting DSM-IV-TR and DSM-5 criteria for diagnosing PE, some differences emerged in percentages of diagnosed cases. The sample consisted of 222 patients aged 18-70. The International Index of Erectile Function (IIEF-15) was used to assess erectile function, and the Premature Ejaculation Severity Index (PESI) was used to assess the severity of PE dysfunction. Neuroticism and Extraversion were measured with the Eysenck Personality Questionnaire-Revised (EPQ-R). In our results, Neuroticism and Extraversion were dependent on the kind of sexual dysfunction and its severity. These results suggest that type, time of onset and severity of sexual dysfunctions should be considered as related to specific personality characteristics and vice versa

Effect of the build orientation on the mechanical properties and fracture modes of SLM Ti–6Al–4V

Recent research on the additive manufacturing (AM) of Ti alloys has shown that the mechanical properties of the parts are affected by the characteristic microstructure that originates from the AM process. To understand the effect of the microstructure on the tensile properties, selective laser melted (SLM) Ti–6Al–4V samples built in three different orientations were tensile tested. The investigated samples were near fully dense, in two distinct conditions, as-built and stress relieved. It was found that the build orientation affects the tensile properties, and in particular the ductility of the samples. The mechanical anisotropy of the parts was discussed in relation to the crystallographic texture, phase composition and the predominant fracture mechanisms. Fractography and electron backscatter diffraction (EBSD) results indicate that the predominant fracture mechanism is intergranular fracture present along the grain boundaries and thus provide and explain the typical fracture surface features observed in fracture AM Ti–6Al–4V

Microstructure of Ti-6Al-4V produced by selective laser melting

Ti-6Al-4V is the most widely used titanium alloy. Manufacturing of Ti-6Al-4V components using novel additive processing techniques such as selective laser melting is of great interest. This study focuses on the microstructure characterisation of Ti-6Al-4V components produced by selective laser melting (SLM) with full (Ti-6Al-4V base plate) and partial (Ti-6Al-4V needle-shaped bed) support. The starting material, a plasma atomised powder, and the component products are studied using various microscopy techniques including optical, scanning electron and transmission electron microscopy and electron backscattered diffraction (EBSD). Powder particles are fully dense, possess a spherical shape and are composed of acicular α phase. The as-built material shows oriented acicular martensitic phase with well defined columnar grains. The morphology of martensitic phase and microstructural evolution will be discussed in relation to the SLM processing parameters employed and the different cooling rates experienced by the components

Refinement of the grain structure of additive manufactured titanium alloys via epitaxial recrystallization enabled by rapid heat treatment

The coarse prior-β grain structure in titanium alloys produced by additive manufacturing is associated to mechanical anisotropy and limited fatigue life. Here we report a novel methodology to refine such structure by rapid heat treatment of Ti-6Al-4V produced by laser powder-bed fusion. The refinement was analysed using high-temperature EBSD that showed, for the first time, how high angle boundary β grains nucleated and grew with quasi-equiaxed morphology by epitaxial recrystallization. These findings show the potential for such heating regime to be applied to control grain size, morphology and distribution in a wider category of structural alloys produced by additive manufacturing

Tracheocutaneous fistula in patients undergoing supracricoid partial laryngectomy: the role of chronic aspiration

The aim of the present retrospective controlled study was to analyse and compare risk factors for tracheocutaneous fistula in patients who received tracheostomy after supracricoid partial laryngectomy with those who received tracheostomy for other causes. We enrolled 39 patients with tracheocutaneous fistulas who were divided into two groups. The first received temporary tracheostomy for supracricoid partial laryngectomies (n = 21), while the control group consisted of patients who received temporary tracheostomy for other causes (n = 18). Risk factors believed to play a role in the pathogenesis of tracheocutaneous fistula were examined including advanced age, cardiopathy, local infections, radiotherapy, elevated body mass index, malnutrition, decannulation time and aspiration grade. The Leipzig and Pearson scale score was significantly higher in the supracricoid partial laryngectomy group (p = 0.006 and 0.031 for univariate and multivariate analyses, respectively). The penetration/aspiration scale score was significantly higher in the supracricoid partial laryngectomy group as determined by univariate analysis (p = 0.014). The decannulation time was significantly lower in the supracricoid partial laryngectomy group (p = 0.004 and 0.0004 for univariate and multivariate analyses, respectively). The number of surgical closures for tracheocutaneous fistula was significantly higher in the supracricoid partial laryngectomy group by univariate analysis (p = 0.027). These results suggest that chronic aspiration and related cough may be important pathogenic factors for tracheocutaneous fistula and could be responsible for the significantly higher rates of closure failure in patients after supracricoid partial laryngectomy

Increasing the build rate of high-strength aluminium alloys produced by laser powder bed fusion

Aluminium alloys from the 2xxx series represent important structural materials due to their optimal combination of strength and corrosion resistance. However, their production by means of laser powder bed fusion (PBF-LB/M) is hampered by the presence of detrimental pores and hot cracks. These defects can be mitigated with appropriate process parameters investigations, often at the expense of the build rate.In this study, a combination of modelling and experimental work is adopted to predict the consolidation behaviour of AA2024, a well-known high-strength Al alloy, in regimes characterised by a progressive increase of build rate. The melt pool geometries are predicted as a function of various process parameters using the Rosenthal solution and variable values of absorptivity. This leads to a facile production of near-full dense crack-free parts at an increasing build rate. The analysis of the specimens produced with such processing parameters reveals that the microstructural features and mechanical properties of AA2024 are largely preserved when build rate is increased.Our approach can be easily implemented in standard PBF-LB/M systems as it does not rely on machine adaptations. Therefore, our new proposed approach might result in a practical solution to increase the productivity of other high-strength Al-alloys in industrial settings