328 research outputs found
Laser offset welding of AZ31B magnesium alloy to 316 stainless steel
In this paper, the feasibility of using a fiber laser to perform a dissimilar metal joining was explored. AZ31B magnesium and 316 stainless steel were autogenously joined in butt configuration. The weldability between different materials is often compromised by a large difference in thermal properties and poor metallurgical compatibility. Thus, the beam was focused onto the top surface of the magnesium plate, at a certain distance from the interfaces (offset), and without using any interlayer or groove preparation. Such a method was called laser offset welding (LOW). Results proved a very good capability. The ultimate tensile strength exceeded the value of 100 MPa, since a resistant and thin layer of hard intermetallic compounds is formed within the fusion zone. The rupture was observed within the magnesium side, far from the centerline. The metallurgy of fusion zone indicated the effectiveness of phases coalescence, without mixing at liquid states. LOW was demonstrated to be a promising technique to join dissimilar metal welds, being capable to produce an effective bonding with good tensile strength
RĂ©sistance primaire aux traitements antituberculeux en CĂŽte d'Ivoire : une enquĂȘte nationale : rapport final
Shapes of leading tunnelling trajectories for single-electron molecular ionization
Based on the geometrical approach to tunnelling by P.D. Hislop and I.M. Sigal
[Memoir. AMS 78, No. 399 (1989)], we introduce the concept of a leading
tunnelling trajectory. It is then proven that leading tunnelling trajectories
for single-active-electron models of molecular tunnelling ionization (i.e.,
theories where a molecular potential is modelled by a single-electron
multi-centre potential) are linear in the case of short range interactions and
"almost" linear in the case of long range interactions. The results are
presented on both the formal and physically intuitive levels. Physical
implications of the obtained results are discussed.Comment: 14 pages, 5 figure
Experimental analysis of spatter generation and melt-pool behavior during the powder bed laser beam melting process
The experimental analysis of spatter formation was carried out on an instrumented SLM set-up allowing the quantification of spatter ejections and possible correlation with melt-pool behavior. Considering nearly similar SLM conditions than those carried out on SLM machines, an increase of large spatters (>80 ÎŒm) with volume energy density (VED) was clearly demonstrated on a 316L stainless steel, which was attributed to the recoil pressure applied on the melt-pool by the metal vaporization and the resulting high velocity vapor plume. In a second step, much lower spattering was shown on Al-12Si powder beds than on 316L ones. Fast camera analysis of powder beds indicated that droplet formation was mostly initiated in the powder-bed near the melt-pool interface. On Al-12 Si alloys, such droplets were directly incorporated in the MP without being ejected upwards as spatters like on 316L. Last, it was shown that a strong reduction of spattering was possible even on 316L, with the use of low VED combined with larger spots (â0.5 mm), allowing to melt sufficiently deep layers in conduction regime and ensure adequate dilution between layers
High temperature oxidation resistance and microstructure of laser-shock peened Ti-Beta-21S
Improving the high temperature (HT) resistance of titanium alloys is currently a technological challenge for extending their use in aerospace engines. Ti-Beta-21S is a metastable ÎČ titanium alloy specifically designed for high temperature applications up to 593 °C. We report the effect of a surface treatment by laser-shock peening (LSP) on the high temperature behavior of Ti-Beta-21S in order to increase further its maximum service temperature. The oxidation kinetics at 700 °C for duration up to 3000 h showed that the LSP treatment increases the oxidation resistance of Ti-Beta-21S. The effects of the LSP treatment on the alloy microstructure, its evolution at high temperature and the diffusion of light atmospheric elements (oxygen and nitrogen) are also reported
Feasibility of trial procedures for a randomised controlled trial of a community based group exercise intervention for falls prevention for visually impaired older people: the VIOLET study
Background Visually impaired older people (VIOP) have a higher risk of falling than their sighted peers, and are likely to avoid physical activity. The aim was to adapt the existing Falls Management Exercise (FaME) programme for VIOP, delivered in the community, and to investigate the feasibility of conducting a definitive randomised controlled trial (RCT) of this adapted intervention. Methods Two-centre randomised mixed methods pilot trial and economic evaluation of the adapted group-based FaME programme for VIOP versus usual care. A one hour exercise programme ran weekly over 12 weeks at the study sites (Newcastle and Glasgow), delivered by third sector (voluntary and community) organisations. Participants were advised to exercise at home for an additional two hours over the week. Those randomised to the usual activities group received no intervention. Outcome measures were completed at baseline, 12 and 24 weeks. The potential primary outcome was the Short Form Falls Efficacy Scale â International (SFES-I). Participantsâ adherence was assessed by reviewing attendance records and self-reported compliance to the home exercises. Adherence with the course content (fidelity) by instructors was assessed by a researcher. Adverse events were collected in a weekly phone call. Results Eighteen participants, drawn from community-living VIOP were screened; 68 met the inclusion criteria; 64 participants were randomised with 33 allocated to the intervention and 31 to the usual activities arm. 94% of participants provided data at the 12 week visit and 92% at 24 weeks. Adherence was high. The intervention was found to be safe with 76% attending nine or more classes. Median time for home exercise was 50 min per week. There was little or no evidence that fear of falling, balance and falls risk, physical activity, emotional, attitudinal or quality of life outcomes differed between trial arms at follow-up. Conclusions The intervention, FaME, was implemented successfully for VIOP and all progression criteria for a main trial were met. The lack of difference between groups on fear of falling was unsurprising given it was a pilot study but there may have been other contributory factors including suboptimal exercise dose and apparent low risk of falls in participants. These issues need addressing for a future trial
Campana points of bounded height on vector group compactifications
We initiate a systematic quantitative study of subsets of rational points
that are integral with respect to a weighted boundary divisor on Fano
orbifolds. We call the points in these sets Campana points. Earlier work of
Campana and subsequently Abramovich shows that there are several reasonable
competing definitions for Campana points. We use a version that delineates well
different types of behaviour of points as the weights on the boundary divisor
vary. This prompts a Manin-type conjecture on Fano orbifolds for sets of
Campana points that satisfy a klt (Kawamata log terminal) condition. By
importing work of Chambert-Loir and Tschinkel to our set-up, we prove a log
version of Manin's conjecture for klt Campana points on equivariant
compactifications of vector groups.Comment: 52 pages; minor revision, changes in the definition of Campana point
Effect of laser shock peening on the high temperature oxidation resistance of titanium
The effect of laser shock peening on the high temperature oxidation resistance of commercial pure titanium at high temperature (700 °C) was studied in long-time (3000 h) exposure under dry air. A reduction of the gain mass by a factor 4 was found for laser-shock peened (LSP) samples compared to untreated titanium, which supports the interest of laser-shock treatment for the improvement of high temperature resistance. Short-durations (10 h and 100 h) oxidation experiments, devoted to investigate the influence of the LSP treatment on the first stages of the oxidation process, were also carried out by TGA. Several techniques as scanning electron microscopy, hardness and roughness measurements, X-ray diffraction and X-ray photoelectron spectrometry, micro-Raman spectroscopy, nuclear reaction analysis and electron backscattered diffraction were used to characterize the sample after laser treatment and oxidations. The formation of a continuous nitrogen-rich layer between the oxide layer and the α-case area in LSP samples appears to be the key factor to explain the reduction of oxygen diffusion, and thus the improvement of the oxidation resistance of laser shocked titanium. Moreover, the grain-texture of LSP samples after oxidation can also explain the improvement of the high temperature oxidation resistance after long times exposures
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