A. Yañez

Modelling of temperature evolution on metals during laser hardening process

Abstract To achieve a precise and controlled laser process, an exhaustive analysis of the thermal behaviour of the material is necessary. In the present paper, a numerical simulation of the laser hardening process has been developed using both analytical solutions and the finite element code ANSYS™ to solve the heat transfer equation inside the treated material. The knowledge of the thermal cycles has enabled suitable processing parameters to be ascertained thus improving surface properties when metallic alloys have been irradiated. A simpler analytical method is also used to determine the mentioned parameters more quickly. This general purpose method has been applied to a specific experimental situation, namely the treatment of cylindrical pieces used in a multistage pump rotary jacket.

Multiplicity dependence of strangeness production in S + Pb collisions at 200 GeV/c per nucleon

Abstract Production of Λ, Λ , and K s 0 has been measured for a wide range of event multiplicity in S+Pb reactions at 200 GeV/c per nucleon. The production of Λ is shown to increase with multiplicity faster than expected from a superposition of p+p collisions. The effect is seen for low multiplicity (below 100 negative particles).

The effect of fit-up geometry on melt flow and weld quality in laser hybrid welding

Hybrid laser-arc welding has a good tolerance to poor fit up as compared to simple laser welding. For a butt joint, the joint fit-up variations can be reduced to two local properties: the gap width and the vertical edge mismatch. The impact of these two properties on the resulting weld quality has been studied systematically in this paper. The original edges as well as the resulting weld surface topography have been scanned in three dimensions in order to study trends. During hybrid welding, the melt flow and the electric arc were observed at the top surface by high speed imaging to analyze the complex fluid flow phenomena.

Production of Λ, Λ, and Ξ−, Ξ+ particles in S+Pb collisions at 200 GeV/c per nucleon☆

Abstract The yields of Λ, Λ , and Ξ−, Ξ + have been measured close to mid-rapidity in sulphur reactions on a Pb target at 200 GeV/c per nucleon beam momentum. Fully corrected production ratios of Λ Λ , Ξ + Ξ , Ξ − Λ and Ξ Λ are presented. The ratios have been analysed both in the context of a rapidly disintegrating quark-gluon plasma and a hadronic resonance gas. Our analysis shows that the hadron gas model is sufficient to explain the observed particle yields.

A measurement of cross sections for 16O Al and 16O Pb interactions at 60 and 200 GeV/c per nucleon

Abstract Cross sections are measured for 16 O collisions with A1 and Pb. Dependences on beam momentum and atomic number are compared with data obtained at much lower beam momenta.

UV laser surface processing of metallic alloys: comparison of experimental and numerical results

The modifications resulting from a laser irradiation and which occur in solid materials are located mainly in the near surface region. An examination of the exposed areas and cross-sections allows to determine the influence of such a treatment. For most of materials, a molten layer is formed during the interaction processes and its depth depends on the irradiation conditions. The thickness of this layer can be measured experimentally but can also predicted by a numerical modelling which provides an analysis of the thermal cycles developed into the sample. In the present study, a KrF laser radiation has been used to irradiate metallic alloys in air and experimental and numerical investigations have been performed in order to compare both results. The aim is to provide complementary information for a more complete understanding of the melt front into solid.

Results from the NA36 experiment on the production of strangeness vbS| = 1 and |S| = 2 particles

Abstract Measurements of the production of singly and doubly strange particles in S + Pb collisions at a beam momentum of 200 GeV c per nucleon are presented. The yield of singly strange particles as a function of negative particle multiplicity has been measured. Rapidity and transverse momentum distributions from events consistent with a maximum impact parameter of 9 fm are compared with the predictions of Fritiof 7.02, Venus 4.10 and RQMD 1.08. The comparison illustrates the importance of re-interactions of secondaries and suggests that additional physical processes are required to be incorporated into the event generators. The analysis of strange particle ratios indicates that strangeness phase space is not saturated in these collisions.

Laser cladding of Ni based cermets

The self fluxing NiCrBSi alloys can produce coating layers by means of laser processing techniques. Main procedures are the laser post-treatment of previously thermal or plasma sprayed coats and the laser cladding, for which preplaced or continuously delivered powder (in this case aided by powder feeders) can be used. NiCrBSi alloys have an interesting property due to the presence of boron and silicon in its composition: they exhibit a relatively low melting point, making the laser cladding process easier. The layers obtained on metallic based materials are resistant to high temperature erosion wear and corrosion. However, if additional abrasive wear resistance is needed, the feeding with ceramic powders such as tungsten carbides (WC) is required. The high melting point of ceramics makes the laser cladding process complicated as the melt pool is made up of liquid metal plus not totally melted ceramic particles and the whole suffers the effect of the shielding and carrying gas flows, producing undesired instabilities. In this paper several combination of WC and NiCrBSi powders were tested. It is shown that the WC fraction in the mixture has a major influence on the obtention of pore and crack free clad layers. Bellow a certain ratio the meltpool appears to be more stable and less affected by the different gas flows used in the process, yielding dense NiCrBSi coatings with rather evenly distributed WC particles. In these conditions, the analysis and characterization of the produced coatings shows that the microstructure gains homogeneity without decreasing too much microhardness if compared with the pure ceramic layers.

Recent results from the NA36 experiment

Abstract Measurements of the production of Λ particles in S+Pb and S+S collisions at a beam momentum of 200 GeV/c per nucleon are presented. The S+Pb data was taken with two different experimental configurations, and the results agree well with each other. The effects of a new Monte Carlo simulation of the NA36 TPC on the efficiency calculations are discussed. The Λ rapidity and multiplicity distributions are then compared to three theoretical models of heavy ion interactions, and to the NA35 results. There is still a significant disagreement between the two experiments. The comparison to the models indicates that reinteractions of secondaries are important in explaining the observed Λ yields. Finally, the yields of doubly strange baryons in S+Pb interactions have been recalculated using the new efficiencies. The ratios of double to singly strange baryons are unchanged from the previously published results.

Strange particle production in 32S+Pb and p+Pb collisions at 200 GeV/c per nucleon

Abstract Cross sections for Λ, Λ , and K° production in S+Pb reaction at 200 GeV/c/n have been measured. Transverse momentum and rapidity distributions as well as the production dependence on event multiplicity are presented. An enhancement of strangeness production in mid-rapidity is observed.