Influence of Sulphur Content on Structuring Dynamics during Nanosecond Pulsed Direct Laser Interference Patterning

The formation of melt and its spread in materials is the focus of many high temperature processes, for example, in laser welding and cutting. Surface active elements alter the surface tension gradient and therefore influence melt penetration depth and pool width. This study describes the application of direct laser interference patterning (DLIP) for structuring steel surfaces with diverse contents of the surface active element sulphur, which affects the melt convection pattern and the pool shape during the process. The laser fluence used is varied to analyse the different topographic features that can be produced depending on the absorbed laser intensity and the sulphur concentration. The results show that single peak geometries can be produced on substrates with sulphur contents lower than 300 ppm, while structures with split peaks form on higher sulphur content steels. The peak formation is explained using related conceptions of thermocapillary convection in weld pools. Numerical simulations based on a smoothed particle hydrodynamics (SPH) model are employed to further investigate the influence of the sulphur content in steel on the melt pool convection during nanosecond single-pulsed DLIP.

Interfacial phenomena between molten iron and molten slag–Effect of nitrogen on the Marangoni convection

In order to investigate the influence of the surface-active element on the interfacial phenomena between molten iron and molten Al2O3-CaO-SiO2 slag, a mildly surface-active element, nitrogen, was introduced, and the interfacial phenomena were directly observed using an X-ray sessile drop method. The multiphysics model was employed to calculate the velocity of the Marangoni convection caused by the surface/interfacial tension gradient along with the contour of the sessile drop. Movement of the sessile drop was observed in the experiment, and the driving force of the movement was discussed from the distribution of surface tension active element viewpoint. The calculated velocity of the Marangoni convection in the droplet was reasonably agreed with the literature data for the metal-gas system, and thus, the same model was applied for the metal-slag system. The velocity of the Marangoni convection for the metal-slag system becomes ten times lower compared to that of metal-gas system. Graphical abstract

Inducing the Effect of a Ga2O3 Nano-Particle on the CsF-RbF-AlF3 Flux for Brazing Aluminum to Carbon Steels

In this study, a Ga2O3 nano-particle was added into CsF-RbF-AlF3 flux to develop a highly active flux for brazing aluminum alloy to steel, and the spreadability and wettability of Zn-Al filler metal that matched the CsF-RbF-AlF3 flux-doped Ga2O3 nano-particle on the steel were investigated. The results showed that the spreadability and wettability of the CsF-RbF-AlF3 flux-doped Ga2O3 nano-particle could be remarkably improved when matching Zn-Al filler metals on both aluminum and low-carbon steel, for which the optimal content is in the range of 0.001–0.003 wt.% of Ga2O3. An investigation and analysis on the mechanism of reactions among CsF-RbF-AlF3-doped Ga2O3 nano-particle flux and filler metal or base metals showed that the Ga2O3 nano-particle is selectively absorbed by the interface of molten Zn-2Al filler metal and base metal, which released the surface-active element Ga to enrich the molten Zn-2Al filler metal and decreased the interfacial tension, so as to promote the enlargement of its spreading area during the brazing process. It was concluded that adding a trace amount of Ga2O3 nano-particle into CsF-RbF-AlF3 flux is a meaningful way to improve the activity of flux for brazing aluminum to steel compared with adding ZnCl2, which poses the risk of corrosion on aluminum.

An experimental study of the inclusion behavior during maraging steel processing

Two maraging steel rods with different sulfur content, deliberately seeded with alumina (Al2O3) inclusions, were remelted in an electron beam furnace. The aim was on one hand to highlight the emergence of non-metallic inclusions at the liquid metal surface and on the other hand to study the effect of the presence of a surface active element (sulfur) on their behavior. The in situ real time observation of Al2O3 clusters was possible using both high-speed video and infrared thermography. The inclusions emerge and tend to collide and quickly agglomerate into rafts. A semi-quantitative analysis confirmed that almost 100% of inclusions are captured at the free surface. The electron beam creates large thermal gradients in the impingement zones; this induces thermocapillary convection (Marangoni effect). The flow direction depends on the sulfur content. This tendency was well-observed by following the trajectories of inclusion rafts.

Study on Interfacial Tension and Flammability of Mg-Ca-Ce Alloy Melt

The Mg-Ca-Ce ignition-proof Mg-alloy with high ignition temperature was developed based on the synergy of flame retardant role of Ca element and Re element ,and the flame retardant mechanism were investigated. The interfacial tension of Mg-Ca-Ce alloy melt was measured with the maximum bubble method. The interfacial tension is found to be remarkably lowered with the increase of Ce element that is a kind of surface active element to Mg-Ca alloy. The results also showed that the compact composite film composed of Ce2O3,CaO and MgO could be formed and reduce the flammability significantly when the Ca content and Re content were 1.2wt.% and 1.5 wt.% respectively, which prevented alloys from oxidation further and raised the ignition point nearly 150°C.

Fundamental Research on the Mechanism of High-Speed Welding Undercut

Undercut is one of the most important problems which restrict welding speed. The research indicated that surface flow direction of welding pool near the periphery is the most important reason of undercut occurred. When the existence of surface active element (e.g. the content of S is about 0.02-0.04% in steel) change the sign of surface tension temperature coefficient to positive, undercut always occurs in TIG moveless welding, and not occurs when the content of S in iron is very low( e.g. S% is less 0.01%). Temperature gradient is the most important factor which influences the depth of undercut. The depth of undercut is bigger with the increase of temperature gradient when the other parameters are the same. When filling metal into welding pool, it restrained the molten metal movement from periphery to the pool center. So it can reduce the tendency of undercut, and eliminate undercut as increasing of filling metal ultimately.

Effects of Surface Active Element on the Biocompatibility of High Nitrogen Stainless Steel

Nickel-free high nitrogen stainless steel has become more important due to its impressive mechanical and corrosion properties. In contrast to high pressure processes, melting plasma in a normal atmosphere is an alternative way of obtaining high nitrogen content at low cost. However, melting in such an atmosphere will bring some surface impurities, like sulfur and oxygen, into the stainless steel through the refractories or the materials themselves. Therefore, this research aims the relationship between the sulfur and nitrogen content, and their influence on biocompatibility. Thermo-Calc is first used to design the composition of Fe-Cr based austenitic nitrogen stainless steel, and melted with different melting techniques, including plasma and an induction melting furnace. The results indicate that the nitrogen content varies with different sulfur content. It is also found that α-Fe (Ferrite) plates are observed near the grain boundaries when the sulfur content reaches a certain level. Besides, the Sulfur content has an obvious influence on the biocompatibility rather than nitrogen contents.