Hydrodynamical phenomena in the process of laser welding and cutting

Abstract

The main objective of the paper is to outline the 'bridges' existing between the outcomes of fundamental researches and the results of investigations in the field of industrial laser materials processing (LMP). An analysis is presented on the models based on non-stationary hydrodynamic phenomena caused by deeply penetrating high power CW laser beam into materials. This is typical of laser welding (LW) and laser cutting (LC). A physical analysis pertaining to melt removal and melt layer instability mechanisms of gas jet assisted CW–CO₂ laser fusion cutting is presented. The models deliberated here are melt squeezing out by gas pressure gradient, melt dragging by the friction force between melt surface and gas flow, formation of moving shelves at the cutting front. In case of high laser intensity, radiative flux interacts with material causing dynamical thermal transport onto the surface and phase transition at solid–liquid–gas interfaces. The solution is based on the non-stationary variables. Under these conditions the Mach number varies significantly due to laser intensity associated with laser flux energy instabilities. The connection among material surface temperature, laser intensity, laser flux and pressure in the plasma cloud is brought out. In addition, novel mechanisms based on hydrodynamics are proposed.     

Error analysis of forward and reverse heat conduction and convection calculations considering uncertainties in welding

Abstract

Numerical models of fusion welding traditionally compute temperature field for a given set of welding conditions in a forward manner. The reliability of computed temperature profile depends on the accuracy of a number of model input parameters, values of which are uncertain in nature. Here, the authors show that a genetic algorithm (GA) assisted integrated numerical model, following either convection or conduction based calculations, can identify the suitable values of the uncertain model input parameters and in turn provide reliable computed results. Powered with GA, the integrated model is used further in a reverse manner to predict multiple sets of welding conditions for a target weld geometry. The convection based calculations have been able to provide more reliable multiple welding variables in reverse calculations.     

Unsteady interfacial phenomena during inward weld pool flow with an active surface oxide

Abstract

Particle image velocimetry (PIV) was applied to estimate the velocity field on a weld pool surface with an oxide layer. A positive surface tension gradient drives an inward flow pattern from the boundary to centre of the weld pool, resulting in particles collecting on the centre line of the pool at the surface. Unsteady flow motions were observed by experimental visualisation of the particle flow, and the computed velocity field shows strong unsteady interfacial movement during inward flow. These unsteady flow motions are related to clockwise and counterclockwise vortices on the weld pool surface, which in turn affect both mass flow and energy transport. Both the qualitative flow motion and quantitative flow velocity are described in this work, which contributes to explaining the characteristic unsteady fluid flow in the weld pool, the measurements also provide experimental data for validation of numerical fluid flow models of the weld pool.     

Urine Advancing Contact Angle on Several Surfaces

Urine wetting properties may influence the design and performance of catheters, urinalysis instruments, and lab-on-a-chip technologies. In this study the advancing contact angle _adv of urine on several materials is characterized. Material type and surface tension have a significant effect on _adv, while pretreatment and aging do not. Mean urine _adv are between ≈78° and ≈89° on hydrophilic surfaces, and up to over ≈105° on hydrophobic surfaces. Expected urine contact angles will decrease from the DI water contact angles by on average 10°, and up to 20°, while urine surface tension will be lower than DI water by 12.12 mN/m and 18.53 mN/m. A unit change (mN/m) in surface tension results in a 0.75° change in _adv. These results indicate that systems attempting to exploit urine wetting must account for highly variable conditions.     

Hydrocarbon condensation modelling to mitigate fluid coker cyclone fouling

FLUID COKING is a continuous process that thermally converts heavy hydrocarbons, such as oil sands bitumen, to lighter and higher‐value products by horizontal spray injection onto a fluidized bed of hot coke particles. The cyclone sections of commercial fluid coker reactors experience fouling during typical operation, which limits unit run lengths. The main objective of this work is to improve fluid coker reliability by proposing cyclone fouling mitigation strategies based on practical operation modifications. This study developed a process simulation in Aspen Plus to establish the combined impact of vapour‐liquid equilibrium, endothermic thermal cracking reactions, pressure changes, and overall fluid dynamics in the selected fluid coker control volumes. The hydrocarbon composition was defined by applying an assay characterization of distillation data for representative hydrocarbon streams. Case studies were performed to determine the sensitivity of the predicted temperatures and hydrocarbon condensate flow rates for: (a) the burner‐to‐fluid coker transfer line temperature; (b) the hot coke flow rate; (c) hot coke entrainment from the freeboard region; and (d) scouring coke flow rate in the horn chamber. The scouring coke flow rate was identified as the most promising process lever to mitigate fluid coker cyclone fouling.     

Studies of microcellular nanocomposites with supercritical fluid assisted injection moulding process

Abstract

In this study, the microstructure, thermal behaviour and mechanical properties of microcellular nanocomposites were studied. Microcell wall structure and smoothness were determined by the size of the crystalline structure, which, in turn, was based on the material system and moulding conditions. Nanoclay in the microcellular, supercrtitical fluid assisted injection moulding process promoted the γ form and suppressed the α form crystalline structure of polyamide 6 (PA6). In the crystallisation kinetics studies, the Avrami equation and the modified Ozawa equation with the Mo method were used to model and analyse isothermal and non-isothermal crystallisation processes respectively. The existence of nanoclay increased the magnitude of the activation energy for both isothermal and non-isothermal crystallisation processes. This suggests the fast crystallisation process and the small crystalline size for microcellular nanocomposite processing. Interestingly, the dissolved gas lowered the crystallinity of the cores of moulded microcellular parts, but the addition of nanoclay reduced the crystallinity of both the cores and the skins of parts. The collective effect of the dissolved gas and nanoclay acted to shorten the moulding cycle time greatly with a reduction in the overall crystallinity of microcellular nanocomposite parts.     

Customization of a Commercially Available Prep Scale SFC System to Provide Enhanced Capabilities

Preparative Scale Supercritical Fluid Chromatography is emerging as a powerful alternative to HPLC for the purification and separation of complex chemical reaction mixtures. Advantages include greatly reduced solvent usage (and thus lower cost and environmental impact), higher throughput, and in some cases higher resolution. While there are commercially available prep SFC instruments, none currently offer all the features desired by many medicinal chemists engaged in the drug discovery process. These include: the ability to collect an unlimited number of fractions per sample with high recovery and negligible carryover, fully automated capacity to collect several hundred fractions, and the ability to collect fractions into the same disposable test tubes and racks which are already employed in HPLC. This article describes the customization of a preparatory scale SFC system purchased from Berger Instruments, Inc., Newark, DE. (a subsidiary Mettler-Toledo International, Inc., of Greifensee, Switzerland) in order to provide these capabilities.     

Water modelling of level fluctuation in thin slab continuous casting mould

Abstract

A water modelling experiment was conducted to study the meniscus instability in a continuous thin slab casting mould using particle image visualisation. The results show that the level fluctuation, circulation centre position and jet impinging depth are unsteady and periodic with a similar period. The probability distributions of the fluctuating meniscus and wave height have been obtained with the highest frequency near the average position. The flow pattern and meniscus profile may be momentarily asymmetrical, and the phase difference of level fluctuation in the two sides of mould centreline is a half period. The average meniscus profile, the highest and lowest meniscus positions are generally symmetrical about the mould centreline. The wave height mainly depends on the jet impinging depth and circulation centre position. The wave height increases as the jet impinging position rises and the circulation centre approaches to the submerged entry nozzle.     

Preparation of body armour material of Kevlar fabric treated with colloidal silica nanocomposite

Abstract

To safeguard bodies of soldiers better, including their necks and joints, a novel armour material was exploited using nanocomposite. Compared with traditional armour materials, this new material possessed superior barrier property and more comfortable characteristics. The new armour material was made of Kevlar cloth and shear thickening fluid (STF). Colloidal silica particles were first synthesised via the Stöber synthesis method and then they were used to prepare a suspension with solvent of polyethylene glycol 200. At last, Kevlar cloth was treated with the suspension and the resultant was named as STF–Kevlar nanocomposite. In this process, the particle sizes were characterised with scanning electron microscopy. The rheological properties were measured with a Physica MCR301 stress controlled rheometer. The results showed that the silica particles could be determined to be monodispersed spherical particles. The suspension, named the shear thickening fluid, had shear thickening characteristic. The property of multilayer STF–Kevlar nanocomposite targets was compared to that of the neat Kevlar cloth and the results indicated that the STF–Kevlar nanocomposite had an improvement in barrier property and was more flexible and comfortable. The mechanism that could improve the barrier property of STF–Kevlar was briefly explained.     

In process monitoring of polymer batch to batch variation in injection moulding

Abstract

Injection moulding studies are reported, during which in process measurements have been used to monitor batch to batch variation of several production grade polymers. Three materials were studied: a polyamide 6, a polyacetal, and a flexible poly(vinyl chloride), all of which were commercial injection moulding polymers supplied by industrial collaborators. Instrumented high precision electric and servohydraulic moulding machines were used to mould parts in house from a number of different material batches. Moulding conditions were kept constant throughout and several process variables were monitored during injection, including melt pressure, melt temperature, and viscosity index –a specific pressure integral calculated from primary injection. Part weights were measured to provide an indication of part quality. Results showed that in each case, variations between batches produced a measurable effect on part quality. These variations were detected by in process measurements, particularly by viscosity index, which tracked significant changes in part weight. Several ‘problem’batches not identified by the compounder's internal quality checks were detected, and the influence of regrind and a development compound were also clearly identified. No simple relationship between viscosity index and part quality was observed for the limited processing range covered. Overall, the studies show the potential of in process measurements to provide a real time, sensitive indication of process variation.