Molten Metal Flow Research Articles

Wetting kinetics and interfacial structure of AgCu alloy spreading on TB8 substrates

AgCu filler metal spreading over TB8 substrate under a high-purity argon atmosphere was conducted at 920 °C. Effects of surface roughness and topography of the substrate on the wetting kinetics were investigated. The interfacial microstructures of AgCu eutectic alloy/TB8 substrate solidified couples was analyzed by optical microscope, scanning electron microscope, energy dispersive spectrometer and transmission electron microscope. Results revealed that the substrate surface roughness and topography had significant influences on the wetting kinetics and spreading process. The spreading was preferential when AgCu molten metal over an anisotropically microstructured surface. When the surface of the substrate was not anisotropic, the spreading morphology was almost a perfect circle. An additional capillary driving force exists when the molten metal flows on the rough surface, which leads to a better wettability. The interfacial microstructure of the resolidified AgCu on the TB8 substrate was Ag(ss)/ Ti3Cu4/Ti2Cu/ /Ti(ss) + Ti2Cu/TB8, and the spreading edge domain near the triple-line is rich in Ag.

Prediction of Laminations in Zinc Alloy Die-Casting by Gas-Liquid Two-Phase Flow Simulation

In the manufacturing process of zinc alloy die-casting, laminations are a serious problem because they cause blister defects and the deposition of zinc on the mold surface. Thus, it is very important to analytically predict the risk of lamination formation before determining the mold design. In this study, we proposed a criterion for predicting the locations of laminations. The criterion value is estimated from the result of a molten metal flow simulation and can indirectly predict the locations of laminations. As the analytic method in the mold-filling simulation, we adopted a gas-liquid two-phase flow model with a finite element method, and the Cahn-Hilliard equation was adopted to determine the interface between the gas and the liquid. We predicted the locations of laminations for two different types of gating system using the mold-filling simulation. The validity of the proposed criterion for laminations was confirmed by comparison of the results of the simulation and the observation of laminations after practical die-casting.

Investigation of heat transfer and fluid flow in high current GTA welding by a unified model

This research is aimed to understand the heat transfer and fluid flow of the arc plasma and weld pool with a significant deformation of the weld pool surface during high current gas tungsten arc welding (GTAW). A unified model including the tungsten electrode, arc plasma and the weld pool of low carbon steel is developed by presetting a free surface deformation from the experimental observations. Buoyance, Lorentz force, plasma drag force and Marangoni force are taken into account to investigate the weld pool dynamics with a great depressed free surface. Heat flux, current density, Marangoni force and plasma drag force at the weld pool surface are presented to understand the heat and momentum transfer from the arc plasma to the weld pool. It is found that the distributions of the heat flux and current density as well as temperature at the depressed weld pool surface are double-peak profiles; the molten metal flow of the weld pool with severe surface depression are determined by the plasma drag force rather than the Marangoni force. The effect of the Lorentz force is more important than that of the buoyance, but weaker than the Marangoni force. The results are consistent with the existing results of both the experiment and the theory.

Modeling and Computation of Casting Process by Particle Method

Since particle-based computational methods, such as smoothed particle hydrodynamics (SPH) method, are suitable to treat free surfaces and moving boundaries, it is expected that the simulation with particle methods can represent more accurate molten metal flow. Hence, we apply the SPH method to some casting processes and validate the numerical results by comparing with experiments. While fluid behaviors of the transportation and the mold filling processes are expressed sufficiently, ladle pouring process of aluminum alloy is not reproduced by the existing scheme. So we developed a new numerical model of surface oxide film, and our numerical results show good agreement with the experimental data.

A comparison of TIG-MIG hybrid welding with conventional MIG welding in the behaviors of arc, droplet and weld pool

The influence of electrode relative position and TIG current variation on arc stability and weld formation of TIG-MIG hybrid welding were studied comparing with that of conventional MIG welding. The welding current-voltage waveform was analyzed to characterize the arc stability. When TIG arc was leading, the hybrid arc was more stable than conventional MIG welding even when the TIG current was as low as 50 A, while the TIG current should be higher than 100 A to avoid spatters when TIG arc was trailing. The arc shape and droplet transfer were investigated to describe the heat and force distribution in weld pool. When TIG arc was leading, the backward arc force and droplet impingement could not decrease the backward flow of molten metal. The increment of MIG arc length dispersed MIG arc heat and resulted in narrower weld width, compared with conventional MIG welding, which was the main factor to suppress undercut defect. When TIG arc was trailing, the forward arc force and droplet impingement significantly decreased the backward flow velocity and gave molten metal more time to spread transversely to suppress undercut defect. Regardless the relative position of TIG and MIG arcs, the hybrid welding speed could reach up to at least 1.5 m/min.

Influence of welding parameters on lap joint between Al and Ti alloys using low power fiber laser

AbstractFiber laser beam welding has always been a user‐friendly and flexible method to join dissimilar materials despite differences in thermal coefficient. Many industrial applications such as automotive has replaced the conventional joining methods towards this because of the flexibility and reduction in time consumption. In the present study, dissimilar titanium alloy; Ti6Al4 V and aluminum alloy; AA2024‐0 were laser welded through a lap joint technique using a low power Yb‐fiber laser without any additional filler. The influence of welding speed on weld morphology was investigated using optical microscopy (OM) and scanning electron microscope (SEM). The cross‐section of the joints revealed that the fusion zone (FZ) and heat affected zones (HAZ) are wider when welding speed decreases with lower laser power. This result shows that the low power fiber laser has sufficient energy to melt the base materials, forming a liquid bridge to facilitate the smooth flow of molten metal between the top and bottom layer. Therefore, at lower welding speeds with constant low laser power, it was shown that there are possibilities of laser welding between two non‐ferrous metals.

A novel electrode-arc-weld pool model for studying the keyhole formation in the keyhole plasma arc welding process

A novel electrode-arc-weld pool model considering the influence of the backside keyhole deviation on the heat source distribution, the influence of the keyhole constraint on the arc pressure and the keyhole geometry dependent plasma shear stress, is developed to study the keyhole formation in the KPAW process. The numerical and experimental results show that: the keyhole depth increases very quickly at first, then smoothly, and then quickly again. In the blind keyhole stage, the arc pressure deepens the keyhole. The plasma shear stress facilitates the fluid flow along the rear keyhole wall and decreases the curvature of the keyhole. The process between the weld pool penetration and the keyhole penetration is defined as the blasting penetration stage, in which the molten bridge under the arc is quickly broken. The pressure balance and mass conservation mechanisms are proposed to be responsible for the blasting penetration. The low melting temperature, low surface tension and bottom surface deformation promote the blasting penetration, while the high thermal conductivity suppresses the blasting penetration. In the penetrated keyhole stage, two contrary convective eddies are found inside the weld pool. At the top surface of the weld pool, the molten metal flows upward and backward near the keyhole, but obviously flows forward at the rear part of the weld pool. At the bottom surface of the weld pool, the molten metal flows downward and backward.

Comparative RANS turbulence modelling of lost salt core viability in high pressure die casting

In this work, the implementation of three turbulence models inside the open source C++ computational fluid dynamics (CFD) library OpenFOAM were tested in 2D and 3D to determine the viability of salt cores in high pressure die casting. A finite-volume and volume of fluid approach was used to model the two-phase flow of molten metal and air, with the latter being treated as compressible. Encouragingly, it is found that, although the choice of turbulence model seems to affect the dispersion of the two-phase interface, the force acting at the surface of the salt core depends only very weakly on the turbulence model used. The results were also compared against those obtained using the commercially available and widely-used casting software MAGMA5.

Improving the Filling System for a Defect Free Casting: A Review

Abstract With the evolution of modern technologies, now a day’s, it is very convenient to find out the primary causes of casting defects. Before going to actual practice, the flow behaviour can be simulated with the help of computer and input parameters can be optimized for obtaining defect free castings. The major defects in casting leading to early failure are due to air porosity and surface turbulence caused by the flow velocity and nature of flow of molten metal. Thus, it is needed to modify the gating system to get the optimised velocity and no surface fluctuations. In this review article, the flow behaviour of some improved gating systems such as submerged gating, uphill filling and trident gating systems have been reviewed and they were found to be turbulent free and the defects due to the surface turbulence have been considerably reduced.

Comparative RANS turbulence modelling of lost salt core viability in high pressure die casting

In this work, the implementation of three turbulence models inside the open source C++ computational fluid dynamics (CFD) library OpenFOAM were tested in 2D and 3D to determine the viability of salt cores in high pressure die casting. A finite-volume and volume of fluid approach was used to model the two-phase flow of molten metal and air, with the latter being treated as compressible. Encouragingly, it is found that, although the choice of turbulence model seems to affect the dispersion of the two-phase interface, the force acting at the surface of the salt core depends only very weakly on the turbulence model used. The results were also compared against those obtained using the commercially available and widely-used casting software MAGMA5.

Simulation of molten metal pouring into the continuous casting machine mold

Abstract Results of metal flow kinematics and temperature field calculation in the molds during molten metal pouring are presented. Navier-Stokes equations, heat transfer equations for flowing fluid and numerical gating technique were applied as the basis for calculations. The developed mathematical model allows studying molten metal flow kinematics in the mold and temperature field in various modes of pouring. Results of solutions are presented in a graphical form. Metal flow kinetics is analyzed and various modes of metal pouring are compared.

Improvement and validation of Zamak die casting moulds

The automotive industry, like many other industries, uses a wide range of parts produced by the die-cast process. Parts like engine blocks, wheel spacers, alternator housings and command cable terminals, are made by die casting with different kinds of materials like aluminium and zinc alloys. Despite being a reliable process both in terms of quantity and quality, it is very important to keep the process parameters controlled, in order to achieve a minimum percentage of defective parts, which may be caused by several factors such as, porosities, segregations, incomplete fill, soldering, cracks, etc. The main goal of the die casting industry is to achieve the zero per cent defects target, a goal that goes along with the automotive industry and its quality system, and to accomplish this objective the stakeholders need to invest in research and development. In the casting industry, for instance, it is very important to have a complete knowledge of the entire process developed inside the casting machine, from the melting pot to the die, in order to obtain data so one can improve the filling parameters, machine parts, and moulds. The focus of the presented study is the improvement of the methodologies used to design moulds for control cable terminals in Zamak alloys. The work starts by characterizing the flow happening inside the mould at the moment of cavity fill by analysing computer fluid dynamics simulations (CFD). The study proceeds by quantifying the porosities detected on cut terminal surfaces, and the ultimate goal is achieved with the modification of molten metal flow systems, like channels and sprues, and the introduction of venting systems, with a resource to mathematical and geometrical calculus developed in MATLAB® specifically for that purpose. The paper ends with the validation of the improvements, by comparing the initial results with the ones obtained through an improved mould, building bases for novel design concepts of moulds for this kind of parts, as well as new studies trying to improve the results now achieved.

Numerical Simulation of Casting Filling Process of Composites Reinforced with Nano SiC Based on Smoothed Particle Hydrodynamics.

The actual casting filling process can be deemed as the coupled flow of air, molten metal and solid-phase particles (such as slag) inside the cavity, which may cause air entrapment, slag inclusion and other casting defects, and affect the quality of casting. Through computation simulation of the casting filling process on the basis of smoothed particle hydrodynamics (SPH) method, it can not only precisely track the motion trajectories of air, molten metal and slag particles, but also precisely predict air entrapment, slag inclusion and other casting defects. This paper established a mathematical model of gas-liquid-solid three-phase flow for the SPH-based casting filling process. The model eliminated the instability of gas-liquid interface pressure by introducing the corrected gas-liquid two-phase momentum equation, and maintained a clear interface between air and molten metal by introducing the surface tension model. In addition, it introduced the motion model of rigid body to deal with the coupled flow process of slag, air and molten metal. The mathematical model established in this paper was used to calculate the bubble floating and the flowing process of the gas-liquid two-phase during mold filling through the bottom side of the injection cavity and of the gas-liquid-solid three-phase flow during mold filling for casting of plate (the plate was composite reinforced with nano SiC, and the viscosity of the composite melt was calculated from the viscosity formula of the composite melt). The calculated results were compared with the experimental results to verify the accuracy and validity of the mathematical model established.

3‐D Numerical Simulation and Analysis of the Flow Pattern in the Shot Sleeve of the Cold Chamber HPDC Process

AbstractThis work presents a 3d‐numerical study of the flow of molten metal in the shot sleeve of a horizontal high‐pressure die casting machine. The analysis of the flow pattern caused by the filling and the plunger motion can be of help in reducing porosity in manufactured parts caused by air entrapment. A particular attention has been paid to the flow and temperature field below the filling hole. The used numerical model is based on the conservation equations of mass, momentum and energy, and describes the free surface using the volume‐of‐fluid method.

Elucidation of the weld pool convection and keyhole formation mechanism in the keyhole plasma arc welding

An electrode-arc model, and a three dimensional weld pool model considering the self-adaptive heat source, the shear stress caused by plasma flow, and the deformation of the top and bottom surfaces, are developed to investigate the weld pool convection and keyhole formation mechanism in the keyhole plasma arc welding. With the help of the X-ray transmission system, high-speed video camera and thermal camera, the fluid flow and the temperature distribution of the weld pool are studied. The numerical and experimental results show that: after the penetrated keyhole formation, two convective eddies are formed inside the weld pool. The molten metal flows backward at the top surface of the weld pool, and flows downward and backward at the bottom surface of the weld pool. The low temperature distribution in the weld pool is attributed to the strong fluid flow and energy transportation caused by the much high arc pressure and plasma shear stress. Both the arc pressure driven weld pool deformation and shear stress driven weld pool deformation are responsible for the keyhole formation, and the first mechanism is especially dominant.

Rapid casting of complex impeller based on 3D printing wax pattern and simulation optimization

Rapid casting is the product of digital, information, and optimization of casting technology. It mainly includes rapid prototyping and virtual manufacturing. In order to shorten the production cycle of a stainless steel closed impeller casting, the wax pattern was made by high impact polystyrene (HIPS) with a selective laser sintering and photosensitive resin with stereolithography (SL). In order to prevent the formation of shrinkage defects, different gating systems designed to examine the molten metal flow and solidification behavior during the pouring and solidification process. The results show that pouring temperature is 1550 °C and pouring speed is 0.75 m/s, which is favorable for filling impeller castings, and can avoid casting defects. The optimized gating system prevented surface shrinkage and interior defects. The optimized gating systems have been verified by experiment, and the rapid casting has been realized based on 3D printing wax pattern and simulation optimization. This rapid casting can reduce processing time and costs, and enhance casting quality in the foundry industry.

Electron beam melting toward inclusion-free titanium alloys

Titanium ingots for airplanes must satisfy high standards of quality, removal of inclusions is essential. For the purpose of reducing the risk of contamination by LDI, a new way of removal of LDI has been studied. In the present study, marangoni flow induced by Intensive Heating of electron beam (IH) has been considered to control a state of a flow of molten metal in a cold crucible. Numerical simulations and an experiment have verified that LDI bounding for a lip was not able to go through the IH and stayed in a hearth.

Fusion zone geometries, cooling rates and solidification parameters during wire arc additive manufacturing

Structure, properties and serviceability of components made by wire arc additive manufacturing (WAAM) depend on the process parameters such as arc power, travel speed, wire diameter and wire feed rate. However, the selection of appropriate processing conditions to fabricate defect free and structurally sound components by trial and error is expensive and time consuming. Here we develop, test and utilize a three-dimensional heat transfer and fluid flow model of WAAM to calculate temperature and velocity fields, deposit shape and size, cooling rates and solidification parameters. The calculated fusion zone geometries and cooling rates for various arc power and travel speed and thermal cycles considering convective flow of molten metal agreed well with the corresponding experimental data for H13 tool steel deposits. It was found that convection is the main mechanism of heat transfer inside the molten pool. Faster travel speed enhanced the cooling rate but reduced the ratio of temperature gradient to solidification growth rate indicating increased instability of plane front solidification of components. Higher deposition rates could be achieved by increasing the heat input, using thicker wires and rapid wire feeding.

Thermo-Fluid Modeling of Selective Laser Melting: Single-Track Formation Incorporating Metallic Powder

Selective laser melting (SLM) utilizes a laser source to melt and fuse metallic particles to form dense solid parts. Since molten metals have surface tension gradients, the thermo-capillary effect becomes a driving force of melt flow, which will subsequently determine the molten/solidified track as well as the build surface morphology. In this study, a 3D volume of fluid model has been developed to simulate heat transfer and fluid dynamic during single-track laser scanning. A sequential powder addition algorithm is applied to obtain random powder distribution over a thick substrate. Temperature-dependent thermo-physical properties of Ti-6Al-4V are used to define the material, and a volumetric heat source is included as an approximation to laser irradiation. As a result of continuous melting and solidifying, the thermal behavior, the molten metal flow and the free surface formation can be numerically analyzed. In addition, a two-layer simulation has been carried out to study the interlayer bonding. Simulations results are compared with SLM experiments using the single-track morphology acquired by white-light interferometry. The melt pool widths obtained from simulations are in good agreement with the measured single-track widths. On the other hand, it is observed that single tracks formed in SLM have an elevated bead height, which is not realized numerically.

Mechanisms for Improvement of Weld Appearance in Autogenous Fiber Laser Welding of Thick Stainless Steels

High-power fiber laser welding is an efficient and effective way to produce heavy section structures. However, there is a significant challenge in producing the welds with free of imperfections such as nail-head-shaped welds, spatters, and root sagging. This is partially due to a lack of understanding of the welding mechanism of high-power fiber laser. In this paper, we were especially interested in the mechanism to improve the appearance of welds, and we focused on the autogenous laser welding on thick stainless steel plates by a 10 kW fiber laser. To look into the relations of process parameters and the quality of welds, a high-speed imaging system was applied to observe the molten pool flow and vapor plume during the welding process. The appearances of welds subjected to different welding conditions were analyzed. The results showed that (1) nail-head-shaped welds were suppressed by using a gas jet during laser welding process. (2) In the forward welding, a gentle upwelling molten metal flow on the rear keyhole wall, a deeper weld pool and a weaker vapor plume resulted in no spatter. (3) The gravity affected the formation of underfills and root sagging significantly during autogenous laser welding of thick plates. (4) When the workpiece was placed vertically in the transverse position, the welding process was stable without an aggregation of molten melt at the back surface. Moreover, the mechanisms of forming root sagging and humps were different at the top surface.