Metal-mold Interface Research Articles

Understanding the metal-mold interfacial reactions during directional solidification of superalloys using Al2O3-rich ceramic mold

This study investigates the reactions occurring at the metal-mold interface during directional solidification of Ni-based superalloys, exerting a significant influence on both the surface quality and the desired composition of the alloy. The origin and nature of these reactions are systematically investigated by integrating thermodynamics and microstructural evolution of the alloy. It was observed that Hf from the melt reacts with the Al2O3-based ceramic mold, leading to the formation of HfO2 at the interface. This compound adheres to the surface of the ingot and also infiltrates into the melt near the interface. In order to validate these findings, an exploration of four alternative Ni-based superalloys, both with and without Hf content, was conducted, revealing an absence of comparable reactions at the interface in Hf-free alloys. Additionally, it was explored that the interface formed due to the metal-mold reaction may serve as a preferential nucleation site for MC-type carbides in C-containing Ni-based superalloys. This study concludes that the Hf content is the primary determinant influencing reactions at the metal-mold interface.

A hybrid approach towards evaluation of average heat transfer coefficient in twin-roll strip casting mold

Continuous casting stands out as one of the most efficient and productive methods for producing billets, slabs, or thin sheets while conserving energy. Understanding the solidification dynamics and achieving defect-free sheets in continuous casting necessitates the accurate prediction of heat transfer coefficient (HTC) at the mold metal interface. Accurate prediction of HTC helps in selecting process parameters such as superheat temperature and casting velocity, which directly affect the production rate and quality of the cast component. Determining the HTC is a complex process, as it is influenced by various process parameters such as casting size, casting speed, superheat temperature, surface roughness, type of water used as a coolant, nozzle design and wettability. Therefore, the present study proposed a hybrid approach for finding HTC in a twin roll strip casting mold through a solidified shell shape, which is influenced by the factors affecting the heat transfer at the mold-metal interface. This approach integrates the experimental and simulation investigations. The numerical investigation used a three-dimensional coupled fluid flow and heat transfer equations with an assumed range of HTC (100–2000 W/(m2K)). The solidified shell shape for different HTCs was compared with the experimental solidified shape form at the outer wall of the mold using the solidification criteria. Results indicate that the initial HTC range of 100–1000 W/(m2K) does not lead to blockage of the graphite mold outlet. However, as HTC values approach 1000 W/(m2K) to 1400 W/(m2K), the melt obstructs the mold exit based on solidification criteria. The HTC values (1350 ± 150 W/(m2K)) obtained from the proposed methodology for mold, align well with the HTC values (1183 ± 593 W/(m2K)) calculated using lamellar spacing of the Al-Mg2Si composite microstructure. This hybrid approach presents a novel and straightforward method for determining HTC, offering accurate predictions that can enhance the precision of continuous casting models and provide a more nuanced understanding of solidification behavior.

Gas Porosity Defect – What It Means and How to Respond

The foundries are facing problem-related to the selection of the parameter’s value for minimum rejection and maximum productivity. The furan no-bake binders system guaranteed dimensional stability and a comparative good surface finish of the casting. Based on past data in the industry, it is found that gas porosity defect is one of the highest. The phenomenon of the formation of the bubble in the fissures of the mould-metal interface, and later on trapping during the solidification leads to gas porosity. The current research work is focused on the minimization of the defect by the selection of the optimum range of input variables. Based on rigorous literature survey and industrial expert’s opinion, it is found that the parameters like grain fineness number (GFN) of the sand, loss on ignition (LoI) of the used sand, the sand temperature at the mixing time, potential of hydrogen (pH) are important parameters for gas porosity defect in the casting.Design-Expert software and particularly response surface methodology (RSM) and sequential approach using the face-centered central composite design is used for the experiments. The results show that a quadratic model with the removal of some insignificant term is a comparatively best fit for gas porosity defects. After analysis, various favorable levels of different parameters are obtained. The research work is based on realistic problems of the foundries and based on the experimental work. Thus, the provided solution is very much useful for foundries to reduce the rejection, particularly for furan no-bake with furfuryl alcohol as resin and sulphonic acid as catalyst. The research problem addressed in the paper is a genuine problem of the foundries and the sole work is based on experimental evidence.

Advancing Aluminum Casting Optimization With Real-Time Temperature and Gap Measurements Using Optical Fiber Sensors at the Metal-Mold Interface

Advancing Aluminum Casting Optimization With Real-Time Temperature and Gap Measurements Using Optical Fiber Sensors at the Metal-Mold Interface

Significant elimination of pore defect and interfacial reaction of sand casted Al–Li alloy castings via a novel inorganic binder coating

Generally, sand casted Al–Li alloy castings have very serious problems in pore defect and interfacial reaction, remarkably weakening the Al–Li alloy castings quality. How to solve these problems is always a challenge. In this paper, a novel inorganic binder coating containing lithium silicate solution binder and silicon carbide powder on the surface of the sand mold was developed to eliminate pore defect and prevent interfacial reaction of the sand casted Al–Li alloy castings. The effective mechanism of the inorganic binder coating on the pore defect and interfacial reaction was discussed. The obtained results showed that the inorganic binder coating formed a thin shell on the surface of the sand mold after pouring, which can effectively prevent the metal-mold interface reaction of the sand casting Al–Li alloy and significantly reduce the porosity defects. Among them, the Al–Li alloy casting prepared using the inorganic binder coating consisting of the lithium silicate binder and silicon carbide powder basically had no pore defects, and the porosity was only 3.73%, which was 66.7% lower than that of the casting prepared by the sand mold without any treatment. Moreover, the interface reaction of the Al–Li alloy and sand mold was fully eliminated, which was attributed to the low overall atomic diffusion rate of the silicon carbide based on molecular dynamics simulation results.

Effect of microstructure on the hardness of ASTM A48 class 20 flaked graphite cast iron

ABSTRACT In foundries, it becomes necessary to minimise the machining allowance for economic reasons. It will save resources and costs by reducing the energy and expenses incurred on re-melting, machining and tooling. A deeper understanding of the complicated microstructures developed within about 3.5 mm of the mould-metal interface is necessary. The current research work uses a unique experimental design that involves performing surface and subsurface microscopy and spectroscopy at 0.5 mm intervals from the as-cast surface to a depth of up to 3.5 mm for three samples. The objective is on evaluating microstructure for determining graphite flake size, distribution, grouping, pearlite and ferrite percentage, and different elements. All three samples were subjected to a hardness test, which is a measure of machinability. The relationship between hardness as a single response to percentage presence of elements, graphite distribution and pearlite has been established using statistical analysis.

Modelling of metal – mould interface resistance in the Al-11.5 wt.% Si alloy casting process

AbstractA computational model is developed to include heat transfer, effects of resistance of the metal –mould interface, and pressure in the simulation of the solidification process. The simulation of the interface resistance is based on the Zero Thickness Element, utilizing the Finite Element Method. The solid boundary conditions, including contact resistances, have been modified by a pressure gradient in each of the Zero Thickness Elements. The pressure gradient has been modelled on experimental data. In order to verify the computational results, an Al-11.5 wt.% Si alloy has been poured into a permanent mould and the temperature of the interface measured. Then we modelled the effect of metallostatic pressure on the overall heat transfer coefficient in the metal–mould interface. The comparison between the experimental and the simulation results during the solidification process shows a good consistency that confirms the accuracy of the model for the effect of the interface resistance on solidification time.

Magnesium stents manufacturing: Experimental application of a novel hybrid thin-walled investment casting approach

The present study addresses a thin-walled investment casting methodology of AZ91D-1 wt% CaO magnesium alloy for stent manufacturing. Additive manufacturing and Yttria-based coating were applied to optimize the process. An induction melting furnace assisted by vacuum was used to cast magnesium alloy stents with 0.4 mm and 0.8 mm wall thickness under an argon-protective atmosphere. According to the proposed methodology and the results obtained for coated models, no metal-mould interface reactions were found, revealing samples with a good surface finish and sane microstructure. The cast magnesium stents without coating were compromised, resulting in samples with a high degree of reactions and impurities. Microscopic examination and hardness measurements demonstrated the effectiveness of the process.

Применение техники цифровой обработки изображений в анализе микроструктуры и исследовании обрабатываемости

Introduction. The world is at the stage of creating an interdisciplinary approach that will be implemented in metallurgical research. The paper formulates the technique of image analysis in the study of processing at different depths from the mold-metal interface. The purpose of the work. Processing of a cast-iron workpiece within the first 3.5 mm of thickness from the mold-metal interface is a serious problem of solid processing. The study of machinability at different depths is a key requirement of the industry for ease of processing. Machinability will determine a number of factors, including tool consumption, workpiece surface quality, energy consumption, etc. The method of investigation. Image analysis is performed to determine the percentage of graphite in etched and non-etched samples. K-means clustering allows to create a new image from a given one with a clear separation of white and black areas by converting a digital image into a binary image using a threshold value for segmentation. The volume fraction of perlite, the volume fraction of graphite and the average size of graphite flakes in microns are used as input variables for the machinability of cast iron. Results and discussion. The output, that is, the segmented image, will be the input function for calculating the workability index using formulas. Thus, microstructural analysis will help predict the workability index of grey cast iron ASTM A48 Class 20. Using this method and the program, based on the microstructure, it is possible to predict in advance the characteristics of the machining of the part, taking into account possible changes in the casting process itself.

Experimental Study Regarding the Possibility of Blocking the Diffusion of Sulfur at Casting-Mold Interface in Ductile Iron Castings

The main objective of this work is to investigate the mechanism of sulfur diffusion from the mold (sand resin P-toluol sulfonic acid mold, sulfur-containing acid) in liquid cast iron in order to limit the graphite degeneration in the surface layer of iron castings. A pyramid trunk with square section samples was cast. On the opposite side of the feed canal of the samples, steel sheets with different thicknesses (0.5, 1, and 3 mm) were inserted with the intention of blocking the diffusion of sulfur from the mold into the cast sample during solidification. The structure evaluation (graphite and matrix) in the surface layer and the casting body was recorded. The experimental results revealed that by blocking the direct diffusion of sulfur at the mold-casting interface, a decrease of the demodified layer thickness (for 0.5 mm steel sheet thickness) is obtained until its disappearance (for steel sheet thicknesses of more than 1 mm). The paper contains data that may be useful in elucidating the mechanism of graphite degeneration in the superficial layer of ductile iron castings. Based on the obtained results, we recommend using such barriers on the metal-mold interface, which are able to limit sulfur diffusion from the mold/core materials into the iron castings, in order to limit or even cease graphite degeneration in the Mg-treated surface iron casting layer. The paper presents additional data related to the interaction of sulfur at the ductile iron casting-mold interface previously analyzed.

Investigation of the Interfacial Heat Transfer Coefficient at the Metal–Mold Interface During Casting of an A356 Aluminum Alloy and AZ81 Magnesium Alloy into Steel and Graphite Molds

Many types of alloys can be cast in graphite molds. The advantage of using graphite molds is that graphite has a higher thermal conductivity than steel and provides a higher alloy cooling rate. In this work, the solidification conditions in graphite and steel molds are compared. Cylindrical ingots of A356 aluminum and AZ81 magnesium alloys were cast into steel and graphite molds, and the temperature was recorded by thermocouples positioned in both the ingots and the molds. Using the error function, which is a measure of the difference between the experimental and simulated temperatures, the interfacial heat transfer coefficient (IHTC) versus the ingot surface temperature curves were obtained. The peak IHTC for ingot casting in the graphite mold was 2–3 times higher than that for the steel mold for both alloys. The solidification time and cooling rate for the solidification of the A356 and AZ81 ingots with varied sizes in the graphite and steel molds were calculated based on the simulation results. The duration of the solidification process in the steel mold was nearly two times longer than that for ingots in the graphite mold. The cooling rate in the graphite mold was nearly 1.5–2 times higher than that in the steel molds.

Control of the Mg-Treated Iron Casting Skin Formation by S-Diffusion Blocking at the Metal–Mould Interface

Having established that sulphur presence in the mould materials appears to have an important contribution in graphite degeneration at least in the casting surface layer, a research program is undertaken to explore the possible beneficial effect of sulphur diffusion blocking at the metal–mould interface. Test samples, with and without a thin steel sheet (up to 3 mm thickness) application on the inner surface of the mould cavity, before iron melt pouring, are considered for structure analysis. A higher nodulizing potential (0.048% Mgres, 0.015% Ceres, and 0.006% Lares) decreases the occurrence of surface graphite degeneration in castings obtained in rigid chemically bonded resin sand moulds, using P-toluol sulfonic acid (PTSA) hardener (S-including), but it is not enough to avoid this phenomenon (200–400 μm skin in present experimental conditions). The casting skin appears to have different values, depending on the evaluation technique (un- and Nital-etching direct measurement, or graphite parameters variation on the casting section). In the presence of a thin steel sheet at the metal–mould interface, the casting skin thickness decreases or is just excluded. It is supposed that it acts as a barrier, blocking S-diffusion from the mould media into the iron melt. Without this S-diffusion, the graphite degeneration in the casting surface layer could be avoided, or at least diminished. For industrial application, the increasing of residual content of nodulizing elements is a limited solution, and it is recommended to use barriers to block S transfer on the mould/metal surface, such as dense coatings or coatings with desulphurization capacity.

IoT feasibility aspects of mold temperature monitoring and casting simulation for smart foundry

The world is moving in the direction of digitalization demanding change to smart manufacturing. Utilization of smart manufacturing with adoption of Internet of Things (IoT) in the foundry industry gives benefits and help to overcome challenges. In cast iron foundry rejection due to solidification defect like sand drop, crack, pinholes, shrinkage cavity, cold shut, misrun which can be reduced with help of directional solidification. For solidification analysis cooling curve monitoring needs time and effort. For analysis the correct method of data collection and monitor is required. In present work authors have made an attempt to use solution that is anticipated is Autocast_X1 software simulation and development of low cost IoT solution for smart foundry. In Autocast-X1 software simulation is performed and defect analysis is carried out. It is the effective way to improve design and helps to reduce trial runs and efforts. Thermocouple directly placed in a mould cavity to monitor temperature. It is helpful to study cooling curves and quality check of solidification defects. Experimental investigations are carried out to map simulation result of mold temperature with real time data with help of thermocouples placed at mold interface. Mold-metal interface data using controllers and amplifier are stored in mobile for live monitoring. Use low cost IoT platform reduces supervisor movement and saves time by providing casting solidification data at his work place. With the help of casting simulation early check of effective gating system and defect prediction enables reasonable productivity and quality. IoT solution helps to monitor foundry practices, reduction in human effort, paperwork, and reduction in cost with rise of profit margin.

Analysis of Permanent Mold Distortion in Aluminum Casting

Mold distortion in permanent mold casting campaigns results in loss of mold productivity and shorter mold life, and can result in out-of-dimensional tolerance castings. Remedies to avoiding distortion during a casting campaign include back face mold machining, bolting, and clamping, but permanent deformation occurs when these restraints are removed. An AFS research program was undertaken using finite element thermal and stress analysis to better understand the causes of permanent mold distortion. Phase 1 of this research, using a simple mold geometry, indicated mold–metal interface thermal shock, and continued mold cycling resulted in cumulative permanent plastic strain. Phase 2 work using a commercial mold insert resulted in good agreement reached between the model plastic strain and the actual measured mold insert distortion. The study showed that mold material properties are significant, with higher mold thermal conductivity (better heat diffusivity beyond the mold–metal interface); and higher creep strength results in less plastic strain per casting cycle and less distortion of the permanent mold.

Heat flow and heat transfer coefficient during crystallization under the pressure

Estimation of the heat flow at the metal-mold interface is necessary for accurate simulation of the solidification processes. For the numerical simulation, a precise prediction of boundary conditions is required to determine the temperature distribution during solidification, porosity nucleation, microstructure development, and residual stresses. Determination of the heat transfer coefficients at the metal-mold interface is a critical aspect for simulation of the solidification process and the microstructure modeling of the castings. For crystallization under the pressure and for thin-walled castings, HTC evaluation is important due to the very limited freezing time.

Qualification of Chemically Bonded Sand Systems Using a Casting Trial for Quantifying Interfacial Defects

Chemically bonded sand cores and molds are an important part of metal casting technology and their interaction with metal (mold–metal interface) is of great interest. Currently, the metal casting industry places a strong emphasis on near-net-shape and thin wall castings, while simultaneously maintaining increasingly stringent dimensional reproducibility requirements. To efficiently and cost effectively accomplish this task, it becomes crucial to ensure or “qualify” high-quality performance at the mold–metal interface. The thermal distortion test (TDT) was used in published American Foundry Society (AFS) studies as a laboratory technique to determine the presence of anomalies and for measuring distortions in chemically bonded sand binder systems. However, these laboratory tests focus purely on sand binder system behavior, without considering the effects of actual mold–metal interfaces. While the TDT provides valuable insight, it is unable to ensure castings will be free (or contain an acceptable amount) of specific defects. Therefore, to maximize casting quality and efficiency, it becomes imperative to incorporate casting trials into a foundry’s quality control system. This will allow the occurrence of potential casting defects to be observed at the mold–metal interface. This paper presents a casting trial technique that provides three specific benefits to foundry quality control systems. First, these trials will qualify sand binder systems against a specific casting surface defect(s), such as surface roughness, veins, penetration, cuts, washes, and erosion. Second, these casting trials are implemented under constant casting, mold, process, and design parameters. This allows the casting trial(s) to provide diagnostic information (i.e., if a specific defect occurs when using a qualified sand binder system, the cause cannot be related to the sand binder system). Third, the casting trial technique provides new opportunities for data analysis and deeper system understanding. Specifically, results in this paper identify the possibility of a potential relation between TDT data (e.g., anomalies and distortions) and actual casting trial data (e.g., defects).

Modeling of Interface Heat Flux and Thermal Field of Mold Materials during Gravity Die Casting

One of the key controllable and influential factors to obtain a casting simulation, representative of reality, is the choice of boundary condition. The thermal boundary condition to be specified at the metal-mold interface must account for complex heat transfer phenomena associated with solidifying casting. The present study aims at estimating the heat flux at the interface of the mold and the solidifying metal by Inverse Heat Conduction Problem (IHCP) approach. Solidification studies were conducted on casting of aluminum reinforced with boron carbide composite. Copper, cast iron and stainless steel were used as mold materials. The temperature data of the mold was recorded from the beginning to end of solidification using k-type thermocouples connected to temperature data logger. This time-temperature history was used as input to the IHCP algorithm to simulate the interface heat flux and thermal field of the mold. The results indicate that the interface heat flux is highly transient and varies with the variation in the thermo-physical properties of the mold materials. The study also demonstrates that heat conduction is one dimensional in copper mold and two dimensional in cast iron and stainless steel mold during phase change.

Effect of Interfacial Heat Transfer Coefficient on Dendritic Growth and Microhardness during Horizontal Directional Solidification of an Aluminum-Copper Alloy

This paper presents a theoretical-experimental study for the prediction of the interfacial heat transfer coefficient during the horizontal directional solidification of an Al-3wt.%Cu alloy on water cooled stainless steel chill under transient heat flow conditions. Eight thermocouples were connected with the casting and the time-temperature data were recorded automatically. The thermocouples were placed at 5, 10, 15, 20, 30, 50, 70 and 90 mm from the metal-mold interface. A numerical technique which compares theoretical and experimental thermal profiles was used to measure the heat transfer coefficient values. This has permitted the evaluation of the variation of this thermal parameter along the solidification which is represented by a power equation that shows the time dependence during the process given by hi = constant (t)-n, which represents the best fit between the experimental and calculated curves. The obtained results also include the variation of both primary and secondary dendritic arm spacings of alloy analyzed as a function of heat transfer coefficient. These dendrite arm spacings were found to decrease as the values of this coefficient are increased. Finally, an experimental law of the Hall-Petch type is proposed relating the resulting microhardness to the heat transfer coefficient investigated.

Fluidity of Sn-based eutectic solder alloys

Eutectic alloys have a great importance in both academic and technological point of view. As regard technological applications such as casting, welding and joining, these systems offer lower melting point than the pure elements, and good fluidity. The fluidity length (\(\hbox {L}_{\text{F}}\)) is known as the distance travelled by the liquid metal forced to flow through a channel of small cross section, until it is stopped by solidification. Physical variables associated with the process are: metallostatic pressure, heat extraction rate at the metal-mold interface, overheating of the liquid metal and the physico-chemical properties of the metal or alloy (latent heat of fusion, density, viscosity, surface tension and solidification mode). In general, pure metals and alloys of eutectic composition have the highest values of fluidity, whilst intermediate composition alloys with greater solidification range show lesser fluidity lengths. Taking into account that the chemical composition plays an important role in the fluidity length by its relationship with the resulting microstructure, the aim of this work is to obtain fluidity values of binary and ternary lead-free metallic alloys in order to determine the relationships between the morphology and the fluidity length, and consequently, the influence on binary and ternary pro-eutectic alloys. Fluidity tests were carried out in a linear fluidity device, using lead-free Sn based alloys, extensively used for important industrial applications. The samples were characterized using optical microscopy, scanning electron microscopy and energy dispersive X-ray micro-analysis. Usually, fluidity length (\(\hbox {L}_{{\text{F}}}\)) depends on solidification mode, latent heat of fusion of the alloy and the fluidity of the phases present in the microstructure.

Low-head direct chill slab casting of aluminium alloy AA4045: 3-D coupled turbulent flow and heat transfer study

In the present study, a low-head direct chill (DC) industrial-scale caster for rolling ingots has been modelled using the advanced computational fluid dynamics approach. Specifically, the 3-D coupled turbulent melt flow and solidification heat transfer in a slab caster has been modelled for the aluminium AA4045 alloy. The turbulence generated in the melt pool is taken into account in the model through the implementation of a popular low-Reynolds (Re) number version of the k-ε eddy viscosity model. The solidification heat transfer for this complex opaque system has been considered through the employment of the well-known ‘enthalpy-porosity’ scheme. A staggered control-volume (CV)-based finite-difference scheme is used to discretise the governing transport equations and the associated boundary conditions. The discretised algebraic equations are then solved sequentially using the tridiagonal matrix algorithm with implicit under-relaxation factors. The SIMPLE algorithm was employed to resolve the velocity–pressure couplings in the momentum equations. Three important process parameters of this casting problem, namely, the casting speed, the pouring temperature (inlet melt superheat), and the convective heat transfer coefficient (HTC) at the metal–mould interface for the low-head 30-mm mould, are varied in their corresponding practical range of 60–180 mm min1, 16–64°C and 1000–4000 W m2°C, respectively. The predicted results are presented in the form of velocity and temperature fields, solid shell thickness at the mould exit, as well as the sump depth and mushy-region thickness at the centre of the ingot. The obtained results are then critically discussed.