In recent years, the production of steel by the continuous castingprocess keeps increasing, the reasons for this growth are the product quality and the cost-effectiveness. This process is described as follows: liquid steel is poured into a copper mould which is kept at a medium temperature thanks to a cooling system. The molten steel in contact with the mould quickly starts to solidify (primary cooling). Extracting rolls under the mould pull the strand and keep it moving forward in the caster while water continues cooling it (secondary cooling). The strand can be cut when all of the section is solidified. Many process parameters influent the quality of the product such as: casting speed, mould level, mould oscillation, composition of the steel, superheat temperature, secondary cooling conditions... A large number of mathematical models of the whole process are reported in the literature. Some analytical solutions for the development of the stress field in the strand in the course of solidification were developed by Weiner and Boley , as well as by Tien and Kaump . Although these analytical solutions provide global reference results for
It is known that DC castingprocess usually consists of two forms — vertical and horizontal direct chills (VDC and HDC). Compared to VDC, HDC process has advantages of lower investment cost, higher flexibility, longer casting times, etc . However, the disadvantages are also obvious, such as inhomogeneous microstructures and macro- segregation resulting from gravity difference between top surface and bottom surface. Considering the noticeable influence of EMFs mentioned above, it is of practical interest to apply an EMF to the horizontal direct chill process.
downward flow, resulting in the inversion of the initial clockwise movement. In contrast, in the TM model (Figure 4.8f), mechanical shrinkage disordered this convective flow, and therefore impeded the directed distribution of the rejected solutes and weakened the accumulation trend of solutes along the centerline and in the solidification front. In addition, this lower positive segregation intensity could also originate from the smaller density variations due to the lower temperature gradient resulting from thermomechanic shrinkage. Ten hours after the filling stage, when the ingot body was almost totally solidified, as illustrated in Figures 4.11c and g, positive segregated zones formed in the center upper region and negative segregated zones at the bottom. Solute-enriched segregation zones were also formed between the center and the ingot wall, as clearly revealed in Figures 4.11c, d, g and h. They extended nearly over the entire length of the ingot, inclined with respect to the ingot boundaries with a pattern similar to those calculated numerically by Schneider and Beckermann (Schneider et Beckermann, 1995). Finally, it should be noted that as shown in Figure 4.11d, large spatial variations in composition are predicted with the TH model. Sang et al. (Sang et al., 2010) also reported similar observations and related it to pure convection- induced flows that are assumed in the thermohydraulic modeling. In contrast, the macrosegregation in the upper section of the ingot and in the hot-top predicted by TM model was comparatively less severe, as seen in Figure 4.11h. The milder segregation finds its origin in the accelerated solidification in the castingprocess due to ingot volume contraction as discussed above.
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Measurement of the heat transfer coefficient under transient and steady state conditions during magnesium semi-solid castingprocess
Weng, Ruey-Jer; Kuo, Jer-Haur; Hwang, Weng-Sing; Moisan, Jean-Francois
𝒙 𝑛𝑒𝑤 = 𝒙 𝑜𝑙𝑑 + 𝛥𝑡𝒗 𝑚𝑠ℎ (25)
It should be reminded that the energy transportation relative to the solid phase in the solid and
mushy zones, is achieved through this mesh updating process under the present ALE framework.
Under such working conditions, the tools are generally damaged through wear and thermal fatigue cracking processes [ 15–18 ]. Ktari et al. [ 19 ] have investigated a diesel engine crankshafts fracture used in train. Mellouli et al. [ 20 ] have investigated the thermal fatigue failure of a prematurely failed brass die-casting.
To improve the structures lifetime several researches have been conducted to study the eﬀect of surface coatings on those structure lives. Panjan et al. [ 12 ] have studied the eﬀect of duplex treatment composed of plasma nitriding and the deposition of 4- mm-thick PVD CrN coating on die-casting tools. They have found that wear of duplex treated tool is smaller than wear of the plasma nitrided tool. In addition, Srivastava et al. [ 21 ] have studied the eﬀect of three-layer coating architecture on die surface: the outer layer consists of a thermal barrier coating composed of rare-earth oxide, the middle one is a TiAlN diﬀusion barrier coating and the inner layer is a thin adhesive Ti layer. These authors have found that this coating can signi ﬁcantly improves the thermal fatigue resistance of the substrate. Klobčar et al. [ 22 ] have studied the eﬀect of cladding with maraging steels; they have found that this treatment provides good thermal fatigue resistance to the treated metal. The functionally graded materials (FGM) were also applied as a solution to improve the structure lifetime. Fazarinc et al. [ 23 ] have studied several FGM surfaces by changing amounts of alloying elements, precisely Si and Mo, they have found that their resistance to thermal fatigue is estimated at 27 times more resistant than the basic material.
Among the different steel families, peritectic steels seem to be very sensitive to the transversal crack formation. This steel family is usually employed for daily applications such as welded constructions or electric household appliances. For these materials, the chemistry and the microstructure, especially the nano-precipitates which form trend to dramatically enhance the crack formation. To optimize the continuous cast process and avoid the crack formation, the fracture mechanisms must be understood and predicted by a numerical model which takes into account both rheological and damage behaviour. The prediction of the damage is based on the microstructural
The average strain and strain variance are presented in Figure 11 for the (111) and (200) reflections. The average strain was significantly reduced with grain refiner additions for the (111) reflection. This was expected, as the grain refined microstructure likely enabled uniform feeding of liquid metal during casting solidification, which compensated the axial contraction of the horizontal bar and alleviated developing strains. In the case of the (200) reflection, mixed strain values were observed for the casting conditions. The strain for the (200) reflection was less sensitive to the effect of grain refinement, as observed in Figure 11a (only a small change in strain magnitude was observed). The average strain variance shown in Figure 11b suggests that grain refiner enhanced strain homogeneity, for both (111) and (200) reflections.
for the related published article.
ABSTRACT. Electromagnetic stirring (EMS) is widely used to increase the efficiency in
continuous castingprocess of steel. In particular, in-mould applications (M-EMS) allow decreasing the free surface fluctuation, controlling the velocity field and decreasing the turbulence of the flow. Since laboratory-scale tests are not fully representative of the process and industrial measurements are both expensive and difficult to carry out, numerical simulation is a strong tool to study and optimize these electromagnetic applications in steel industry. These simulations cannot fully model the process of its multiphysical nature and the simulation of all the phenomena involved would lead to huge computational costs, which is one of the main limits in the current situation. For this reason, this work aims at starting searching a coupling algorithm which could guarantee both computational efficiency and accuracy of the final results.
Recently, considerable amount of studies was performed to meet alloy requirements and further increase in mechanical properties of high pressure die castings. In this regard, using a vacuum assistant system during castingprocess has brought new insight in high pressure die casting technologies. The presence of vacuum assistant allowed to perform T5 and T6 heat treatments, without the risk of blistering. Mainly, T5 temper used for the castings which are artificially aged at temperatures between 170°C and 210°C. Due to precipitation hardening, higher mechanical properties can be achieved by T5 heat treatment applications. Also, low heat treatment temperatures inhibit to formation of distortion and blistering which have harmful influence on the mechanical properties of alloys. On the other hand, in T6 temper, the alloys are subjected to solution heat treatment at high temperatures (500-540°C), then it is followed by an artificial aging treatment. Due to very high treatment temperatures, highest strength and good ductility can be achievable in T6 tempers.
S. Castagne, F. Pascon, G. Blès and A.M. Habraken
Abstract. Two complementary approaches of steel continuous casting modelling using the finite element code
LAGAMINE have been developed in the M&S Department. We propose here a description of the context in which the study started, then a description of both macroscopic and mesoscopic approaches. The first one describes the whole continuous castingprocess, from the free surface in the mould and through the entire machine, including thermal and mechanical behaviour of the steel. The second approach focuses on the prediction of cracks and is developed at the grain scale. Some results are also presented for both models.
Industrial context of the project
SMC is characterised by a strong product mix with 7 different business lines, about 1000 revolving references per year and an average of 15 new tooling per month. Furthermore, the sand castingprocess needs, at the moment from 20 to 40 hours to complete a CAD study; that is to say, model the part, the master pattern, the pattern-plates, the cluster and simulate the fill up and the solidification. To generalise this kind of study to all the parts is not possible at the moment compared to the technical office capacities. Therefore, the toolmaker suppliers use the traditional moulding techniques to manufacture the pattern plates from the 2D drawing or from the part CAD model. The random repeatability of the tooling penalises the simulation accuracy. Therefore, there are only some clusters that are simulated and the results are interpreted with cautiousness.
Numerical modeling is an essential tool to optimize casting processes but the accuracy of the results is largely dependent on the boundary conditions that are specified. An important boundary condition in the modeling of high pressure die castingprocess is the interfacial heat transfer coefficient (IHTC) located between the casting and the die. This coefficient refers to the thermal conductance whose inverse is the thermal resistance. In high pressure die casting, the greatest thermal resistance in the entire process is at this location 1 and it is thus fundamental to characterize it as accurately as possible. The evolutions of the casting-die contact and the corresponding heat transfer have been described in several studies 2-10 . Typically, the heat transfer at the casting-die interface is characterized in term of 3 physical mechanisms 2, 4, 5 : 1) the IHTC reaches a maximum value as the metal contacts the die, a consequence of a conforming contact that provides a large heat transfer, 2) solidification creates a nonconforming contact, impairing the heat transfer at the casting-die interface and decreasing the IHTC and 3) a gap develops between the casting and the die, leading to a poor heat transfer and low values in the IHTCs.
URL: www.ulg.ac.be/matstruc/ e-mail: F.Pascon@ulg.ac.be ; Anne.Habraken@ulg.ac.be
ABSTRACT: Two complementary approaches of steel continuous casting modelling using the finite element code LAGAMINE have been developed in the M&S Department. The first one (at a macroscopic scale) describes the whole continuous castingprocess, from the free surface in the mould and through the entire machine, including thermal and mechanical behaviour of the steel. The second approach focuses on the prediction of cracks and is developed at grain scale. We present in this paper a description of the context in which the study started, the description of the macroscopic approach and some results.
During this work we have identified a principal difficulty, which for this modelling impact the multiplicity of the elements on the availability for their characterization and for their organization. Even if the manufacturing sand castingprocess seems simple, it uses many components (alloy, cores, mould…), and we limited the definition in term of model refinement since each one of these components could be the subject of a finer modelling. We tried to choose this limitation in order to represent the general process without going closer into enterprise specificities. For example, the cores or fusion process of realization are not completely defined since depending on the machines, uses and other specificities of the workshop. In fact this remark is shareable for all the components entering into the realization of the finished part. But we think that we defined a basic minimal skeleton, transposable from one company to another using the sand castingprocess.
In order to represent intergranular creep fracture, the developed model contains solid finite elements for the grains and interface elements for their boundaries (see also ). Inside the grains, an elasto-visco- plastic law without damage is used, and at its boundaries, a law with damage is preferred.
The mesoscopic approach allows a parametrical study of various factors such as grain size, precipitation state or oscillation marks geometry. The model can be applied to the continuous castingprocess as we know the loading to apply in the critical zone thanks to the macroscopic approach. As we want to analyse the process at the grain scale, we have to concentrate on specific zones.
In the present paper, a 2D thermal finite element approach models the stationary state of a continuous castingprocess. It allows predicting the thermal field within the mold wall and the thickness of the solidified steel shell of the strand. A second thermal model is established in order to simulate the propagation of a crack due to the sticking break out phenomenon. A specific remeshing procedure has been developed. The model allows predicting hot tearing due to sticking phenomena and its propagation. The model sensitivity has been checked. It increases the understanding of the phenomena. In future work, parametric studies of crack events will improve the knowledge of this phenomenon and a 3D mechanical approach will help to identify the mechanical stress and strain fields in 3D at the rupture events.
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pattern cavities, high risk of forming defects, high tendency for hot tearing, etc. Aluminum alloy AA6061 is a wrought alloy with low silicon content. Its strong mechanical properties and market availability make it one of the most popular alloys for a wide range of applications, including the automotive industry. Conventional pressure die casting (liquid state) is a high-productivity shape castingprocess used to chain produce complex- shaped aluminum parts at very low cost in high volumes. Generally, pressure die cast part manufacturing is much cheaper than wrought processes. On the other hand, such parts have limited mechanical properties. Semi-solid pressure die casting is a new approach which can be used to make parts that are comparable in quality to those produced with wrought processes and comparable in cost with conventional pressure die- casting. Indeed, this technology eliminates casting defects and opens possibility for additional improvement, such as heat treatment, to the production process.[3, 4]
shaped, sintering additives, water if aqueous tape-casting). All the organic components remain in the green tape after drying. Since they are removed when heated at elevated temperature in air (between 300 °C and 700 °C, from polymers to graphite), they give rise to pores, which cannot -or must not- always be eliminated during the sintering treatment. Consequently, the ratio of the amount of powder to the organic compounds, and hence the final formulation of the slurry, must be fine-tuned in order to tailor the final microstructure and density. Concerning water based tape-casting, the slurry is gelled after casting and the water is removed during drying; the gel is decomposed at 350 °C at the onset of the thermal treatment giving place to porosity. Here again the ratio between the solid phase and the liquid in the slurry will tailor the level of porosity, in addition to pore formers. In both cases, organic or aqueous tape-casting, the ratio of the solid phase to the liquid/or organic phases will lead to the control of a 1-2 µm size interconnected porosity, which can be extremely useful for increasing the quantity of triple phase boundaries. Amongst the slurry characteristics, the stability is of utmost importance. Polarization interactions must take place at the solid-liquid interface, interactions whose intensity governs the slurry stability (Moreno, 1992). Consequently, the value of the dielectric constant of the liquid determines the slurry stability, and hence the choice of solvent, which in turn determines the choice of all the other additives. The other forces acting on the particles in the slurry are gravity, which depends on the particles mass (and indirectly size), and the attractive Van der Waals interactions, which promote flocculation and act against the stability of the slurry. On the other hand, thermal agitation, electrostatic and steric repulsive forces promote the dispersion of the particles and therefore increase the stability of the slurry. The role of the dispersant agent is precisely to enhance the intensity of these dispersive forces. The second important slurry characteristic is the viscosity, which determines the operability of the process to cast green tapes. The slurry viscosity varies as a function of the amount of solvent per unit volume; the solvent quantity needs to be precisely adjusted to allow for a good dispersion of the powder as well as for an efficient dissolution of the binder.