DC Casting Research Articles

An investigation on DC casting of a wrought aluminium alloy at below liquidus temperature by using melt conditioner

The application of Melt Conditioner (MC) for production of high-quality DC billets and slabs at semisolid state is examined. In this technique, liquid metal is continuously fed into a twin-screw melt conditioning device, where the liquid metal or semisolid slurry is subjected to high shear rate, and then delivered continuously into a DC caster. In this paper, the feasibility of MCDC of a wrought aluminium alloy in the semi solid state has been investigated and the possibility of DC casting by MC technique below the liquidus temperature as a novel idea was explored. This idea may challenge the conventional DC casting which is normally performed above liquidus temperature.

Effect of water cooling conditions on splash break occurance during aluminum DC casting

The cooling water used in vertical aluminum DC casting is generally induced to just under the mold to prevent molten aluminum break out. However, the cooling water leaks into the air gap between the mold and the surface of ingot, so water splashes over the top of the molten aluminum, resulting in uneven cooling. But the quantitative survey on the splash break behavior and the change in the cooling condition has not been conducted. Hence, to quantify the splash break condition and the cooling capability, we have made the “Cooling water simulator” capable of freely controlling the potential splash causing factors such as temperature of cooling water, water flow rate, air gap amount, and angle of the water flow, reproducing the splash, and measuring the heat transfer value. The experimental result reveals that the temperature of cooling water is not a factor highly influencing on heat flux and splash, but the splash behavior highly depends on three factors which are the water flow rate, the angle of water flow, and the distance between the cooling water hit point level and the cooling water exit level. And that the heat flux also increases when splash occurs. In addition, the splash break parameter was drawn based on the above three factors, and the correlation between splash break parameter and heat flux was clarified. Use of splash break parameter enables setting of the casting condition where the maximum heat flux is obtained while preventing splash.

Billet Sump Modification of Al-Billets for the Prevention of Starting Cracks

Depending on the cast alloy, starting cracks are a common problem while casting aluminium extrusion billets. Hot crack formation is well investigated in theory as well as the formation of the sump shape using different casting technologies. Nevertheless, starting cracks are a problem in DC casting. Because TRIMET ALUMINIUM AG Essen uses the spout and float technology for DC-casting of aluminium this paper presents an investigation into methods for the prevention of starting cracks in Al-billets using float and spout DC-casting and covers the research methods, the results of crack prevention and the results of implementing these methods. The main topics to be considered are the control of sump formation and the correlation between sump geometry and hot cracks in the billet. Our experimental research covers DC-casting with varied casting parameters (stop-and-go casting, casting speed etc.), sump depth measurements, ultrasonic tests and metallographic examinations. We discuss crack prevention in the analysis and interpretation sections given consideration of the sump depth measurements. On the basis of these measurements the effect of the different casting technologies and parameters are shown. Finally, the methods we describe have to be implemented in production. The essential points are the upscaling and the automation of the casting process, especially the start-up phase. The results of this investigation provide excellent possibilities for crack prevention for DC-casting.

Numerical issues in modelling macrosegregation during DC casting of a multi‐component aluminium alloy

PurposeThe purpose of this paper is to investigate the ways to diminish or eliminate numerical diffusion and dispersion. Numerical dispersion and diffusion are present in the predicted macrosegregation profiles reported in the literature and they hinder the interpretation of the simulation results. With the motivation to eliminate these numerical problems by employing appropriate meshes, simulations of macrosegregation in a billet direct‐chill cast from a multi‐component aluminium alloy has been performed.Design/methodology/approachFirst the idea that numerical dispersion could be alleviated by refining the structured mesh size is tested and the extent of this mesh refining to overcome these numerical problems is discussed. Second the link of numerical dispersion and diffusion to the type of mesh used is investigated.FindingsUnstructured mesh eliminates the numerical dispersion present in the structured mesh while it introduces the numerical diffusion. It is concluded by performing calculations with the same settings but different meshes that, although refining the structured mesh could alleviate the numerical oscillation, it increases the computation time dramatically. Therefore the best solution to overcome these numerical problems is the employment of a hybrid mesh consisting of both structured and unstructured mesh.Originality/valueThis work reveals the reasons behind the numerical dispersion and diffusion in macrosegregation modelling and gives a practical solution.

Analysis of the impact of macrosegregation on hot tearing conditions during DC casting of aluminium alloys

It is well known that the chemical composition has a strong impact on the hot tearing susceptibility of aluminium alloys. Macrosegregation formation during DC casting of aluminium may lead to large variations of the alloy composition. Assessments of the cracking susceptibility during DC-casting do not take into account composition variations within the ingot. A two-phase model where the main phenomena associated with the formation of hot tears are accounted for, i.e., the lack of feeding at the late stages of solidification and the localization of viscoplastic deformation, has recently been proposed by the authors. In the present study, the two-phase model is extended to allow for the computation of macrosegregation and its interaction with hot tearing formation. The impact of macrosegregation on mechanical fields often associated with the cracking tendency is assessed through case studies where both macrosegregation and the mechanical analysis are computed simultaneously. The case studies are carried out for the start-up of the DC casting process.

Investigation of residual strains by neutron diffraction in an AZ31 direct chill cast billet

Neutron diffraction data was collected, showing the strain distributions along radial, axial and hoop directions in a direct chill cast billet of AZ31 magnesium alloy. Strain measurement by neutron diffraction is a non-destructive technique that uses the diffraction of a beam of thermal neurons to determine the atomic spacing within a small gauge volume inside polycrystalline materials. The benefits of using neutrons versus X-rays lie in the increased penetration depth achieved with neutrons, which can allow measurement of the internal strains several centimeters away from the material surface. A data-processing technique was proposed to assess and remove the irregular points, and the point-to-point fluctuations were evaluated. Residual strain measurements on the as-cast billet contribute direct observation of the stress/strain state in the billet. And the results also provide the data necessary to validate a thermo-mechanical model that predicts the evolution of stress/strain during the DC casting and subsequently to investigate the cracking defects in the billets.

Prediction method of crack sensitivity during DC casting of Al–Mn and Al–Mg series aluminum alloys

Since it has been difficult to predict a crack sensitivity depending on alloy composition during aluminum DC casting, a new prediction method based on a relation between calculated solid fraction and temperature was developed concerning Al–Mn and Al–Mg series aluminum alloys. In order to understand that the surface crack is located at ingot surface, two cracking modes were considered. They were due to “difference in strain between water chilling surface and mushy surface (crack mode Z)” and “difference in strain rate between surface and just inner surface (crack mode Y)”. It's common knowledge that the material is cracked at high solid fraction region since melt cannot go into a crack gap and ductility is very low. Therefore we assumed the high solid fraction region was from 0.75 to 0.95 (Region II) and set up two indexes which are calculated from temperature difference (ΔTII) and difference in a temperature drop per unit solid fraction (ΔRII/ΔTII) within the Region II. By using the both indexes, which were called “Brittle temperature range” and “Parameter of the strain rate difference” respectively, the crack sensitivities through DC casting experiments were suitably represented.

Evolution of precipitation during different homogenization treatments in a 3003 aluminum alloy

The evolution of the precipitation of second phase particles dispersed in a DC cast 3003 aluminum alloy during different homogenization treatments was investigated. Eight kinds of homogenization conditions were designed. We conclude that the evolution of precipitated dispersed particles during homogenization is controlled by nucleation, growth, Ostwald ripening process and hetero-precipitation. Nucleation of the particles would occur first during the initial phase of homogenization. They would then undergo a process of growth, dissolution and coarsening, before reaching the final state of precipitation. Two-color particles usually appear at step-homogenization, which has a lower later temperature, 600 °C × 9 h → 460 °C × 3 h, due to a hetero-precipitation behavior.

Mechanical Properties and Metallurgical Qualities of Magnesium Alloy Sheets Manufactured by Twin-Roll Casting

This paper is concerned with a manufacturing process and technology to facilitate the economical manufacture of high-quality magnesium alloy sheets. Magnesium alloys AZ31, AZ61, AM60 and AZ91 were used to investigate the appropriate anufacturing conditions for use in twin-roll strip casting. Temperatures of the molten materials and roll speeds were varied to find the appropriate manufacturing conditions. The effects of manufacturing conditions on possible forming were clarified in terms of roll speeds and roll gaps between upper and lower rolls. In addition, microscopic observation of the microstructure of the finished casting was performed. It was clarified that a magnesium sheet of 2.5 to 4.5mm thickness could be produced at a speed of 20 m/min by a horizontal copper roll caster. It was also found that the cast magnesium sheet of AZ31, AZ61, AM60 and AZ91 manufactured by roll strip casting could be used for plastic forming if the appropriate magnesium sheets were produced after the roll casting process. By a warm deep drawing test, it also demonstrated that a limiting drawing ratio of 2.4 was possible in the case of AZ91 sheet that was difficult to be manufactured by conventional extrusion process or DC casting and hot-rolling process for magesium alloy sheets with high aluminium contents.

Microstructures and Mechanical Properties of AZ31B Extruded Sheets from Different Casting Processing

A novel way producing magnesium billets, LFEC (low frequency electromagnetic casting processing), was developed in Northeastern University in China. The high-quality magnesium billets with less macrosegregation, refined microstructure, and better surface quality were achieved because the temperature field and the flow pattern of magnesium DC casting were improved significantly after applying low frequency electromagnetic field. Extrusion is an important plastic deformation process for magnesium alloys. In this research, the magnesium billets from LFEC were extruded through a special designed die into sheets. The results of investigation on AZ31B indicated that the extrusion velocity has obvious effects on their microstructures and mechanical properties and the sheets from LFEC had finer microstructure and higher mechanical properties than that from conventional DC casting.

Feasibility study of twin roll casting process for magnesium alloys

Abstract This paper is concerned with the development of a strip casting technology for manufacturing magnesium alloy sheets. The aim of the work is to establish a manufacturing process and technology to facilitate the economical manufacture of high-quality magnesium alloy sheets. Magnesium alloy AZ31, AZ61, AM60 and AZ91 were used to investigate the appropriate manufacturing conditions for use in twin roll strip casting. Temperatures of the molten materials and roll speeds were varied to find the appropriate manufacturing conditions. The effects of manufacturing conditions on possible forming were clarified in terms of roll speeds and roll gaps between upper and lower rolls. In addition, microscopic observation of the microstructure of the finished casting was performed. It was clarified that a magnesium sheet of 2.5–4.5 mm thickness could be produced at a speed of 20 m/min by a horizontal copper roll caster. Mill stiffness and a method of predicting the cast sheet's thickness were investigated to determine the appropriate manufacturing conditions. It was also found that the cast magnesium sheet of AZ31, AZ61, AM60 and AZ91 manufactured by roll strip casting could be used for plastic forming if the appropriate magnesium sheets were produced after the roll casting process. By a warm deep-drawing test, it also demonstrated that a limiting drawing ratio of 2.4 was possible in the case of AZ91 sheet that was difficult to be manufactured by conventional extrusion process or DC casting and hot-rolling process for magesium alloy sheets with high aluminium contents.

Analysis of solidification in Bridgman furnace as simulation of DC casting of aluminium alloy slabs

The vertical Bridgman directional solidification equipment has been used in several investigations to simulate direct chill casting of wrought aluminium alloys. As a basis for such investigations and alloy developments, it is important to have an understanding of the performance of the furnace used during simulation of the casting conditions. In this investigation the thermal conditions in the furnace have been analysed in detail, both by measurements and by mathematical modelling. The growth characteristics of the furnace, such as gradient, growth rates and cooling rates have been compared to conditions in large ingots. The direct chill casting conditions, which the simulations have been compared to, are casting of slabs of 330 and 600 mm thickness of an aluminium AA3003 type alloy. The results show that the experiments are able to simulate the cooling conditions in the ingots except from the surface zone. Comparisons of the microstructures have been made and a good agreement has been obtained for structure parameters such as grain size and DAS.

Coupled modeling of electromagnetic field, fluid flow, heat transfer and solidification during low frequency electromagnetic casting of 7XXX aluminum alloys: Part 1: Development of a mathematical model and comparison with experimental results

A comprehensive mathematical model has been developed to describe the interaction of the multiple physics fields during the conventional DC casting and low frequency electromagnetic casting (LFEC) process. The model is based on a combination of the commercial finite element package ANSYS and the commercial finite volume package FLUENT, with the former for calculation of the electromagnetic field and the latter for calculation of the magnetic driven fluid flow, heat transfer and solidification. Moreover, the model has been verified against the temperature measurements obtained from two 7XXX aluminum alloy billets of 200 mm in diameter, during the conventional DC casting and the LFEC casting processes, respectively. There was a good agreement between the calculated results and the measured results. Further, comparison of the calculated results of the LFEC process with that of the conventional DC casting process indicated that velocity patterns, temperature profiles and the sump depth are modified remarkably by the application of a low frequency electromagnetic field during the DC casting.

Coupled modeling of electromagnetic field, fluid flow, heat transfer and solidification during low frequency electromagnetic casting of 7XXX aluminum alloys: Part II: The effects of electromagnetic parameters on casting processes

A comprehensive mathematical model for the interaction of the multiple physics fields during the conventional DC casting and LFEC processes has been developed and verified against experimental results in Part I. In this second article of a two-part series, the effects of electromagnetic parameters on electromagnetic field, fluid flow, temperature field and solidification during LFEC process have investigated numerically by using the mathematic model in Part I. The choosing criterion of the electromagnetic parameters during LFEC process has been used in order to obtain the best structure of the billets by analyzing the effects of fluid flow and temperature field on the solidification process in the presence of electromagnetic field.

Modeling the Role of Microstructure on Shear Instability with Reference to the Formability of Automotive Aluminium Alloys

This paper addresses the effect of microstructure on the formability of aluminium alloys of interest for automotive sheet applications. The bulk of this work has been on the alloy AA5754 – both conventional DC cast alloys and continuous cast alloys made by twin belt casting. It is known that alloys such as these contain Fe as a tramp impurity which results in Fe-based intermetallic particles distributed through microstructure as isolated particles and in stringers aligned along the rolling direction. It is thought that these particles are the cause, both of the reduced ductility that is observed as the Fe level rises, and the relatively poor formability of strip cast alloys, as compared with those made by DC cast. Conventional wisdom suggests that the reduction of ductility is due to the effect of particles as nucleating sites for damage. However, most studies show that these materials are resistant to damage until just before fracture. We now believe that effect is actually related to the development of shear bands in these materials. We present experimental data which supports this conclusion. We then show how the FE models we have developed demonstrate the role of shear instability on fracture and the role played by hard particles. We show how a unit cell approach can be used to incorporate the effect of particle density and morphology on shear localization in a way that includes statistical variability due to microstructural heterogeneity. This leads to a set of constitutive equations in which the parameters are distributed from one region to another. These are then fed into a macroscopic FE model at the level of the specimen or the component in order to determine the effect of microstructural variability on shear instability and ductility.

Fatigue Behaviour in Fine Grained Aluminium Alloys

The effects of alloy production method on microstructure and hence fatigue crack growth rate and fracture mechanism have been examined for a variety of fine-grained/high dispersoid Al-Li-Mg-Zr and Al-Li-Cu-Mg-Zr alloys. Microstructures have been assessed by scanning and transmission electron microcopy, together with electron back scattered diffraction pattern assessment. In these fine-grained/high dispersoid materials, high crack growth rates are seen in comparison to the traditional DC cast alloys, excepting a material with high volume fraction of shearable precipitate. The effects of fracture surface roughness and closure levels in determining crack growth rates has been assessed.

The performance of Al–Ti–C grain refiners in twin-roll casting of aluminium foilstock

The master alloys based on the Al–Ti–B system have been used extensively for refining the grain structure of aluminum alloys. The quality-related problems linked with the TiB 2 particles, however, have generated an interest in the Al–Ti–C grain refiners as an acceptable replacement for Al–Ti–B master alloys. TiC particles are smaller than the TiB 2 particles and are less prone to agglomeration. Al–3Ti–0.15C grain refiners have been in use for some time in several alloy systems. Much of the work reported on this alloy, however, has been from DC casting while performance data in strip casting is not available. In the present work, a commercial Al–3Ti–0.15C grain refiner was employed in the twin-roll casting of AA8111 foilstock. Its grain refining efficiency was compared with that of the Al–5Ti–0.2B master alloy, the standard grain refiner in aluminium industry for the manufacture of aluminium foil products.

Fatigue crack growth and closure in fine-grained aluminium alloys

The effects of microstructure on fatigue crack growth rate and fracture mechanism have been examined for several relatively fine-grained/high dispersoid aluminium alloys. Particular attention has been paid to the behaviour of spraycast Al–Li alloys, as such materials may present novel property combinations for light weight structural aerospace applications, with extended conventional composition ranges. Fracture resistance has been assessed in terms of both intrinsic and extrinsic factors, including roughness induced closure (RICC). In these fine-grained/high dispersoid materials, high crack growth rates are typically seen in comparison to traditional DC cast alloys, consistent with low fracture surface roughness levels and correspondingly reduced levels of RICC. The competitive influence of shearable precipitates on crack roughness is however noted in a high-Li alloy variant. The effects of fracture surface roughness and closure levels on long crack fatigue crack growth rates have been compared with recent novel modelling of closure levels.

X-ray micro-tomographic observations of hot tear damage in an Al–Mg commercial alloy

X-ray micro-tomography and three-dimensional image analysis were used to characterize hot tearing. The formation and interconnectivity of deformation induced damage during hot tearing was quantified in specimens of DC cast aluminum alloy AA5182 (of composition Al–4.63%Mg–0.49%Mn–0.17%Fe–0.04%Cu) subjected to tensile load at a temperature of 528 °C (fraction solid ∼0.98) over a range of strains varying from 0 to 0.20 (failure). This technique also allowed for quantification of the porosity formed during DC casting.

Mesh-Free Simulation of Transport Phenomena in Continuous Castings of Aluminium Alloys

This paper introduces a general numerical scheme for solving convective-diffusive problems that appear in the solution of microscopic and macroscopic transport phenomena in continuous castings and the heat treatment of aluminium alloys. The numerical scheme is based on spatial discretisation that involves pointisation only. The solution is based on diffuse collocation with multi-quadric radial basis functions. The application of the method is demonstrated in a simplified model of a billet DC casting and verified by a comparison with the classical finite volume method.