Melt Conditioned Direct Chill Research Articles

In-situ microstructural control of A6082 alloy to modify second phase particles by melt conditioned direct chill (MC-DC) casting process – A novel approach

• High shear melt conditioning in the sump during direct chill casting of A6082 billet. • Novelty of maintaining 2–7 °C above alloy liquidus as the sump thermal condition. • In-situ control of solidification during casting by intensive melt shearing. • Precise tailoring of as-cast microstructure to modify second phase particles. • Potential for industries to eliminate homogenisation and use recycled Al alloys. Controlling the formation of noncompact second phase particles during direct chill (DC) casting of aluminium alloys with grain refiner addition remains challenging, as it results in energy intensive homogenisation and deformation problems. In this work, we employed a novel strategy in the DC casting of A6082 alloy to produce billets with a fine-scale dispersion of second phase particles. The strategy involves maintaining 2–7 °C above alloy liquidus as a thermal condition in the sump by in-situ melt conditioning (MC) using a rotor-stator high-shear device operated at a critical rotation speed. As a result, in-situ control of solidification behaviour is achieved to precisely tailor the as-cast microstructure. The billet grain refinement is attained by MC-DC casting without the deliberate addition of chemical grain refiners. The microstructure of the MC-DC cast billet at the critical rotation speed showed a fine-scale dendritic structure with refined secondary dendrite arm spacing (SDAS). The solidification front proceeded with a shallow sump and a corresponding shorter solidification time, higher cooling rate, higher temperature gradient, and smooth solidification rate profile. The ideal fine-scale dendrites with low SDAS divided the remaining eutectic liquid into fine-scale and isolated liquid pockets, resulting in fine-scale, compact morphology, and uniform distribution of second phase particles in the as-cast microstructure. The MC-DC casting process showed the ability to increase the cast house production rate by increasing the casting speed without bleeding the billet. The present approach could be beneficial for eliminating or reducing the homogenisation practice and may also introduce significant flexibility in using recycled Al alloys in the industry.

Direct Chill Casting and Extrusion of AA6111 Aluminum Alloy Formulated from Taint Tabor Scrap.

AA6111 aluminum automotive body-sheet alloy has been formulated from 100% Taint Tabor scrap aluminum. Direct chill casting with and without high shear melt conditioning (HSMC) was used to produce the AA6111 alloy billets. Both homogenized and non-homogenized billets were extruded into sheets. The optical micrographs of the melt conditioned direct chill (MC-DC) samples showed refined equiaxed grains in comparison to direct chill (DC) cast and direct chill grain refined (DC-GR) samples. Optical metallography showed extensive peripheral coarse grain (PCG) for the DC, DC-GR and MC-DC planks extruded from the homogenized standard AA6111 billets while planks extruded from modified AA6111 billets (with recrystallization inhibitors) showed thin PCG band. The co-addition of recrystallization inhibitors Mn, Zr, and Cr with elimination of the billet homogenization step had a favorable impact on the microstructure of the AA6111 alloy following the extrusion process where a fibrous grain structure was retained across the whole section of the planks. The mechanical properties of as-cast planks extruded from non-homogenized billets were similar to those extruded from homogenized billets. Eliminating the homogenization heat treatment step prior to extrusion has important ramifications in terms of processing cost reduction.

Enhancement of chip breakability of aluminium alloys by controlling the solidification during direct chill casting

Refinement and uniform dispersion of the free machining elements within the matrix is always desirable for aluminium billets produced by direct chill (DC) casting. The conventional grain refiner addition cannot ensure uniformity of microstructure and homogeneous distribution of insoluble or intermetallic phases. Melt conditioned direct-chill (MC-DC) casting is a key technology for producing uniform microstructure and refinement of secondary phases. This physical approach combines conventional vertical direct chill (DC) casting with a high shear device directly immersed in the sump for in situ microstructural control. Increased heat extraction rate due to forced convection, a larger temperature gradient at the solidification front and a shallower sump contribute to fine equiaxed dendritic grains with fine dendritic arm spacing that favor the fine and uniform distribution of second phase particles in the as-cast billet. This trend of the microstructural features was preserved even after thermomechanical downstream processing, giving rise to much improved machinability for the extruded solid bars.

Melt Conditioned Direct Chill (MC-DC) Casting and Extrusion of AA5754 Aluminium Alloy Formulated from Recycled Taint Tabor Scrap.

The melt conditioned direct chill (MC-DC) casting process has been used to produce billets and extruded planks of AA5754 alloy formulated from 100% recycled Taint Tabor scrap aluminum. The billets were homogenized and then extruded into flat planks. Optical metallography of the MC-DC cast billets showed equiaxed refined grains in comparison to conventional direct chill (DC) cast and direct chill grain refined (DC-GR) cast billets formulated from the same Taint Tabor scrap. Microstructural evaluation of the extruded planks showed extensive peripheral coarse grain (PCG) for the DC, DC-GR and MC-DC cast planks. The 2 mm and 1 mm MC-DC cast planks produced after cold rolling and heat treatment showed a fully recrystallized microstructure at 380 °C and 300 °C for 10 min respectively with an improvement in mechanical properties over DC-GR cast and similarly processed planks. The as-extruded MC-DC cast planks tensile tested in the transverse direction showed 34% elongation and 213 MPa ultimate tensile strength. These tensile results showed 5.8% higher elongation and 1.2% higher ultimate tensile strength compared with the DC-GR planks after applying high shear melt conditioning.

Melt Conditioned Direct Chill (MC-DC) Casting of AA-6111 Aluminium Alloy Formulated from Incinerator Bottom Ash (IBA)

The melt conditioned direct chill (MC-DC) casting process has been used for the production of billets of AA-6111 alloy formulated from recycled aluminium derived from incinerator bottom ash (IBA). The billets were homogenised and then extruded into planks. Optical metallography of the MC-DC billets showed equiaxed refined grains in comparison to DC and grain refined (DC-GR) billets formulated from the same scrap source. Microstructure evaluation for the extruded planks showed a less extensive peripheral coarse grain (PCG) for the MC-DC sample.

Numerical modelling of melt-conditioned direct-chill casting

Melt conditioned direct-chill (MC-DC) casting is a novel technology which combines direct-chill (DC) casting with a high shear device directly immersed in the sump for in situ microstructural control. A numerical model of melt-conditioned direct-chill casting (MC-DC) is presented in this paper. This model is based on a finite volume continuum model using a moving reference frame (MRF) to enforce fluid rotation inside the rotor-stator region and is numerically stable within the range of processing conditions. The boundary conditions for the heat transfer include the effects of the hot-top, the aluminium mould, and the direct chill. This model is applied to the casting of two alloys: aluminium-based A6060 and magnesium-based AZ31. Results show that MC-DC casting modifies the temperature profile in the sump, resulting in a larger temperature gradient at the solidification front and a shorter local solidification time. The increased heat extraction rate due to forced convection in the sump is expected to contribute to a finer, more uniform grain structure in the as-cast billet.

CFD simulation of direct chill casting process of magnesium alloy billets

This work demonstrates and simulates the industry scale direct chill casting of magnesium alloy (AZ31) billets processed under various conditions. The solidification mechanisms involved the conventional and melt-conditioned direct chill casting processes are simulated using the computational fluid dynamics. The heat flow and the solid-liquid interface are clearly identified for various casting speeds, depth of the rotor-stator device and the device geometry. The results also indicate that the shape and depth of the solid-liquid interface sump is sensitive to the casting parameters.

Grain Refinement and Improvement of Solidification Defects in Direct-Chill Cast Billets of A4032 Alloy by Melt Conditioning

Melt-conditioned, direct-chill (MC-DC) casting is an emerging technology to manipulate the solidification process by melt conditioning via intensive shearing in the sump during DC casting to tailor the solidification microstructure and defect formation. When using MC-DC casting technology in an industrial scale DC cast billet of an A4032 aluminum alloy, significant grain refinement and uniform microstructure can be achieved in the primary α-Al phase with fine secondary dendritic arm spacing (SDAS). Improved macrosegregation is quantitatively characterized and correlated with the suppression of channel segregation. The mechanisms for the prevention of channel segregation are attributed to the increased local cooling rate in the liquid–solid phase region in the sump and the formation of fine equiaxed dendritic grains under intensive melt shearing during MC-DC casting. A critical cooling rate has been identified to be around 0.5 to 1 K/s (°C/s) for the channel segregation to happen in the investigated alloy based on quantitative metallographic results of SDAS. Reduction and refinement of microporosity is attributed to the improved permeability in the liquid–solid phase region estimated by the Kozeny–Carman relationship. The potential improvement in the mechanical properties achievable in MC-DC cast billets is indicated by the finer and more uniform forging streamline in the forgings of MC-DC cast billet.

Microstructure Evolution in Melt Conditioned Direct Chill (MC-DC) Casting of Fe-Rich Al-Alloy

In order to fabricate high quality aluminium products, it is first essential to produce high quality billets/slabs. One of the key objectives associated with casting processes is to be able to control the as-cast structure. A novel direct chill (DC) casting process, the melt conditioned direct chill (MC-DC) casting process, has been developed for production of high quality aluminium billets. In the MC-DC casting process, a high shear device is submerged in the sump of the DC mould to provide intensive melt shearing, which in turn, disperses potential nucleation particles, creates a macroscopic melt flow to uniformly distribute the dispersed particles, and maintains a uniform temperature and chemical composition throughout the melt in the sump. The effect of intensive shearing on the complex microstructure evolution observed after MC-DC is explained on the basis of nucleation and growth behavior. Complete suppression of typical columnar grain growth and significant equiaxed grain refinement is observed. The solidification mechanisms responsible for the significant grain refinement by intensive shearing and the morphological evolution of Mg2Si and Fe–containing intermetallic phases are discussed.

Understanding the Evolution of the Microstructure in Melt-Conditioned Direct-Chill Cast Al Alloys

Present work focuses on understanding the solidification mechanism responsible for the grain-refinement in melt-conditioned direct chill (MC-DC) cast Al-alloy. It has been found that the melt-conditioning performed by stirring the liquid above the solidification front using a rotor–stator type melt-conditioning device, results in substantial fragmentation of dendrites due to forced convection and thermo-solutal distribution across the billet sump. Results also indicate that an isothermal region is created in the MC-DC regime of the billet sump, which further enhances the nucleation and restricts growth of the α-Al phase. The forced convection together with the solute transport lead to catastrophic dendrite fragmentation.

Melt Conditioned Direct Chill Casting (MC-DC) Process for Production of High Quality Aluminium Alloy Billets

A novel direct chill (DC) casting process, melt conditioned direct chill (MC-DC) casting process, has been developed for production of high quality aluminium alloy billets. In the MC-DC casting process, a high shear device is submerged in the sump of the DC mould to provide intensive melt shearing, which in turn, disperses potential nucleating particles, creates a macroscopic melt flow to uniformly distribute the dispersed particles, and maintains a uniform temperature and chemical composition throughout the melt in the sump. Experimental results have demonstrated that, the MC-DC casting process can produce aluminium alloy billets with significantly refined microstructure and reduced cast defects. In this paper, we give an overview of the MC-DC casting process and report on results obtained from an industrial scale trial.

Solidification Mechanisms in Melt Conditioned Direct Chill (MC-DC) Cast AZ31 Billets

The solidification mechanism in the melt conditioned direct chill (MC-DC) casting process was investigated by pouring liquid Pb into the sump during the casting process. This preserved the solidification front for subsequent metallographic examination. The results demonstrated that the solidification in the MC-DC casting process was preceded by the sedimentation of rosettes, which were nucleated in the sump and then grew under intensive melt flow.

Melt Conditioned Direct Chill (MC-DC) Casting of Al Alloys

A fine and uniform microstructure is always desirable for aluminium billets produced by direct-chill (DC) casting. Although this is usually achieved by grain refiner addition during DC casting, in this paper we demonstrate a physical approach for grain refining DC cast billets. Intensive melt shearing is applied to DC casting by submerging a rotor–stator based high shear device in the sump of a conventional DC caster. It has been shown that DC casting with intensive melt shearing can achieve one order of magnitude grain size reduction without using chemical grain refiners. Such fine and equiaxed grain structure in the Al billets can be attributed to enhanced heterogeneous nucleation and growth under conditions with reduced temperature gradient and increased growth velocity.

Grain refinement of DC cast magnesium alloys with intensive melt shearing

A new direct chill (DC) casting process, melt conditioned DC (MC-DC) process, has been developed for the production of high quality billets/slabs of light alloys by application of intensive melt shearing through a rotor-stator high shear device during the DC casting process. The rotor-stator high shear device provides intensive melt shearing to disperse the naturally occurring oxide films, and other inclusions, while creating a microscopic flow pattern to homogenize the temperature and composition fields in the sump. In this paper, we report the grain refining effect of intensive melt shearing in the MC-DC casting processing. Experimental results on DC casting of Mg-alloys with and without intensive melt shearing have demonstrated that the MC-DC casting process can produce magnesium alloy billets with significantly refined microstructure. Such grain refinement in the MC-DC casting process can be attributed to enhanced heterogeneous nucleation by dispersed naturally occurring oxide particles, increased nuclei survival rate in uniform temperature and compositional fields in the sump, and potential contribution from dendrite arm fragmentation.

Microstructures of DC Cast Light Alloys under the Influence of Intensive Melt Shearing

A new direct chill (DC) casting process, melt conditioned DC (MC-DC) process has been developed for production of high quality ingots and billets of light alloys. In the MC-DC casting process, intensive melt shearing provided by a newly developed rotor-stator unit is used to control the solidification process during the DC casting with a conventional DC caster. Experimental results of DC casting of Al- and Mg-alloys with and without intensive melt shearing have demonstrated that the MC-DC casting process can produce light alloy billets with significantly refined microstructure and substantially reduced cast defects. The effect of intensive melt shearing on grain refinement has been mainly attributed to the enhanced heterogeneous nucleation on well dispersed oxides occurring naturally in the alloy melt.

Grain refinement of DC cast AZ91D Mg alloy by intensive melt shearing

Melt conditioning by advanced shear technology (MCAST) is a new process for microstructural refinement of both cast and wrought magnesium alloys. Melt conditioned direct chill (MCDC) casting combines the MCAST process with conventional direct chill (DC) casting. In the present work, melt conditioning has been combined with permanent mould casting to simulate the production of DC cast AZ91D billets and slabs. The results show that the MCDC process can achieve significantly finer grain size and more uniform microstructure than conventional DC process for both billets and slabs. Grain refinement in the MCDC process is due to the fine and well dispersed oxide particles produced after processing in the MCAST unit.

Melt Conditioned Direct Chill Casting (MC-DC) of Wrought Al-Alloys

Melt Conditioned Direct Chill (MC-DC) casting is a new development for producing high-quality billets and slabs. In the MC-DC process, liquid metal is continuously fed into a MCAST (melt conditioning by advanced shear technology) machine, where the liquid metal is subjected to high shear rate and high degree of turbulence provided by a twin screw mechanism at temperatures either above or below the alloy liquidus, and the conditioned liquid metal is then fed continuously into a Direct Chill (DC) caster to produce billets or slabs. The MC-DC process is applicable to both Al- and Mg-alloys. In this paper we present our experimental investigations of the effects of processing parameters on the microstructural and compositional uniformity of 5xxx and 7xxx series Al-alloys. It has been confirmed by our experiments that the MC-DC process can produce billets and slabs with fine and uniform microstructure, uniform chemical compositions and much reduced cast defects, such as porosity and cracks.