Melt Conditioned Direct Chill Casting 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.

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.

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.

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.

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.

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.