MELT CONDITIONED CASTING OF ALUMINUM ALLOYS

Geoff Scamans, Hu-Tian Li and Zhongyun Fan

Brunel Centre for Advanced Solidification Technology, Brunel University, Uxbridge, Middlesex UB8 3PH, UK

Keywords: Melt conditioning, Aluminum alloys, Grain refinement

AbstractHigh shear melt conditioning of aluminum alloy melts disperses oxide films and provides potent nuclei to promote non-dendritic solidification leading to refined as cast microstructures for shapecastings, semis or continuously cast product forms. A new generation of high shear melt conditioning equipment has been developed based on a dispersive mixer that can condition either a batch melt or can provide a continuous melt feed. Most significantly the melt conditioner can be used directly in the sump of a DC caster where it has a dramatic effect on the cast microstructure. The present goals are to expand the castable alloy range and to increase the tolerance of alloys used in transport applications to impurities to increase the use of recycled metal. The paper will review the current status of the melt conditioning technology across the range of casting options and will highlight development opportunities.

IntroductionLiquid aluminum alloys oxidize rapidly when exposed to oxygen-containing atmosphere. This means that aluminum alloy melts inevitably contain oxides as inclusions. Such oxides in the from of thin films reduce melt fluidity and cause blockages during die filling [1] and if they are entrained in the melt as bi-films these become cracks in the solidified metal that degrade mechanical properties. Severe oxidation decreases yield and this increases component cost. Consequently, oxide inclusions in alloy melts are usually considered as undesirable. Oxidation isprevented or reduced by using a protective atmosphere or by vacuum casting, or oxide films are removed from melts by either chemical or physical means [2]. The present refining techniques, such as filtration and rotary degassing, can refine melts to some extent by removing inclusions but they are relatively costly and time-consuming. This motivated the development of the Melt Conditioning by Advanced Shearing Technology (MCAST) process [3, 4]. High shear melt conditioning has been achieved using either a twin screw mechanism or a rotor-stator mechanism [3]. In both cases the liquid metal in is subjected to intensive shearing under a high shear rate with a high intensity of turbulence. The conditioned liquid metal has extremely uniform temperature, uniform composition and well dispersed inclusion particles. These individual particles have a both a fine size and a narrow size distribution, and most importantly, are likely to be completely wetted by the liquid metal under the intensive forced convection. In the new generation of melt conditioners the high shear mixer is a rotor-stator unit. This simple device comprises of a rotor, a stator and a housing for holding the stator and the rotor. A motor drives the rotor at speeds up to 104 rpm and shears the liquid metal in the gap between the rotor and the stator and also in the openings of the stator. The high shear device provides not only flow for distributive mixing but also intensive shearing for dispersive mixing. The main advantages of the high shear devices include significantly enhanced kinetics for any chemical reactions or phase transformations, well dispersed and uniformly distributed solid particles or gas bubbles, size reduction of solid particles or gas bubbles, improved homogenization of chemical composition and temperature fields, and forced wetting of the usually difficult-to-wet solid particles in the liquid metal. Hence, high shear devices can be used for conditioning alloy melts prior to

13th International Conference on Aluminum Alloys (ICAA13) Edited by: Hasso Weiland, Anthony D. Rollett, William A. Cassada

TMS (The Minerals, Metals & Materials Society), 2012

1395

solidification processing, physical grain refinement by dispersing naturally occurring oxides,degassing, preparation of metal matrix composites, and the supply of semisolid slurries.

Oxides in Aluminum Alloy MeltsThe surface of liquid aluminum oxidizes rapidly when it is exposed to an atmosphere containing oxygen and/or water vapor [5]. The oxidation process starts with the formation of a thin amorphous layer on the melt surface. However, this amorphous layer becomes thermodynamically unstable beyond a critical thickness and the more stable crystalline oxidenucleates and grows rapidly at the melt/oxide surface. This occurs after an incubation time of 5-10 minutes at 750°C [6]. The crystalline form is -Al2O3 or other structural forms, depending on both the alloying elements in the melt and the oxidation conditions [6,7]. It has been reported

-Al2O3 film the alumina will be subjected to further transformation to - -

-Al2O3 is very sluggish and there is no consensus on the time and temperature for this transformation [8, 9]. Oxidation rates increase sharply with magnesium content [5] and theamorphous alumina films is rapidly broken down by reaction to form MgO and crystals of MgOand MgAl2O4 also nucleate and grow rapidly. Studies have shown that MgO is progressively replaced by the more stable MgAl2O4 with time [7].In our studies, a pressurized melt filtration technique was used to collect oxides from the melt foranalytical microscopy. Figure 1a shows the collected oxide in commercial purity (CP) aluminumat 700°C, which was identified as -Al2O3 by X-ray diffraction [10]. Closer examination showsthat -Al2O3 films consist of platelets and sometimes are bifilms entrained in the melt. The -Al2O3 platelets are held together by liquid metal. This explains why -Al2O3 films can be so flexible in alloy melts. The main oxide in liquid LM24 and binary Al-Mg alloys has been identified as MgAl2O4. Examination at high magnification has shown that MgAl2O4 films consist of discrete MgAl2O4 particles held together by a liquid matrix (Figure 1b). With further oxidation the MgAl2O4 films can crumple together forming particle clusters. Significantly with increased magnesium content the MgAl2O4 particles can be naturally dispersed, and are more likely to be uniformly distributed in the alloy melt. For instance, MgAl2O4 collected by melt filtration from aliquid Al-5%Mg alloy are discrete individual platelets [11]. Even a mild shear flow, as created by melt handling and pouring, will be enough to disperse the MgAl2O4 particles in the alloy melt.

Figure 1. SEM micrographs (a) -Al2O3 oxide films in a CP aluminum melt at 700°C; and (b) typical morphology of MgAl2O4 oxide films in LM24 at 700°C

The shear rate achievable by the high shear techniques [3,4,]is orders magnitude higher than that provided by propeller based mixers, electromagnetic stirring and other existing physical fields. The local shear force provided by the intensive melt shearing is high enough to overcome the capillary force. Consequently, oxide films are dispersed into individual particles, which are then

1396

distributed uniformly in the alloy melt. As shown in Figure 2, after intensive melt shearing, -Al2O3 films are converted into submicron-sized platelets (Figure 2a) and MgAl2O4 films are dispersed into micron- or submicron-sized MgAl2O4 particles (Figure 2b). The clustering of oxide particles in Figure 2 is a result of pressurized melt filtration.

1 m

(a)

Figure 2. SEM micrographs (a) Platelets of -Al2O3 in sheared CP Al; (b) MgAl2O4 in sheared LM24 melts.

Dispersing oxide films facilitates casting processes, since it increases the fluidity of alloy melts at a given melt temperature, and avoids potential blockage by large oxide films during die filling. Additionally, dispersing oxide films can improve the quality of cast products by eliminatinglarge oxide films as defects in the castings, and improving mechanical performance. Intensive melt shearing prior to casting can significantly improve the ductility of cast components [10]. Most importantly, dispersing oxide films can provide several orders of magnitude higher number density of potential nucleating particles to enhance heterogeneous nucleation for grain refinement. Dispersing oxide films also allows an increased content of recycled scrap to be used in alloy formulation without degradation of the quality of cast components. Increased scrap metal content usually leads to increased oxides and other inclusions in the alloy melt, causing processing difficulties and inferior casting quality. Intensive melt shearing can mitigate such problems by reducing/eliminating the harmful effects of oxides and inclusions without the use of expensive chemical treatment.

Grain Refinement of Aluminum Alloys by Melt Conditioned CastingFigure 3a shows the large average grain size of CP Al assessed by the standard TP-1 test as a function of casting temperature (or melt superheat) without intensive melt shearing [10]. Meltshearing leads to significant grain refinement that shows a complex grain size dependence on melt superheat (Figure 3b).

Figure 3. Average grain size of CP Al as a function of processing temperature. (a) Without intensive melt shearing; (b) with intensive melt shearing.

1397

The grain size increases initially with the increase in melt superheat, decreases sharply at 740°Cfrom 500µm to about 150µm and then increases with further increase of melt superheat. In addition, it was found that the effect of intensive melt shearing on grain refinement was alsodependent on alloy composition [11]. Figure 4 shows the grain size of binary Al-Mg alloys solidified in the TP-1 mould with and without intensive melt shearing as a function of magnesium content. Without intensive melt shearing, the grain size decreased sharply with magnesium content in dilute Al-Mg alloys (Mg<1%), and then levels off at around 200µm.Intensive melt shearing of Al-Mg alloys is only effective for melts with less than 1% magnesium.

Figure 4. Average grain size as a function of magnesium content with and without shearing.

The potency of a solid substrate in a liquid metal can be defined as the degree of lattice matching across the interface between the substrate and the solid phase to be nucleated [13]. The lattice misfit f can be used as a quantitative measure of the potency for heterogeneous nucleation, and is defined as f = (dS-dN)/dS, where dS and dN are the atomic spacing along a closely packed direction on a closely packed plane of the solid and the nucleating substrate, respectively. The calculated lattice misfit with solid aluminum at 660°C is 3.38% for -Al2O3, -0.48% for -Al2O3, and 1.41% for MgAl2O4, indicating that these oxides are highly potent for nucleation of -Al. The corresponding lattice misfits are -4.22% for TiB2 and 0.09 % for Al3Ti [13]. It is the lattice misfit of the solid/substrate interface at the moment of nucleation that determines the potency of the nucleating system, not the theoretically calculated misfit for pure metals. Adsorption of solute elements at the liquid/substrate interface is a thermodynamically favorable process if suchsegregation lowers the interfacial energy [14]. Segregated solute at the interface may increase or decrease the potency of the substrate depending on weather they reduce or increase the lattice misfit [13]. In addition, for enhancing heterogeneous nucleation to achieve effective grain refinement, the nucleating particles not only need to be potent, but also need to have an adequate particle number density, a suitable particle size and a narrow size distribution [15,16]. Nucleation on potent substrate will result in a coherent or semi-coherent solid/substrate interface. Therefore, the most direct evidence of heterogeneous nucleation on potent substrate is the existence of an orientation relationship between the solid and the substrate obtained by electron diffraction. In Al-Mg alloys, TEM examination has confirmed that the MgAl2O4 particles have {111} planes as their naturally exposed surface [11]. Selected area diffraction patterns have determined the following cube-on-cube orientation relationship (OR) between MgAl2O4 -Al:

(111)[110] MgAl2O4 // (111) [110] -Al (1)This is very much expected since both MgAl2O4 -Al have the same crystal structure and closely matched atomic spacings along the closely packed directions on the closely packed planes. The calculated lattice misfit at 660°C is 1.41% along the [110] direction on the (111)

0

400

800

1200

1600

0 1 2 3 4 5 6 7 8

Ave

rage

gra

in s

ize,

(µm

)

wt.% Mg

Non-ShearedSheared

1398

plane. It is clear that the lattice misfit between MgAl2O4 -Al is relatively small compared with that for the Al/TiB2 system (-4.22%).With this understanding, the experimental results can be rationalized. In CP aluminum, the oxide exists is in the form of -Al2O3 films (Figure 1a). Although -Al2O3 is potent for nucleation of -Al, there is not a sufficient number density of -Al2O3 particles as nucleation sites, resulting in a coarse grain structure (Figure 3a). With intensive melt shearing, -Al2O3 films are dispersed into submicron-sized -Al2O3 platelets (Figure 2a). Consequently, the number density of -Al2O3

particles is increased by several orders of magnitude, leading to a significant grain refinement (Figure 3b). In addition, intensive melt shearing enhances the kinetic condition for mass transport in the melt and this accelerates the transition from -Al2O3 to -Al2O3. Based on the experimental results in Figure 6b, the equilibrium temperature for the -Al2O3 to -Al2O3

transition is 740±20°C. Since -Al2O3 is more potent than -Al2O3 the grain size reduction after the - to -Al2O3 transition is predictable, as shown in Figure 3b. Similarly, grain refinement by intensive shearing of dilute Al-Mg alloys (Mg<1%) can be explained by increased number density of potential nucleating particles. However, in the concentrated Al-Mg alloys (Mg>1%), intensive melt shearing does not produce further grain refinement as the MgAl2O4 particles are already dispersed naturally.

Development of the Melt Conditioned Casting Technology

The high shear device has been directly incorporated into the direct chill (DC) casting process as shown in Figure 5a [12]. Figure 5b shows the microstructure of the melt conditioned (MC)-DC cast 80mm diameter AA7075 billet. The microstructure dramatically changes from a coarse dendritic microstructure without intensive melt shearing (bottom of Figure 5b) to a fine and uniform equiaxed microstructure with melt conditioning (top of Figure 5b). Similarly, intensive melt shearing has also been applied in twin roll casting (TRC) process as the MC-TRC process [17], which can provide aluminium alloy strip with fine and uniform microstructure without centreline segregation. Intensive melt shearing has also been applied to shape casting processes. Sand casting is a slow cooling process, often producing an extremely coarse microstructure. Figure 6 compares the microstructures of sand cast LM25 with and without intensive melt shearing. Similar results have also been demonstrated for high pressure die casting [17, 18].

Figure 5. (a) Schematic illustration of the melt conditioned direct chill (MC-DC) casting process using a rotor-stator device; (b) Microstructure of the DC cast AA7075 alloy billet

1399

Figure 6. Microstructures of sand cast LM25 components (a) Conventional sand casting (SC); (b) melt conditioned sand casting (MC-SC).

Concluding Remarks Due to their high affinity to oxygen Al alloy melts oxides readily in atmosphere containing oxygen. We found that oxides formed naturally in Al alloy melts are in the form of continuous liquid oxide films consisting of densely populated oxide particles in a liquid matrix. Such oxidefilms are amenable to physical manipulation by intensive melt shearing to form dispersed oxide particles uniformly distributed in the alloy melt. Theoretical analysis and experimental results have confirmed that most of the oxide particles in the sheared alloy melts are potent substrates for heterogeneous nucleation, have adequate number density, suitable size and size distribution for grain refinement. It is demonstrated that oxides in Al alloy melts can be harnessed to mitigate their harmful effects and to be deployed for grain refinement during a variety of casting processes, where it has a dramatic effect on the cast microstructure. The long-term goals are to expand the castable alloy range and to increase the tolerance of alloys used in transport applications to impurities to increase the use of recycled metal.

Acknowledgements This work was carried out at BCAST in the EPSRC supported LiME IMRC and the support of EPSRC is gratefully acknowledged.

References1. J. Campbell, Castings. 2nd Ed. (Oxford: Butterworth-Heinemann, 2003), 2-7.2. D. V. Neff, ASM Handbook, vol. 15 Casting, (ASM, 2008), 194.3. Z. Fan et al., Acta Mater, 2009; 57: 4891-4901.4. Z. Fan, Y. B. Zuo and B. Jiang, Mater Sci Forum, 690 (2011), 141-144.5. S. A. Impey, D. J. Stephenson and J.R. Nicholls, Mater Sci Tech, 4 (1988), 1126-1132.6. S. A. Impey, D. J. Stephenson and J. R. Nicholls, (Paper presented at the first international conference on the microscopy of oxidation, University of Cambridge,1991).7. I. Haginoya and T. Fukusako, Trans Japan Ins Metals,; 24 (1983), 613-619.8. K. Wefers and C. Misra, Oxides and Hydroxides of Aluminium. Alcoa, USA: Alcoa Laboratories; 1987.9. L. A. Narayanan, F. H. Samuel and J. E. Gruzleski, Metall Mater Trans A, 25 (1994), 1761-1773.10. H. T. Li, PhD Thesis. Brunel University, Uxbridge, UK; 2011.11. H. T. Li, Y. Wang and Z. Fan, Acta Mater, 60 (2012), 1258-1537.12. Y. B. Zuo, B. Jiang and Z. Fan, Mater Sci Forum, 690 (2011), 137-140.13. Z. Fan, Proceedings of the John Hunt Symposium, Brunel University, 2011, p29-44.14. J. Gibbs, In: Collected Works of J. Willard Gibbs, V1. NY: Langman, Green and Co; 1928.15. A. L. Greer, et al., Acta Mater, 48 (2000), 2823-2835.16. T. E. Quested and A.L. Greer, Acta Mater, 52 (2004), 3859-3868.17. S. Kumar, et al., Mater Sci Tech, 27 (2011), 1833-1839.18. X. Fang, et al., Mater Sci Eng A, 445-446 (2007), 65-72.

1400