ANALYSIS AND INTERPRETATION OF MAXUS-7 EXPERIMENTS MACE ON COLUMNAR-EQUIAXED SOLIDIFICATION IN AL-SI ALLOYS

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1 ANALYSIS AND INTERPRETATION OF MAXUS-7 EXPERIMENTS MACE ON COLUMNAR-EQUIAXED SOLIDIFICATION IN AL-SI ALLOYS Laszlo Sturz, Gerhard Zimmermann ACCESS e.v., Intzestrasse 5, Aachen, Germany, ABSTRACT Directional solidification experiments in metallic AlSi7 alloys have been carried out onboard MAXUS-7 in 2006 to investigate the columnar-equiaxed transition in dendritic grain growth. Within the 12 minutes of available microgravity time three rod-like samples have been processed to investigate the effect of microgravity, additions of grain refiner particles and processing conditions like cooling rate. The columnarequiaxed-transition was obtained clearly in two of the samples. Here we present a sound comparison between selected results from the three experiments and their corresponding reference experiments on ground. Different modelling approaches like phase-field method are compared to the results to improve the understanding and modelling of the transition in these Al-based alloys. 1. INTRODUCTION Solidification of the primary solid phase from the melt is the first step in microstructure formation in metallic alloy systems. The microstructure of this phase is often dendritic, and the dendrites may be subject to subsequent coarsening or phase transformations during the solidification path towards the final microstructure at ambient. Nevertheless, the primary phase- and its microstructure formation are of key importance for the mechanical properties. The columnar-to equiaxed transition (CET) is related to a change in microstructural and thus mechanical features of the primary phase. In columnar dendritic growth grains are elongated and have a preferred growth direction, while in equiaxed dendritic growth the orientation is random with smaller shape factors. Columnar dendritic growth is favoured for example in single crystal growth [1] or in turbine blades, where alignment of grain boundaries in a certain blade direction is required [2]. On the other hand, equiaxed structures with small grains have more isotropic properties. CET is generally undesired in a cast and solidified part, making investigations on the transition valuable in order to understand and control the physical phenomena that govern the transition. When columnar dendritic growth is present, the dendrite tips of the columnar grains grow more or less opposite to the direction of heat extraction at some level of undercooling, when compared to thermal equilibrium conditions for a given alloy system. Ahead of the columnar dendrites tips an undercooled zone is built up, which can extend even to the region between the tips. Size of the zone and level of undercooling are related to thermodynamical (for example liquidusslope, partition coefficient), thermophysical (diffusion constants) and experimental parameters (cooling rate, temperature gradient). More details on CET phenomenology can be found in [3-4]. Dendritic fragments can be released from the growing columnar front by mechanical shear stress from melt flow or due to remelting of dendritic necks. The fragments may be transported into the undercooled melt, where they can grow and participate to or dominate equiaxed growth [5]. Another source for equiaxed dendrites is heterogeneous nucleation of the primary phase on suitable substrates or impurities in the undercooled melt. This may happen, if the critical undercooling for this process is lower than the local bulk undercooling at the nucleation site [6]. Despite the source of equiaxed grains, they tend to grow with random orientation and low aspect ratio even in presence of a temperature gradient in the bulk melt. If the equiaxed grains ahead the columnar dendrites are sufficient in number or size, they can arrest the columnar growth front, and a CET is found, sometimes sharp and sometimes more gradual under the occurrence of a mixed columnar and equiaxed zone. In literature empirical or analytical criteria are found to define the conditions for the CET. One example is the criteria for mechanical blocking of Hunt [7], in which CET has occurred, when the equiaxed volume fraction φ is larger than Gravity acts as a complicating factor in CET investigations, since natural convection may be present additionally to diffusive transport and sedimentation/buoyancy of solid grains may change the conditions for CET significantly. In microgravity the CET-phenomenon is reduced to a few processing and alloy parameters, which can be tackled by numerical or analytical descriptions. We have performed a series of experiments called MACE (Metallic Alloys in Columnar Equiaxed Solidification) on the sounding rocket MAXUS-7 in In the reduced gravity environment three different experiments have been carried out using the binary alloy AlSi7 with and without grain refiner Proc. 19th ESA Symposium on European Rocket and Balloon Programmes and Related Research, Bad Reichenhall, Germany, 7 11 June 2009 (ESA SP-671, September 2009)

2 particles. We give a very brief summary of the experiments and their results, while focusing here on their interpretation and numerical modelling. 2. EXPERIMENTAL PART The binary alloy AlSi7 was used, which serves as a model alloy for technical cast alloys A356 and A357. The alloys were prepared by Hydro Aluminium GmbH Deutschland and amounts of 215 µg/g Ti and 30 µg/g B were found to be present as grain refiner additions (mainly as TiB2 particles). Cylindrical rods of 7,90*10-3 m diameter and 0,205 m length were machined to fit into ceramic Al 2 O 3 crucibles. Fig. 1 shows a schematic drawing of the main parts of the furnace. Melting and solidification is controlled by the three heaters H1-H3 and the temperature evolution measured with means of Ni-CrNi thermocouples attached to the crucible walls. Figure 1: Schematic drawing of sample-rod, furnace set-up and thermocouple positions. into the small thermal gradient. CET is provoked by this gradient decrease, while keeping the external cooling rate at all three heaters constant and equal. Tab. 1 summarizes the experimental parameters. While MACE-A and MACE-B investigate the effect of grain refiner particles on the CET in low-gravity, MACE-C is a parameter variation, when compared to MACE-A. Fig. 3 shows the temperature profiles along the samples axes for MACE-A to MACE-C at the beginning and the end of the low-gravity period. The time-dependent temperature change measured by all thermocouples for MACE-A is given in Fig. 4. Both in Fig. 3 and Fig. 4 the gradient changes are clearly visible. Table 1: Experimental parameters for MAXUS-7, (g.r.=grain refiner). Experiment MACE-A MACE-B MACE-C Alloy AlSi7 AlSi7+g.r. AlSi7 Gradient H1-H [Km -1 ] Gradient H2-H [Km -1 ] Cooling rate [Ks -1 ] 0,195 0,195 0,1 For mechanical stability all parts are fixed into an argon- and cotton-wool filled housing, shown in Fig. 2. Figure 2: Experimental module TEM 01-2M for the experiments MACE ; onboard MAXUS-7. The three furnaces are shown assembled to the experimental module TEM 01-2M for MAXUS built by EADS-Astrium. Two different thermal gradients were established between H1-H2 and H2-H3 at the beginning of the solidification experiments. Solidification was initiated immediately after the beginning of the low-gravity period (<10-3 g) in the partly molten samples. The solidification front moves from the high thermal gradient with columnar dendrites Figure 3: Measured temperature profiles for MACE-A to MACE-C at the beginning and the end of the low-gravity period. T L =liquidus-temperature, T E =eutectictemperature. Circles denote the positions of the thermocouples. The cooling curves (Fig. 4) have been used to estimate the columnar and equiaxed front-velocity (at liquidustemperature minus 5K undercooling, here 888 K, and the axial temperature gradient ahead of the front in the liquid. It is assumed, that the columnar/equiaxed front velocity is equal to the isotherm velocity at 888 K and linear interpolation between neighbouring

3 thermocouples is sufficient to calculate the temperature gradient. More details for the experimental set-up, sequence and analysis of data can be found in [9-12]. Figure 4: Measured cooling curves for MACE-A, T L, T E like in Figure 3. From longitudinal and transversal polished and etched sections of the processed sample the CET position was revealed with means of a grain-size analysis [11-12]. In MACE-C no CET was observed. Here the temperature gradients are smaller, when compared to MACE-A (Tab. 1). In the rod with diameter 7.90*10-3 m radial growth from the crucible walls has started, when the order of the axial temperature gradient was similar to the radial one. As a consequence radial columnar growth finally blocked the axial columnar growth [11]. Therefore, we do not discuss the results for MACE-C here. Fig. 5 shows two selected longitudinal sections of REF-A, where CET is obtained in between the sections and calculated to be at m. For MACE-A, MACE-B and their corresponding 1greference experiments REF-A and REF-B from the axial positions of the CET s and the temperature gradient and isotherm velocity calculations, the critical values at the CET were calculated. These critical parameters are the benchmarks to be compared to numerical and analytical CET models. Figure 5: Selected longitudinal sections from REF-A (AlSi7 without grain refiners). Top: ,1619 m; columnar growth. Bottom: ,1721 m, equiaxed growth. Different colours denote different grain orientations. 3. RESULTS The critical parameters from the low-gravity and their corresponding reference experiments are shown in Fig. 6. The effect of grain refiner particles is to lower the critical undercooling for heterogeneous nucleation of primary equiaxed Al-dendrites in the bulk. CET is thus present already at higher gradients and lower front velocities [4]. Figure 6: Critical parameters (G, v) at the CET. Dashed lines highlight values for MACE-A (G=750 Km -1, v=2,57*10-4 ms - 1 )

4 The explanation of the difference between low-gravity and earth-bounded experiments concerning critical parameters is not straightforward. On one hand different nucleation mechanisms contribute to the different alloys (mostly heterogeneous nucleation on MACE-B/REF-B; less heterogeneous nucleation and eventually fragmentation in MACE-A/REF-A), on the other hand different transport mechanisms for solute, heat and grains have to be accounted for Microstructure simulations Microstructure simulations using the phase-field method investigate the conditions for CET on the dendritic level numerically. In a given alloy with known thermodynamical and thermophysical parameters the phase-field method is able to calculate the microstructure evolution in space and time in dependence of experimental parameters (here thermal gradient and cooling rate). Tab. 2 summarizes the simulation parameters used for MACE-A Fig. 7 shows the beginning of the simulations after t=5 s with three positioned seeds, which grow towards steady-state at t=300 s. The seeds were orientated in growth direction to give stable columnar growth. The orientation is vertical parallel to the thermal gradient. From steady-state growth at t=300 s nucleation is initiated at random positions with random orientations and given nuclei distances. Nucleation undercooling was a free simulation parameter and finally CET was found with T N =29.0±0.5 K. At higher values the structure remained columnar. Fig. 8 shows at t=309 s the transition to equiaxed structures, while Fig. 9 is an overview image from t=300 s to t=314 s. t=5 s: Orientation t=5 s: Undercooling Table2: Simulation parameters for the phase-field simulations in MACE-A. Parameter Value/Unit Domain (width x hight) 2667 x 1667 cells Spatial resolution x 0.75*10-6 m (per cell) Time resolution t adaptive, approx s Number cells in interface 5 (=3,75*10-6 m) Surface energy σ Jm -2 Mobility surface µ 5*10-11 m 4 /Js Anisotropy surfacetension 0.25 (static) Anisotropy mobility (dynamic) Diffusion-coefficient D L 3*10-9 m 2 s -1 Al-Si Solid state diffusion D S 3*10-12 m 2 s -1 Al-Si Temperature gradient G, 750 Km -1, 0.2 Ks -1 cooling rate Nucleation: nuclei 2*10-3 m (MACE-A) distance Nucleation: critical Simulation-parameter undercooling t=300 s: Orientation t=300 s: Undercooling Figure 7: Dendritic orientation against the vertical thermal gradient and solutal undercooling in the bulk melt for columnar growth in MACE-A. Calculation domain: 2.00 *10-3 m x 1.25*10-3 m. t=309 s: Orientation t=309 s: undercooling Figure 8: Transition to equiaxed structures at t=309 s for MACE-A. Here, two-dimensional simulations have been carried out to investigate the conditions for columnar and equiaxed growth. As the critical front velocity is close to the equilibrium value v EQ =K/G=2.67*10-4 ms -1, the microstructures have been calculated at thermal equilibrium with constant thermal gradient and constant cooling rate. The 2D nucleation density has been taken from 2D metallographical sections.

5 columnar growth front. This is one of the fundamental assumptions made by Hunt in his model. As the CET in MACE-A is a 3D phenomenon and phase-field simulations in 3D are too time-consumptive for the time being, we make use of the similarity of phase-field simulations in 2D and the Hunt model in 2D. We extend the Hunt model into 3D; the comparison of both Hunt-predictions is given in Fig. 11. An increasing deviation for high thermal gradients towards higher critical front velocities is found for the 3D case. Furthermore the result for experiment REF-A is given, showing the small but significant difference between earth and reduced gravity experiments. Figure 9: Overview from t=300 s to t=314 s for MACEA with CET at a critical undercooling of TN=29.0±0.5 K Comparison with analytical model For the nucleation undercooling found from 2D numerical phase-field simulations in MACE-A of TN=29.0±0.5 K, parameter variations have been carried out for different cooling rates (front velocities in steady-state) and thermal gradients. The results are shown in Fig. 10 in a CET map. In addition to these results for the phase-field simulation the experimental critical (G,v)-parameters and predicted curves from the analytical Hunt-model [7] are plotted. In the Huntmodel the same parameters are used as in the 2D phase-field simulations. As can be seen, the predicted curves from the Huntmodel are in good agreement with the phase-field simulations and divide the processing map into a columnar, a mixed and an equiaxed growth region. Both phase-field and analytical calculation have been carried out in 2D; with the kinetic growth model (LGK) following Lipton et al. [13] for the Hunt calculations. As hypothesis for this agreement we assume, that with the low nucleation density in case of MACE-A (without grain refiner particles) the equiaxed grains do not interact with each other or with the Figure 10: Comparison between experimental (G,v) from MACE-A, 2D-phase-field simulations and predictions from the analytical 2D-Hunt-model. Figure 11: Comparison between the model predictions of the Hunt-model in 2D/3D with results from MACE-A experiment REF-A.

6 4. SUMMARY The transition from columnar to equiaxed grain growth during solidification of the primary phase from the melt is an undesired phenomenon. It is based on a complex interplay between transport of heat and mass, segregation, nucleation und gravity-induced effects like convection and sedimentation. The investigation of the CET under reduced gravity gives the opportunity to perform benchmark experiments under simplified diffusive transport conditions to be tackled with numerical methods. On MAXUS-7 CET was investigated in the model alloys AlSi7 and AlSi7+TiB2 and compared to phasefield simulations and the analytical Hunt-model [7]. The results can be summarized as follows: CET in grain-refined alloys is found to occur at higher thermal gradients and lower front velocities in all cases (µg, 1g), when compared to non-refined alloys. This behavior is due to the reduced critical nucleation undercooling. In all alloys microgravity has the same effect as stated above. The difference is small but significant. The underlying mechanisms are not understood completely yet. For MACE-A adapted phase-field simulations and the Hunt-model show good agreement in 2D. An extension of the Hunt-model to 3D shows deviations towards higher gradients, when compared to the 2D case. In summary the results presented here show, that rather simple models can help to predict CET in the alloy investigated here. Implementation of the predicted columnar/equiaxed growth regimes into processing tools may help to identify suitable processing windows or optimize a process. Nevertheless, the effect of melt flow on the CET-predictions needs to be accounted for in more detail in the future. 5. ACKNOWLEDGEMENTS This work was financially supported by the German Space Agency DLR and the European Space Agency ESA, which is gratefully acknowledged. The authors appreciate the Team of Astrium Space Transportation GmbH in Trauen for the hardware development and support during the preparatory experiments. SSC is acknowledged for mission support in Kiruna/Sweden. 6. REFERENCES 1. Pötschke M., Gaitzsch U., Roth S., Rellinghaus B., Schultz L., Preparation of melt textured Ni Mn Ga, J. Magnetism and Magnetic Materials, Vol. 316(2), , Wagner A., Shollock B.A., McLean M., Grain structure development in directional solidification of nickel-base superalloys, Mat. Sci. Eng. A, Vol. 374(1-2), , Martorano M.A., Beckermann C., Gandin Ch.-A.. Met. Mat. Trans. Vol. 34A, , Spittle J.A. Int. Materials. Rev. Vol. 51, , Mathiesen R.H., Arnberg L., Stray crystal formation in Al 20 wt.% Cu studied by synchrotron X-ray video microscopy, Mat. Sci. Eng Vol. A , , Greer A.L., Bunn A.N., Tronche A., Evans P.V., Bristow D.J., Modelling of inoculation of metallic melts: application to grain refinement of aluminium by Al Ti B, Acta Mat., Vol. 48(11), , Hunt J.D. Mat. Sci. Eng. Vol. 65, 75-83, McFadden S., Browne D.J. & Gandin Ch.-A. Met. Mat. Trans. Vol. 40A, , Sturz L., Drevermann A., Pickmann C., Zimmermann G., Influence of grain refinement on the columnar-to-equiaxed transition in binary Al alloys, Mat. Sci. Eng. A, Vol , , Sturz L., Zimmermann G., Investigations on columnar-to-equiaxed Transition in Binary Al Alloys with and without Grain Refiners, Mat. Sci. Forum Vol. 508, pp , Sturz L., Zimmermann G., Jung H., Mangelinck- Noel N., Nguyen-Thi H., Billia B., Investigations on the columnar-equiaxed transition in AlSi7 alloys onboard MAXUS-7 in low gravity environment, Proceedings of the 18th ESA Symposium on European Rocket and Balloon Programmes and Related Research, ESA SP-647, , Sturz L., Zimmermann G., Microgravity experiments on the columnar-equiaxed transition in Al-based alloys Proc. of the 5th Decennial International Conference on Solidification Processing SP07, Sheffield, , Lipton J., Glicksman M.E. and Kurz W., Dendritic growth into undercooled alloy metals, Mater. Sci. Eng. Vol. 65,