Microstructure Evolution in Al-7Si-0.3Mg Alloy During Partial Melting and Solidification from Melt: A Comparison

This introduction paper is based on the paper "Microstructure Evolution in Al-7Si-0.3Mg Alloy During Partial Melting and Solidification from Melt: A Comparison" published by "world foundry congress (wfc06)".

1. Overview:

• Title: Microstructure Evolution in Al-7Si-0.3Mg Alloy During Partial Melting and Solidification from Melt: A Comparison
• Author: Shankargoud Nyamannavar, M. Ravi and K. Narayan Prabhu
• Year of publication: The paper is associated with wfc06 (World Foundry Congress 2006).
• Journal/academic society of publication: world foundry congress (wfc06)
• Keywords: SSM, partial melting, modification, α-particles, and instability.

2. Abstract:

In the present work, a comparative study of microstructure evolution in Sr modified Al-7Si-0.3Mg alloy isothermally held at semi-solid state under conditions of (i) cooling from liquid state and (ii) partial melting from solid state to the semi-solid temperature was carried out. The effect of cooling rate (0.01 to100 K/s) on the microstructure during solidification of semi solid alloy is studied. Partial melting of alloy results in the fine and more spherical solid phase compared to cooling of the same alloy from liquid state. Chemical modification of the eutectic silicon by Sr addition was found to remain same for both cooling the melt from liquid as well as partial melting from solid state, contrary to the reported results. The morphology of eutectic silicon corresponding to the liquid entrapped in solid phase is finer compared to that in interconnected liquid channel.

3. Introduction:

Semi-solid metal (SSM) processing involves processing an alloy in a temperature range where it is partly liquid and partly solid. This method utilizes a semi-solid slurry with spherical solid phase particles (α-particles) dispersed in a liquid matrix. This slurry can be obtained either by controlled solidification (Rheocasting/Rheoforming) or by partial melting and isothermal holding of a preprocessed solid (Thixocasting/Thixoforming). SSM processing of aluminium-silicon alloys offers advantages of both casting and wrought forming. In Thixocasting/Thixoforming, a preprocessed billet is heated to a semi-solid temperature, held isothermally for desired solid fraction and microstructure, and then die-cast/forged. Partial melting is thus a crucial step. During isothermal holding, the solid phase morphology changes from dendritic to spherical to reduce surface energy, as illustrated in Figure 1 [6].

Al-Si alloys like A356 (Al-7Si-0.3Mg) and A357 (Al-7Si-0.6Mg) are widely used in SSM processing. The mechanical properties of these hypoeutectic Al-Si alloys can be enhanced by modifying the coarse acicular eutectic silicon. Strontium (Sr) is a common modifier due to its semi-permanent effect, ease of handling, and non-toxic nature. However, holding the modified alloy in a melt can lead to "fading" of the modification effect. Stucky et al. [12] reported that partial melting and air cooling of Sr-modified A356 alloy nullified the chemical modification, while full melting and air cooling retained it. This study aims to compare microstructure evolution in Sr-modified Al-7Si-0.3Mg alloy under conditions of partial melting from solid state versus cooling from liquid state to the semi-solid temperature, and to study the effect of cooling rate on the microstructure.

4. Summary of the study:

Background of the research topic:

The research focuses on semi-solid metal (SSM) processing of Al-7Si-0.3Mg alloy, a common material for automotive and general applications. A key aspect of SSM processing is achieving a microstructure with spherical solid particles in a liquid matrix. Modification of eutectic silicon, typically with Strontium (Sr), is important for enhancing mechanical properties.

Status of previous research:

Previous studies have shown that Sr modification can "fade" over time in molten metal. Stucky et al. [12] reported that for Sr-modified A356 alloy, partial melting followed by air cooling nullified the modification effect, whereas complete melting and solidification retained it. This finding suggested that the processing route to the semi-solid state could significantly impact the final microstructure, particularly the eutectic silicon morphology.

Purpose of the study:

The purpose of this study was to conduct a comparative study of microstructure evolution in Sr-modified Al-7Si-0.3Mg alloy when isothermally held at a semi-solid state. Two conditions were compared: (i) cooling from the liquid state to the semi-solid temperature, and (ii) partial melting from the solid state to the semi-solid temperature. Additionally, the study aimed to investigate the effect of cooling rate (0.01 to 100 K/s) on the microstructure during solidification from the semi-solid state.

Core study:

The core of the study involved preparing Sr (0.02%) modified Al-7Si-0.3Mg alloy samples and subjecting them to two different thermal paths to reach a semi-solid temperature of 590°C (37% solid fraction). These paths were:

1. Heating to 680°C (complete melting) and then cooling to 590°C.
2. Partially melting the sample by controlled heating from room temperature to 590°C.
   Samples were held isothermally at 590°C for 8000 s and then cooled at different rates (0.01 K/s to 100 K/s). The resulting microstructures were examined to compare the morphology of the primary α-Al phase and the eutectic silicon, particularly focusing on the retention of Sr modification.

5. Research Methodology

Research Design:

The study employed an experimental research design. Sr (0.02%) modified Al-7Si-0.3Mg alloy was prepared from commercial A356 alloy by adding Al-10Sr master alloy. The chemical composition is given in Table 1. Gravity die-cast cylindrical samples (10mm diameter, 10mm height) were used. Samples were coated with ceramic slurry, and a Chromel-Alumel thermocouple was inserted.
Samples were brought to the semi-solid temperature of 590°C (37% solid fraction) by two methods:

1. Heating to 680°C (complete melting) and then cooling to 590°C.
2. Partially melting from solid state by controlled heating to 590°C from room temperature.
   After an isothermal hold of 8000 s at 590°C, samples were cooled at different rates.

Data Collection and Analysis Methods:

• Temperature Measurement and Control: A Chromel-Alumel thermocouple connected to a Keithley data acquisition system interfaced with a computer was used to monitor and record temperature. Samples were heated in a vertical gradient furnace.
• Cooling Methods: Different cooling rates (0.01 K/s to 100 K/s) were achieved by quenching in water, blowing air, air cooling, and furnace cooling.
• Microstructural Analysis: Specimens were sectioned, polished, and etched with 0.5% HF solution. Microstructure examination and analysis were performed using a Leica DMRX microscope and a Clemax image analysis system.

Research Topics and Scope:

• Material: Sr (0.02%) modified Al-7Si-0.3Mg alloy.
• Processing Conditions:
  • Isothermal holding at semi-solid temperature: 590°C (37% solid fraction) for 8000 s.
  • Two routes to semi-solid state: cooling from melt (680°C) vs. partial melting from solid.
  • Cooling rates from semi-solid state: 0.01 K/s to 100 K/s.
• Focus of Analysis:
  • Comparison of α-particle morphology (sphericity, size).
  • Effectiveness of Sr modification on eutectic silicon morphology under different processing routes and cooling rates.
  • Morphology of eutectic silicon in entrapped liquid versus interconnected liquid channels.

6. Key Results:

Key Results:

• Sr addition effectively modified the eutectic silicon from coarse acicular to fine fibrous morphology (Figure 2).
• Isothermal holding at 590°C resulted in a non-dendritic microstructure with α-particles. Samples partially melted from the solid state exhibited finer and more spherical α-particles compared to those cooled from the liquid state to the semi-solid temperature (Figure 3). This difference is attributed to the finer initial microstructure of the die-cast samples used for partial melting.
• The chemical modification of eutectic silicon by Sr remained effective for both processing routes (cooling from melt and partial melting from solid state) across all investigated cooling rates (0.01 K/s to 100 K/s) (Figure 4). This contradicts the findings of Stucky et al. [12] who reported a loss of modification during partial melting.
• The average size of eutectic silicon particles in the interconnected liquid region decreased with an increase in cooling rate (Figure 5). There was no significant difference in eutectic silicon particle size between the two processing routes (partial melting vs. solidification from melt) at any given cooling rate.
• The morphology of eutectic silicon in the liquid entrapped within the solid α-particles was finer compared to that in the interconnected liquid channels, especially at lower cooling rates (e.g., 0.01 K/s). This is attributed to different solidification conditions, possibly higher undercooling or quench modification effect due to the surrounding solid acting as a heat sink for the entrapped liquid.

Figure Name List:

• Figure 1: Mechanisms of morphology change in the solid phase during isothermal holding of semi-solid alloy. [6] a) Coarsening mechanisms (b) Coalescence mechanisms
• Figure 2: Microstructure of Al-7Si-0.3Mg alloy a) Unmodified (b) 0.02% Sr modified
• Figure 3: Microstructures of 0.02% Sr modified Al-7Si-0.3Mg alloy isothermally held at 590°C for 8000 s and quenched in water a) Cooled from liquid b) Partial melted
• Figure 4: Microstructures of 0.02%Sr modified Al-7Si-0.3Mg alloy isothermally held at 590°C for 8000 sec. and cooled at different cooling rates.
• Figure 5: Variation of size of eutectic silicon particle in the interconnected liquid region with cooling rate.
• Table 1: Chemical composition of the alloy (by weight pct.).

7. Conclusion:

1. Partial melting of the alloy results in fine and more spherical solid phase compared to cooling of the same alloy from liquid state and this is influenced by the initial microstructure of the alloy.
2. Chemical modification of the eutectic silicon by Sr remains the same for both cooling the melt from liquid as well as partial melting from solid state.
3. The morphology of eutectic silicon corresponding to the liquid entrapped in solid phase is finer compared to that in the interconnected liquid channel. The effect is significant at lower cooling rates during solidification of the semi-solid alloy.

8. References:

• [1] M. C. Flemings, R. G. Riek and K. P. Young, Rheocasting, Material Science and Engineering, 25, 1976, pp103-107.
• [2] M. C. Flemings, Behavior of metal alloys in the semi solid state, Met. Trans. 22A, 1991, pp957-981.
• [3] D. H. Kirkwood, Semisolid metal processing, Int. Mat. Rev. 39, 1994, pp173-189.
• [4] Z. Fan, Semisolid metal processing, Int. Mat. Rev. 47, 2002, pp49-85.
• [5] W. R. Loue, M. Suery, Microstructural evolution during partial remelting of Al7Si-Mg alloys, Material Science and Engg. A203, 1995, pp1-13
• [6] Andreas Mortensen, On the influence of coarsening on micro-segregation, Met. Trans. 20A, 1989, 247-253.
• [7] P. R. G. Anderson, J. C Summerill and A.R.A. McLelland, The view of potential users, Proceedings of 4th International Conference on Semi Solid Processing of Alloys and Composites, The University of Sheffield, England, 9-21 June 1996, pp.208-214.
• [8] G. Chiarmetta, Thixoforming of automobile components, Proceedings of 4th International Conference on Semi Solid Processing of Alloys and Composites, The University of Sheffield, England, 9-21 June 1996, pp204-207.
• [9] R. Moschini, Mass production of fuel rails by pressure die casting in the semi-solid state, Proceedings of 4th International Conference on Semi Solid Processing of Alloys and Composites, The University of Sheffield, England, 9-21 June 1996, pp248-250.
• [10] T. Basner, Rheocasting of semi-solid A357 Aluminum, SAE 2000 World Congress Detroit, Michigan, 6-9 March, 2000
• [11] John E. Gruzleski, Bernard M. Closset, The treatment of liquid aluminum-silicon alloys, American Foundrymen's Society, Inc. des Plaines, Illinois. USA 1990 ISBN 0-87433-121-8.
• [12] M. Stucky, M. Richard, L. Salvo and M. Suery, Influence of electromagnetic stirring, partial remelting and thixoforming on Mechanical properties of A356 alloys, Proceedings of 5th Int. conference on semi solid processing of alloys and composites. Colarado, 23-25 June 1998, pp.513-520.
• [13] Lu, Shu-Zu and Hallawell, The mechanism of silicon modification in aluminium-silicon alloy: Impurity Induced Twinning, Met. Trans. 18A.1987, pp.1721-23.
• [14] M. Rettenmayr, O. Pompe Interface instabilities on solidifying globuletic particles, Journal of Crystal growth, 173, 1997, pp182-188.
• [15] O. Pompe, M. Rettenmayr, Microstructural changes during quenching, Journal of Crystal growth, 192, 1998, pp300-306.
• [16] J. Valer, F. Sant-Antoinin, P. Meneses and Suery, Influence of processing on microstructure and semi-solid Behavior of Al-Ge alloys, Proceedings of 5th International Conference on Semi Solid Processing of Alloys and Composites. Colorado, 23-25 June 1998, pp.513-520.

9. Copyright:

• This material is a paper by "Shankargoud Nyamannavar, M. Ravi and K. Narayan Prabhu". Based on "Microstructure Evolution in Al-7Si-0.3Mg Alloy During Partial Melting and Solidification from Melt: A Comparison".
• Source of the paper: [DOI URL not provided in the document]

This material is summarized based on the above paper, and unauthorized use for commercial purposes is prohibited.
Copyright © 2025 CASTMAN. All rights reserved.