- Email: [email protected]
Influence of Li addition on intermetallic compound morphologies in Al–Si–Cu–Fe cast alloys
Scripta Materialia 51 (2004) 43–46 www.actamat-journals.com
Influence of Li addition on intermetallic compound morphologies in Al–Si–Cu–Fe cast alloys P. Ashtari
a,*
, H. Tezuka b, T. Sato
b
a
b
Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, S8 Building, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8552, Japan Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8552, Japan Received 26 November 2003; received in revised form 16 March 2004; accepted 18 March 2004 Available online 17 April 2004
Abstract The influence of Li addition on the Fe-containing intermetallic compounds in Al–6.5%Si–3.5%Cu–1%Fe cast alloys (in wt%) has been investigated. Li successfully modifies the morphology of the b-Al5 FeSi phase from coarse intersected and branched platelets into finer and independent ones. Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Aluminum alloys; Intermetallic phases; Differential Thermal Analysis (DTA); Modification
1. Introduction Aluminum cast alloys are widely used to produce a large number of components for the automotive industry. The mechanical properties of aluminum alloys are adversely affected by iron content. Fe forms complex intermetallic compounds with Al, Si, Mn and Cr. The platelet Fe-containing phase (b-Al5 FeSi) is the most harmful one for the mechanical properties of Al–Si based cast alloys. Addition of trace elements can modify the b-phase morphology to less harmful forms. Sr is a well known element to modify the eutectic Si phase in the aluminum–silicon alloys. Sigworth [1] reported that the formation of Fe-containing brittle phases is retarded in the Sr modified 319 alloy. Samuel et al. [2] reported that the addition of Sr leads to the reduction of the platelet b-phase volume fraction in the Mg-containing 319 alloy. Previous study by the present authors [3] showed that Sr is an effective element to modify the size and morphology of the intermetallic compounds. The combined addition of Sr and Mn is also found to cause the modification, reduction of the size and volume
*
Corresponding author. Tel.: +81-3-5734-3140; fax: +81-3-57343147. E-mail address: [email protected] (P. Ashtari).
fraction of the b-phase as well as to enhance the formation of Chinese script and sludge type morphologies. The purpose of this study is to find other effective elements to modify the b-phase. Li is also an effective element to modify the eutectic Si phase in Al–Si alloys [4]. Li addition has been reported to increase the hydrogen solubility in molten aluminum [5] and to increase surface oxides [6,7] which alter the characteristics of wrought products. In this study, the influence of Li addition on the intermetallic compound morphologies in an Al–Si– Cu–Fe cast alloy has been studied. The Fe concentration was as high as 1 wt% to make clear the morphological changes of the Fe-containing intermetallic compounds.
2. Experimental procedure Three alloys with the basic composition of Al– 6.5%Si–3.5%Cu–1.0%Fe (in wt%) were melted in a graphite crucible by an electrical resistance furnace. Two of the melts were treated by Li, added in levels of 0.12 and 0.33 wt% in the form of the Al–18.5wt% Li master alloy. In order to have different cooling rates the alloys were cast into cast-iron and graphite molds where the alloys were solidified at average cooling rates of 4.2 and 1.4 °C/s, respectively. The effect of Li on the microstructures was studied using an optical microscope and
1359-6462/$ - see front matter Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2004.03.022
44
P. Ashtari et al. / Scripta Materialia 51 (2004) 43–46
SEM. An image analysis method was utilized to measure the size and volume fraction of the phases. Differential thermal analysis (DTA) was performed to reveal the influence of Li on the solidification phenomena.
3. Results and discussion Fig. 1 shows the microstructures of the alloys cast into an iron mold. These microstructures contain flakelike eutectic Si particles and coarse b-phase platelets with an intersected and branched structure in the Li-free alloy (Fig. 1(a)), however, in the Li-containing alloys the eutectic Si is well refined and the b-phase has a finer and individual structure (Fig. 1(b) and (c)). This can be clearly observed in the SEM images of the deep-etched samples as shown in Fig. 2, where a branched b-phase is observed in Fig. 2(a) as highlighted by an arrow. A compound described as AlLiSi is also formed in the Licontaining alloys (see Fig. 1). The image analysis results shown in Fig. 3 clearly indicate that the length of the bphase decreases by the addition of Li. The average sizes of the b-phase for the Li-free, 0.12 Li and 0.33 Li alloys are 37.0, 19.1, and 14.5 lm, respectively. Thus, Li is an effective element in modifying and shortening the platelet b intermetallic compound. The result of the image analysis also shows that the volume fraction of the b intermetallic compound remains unchanged by addition of Li to the alloys.
Fig. 3. Effect of Li addition on the length of the b-phase particles in the iron mold cast alloys.
The mechanisms of modification of the b-phase by Li addition can be explained with regards to the nucleation behavior of the b-phase. Fig. 4 shows the DTA results for samples of Li-free and Li-containing alloys. The b-phase starts to crystallize after crystallization of the a-Al phase (the pre-eutectic b-phase) and continues to crystallize until the end of the ternary eutectic reaction. The eutectic phase consists of the a-Al, Si and b-phases (the coeutectic b-phase). The addition of Li increases the liquidus temperature of the alloy and lowers both the crystallization temperature of the b-phase and the eutectic reaction temperature. In other words, the b-phase crystallization and eutectic reaction take place under higher undercoolings in the Li-containing alloys. The increased undercooling is considered to be the result of suppressed nucleation due to the eliminated nucleation
Fig. 1. Microstructures of the alloys cast into an iron mold, showing the effect of Li addition. (a) Li-free, (b) 0.12 wt% Li and (c) 0.33 wt% Li.
Fig. 2. SEM images showing the effect of Li addition on the microstructure of the iron mold cast alloys. (a) Li-free, (b) 0.12 wt% Li and (c) 0.33 wt% Li.
P. Ashtari et al. / Scripta Materialia 51 (2004) 43–46
Fig. 4. DTA analysis of the Li-free and Li-containing samples cooled at 10 °C/min.
sites. This is well consistent with the recent findings by Nogita and Dahle [8–10]. They investigated the solidification mode of the eutectic phase in Al–Si cast alloys by electron back-scattering diffraction and found that the eutectic nucleation mode is strongly dependent on the additive elements. Their results indicated that the eutectic cells grew from the primary a-Al phase in an unmodified alloy, however, when the eutectic cells were modified by Sr or Sb the eutectic cells nucleated and grew separately from the primary a-Al dendrites. It is suggested that the co-eutectic b and Si phases are modified as a result of the activation of some other nucleation sites for the eutectic cells under a higher undercooling. The nucleation sites, however, are not identified yet. Fig. 5 shows SEM images of the alloys cast into a graphite mold. These microstructures contain flake-like Si particles and the coarse platelet b-phase in the Li-free alloy, whereas both the flake-like Si and platelet b-phase are changed into a refined Si and a modified b-phase in the Li-containing alloys. A compound described as AlLiSi is also formed in the Li-containing alloys. The microstructures of the alloys solidified at a lower cooling rate are coarser than the microstructures of the alloys solidified at a higher cooling rate (see Figs. 2 and 5). The effect of Li on the b-phase modification is less in the alloys solidified at lower cooling rates. The above trends can be clearly noticed in the image analysis results as shown in Fig. 6. The average sizes of the b-phase for the
45
Fig. 6. Effect of Li addition on the length of the b-phase particles in graphite mold cast alloys.
Li-free, 0.12 Li and 0.33 Li alloys are 47.4, 37.1, and 27.5 lm, respectively. The length of the b-phase decreases by addition of Li, however higher cooling rate solidification exerted by an iron mold decreases the length of the b-phase dramatically compared with the values obtained during the lower cooling rate solidification exerted by a graphite mold (see Figs. 3 and 6). Thus, the cooling rate is an important factor to control the size of the b-phase and Li is more effective in the case of high cooling rates. It can be suggested that Li is rejected from the solid–liquid interface during the high cooling rate solidification in the iron mold. Therefore, during the subsequent cooling and solidification, the liquid in front of the solid–liquid interface becomes enriched in Li and as a result, effective Li is increased. However, the amount of Li enrichment in front of the solid–liquid interface is lower because the rejected Li has much more time to diffuse into the liquid when the cooling rate is low. Therefore, Li is less effective in the case of graphite mold cast alloys. Despite the advantageous effect of Li addition on the modification of the eutectic silicon and intermetallic compounds, the castings of Li-containing alloys are prone to casting defects such as porosity. Therefore, proper degassing and melting under an inert gas atmosphere must be taken into account as precautions to produce sound castings. The addition of Li also causes another compound of AlLiSi, which is undesirable for the mechanical properties. Image analysis shows that the
Fig. 5. SEM images showing the effect of Li addition on the microstructure of graphite mold cast alloys. (a) Li-free, (b) 0.12 wt% Li and (c) 0.33 wt% Li.
46
P. Ashtari et al. / Scripta Materialia 51 (2004) 43–46
volume fraction of the AlLiSi phase increases with increasing the Li content in both iron and graphite molds. However, increasing the Li amount does not change the length of the b-phase remarkably as shown in Figs. 3 and 6. Therefore, to avoid the AlLiSi phase formation, it can be concluded that low addition of Li is preferable. 4. Conclusions (1) In Al–Si–Cu–Fe alloys Li was found to be effective to modify the b-Al5 FeSi intermetallic compound into finer and independent ones. The effect of Li was more pronounced during a high cooling rate solidification. (2) The addition of Li causes decreased crystallization temperature of the eutectic Si and b-phases. It is suggested that Li suppresses the nucleation of these phases. Thus, the crystallization of the eutectic Si and b-phases takes place at higher undercoolings. (3) The addition of Li should be carefully controlled to add minimum amount to avoid the AlLiSi phase formation.
Acknowledgements P. Ashtari gratefully acknowledges financial support from the Ministry of Education, Science, Culture and Sports, Japan, as ‘‘Honors scholarship for privately financed international students’’.
References [1] Sigworth GK. Mod Cast 1987;77:23. [2] Samuel FH, Ouellet P, Samuel AM, Doty HW. Metall Trans A 1998;29A:2871. [3] Ashtari P, Tezuka H, Sato T. Mater Trans JIM 2003;44:2611. [4] Ashtari P, Influence of Li on the aluminum–silicon alloys. Master’s thesis. Iran University of Science and Technology (IUST), Tehran, Iran. 1996. [5] Saikawa S, Sugioka S, Nakai K, Kamio A. J Jpn Inst Light Met 1991;41:596. [6] Prasad NH, Balasubramaniam R. J Mater Process Technol 1997; 68:117. [7] Jayaram V. J Mater Sci 1996:314591. [8] Nogita K, Dahle AK. Mater Charact 2001;46:305. [9] Nogita K, Dahle AK. Mater Trans 2001;42:207. [10] Nogita K, Dahle AK. Mater Trans 2001;42:393.
Influence of Li addition on intermetallic compound morphologies in Al–Si–Cu–Fe cast alloys P. Ashtari
a,*
, H. Tezuka b, T. Sato
b
a
b
Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, S8 Building, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8552, Japan Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8552, Japan Received 26 November 2003; received in revised form 16 March 2004; accepted 18 March 2004 Available online 17 April 2004
Abstract The influence of Li addition on the Fe-containing intermetallic compounds in Al–6.5%Si–3.5%Cu–1%Fe cast alloys (in wt%) has been investigated. Li successfully modifies the morphology of the b-Al5 FeSi phase from coarse intersected and branched platelets into finer and independent ones. Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Aluminum alloys; Intermetallic phases; Differential Thermal Analysis (DTA); Modification
1. Introduction Aluminum cast alloys are widely used to produce a large number of components for the automotive industry. The mechanical properties of aluminum alloys are adversely affected by iron content. Fe forms complex intermetallic compounds with Al, Si, Mn and Cr. The platelet Fe-containing phase (b-Al5 FeSi) is the most harmful one for the mechanical properties of Al–Si based cast alloys. Addition of trace elements can modify the b-phase morphology to less harmful forms. Sr is a well known element to modify the eutectic Si phase in the aluminum–silicon alloys. Sigworth [1] reported that the formation of Fe-containing brittle phases is retarded in the Sr modified 319 alloy. Samuel et al. [2] reported that the addition of Sr leads to the reduction of the platelet b-phase volume fraction in the Mg-containing 319 alloy. Previous study by the present authors [3] showed that Sr is an effective element to modify the size and morphology of the intermetallic compounds. The combined addition of Sr and Mn is also found to cause the modification, reduction of the size and volume
*
Corresponding author. Tel.: +81-3-5734-3140; fax: +81-3-57343147. E-mail address: [email protected] (P. Ashtari).
fraction of the b-phase as well as to enhance the formation of Chinese script and sludge type morphologies. The purpose of this study is to find other effective elements to modify the b-phase. Li is also an effective element to modify the eutectic Si phase in Al–Si alloys [4]. Li addition has been reported to increase the hydrogen solubility in molten aluminum [5] and to increase surface oxides [6,7] which alter the characteristics of wrought products. In this study, the influence of Li addition on the intermetallic compound morphologies in an Al–Si– Cu–Fe cast alloy has been studied. The Fe concentration was as high as 1 wt% to make clear the morphological changes of the Fe-containing intermetallic compounds.
2. Experimental procedure Three alloys with the basic composition of Al– 6.5%Si–3.5%Cu–1.0%Fe (in wt%) were melted in a graphite crucible by an electrical resistance furnace. Two of the melts were treated by Li, added in levels of 0.12 and 0.33 wt% in the form of the Al–18.5wt% Li master alloy. In order to have different cooling rates the alloys were cast into cast-iron and graphite molds where the alloys were solidified at average cooling rates of 4.2 and 1.4 °C/s, respectively. The effect of Li on the microstructures was studied using an optical microscope and
1359-6462/$ - see front matter Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2004.03.022
44
P. Ashtari et al. / Scripta Materialia 51 (2004) 43–46
SEM. An image analysis method was utilized to measure the size and volume fraction of the phases. Differential thermal analysis (DTA) was performed to reveal the influence of Li on the solidification phenomena.
3. Results and discussion Fig. 1 shows the microstructures of the alloys cast into an iron mold. These microstructures contain flakelike eutectic Si particles and coarse b-phase platelets with an intersected and branched structure in the Li-free alloy (Fig. 1(a)), however, in the Li-containing alloys the eutectic Si is well refined and the b-phase has a finer and individual structure (Fig. 1(b) and (c)). This can be clearly observed in the SEM images of the deep-etched samples as shown in Fig. 2, where a branched b-phase is observed in Fig. 2(a) as highlighted by an arrow. A compound described as AlLiSi is also formed in the Licontaining alloys (see Fig. 1). The image analysis results shown in Fig. 3 clearly indicate that the length of the bphase decreases by the addition of Li. The average sizes of the b-phase for the Li-free, 0.12 Li and 0.33 Li alloys are 37.0, 19.1, and 14.5 lm, respectively. Thus, Li is an effective element in modifying and shortening the platelet b intermetallic compound. The result of the image analysis also shows that the volume fraction of the b intermetallic compound remains unchanged by addition of Li to the alloys.
Fig. 3. Effect of Li addition on the length of the b-phase particles in the iron mold cast alloys.
The mechanisms of modification of the b-phase by Li addition can be explained with regards to the nucleation behavior of the b-phase. Fig. 4 shows the DTA results for samples of Li-free and Li-containing alloys. The b-phase starts to crystallize after crystallization of the a-Al phase (the pre-eutectic b-phase) and continues to crystallize until the end of the ternary eutectic reaction. The eutectic phase consists of the a-Al, Si and b-phases (the coeutectic b-phase). The addition of Li increases the liquidus temperature of the alloy and lowers both the crystallization temperature of the b-phase and the eutectic reaction temperature. In other words, the b-phase crystallization and eutectic reaction take place under higher undercoolings in the Li-containing alloys. The increased undercooling is considered to be the result of suppressed nucleation due to the eliminated nucleation
Fig. 1. Microstructures of the alloys cast into an iron mold, showing the effect of Li addition. (a) Li-free, (b) 0.12 wt% Li and (c) 0.33 wt% Li.
Fig. 2. SEM images showing the effect of Li addition on the microstructure of the iron mold cast alloys. (a) Li-free, (b) 0.12 wt% Li and (c) 0.33 wt% Li.
P. Ashtari et al. / Scripta Materialia 51 (2004) 43–46
Fig. 4. DTA analysis of the Li-free and Li-containing samples cooled at 10 °C/min.
sites. This is well consistent with the recent findings by Nogita and Dahle [8–10]. They investigated the solidification mode of the eutectic phase in Al–Si cast alloys by electron back-scattering diffraction and found that the eutectic nucleation mode is strongly dependent on the additive elements. Their results indicated that the eutectic cells grew from the primary a-Al phase in an unmodified alloy, however, when the eutectic cells were modified by Sr or Sb the eutectic cells nucleated and grew separately from the primary a-Al dendrites. It is suggested that the co-eutectic b and Si phases are modified as a result of the activation of some other nucleation sites for the eutectic cells under a higher undercooling. The nucleation sites, however, are not identified yet. Fig. 5 shows SEM images of the alloys cast into a graphite mold. These microstructures contain flake-like Si particles and the coarse platelet b-phase in the Li-free alloy, whereas both the flake-like Si and platelet b-phase are changed into a refined Si and a modified b-phase in the Li-containing alloys. A compound described as AlLiSi is also formed in the Li-containing alloys. The microstructures of the alloys solidified at a lower cooling rate are coarser than the microstructures of the alloys solidified at a higher cooling rate (see Figs. 2 and 5). The effect of Li on the b-phase modification is less in the alloys solidified at lower cooling rates. The above trends can be clearly noticed in the image analysis results as shown in Fig. 6. The average sizes of the b-phase for the
45
Fig. 6. Effect of Li addition on the length of the b-phase particles in graphite mold cast alloys.
Li-free, 0.12 Li and 0.33 Li alloys are 47.4, 37.1, and 27.5 lm, respectively. The length of the b-phase decreases by addition of Li, however higher cooling rate solidification exerted by an iron mold decreases the length of the b-phase dramatically compared with the values obtained during the lower cooling rate solidification exerted by a graphite mold (see Figs. 3 and 6). Thus, the cooling rate is an important factor to control the size of the b-phase and Li is more effective in the case of high cooling rates. It can be suggested that Li is rejected from the solid–liquid interface during the high cooling rate solidification in the iron mold. Therefore, during the subsequent cooling and solidification, the liquid in front of the solid–liquid interface becomes enriched in Li and as a result, effective Li is increased. However, the amount of Li enrichment in front of the solid–liquid interface is lower because the rejected Li has much more time to diffuse into the liquid when the cooling rate is low. Therefore, Li is less effective in the case of graphite mold cast alloys. Despite the advantageous effect of Li addition on the modification of the eutectic silicon and intermetallic compounds, the castings of Li-containing alloys are prone to casting defects such as porosity. Therefore, proper degassing and melting under an inert gas atmosphere must be taken into account as precautions to produce sound castings. The addition of Li also causes another compound of AlLiSi, which is undesirable for the mechanical properties. Image analysis shows that the
Fig. 5. SEM images showing the effect of Li addition on the microstructure of graphite mold cast alloys. (a) Li-free, (b) 0.12 wt% Li and (c) 0.33 wt% Li.
46
P. Ashtari et al. / Scripta Materialia 51 (2004) 43–46
volume fraction of the AlLiSi phase increases with increasing the Li content in both iron and graphite molds. However, increasing the Li amount does not change the length of the b-phase remarkably as shown in Figs. 3 and 6. Therefore, to avoid the AlLiSi phase formation, it can be concluded that low addition of Li is preferable. 4. Conclusions (1) In Al–Si–Cu–Fe alloys Li was found to be effective to modify the b-Al5 FeSi intermetallic compound into finer and independent ones. The effect of Li was more pronounced during a high cooling rate solidification. (2) The addition of Li causes decreased crystallization temperature of the eutectic Si and b-phases. It is suggested that Li suppresses the nucleation of these phases. Thus, the crystallization of the eutectic Si and b-phases takes place at higher undercoolings. (3) The addition of Li should be carefully controlled to add minimum amount to avoid the AlLiSi phase formation.
Acknowledgements P. Ashtari gratefully acknowledges financial support from the Ministry of Education, Science, Culture and Sports, Japan, as ‘‘Honors scholarship for privately financed international students’’.
References [1] Sigworth GK. Mod Cast 1987;77:23. [2] Samuel FH, Ouellet P, Samuel AM, Doty HW. Metall Trans A 1998;29A:2871. [3] Ashtari P, Tezuka H, Sato T. Mater Trans JIM 2003;44:2611. [4] Ashtari P, Influence of Li on the aluminum–silicon alloys. Master’s thesis. Iran University of Science and Technology (IUST), Tehran, Iran. 1996. [5] Saikawa S, Sugioka S, Nakai K, Kamio A. J Jpn Inst Light Met 1991;41:596. [6] Prasad NH, Balasubramaniam R. J Mater Process Technol 1997; 68:117. [7] Jayaram V. J Mater Sci 1996:314591. [8] Nogita K, Dahle AK. Mater Charact 2001;46:305. [9] Nogita K, Dahle AK. Mater Trans 2001;42:207. [10] Nogita K, Dahle AK. Mater Trans 2001;42:393.