Optimizing AlSi9Mg Alloys: A Deep Dive into Lithium Additions and Process Holding Time
This technical summary is based on the academic paper "Effect of Li additions and holding time on the mechanical properties of the AlSiM9mg alloys" by O. Özaydın, Y. Kaya, and D. Dışpınar, published in La Metallurgia Italiana (2021).
Keywords
- Primary Keyword: AlSi9Mg alloys
- Secondary Keywords: Lithium additions, holding time, mechanical properties, aluminum alloys, die casting, fluidity, UTS, elongation
Executive Summary
- The Challenge: To improve the mechanical properties of AlSi9Mg aluminum alloys for demanding applications without introducing process issues like slag formation or reduced fluidity.
- The Method: Introducing low levels of lithium (0.1% and 0.2% wt.) to an AlSi9Mg melt and evaluating the mechanical properties immediately after addition and after a one-hour holding period.
- The Key Breakthrough: A one-hour holding time causes a significant loss of lithium (~70%), which paradoxically leads to a substantial increase in elongation while generally decreasing tensile and yield strength.
- The Bottom Line: Lithium's high reactivity and its significant loss over time make melt holding time a critical process variable that drastically alters the final mechanical properties of AlSi9Mg castings.
The Challenge: Why This Research Matters for HPDC Professionals
In the automotive industry, the drive for fuel efficiency and lower emissions has made lightweight aluminum alloys essential. Materials like AlSi9Mg are prized for their low density, excellent castability, and high specific strength, making them ideal for parts like wheels, engine blocks, and cylinder heads. However, the push for even better performance requires continuous material development.
One avenue of exploration is the addition of elements like lithium (Li), traditionally used in aviation to lighten alloys. But adding highly reactive elements like Li into an aluminum melt isn't straightforward. It can lead to process challenges, such as excessive slag formation, and can have complex, sometimes counterintuitive, effects on mechanical properties and casting behavior like fluidity. This study was initiated to understand the precise effects of low-level Li additions and, critically, the impact of melt holding time on the final properties of AlSi9Mg alloys.
The Approach: Unpacking the Methodology
The researchers conducted a controlled experiment to isolate the effects of lithium concentration and holding time. The methodology provides a clear and reliable basis for the findings.
Method 1: Alloy Preparation and Modification
The base material was an AlSi9 alloy, melted at 720°C in 10 kg silicon carbide (graphite) crucibles. Lithium was introduced using an AlLi5 master alloy at two target levels: 0.1 wt.% and 0.2 wt.%. This controlled approach ensured a consistent starting point for all trials.
Method 2: Variable Testing Conditions
To understand the impact of lithium's high reactivity over time, the experiment was divided into two key scenarios for each lithium level:
1. Time Zero (t=0h): Samples were cast immediately after the lithium addition.
2. Time One Hour (t=1h): The molten alloy was held at temperature for one hour before casting.
This design allowed for a direct comparison, revealing how properties change as lithium "burns off" from the melt.
Method 3: Comprehensive Analysis
The resulting cast samples were subjected to a full suite of tests to characterize their properties. This included chemical analysis to confirm the exact elemental composition (including the final Li content), microstructural examination, fluidity tests, and tensile tests to determine Ultimate Tensile Strength (UTS), Yield Strength (YS), and percentage elongation (%E).
The Breakthrough: Key Findings & Data
The study revealed a complex relationship between lithium, holding time, and the final performance of the AlSi9Mg alloy.
Finding 1: Holding Time Drastically Alters Lithium Content and Mechanical Properties
The most significant finding was the dramatic effect of the one-hour holding period. As shown in Tables 3 and 4 of the paper, the initial 0.230% Li addition was reduced to just 0.029% after one hour—a loss of over 85%. This chemical change had a profound impact on mechanical properties.
According to Table 6, for the alloy with an initial 0.2% Li addition:
- Elongation (%E) more than doubled, increasing from 3.36% at t=0h to 6.95% at t=1h.
- Yield Strength (YS) saw a slight decrease from 80.56 N/mm² to 77.25 N/mm².
- Ultimate Tensile Strength (UTS) unexpectedly increased from 144.85 N/mm² to 159.29 N/mm².
This demonstrates that as lithium is lost from the melt, the alloy's ductility significantly improves. The abstract notes a general decreasing trend for UTS and YS, which was observed in the 0.1% Li case, highlighting the complexity of the interactions.
Finding 2: Increased Lithium Content Negatively Impacts Fluidity
Fluidity is a critical property for die casting, as it determines the ability of the molten metal to fill intricate mold cavities. The study concluded that adding lithium had a detrimental effect on this property. As stated in the paper's conclusion, "the fluidity was negatively affected by the increasing Li level." The visual results from the fluidity test apparatus shown in Table 6 support this finding, indicating shorter flow lengths for the higher lithium content samples. This is a crucial consideration for producing complex, thin-walled components.
Practical Implications for R&D and Operations
- For Process Engineers: This study suggests that adjusting melt holding time is a critical process parameter when Li-modified alloys are used. The ~70% loss of lithium in one hour means that any delay between melt treatment and casting will significantly alter the final product's mechanical properties, particularly its ductility. Tightly controlling this time is essential for process consistency.
- For Quality Control Teams: The data in Table 6 of the paper illustrates the strong effect of holding time (and resultant Li content) on elongation. This relationship could inform new quality inspection criteria, where a specific elongation target might be linked back to a maximum allowable melt holding time.
- For Design Engineers: The findings indicate that while lithium is known for lightening alloys, its application in AlSi9Mg does not provide a simple, across-the-board improvement in mechanical properties. The trade-off between strength, ductility, and castability (fluidity) must be carefully considered. This research suggests that achieving high ductility is possible, but it comes at the cost of process complexity and tight control.
Paper Details
Effect of Li additions and holding time on the mechanical properties of the AlSiM9mg alloys
1. Overview:
- Title: Effect of Li additions and holding time on the mechanical properties of the AlSiM9mg alloys
- Author: O. Özaydın, Y. Kaya, D. Dışpınar
- Year of publication: 2021
- Journal/academic society of publication: La Metallurgia Italiana - International Journal of the Italian Association for Metallurgy
- Keywords: ALSI9MG, ALUMINUM ALLOYS, HOLDING TIME, ALLI5 ADDITIONS, MECHANICAL PROPERTIES
2. Abstract:
Aluminum alloys are widely used in industry due to their lightness and ease of machining. Concurrently, the mechanical properties of aluminum alloys can be increased by different development studies. Grain refining, modification and heat treatment can be given as examples of these development studies. In this study, the influences that may occur in mechanical properties by adding lithium (Li) additions, which are generally used for lightening in aviation applications, to AlSi9Mg alloy at different levels have been investigated. Due to the slag problems especially at high Li ratios, a study with relatively low Li addition ratios has been developed. At the same time, the effect of Li additions on high temperature and holding times was also studied. According to the results of the study, it was observed that a high amount of slag was formed at the high Li addition level and the mechanical properties were in a decreasing trend. In addition, it has been observed that the increase in Li level negatively affects the fluidity. It was also observed that the holding times dramatically decreased the Li values in the aluminum alloy structure, this decrease occurred in the ultimate tensile strength (UTS) and yield strength (YS), while the unit elongation (%) increased slightly.
3. Introduction:
The use of aluminum and its alloys has increased across many industries, with the automotive sector being the largest market for castings. The demand is driven by the need for increased fuel efficiency and stricter emission regulations, making aluminum's low density, excellent castability, high specific strength, and easy processability highly desirable for components like wheels, cylinder heads, and engine blocks. This study builds upon previous research investigating the effects of Li on various aluminum alloys, aiming to characterize its influence on the microstructure and mechanical properties of the AlSi9Mg alloy specifically.
4. Summary of the study:
Background of the research topic:
The continuous demand for high-performance, lightweight materials in the automotive industry necessitates ongoing research into improving the properties of aluminum alloys. Modification with various elements is a common strategy to enhance mechanical characteristics.
Status of previous research:
Prior studies have investigated the effects of Li on different aluminum alloys. Lei et al. studied its effect on hypoeutectic Al-7Si alloy, noting changes in eutectic Si morphology. Ashtari et al. examined Li's influence on intermetallic compounds in Al-Si-Cu-Fe alloys. Karamouz et al. investigated Li's impact on the microstructure and properties of A380 aluminum alloy, both with and without heat treatment. Din et al. explored a range of Li additions to Al-Mg-Si alloys, observing density reduction and strength increases. These studies provide a foundation for understanding Li's complex role in aluminum systems.
Purpose of the study:
The purpose of this study was to investigate the influences of adding lithium (Li) at different levels (0.1 wt.% and 0.2 wt.%) to an AlSi9Mg alloy. A primary focus was to also understand the effect of holding the molten alloy at high temperature for one hour on the final Li content and the resulting mechanical properties.
Core study:
The core of the study involved the experimental casting of an AlSi9Mg alloy under four distinct conditions: with 0.1% Li addition at zero holding time (t=0h) and one-hour holding time (t=1h), and with 0.2% Li addition at t=0h and t=1h. The resulting samples were then subjected to chemical, microstructural, and mechanical analysis to quantify the effects of the variables.
5. Research Methodology
Research Design:
The study employed a comparative experimental design. An AlSi9Mg base alloy was modified with two different levels of Li. For each Li level, samples were produced at two different holding times (0 and 1 hour) to assess the change in properties over time at melting temperature.
Data Collection and Analysis Methods:
Base metal was melted at 720 °C in 10 kg SiC (graphite) crucibles. Li was added via an AlLi5 master alloy. Chemical compositions were determined for each of the four experimental conditions (Tables 1-4). Microstructures were examined at various magnifications (Table 5). Mechanical properties, including Ultimate Tensile Strength (UTS), Yield Strength (YS), and Elongation (%E), were measured via tensile testing, and fluidity was assessed using a dedicated test apparatus (Table 6).
Research Topics and Scope:
The research is focused on the as-cast AlSi9Mg alloy. The scope is limited to the investigation of two low-level Li additions (0.1 wt.% and 0.2 wt.%) and a single holding period of one hour. The study evaluates the resulting chemical composition, microstructure, mechanical properties (tensile and elongation), and fluidity.
6. Key Results:
Key Results:
- A high amount of slag was observed to form at the higher Li addition level.
- An increase in the Li level was found to negatively affect the alloy's fluidity.
- A one-hour holding period at melting temperature dramatically decreased the Li content in the final alloy, with an approximate reduction of 70%.
- This reduction in Li content during holding generally resulted in a decrease in ultimate tensile strength (UTS) and yield strength (YS).
- The unit elongation (%) increased slightly after the one-hour holding period.
Figure Name List:
- Fig.1 - Macrostructures.
7. Conclusion:
It was concluded that Li addition could not reach the desired improvement in the elongation values in AlSi9 alloy, the fluidity was negatively affected by the increasing Li level, and as the holding time was increased, the Li ratio was decreased approximately 70%.
8. References:
- [1] Dispinar, D.: "Determination of Metal Quality of Aluminium and Its Alloys (PhD Thesis)" (pp.1-4). School of Metallurgy and Materials - The University of Birmingham. January 2005.
- [2] Davis J R (editor), ASM Specialty Handbook: Aluminum and Aluminum Alloys, 1993, ASM International.
- [3] Lei, W., Liu, X., Wang, W., Sun, Q., Xu, Y., Cui, J.: "On the influences of Li on the microstructure and properties of hypoeutectic Al-7Si alloy" (pp.703-706) Journal of Alloys and Compounds. 30 December 2017. DOI: 10.1016/j.jallcom.2017.04.295
- [4] Ashtari, P., Tezuka, H., Sato, T.: "Influence of Li addition on intermetallic compound morphologies in Al-Si-Cu-Fe cast alloys" (pp.43-46) Scripta Materialia. 17 April 2004. DOI: 10.1016/j.scriptamat.2004.03.022
- [5] Karamouz, M., Azarbarmas, M., Emamy, M.: "On the conjoint influence of heat treatment and lithium content on microstructure and mechanical properties of A380 aluminum alloy” (pp.377-382) Materials and Design. 20 February 2014. DOI: 10.1016/j.matdes.2014.02.033
- [6] Din, ud S., Kamran, J., Tariq, N.H., Hasan, B.A., Petrov, R.H., Bliznuk, V., Zuha, uz S.: "The synergistic effect of Li addition on microstructure, texture and mechanical properties of extruded Al-Mg-Si alloys" (pp.11-22) Materials Chemistry and Physics. 28 February 2016. DOI: 10.1016/j.matchemphys.2016.02.029
- [7] Karamouz, M., Azarbarmas, M., Emamy, M., Alipour, M.: "Microstructure, hardness and tensile properties of A380 Aluminum alloy with and without Li additions” (pp.409-414) Materials and Engineering A. 29 June 2013. DOI: 10.1016/j.msea.2013.05.088
- [8] Mørtsell, A.E., Marioara, C.D., Andersen, S.J., Ringdalen, I.G., Friis, J., Wenner, S., Røyset, J., Reiso, O., Holmestad, R.: "The effects and behaviour of Li and Cu alloying agents in lean Al-Mg-Si alloys" (pp.235-242) Journal of Alloys and Compounds. 23 December 2016. DOI: 10.1016/j.jallcom.2016.12.273
- [9] Koshino, Y., Kozuka, M., Hirosewa, S., Ariga, Y.: "Comparative and complementary characterization of precipitate microstructures in Al-Mg-Si(-Li) alloys by transmission electron microscopy, energy dispersive X-ray spectroscopy and atom probe tomography" (pp.765-770) Journal of Alloys and Compunds. 7 November 2014. DOI: 10.1016/j.jallcom.2014.10.199
- [10] Özaydın, O., Arman, E., Kaya, A., Dokumacı, E., (2019) The Effects of Artificial Ageing Conditions on A356 Aluminum Cast Alloys. ECHT 2019 European Conference on Heat Treatment, Bologna / Italy
- [11] Nikolay A. Belov, Dmitry G. Eskin, Andrey A. Aksenov. (2005) Multicomponent Phase Diagrams: Applications for Commercial Aluminum Alloys (pp.257-284) Elsevier Science eBook ISBN: 9780080456966 DOI:10.1016/B978-0-08-044537-3.X5000-8
- [12] Xiaokun Yang, Baide Zhang, Mawu Li, Lizhong Yan, Chihui Liu, Yongfu Zhang, Hongwei Liu, Hongwei Yan and Kai Wen (2019) Mater. Res. Express 6 IOP Publishing Ltd https://doi.org/10.1088/2053-1591/ab440e
Expert Q&A: Your Top Questions Answered
Q1: Why were relatively low lithium addition ratios (0.1% and 0.2%) chosen for this study?
A1: The paper's abstract states that the study was developed with relatively low Li addition ratios specifically "due to the slag problems especially at high Li ratios." This indicates that higher concentrations of lithium are known to cause excessive dross and slag in the melt, which creates significant processing challenges and can lead to inclusions in the final casting. The researchers chose these lower levels to investigate the effects of Li while remaining in a more manageable process window.
Q2: What was the observed effect of the one-hour holding time on the lithium content in the alloy?
A2: The effect was dramatic. The paper's conclusion states that the Li ratio decreased by approximately 70% during the one-hour holding time. The chemical analysis tables (Tables 1-4) provide specific data: for the 0.2 wt.% addition, the measured Li content dropped from 0.230% at t=0h to just 0.029% at t=1h. This confirms that lithium is highly reactive and "burns off" rapidly from the molten aluminum at casting temperatures.
Q3: How did the loss of Li after holding affect the ultimate tensile strength (UTS) and yield strength (YS)?
A3: According to the abstract, the holding time and subsequent decrease in Li content caused a decrease in both UTS and YS. This trend is clearly visible in the 0.1% Li trial data from Table 6, where UTS dropped from 154.14 to 149.15 N/mm² and YS dropped from 82.78 to 75.16 N/mm². While the 0.2% Li trial showed an anomalous increase in UTS, the authors' overall conclusion points to a general decrease in strength as Li is lost.
Q4: What was the most significant positive change in mechanical properties observed after the holding period?
A4: The most significant positive change was the increase in unit elongation (ductility). The abstract notes that "the unit elongation (%) increased slightly." However, the data in Table 6 for the 0.2% Li addition shows a substantial improvement, with elongation more than doubling from 3.36% at t=0h to 6.95% at t=1h. This suggests that while the initial Li addition may reduce ductility, allowing it to burn off can restore or even enhance it.
Q5: Did adding lithium improve the alloy's fluidity for casting?
A5: No, it had the opposite effect. The paper's conclusion is unequivocal: "the fluidity was negatively affected by the increasing Li level." This is a critical finding for die casting applications, as reduced fluidity can prevent the complete filling of complex molds, leading to misruns and other defects. This suggests that any potential benefits from Li addition must be weighed against this negative impact on castability.
Conclusion: Paving the Way for Higher Quality and Productivity
This research provides valuable insights into the complex role of lithium in AlSi9Mg alloys. It highlights that while Li additions are explored for property modification, their high reactivity makes process control paramount. The study's key breakthrough is demonstrating that melt holding time is not a passive variable but an active factor that dramatically alters the alloy's chemistry and, consequently, its final mechanical properties—especially ductility. For engineers working with modified aluminum alloys, this underscores the critical need to standardize and minimize the time between melt treatment and casting to ensure product consistency and performance.
At CASTMAN, we are committed to applying the latest industry research to help our customers achieve higher productivity and quality. If the challenges discussed in this paper align with your operational goals, contact our engineering team to explore how these principles can be implemented in your components.
Copyright Information
This content is a summary and analysis based on the paper "Effect of Li additions and holding time on the mechanical properties of the AlSiM9mg alloys" by "O. Özaydın, Y. Kaya, D. Dışpınar".
Source: The paper was published in La Metallurgia Italiana - November/December 2021, pages 19-24.
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