Actual chemical composition (in wt.%) of synthetic Al-Si-Cu alloys.
Abstract
This chapter presents the potential of the cooling curve analysis to characterize the solidification path of the cast hypoeutectic series of Al-Si-Cu alloys and to quantify their feeding regions. The aim of this work is to examine how variations in the chemical composition of Si (5, 7 and 9 wt.%) and Cu (from 0 to 4 wt.%) might affect the characteristic solidification temperatures, their corresponding fraction solid, and feeding regions of investigated alloys. These parameters collected from the cooling curve analysis can be used for better understanding of the solidification paths of Al-Si-Cu alloys and could easily be incorporated into existing simulation software packages to improve their accuracy.
Keywords
- aluminum alloys
- thermal analysis
- cooling curves
- fraction solid
- feeding
1. Introduction
Al-Si-Cu casting alloys show a great promise for several fields of engineering applications. Over the past few years, these alloys have been widely used in the automotive industry due to their suitable properties such as their lightness, strength, recyclability, corrosion, resistance, durability, ductility, formability and conductivity. Their good metallurgical properties, such as castability and fluidity, further enhance the applicability of these alloys for the production of intricate castings such as, e.g., the engine parts and cylinder heads. The chemical compositions of these alloys have a significant impact on all of the aforementioned properties. The alloying elements are usually added with the intent to improve the specific properties of casting parts. The main alloying elements: Si and Cu are primarily responsible for defining the microstructure and mechanical properties of aluminum alloys [1, 2, 3, 4, 5, 6, 7]. The castability and fluidity of these alloys have improved through Si addition. Additionally, the presence of Si leads to the reduction of shrinkage porosity, giving those alloys superior mechanical and physical properties.
Copper, as a second major alloying element, has been added to considerably increase strength and hardness of Al-Si-Cu alloys in as cast and heat-treated conditions. In addition, Cu reduces the corrosion resistance of aluminum alloys, and in certain alloys increases stress corrosion susceptibility. This element is generally responsible for reducing the casting characteristics, especially the feeding ability of Al-Si-Cu alloys [8, 9, 10].
Any cast aluminum alloy during the transition from liquid to solid condition characterizes reduction in its volume. That reduction is usually in the range between 4 and 8 wt.% (higher Si content corresponds to lower reduction in the volume and vice versa). In order to eliminate the potential formation of shrinkage porosity by maintaining a path for fluid flow from the higher heat mass and the pressure of the riser to the isolated liquid pool, cast parts need to be additionally fad with a new volume of the liquid melt. According to Campbell [11], during directional solidification, it can be recognized five feeding mechanisms. They are, as Figure 1 illustrates liquid feeding, mass feeding, interdendritic feeding, burst feeding, and solid feeding [11].
The liquidus (
Depending on the solidification interval of alloys, chemical compositions, cooling rates, amount of master alloys, hydrogen content and other, Al-Si-Cu alloys are prone to developing a considerable amount of shrinkage porosity. The solidification interval of Cu free alloys is very narrow; typically around 60°C, containing approximately 50% eutectic liquid. Usually, the level of porosity in such type of aluminum alloys is very low due to no feeding constraint during solidification of the last portion of eutectic liquid. The presence of Cu in the aluminum silicon alloys considerably extend their solidification range (reaching more than 100°C), making them more prone to the formation of shrinkage porosity [31].
Recently, it has shown [31, 32] sensitivity of aluminum-silicon alloys to porosity based on the content of Cu in these alloys. Addition up to 1 wt.% of Cu resulted in a significant increase in the porosity level. Surprisingly, further Cu addition up to 4 wt.% did not have such a significant impact on the porosity level at the same aluminum silicon alloy. It looks that development of porosity by cast aluminum-silicon alloys does not depend only on the concentration of Cu. It is also still not entirely clear which feeding regions is more responsible for the formation of shrinkage porosity. The impact of various major alloying elopements (Si and Cu) on the feeding regions has not yet been fully analyzed. There is a lack of data, in the available literature, regarding quantification of feeding regions. The objective of this work is to examine how variation in chemical composition of Al-(5, 7, 9)Si-(0–4)Cu (wt.%) alloy may affect its characteristic solidification temperatures and corresponding fraction solid related to each temperature, as well as to quantify the effect of various contents of Si and Cu on the corresponding feeding regions. This analysis should help foundry professionals to understand better which feeding regions are more responsible for the formation of shrinkage porosity. To accomplish this, several experimental tests were carried out by applying the TA technique. All experimentally obtained data (the characteristic solidification temperatures and solid fraction) will be applied to quantify the five feeding regions of these alloys.
2. Experimental procedure
Twenty-five different Al-Si-Cu alloys with the chemical compositions, as presented in Table 1, are synthetically produced. Pure aluminum (commercial purity 99.7 wt.%) and pure copper (commercial purity 99.9 wt.%) have been used as impute materials. The content of the main alloying elements varied between 4.96–8.93 wt.% of Si and 0.0–4.30 wt.% of Cu. Their chemical compositions have been determined using optical emission spectroscopy (OES).
Alloy | Si | Cu |
---|---|---|
Al-5Si | 4.96 | 0 |
Al-5Si-1Cu | 5.22 | 1.12 |
Al-5Si-2Cu | 5.12 | 1.88 |
Al-5Si-3Cu | 5.08 | 3.11 |
Al-5Si-4Cu | 5.01 | 4.30 |
Al-7Si | 6.80 | 0 |
Al-7Si-1Cu | 7.32 | 0.89 |
Al-7Si-2Cu | 7.32 | 2.04 |
Al-7Si-3Cu | 7.32 | 3.28 |
Al-7Si-4Cu | 7.13 | 4.30 |
Al-9Si | 8.80 | 0 |
Al-9Si-1Cu | 8.93 | 0.92 |
Al-9Si-2Cu | 8.93 | 2.17 |
Al-9Si-3Cu | 8.82 | 2.93 |
Al-9Si-4Cu | 8.92 | 4.02 |
The alloys were melted in an electric resistance furnace, capacity 8 kg. No grain refining and modifier agents were added to the melt. During all experiments, degassation was not applied. Samples with masses of approximately 250 g were poured into coated stainless-steel cups. The height of the thermal analysis test cup was 60 mm, its diameter was 50 mm, while the weight of the steel test cup was 50 g.
Two calibrated commercial N type thermocouples with an accuracy of ±0.10°C were inserted into thermal analysis cup and used during all experiments. One thermocouple was placed in the center of the thermos analysis cup while second 5 mm away from the cup inner wall. They recorded temperature during solidification of an investigated alloy (especially between 750 and 400°C temperature range). The National Instrument data acquisition system has been applied to collect temperature-time data. During all trials, the sampling rate was five data per second. The cooling conditions were maintained constant during all experiments, but due to various Si and Cu contents, the solidification rates slightly varied between maximal 0.26°C/s for Al-5Si-4Cu (wt.%) alloy and minimal 0.11°C/s for Al-9Si (wt.%) alloy. The cooling rate has been calculated as the ratio of the temperature difference between
3. Results and discussion
Porosity is one of the most common defects in aluminum cast parts caused mostly due to insufficient feeding and hydrogen precipitation during solidification. The amount of dissolved hydrogen in cast Al-Si alloys can be kept very low by degassing the melt. However, shrinkage porosity can still be a problem in the cast parts caused by non-proper feeding ability. Consequently, understanding the feeding behavior of hypoeutectic Al-Si-Cu alloys is an important aspect of sound casting production. In this paper, the impact of various contents of Si and Cu on different feeding regions has been analyzed by applying the TA technique. The main objective of this work was to better understand their impact on the feeding ability of Al-Si-Cu alloys and to quantify each feeding region regarding the characteristic solidification temperatures and/or the corresponding amount of fraction solid precipitated between those temperatures.
3.1 Analysis of characteristic solidification temperatures
The results of the cooling curve analysis are summarized in Table 2. The values of characteristic solidification temperatures (
Alloy |
|
|
|
|
---|---|---|---|---|
Al-5Si | 632.9 | 624.1 | 575.7 | 553.4 |
634.2 | 624.9 | 576.7 | 555.5 | |
Al-5Si-1Cu | 631.5 | 623.1 | 571.4 | 500.1 |
628.1 | 623.4 | 571.7 | 499.7 | |
Al-5Si-2Cu | 625.4 | 619.5 | 567.2 | 497.8 |
624.9 | 619.1 | 568.0 | 496.8 | |
Al-5Si-3Cu | 622.5 | 616.2 | 562.8 | 500.6 |
621.8 | 617.0 | 562.0 | 499.2 | |
Al-5Si-4Cu | 617.0 | 613.2 | 558.7 | 498.5 |
617.1 | 613.2 | 558.7 | 501.9 | |
Al-7Si | 617.8 | 610.7 | 576.7 | 552.0 |
617.6 | 611.5 | 576.8 | 553.4 | |
Al-7Si-1C | 612.6 | 604.5 | 573.8 | 498.0 |
611.8 | 604.5 | 574.0 | 497.9 | |
Al-7Si-2Cu | 607.4 | 602.3 | 570.6 | 495.3 |
607.2 | 603.3 | 570.2 | 495.0 | |
Al-7Si-3Cu | 603.5 | 598.0 | 567.1 | 494.3 |
603.2 | 596.8 | 566.5 | 494.0 | |
Al-7Si-4Cu | 599.6 | 594.0 | 563.4 | 497.1 |
599.1 | 593.6 | 563.8 | 496.1 | |
Al-9Si | 600.2 | 595.7 | 575.0 | 549.3 |
600.5 | 597.6 | 575.2 | 552.3 | |
Al-9Si-1Cu | 597.3 | 593.9 | 573.1 | 494.7 |
595.8 | 593.7 | 572.,4 | 493.6 | |
Al-9Si-2Cu | 591.9 | 589.,2 | 569.5 | 493.6 |
591.9 | 589.6 | 569.6 | 494.5 | |
Al-9Si-3Cu | 589.4 | 587.2 | 567.1 | 492.7 |
588.7 | 587.0 | 566.5 | 493.7 | |
Al-9Si-4Cu | 582.8 | 581.8 | 564.8 | 493.0 |
582.4 | 581.7 | 564.6 | 492.6 |
The
During the solidification of any aluminum hypoeutectic Al-Si-Cu alloys, a dendritic network of primary α-aluminum crystals will be developed. However, as the melt cools, the dendrite tips of the growing crystals begin to impinge upon one another until a coherent dendritic network is formed [4]. The temperature at which the dendrite tips start to touch each other is called dendrite coherency temperature
The rigidity point/temperature indicates the moment during solidification at which the flow of residual melt through interdendritic channels is completely restricted. As Figure 2 shows, the
From Table 2 and Figure 3, it is obvious that any changes in the content of Si have no significant impact on the value of
Finally, the
The addition of Si and Cu into aluminum alloys considerably changes the solidification ranges of these alloys (the difference between
3.2 Fraction solid analysis
The term fraction solid is related to the amount of solid phase(s) formed during melt solidification between liquidus and solidus temperatures, expressed in percentage. Correct information regarding fraction solid is necessary to accomplish computer simulation of casting feed ability as well as to characterize the solidification process and make a prediction concerning the casting structure.
Various methods for determining the fraction solid of casting alloys are presented in the literature [30, 31, 32, 33, 34, 35, 36, 37, 38]. The most commonly used technique employs quantitative metallography. The image analysis system is used to measure the volume fraction of phases formed prior to quenching in a set of melt specimens obtained between the
No | Type of models | Method | Comments |
---|---|---|---|
1. |
|
LINEAR [30] | Latent heat is assumed to vary linearly between liquidus and solidus temperatures. This model has no theoretical basis but is frequently used due to its simplicity. |
2. |
|
LEVER RULE [30} | Solidification in this model is assumed to progress very slowly and the solid and liquid phases coexist in equilibrium in the mushy zone. |
3. |
|
SCHEIL’S [30] | In this model, it is assumed that no solute diffusion occurs in the solid phase and also that the liquid is perfectly homogeneous. |
4. |
|
GRAIN NUCLEATION [32, 37] | The calculation of fraction solid is based on the grain nucleation law and on the assumption that the shape of the grains is spherical. |
5. |
|
HEAT BALANCE [9, 10, 35, 37] | Fraction solid can be calculated by determining the cumulative area between the first derivative of the cooling curve (cc), and the “zero” cooling curve (hypothetical cooling curve without phase transformations) (zc). |
The TA technique has been applied in this work to calculate the distribution of fraction solid between the
Table 4 and Figure 4 summarize the impact of various content of Si and Cu on the distribution of fraction solid at characteristic solidification temperatures (
Alloy |
|
|||
---|---|---|---|---|
|
|
|
|
|
Al-5Si | 0 | 27.5 | 95.2 | 100 |
24.6 | 93.1 | |||
Al-5Si-1Cu | 26.0 | 85.7 | ||
23.2 | 82.5 | |||
Al-5Si-2Cu | 24.0 | 79.7 | ||
21.3 | 76.7 | |||
Al-5Si-3Cu | 19.3 | 73.7 | ||
17.6 | 74.7 | |||
Al-5Si-4Cu | 15.4 | 65.3 | ||
16.5 | 68.7 | |||
Al-7Si | 17.8 | 91.9 | ||
17.5 | 91.2 | |||
Al-7Si-1Cu | 16.7 | 76.4 | ||
18.7 | 78.2 | |||
Al-7Si-2Cu | 14.7 | 65.6 | ||
14.5 | 66.7 | |||
Al-7Si-3Cu | 14.5 | 61.8 | ||
15.8 | 65.5 | |||
Al-7Si-4Cu | 13.9 | 56.4 | ||
14.1 | 55.4 | |||
Al-9Si | 14.1 | 89.1 | ||
12.3 | 90.2 | |||
Al-9Si-1Cu | 12.2 | 73.1 | ||
13.1 | 77.8 | |||
Al-9Si-2Cu | 12.3 | 65.8 | ||
12.8 | 63.1 | |||
Al-9Si-3Cu | 13.3 | 61.3 | ||
13.8 | 60.1 | |||
Al-9Si-4Cu | 12.6 | 56.0 | ||
12.3 | 57.1 |
From Figure 4, it is obvious that with the Cu free Al-Si alloys, the interdendritic feeding region is dominantly independent of the content of Si in the investigated alloy. Increase in the Si content from 5 to 9 wt.% decreased the amount of fraction solid at the DCT from approximately 27% up to 13%. For the same increase of the Si content, the amount of fraction solid which precipitated at Rigidity point decreased from 94% to 89.5%. This means that around 70% of fraction solid precipitated during solidification between Dendrite Coherency and Rigidity temperatures. At the same time, an increase in the content of silicon from 5 to 9 wt.% decreases the amount of fraction solid by almost 50%, which precipitated between
Simultaneously, the amount of fraction solid formed between
4. Conclusion
In the available literature, information related to a quantitative description of the five feeding mechanisms proposed by Campbell is limited. In this paper, the impact of the main alloying elements Si and Cu on different feeding regions of hypoeutectic Al-Si-Cu cast alloys has been studied using the TA technique. It has been shown that both elements have a significant impact on the characteristic solidification temperatures as well as on the amount of fraction solid precipitated at given temperatures. This work has also shown that TA is a valuable tool widely used in aluminum foundries that can collect numerous parameters (characteristic solidification temperatures, fraction solid distribution and others), which are beneficial for a better understanding of the solidification path of hypoeutectic Al-Si-Cu alloys. Applying TA technique as presented in this paper, it is now possible to describe each feeding region quantitatively through a temperature difference related to the total solidification interval or through a different amount of fraction solid that precipitated in each region. It can be assumed that calculated fraction solid at the DCT and fraction solid at
Acknowledgments
Publication of the manuscript is funded by the Lola Institute Ltd. (www.li.rs).
Conflict of interest
All authors declare that they have no conflict of interest in this research.
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