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 [1–7]. 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 [8–10].

applied at numerous aluminum foundry plants. The TA method is simple, inexpensive and provides consistent results. Applying thermal analysis technique some fundamental relationship between cooling or its derivatives curves characteristics, alloy composition and melt treatment can be easier recognized and even better understood. Additionally, the first derivative of the cooling curve has been applied to calculate solid fraction distribution between Tliq and Tsol emperatures [29, 30]. 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

Quantification of Feeding Regions of Hypoeutectic Al-(5, 7, 9)Si-(0-4)Cu (wt.%) Alloys Using…

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 aluminumsilicon 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.

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

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

been determined using optical emission spectroscopy (OES).

them more prone to the formation of shrinkage porosity [31].

DOI: http://dx.doi.org/10.5772/intechopen.90337

2. Experimental procedure

23

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 (TliqÞ, dendrite coherency temperature (TDCTÞ, rigidity (TRigidity) and solidus temperature (Tsol) are important characteristic solidification temperatures of any aluminum alloys, which could be successfully used to delineate transition between various types of feeding mechanisms. All of these characteristic solidification temperatures, as Figure 2 illustrates, can be easily determined using the thermal analysis (TA) technique [12]. The TA has been used for many years in aluminum casting plants as a quality control tool [3, 4, 13–28]. There are many reasons why this more than hundreds of years old technique has commercially

Figure 1.

Five feeding mechanisms recognized during directional solidification.

#### Figure 2.

Characteristic solidification temperatures, determined from the cooling curve, are bordering five feeding mechanisms.

#### Quantification of Feeding Regions of Hypoeutectic Al-(5, 7, 9)Si-(0-4)Cu (wt.%) Alloys Using… DOI: http://dx.doi.org/10.5772/intechopen.90337

applied at numerous aluminum foundry plants. The TA method is simple, inexpensive and provides consistent results. Applying thermal analysis technique some fundamental relationship between cooling or its derivatives curves characteristics, alloy composition and melt treatment can be easier recognized and even better understood. Additionally, the first derivative of the cooling curve has been applied to calculate solid fraction distribution between Tliq and Tsol emperatures [29, 30].

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 aluminumsilicon 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).

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

responsible for reducing the casting characteristics, especially the feeding ability of

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 feed-

The liquidus (TliqÞ, dendrite coherency temperature (TDCTÞ, rigidity (TRigidity) and solidus temperature (Tsol) are important characteristic solidification temperatures of any aluminum alloys, which could be successfully used to delineate transition between various types of feeding mechanisms. All of these characteristic solidification temperatures, as Figure 2 illustrates, can be easily determined using the thermal analysis (TA) technique [12]. The TA has been used for many years in aluminum casting plants as a quality control tool [3, 4, 13–28]. There are many reasons why this more than hundreds of years old technique has commercially

Al-Si-Cu alloys [8–10].

Mass Production Processes

ing, and solid feeding [11].

Figure 1.

Figure 2.

22

mechanisms.

Five feeding mechanisms recognized during directional solidification.

Characteristic solidification temperatures, determined from the cooling curve, are bordering five feeding


3.1 Analysis of characteristic solidification temperatures

DOI: http://dx.doi.org/10.5772/intechopen.90337

The results of the cooling curve analysis are summarized in Table 2. The values of characteristic solidification temperatures (Tliq, TDCT, TRigidity and Tsol) have been determined from the cooling curves or their corresponding first derivatives curves. The dendrite coherency [3] and rigidity [12] temperatures have been determined by applying the two thermocouples method (one thermocouple located at the center

Quantification of Feeding Regions of Hypoeutectic Al-(5, 7, 9)Si-(0-4)Cu (wt.%) Alloys Using…

634.2 624.9 576.7 555.5

628.1 623.4 571.7 499.7

624.9 619.1 568.0 496.8

621.8 617.0 562.0 499.2

617.1 613.2 558.7 501.9

617.6 611.5 576.8 553.4

611.8 604.5 574.0 497.9

607.2 603.3 570.2 495.0

603.2 596.8 566.5 494.0

599.1 593.6 563.8 496.1

600.5 597.6 575.2 552.3

595.8 593.7 572.,4 493.6

591.9 589.6 569.6 494.5

588.7 587.0 566.5 493.7

582.4 581.7 564.6 492.6

Alloy Tliq TDCP TRigidity Tsol Al-5Si 632.9 624.1 575.7 553.4

Al-5Si-1Cu 631.5 623.1 571.4 500.1

Al-5Si-2Cu 625.4 619.5 567.2 497.8

Al-5Si-3Cu 622.5 616.2 562.8 500.6

Al-5Si-4Cu 617.0 613.2 558.7 498.5

Al-7Si 617.8 610.7 576.7 552.0

Al-7Si-1C 612.6 604.5 573.8 498.0

Al-7Si-2Cu 607.4 602.3 570.6 495.3

Al-7Si-3Cu 603.5 598.0 567.1 494.3

Al-7Si-4Cu 599.6 594.0 563.4 497.1

Al-9Si 600.2 595.7 575.0 549.3

Al-9Si-1Cu 597.3 593.9 573.1 494.7

Al-9Si-2Cu 591.9 589.,2 569.5 493.6

Al-9Si-3Cu 589.4 587.2 567.1 492.7

Al-9Si-4Cu 582.8 581.8 564.8 493.0

collected for each alloy).

Table 2.

25

curve analysis.

Two sets of the characteristic temperatures have been collected for each analyzed alloy (two cooling curves have been

Characteristic solidification temperatures of Al-(5, 7, 9)Si-(0–4)Cu (wt.%) alloys determined using cooling

Table 1.

Actual chemical composition (in wt.%) of synthetic Al-Si-Cu alloys.

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 Tliq and Tsol to the total solidification time between these two temperatures. Each TA trial was repeated two times. Consequently, a total of 50 cooling curves were gathered.
