Experiments

3.2.1. Thermal Analysis

Thermal analysis became widespread technique for evaluating melt preparation of aluminum alloys in the 1980’s [1, 2]. Basically, thermal analysis is performed by pouring the molten metal into a cup equipped with a thermocouple to record the temperature changes against the time. The solidification reactions occurring during the process are detected as changes in thermal profile of the curve. The cooling rate is only controlled by the heat loss by the cup wall to the surrounding environment. Several parameters have been proposed in the literature for characterizing the thermal effects seen on the cooling curves which are associated to nucleation and growth of (Al) phase (grain refinement) and to the (Al)-Si eutectic (eutectic modification) [3-8]. Development of computer aided cooling curve analysis also extended the use of the thermal analysis with the capability to detect minor reaction through the use of cooling curve first derivative [9]. For quaternary alloy or commercial alloy the solidification process continues after the (Al)-Si eutectic until the final reaction where secondary precipitation such as phi phase, Al2Cu and/or Mg2Si occurs. With high cooling

rates, the thermal arrest associated with secondary reaction is difficult to detect. Therefore, extrapolation from the final solidification reaction is used to track the end of eutectic plateau.

Grain size prediction in cooling curve thermal analysis is usually linked with undercooling and recalescence during the nucleation phenomena, where no or lack of recalescence indicates a good nucleation which leads to fine grain size assessment [8].

Thermal analysis can also be used to predict the modification level of the alloys by analyzing the cooling and the eutectic temperature depression. Other than temperature parameters, time parameters can be used to characterize the cooling curves, see section 3.2.3.

3.2.2. Instrumented casting

Cooling curve analysis was performed on a series of A356 alloys with chemical composition listed in Table 3.1. Most of the grain size predictions in the cast house are made by using standard thermal analysis cup (quick cup) with very low thermal coefficient, in which the cooling rate is controlled at low or moderate value. However, in real castings, the shape of the casting components varies and this could induce different cooling rates across the part affecting the final grain size and modification of the materials. Therefore, experimental castings were prepared to study the effect of cooling rate.

The cooling curves were recorded using Thermolan-Al system. The experimental procedure consisted in casting the liquid alloys into sand and metallic moulds with different casting modulus (ratio of outer surface to volume) according to the schematic in Figure 3.2.

These moulds were designed for casting cylindrical samples with various thermal moduli (TM) through variation in h =. The TM values and the cooling rates achieved are listed in Table 3.2.

Figure 3.2. Schematic of the mould used showing six cylinders with different thermal moduli.

At the time of casting, a standard cup for thermal analysis (TA) with TM=0.605 cm was also poured and its cooling curve was recorded along with that of other moduli. Type K thermocouple was used in this experiment with temperature error from -0.9 to 1.1 °C.

Table 3.2. Thermal moduli (TM) and ranges of eutectic cooling rate (CR_e) of cylinder test samples.

Sand mould TM (cm) 1.5 1.15 1 0.8 0.6 0.4

-CRe(°C/s) 0.13-0.20 0.17-0.32 0.21-0.39 0.28-0.72 0.65-1.1 2.4-2.9

-Metallic mould TM (cm) - 1.15 1 0.8 0.6 0.4 0.3

CRe(°C/s) - 1.5-2.8 2.6-3.0 3.5-5.6 4.7-6.5 9.4-12.5 9.9-18.1

3.2.3. Cooling curve analysis

The characteristic parameters obtained from the cooling curves were extracted according to the nomenclature shown in Figure 3.3. All values are listed in Appendix 2.

Figure 3.3. Parameters taken from cooling curves for characterizing primary (Al) precipitation and (Al)-Si eutectic. Insert image showed enlarged area of liquidus.

Not all the data from the series was used in the calculation because some of the cooling curves could not be recorded due to thermocouple failure. Moreover, high cooling rate for small modulus is causing a fast heat release by the mould and large time gap interval for thermocouple data acquisition. Thus, the initial solidification temperature of the small cylinders could not be recorded properly. Some parameters related to the whole solidification process were selected to check for the overall reproducibility of the data records. Finally,

some of the data when the initial temperature was less than 620°C were excluded from further analysis. The cooling curve parameters considered in present work are defined as follows:

 For the nucleation of (Al), parameters related to temperature: minimum temperature (TN,min), maximum temperature (TN,max) and recalescence (TN= TN,max - TN,min), as seen in Figure 3.3. In the case of no nucleation recalescence, the TN,max was obtained from the maximum value of the first derivative of the cooling curve, as seen in Figure 3.4.

 total nucleation time (tN) was measured from the primary (Al) nucleation time start (t1) to the extrapolation time (t2)

 Vmaxrecalescence was acquired from the maximum value of the first derivative of the cooling curve.

 Absolute nucleation undercooling was obtained from temperature difference between calculated liquidus (Tliq) and maximum nucleation temperature, max (TN,max).

 Liquid cooling rate (CR_L) was acquired from the slope of the cooling curve (625°C -620°C) before the primary (Al) nucleation reaction (T_N,min).

 Eutectic cooling rate (CRe) was measured from the slope of the cooling curve (600°C to 575°C) after primary (Al) nucleation and prior to the (Al)-Si eutectic reaction.

 t_coales was measured from the primary (Al) nucleation start (t₁) to (Al)-Si eutectic start (t3). In the case of no recalescence, the time was evaluated between the maximum values of the first derivative of the cooling curve, see Figure 3.4.

 tf was measured from the primary nucleation time start (t₁) to the final solidification reaction (t4).

 For the eutectic reaction, parameter related to temperature: minimum eutectic temperature (Te,min), maximum eutectic temperature (Te,max) and the recalescence (T_e= T_e,max- T_e,min). In the case of no eutectic recalescence, the T_e,max was obtained from the maximum value of the first derivative of the cooling curve, as seen in Figure 3.4.

 teut (eutectic time) was measured from the (Al)-Si eutectic start (t₃) to the final solidification reaction (t4).

 Td = TR - Te,max, is the eutectic depression, where TR is the equilibrium eutectic temperature calculated using an equation obtained by updating the one proposed by Mondolfo (Appendix 3).

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Figure 3.4. Method to extract the characteristic T_N,_maxand T_e,maxfrom the cooling curve when no recalescence shows up.

3.2.3. Eutectic reference temperature

Concerning eutectic modification, as already stated before the most used method is to correlate the effectiveness of modification with increased eutectic undercooling or eutectic depression. Evaluation of this undercooling requires the knowledge of a eutectic reference temperature which should be given by the relevant phase diagram and has often been evaluated experimentally as the eutectic temperature of the unmodified alloy. However, in many cases, this latter cannot be obtained in a cast shop due the possibility of prior modification treatment to the alloys or the use of returns in the charge.

One of the most used methods to evaluate the reference temperature of the (Al)-Si eutectic was proposed by Apelian et al. [5] who derived an equation expressing the eutectic temperature as a function of alloy's composition from ternary phase diagrams compiled by Mondolfo [10]. The so-called “Mondolfo's equation” is still widely accepted as seen in the recent work by Wang and Lu [11]. In this work, we first reconsider the derivation of the equation and then update it according to more recent assessed phase diagram information (Appendix 3).

There are several other approaches proposed to calculate the reference eutectic temperature. Study by Joenoes and Gruzleski [12] focused on the magnesium effect and proposed a series of empirical calculations with a coefficient depending on the Si content in the alloys. While those equations only consider one element in the calculation and are limited to one type of chemical composition, Sthuldreier et al. [13] considered 3 major elements, Mg, Cu and Fe, based on their experimental data. A different approach was used by Djurdjevic

binary Al-X systems of interest and then defined a silicon equivalent (Sieq) for each element X.

We then review other approaches proposed in the literature and finally compare them to available experimental data from literature. Detailed work can be seen Appendix 3.