TRABALHO COBEM 2013 - FINAL.pdf

8/12/2019 TRABALHO COBEM 2013 - FINAL.pdf

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1. INTRODUCTION

Aluminum alloys castings had a fundamental role in the growth of the metal-mechanics industry. Nowadaysthese alloys are supplied in a wide range of chemical compositions. We highlight the AlCuSi ternary system becauseof particular outstanding properties such as high mechanical strength, low weight and very good fluidity. Thesequalities make them a good choice for applications in the automotive and aerospace industry. The potential of such

alloys has attracted much attention of researchers with a view to investigating the microstructure evolution during thesolidification process. The presence of dendritic structures during solidification, with concomitant microsegregation, is

of great interest since these solidification features are commonly found in many engineering materials and furthermore,greatly influence the mechanical behavior.

A number of directional solidification studies characterizing primary (1) and secondary (2) dendrite arm

spacings as a function of alloy concentration (C0), tip growth rate (VL) and temperature gradient ahead of themacroscopic solidification front (GL) can be found in the literature (Gunduz and adirli, 2002; Rocha et al., 2003;Ferreira et al., 2010). Bouchard and Kirkaldy (1997) and Garcia (2007) summarized theses studies and grouped them

into two categories: those involving steady state heat flow solidification and those in unsteady-state regime. In theformer category solidification is controlled and the significant controllable variables, GLand VLare kept constant andare independent of each other. In the latter group, which characterizes, for instance, the solidification conditions of a

body of irregular shape, these variables are interdependent and vary freely with time (Ferreira et al., 2010). The analysisof dendritic structures in the unsteady-state regime is very important, since it encompasses the majority of industrialsolidification processes. Investigations on primary and secondary dendritic growth of binary alloy systems during

transient solidification are reported in the literature: Sn-Pb (Rocha et al, 2003A; Rocha et al, 2003B), Al-Cu (Gunduzand adirli, 2002; Rocha et al, 2003A), Pb-Sn (Li et al., 1999), Mg-Al (Zhang et al., 2007). On the other hand, studiesin the field of transient solidification of ternary alloys related to microstructural parameters, solidification modeling,

solute segregation and porosity formation are very scarce (Moutinho et al., 2012). The main difficulty is related to thedetermination of the ternary solidification path and the intermediate phase reactions. The microstructural evolution andgrowth models for ternary alloys cannot be found in the literature.

The gravity effects in relation to the dendritic growth have been investigated with the chill placed in general onthe bottom or top of the mold. In the case of vertical upward directional solidification, the influence of the convection isminimized when solute is rejected for the interdendritic regions, providing the formation of an interdendritic liquid

denser than the global volume of liquid metal. When the process is carried out vertically downward, the systemprovides the melt convection which arises during the process. In the horizontal unidirectional solidification, when thechill is placed on the side of the mold, the convection in function of the composition gradients in the liquid alwaysoccurs. An interesting feature of the horizontal configuration is the gradient of solute concentration and density in

vertical direction because solute-rich liquid falls down whereas free solvent-crystals rise due to buoyancy force.Moreover, there will also be a vertical temperature gradient in the sample as soon as a thermosolutal convection roll

emerges. In spite of these particular physical characteristics, only a few studies (Nogueira et al., 2012) have reportedthese important effects of melt convection and direction of growth on dendrite arm spacings for this particular case.

In this work an experimental approach is developed to quantitatively determine and correlate the solidification

thermal variables such as, tip growth rates (VL) and cooling rates (TR), with primary dendrite spacings (1) of Al-6%wt.Cu-4%wt.Si ternary alloy solidified in unsteady state heat flow conditions. For this purpose, a water-cooledsolidification experimental apparatus was developed.

2. EXPERIMENTAL PROCEDURE

The Al-6wt.%Cu-4wt.%Si alloy was solidified directionally using the casting assembly schematically shown inFigure 1. It was designed in such a way that the heat was extracted only through the water-cooled system placed in the

lateral mold wall, promoting horizontal directional solidification. The carbon steel mold used had a wall thickness of 3mm, a length of 110 mm, a height of 60 mm and a width of 80 mm. The lateral inner mold surfaces were covered with alayer of insulating alumina and the upper part of the mold was closed with refractory material to prevent heat losses.The thermal contact condition at the metal/mold interface was also standardized with the heat extracting surface being

polished.The alloy was melted in situ and heated until a superheat of 10% above the liquidus temperature (TLiq) using an

electrical furnace. Approaching the superheat temperature, the mold was taken from the heater and set immediately on awater cooled carbon steel chill. Water was circulated through this cooling jacket keeping the carbon steel plate during

the solidification at a constant temperature of about 25 0C and thus inducing a longitudinal heat transfer from the mold.Solidification occurred dendritically from the lateral chill surface, forming a columnar structure. During thesolidification process, temperatures at different positions in the alloy samples were measured and the data were acquired

automatically. For the measurements, a set of five fine type K thermocouples, arranged as shown in Figure 1, was used.The thermocouples were sheathed in 1.6 mm diameter steel tubes, and positioned at 5, 10, 15, 30 and 50 mm from the

heat-extracting surface. The thermocouples were calibrated at the melting point of Al, exhibiting fluctuations of about0.40C and 10C respectively, and connected by coaxial cables to a data logger interfaced with a computer. Previousmeasurements of the temperature field were carried out confirming that the described experimental set-up fulfills therequirement of an unidirectional heat flow in horizontal direction.


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(a)

(b)

Figure 1. (a) Scheme of the experimental apparatus for directional solidification and (b) Furnace schematic showingthermocouples located at different positions from the metal-cooling chamber interface. (Silva et al., 2009).

Temperature controller

Data acquisition system

Experimental setup


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Selected transverse (perpendicular to the growth direction) sections of the directionally solidified specimens at5, 10, 15, 20, 30, 40, 50 and 60 mm from the metal-mold interface were polished and etched with a solution of 5%

NaOH in water for micrograph examination. Image processing system Olympus BX51 and Image Tool (IT) softwarewere used to measure primary arm spacings (about 20 independent readings for each selected position, with the averagetaken to be the local spacing) and their distribution range. The method used for measuring the primary arm spacing onthe transverse section was the triangle method (Gunduz and adirli, 2002; Rocha et al., 2003A).

3. RESULTS AND DISCUSSION

Experimental cooling curves for the five thermocouples inserted into the casting during solidification of thealloy investigated in this study are shown in Figure 2.

Figure 2. Experimental thermal responses of temperature vs. time for five thermocouples located at different positions

from the metal-cooling chamber interface. TVis the initial melt temperature.

It is well known that the primary dendritic arm spacings are dependent on solidification thermal variables suchas VLand TRall of which vary with time and position during solidification. In order to determine more accurate valuesof these parameters, the results of experimental thermal analysis have been used to determine the displacement of the

liquidus isotherm, i.e., the thermocouples readings have also been used to generate a plot of position from the metal/mold interface as a function of time corresponding to the liquidus front passing by each thermocouple. A curve fittingtechnique on such experimental points has generated power functions of position as a function of time. Experimentalpositions of liquidus isotherms as a function of time are shown in Figure 3.

Figure 3.

Experimental position of liquidus isotherm from the metal-mold interface as function of time.

0 20 40 60 80 100 120 140 160 180 200 220 240300

350

400

450

500

550

600

650

700

Al- 6wt.% Cu- 4wt.% Si

TV= 684C

TL= 622C

Temp

erature(C)

Time (s)

Termopar 5mm

Termopar 10mm

Termopar 15mm

Termopar 30mm

Termopar 50mm

TL=622C

0 10 20 30 40 50 60 70 80 900

10

20

30

40

50 Al- 6wt.% Cu- 4wt.% Si

Position,P(mm)

Time, t (s)

Experimental

P=1.4(t)0.79

R2= 0.96


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The derivative of this function with respect to time has yielded values for VL. The TRprofile was calculated byconsidering the thermal data recorded immediately after the passing of the liquidus front by each thermocouple. The

method used for measuring the tip cooling rate was used recently by Rocha (Rocha et al., 2003A). Figures 4 and 5show, respectively, these results.

Figure 4.Tip growth rate as a function of position from the metal-mold interface.

Figure 5.Tip cooling rate as a function of position from the metal-mold interface.

Figure 6 presents microstructures of cross section of samples at 10, 30, and 60 mm from metal/mold interface,showing the primary dendrite arms. The dendrite arm spacings were sufficiently distinct to make reasonably accurate

measurements along the casting length. Figure 7 shows the average experimental values of primary dendritic spacingsas a function of distance from the metal-mold interface obtained in this work. It is observed that these dendrite armspacings increase with the distance from the heat-extracting surface of Al-6wt.%-4wt.%Si alloy investigated. In order tocorrelate the primary dendrite arm spacings measured from the afore-mentioned microstructures with solidification

thermal variables, they are plotted as a function of V Land TR in Figures 8 and 9. The average dendritic spacings alongwith the standard variation are presented in these figures, with the lines representing an experimental power function fitwith the experimental points. It is observed that the use of a water-cooled mold imposes higher values of tip growth

rates and cooling rates near the casting surface and a decreasing profile along the casting due to the increasing thermalresistance of the solidified shell with distance from the cooled surface. This influence translates to the observedexperimental values of primary dendritic spacings. As shown in Figure 8, the primary dendrite arm spacing was found

to decrease as the VLis increased. Most of the results from the literature pertaining to 1in binary (Rocha et al,, 2003Aand 2003B) and ternary (Moutinho et al., 2012) alloys also indicate a decrease in spacing with decreasing V L.Furthermore, a power law function characterizes the experimental variation of primary spacings with tip growth ratewith an index of 1.1, i.e. 1 (VL)1.1. It can be observed in Figure 9 that a 0.55 power law characterizes theexperimental variation of primary spacings with cooling rate. This is in agreement with observations reported by Rochaet al (2003A) and Moutinho et al.(2012) that exponential relationships 1 = constant (TR)0.55 best generate the

0 10 20 30 40 50

0.4

0.6

0.8

1.0

1.2

.


VL

(mm/s)

Position, P (mm)

Experimental

VL=1.2(P)

-0.27

R2

= 1

0 10 20 30 40 50 600

5

10

15

20

25

30


TR

(C/s)

Position, P (mm)

Experimental

TR=40(P)-0.93

R2= 0.98


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Figure 7.

Primary dendrite arm spacing as a function of distance from metal-mold interface.

Figure 8.

Primary dendrite arm spacing as a function of tip growth rate.

Figure 9.

Primary dendrite arm spacing as a function of tip cooling rate.

0.4 0.5 0.6 0.7 0.8 0.910

1

102

103

Al-6wt.% Cu- 4wt.% Si

Experimental

103 (V

L)-1.1

R2= 0.89Primarydendritearm

spacing,

1

m)

Tip growth rate, VL(mm/s)

1 1010

1

102

103


Experimental

1=303 (TR)-0.55

R2=0.98

Primarydendritearm

spa

cing,

1(m)

Tip cooling rate, TR(

oC/s)


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4. CONCLUSIONS

The following major conclusions can be drawn from this study, in which Al-6wt.%.-4wt.% Si alloy has beendirectionally solidified under unsteady-state heat flow conditions: Primary dendrite arm spacings were observed todecrease as the tip growth rate or the tip cooling rate is increased. A power law function characterizes the experimental

variation of primary spacings with tip growth rate with an index of 1.1 as well as a0.55 power law characterizes theexperimental variation of primary spacings with cooling rate. The equations obtained in this work that correlates 1as a

function of VLand TR, respectively, are given by 1= 103(VL)-1.1and 1= 303(TR)

0.55, respectively.

5. ACKNOWLEDGEMENTS

The authors acknowledge the financial support provided by IFPA (Federal Institute of Education, Science andTechnology of Par), UFPA (Federal University of Par) and CNPq (The Brazilian Research Council), Brazil.

6. REFERENCES

Okamoto, T.; Kishitake, K., 1975, Dendriticstructure in unidirectionally solidified aluminum, tin, and zinc base binaryalloys,Journal of Crystal Growth, Vol. 29, p. 137-146.

Li, M.; Mori, T.; Iwasaki, H., 1999, Effect of solute convection on the primary arm spacings of Pb- Sn binary alloys

during upward directional solidification,Materials Science Engineering A, Vol. 265, p. 217-223.

Gndz, M.; adirli, E., 2002, Directional solidification of aluminumcopper alloys, Materials Science andEngineering, Vol. A327, p. 167185.

Zhang, C.; Ma, D.; Wu, K.S.; Cao, H. B.; Cao, G. P.; Kou, S.; Chang, Y. A.; Yan. X. Y., 2007, Microstructure and

microsegregation in directionally solidified Mg-4Al alloy,Intermetallics, Vol. 15, p. 1395-1400.

Ferreira I.L.; Moutinho, D.J.; Gomes, L.G.; Rocha, O.L.; Goulart, P.R.; Garcia, A., 2010, Microstructural Development

in a Ternary Al-Cu-Si Alloy during Transient. Solidification.Materials Science Forum, Vols. 636-637, p. 646-650.

Moutinho, D.J.; Gomes, L.G.; Rocha, O.L.; Ferreira, I.L.; Garcia, A., 2012 Thermal parameters, microstructure andporosity during transient solidification of ternary AlCuSi alloy,Materials Science Forum,Vols. 730-732, p.883-

8868.

Rocha, O.L., Siqueira, C.A., Garcia, A., 2003A, Heat Flow Parameters Affecting Dendrite Spacings During Unsteady-

State Solidification of Sn-Pb and Al-Cu Alloys,Metallurgical and Materials Transactions A, Vol.34A, p. 995-1006.

Rocha, O.L., Siqueira, C.A., Garcia, A., 2003B, Cellular/Dendritic Transition During Unsteady-State Unidirectional

Solidification of Sn-Pb Alloys,Materials Science and Engineering A, Vol.347, p. 59-69,.

Garcia, A., 2007, Solidificao:Fundamentos e Aplicaes, Campinas, Editora da Unicamp, 2 ed

Bouchard, D., kirkaldy, J.S., 1997, Prediction of Dendrite Arm Spacings in Unsteady and Steady-State Heat Flow of

Unidirectionally Solidified Binary Alloys,Metallurgical and Materials Transactions B, Vol. 28B, p. 651-663.

Nogueira, M.R.; Carvalho, D.B.; Moreira, A.L.; Dias Filho; J.M.2, Rocha, O.L., 2012, Espaamentos dendrticosprimrios da liga Sn-5%Pb solidificada direcionalmente em um sistema horizontal. Revista Matria, Vol. 17, N. 2,p. 1009-1023.

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