Microstructure evolution during sand casting in AZ80 Mg alloy

Transcription

1 Microstructure evolution during sand casting in AZ80 Mg alloy Rahul R. Kulkarni 1 *, Nithyanand Prabhu 1, Bhagwati P. Kashyap 1 Indian Institute of Technology Bombay, Powai, Mumbai * Corresponding Author: rrk2082@gmail.com Abstract- AZ80 Mg alloy is popular because of its low cost, low weight and good castability. Evolved morphologies and the mechanism of evolution during sand casting of this alloy are studied in present research. The as-cast microstructure of this alloy consists of partially divorce eutectic, fully divorce eutectic and lamellar eutectic. These morphologies are due to differential cooling rates during casting. Keywords: As-cast AZ80 Mg alloy, eutectic microstructures, differential cooling rate I. INTRODUCTION Aluminum is the major alloying element in light weight Mg alloy due to lower cost, good corrosion resistance, and good castablity and strength properties [1-5]. AZ80 Mg alloy is one of the alloys with Al as a major alloying element. Mg-Al phase diagram(figure 1) shows the two equilibrium phases as α-mg and γ- Mg17Al12. Further, these phases are due to eutectic reaction occurring at 437 C and at 33 wt% Al. Due to good casting characteristics, usual method of manufacturing of this alloy is casting. 198

2 Figure 1. Mg-Al phase diagram. [6] Previous study in AZ91 alloy shows various morphologies of the above phases during solidification of casting [7-8]. The study of these morphologies is important because it influences the mechanical properties of these alloys. Sand casting is one of the casting methods used for AZ80 Mg alloy. In present investigation, the morphologies of these phases are studied to get better understanding about the microstructure evolution during solidification of AZ80 Mg alloy during sand casting. II. EXPERIMENTAL PROCEDURE AZ80 Mg alloy with 8.5 wt% Al, 0.5 wt% Zn was used for experimentation. Alloy is made using sand casting procedure. The specimen of size 10 mm x 10 mm x 10 mm was cut from ingot and used for metallography study. Specimen for microstructure study was prepared by mechanical polishing, and final polishing was done using 0.25 µm diamond paste. Then, specimen was cleaned by ethanol using ultrasonic cleaning.etching was done with 10 ml HF and 90 ml water (which makes α-mg as white and γ-mg17al12 as dark) and acetic picral (5 ml acetic acid, 6 gm picric acid, 10 ml water, and 100 ml ethanol) to reveal the grain boundaries. Microstructure study was done using optical microscope and SEM (Scanning Electron Microscopy)- EDS (energy dispersive spectrometry) of model Hitachi S-3400N. III. RESULTS AND DISCUSSION A. Microstructure evolution The microstructure of sand cast AZ80 Mg alloy was analyzed at three positions. As the aim of the present study is to study the morphologies evolved during sand casting, there will not be further distinction about these positions investigated. The polished and etched specimens were observed under optical microscope at different magnifications, using etchants as acetic picral (Figure 2 (a)) and HF (Figure 2 (b)). The grain boundaries are revealed by acetic picral. The optical images were taken at different magnifications with Olympus software attached to image analyzer. The microstructure of the as-cast sample clearly shows the presence of both matrix α-mg phase and γ-mg17al12 precipitate. Further, the presence of γ-mg17al12 is confirmed by the energy dispersive spectroscopy (EDS) attached to the scanning electron microscope (SEM). EDS analysis shows that atomic ratio of Mg to Al in white phase is 60.39:37.57 = 1.61, which nearly equals to actual atomic ratio in Mg17Al12, viz. 17:12 = This confirms the white phase to be γ-mg17al12. Further, it is confirmed by line scan which was taken on the microstructure. Figure 3 reveals the aluminum rich and aluminum depleted phases; the line scan shows the depletion of Mg in the phase where Al is rich and vice-versa. As cast microstructure consists of mainly three parts: α-mg (both primary and eutectic), γ-eutectic particles and lamellar γ-precipitates (cooling rate of casting being sufficiently low, discontinuous precipitation can occur). When an alloy is solidified, it follows the primary solidification metallurgy as phase α-mg will grow dentrically by rejecting the Al and Zn 199

3 into the inter-dendritical liquid. When this region accumulates the Al, which is having eutectic composition, γ- Mg17Al12 will grow with further under cooling of the alloy. This is known as divorce eutectic solidification. (a) 200

4 (b) Figure 2. Optical micrographs showing various morphologies by using etchants (a) Acetic picral (b) HF. Figure 3. Line scan in EDS showing Al rich and depleted region in microstructure of AZ80. This eutectic growth morphology has been studied by Nave et al [6]. According to them, such morphologies were developed because of the differential cooling rate during solidification. The detailed mechanism for these morphologies is described as follows. The evolution of these morphologies can be explained on the basis of phase diagram and differential cooling rate. Conventionally, the evolution of equilibrium phases are due to isothermal cooling, i.e. it is evolved by considering very slow cooling rate to reach stable phase condition. But, in actual practice, there occurs a variation in cooling rate during solidification of Mg-Al-Zn alloy casting. Nave et.al (2000) reported the changes in the morphologies of second phase (i.e. Mg17Al12) with the cooling rate due to non-equilibrium cooling. 201

5 Further, solidification of this alloy could be studied with the help of phase diagram. Since AZ80 alloy contains zinc (0-0.5 wt %), it is important to study the effect of Zn addition on the Mg-Al phase diagram. Figure 1 shows the Mg-Al phase diagram/equilibrium diagram which shows the presence of α-mg and γ-mg17al12 at room temperature. The microstructure of AZ80 Mg alloy after solidification can be studied fromthe non-equilibrium phase diagram presented in Figure 4. As mentioned above, as-cast microstructure of AZ80 alloy can be characterized by three morphologies as- 1. α-mg (both primary and eutectic) 2. γ-eutectic particles 3. Lamellar γ-precipitates (with cooling rate of casting being sufficiently low,discontinuous precipitation occurs) When this alloy is solidified, it follows the primary solidification metallurgy. The phase α-mg will grow dendritically by rejecting the Al and Zn into inter-dendritical liquid. When this region accumulates the Al, which is having eutectic composition, γ-mg17al12 will grow with further under-cooling of the alloy. This is known to be as divorce eutectic solidification [9]. This eutectic growth has been studied by Nave et.al (2000) in AZ91 Mg alloy. The chart presented below gives the two parts of eutectic growth (i) fully divorce eutectic (ii) partially divorce eutectic. The mechanism behind formation of these morphologies was proposed by Nave et al (2000) in AZ91 alloy [6], which is explained in subsequent paragraph. Eutectic Growth Partially divorce eutectic Fully divorce eutectic Temperature TE to T2 Temperature TE to T3 A partially divorce eutectic morphology is characterized by island of eutectic α-mg within the γ-mg17al12 phase, but the bulk of the α-mg within the γ-mg17al12 particle is much lower than the proportion predicted by the equilibrium phase diagram. A fully divorce eutectic morphology is where the two eutectic phases are completely separate in the microstructure. Each interdentric region consists of a single γ- Mg17Al12 particle surrounded by eutectic α- Mg which has grown from the primary dentrites. 202

6 Figure 4. Mg-Al phase diagram (dotted lines indicate non-equilibrium solidus and liquidus) [10]. B. Partially divorce eutectic: This morphology is favorable when the cooling rate of casting is low. This type of eutectic can be discussed with the help of Figure. Consider that this alloy is cooled (TE-T2) within the short time Δt. For the slow cooling rate (TE-T2)/ Δt, α-mg will grow following the non-equilibrium cooling. Second phase γ-mg17al12 will form with 40% Al composition after Δt. It will grow by making Al depleted region in the liquid. It is obvious that the last liquid which solidifies has depletion of Al and this, in turn, favors the α-mg to form the island in the matrix of Mg17Al12. Therefore, this morphology is characterized by island of α-mg in the γ- Mg17Al12. C. Fully divorce eutectic: This morphology is favorable when the cooling rate during solidification of casting is high (TE-T3)/Δt. When cooling rate is high, formation of Mg17Al12 is difficult. It will grow without much change in the Al content (nearly 37%) in the remaining cooling rate. Therefore, this morphology is characterized by γ-mg17al12being surrounded by α-mg as shown in Figure

7 D. Lamellar eutectic: This eutectic is formed due to very slow cooling rate i.e. at the end of the solidification. The various mechanisms are proposed previously in Mg-Al alloy [11-14]. One of the mechanisms is due to change in chemical composition by diffusion process. Cellular precipitation is also known as discontinuous precipitation because of the sudden change in composition across the moving grain boundary as shown in Figure 2.On the other hand, non-cellular precipitation is known as continuous because the composition of the matrix phase decreases continuously at any point. In general, continuous precipitation leads to much better mechanical properties. This can be attributed to the more uniform distribution of precipitates, which nucleate throughout the matrix (on dislocations for example) and hence, much smaller size distribution as opposed to discontinuous precipitation. IV. CONCLUSIONS As cast AZ80 alloy consists of α-mg, partially and fully divorce eutectic of α and γ-mg17al12, and lamellar eutectic of α and γ phases. These morphologies are dependent on cooling rate during solidification of casting. Partially divorce eutectic morphology in the microstructure is characterized by islands of α-mg in the matrix of γ-mg17al12 which is in the matrix of α-mg. No such characterization is observed in case of fully divorce eutectic.lamellar eutectic of γ-mg17al12 and α-mg is due to very slow cooling rate as specified by the usual eutectic phase diagram. V. REFERENCES [1]H. Friedrich, S. Schumann, Research for a New age of Magnesium in the Automotive Industry, J. Mater. Process. Technol.,vol. 117, 2001, pp [2] B.L. Mordike, T. Ebert, Magnesium: Properties-applications-potential, Mater. Sci. Eng. A, vol. 302, 2001, pp [3] B. Smola, I. Stulikova, F. von Buch, B.L. Mordike, Structural aspects of high performance Mg alloys design, Mater. Sci. Eng. A, vol. 324, 2002, pp [4] S. Kleiner, E. Ogris, O. Beffort, P.J. Uggowitzer, Semi-Solid Metal Processing of Aluminum Alloy A356 and Magnesium Alloy AZ91: Comparison Based on Metallurgical Consideration, Adv. Eng. Mater., Vol. 9, 2003, pp [5]K.N. Braszczyn ska-malik, The Study on the Shaping of the Microstructure ofmagnesium Aluminum Alloys,WIPMiFSPCz., [6] ASM Handbook Committee, Alloy Phase Diagram, ASM International, Ohio, USA, [7] M.D. Nave, A.K. Dahle, D.H. StJohn, Magnesium technology 2000, in: H.I. Kaplan, J.N. Hryn, B.B. Clow (Eds.), the Minerals, Metals and Materials Society (TMS), Warrendale, PA, USA, 2000, pp [8]M.D. Nave, A.K. Dahle, D.H. StJohn, Magnesium technology 2000, in: H.I. Kaplan, J.N. Hryn, B.B. Clow (Eds.), the Minerals, Metals and Materials Society (TMS), Warrendale, PA, USA, 2000, pp [9]Y. K. Yang, H. Dong, H. Cao, Y. A. Chang, S. Kou, Liquation of Mg Alloys in Friction Stir Spot Welding, supplement to the welding Journal, Vol. 87, 2008, pp [10] T. Zhu, Z. W. Chen, W. Gao, Microstructure formation inpartially melted zone during gas tungsten arc welding of AZ91 Mgcast alloy, Mater. Char., Vol. 59, 2008, pp [11] K. N. Tu and D. Turnbull, Acta metall. 15 (1967) [12] K. N. Tu and D. Turnbull, Acta metall. 15 (1967) [13] K. N. Tu, Metall. Trans. 3 (1972) [13]R. A. Fournelle and J. B. Clark, Metall. Trans. 3 (1972)