The Al-Si cast alloys are widely used in the automotive

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1 Microstructural evolution of direct chill cast Al-15.5Si-4Cu-1Mg-1Ni-0.5Cr alloy during solution treatment He Kezhun 1, *Yu Fuxiao 2, Zhao Dazhi 2 and Zuo Liang 1 (1. Key Lab for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang , China; 2. School of Materials and Metallurgy, Northeastern University, Shenyang , China) Abstract: Heat treatment has important influence on the microstructure and mechanical properties of Al-Si alloys. The most common used heat treatment method for these alloys is solution treatment followed by age-hardening. This paper investigates the microstructural evolution of a direct chill (DC) cast Al-15.5Si-4Cu-1Mg-1Ni-0.5Cr alloy after solution treated at 500, 510, 520 and 530, respectively for different times. The major phases observed in the as-cast alloy are α-aluminum dendrite, primary Si particle, eutectic Si, Al 7 Cu 4 Ni,, and Al 2 Cu. The Al 2 Cu phase dissolves completely after being solution treated for 2 h at 500, while the eutectic Si, and phases are insoluble. In addition, the Al 7 Cu 4 Ni phase is substituted by the phase. The α-aluminum dendrite network disappears when the solution temperature is increased to 530. Incipient melting of the Al 2 Cu-rich eutectic mixture occurrs at 520, and melting of the and phases is observed at a solution temperature of 530. The void formation of the structure and deterioration of the mechanical properties are found in samples solution treated at 530. Key words: Al-Si alloy; solution treatment; microstructure; intermetallic compound CLC numbers: TG Document code: A Article ID: (2011) The Al-Si cast alloys are widely used in the automotive industry due to their outstanding castability, good wear resistance and low thermal expansion, together with their low specific weight [1-3]. Heat treatment exerts a significant influence on the microstructure and mechanical properties of these alloys. The most common heat treatment process used with these alloys is solution treatment followed by agehardening [4]. During solution treatment, the alloy is subjected to high temperatures for relatively long periods of time with two main objectives: first, to obtain maximum solubility and homogeneity of the alloying elements in the matrix; and second, to modify the acicular morphology of the eutectic Si to a less detrimental, rounded one [5]. The solution temperature should be high enough and the treatment time long enough to obtain maximum solubility and homogeneity of the alloying elements. However, control of the solution treatment temperature and time is very critical because, if the melting point is exceeded, there is localized melting at the grain boundaries and the mechanical properties are reduced [6] ; while increasing the solution time may result in the coarsening of the structure. The major components in the as-cast Al-Si-Cu-Mg-Ni *Yu Fuxiao Male, born in 1965, Ph.D, Professor. Research interests: direct chill casting, forming and strengthening of Al-Si alloys. fxyu@mail.neu.edu.cn Received: ; Accepted: alloy system are known to be very complex as all the constituent elements can form different phases whose selection depends on the ratio between these alloying elements [7]. The diverse phase constituents in these alloys required a systematic study of the effect of the solution treatment on the microstructure. However, a few studies have been devoted to the microstructural evolution of these alloys during solution treatment. In the present investigation, different solution temperatures (500, 510, 520 and 530 ) and solution times (2, 4, 10 and 24 h) were used to study the microstructural evolution of a direct chill (DC) cast Al-15.5Si-4Cu-1Mg-1Ni- 0.5Cr alloy during solution treatment. 1 Experimental procedure The experimental alloy (nominal chemical composition is listed in Table 1) was prepared using 99.7wt.% pure Al, 99.4 wt.% pure Si, 99.9 wt.% pure Mg, and Al-49.7wt.%Cu, Al-9.85wt.%Ni, Al-9.7wt.%Cr master alloys. The alloy was melted in an induction furnace and DC cast to billets of 100 mm in diameter at a pouring temperature of 830 and a casting velocity of 140 mm min -1. The DC casting process can be described in brief as follows: First, the melt was poured into a water-cooled mold, and the melt started to solidify and formed a solid shell, then the solid bottom part was withdrawn from the mold and the solid side surface was directly cooled by water jets; in such a way that the billet was produced. 264

2 August 2011 Table 1: Nominal chemical composition of experimental alloy studied in present work (wt.%) Si Cu Mg Ni Cr Al Bal. Samples sectioned from the ingot were solution heat treated at different temperatures (500, 510, 520 and 530 ) in a salt bath for 2, 4, 10 and 24 h, respectively. After solution treatment, the samples were quenched in water. The samples were ground and polished following standard metallographic practices. The microstructure was examined using an optical microscope (Leica DMI5000M) and a scanning electron microscope (SUPRA35.LEO). The eutectic Si particle characteristics (area and density) were measured for as-cast and solution treated samples using software of Image-Pro Plus 6.0. For each sample, three random fields were examined on the sample surface at a magnification of Results and discussion Research & Development 2.1 Microstructures of as-cast alloy Figure 1 shows the as-cast microstructure of Al-15.5Si- 4Cu-1Mg-1Ni-0.5Cr alloy. It consists of primary Si and α -aluminum dendritic halos with eutectic Si and complex inter-metallic compounds segregated into the inter-dendritic regions. The polygonal primary Si particles and irregular eutectic Si are dark grey, while the inter-metallic phases are light grey in color. The high-magnification micrograph in Fig. 1 reveals that the eutectic Si flakes are unmodified and that the inter-metallic phases precipitate in fibrous or blocky morphologies in the inter-dendritic regions. Fig. 1: Microstructures of as-cast alloy: low-magnification; high-magnification of the region in To study the complex eutectic mixture, EDS analysis using the scanning electron mcroscope (SEM) was carried out to identify the finer intermetallic compounds. As shown in Fig. 2, four intermetallic phases were quantified: Al 2 Cu (θ), Al 7 Cu 4 Ni (γ), Al 5 Cu 2 Mg8Si 6 (Q) and. The volume fractions of the γ and Q phases are large, while that of the θ phase is small. The blocky phase has a complex elemental constitute, and the EDS analysis reveals its chemical composition is close to. The extremely fine eutectic pocket (as fine fibers intermixed with aluminum) comprises Al, Si, Q and γ phases. Al 7 Cu 4 Ni Al+Si+Q+γ Al 2 Cu Eutectic Si Al 2 Cu Al 7 Cu 4 Ni Fig. 2: Details of inter-metallic phases with their corresponding EDS analysis 265

3 2.2 Microstructural evolution α-aluminum dendrite Figure 3 shows the microstructures of the solution treated samples. For samples solution treated below 530, the α-aluminum dendrite network did not undergo much change, even with a prolonged solution time of 24 h at a solution temperature of 520. However, the dendrite network disappeared completely [see Fig. 3(c)] when the solution temperature was increased to 530. Fragmentation Incipient melting Necking (c) (d) Void formation Coarsening h; h; (c) h; (d) h Fig. 3: Microstructures of samples solution treated under different conditions Eutectic Si The morphology change of the eutectic Si is obvious after solution treatment. The plate-like eutectic Si in as-cast case [shown in Fig. 1] was broken into small particles. The fragmentation process was accelerated rapidly by increasing the solution temperature, as can be seen in Fig. 3 and Fig. 3. For a given solution temperature, the aspect ratio of the eutectic Si decreases with the increase of solution time [see Fig. 3 and Fig. 3(d)], which indicates that the Si particles underwent coarsening. Generally speaking, the eutectic Si experienced two stages during solution treatment: fragmentation and coarsening. When the alloy was kept at a solution temperature for a period of time, shape perturbation in the Si particles began to arise until the particles were broken into a series of spherical crystals. This process happened due to the instability of the interfaces between two different phases and was driven by a reduction in the total interfacial energy. Subsequently, the Si particle size began to increase as a result of particle coarsening if its size was larger than the critical volume, whereas smaller particles dissolved into the larger ones according to Ostwald ripening [8]. Figure 4 gives the parameters of the eutectic Si particles after solution treatment. The average Si particle area increases with the increase of solution time, while that of the Si particle density decreases. It can be inferred from Fig. 4 that the fragmentation stage of eutectic Si did not finish until the solution time reaches 4 h when solution treated at 500 as the average Si particle area decreased with solution time from 2 h to 4 h; while the opposite situation was found with the samples solution treated at 510 and 520. After solution treated at 520 for 24 h, the average Si particle area increased to μm 2, while the density decreased to 5,165 particles per square meter. The decrease in eutectic Si particle density supports the assumption that the smaller Si particles dissolved in the larger ones, which resulted in the coarsening of the particles. This is in accordance with the phenomenon of Ostwald ripening mentioned before Intermetallic phases Figure 5 shows the characteristic of the intermetallic phases after solution treatment. The complex eutectic mixture experienced fragmentation in the early stage of solution treatment, and a large number of individual particles were formed. EDS analysis revealed that the θ phase dissolved completely after solution treatment for 2 h at 500, whereas the γ phase was substituted by the (d ) phase, which can still be detected after solution treatment for 24 h at 520 ; the EDS spectra of the phase is shown in Fig.6. On the other hand, the Q phase underwent spheroidization during solution treatment at temperature below 520. Other researchers reported that the Q phase has a low solubility in solid solution [9], being insoluble at solution temperature as high as 530 [10]. In addition, the Al 15 (Cr,Fe,Ni,Cu) 4 Si 2 phase retained its blocky morphology, being insoluble at all the experimental solution temperatures. The 266

4 August 2011 Research & Development Fig. 4: Evolution of eutectic Si particle parameters of the solution treated alloy as a function of solution time in the temperature range of 500 to 530 : average Si particle area and Si particle density (c) (d) Fig. 5: Evolution of intermetallic phases solution treated at 500, 510, 520 (c) and 530 (d) for 2 h, respectively Fig. 6: EDS spectra of phase stability of this phase can be attributed to the multiple elements it consists of. This observation is in agreement with the findings of Moustafa et al. [11], who reported that increasing the number of elements in an intermetallic compound increases its stability during solution heat treatment Incipient melting Incipient melting occurred as soon as the solution temperature rose to 520, which is evidenced by the black spots observed in the eutectic mixture shown in Fig. 3 (marked by the arrows); it corresponds to the melting of the θ-rich eutectic mixture. Further increasing the solution temperature to 530 caused the microstructure to change again [see Fig. 5(d)]. Melting of the Q phase was almost complete; a fraction of the d phase melted and this led to liquid droplets within the grains; the formation of voids was severe which caused a deterioration in the structure of the material. 3 Conclusions Microstructural evolution of DC cast Al-15.5Si-4Cu-1Mg-1Ni- 0.5Cr alloy after solution treated at 500, 510, 520 and

5 for different times were studied. Based on the present findings, the following conclusions can be drawn: (1) The major phases formed in the as-cast alloy were α -aluminum dendrite, primary Si particle, eutectic Si, A l7 Cu 4 Ni, A l5 Cu 2 Mg 8 Si 6, Al 15 (Cr, Fe, Ni, Cu ) 4 Si 2 and Al 2 Cu. (2) During solution treatment, the eutectic Si, and phases were insoluble; the A l7 Cu 4 Ni phase was substituted by the phase; the Al 2 Cu phase dissolved completely after solution treatment for 2 h at 500 ; and the α -aluminum dendrite network disappeared when the solution temperature was increased to 530. (3) Incipient melting of the Al 2 Cu-rich eutectic mixture occurred as soon as the solution temperature was increased to 520 ; melting of the A l5 Cu 2 Mg 8 Si 6 and phases was observed at a solution temperature of 530. References [1] Timmermans G and Froyen L. Fretting wear behavior of hypereutectic P/M Al Si in oil environment. Wear, 1999, 230(2): [2] Lasa L and Rodriguez-ibabe J M. Effect of composition and processing route on the wear behaviour of Al-Si alloys. Script. Mat., 2002, 46(2): [3] Kong Fanxiao. Study on Hypereutectic Al-Si Piston Alloy [Dissertation]. Chongqing, College of Mechanical Engineering, Chongqing University, (in Chinese) [4] Lasa L and Rodriguez-ibabe J M. Evolution of the main intermetallic phases in Al-Si-Cu-Mg casting alloys during solution treatment. J. Mater. Sci., 2004, 39(4): [5] Prasad B K. Structure-property related changes in a hypoeutectic aluminium-silicon alloy induced by solutionizing. Mat. Trans., 1994, 35(12): [6] Apelian D, Shivkumar S and Sigworth G. Fundamental aspects of heat treatment of cast Al-Si-Mg alloys. AFS Trans., 1989, 97: [7] Belov N A, Eskin D G and Avxentieva N N. Constituent phase diagrams of the Al-Cu-Fe-Mg-Ni-Si system and their application to the analysis of aluminium piston alloys. Act. Mater., 2005, 53(17): [8] McAdam C L and Jenkinson D C. The stability of silicon fibres in aluminium-silicon alloys and their influence on mechanical properties. In: Proc. 27th Annual Congress of the Australian Institute of Metals, Melbourne, Australia, Australian Institute of Metals., 1974: [9] Ouellet P and Samuel F H. Effect of Mg on the ageing behaviour of Al-Si-Cu 319 type aluminium casting alloys. J. Mater. Sci., 1999, 34(19): [10] Gupta A K, Jena A K and Chaturvedi M C. Insoluble phase in Al-1.52Cu-0.75Mg alloys containing silicon. Mater. Sci. Tech., 1987, 3(12): [11] Moustafa M A, Samuel H and Doty H W. Effect of solution heat treatment and additives on the microstructure of Al-Si (A413.1) automotive alloys. J. Mater. Sci., 2003, 38(22): This work was financially supported by the National High-Tech Research and Development (863) Program of China under grant No. 2007AA03Z516, No. 2008AA and No. 2009BAE80B01, and the National Natural Science Foundation of China under grant No & No