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Microstructural Evaluation of Th ixoforged A380 Alloy Billets Produced by Low Superheat Casting Process

Kerem Altuğ Güler, Alptekin Kısasöz, Ahmet Karaaslan, Gökhan Özer

Yıldız Technical University - Türkiye

Abstract Thixoforging combined with low superheat casting is a kind of semi-solid technique which is especially used for shaping of aluminium casting alloys. In this work first, ingots were cast in low superheat conditions then they were heat up to semi-solid state and subjected to compression deformation. A380

cylindrical steel die to produce an ingot in dimensions 40 mm diameter and 310 mm height. Four pieces in 40 mm height were cut out from the ingot, placed into a furnace and raiSpecimens were hold at this semi-solid temperature in 20, 40, 60 and 80 minutes respectively then placed into preheated open die and uniaxial compression pressure were applied parallel to main axis direction by using a hydraulic press. All specimens were compressed with 50 % deformation ratio. Microscope observations on the as-cast specimen show that, almost non-dendritic structure was formed with low superheat ingot casting. Deformation texture can be recognised in 20 minutes holding specimen and formation of spherical grains can be seen as obvious in micrographs of 40 and 60 minutes holding specimens and 80 minutes holding causes grain growth as well. Moreover Brinell hardness tests were applied for mechanical evaluation of specimens and process. 1. Introduction The semi-solid metal processing has developed as an alternative to the conventional casting and forging processes. It is a kind of net shape fabrication technology in which the shaping alloy takes place at a temperature range that the solid and the liquid phases coexist [1]. The semi-solid metal processing includes two routes; rheo and thixo. Thixoforging belongs to the thixo route. It defines the technique where right material is heated into the two phase region (liquid and solid) and placed into a die then shaped the material in semi-solid state. For successful thixoforging process, microstructure of the alloy must have appreciable melting range and the microstructure of the alloy must consist of solid spheroid grains in a molten matrix. In this state, the

material shows thixotropic behaviour [2, 3, 4]. Thixoforging is a significant producing technique for aluminium based components with high mechanical properties. Especially thixoforged parts have higher densities and mechanical properties than traditional cast parts. In thixoforging process, a semi-solid alloy billet with non-dendritic microstructure is transferred into a preheated die cavity and forged into near-net shaped parts [5, 6, 7]. The low superheat casting is a very suitable technique for preparing semi-solid billet for thixoforging process. It is extremely simple process, low cost and no need for complex equipment. The low superheat pouring leads formation of equiaxed grains and regarded as the simplest alternative to produce thixo billets in non-dendritic spherodized microstructure [5, 8, 9]. In this study, thixoforging process combined with low superheat casting is applied to A380 alloy, which is well known and commonly used in aluminium casting industry. 2. Experimental Procedure 1000 g A380 aluminium alloy was charged into a

resistance furnace. Chemical composition of the alloy is given in Table 1. After dross removal, low superheat casting was carried out by pouring the molten metal into steel cylindrical billet (ingot) mould in 40 mm diameter. The steel billed mould and as cast billet in 310 mm length is shown in Fig 1. Table 1. Chemical composition of A380 aluminium

casting alloy (wt%) Si Fe Cu Mn Mg Zn Al 8.620 0.824 3.091 0.224 0.280 0.9 Bal.

Upper section approximately in 80 mm length, which includes central shrinkage cavity, was cut out form the billet and five specimens in 40 mm length were sliced from remaining part. One of them was hold to examine as cast microstructure. Other four round billet specimens were placed into a heat

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at this temperature for 20, 40, 60 and 80 min. respectively before thixoforging.

Figure 1. Steel billet mould and as cast A380 alloy

billet

Forging is applied to specimens, which were heated to semi-solid zone, by using a simple unidirectional hydraulic press machine. Compaction was carried out in an open die, which was preheated by oxy-propane torch flame, on main billet axis direction with 50 % deformation ratio. As a result, 40 mm heights became 20 mm. A photography of thixoforging process is seen Fig. 2.

Figure 2. A moment of thixoforging process

For microstructural evaluation, as cast and forged specimens were sectioned and cold mounted then

grinded, polished and etched with 0.5 % HF solution. Observations were carried out by using image analyser assisted light metal microscope (Leica ICM 1000). Additionally Brinell hardness values of specimen sections were measured. 3. Results and Discussion

Thixoforging in the open die is a helpful process to observe deformation behaviour of the semi-solid material and differences of this behaviour according to liquid phase ratio. In Fig.3 as cast and thixoforged

and hold 20, 40, 60 and 80 min. respectively, are seen side by side.

Figure 3. As cast and thixoforged round billet

specimens

It is evident that, fragmentation occurrence during open die thixoforging is proportionally related with liquid phase volume, which increases with holding time. Naturally this flow caused fragmentation behaviour converts into shaping ability in closed die forging process. In Fig. 4 as cast micrograph of low superheat casting is given. Low casting temperature combined with cold steel die showed modification effect on

-Al grains and dark zones are fine eutectic structure.

Figure 4. As cast microstructure of round billet

specimen

It is obvious that, low superheat casting and subsequent rapid solidification suppress dendrite growth. In some places secondary dendrite arms were separated and in some other places primary

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-Al grains were formed in shape near-spherical. Solidus-liquidus range of A380 aluminium alloy is

-solid

it is expected that, for sufficient holding time whole eutectic phase is going to be melt. Micrographs of thixoforged round billets are given together in Fig. 5. Microstructure in Fig. 5a belongs to 20 min. heated specimen. It is understood that, there was not any prominent liquefaction occurs in this holding period and because of rapid cooling after forging, recrystallization was not completed so, deformation texture retained in microstructure.

Figure 5. Micrographs (100X) of thixoforged specimens; a) 20 min., b) 40 min., c) 60min.

Examination of 40 min heated specimen microstructure (Fig. 5b) is showed that, spherodizing

-Al was started however eutectic phase did not melt completely and as cast eutectic zones still existing. Mean spherical grain diameter measured form this microstructure is approximately

ng to micrograph given in Fig. 5c, 60 min heated was matured the spherodizing and provides sufficient time for complete eutectic melting. Sphere shaped grain diameter of this

growth can be seen in microstructure of 80 min

Brinell hardness values of as cast and thixoforged specimens are given in Table 2. In these values hardness of 20 min heated specimen is not unexpected. After heating and forging final hardness is a bit higher than hardness of low superheat casting state. Dramatic decrease in hardness of 80 min. heated billet is directly related with grain growth and precipitation growth which are the results of excessive heating. When compared the hardness values of 40 min. and 60 min. heated specimens, it

seems to be contradicted by the grain sizes. In Fig. 6 microstructure of 40 min. and 60 min. heated specimens are given again in lower magnification.

Table 2. Brinell hardness values of as cast and thixoforged specimens

Specimen Brinell Hardness (HB)

As cast 111 20 min heated 112 40 min heated 93 60 min heated 109 80 min heated 82

Figure 6. Micrographs (50X) of 40 min. (left) and

60 min. (right) heated specimens

Liquid phase proportion occurred in 40 min heating is low thus; some of the coarse eutectic zones of as cast structure were survived. However spherodizing was advanced and grains became larger in 60 min. heating time, eutectic phase completely melt down and flow around the sphere grains as the effect of forging. Then solidify in finely dispersed form through the whole structure. Consequently hardness is increased rather than decreased and become nearly equal to hardness of as cast structure. 4. Conclusion Thixoforging combined with low superheat casting gives successful results for A380 aluminium casting alloy.

selected for semi-solid holding temperature. Round billet specimens were hold at semi-solid state for 20, 40, 60 and 80 min. Thixoforging was carried out in 50 % deformation ratio. Low heating times is not sufficient for liquid phase formation and excessive periods cause grain coarsening.

thixoforging gives optimal results. Whole structure transforms into spherical form with avoiding hardness decrease from as cast structure.

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Acknowledgment The work has been supported by the Yildiz Technical University Scientific Research Projects Coordination Department. Project Number 2015-07-02-GEP03. References [1] G. Chung, A. Bolouri and C. Kang, International Journal of Manufacturing Technology, 58 (2012) 237-245. [2] Z. Zhao, Q. Chen, C. Hu, S. Huang and Y. Wang, Journal of Alloys and Compounds, 485 (2009) 627636. [3] S. Chayong, H. V. Atkinson and P. Kapranos, Materials Science and Engineering A, 390 (2005) 312. [4] S. Chayong, H. V. Atkinson and P. Kapranos, Materials Science and Technology, 20 (2004) 490-496. [5] S. C. Wang, N. Zhou, W. J. Qi and K. H. Zheng, Transactions of Nonferrous Metal Society of China, 24 (2014) 2214-2219. [6] Q. Zhu and S. P. Midson, Transactions of Nonferrous Metal Society of China, 20 (2010) 1042-1047. [7] S. Tahamtan and A. Fadavi Boostani, Transactions of Nonferrous Metal Society of China, 20 (2010) 781-787. [8] M. A. Easton, H. Kaufmann and W. Fragner, Materials Science and Engineering A, 420 (2006) 135-143. [9] O. Lashkari, S. Nafisi and R. Ghomashchi, Materials Science and Engineering A, 441 (2006) 49-59.