UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

1110 IMMC 2018 | 19th International Metallurgy & Materials Congress

Melt Quality Increasing Study with Grain Refi nement in 356 Aluminium Alloy

Özen Gürsoy, Eray Erzi, Derya Dışpınar

Istanbul University, Faculty of Engineering, Department of Metallurgical and Materials Engineering, İstanbul, Turkey

Abstract

Grain refinement with Ti-based master alloys is a classic application for aluminium casting industry. It is claimed that the grains are refined and thus improved mechanical properties through the constitution of heterogeneous nucleation center Ti compounds such as TiAl3 in liquid aluminium. However, the relation between the presence of bifilms in liquid aluminium and grain refiners is not adequately mentioned. In this study, the ratio of wt.0.1% Al-5Ti-1B master alloy is added into 356 aluminium alloy has high bifilm index (BI). Ti ratio and BI chancing of the melt are succeeded with produced reduce pressure test (RPT) samples. Into die and sand moulds step samples for microstructure analyses and tensile samples for tensile tests are poured. As a result, it is found that titanium compounds rapidly migrate to the bottom of the crucible after addition and also sweep down the bifilms.

1. Introduction

A356 aluminium alloy is most commonly used and researched casting alloy in the aerospace and automotive industries. In practice, A356 is cast basically two different moulds: die and sand. If it’s necessary to compare, it has been claimed that much higher mechanical properties are obtained by using die mould in casting. It is known that a similar effect is observed by using Al-Ti-B grain refiner master alloy [1-3]. Microstructural differences have been shown as the main reason of this.

Grain refinement with Al-Ti-B master alloy is the most commonly used method in aluminium casting processes. With the addition of grain refiner, aluminium matrix is dissolved and released TiB2 and TiAl3 particles heterogeneously nucleate -Al during solidification [1]. Thus, higher mechanical properties are obtained by means of fine equiaxed grains. However, due to the density differences between liquid aluminium and grain refiners, the particles show settling behaviour when they are added to liquid aluminium. Therefore, the efficiency of Al-Ti-B master alloys has been a long going discussion by many researchers [2,3].

Bifilms whose density is almost equal to the molten metal is suspended in liquid aluminium [4,5]. Time dependently, settling of TiB2 and TiAl3 particles sweep down the bifilms to bottom of the crucible after grain refiner addition. This phenomenon directly influences the melt quality, microstructure, and mechanical properties.

2. Experimental Procedure

In this study, the effect of Ti-based inoculant on bifilm existence was investigated. 356 aluminium alloys were supplied from automotive industry’s scraps. The chemical composition of the 356 alloy as can be seen in Table 1. The sand moulds were prepared by mixing silica sand, resin, and hardener. The charge was pre-heated at 400 ºC for 1 hour and then the casting temperature was set at 725 ºC. Two different casting conditions, with (ratio of wt. 0.1%) and without Al-5Ti-1B addition, were applied. The pouring was begun 10 minutes after grain refinement master alloy addition. In these conditions, tensile bars and step samples are poured to analyze mechanical properties and microstructure respectively into either die or sand moulds. At the same time, bifilm index is determined for each condition by Reduce Pressure Test (RPT) machine to determine the melt quality changing. No sample was heat treated.

Table 1. Chemical composition of the 356 alloy (wt. %). Si Fe Mg Ti B Sr

7.31 0.13 0.26 0.11 0.001 0.008

3. Results and Discussion

Residual Ti is a usual situation in seconder aluminium alloys. Therefore, although no grain refiner is added the Ti ratio is over 0.1% in the as-received alloy As can be seen in Figure 1, according to spectral analyses there is a slight decreasing of Ti ratio in the melt for the as-received alloy. However, in the Al-5Ti-1B grain refiner added melt, noticeable increasing of Ti content toward the bottom zone of the molten metal was observed.

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Figure 1. Change of Ti ratio in different zones of the crucible during casting prosses.

BI analyses show clear time depending trend which is decreasing in both conditions. Interestingly, although the molten alloy had the same initial BI, Al-5Ti-1B master alloy added melt had lower quality. Master alloy addition decreases the melt quality because of oxide film entrapment during addition. Also, in both melts, BIs increase in the bottom zone of the molten metal, as can be seen in Figure 2. This is a good evidence of bifilm accumulation in the bottom of the crucible accompanied by the Ti settlement.

Figure 2. BI changes of molten metals.

In the microstructure analyses, it is observed that Al-5Ti-1B addition is not worked. As can be seen in Figure 3, there is no any remarkable difference between as received and grain refiner added images in both two mould. Also, heterogeneity in microstructures is discerned. However, the effect of variation of the cooling rate because of mould material on microstructure can be observed. Conventional results cannot be observed in SDAS analyses except for few data. Despite the grain refiner addition, there is no successful refinement. Figure 4 also shows the importance of cooling rate on grain size during solidification. SDAS data of die-cast samples are higher than sand cast samples’.

Figure 3. Microstructural images of castings (a: as-received die, b: grain refiner added die, c: as-received

sand, d: grain refiner added sand).

UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

1112 IMMC 2018 | 19th International Metallurgy & Materials Congress

Figure 4. Microstructural analyses of all conditions.

Despite the finer grains, the worst and inconsistent UTS and elongation values are observed in die casting (Fig. 4-5).

Figure 4. UTS results of tensile tests.

According to UTS results, grain refiner addition cannot rely a better result for tensile properties and it has an adverse effect on elongation values in die cast samples. The best tensile test results are obtained distinctly in sand casting.

Figure 5. Elongation results of tensile tests.

Results of two different conditions (no add (i.e.: as-received) and Al-5Ti-1B add.) are close to each other. Al-5Ti-1B master alloy addition in sand casting has increased stability of UTS and elongation values. Low cooling rate results in unravelling o bifilms during solidification. So that, the notch effect of the failures is decreased in tensile testing compare to the compacted-scrambled bifilms.

4. Conclusion

• Addition of master alloy increases the bifilm index and decreases the melt quality.

• In these conditions, no clear relation between grain refiner addition and microstructure was detected.

• Duration (actually gravity) and particles from grain refiner, such as TiB2 and TiAl3, settle the bifilms.

• The shape factor of bifilm effects considerably on mechanical properties.

Acknowledgement

The authors wish to thank for laboratory support from Alcast Metal, Bursa.

References

[1] P. S. Mohanty & J. E. Gruzleski, Acta Materialia, 44 (9) (1996) 3749-3760. [2] P. L. Shaffer & A. K. Dahle, Materials Science & Engineering A. 413 (2005) 373-378. [3] C. Limmaneevichitr & W. Eidhed, Materials Science & Engineering A. 349 (2003) 197-206. [4] D. Dispinar & J. Campbell, Materials Science & Engineering A. 528 (2011) 3860-3865. [5] D. Dispinar, S. Akhtar, A. Nordmark, M. Di Sabatino, L. Arnberg, Materials Science & Engineering A. 527 (2010) 3719-3725.