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114719. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2018

Investigation of the Hot Tearing Degree in A380.1 Alloy with Sr and Ti Addition

Muhammet Uludağ¹, Remzi Çetin², Murat Tiryakioğlu³, Derya Dışpınar4

¹Bursa Technical University, Metallurgical and Materials Eng. Dept., Bursa, Turkey²KTO Karatay University, Materials Eng. Dept., Konya, Turkey

³University of North Florida, School of Engineering, Jacksonville, FL 32224, USA4Istanbul University, Metallurgical and Materials Eng. Dept., İstanbul, Turkey

Abstract

One of crucial defects for aluminum alloys is hot tearing. In this work, HTD formulation was applied on A380.1 alloy casted under different addition parameters which are Sr and Ti addition. Effect of Sr modification and Ti grain refinement on hot tearing in A380.1 alloy were investigated by using T-shape mold. It was concluded that the HTD formulation for T shape mold seems to be suitable for define hot tearing tendency of Al alloys. Sr addition is not suggested to prevent hot tearing but Ti addition can be suggested.

1. Introduction

Casting defects affect properties of the alloys seriously. One of crucial defects for aluminum alloys is hot tearing [1]. In the literature, pretty much studies [2-10] about this defect have been performed. Hot tearing formation is seen in mushy zone during solidifcaiton because of strength, constrained shrinkage and lack of feeding [1, 2]. This is usually connected with the grade of segregation that result in a microscopic scale [11]. The liquid film perfectly wets the solid grains and acts as a lubricant to aid hydrostatic pressure to decrease [4]. Uluda et al [7] claimed a new approach for hot tearing which is bifilm content and freezing range is the main reason for this defect. Lin et al [12] studied on hot tearing sensitivity (HTS) using Constrained Rod Casting (CRC) test. They published a formulation to calculate HTS with four different severities. The arms of cast part were numbered 1, 2, 3, 4 from bottom to top and each arm number multiplies its hot tear severity. Total value from four arms give us HTS for the casting. Formulation is given below.

𝐿𝑖 𝑋 𝐶𝑖

Where; 𝐿𝑖 Number of arms for CRC mold (from

bottom to above) and 𝐶𝑖 Severity of hot tearing

On the other hand, Uluda et al [8] tried to create a new formulation to define hot tearing degree of the Al alloys using T-shape mold. They used five different severity that are 2, 4, 6, 10 and 30 was called as ST,

surface sink factor that is 3 was called as Fsc and distance of the tearing was called as DVT. The formulation is given below.

HTD = ST×(FSC+DVT/10)

In this work, HTD formulation was applied on A380.1 alloy casted under different addition parameters which are Sr and Ti addition. Effect of Sr modification and Ti grain refinement on hot tearing in A380.1 alloy were investigated by using T-shape mold.

2. Experimental Procedure

Chemical composition of the alloy that was used in this study are given in Table 1.

Table 1. Chemical composition (in wt. %) of the alloy used in the study.

Alloy Si Fe Cu Mn Mg Zn Ti Al

A380.1 8.14 0.64 3.115 0.44 0.22 0.49 0.02 Rem.

An electrical furnace that has 5 kW power and a SiC crucible that has 22 kg capacity were used to melt the alloy. To investigate hot tearing in this study, T shape permanent mold was used. Image of the cast part with dimensions is given in Figure 1.

Figure 1. Image of cast parts with dimensions of T shape mold

Castings were started firstly for as-received condition when the alloy in the crucible is melted. The second

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and third casting was carried out for Sr modification and Ti addition respectively. The liquid alloy was kept for 10 minutes after Sr addition and 5 minutes after Ti addition. 740 C ± 15 C was preferred as pouring temperature for A380.1 alloy. Three different casting into the T-shape mold were applied from the same melt for each parameters. Analysis of microstructure and scanning electron microscopy (SEM) were done on every parameters. Calculation of HTD were done on T zone that is hot tearing area of the T-shape mold.

3. Result and Discussion

Microstructural images for three casting conditions as a representative are given in Figure 2.

Figure 2. Microstructure images for casting parameters

It can be seen on the images that addition parameters changed microstructure morphology. As know that A380.1 alloy has 8 % Si and 3 % Cu and is solidified with a-Al dendrites, Al-Si eutectic and Al-Cu eutectic phases in as-received condition similar Figure 2. Sr addition affect silicon morphology which is changed from coarse and long size to fine and short size. This Sr effect can be seen on the images clearly. Other addition parameters is Ti grain refinement that plays an important role on secondary dendrites arm spacing (SDAS). It representatives grain size of the alloys. When all images of this parameters obtained experimental study are analysis, it can be understood that Ti addition help to get grains in small size for the alloy.

Images of the T zone to show hot tearing of A380.1 alloy under different addition parameters present in Figure 3. It can be seen that there can be a sever hot tearing on T zone of the alloy and this alloy can be torn easily if the oxide structures are available at the around of the T zone [7].

TMMOB Metalurj i ve Malzeme Mühendisleri Odas ı Eğ i t im MerkeziBildir i ler Kitab ı

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Figure 3. Hot tearing areas on T shape mold

HTD calculations were done from the T-shape casting parts and the results are given in Figure 4. Uluda et al [9] studied on hot tearing of A380 alloy and used Constrained Rod casting (CRC) mold. They calculated hot tearing sensitivity of the alloy with different addition parameters by using the formulation published by Lin et al [12]. They claimed that while Ti addition decreases the hot tearing formation, Sr addition increases hot tearing formation. The results of this study is similar to results of Uluda et al’s study [9]. The average value of the results seems to Sr addition decreases hot tearing like Ti addition. When the three casting trials are evaluated together, all casting of Sr addition have hot tearing or hot cracking. Severity of the casting parts of Sr addition can be low but all castings have hot cracking. However, hot tearing is either occurred or no occurred with Ti addition. If there is no hot tearing, surface sink can be seen on T zone. This is an evidence for T zone has not any oxide structure and because of this surface sink is occurred during the solidification.

Figure 4. HTD results of the addition parameters

SEM examinations were done on surface of hot tearing of T zones. Images to present tearing on T zone are given in Figure 5. Hot tearing formation between dendrites arms can be seen easily on the SEM images. As it was previously claimed [1, 7-9] that there are oxide structures on hot tearing surface. It can be said that when the casting quality of the a alloy is improve namely oxide structures are removed from the melt, hot tearing can be prevented. Sr and Ti addition play an important role on hot tearing formation. Sr addition is not suggested with this study, because though HTD of Sr addition parameters is low, all castings of this parameters have hot tearing or hot cracking. However, Ti addition can be suggested, because Ti addition has an effect on casting quality by cleaning oxide structures [1, 13].

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Figure 5. SEM images from hot torn areas

4. Conclusion

Effect of Sr modification and Ti grain refinement on hot tearing in A380.1 alloy were investigated with this study and conclusions of the study summarized below:

1- The HTD formulation for T shape mold seems to be suitable for define hot tearing tendency of Al alloys.

2- Sr addition is not suggested to prevent hot tearing but Ti addition can be suggested.

3- The main reason for hot tearing is oxide structures [7] and this defects must be removed from the liquid before casting.

AcknowledgmentThis work has been supported by the Scientific Research Projects Coordination Unit of Selcuk University (Project number: 13101026). Author would like to thanks to TUBITAK (Turkey) for its supporting.

References

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