UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

1048 IMMC 2016 | 18th International Metallurgy & Materials Congress

Hot Tearing Tendency of A356, A413 and A380.1 Alloy Muhammet Uludağ¹, Remzi Çetin², Derya Dışpınar³

¹Selçuk University, ²Haliç University, ³İstanbul University - Türkiye

Abstract There is long going discussion in casting industry on hot tearing which is known to be one of top three casting defects. In ferrous alloys, the phenomenon is mostly defined. However, in light alloys, the tendency to hot tearing has become a complex issue simply because there is a huge scatter in results. In this work, the effect of melt quality and melt treatment has been studied in a way to add another perfective to the field. Three Al-Si alloys with various solidification ranges were selected. 1. Introduction Hot tearing is one of the most important defects in aluminium alloys [1]. There are pretty much studies [2-6] about this defect in literature but the main reason for hot tearing still undisclosed point. It is known that hot tear occurs in mushy zone when the alloy solidifies because of strength, constrained shrinkage and lack of feeding [1, 2]. This defect is usually linked with the degree of segregation that result in a microscopic scale [7]. The liquid film perfectly wets the solid grains and acts as a lubricant to aid hydrostatic pressure to decrease [4]. Verö [8] studied on effect of chemical composition of alloys on hot tearing by using U shape permanent mold and claimed that hot tear sensitivity gets better with silicon content until 1.9% but it gets worse above this silicon content. Length of crack can be decreased by microstructure converting from columnar to equiaxed [9]. Spittle and Cushway [5] studied on effect of Cu content on hot tearing until 15% Cu content. They found that hot tearing sensitivity was increased when Cu contents was less than 7% but above 7% Cu contents, hot tearing can get better. Lin et al [10] 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 (1)

Where; Li Number of arms for CRC mold (from bottom to above) and Ci Severity of hot tearing.

In present study, three different alloys that are A356, A413 and A380.1 were used to examine hot tearing tendency for both no degassing and degassed castings by using two different molds that are T shape hot tear test mold and Constrained Rod Casting mold (CRC). 2. Experimental Procedure Chemical composition of A356, A413 and A380.1 alloys that were used in this study are given in Table 1. Table 1. Chemical composition (in wt. %) of the alloys

used in the study. Alloy Si Fe Cu Mn Mg Zn Ti Al

A356 6.80 0.19 0.003 0.001 0.30 0.011 0.108 Rem.

A413 13.00 0.19 0.018 0.001 0.02 0.012 0.006 Rem.

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

Alloy was melted in a crucible that has 22 kg capacity using an electrical furnace. CRC and T shape permanent molds were used to examine hot tearing in this study. Images of the cast parts with dimensions are given in Figure 1.

a

b

Figure 1. Image of cast parts with dimensions a) CRC mold, b) T shape mold

TMMOB Metalurj i ve Malzeme Mühendisleri Odas ıBildir i ler Kitab ı

104918. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2016

Castings were done before and after degassing that was done 20 minutes with Ar. Pouring temperature were 740 C ± 15 C for A356 and A380.1 and 700 C ± 15 C for A413 alloy. Three trials were applied for each

parameters. Microstructure was examined on every alloys. Scanning Electron Microscopy (SEM) was examined on A380.1 alloy that was tore only. 3. Result and Discussion: Microstructural images for three alloys as a representative are given in Figure 2. As seen from images, A356 has dendritic+eutectic microstructure with low eutectic and high dendritic distribution whereas A380 has high eutectic and low dendritic structure. On the other hand, A413 is a typical 100% eutectic.

A356 A413

A380.1

Figure 2. Microstructure images for alloys In Figure 3, T junctions of cast samples images are given in accordance with their melt treatment sequence. Hot tearing was only observed in A380 alloy. Similarly, in CRC moulds (Figure 4), hot tearing tendency was high in A380 and almost no cracks were observed in A356 and A413.

Figure 3. Hot tearing areas in T shape mold

Figure 4. Casting parts from CRC mold

CRC mould can numerically give an indication of Hot Tearing Susceptibility (HTS) as given in Equation 1. The results are given in Figure 5. It can be seen that A380 hot tearing tendency was high when no degassing was carried out in the melt. After degassing, HTC was decreased from 7 to 3. SEM images (Figure 6) revealed dendritic structure on the fracture surface of hot tear sample of the non-degassed melts. However, the degassed melt fracture surface was observed to be more brittle-like which indicated a defect originated fracture mechanism.

UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

1050 IMMC 2016 | 18th International Metallurgy & Materials Congress

Figure 5. Hot tearing sensitivities for all parameters.

A380.1

No degassing A380.1

Degassed

Mac

ro im

ages

SEM

imag

es

Figure 6. Macro and SEM images from hot torn areas. 4. Conclusion Increased Cu content in Al-Si alloys increased hot tearing tendency. Melt quality has a significant effect on hot tearing. There is also a good correlation between solidification range ad hot tearing. Eutectic alloy has zero freezing range thus no tear was observed in any of the castings. Acknowledgment This 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 1. Campbell, J., Castings : [the new metallurgy

of cast metals]. 2003, Butterworth Heinemann.

2. Eskin, D. and L. Katgerman, A quest for a new hot tearing criterion. Metallurgical and

Materials Transactions A, 2007. 38(7): p. 1511-1519.

3. M’Hamdi, M. and A. Mo, Light metals, 2002. 4. Rappaz, M., J.M. Drezet, and M. Gremaud, A

new hot-tearing criterion. Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 1999. 30(2): p. 449-455.

5. Spittle, J. and A. Cushway, Influences of superheat and grain structure on hot-tearing susceptibilities of AI-Cu alloy castings. Metals Technology, 1983. 10(1): p. 6-13.

6. Warrington, D. and D. McCartney, Hot-cracking in Aluminium Alloys 7050 and 7010-a Comparative Study. Cast metals, 1991. 3(4): p. 202-208.

7. Larouche, D., et al., A constitutive model for the tensile deformation of a binary aluminum alloy at high fractions of solid. Metallurgical and Materials Transactions B: Process Metallurgy and Materials Processing Science, 2006. 37(3): p. 431-443.

8. Verö, J., The Hot-Shortness of Aluminum Alloys. The Metals Industry, 1936. 48.

9. Pumphrey, W.I. and P.H. Jennings, A Consideration of the Nature of Brittleness at Temperature above the Solidus in Castings and Welds in Aluminum Alloys. J. Inst. Metals, 1948. 75.

10. Lin, S., C. Aliravci, and M.O. Pekguleryuz, Hot-Tear Susceptibility of Aluminum Wrought Alloys and the Effect of Grain Refining. Metallurgical and Materials Transactions A, 2007. 38(5): p. 1056-1068.