Correlation Between Mechanical Properties and Porosity Distribution of A356 in Gravity Die Casting and Low Pressure Die Casting

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1 Advanced Materials Research Online: ISSN: , Vol. 445, pp doi: / Trans Tech Publications, Switzerland Correlation Between Mechanical Properties and Porosity Distribution of A356 in Gravity Die Casting and Low Pressure Die Casting D. Dispinar 1, a, S. Akhtar 2,b, A. Nordmark 1,c, F. Syvertsen 1,d, M. Di Sabatino 2,e, L. Arnberg 2,f 1 SINTEF Materials and Chemistry, Trondheim, Norway 2 NTNU, Department of Materials Science, Trondheim, Norway a derya.dispinar@gmail.com, b s.akhtar@ntnu.no, c arne.nordmark@sintef.no, d freddy.syvertsen@sintef.np, e m.di.sabatino@ntnu.no, f lars.arnberg@ntnu.no Keywords: aluminium; GDC; LPDC; porosity; melt cleanliness; tensile properties Abstract. Gravity die casting (GDC) and low pressure die casting (LPDC) methods were used to compare the mechanical properties and porosity distribution in a 5-step mould design. Commercially available A356 alloy was used for the experiments. Ar and Ar+H 2 mixture were used to achieve two different hydrogen levels, i.e. 0,1 and 0,2 ml/100g Al, respectively. Although the porosity level was lower in LPDC, the tensile properties were lower than GDC due to the fact that LPDC melt had 50 mm bifilm index, whereas GDC melt had 20 mm. This investigation has shown that the metal quality has a larger effect over the mechanical properties than the porosity content. Introduction Gravity casting has been one of the most traditional and simplest ways of casting metals and alloys. It is practical and economical. However, it has few disadvantages besides its common use. The folding of the advancing front of the liquid metal causes many problems such as entrained air, bubbles, oxide bifilms and so on. Therefore, it is important to design a gating system that would prevent these defects. These can be overcome by critical calculations and designs with downhill sprue; horizontal runner bars and uphill filling of the cast part. The optimised effective system was proposed by Campbell [1]. Since the velocity of the liquid metal during filling is the critical step in the casting operations, many counter-gravity systems have been developed. The most common one is called low pressure casting where the liquid metal is carried into the mould cavity through a riser tube by pressure applied to the surface of the melt in an enclosed furnace. The advantages of the system are the control of the speed and elimination of sprue and runners. Presence of defects in any production part caused by bad gating and wrong feeder placement can make the properties unpredictable and significantly affect the mechanical properties of aluminium castings, especially the ductility and fatigue properties. Caceres [2-3] investigated the effects of phases, pores and inclusions and concluded that not the type of the defect but the area fraction of the defect had a significant effect over the properties. Lui [4] concluded that oxides have deleterious effect over the mechanical properties whereas Wang [5-6] suggested that porosity is more detrimental than oxides. All in all, these defects would result in an increase in the rejection rates [7-9]. Depending on several parameters, these rejection rates could go up to 20%. In castings, the nucleation of pores can be expected to occur primarily at heterogeneous sites. Heterogeneous sites are inherent in almost all liquids, and inclusions are forming the most important category of such sites in the melt. Since oxides and other non-metallic inclusions are not All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (# , Pennsylvania State University, University Park, USA-18/09/16,21:16:45)

2 284 Materials and Manufacturing Technologies XIV wetted by the metal, the microscopically rough surfaces with free cavities serve as nuclei for bubbles to grow. It has been well demonstrated [10-14] that the presence of inclusions and oxides greatly enhances the porosity formation in aluminium alloys. In this work, the aim was targeted to check the effect of melt quality over the mechanical properties and porosity distribution of step mould castings that are cast in gravity and low pressure die casting units. Experimental work In the experiments, a step mould was used where the geometry (Fig. 1) consists of 5 steps by 120 mm x 50mm and 5, 10, 15, 20 and 30 mm thicknesses. The die halves are made of steel and equipped with cooling channels. The channels are connected to an oil heating/cooling unit by flexible tubes. In the experiments, the die was kept constant at 300 C. Figure 1: Step mould die used in the casting experiments The die could be positioned in a way such that it can be used for both gravity and low pressure castings. Position A in Fig. 1 is used as a feeder when gravity die casting (GDC) and position B is closed by a block. For Low pressure casting (LPDC), position A is closed and position B is used as the riser tube. Table 1. Chemical composition of A356 used in the experiments Si Mg Mn Fe Ti Na Sr P Al rem. A resistance furnace was used to melt 70 kg of A356 in a crucible. The composition of the studied alloy is given in Table 1. The melt was gas fluxed with an impeller in order to achieve hydrogen contents of 0,1 and 0,2 ml/100g Al using pure Ar and Ar+H 2 mixture (5 l/min). The hydrogen content was measured with ALSPEK-H from Foseco [15]. At each casting series, reduced pressure test samples were collected and bifilm index [16] was measured for metal quality check. Archimedes principle was used to measure the volumetric porosity in castings. Samples from each step were then machined into test bars for mechanical testing. Results Reduced pressure test samples collected from the melts that were prepared for gravity die casting and low pressure die casting showed that bifilm index values were 13 mm and 50 mm for 0,1 ml/100g Al respectively; while these values were 21 mm and 56 mm for 0,2 ml/100g Al hydrogen level (Fig. 2).

3 Advanced Materials Research Vol Figure 2. Bifilm index change with different casting methods Porosity percentage of the gravity die casting samples were 0,24% and 0,39% for 0,1 and 0,2 ml/100g Al, respectively. These values were slightly lower in low pressure die casting samples: 0,12% and 0,29% which is given in Fig 3. Figure 3. Porosity change of castings at different hydrogen levels For the comparison of the tensile test results, Weibull distribution of data from gravity and low pressure die castings were used. Fig 4 shows the results for 0,1 ml/100g Al hydrogen level and Fig 5 shows for 0,2. (a) (b) Figure 4. Comparison of the tensile test results for 0,1 ml/100g Al hydrogen level (a) UTS, (b) elongation

4 286 Materials and Manufacturing Technologies XIV (a) (b) Fig 5. Comparison of the tensile test results for 0,2 ml/100g Al hydrogen level (a) UTS, (b) elongation Figure 6: SEM images from the fracture surface of tensile bars showing the bifilms in LPDC castings Figure 7: SEM images from the fracture surface of tensile bars showing the bifilms in GDC casting

5 Advanced Materials Research Vol Discussion There has been a long going research over the relationship between hydrogen, oxides, porosity and mechanical properties [2-3, 5-6, 13, 17-20]. Models have been introduced to predict the porosity by means of hydrogen diffusion and nucleation. However, none of the studies suggested that porosity was effectively growing on oxides; i.e. bifilms [1]. Bifilms are often seen at grain boundaries since the dendrites cannot grow across the air film. With the precipitation of more gas evolving during solidification, the pore may grow more rounded. Finally, the size may be such that the original bifilm will become relatively insignificant in size, effectively tucked in a corner of the pore, while the pore expands into (i) the free liquid to become a spherical pore or (ii) among the dendrites to become an interdendritic pore. Both types could be gas or shrinkage or a mixture of the two, depending on whether the pore grows freely in the liquid, or gases surround by dendrites. The essential idea about the bifilms is their ability and the potential behaviour to be able to open themselves under certain circumstances: as in Campbell s words unfurling of bifilms. The simplest conditions are diffusion of rejected hydrogen into the bifilms and/or negative pressure generated by the shrinkage. In such cases, the presence of bifilms becomes more easily detected (as pores). On the other hand, if unopened, they may remain in the structure. In this work, it was found that it was the metal quality (i.e. bifilms) that had a significant affect over tensile properties instead of the porosity content. Despite, majority of the inspection methods involve the detection of porosity on the finished cast part. During gravity casting, the only external pressure present is the liquid head from the feeder (Location A in Fig. 1) which is relatively low compare to the constant pressure applied during solidification in the die in LPDC. In LPDC, the cast part is kept under pressure due to the design of the system until it solidifies. Therefore, as long as the pressure is applied, bifilms may find it hard to unfurl to form the porosity. This is clearly seen in the porosity distribution of step mould castings (Fig 3) where LPDC has few pores compare to GDC. Nevertheless the affect of higher bifilm index was observed pronouncedly in the mechanical test results (Figs 4-5). The low porosity levels in low pressure die casting (Fig 3) have not resulted in high mechanical properties. Liu and Samuel [4] also concluded that the oxides had a deleterious effect over mechanical properties than any other inclusions. Caceres [3] investigated the effect of casting defects on tensile properties and showed that the dominant parameter was the area of fraction of defects in cross section, regardless of their shape and distribution. As seen in Weibull distribution of the tensile test results in Figs 4 and 5, the castings with high bifilm index had higher scatter with low values. The SEM images on the fracture surface of the tensile bars also show that LPDC castings (Fig 6) had flat-like, larger surface area of oxides compare to the bifilms in GDC (Fig 7) where they were more compact and crumpled. In theory, it would have been expected to find that mould filling is much more controlled in LPDC, so that the properties would be better. However, here, the importance of bifilm index was seen clearly. Regardless of the casting method (no matter how perfect the method or the filling could be) and the gas content, it is the bifilm content of the melt that affects the mechanical properties. When the bifilm index of the melt is high (i.e. bad quality melt to start with), the properties of the casting would fail.

6 288 Materials and Manufacturing Technologies XIV Conclusion In these experiments LPDC castings had lower porosity than GDC castings. However, mechanical properties were not directly related to the porosity levels in the castings but to the bifilm index. The cleanliness of the melt has a major affect over the properties of the cast part. Higher the bifilm index, higher the scatter and lower the tensile test values. Acknowledgement The European Integrated Project NADIA (New Automotive components Designed for manufactured by Intelligent processing of light Alloys - Contract NMP2-CT ) is gratefully acknowledged for financial support. Authors would also like to acknowledge the help of Mr. Kurt Sandaunet during the experiments. References: [1] Campbell, J., Castings. 2nd ed. 2003, Oxford: Butterworth Heinemann. VIII, 335 [2] Caceres, C.H., C.J. Davidson, and J.R. Griffiths, The deformation and fracture behaviour of an Al-Si-Mg casting alloy. Materials Science and Engineering A, (2): p [3] Caceres, C.H. and B.I. Selling, Casting defects and the tensile properties of an Al-Si-Mg alloy. Materials Science and Engineering, A220: p [4] Liu, L. and F.H. Samuel, Effect of inclusions on the tensile properties of Al 7% Si 0.35% Mg (A356.2) aluminium casting alloy. Journal of Materials Science, (9): p [5] Wang, Q.C., D. Apelian, and D.A. Lados, Fatigue behavior of A356/357 aluminum cast alloys. Part II: effect of microstructural constituents. Journal of Light Metals, : p [6] Wang, Q.G., D. Apelian, and D.A. Lados, Fatigue behavior of A356-T6 aluminum cast alloys. Part I. Effect of casting defects. Journal of Light Metals, (1): p [7] Zhang, B., et al., Casting defects in low-pressure die-cast aluminum alloy wheels. JOM Journal of the Minerals, Metals and Materials Society, (11): p [8] Bonollo, F., et al., Gravity and low pressure die casting of aluminium alloys: a technical and economical benchmark. La Metallurgia Italiana, : p [9] Merlin, M., et al., Impact behaviour of A356 alloy for low-pressure die casting automotive wheels. Journal of Materials Processing Technology, (2): p [10] Chen, X.-G. and S. Engler, Formation of gas porosity in aluminum alloys. AFS Transactions, : p [11] Emadi, D., J.E. Gruzleski, and M. Pekguleryuz, Melt oxidation behavior and inclusion content in unmodified ad Sr-modified A356 alloy- their role in pore nucleation. AFS Transactions, : p [12] Kaye, A. and A.Street, Die Casting Metallurgy. 1982, London: Butterworth. [13] Samuel, A.M. and F.H. Samuel, Porosity factor in quality aluminum castings. AFS Transactions, : p [14] Simenson, C.J. and G. Berg, A survey of inclusions in aluminum. Aluminium, : p