CHARACTERIZATION OF MICROSTRUCTURE AND SHRINKAGE POROSITY OF A SEMI-SOLID METAL SLURRY IN GRAVITY DIE CASTING. and J.Wannasin 5 *

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1 CHARACTERIZATION OF MICROSTRUCTURE AND SHRINKAGE POROSITY OF A SEMI-SOLID METAL SLURRY IN GRAVITY DIE CASTING S.Thanabumrungkul 1, W.Jumpol 2, R.Canyook 3, N.Meemongkol 4 and J.Wannasin 5 * 1 Department of Mining and Materials Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand sangop403@gmail.com 2 GISSCO Company Limited,170/4 Moo 3 Phawong, Muang Songkhla, Songkhla 90110, Thailand info.gissco@gmail.com 3 Department of Materials and Production Technology Engineering, Faculty of Engineering, King Mongkut s University of Technology North Bangkok,Thailand rungsinee.c@eng.kmutnb.ac.th 4 Department of Industrial Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand napisphon.m@psu.ac.th 5 The Southern Thailand Science Park/ Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand jessada@mit.edu Keywords: Semi-Solid Gravity Casting, Slurry Casting, GISS Technology Abstract. Current aluminum automotive parts such as wheels, engine and transmission components are produced by tilted gravity die casting for control gas porosity. But, there are still problems resulting in inefficient production: shrinkage porosity, microstructure size and uniformity. Shrinkage porosity is one of the major issues which affect mechanical properties such as strength and elongation in tilted pour permanent mold. Recent work using slurry casting technique has shown potential in gravity sand casting. Results show that the casting parts complete filling at low solid fraction and the casting yield also higher than conventional gravity sand casting.this paper extends important work for potential industrial applications in gravity die casting: microstructure size, uniformity, solid fraction control for micro shrinkage level. Introduction Current aluminum automotive parts such as wheels, engine and transmission components are produced by tilted gravity die casting for control gas porosity [1]. But, there are still problems resulting in inefficient production: shrinkage porosity, microstructure size and uniformity. Shrinkage porosity is one of the major issues which affect mechanical properties such as strength and elongation in tilted pour permanent mold [2].Gunasegaram at el [3] have studied the factors that affecting shrinkage porosity in permanent mold casting in commercial mass production. The results shown that thicker mold coat and higher mold

2 temperature for temperature gradients could manage and move shrinkage porosity away from critical location, however, this prolong casting cycle time and creating a coarser grain structure., Also, microstructure size and uniformity affect heat treatment times which add production costs [4].For Al-Si alloy grain refining, TiBor (Al-5Ti-1B master alloy) is a most widely used throughout the casting industry [5-7]. Recent work using slurry casting technique has shown potential in gravity sand casting. Results show that the casting parts complete filling at low solid fraction and the casting yield also higher than conventional gravity sand casting [8]. This paper extends important work for potential industrial applications in gravity die casting: cycle times, microstructure size, uniformity, solid fraction control for micro shrinkage level. Experimental Semi-solid slurry gravity die casting Commercial cast aluminum A356 alloy was used in this study. The chemical composition of the alloy is given in Table 1. The aluminum alloy was melted in a graphite crucible using an electric furnace. The melt was treated with commercial cleaning flux and degased by purging nitrogen gas through porous graphite for 10 minutes. To perform semi-solid slurries, the gas-induced semi-solid (GISS) technique was used [9]. This simple rheocasting technique, fine inert nitrogen gas bubbles are introduced through porous and cold graphite rod into the melt to crated the desired solid particle then the melt is quickly poured to a gravity die casting mold. Fig.1 shows the Schematic illustration of GISS slurry gravity die casting process. Table 1. Chemical compositions of the A356 aluminum alloy used in this study (wt.%) Alloy Si Mg Cu Zn Fe Mn Al A Balance Fig. 1 Schematic illustration of the GISS slurry gravity die casting process

3 Gravity Mold The gravity mold was coated by commercial ceramic coating before casting. The mold was controlled the temperature in 3 positions top, middle and bottom of the mold with 7 thermocouples and heating elements (three each side, one at the top) and all connect to the control unit. The Schematic of the experimental setup used in this study is shown in Figure 2 For preliminary studies, the melts at 680 C was poured into the mold at 375 C. Cross section found the macro-shrinkage along the sample that shown as Figure 3. This may because of not enough feeding [4]. The macro-shrinkage disappear when the mold temperature reach 475 C that using in this experiment. Table 2 shows the summary of parameters used in this study. The percentage of initial solid fraction was used rapid quenching technique [10]. Fig 4(a) and Fig 4 (b) shows the representative microstructure of initial solid fraction of 2 and 5 seconds rheocasting time (SGDC-2 and SGDC-5) and the percentages of initial solid fraction are 3.5 and 7.1, respectively. Fig. 2 Schematic of the experimental setup used in this study Fig. 3 Macro-shrinkage along the sample at mold temperature 375 C

4 Table 2. Summary of parameters used in this study Condition Pouring/Rheo-casting temperature ( C) Rheocasting % Initial solid fraction time (s) Liquid S-GDC S-GDC (a) (b) Fig. 4 The representative microstructure of initial solid fraction (a) S-GDC-2 (b) S-GDC- 5 Microstructure analysis Macro Uniformity After remove the cast samples from the mold. The samples were cross section machined and using standard grinding and polishing procedures. The digital camera was used to observed the macrostructure uniformity. Micro Uniformity The microstructure were observed under microscope. The average particle size were measured using standard image tool. Four position were analyzed as shown in Fig5. Position T were taken from the top just below the sprue, Position M were taken from the middle and position B were taken from the bottom of the samples. For position C the samples were analyzed along the horizontal direction between position M and position B.

5 Fig. 5 The Schematic of the area of microstructure analysis Micro Shrinkage porosity analysis In this study, five samples of each condition were assessed for micro shrinkage porosity area. The samples were photographed by using the camera. Brightness and contrast adjustments on the photographs were carried out using Photoshop software. Image analysis was then performed on the micrographs using Image Tool software.. Fig 6 shows the representative procedure of analyzed the micro shrinkage. The percentage of porosity was presented of different zone (zone II and zone III on Fig 6 b) Fig. 6 The representative procedure of analyzed the micro shrinkage (a) Sample was taken from camera (b) The analysis zone (c) Sample after adjust brightness and contrast

6 3. Results and discussion Microstructure Macro Uniformity Fig.7 shows the macrostructure comparison between normal liquid casting as shown in Fig.7 (a) and slurry casting at rheocasting time 2 seconds as shown in Fig.7 (b) and 5 seconds as shown in Fig.7 (c). The results show that macrostructure of S-GDC-5 is more uniformity than S-GDC-2 and it has significantly uniformity than Liquid. The results also confirmed with the particle size distribution in microstructure analysis. Fig. 7 Representative macrostucture of (a) Liquid (b) S-GDC-2 (c) S-GDC-5 Micro Uniformity Fig.8 show the representative microstructure of three positions in Fig.5. from the top (Position T) near the sprue, middle (Position M) and the bottom (Position B). Fig.8 (a) show the liquid casitng microstructure with large dendritic structure of α phase along the sample part. The average particle sizes of liquid casting are 378.3±103.3 µm, 328.1±83.3 µm and 270.9±127.9,respectively. The average particle sizes of S-GDC-2 are 140.9±23.1 µm, 114.6±26.6 µm and 89.2±25.7 µm, respectively. The average particle sizes of S-GDC-5 are

7 116.1±18.0 µm, 107.6±14.6 µm and 81.0±14.7 respectively. The results are shown in Fig.9 and the summary experiment and data results are shown in Table 4. The results show that the slurry casting has lower average particle size compare with liquid casting in every position in the part. The slurry casting has significantly uniformity than liquid casting. These also observed by the representative microstructure of position C in Fig.10 T T T M M M B B B (a) (b) (c) Fig.8 Representative microstructure of three positions in Fig.5: (a) Liquid (b) S-GDC-2 (c) S- GDC-5

8 Liquid S-GDC-2 S-GDC-5 Average Particle Size (um) B M T Position Fig.9 Average particle size of Liquid, S-GDC-2 and S-GDC-5 in three positions Table 4 Summary of experimental and data results Condition Average particle size (µm) Shrinkage Porosity (%) Position T Position M Position B Zone II Zone III Liquid 378.3± ± ± S-GDC ± ± ± S-GDC ± ± ± (a) (b) (c) Fig.10 Representative microstructure of position C in Fig.5: (a) Liquid (b) S-GDC-2 (c) S-GDC-5

9 Micro Shrinkage porosity The shrinkare porosity data of liquid,s-gdc-2 and S-GDC-5 in zone II are 2.1%, 1.0% and 0.2%, respectively and zone III are 0.4%,0% and 0% respectively. The results are shown in Fig.11. Zone II has more shirnkage porosity than zone III that may because of zone II is the thick section in the center of the part so the last solidification appear there, However, less micro shrinkage porosity happened when increased the rheocasting time. The results shows that no micro shrinkage in zone III with condition S-GDC-2 and S-GDC-5. Although S-GDC-2 and S-GDC-5 still have small amount of porosity in zone II (1.0% and 0.2%), this may be solved with optimize solid fraction and mold temperature controlled, which will be conducted in future works. Shrinkage Porosity (%) Zone II Zone III 0.0 Liquid S-GDC-2 S-GDC-5 Fig.11 Shrinkage porosity of Liquid,S-GDC-2 and S-GDC-5 at Zone II and Zone III 4. Conclusions This study has shown with the use of GISS slurry gravity die casting for A356 casting samples. The results are summarized as follows (1) The slurry gravity die casting parts have less micro shrinkage than liquid casting parts. The microstructure analyzed and percentage of porosity along the parts confirmed the results. (2) The slurry gravity die casting parts have lower average particle size than liquid casting parts. The slurry gravity die casting parts also have more uniformity. (3) To optimize the shrinkage porosity, the solid fraction and mold temperature controlled have been controlled.

10 Acknowledgement The authors would like to acknowledge the financial support from the Thailand Research Fund through the Royal Golden Jubilee Ph.D. Program (Grant No.PHD/0212/2552). The authors would like to thank Mr.Jijareon Sawadiwong, Mr.Saifar Suwannasam, Mr.Piman Chareamsrisuk from GISSCO CO.,LTD. References [1] R.Bain, M.Cunningham, Aluminum Permanent Mold Handbook Published by the American Foundry Society (2001) [2] A.M. Gokhale, G.R.Petel, Scr. Mater.52 (2005) [3] D.R.Gunasegaram, D.J.Farnsworth, T.T.Nguyen. J.Matererials Processing Technology 209 (2009) [4] J.Campbell,Castings,Butterworth-Heinemann (1999) , [5] P.S.Mohanty, J.E. Gruleski, Acta. Mater.44 (1996) [6] P.L.Schaffer, A.K. Dahle, Mater Sci Eng A (2005) [7] P.Li,S.Liu,L.Zhang,X.Liu, Mater and Design 47 (2013) [8] T.Chucheep, R.Burapa, S.Janudom, S.Wisutmethangoon, J.Wannasin, Trans Nonferrous Met Soc China 20 (2010) [9] J.Wannasin, R.A. Martinez, M.C.Flemings, Scr. Mater.55(2006) [10] J.Wannasin, R Canyook, S.wisutmethanggoon, M.C.Flemings, Acta.Mater.61(2013) 3897.