Advanced Materials Research Vols. 264-265 (2011) pp 295-300 Online available since 2011/Jun/30 at www.scientific.net © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.264-265.295
Effect of pouring temperature and melt treatment on microstructure of lost foam casting of AL-Si LM6 Alloy Majid Karimian1,a, A. Ourdjini2,b, M. H. Idris3,c, M. Bsher. A . Asmael4,d 1,2
Department of material, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, Skudai, 81310, Johor Bahru, Malaysia
3,4
Department of manufacturing, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, Skudai, 81310, Johor Bahru, Malaysia 1
Department of Mechanical Engineering, Khomeinishahr branch, Islamic Azad UniversityKhomeinishahr- Isfahan-Iran) a
[email protected],
[email protected],
c
[email protected], d
[email protected]
Key words: Lost foam casting, pouring temperature, Al-Si alloy
Abstract. This paper presents the experimental investigation conducted on Al-Si cast alloy (LM6) cast using lost foam process. The main objective of the research is to investigate the effect of pouring temperature, section thickness and melt treatment on the microstructure of the lost foam casting of Al-Si alloy. Step pattern with five different sections was prepared from 20 kg/m³ density foam and poured at five different temperatures; 700, 720, 740, 760, and 780℃ with and without the addition of AlTiB as grain refiner. Analysis on microstructure, eutectic silicon spacing and porosity percentage were conducted to determine the effect of both parameters. The results show that pouring temperature has significant influence on the quality as well as microstructure of the lost foam casting of LM6 Al-Si alloy. Lower pouring temperature was found to produce finer microstructure casting. However, the addition of AlTiB as grain refiner did not affect the produced castings significantly whether in terms casting quality or microstructure. Introduction Aluminium alloy castings are widely used in the automobile and aerospace industries and are replacing heavier forged steel or cast iron for the lighter and lower fuel consumption vehicles. The lost foam casting (LFC) is one of the casting processes mostly used in the automobile industries to produce defect free castings, complex shape and good surface finish[1]. The properties of Al- Si alloys are controlled by the phases that constitute the alloy (Al and Si) and processing parameters. The pouring temperature for instance directly affects the nucleation and growth of primary phases (Al or Si) in Al–Si alloy[2]. Pouring temperature determines the cooling rate, nucleation, controls the size of formed grains during growth, the grain morphology and the distribution of alloying elements within the matrix[3]. Parameters that have influence on LFC quality are shown in the Ishikawa’s diagram in Fig. 1[4, 5]. Pouring temperature and melt treatment are two of the parameters that have been identified to affect the quality of the Al-Si alloy LFC. This paper is aimed at investigating the effect of these two parameters (pouring temperature and addition of grain refiner) on the quality of lost foam casting of LM6 Al-Si alloy.
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Evaporative pattern casting process (surface roughness) Coating
Pattern
Sand
Vibration
Vacuum
Alloy
Thickness
Density
Type
Frequency
Degree of vacuum
Pouring temperature
Slurry viscosity
Bead size
Shape
Amplitude
Size
Time
Pouring time
Figure 1 Ishikawa cause effect diagram of EPC process [4, 5] Experimental Procedure The pattern used was a step polystyrene pattern sized 150mm × 250mm and with cross-sectional thickness of 3, 6, 12, 18, 24 mm as shown in Fig. 2. The riser was made with 24×38×36 mm dimensions for better feeding with added extra 10 mm foam on top of the pattern and assembled with the aid of glue. The patterns made of polystyrene foam with the density of 20Kg/m3 were cut using hot wire with accuracy of ±0.5 mm. Two types of castings were made: untreated and AlTiB grain-refined LM6 alloys. Liquid pouring was conducted at five different temperatures: 700, 720, 740, 760 and 780°C.
24
250 50 3 100
Figure 2 Pattern dimensions (mm) The polystyrene pattern was coated with a layer of Zircon flour (200 mesh) and colloidal silica (30% concentration) slurry in the ratio of 80:20. The viscosity of the slurry was maintained at 27 seconds measured using Zahn flow cup no.5. The pattern that has been coated was left to dry for 24 hours in a controlled room temperature (27°c). The coating thickness was measured between 0.30.5 mm. The mould was prepared by filling unbonded unbounded silica sand with AFS grain fineness number 70. The sand is slowly introduced into the flask by hand and filling is accomplished by gravity. The flask was thus subjected to a horizontal vibration of 50 Hz for 3 minutes. Uniform vibration of the flask is facilitated by clamping the flask to the vibration table at 4 points. Fig. 3 shows the position of the pattern inside the flask after sand filling.
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Figure 3 Pattern orientation and position in the flask The alloy used is an Al-Si alloy (British Standard designation - LM6). The chemical composition of the alloy is shown inTable1. Table 1 The chemical composition of LM6 alloy investigated Elements
Si
Fe
Mg
Mn
Cu
Ti
Zn
Balance
Wight (%)
10.55
0.62
0.27
0.23
1.79
0.043
0.852
Al
The LM6 alloy ingot was melted in an induction furnace. Two melts were prepared; untreated and treated melts. For the treated melt, the melt was added with titanium-boride (TiB) as a grain refiner which was introduced into the melt as an AlTiB 3/1 master alloy (ratio of Ti:B is 3:1). TiB is practically used to improve many metallurgical properties such as resistance to cracking, ductility and surface finishing characteristics in addition to refining the grain size. Once the LM6 ingot has melted the grain refiner (0.2% AlTiB) wrapped in an aluminium foil is introduced into the bottom of the melt using a steel shaft precoated with zircon-colloidal silica mixture. Both melts; treated and untreated melt were poured at five different temperatures; 700, 720, 740, 760 and 780°C. The temperature was controlled within ±5°C. Analysis conducted includes microstructure analysis and eutectic silicon spacing measurements. Prior to the microstructure analysis the samples were prepared acording to the standard procedures for metallographic analysis. The samples for analysis were taken from the middle of each section as shown in Fig. 4. The measurement of eutectic silicon spacing was performed on micrographs magnified 100X using the linear intercept technique Fig. 5. The final reading of the eutectic spacing is the average of at least 10 measurements.
Figure 4 Sample preparation for microstructure analysis
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Figure5 Measurement of eutectic silicon spacing using linear intercept technique Results and Discussion The microstructures of the untreated and TiB-treated castings for 3, 12 and 24 mm cross-sections poured at the temperatures of 700 and 780oC are shown in Fig. 6 and 7 respectively. It can be observed that for all sections, the microstructure becomes finer with the addition of the TiB. The effect of pouring temperature is also evident as castings produced from higher pouring temperature of 780oC were coarser compared with those cast at 700oC. This is also obvious as casting poured at lower pouring temperature and thinner section results in higher cooling rates and hence produces finer microstructure. The effect of adding the grain refiner (TiB) is illustrated in Fig. 7, which shows the microstructures of castings produced at a constant pouring temperature of 780oC for the 3, 12, and 24 mm sections, with and without the addition of TiB grain refiner. It is again clear that for the same conditions the castings treated with TiB grain refiner showed finer structure compared to those untreated castings. By increasing the section thickness from 3 to 24 mm, the structure would become coarser due to slower rate of solidification, Fig. 8.
780°c- 3mm
780°c - 12mm
780°c - 24mm
700°c -3mm
700°c -12mm
700°c -24mm
Figure 6 Effect of pouring temperature and section thickness on microstructure of untreated alloy (magnification100X)
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700°c -24mm
700°c -12mm
780°c -3mm
780°c -24mm
780°c -12mm
Figure 7 Effect of pouring temperature and section thickness on microstructure of TiB-treated alloy. (magnification100X)
Without TiB-3mm
With TiB-3mm
Without TiB-12mm
Without TiB-24mm
With TiB-12mm
With TiB-24mm
Figure8: Effect different sections thickness on microstructure of untreated and TiB-treated LM6 alloy poured at 740°C. (magnification100X) Effect of pouring temperature, section thickness and grain refiner on eutectic spacing is tabled in Table 2. Table2 Effect of pouring temperature, section thickness and grain refiner on eutectic spacing Pouring temperature (°C)
Section thickness (mm)
Without Ti-B
With Ti-B
3 12 24 3 12 24
700
740
780
8.4 15.93 17.3 8.1 11.2 12.12
10 16.4 21.25 8.4 13 14.45
14 18.2 30.87 8.9 13.5 18.1
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The graph of the result of the eutectic silicon spacing measured for 3, 12, and 24 mm sections for treated and untreated castings is shown in Fig. 9. It can be clearly observed that the eutectic spacing increases with the increased in pouring temperature. This is most likely due to the decreased in the cooling rate and hence coarser structures are obtained for higher pouring temperatures. It is also observe that decreasing section thickness from 24 mm to 3 mm decreases the eutectic spacing which due to the fact that thinner section solidify faster and produce finer structures and vice versa. Eutectic silicon spacing (µm)
35 30 25
3
20
12
15
24
10
3-TiB
5
12-TiB
0
24-TiB 700
740
780
Pouring temperature (°c)
Figure 9 Effect of pouring temperature, section thickness and grain refiner on eutectic spacing Conclusions Based on the investigation conducted it can be concluded that pouring temperature and section thickness have great influence on the microstructure of untreated and TiB-treated Al-Si LM6 alloy. However, TiB-treated castings shows shorter eutectic silicon distance in its microstructure compared to the untreated ones. The shortest eutectic silicon distance was registered in the TiBtreated casting at the 3mm section thickness with the average value of 8.1 whilst the longest was at the 24mm section thickness of the untreated castings at the value of 17.3. Acknowledgement The authors are thankful to the Ministry of Science, Technology and innovation, Malaysia for providing the financial support and the Faculty of Mechanical Engineering, Universiti Teknologi Malaysia (UTM) for providing resources and facilities. References [1] Stuart D McDonald, K.N., Arne K Dahle, Eutectic nucleation in Al–Si alloys. Acta materialia, 2004. 52(14). [2] Shivkumar, S., Casting characteristic of aluminum alloys in the EPC process. Am. Foundryman Soc. Trans., 1993. 101: p. 519–524. [3] Kiyoung Kim, K.L., Effect of Process Parameters on Porosity in Aluminum Lost Foam Process. Material science technology, 2005. 21(5): p. 681. [4] Kumar, S., P. Kumar, and H.S. Shan, Effect of evaporative pattern casting process parameters on the surface roughness of Al-7% Si alloy castings. Journal of Materials Processing Technology, 2007. 182(1-3): p. 615-623. [5] M. SANDS, S.S., EPS molecular weight and foam density effects in the lost foam process. JOURNAL OF MATERIALS SCIENCE, 2003. 38: p. 2233-2239.
Advances in Materials and Processing Technologies II 10.4028/www.scientific.net/AMR.264-265
Effect of Pouring Temperature and Melt Treatment on Microstructure of Lost Foam Casting of AL-Si LM6 Alloy 10.4028/www.scientific.net/AMR.264-265.295
