IJIRST –International Journal for Innovative Research in Science & Technology| Volume 3 | Issue 11 | April 2017 ISSN (online): 2349-6010
Effect of Mg Addition on the Microstructure, Secondary Dendritic Arm Spacing & Hardness of A356 Al Alloy Dr. Cherian Paul Assistant Professor Department of Mechanical Engineering Saintgits College of Engineering, Kottayam-686532, Kerala, India
Joel George Thomas UG Student Department of Mechanical Engineering Saintgits College of Engineering, Kottayam-686532, Kerala, India
Arun U UG Student Department of Mechanical Engineering Saintgits College of Engineering, Kottayam-686532, Kerala, India
Justin Joseph George UG Student Department of Mechanical Engineering Saintgits College of Engineering, Kottayam-686532, Kerala, India
Anandhu A S UG Student Department of Mechanical Engineering Saintgits College of Engineering, Kottayam-686532, Kerala, India
Abstract An investigation was carried out to understand the effect of Mg addition on the dendritic arm spacing and hardness of A356 aluminium alloy. The effect of Mg content was assessed by melting the A356 Al alloy in a muffle furnace with the addition of varying weight percentage Mg (1, 1.5, 2.5, 3.5 % wt) and casting in both permanent metal die and sand mould. It was observed that the secondary dendritic arm spacing reduces with Mg content concluding that Mg addition improves the formation of nucleation sites and thereby results in grain refinement. The Brinell hardness tester was used to measure the hardness of the alloy. It was found that the hardness increased with Mg content and comparatively, die cast samples showed improved hardness when compared with that of sand cast samples. Keywords: Al-Mg Alloys, Dendrites, Hardness, Microstructure, SDAS _______________________________________________________________________________________________________ I.
INTRODUCTION
Cast A356 Aluminium Alloy is taken into consideration owing its application in vehicle structures as they offer attractive combinations of excellent castability and good weldability, pressure tightness, and good resistance to corrosion. It can be used for aircraft pump parts, automotive transmission cases, aircraft fittings and control parts, water cooled cylinder blocks, aircraft structures and engine controls, nuclear energy installations, and other applications where high strength permanent mould or investment castings are required. However, in severe service circumstances like those in mining and rolling sectors, the low hardness and poor wear resistance limit its further industrial applications. An investigation was carried out to assess the effect of magnesium addition on the properties of A356 Al alloy and to optimize the melting parameters of the process. Further property evaluation including microstructure, secondary dendritic arm spacing, hardness test, density test, tensile test, wear test, corrosion test, spectroscopy, SEM and EDAX were carried out on cast A356 alloy with varying Mg weight percentage. The addition of magnesium allows the castings to be heat treated whereby magnesium silicide precipitates are formed which harden the alloy and impart specific combinations of strength and ductility. The iron, present as an impurity, solidifies into a number of brittle phases from the eutectic liquid. The composition and structure of these phases is affected by the solidification rate of the casting and the alloy chemistry. The coefficient of thermal expansion and its electrical resistivity increases a little. AlMg alloys have high strength, good ductility and excellent corrosion resistance. Al-Mg alloys respond well on heat treatment and a higher ultimate tensile strength and yield strength is achieved. The purpose of magnesium in A356 Al alloy is to precipitate Mg2Si particles but a disadvantage is that big intermetallic compounds can appear; those phases reduce the ductility. In alloys that have an amount of magnesium between 0.05 % to 0.3 % seems to decrease the amount of porosity.
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Effect of Mg Addition on the Microstructure, Secondary Dendritic Arm Spacing & Hardness of A356 Al Alloy (IJIRST/ Volume 3 / Issue 11/ 057)
II. EXPERIMENTAL DETAILS Materials Used Short descriptions of the materials used for conducting the experiments are given below: A356 Aluminium Alloy A356 Al alloy is used to make the melt. The melting temperature of pure Al is 660.3 oC and that of A356 Al alloy is 625oC. The composition of the alloy used is given below in table 3.1. The following composition is obtained as a result of spectroscopy. Table – 1 Composition of A356 Element Weight Percentage Element Weight Percentage Mg ≈ 0.3% Ti ≈ 0.25% Si ≈ 7% Mn ≈ 0.35% Fe ≈ 0.6% Other ≈ 0.15% Cu ≈ 0.25% Al Balance Zn ≈ 0.35%
Fig. 1: A356 Al alloy
Magnesium Mg is added to the above composition with varying weight percentages 1, 1.5, 2.5 and 3.5. The melting temperature of Mg is 650oC
Fig. 2. Magnesium Experimental Procedure The crucible is pre-heated in the muffle furnace to 650oC. Both the dies, tongs, A356 and Mg are pre-heated in the hot air oven upto 100oC. According to the charge calculation, the first weight percentage of A356 is loaded into the crucible and heated upto 750oC in the muffle furnace and the temperature is maintained constant for 20 minutes for the A356 to melt. Mg is added to the melted A356 according to the charge calculation for the varying weight percentages. Al-Mg is again heated in the furnace to 750oC. Graphite powder is applied inside the metal die for easy removal of the casted alloy. The melted alloy is then casted in the metal die and sand die. The developed alloy is then tested for microstructure, secondary dendritic arm spacing and hardness.
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Effect of Mg Addition on the Microstructure, Secondary Dendritic Arm Spacing & Hardness of A356 Al Alloy (IJIRST/ Volume 3 / Issue 11/ 057)
III. RESULTS AND DISCUSSIONS Microstructural Evaluation
1 Wt% Metal Cast
1 Wt% Sand Cast
Fig. 3. Microstructure of alloy with 1 weight percentage Mg
1.5 Wt% Metal Cast
1.5 Wt% Sand Cast
Fig. 4: Microstructure of alloy with 1.5 weight percentage Mg
2.5 Wt% Metal Cast
2.5 Wt% Sand Cast
Fig. 5: Microstructure of alloy with 2.5 weight percentage Mg
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Effect of Mg Addition on the Microstructure, Secondary Dendritic Arm Spacing & Hardness of A356 Al Alloy (IJIRST/ Volume 3 / Issue 11/ 057)
3.5 Wt% Metal Cast
3.5 Wt% Sand Cast
Fig. 6: Microstructure of alloy with 3.5 weight percentage Mg
It is observed from the above microstructures that the white portion is the aluminium matrix and the black lines are silicon in the form of eutectic silicon. The clusters of black lines are intermetallic Mg 2Si and the dark black spots represents the porosity in casting. Comparing the microstructure of the alloy with 1, 1.5, 2.5 and 3.5 weight percentage Mg, it is observed that the grain size decreases as the weight percentage of Mg increases. It is also observed that intermetallic Mg 2Si formed is more in case of 3.5 weight percentages Mg when compared with 1, 1.5 and 2.5 weight percentage Mg castings. In all the above cases it is also observed that the sand cast alloy has more porosity than metal cast alloy. The porosity is due to the casting defect in sand mould. Also the grain size is larger in the case of sand cast as the cooling rate is slower for sand cast than metal cast. Secondary Dendritic Arm Structure Secondary dendritic arm spacing is the distance between successive dendritic arms. It is observed that the SDAS of the metal casting is less than that of the sand casting. This is due to the cooling rate and the refinement of the grain size. Sand cast has coarse dendritic arm spacing and metal cast has fine dendritic arm spacing. It is also observed that as the weight percentage increases the grain size decreases.
Fig. 7: SDAS Table – 2 SDAS SDAS Wt % Sand Metal 1 135µm 51µm 1.5 86µm 45µm 2.5 65µm 40µm 3.5 58µm 36µm
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Effect of Mg Addition on the Microstructure, Secondary Dendritic Arm Spacing & Hardness of A356 Al Alloy (IJIRST/ Volume 3 / Issue 11/ 057)
Fig. 8: SDAS vs. Weight Percentage
Hardness Table – 3 Brinell hardness Brinell Hardness (HB) Wt % Sand Metal 1 83 93 1.5 88 103 2.5 94 124 3.5 100 136
It is observed that the metal casting depicts more hardness than sand casting. It is also evident that as the percentage of Mg increases the hardness also increases. The hardness of the developed alloy is more than that of A356. The hardness increases due to the formation of nucleation sites and grain refinement.
Fig. 9: Hardness vs. Weight Percentage
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Effect of Mg Addition on the Microstructure, Secondary Dendritic Arm Spacing & Hardness of A356 Al Alloy (IJIRST/ Volume 3 / Issue 11/ 057)
IV. CONCLUSION A356 alloy with varying Mg content was developed and property evaluation was carried out. Alloys cast in metal die showed refined grain structure when compared with sand cast alloy. Alloys cast in metal die were found to exhibit more hardness than sand cast alloys. SDAS was found to reduce with the addition of Mg. Hardness was found to increase with Mg content. Thus we can conclude that as the SDAS decreases the hardness increases. This is due to the formation of nucleation sites and grain refinement which prevents crack propagation. REFERENCES [1] [2]
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