Influence Of Combined Grain Refinement And Modification On Microstructure And Mechanical Properties

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Influence Of Combined Grain Refinement And Modification On Microstructure And Mechanical Properties Of Al-Si18% Alloy.

Ashish Dahala, Debesh karkia, Nischal P. Rajbhandaria, ,Kotgi Kotreshb a Department of Mechanical Engineering, NitteMeenakshi Institute of Technology, Bangalore, 560064,India Asst.Proffesor ,Department of Mechanical Engineering, NitteMeenakshi Institute of Technology, Bangalore, 560064,India

b

Abstract This paper attempts to investigate the influence of the microstructure and mechanical property changes on Al-Si 18% alloy by combined action of both (Al-TiB2+ Na2SiF6) grain refinement and modification and without grain refinement and modification effect by cooling under three different conditions very fast cooling, moderate cooling and slow cooling.. The microstructures of the castings are studied by optical microscopes. The microstructure and mechanical properties (tensile strength, hardness and wear) was tested of as cast, treated (grain refined and modified) samples. The result demonstrated that significant refinement of Îą Al was due to the addition of refiner.

Similarly addition was responsible for the modification of Si particles. These refinement and modification in microstructure resulted in improvement of hardness value, tensile strength and resistance for wear.

Introduction Al-Si eutectic alloy are the mostly used alloys for industrial applications due to their unique mechanical properties such as high strength to weight ratio and corrosion resistance. In addition, the low coefficient of thermal expansion and good wear resistance make these alloys suitable for manufacture of components such as cylinder blocks and piston [1].Grain refinement is considered to be one of the most important and popular melt treatment processes for aluminum–silicon alloys castings. The use of grainrefiners to improve castings mechanical properties is widespread in aluminium industry, and its associated benefits on final products are well documented [2].The advantages of grain refinement of aluminum alloys are both technical and economic, which include reduced ingot cracking, better ingot homogeneity [3], being less susceptible to hot cracking [4] and mechanical properties are improved significantly. Grain refinement improves the quality of castings by reducing the size of primary ι-Al grains nucleated in the as-cast product, which otherwise will solidify naturally with coarse colaminar grain structure in the absence of grain refiner.

Fine equiaxed grain structure is desired because it comes along with several benefits such as uniform IDL - International Digital Library

distribution of second phases and microporosity to improve homo-geneity, improved feeding ability to avoid incomplete filling of mold [5], reduced porosity and the elimination of hot tearing, high yield strength, high toughness, im-proved machinability and excellent deep drawability of the products [6].

The present investigation is an attempt to improve the mechanical properties of Al-Si 18% by cooling under different conditions with and without grain refiner and modifier Al-TiB2& Na2SiF6.The microstructure, tensile strength, wear hardness was studied with and without using grain refiner and modifier.

Fig1. Stir casting setup 1|P a g e

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International e-Journal For Technology And Research-2017 II. EXPERIMENTAL PROCEDURE: The Al-Si alloy was melted in graphite crucible in induction melting furnace and the melt was held at 850°C. Titanium-diboride powder, sodium silico flouride, 2% of the total volume, duly packed in aluminum foil were added to the melt for grain refinement and modification. The melt was stirred for 2 minutes after the addition of grain refiner and/or modifier. And the molten metal was cooled under different conditions. The melt with the grain and refiner were prepared under three different cooling conditions. One of it was poured into a rectangular cast iron mould for fast cooling and another one was left in mold itself for very slow cooling until it gets the room temperature and to achieve moderate cooling the graphite crucible was taken out of the furnace and buried under sand until it gets solidified.

After solidification the casting was taken out from the mould and are cut to required shape and sizes for microstructure, wear testing, hardness testing and tensile strength testing. And the same process was repeated without using grain refiner and modifier.

Casting

Very cooling

Under fast cooling the cast material, after addition of Al-TiB2& Na2SiF6 the primary silicon is refined and large eutectic are visible.

Under moderate cooling the cast material has large silicon structure before addition of Al-TiB2& Na2SiF6 .after the addition the significantly refine coarse primary aluminum and primary silicon to fine grain size.

Under slow cooling the cast material has large eutectic crystals and primary silicon is large. After the addition of Al-TiB2& Na2SiF6, the Modification refines the primary and eutectic silicon crystals and changes the morphology of these crystals

Specimen

fast

Moderate cooling

Very cooling

modifier to Al-Si 18% alloy significantly refine coarse αaluminum dendrites to fine equiaxed αaluminum dendrites. Modification refines the primary and eutectic silicon crystals and changes the morphology of these crystals. The change in microstructure from coarse columnar grain structure to fine equiaxed grain structure and coarse dendritic structure to fine dendritic structure.

slow

Without grain refinement and modifier

With grain refinement and modifier

Without grain refinement and modifier

With grain refinement and modifier

Without grain refinement and modifier

With grain refinement and modifier

Fig: 2 as cast (fast cooling)

Table 1. Specimen condition III. RESULTS & DISCUSSION 1. Microstructure Fig. (2-7) show photomicrographs of Al-Si 18% alloy. It is observed that the addition of 2% of total volume of Al-TiB2& Na2SiF6 grain refiner and IDL - International Digital Library

Fig: 3 using grain refiner& modifier (Fast cooling) 2|P a g e

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International e-Journal For Technology And Research-2017

Fig: 4 as cast (moderate cooling)

Fig: 7 using grain refiner& modifier (Slow cooling) 2. Mechanical properties a) Hardness properties Following graph shows the hardness value of the AlSi18% in different conditions with addition of grain refiner & grain modifier and without using of it.

Fig: 5 using grain refiner& modifier (Moderate cooling)

I) as cast Materials Al-Si 18%

Diameter under indentation (mm)

Brinell hardness number N/mm2

Fast cooling

3.5

246.568

Moderate cooling

3.6

232.79

Slow cooling

3.7

219.94

Table2. Hardness value of as cast Fig: 6 as cast (slow cooling) b) Using grain refiner and modifier

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International e-Journal For Technology And Research-2017 Materials Al-Si 18%

Diameter under indentation (mm)

Brinell hardness number (BHN) N/mm2

Graph below shows the comparison of wear properties of Al-Si18% alloy in different conditions with and without addition of grain refiner and modifier.

wear comparision Fast cooling

3.42

Moderate cooling

3.56

Slow cooling

3.62

258.92

80

Column2

Column1

70

238.316

60 50

232.86

Table3. Hardness value after grain refinement and modification.

40 30 20 10 0 fast cooling moderate cooling slow cooling

Fig 9. Wear vs. Casting Condition c) Tensile properties: Graph below shows the tensile value of the Al-Si18% in different conditions with and without addition of grain refiner & grain modifier.

Fig 8. Hardness vs. Casting Condition From the table and the graph, the specimen obtained from very fast cooling has more hardness in comparison to moderate and slow cooling. Addition of grain refiner and modifier increases the hardness value. b) Wear properties

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Fig 10. Engg. UTS vs. Casting Condition

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Breaking load 1500 1000

IV. CONCLUSION From the tests conducted to determine the mechanical properties, wear behavior, microstructure, by addition of grain refiner and without addition of grain refiner and modifier it can be concluded that:

500 0 fast cooling moderate cooling

slow cooling



The specimen obtained from very fast cooling has more hardness, less wear, more tensile strength, more breaking load in comparison to other cooling conditions.

Fig11. Breaking load vs. Casting Condition



Addition of grain refiner and modifier increases the hardness, tensile strength, breaking load and decreases the wear rate.

Case1: As cast:



In case of fast cooling we obtain value hardness 246.568BHN, Engg. Ultimate tensile strength 93.6 (N/mm2), breaking load 1235.7 N and wear 53 µm.

In case of moderate cooling we obtain value hardness 232.719 BHN, Engg. Ultimate tensile strength 79.6 (N/mm2), breaking load 705.6 N and wear 68 µm. In case of slow cooling we obtain value hardness 219.94 BHN, Engg. Ultimate tensile strength 63.2 (N/mm2), breaking load 504 N and wear 61 µm.

In addition of grain refinement, modification combined action of both (Al-TiB2+ Na2SiF6) to Al-Si18% alloy significantly refines coarse of primary aluminum and primary silicon to fine grain size and are properly refined.

References

Case 2: Al-Si alloy (with grain refinement and modification)

[2]

as cast

using grain refiner & modifier

In case of fast cooling Increase hardness is 246.568to 258.92 BHN, Engg. Ultimate tensile strength 93.6 to 108.4 (N/mm2) and breaking load 1235.7 to 1368.2 N and wear 53 to 51 µm. In case of moderate cooling Al-Si alloy (with grain refinement and modification).

Increase hardness is 232.719 to 238.316 BHN, Engg.

Ultimate tensile strength 79.6 to 62(N/mm2), breaking load 705.6 to 774.8 N and wear 68 to 52 µm. In case of slow cooling Al-Si alloy (with grain refinement and modification) Increase hardness is 219.94 to 232.86 BHN, Engg. Ultimate tensile strength 63.2 to 67.9 (N/mm2), breaking load 504 to 537 N and wear 61 µm as compare to case 1. Here increase in mechanical properties, due to addition of grain refinement and modification occurred.

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IDL - International Digital Library Of Technology & Research Volume 1, Issue 3, Apr 2017

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