Study on effects of T6 heat treatment on grain refined A319 alloy with Magnesium and Strontium addition

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ISSN : 2319 – 3182, Volume-2, Issue-3, 2013

59

Study on effects of T6 heat treatment on grain refined A319 alloy with Magnesium and Strontium addition

Gopikrishna. S & Binu. C. Yeldose

Department of Mechanical Engineering, Mar Athanasius College of Engineering, Kothamangalam, Kerala, India E-mail : [email protected], [email protected]

Abstract – Effect of heat treatment on A319 alloy with 0.45%wt Magnesium, 0.02%wt Strontium and grain refinement using Titanium has been investigated by hardness measurement and tensile testing. Experiments have been conducted at ageing temperatures 150°C, 160°C and 170°C. Hardness has been estimated up to 24 hours aged samples. The results indicate that hardness of 319 alloy increases with Sr addition and grain refinement.

When Mg is added hardness is again found to be increasing progressively up to a maximum value and then varying non – uniformly. The tensile strength and microstructure after Mg modification and heat treatment have been discussed.

Keywords-heat treatment; precipitation hardening; grain refinement;

I. INTRODUCTION

The use of aluminum castings in aerospace, automotive and general engineering industry has increased dramatically over the past three decades.

Automotive industry strives to achieve light weight components to reduce fuel consumption, to improve overall performance and to meet environmental requirements. Weight reduction can be achieved by replacing steel and cast iron products by aluminum.

Aluminum readily forms alloys with many elements such as copper, zinc, magnesium, manganese and silicon. The alloy known as A319 (Al-6.5%Si-3.5%Cu) is a commercially popular alloy used in various applications due to their excellent combination of properties such as fluidity, low coefficient of thermal expansion, high wear resistance, high strength to weight ratio, good corrosion resistance etc.

Chemical and thermal treatments are applied to this alloy in order to obtain improved mechanical properties.

The morphology of eutectic silicon in A319 alloy has a strong influence on the mechanical properties of the casting. The silicon particle morphology can be

controlled by the addition of strontium containing master alloy to the melt. The addition of strontium results in a fine and fibrous silicon structure during the solidification and produces several benefits. The strontium modification may improve the ductility, fracture and impact properties. Furthermore strontium modification can be effectively used to reduce the solution treatment time of the alloy. Despite these benefits there is a degree of apprehension associated with the modification primarily because of the apparent increase of porosity in the casting. Strontium addition can reduce the rejection rate and improves the casting quality.

In 319 alloy silicon and copper are the major alloying elements and magnesium is added for improving mechanical properties. The presence of magnesium improves strain hardenabilty and enhances material strength by solid solution. Presence of Ti enhances grain refinement. In order to improve mechanical properties of cast components, A319 alloys are heat treated. The typical heat treatment for A319 alloy is T6 heat treatment. T6 heat treatment comprises of solution treatment followed by quenching and precipitation hardening.

II. EXPERIMENTAL DETAILS A. Casting Preparation

The 319 ingots were cleaned using acetone to make it free from moisture. Cast iron moulds are used for casting. Graphite coatings are provided inside the moulds for easy separation of the castings from the mould after solidification. The moulds are then preheated to a temperature of 250°C.

B. Sequence of casting operation

10kg of A319 alloy is weighed using a weight balance. The pit furnace is heated to 700°C to become red hot and the alloy is charged in the crucible. Coverall flux of 100gm is also added in to the crucible while

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International Journal on Theoretical and Applied Research in Mechanical Engineering (IJTARME)

60 charging the 319 alloy. Hexa chloro ethane tablets were added to degas the melt after 319 ingots gets completely melted. 0.45%wt Mg is added in to the melt after degassing followed by 0.02%wt Sr and TiB addition.

After all the additions are over the melt is subjected to nitrogen degassing for 1hour.

After degassing the molten metal is poured in to the pre heated moulds. While pouring the temperature of melt should be at 720°C. Then the moulds are allowed to solidify as shown in Fig.1. After solidification, the castings are removed from the mould.

C. Specimen Preparation

Samples are cut from random portions of castings for metallographic studies.

D. Heat Treatment

T6 heat treatment includes 3 main steps. Solution treatment at 500°C for 8 hours in an air circulated furnace as per ASTM standards. Next step is quenching.

Quenching is done in water at 60°C and then natural aging for 12 hours. Last step is the artificial ageing. 3 sets of samples were taken and aged at 3 different temperatures at 150°C, 160°C and 170°C for 24 hours.

E. Metallographic samples

Initially the samples were polished using different grades of silicon carbide papers of progressively fine grades of 220, 400 and 600 grits. During paper polishing water is used as the cleaning agent.

F. Tensile testing

Tensile testing is carried out using computer controlled universal testing machine. The ends of specimen were gripped in the UTM and load is applied till it fractures. The tensile samples were also prepared according to the ASTM standards.

III. RESULTS AND DISCUSSION A. Microstructure

The alloy cast in permanent mould is cut to make samples for evaluating metallographic studies. Fig.1 shows microstructure of unmodified A319 alloy which consists of aluminum network and eutectic silicon. The morphology of silicon is plate like structure (acicular structure). This needle like structure acts as stress raisers in the microstructure making the material to fracture.

Fig.2 shows microstructure of unmodified A319 alloy after heat treatment. The structure of silicon is changed to round structure after heat treatment. Fig.3 shows microstructure of A319 alloy with 0.02%wt strontium modification and heat treatment. In the unmodified alloy the silicon particles had a coarse plate like form.

Microstructure reveals that addition of strontium modifies the coarse and acicular silicon to finer fibrous structure.

Fig.1 shows microstructure of unmodified A319 alloy in which the Al2Cu phase appears in blocky form and distributed very non-uniformly. Fig.3 shows as cast microstructure of strontium modified heat treated A319 alloy. The blocky form of Al2Cu has reduced considerably and the distribution was also found to be better indicating that Sr addition was effective to some extent in distributing the Cu atoms in the matrix. As regards porosity in the casting is concerned, it has been observed that less than around 1% porosity was present in both unmodified and modified alloy casting indicating that Sr addition has no effect on the porosity content on the casting.

The as cast microstructure of A319 alloy consists of α Al dendrites, needle shaped eutectic silicon, β shaped Al5FeSi inter-metallic and equilibrium Al2Cu. The morphology of eutectic silicon, namely size and shape plays an important role in determining the mechanical properties of this alloy. Under normal cooling conditions the eutectic Si appears as long acicular needles which act as stress raisers, thereby decreasing the mechanical properties. It has been reported that the addition of elements such as Sr/Na can modify the morphology of eutectic silicon from needle shaped one to fibrous one. The modified eutectic Si in fibrous form will not act as stress concentrator and hence the mechanical properties of the alloy will be improved.

Strontium addition improves the Cu distribution in the matrix which will lead to the better distribution of the hardening Al2Cu precipitate on heat treatment.

B. Hardness

Hardness of A319 alloy increases with strontium addition and grain refinement. When Mg is added hardness is again found to be increasing. Hardness values plotted at 150°C of strontium modified and grain refined alloy is shown in Fig.4. At each ageing temperature hardness increases with ageing time, reaches a maximum value and thereafter decreases as shown in Fig.5. Top hardness is found at 170°C for 8 hours. At this time and temperature hardness is 125 BHN. The size, morphology and the distribution of silicon particles could affect the hardness of the eutectic mixture. Modified and grain refined A319 alloy without Mg shows a maximum hardness of 102 BHN at 150°C.

Increase in hardness is due to the cooperative precipitation of Al₂Cu and Mg₂Si precipitates. So a solutionisng temperature of 500°C is appropriate for the alloy. Below this temperature solutionisation is insufficient, whereas coarsening of silicon particles and melting of Al₂Cu may occur at higher temperatures.

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61 C. Tensile strength

Fig.6, Fig.7 and Fig.8 shows the ultimate tensile strength, yield strength and elongation of as cast as well as heat treated samples of base alloy and modified alloy.

IV. CONCLUSION

The work is mainly focused on effect of T6 heat treatment on Mg added 319 alloy. Heat treatment is done at different ageing temperatures for 24 hours. Base alloys show optimum mechanical property at 150°C for 12 hours of ageing. Base alloy with Mg is heat treated at different ageing temperatures. From the ageing curve it can be seen that hardness values increases, reaches a maximum value and then varying non- uniformly.

Fig.1 Microstructure of unmodified A319 alloy. Silicon particles are having needle shape structure which acts as stress

raisers.

Fig.2 Microstructure of unmodified A319 alloy after heat treatment. Needle shaped silicon structure is transformed to

round shaped structure.

Fig.3 Microstructure of strontium modified A319 alloy after heat treatment. Coarse structural silicon changes to fine round

shaped silicon.

Increase in hardness is due to an additional phase called Q- Al₅Mg₈Si₆Cu₂ with the increase in percentage addition of magnesium. Alloy with Mg shows optimum mechanical property at 170°C for 8 hours of ageing. The tensile test shows that with the addition of Mg, the ultimate tensile strength and yield strength increases.

Fig.4 Hardness of strontium modified and grain refined alloy at 150°C

Fig.5 Artificial aging curve of Al 319 + 0.45% Mg

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Fig. 6: Ultimate tensile strength of as cast and heat treated samples

Fig.7 : Yield tensile strength of as cast and heat treated samples

Fig.7 Elongation of as cast and heat treated samples Mg added 319 alloy with heat treatment shows highest tensile strength and yield strength. Mg added A319 alloy with heat treatment shows highest tensile strength of 372Mpa at 170°C for 8 hours of ageing. The magnesium addition decreases elongation due to the formation of brittle phases.

V.ACKNOWLEDGMENT

Thanks to Mr. M.C Shaji, NIIST, Trivandrum for his technical assistance. Technical support got from NIIST, Trivandrum is gratefully acknowledged.

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