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An Investigation Of The Effects Of Solution Heat Treatment On Mechanical Properties Of Aluminium 7075 Alloys

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6982

An Investigation Of The Effects Of Solution Heat

Treatment On Mechanical Properties Of

Aluminium 7075 Alloys

Mohan Kumar S, Govindaraju H K

Abstract: The present works describes the Effect of heat treatment on grain size of Al 7075 alloys. The melting furnace utilized for the casting of the aluminum is an electrical resistance heater with a mechanical stirrer coated with tungsten carbide. The ageing time was varied from 1 to 20 hours in the steps of 1 hour. Mechanical characterization was performed as per ASTM standards such as tensile strength hardness tests. Results of the study reveal that Ultimate tensile strength and hardness properties are enhanced as the ageing time increases. The technique of optical microscopy was used to examine sample specimen for microstructures, and the microstructure indicated uniform particle distribution in the matrix state. Experimental results shows that there is a fall in the micro-hardness number after 14 hours of ageing due to an increase in the size of fine π precipitates as well as precipitate coarsening. The ultimate tensile strength was found to be 417.29 MPa after 20 hours of ageing, which is approximately 83.44% higher than that observed for 1 hour ageing specimen. After heat treatment, the strengthening of Al7075 alloy depends on several factors such as precipitation hardening and grain size. Furthermore to examine the fracture a scanning electron microscopy was used to examine the tested samples

Index Terms: Al 7075, Stir casting, Heat Treatment, Microstructure, Scanning Electron Microscope, Yield Strength. ——————————  ——————————

1

INTRODUCTION

Due to their desirable properties such as lower density and sensitive strength, aluminum and its alloys have found wide-spread applications. They have been used in many areas such as food utensils, storage cans, heat retention foils, support structures, military and commercial vehicle structural components. The strength of aluminum and its alloys is due to its density property varying from ~2.7 g / cm3 and having the face centre cube shape, thereby preserving the stability until melting. The characteristics of aluminum alloys are highly dependent on microstructure behaviour, binding properties between the particles, different secondary techniques and heat treatment. The aluminum alloy categories are dependent on the use of alloying elements. 7xxx series consisting of Al-Zn-Mg-Cu fulfilled the use of aluminum alloys in structural applications. The components must be robust as they are subjected to various operating conditions. Ductility, strength, modulus and damage tolerance are some key aspects that must be considered for the use of material as structural applications. By following certain alloying processes, these properties can be established. Alloys for high-performance structural usages undergo age hardening process that is developed in the forming process, plates or extrusions. The sequential cycle has to be followed, i.e., for the design of alloys with high thickness. Casting, homogenizing, warm rolling, solution treatment, quenching and stress relief by stretching and ageing. [3]. The heat treatment mechanism for aluminum alloy begins at elevated temperatures with solution heat treatment to accommodate the elements in solid solution.

An example of 7075 alloy shows that it is heated to 4800C and the temperature is instantly decreased by keeping it in water called quenching. The alloy is very appropriate at this temperature to undergo different processes. Once this alloy is kept at a higher temperature than room temperature, the alloy strength increases with decreased ductility. Precipitate formation begins as it is kept at a super saturated level [4]. Alloy has higher strength, lower density, greater toughness and lighter weight along with corrosion resistant properties. Their application in automotive, aviation, structures and models has been prominent. [2]. Al and its alloys are used in the form of cast and wrought. This alloy is 3rd still used in its iron and steel application [3]. This material's ability to display the property of light weight and high strength compared to other Al alloys makes it a sought-after material for applications in airframes and high stress components [4]. The aluminum's reaction capacity depends on its ability to form an oxide layer to display the corrosion's positive effects. The decrease in resistance to corrosion depends on the percentage of the alloying element in it. Often there may be some solid precipitations or phases that interfere with the continuity of the developed oxide layer and significantly affect the matrix [5]. The method adopted to change the state of metal specimen's with respect to its structure of metallurgy, mechanical properties and variations in stress is considered as heat treatment. The alloys are classified into two subgroups; treatable alloys and alloys that non heat-treated. Aluminum alloy heat treatment falls under the category of heat treatable alloys that improves the materials hardness and strength. Alloys whose properties must be altered by cooling is classified as non-heat treatable alloys. Solutionizing, quenching and ageing are the main steps taken in the heat treatment process. Solution heat treatment is heating the alloy close to its eutectic temperature and keeping it for a specified long time to form a solid solution at that temperature. After solution heat treatment, quenching is followed to maintain the system in a super-saturated state [15]. The strength of the formed alloy also increases as the temperature increases. In some specimens that have the eutectic phase if the temperature rises above the required eutectic temperature than the results will affect this phase's ______________________________

• Mohan Kumar S Department of Mechanical Engineering, Amrita School of Engineering, Bengaluru, Amrita Vishwa Vidyapeetham India. E-mail:s_mohankumar@blr.amrita.edu.

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6983 melting. Therefore, with consideration of this aspect, the

choice of temperature should also be made.

2

EXPERIMENTAL

PROCEDURE

Al7075 alloy is an incredibly interesting material that is primarily used for structural components that are extremely stressed. This material has a range of applications in the field of aerospace, automobile, marine and space. Table.1 demonstrates chemical composition of Al7075 alloy.

Table: 1 Chemical composition of Al7075 alloy

Element Cu Cr Mn Mg Si Ti Zn Fe Al

Wt. % 1.8 0.2 0.4 1.9 0.5 0.15 3.25 0.5 Balance

The melting furnace used for aluminum casting is an electrical resistance heater with a tungsten carbide-coated mechanical stirrer to ensure the stirrer's better life. Aluminum melting furnace shown in figure 1.

Fig. 1. Melting Furnace

2.1 Heat Treatment

Al7075 alloy specimens are machined to the desired size and subjected to the process of heat treatment (i.e. T6 condition). For two hours, the specimens were solutionised at 550 ° C and quenched in water. Upon quenching, both natural and artificial ageing was adopted. Artificial ageing of water-quenched specimens was completed at 190°C for various ageing hours ranging from 1 to 20 hours in step of 1 hrs. Fig.2 shows furnace used for heat treatment used in the present study.

.

2.3 Optical Micrograph studies

Fig. 2. Heat treatment Process

Small pieces of size 10x10x10 mm were machined as cast alloy for Optical micrographic studies. Emery polishing was used to prepare sample samples using silicon carbide abrasive sheets, of course, medium and fine grit scale 300,600,800 & 1200. Using alumina as abrasive samples were cleaned dried and polished on a velvet cloth on a double-disk polishing machine using a diamond paste to get a mirror finish. In order to reveal the grain structure, further samples were etched using the keller's reagent (3%HCl+HNO3+HFThe Meiji microscope shown in figure 3. has been used to examine the highly polished surfaces and the Samsung camera has been used to capture microphotos.

Fig. 3. Photograph of Metallurgical Microscope

2.4 Micro Hardness Test

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6984

Fig. 4. Photograph of Vickers Microhardness Tester

2.5 Tensile Test

Tensile tests were conducted using TUE-C-400 universal testing machine as per ASTM E8 standards at M/s. Advanced Metallurgical Laboratory, Peenya, Bangalore, India. To quantify the ultimate tensile strength and ductility of each sample, an average of 3 results was considered. The strength of each material was taken as an average of three results. Figure 5 shows the tensile test specimen dimensions used in this present investigation.

Fig. 5. Tensile Test Specimen (ASTM E8-M)

3

RESULTS

AND

DISCUSSION

3.1 Microstructure Study

Al7075 alloy casting microstructure study shows improved fluidity, grain refinement, decreased dendrite arm size, different aluminum and eutectic boundary phases. Different artificial aging conditions microstructure study picture are shown in Figure 6. Enhanced mechanical, age-hardening and fracture properties showed in grain refining. The effects on the microstructure of heat treatment greatly improve the transgranular fracture ratio. It results in uniform strain within grains and improved grain boundaries bonding. The fracture toughness and ductility were enhanced because of increased

resistance to fracture. Increased mechanical properties due to enhancing homogenity of precipitated particles and Adequate crystallographic texture distribution.

3.2 Hardness Properties.

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6985

Fig. 7. Effect of Heat treatment on Hardness

3.3 Tensile Properties

The heat treatment effect has been studied on the yield strength of Al7075 alloy. In the steps of 1 hour ageing time is varied from 1 to 20 hours and the yield strength obtained is shown in Fig. 8. The trend in yield strength was found to gradually increase up to 20 hours of ageing period. To start with, the yield strength was found to be 197.9 MPa and 211.9 MPa respectively after ageing time of 1 and 2 hours. The yield strength was observed to rise further to 217.17 MPa after 3 hours of ageing and continued to rise until 20 hours of ageing. The yield strength was observed 397.57 MPa after 20 hours of ageing, which is nearly double that observed for specimen aged 1 hour. After heat treatment, the strengthening of Al7075 alloy depends on many factors such as grain size and hardening of precipitation. Al7075 alloy's yield and ultimate tensile strength depend largely on the material's grain size. This form of strengthening is also known as reinforcement of Hall-Petch. Accordingly, the size of the grain is inversely proportional to the material yield strength.

Fig. 8. Tensile strength properties

3.4 Study on Fractured Surfaces

By scanning the electron microscope tensile specimens fractured surfaces were investigated. Figure 9 shows SEM images of alloy surfaces with tensile fractures. In the ageing specimen, the fracture surface of alloys Al 7075 reveals even more pulled regions with dimpled rupture morphology and represents the transgranular fracture as shown in Figure().This difference was due to enhanced heat treatment effect on the alloy's strength and toughness. Depending on the material, the

final fracture was different. Presence of deep dimples evidents ductile failure was shown as pulled surfaces. This illustrates the heat treatment effect and changes in alloy microstructure.

Fig. 9. SEM Analysis

4 CONCLUSION

It has been found from the investigation that Al 7075 alloy mechanical properties have been enhanced by increasing the ageing time after solution treatment. In 20 hours of ageing treatment, there was a considerable increase in ultimate tensile of strength of 417.29 MPa which is almost double the times of 1 hour ageing heat treatment As well as a decrease in hardness of alloy after 14 hours of ageing. All samples failed in ductile manner, revealing an increased ultimate strength with lowered structural defects as the ageing period increased. With few structural defects, tensile specimens are likely to achieve high peak strength values. It can be seen from the microstructure that the intermetallic precipitates are present and distributed evenly throughout the alloy material.

REFERENCES

[1]. S. Mohan Kumar, et al. "Evaluation of fracture toughness and mechanical properties of aluminum alloy 7075, T6 with nickel coating." Procedia Engineering 97 (2014): 178-185

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6986 Characterization of Al7075-Silicon Nitride MMC’s.‖

International Journal of Mechanical and Production Engineering Research and Development (IJMPERD) ISSN(P): 2249-6890; ISSN(E): 2249-8001 Vol. 8, Issue 4, Aug 2018, 575-584

[3]. Paul A. ROMETSCH, Yong ZHANG, Steven KNIGHT. Heat treatment of 7xxx series aluminium alloys— Some recent developments. Trans. Nonferrous Met. Soc. China 24(2014) 2003−2017.

[4]. D. S. Mackenzie, G. E. Totten, Eds., Handbook of Aluminum, volume 1:Physical Metallurgy and Processes, Marcel Dekker, New York, 2003, pp. 103, 106, 108, 121-129, 882-885, 908, 1002.

[5]. S, Mohan Kumar. Et al. ―Electrochemical corrossion studies and mechanical characterization of al7050 metal matrix composite reinforced with Zro2 particulate by stir casting method,‖ International Journal of Mechanical Engineering & Technology (IJMET), Volume 8, Issue 11, November 2017, pp. 949–957

[6]. R. E. Smallman, R. J. Bishop, Modern Physical Metallurgy and Materials Engineering - Science, Process, Applications, 6th ed. Elsevier, 1998, pp. 265- 266.

[7]. Sairam Varma G, et al. ―The Effect of TiC on Microstructure and Mechanical Properties of AA 2024 MMCs,‖ IOP Conf. Series: Materials Science and Engineering 376 (2018) 012085.

[8]. Ryen, Øyvind & Holmedal, Bjørn & Nijs, Oscar & Nes, Erik & Sjölander, Emma & Ekström, Hans-Erik. ‖Strengthening mechanisms in solid solution aluminum alloys. Metallurgical and Materials Transactions A‖: Physical Metallurgy and Materials Science. 37. 1999-2006.

[9]. Aditya Ranganathan, et al. ―Mechanical Characterization of AA7050-TiC Metal Matrix Composites under as Cast as condition‖, Materials Today: Proceedings 5 (2018) 25368–25375

[10]. S. Mohan Kumar, et al. ―Evaluation of Mechanical and Wear Properties of Aluminium AA430 Reinforced with SiC and Mgo‖, Materials Today: Proceedings 4 (2017) 509–518.

[11]. V. Ravi Kumar,et al. ―Effect of metal coatings on mechanical properties of aluminium alloy‖, AIP Conference Proceedings 1859, 020037 (2017). [12]. S. Mohan Kumar et al. ―Mechanical characterization

Figure

Fig. 1 . Melting Furnace
Fig. 4. Photograph of Vickers Microhardness Tester
Fig. 7. Effect of Heat treatment on Hardness

References

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