UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

674 IMMC 2016 | 18th International Metallurgy & Materials Congress

Eff ect of Chemical Composition and Heat Treatment on Hardness and Wear Properties of 7050 Al Alloy

Erdem Karakulak, Yusuf Burak Küçüker, Adem Karaaslan

Kocaeli University - Türkiye

Abstract:

7050 aluminum alloys with different Zn/Mg ratios were cast into metal molds using and induction furnace. Solution heat treatment at 470 °C for 8 hours and aging treatment at 150 °C for different durations between 6 and 48 hours were applied to cast samples. Microstructural investigations, hardness tests and dry sliding wear tests were conducted on as cast and heat treated samples. Hardness values of the specimens with higher Mg contents was higher in all conditions. For all samples over aging was reported after 24 hours of aging. Wear resistance of all alloys was consistent with hardness values. Specimens with higher hardness showed higher wear resistance. Wear mechanisms were identified by investigation of worn surfaces of the specimens after wear tests.

1. Introduction

7xxx (Al-Zn-Mg-Cu) series aluminum alloys show the higher strength among all aluminium alloys. Because of their high strength and low density these alloys are widely used in aerospace and automotive industries [1,2]. These alloys are age hardenable and heat treatments can affect mechanical properties of these alloys dramatically [3-5]. Usually T6 heat treatment is applied to 7xxx alloys to achieve maximum hardness and strength. Also some other heat treatments (T7451, T76 etc.) can be applied to increase corrosion resistance of 7xxx aluminum alloys. Chemical composition of the alloys also has an enormous effect on the properties of the material [6,7]. Amount of alloying elements can change number, size and type of precipitates that formed after aging treatment. 7050 aluminum alloy has a combination of high strength and fracture toughness, low density and good corrosion resistance [8-10]. These properties makes 7050 alloy as a candidate material for numerous applications. Some of these applications may require high wear resistance. In this study two different 7050 alloy is produced with different magnesium contents. Microstructure,

hardness and wear properties of these alloys investigated after different heat treatments.

2. Experimental Study

Pure metals were used to produce 7050 alloys with two different chemical compositions. The results of chemical analyses of the specimens are given in Table 1. The main difference between two samples is the magnesium amount in the alloy. An induction furnace with graphite crucible was used for melting process. After pure aluminum is melted different alloying additions were made for different castings. For all castings a metal mold was used and casting temperature was kept stable at 700 °C. Solution treatment was applied to the samples at 470 °C for 8 hours and specimens were water quench immediately. Specimens then aged at 150 °C for different durations between 6 to 48 hours. Microstructures of cast, solution treated and aged samples were investigated using an optical microscope. Hardness tests were conducted using a Future-tech Vickers hardness testes. All reported hardness value is an average of 10 measurements. To understand the effect of heat treatments on wear properties of the materials dry sliding wear tests were realizied. A ball-on-disc type wear aparatus was used during the tests. The load was selected as 25 N for wear tests, and sliding distance was kept constant for all tests as 500 m. Specimens were cleaned with alcohol and weighed before and after wear tests to obtain weight loss data. Obtained weight loss data is used to calculate wear rate of the specimens. To understand the wear mechanisms operated during wear tests worn surfaces of the specimens were investigated under scanning electron microscope.

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67518. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2016

Table 1. Results of chemical analyses of the cast samples

Element Zn Mg Cu Fe Al

6.7Zn-2Mg-2Cu 6,06 2,15 2,07 0,12 Bal.

6.7Zn-2.5Mg-2Cu 6,12 2,67 1,96 0,11 Bal.

7050 Standard 5,7-6,7

1,9-2,6 2-2,6 <0,15 Bal.

3. Results

3.1. Microstructural Investigations

Micrographs of 7050 alloy taken with optical microscope in as-cast and aged condition are given in Fig. 1. In the as cast condition microstructure is consisting of -Al dentrides and intermetallics in grain boundaries.

(a)

(b)

Figure 1. Microstructures of 6.7Zn-2Mg-2Cu alloy in (a) as-cast and (b) 24 hours aged conditions

During solution treatment intermetallic phases are dissolved in aluminum matrix and in aged situation very small amount of intermetallics in grain boundaries remains. The dissolved atoms forms fine precipitates inside grains of -Al.

3.2. Hardness Tests

Hardness tests results are given in Fig. 2. As can be seen on graph hardness of the sample with higher magnesium is higher as expected. For all specimens peak hardening was reported at 24 hours of aging. Longer aging times causes a decrease in the hardness of the material because of overaging.

Figure 2. Change of hardness of 7050 alloys with aging time

3.3. Wear Tests

Results of wear test are given in Fig. 3. As seen on the figure wear tests results are in good harmony with hardness tests reults. Increasing hardness of the specimen causes an increase in the wear resistance of the specimen which results with a lower wear rate value. As in the hardness tests specimens with higher Mg always have superior wear results compared to the ones with lower Mg addition. The drop in the hardness caused by overaging also causes an increase in the wear rate of the over aged samples. Wear rate of the as cast samples was calculated as 16,7 and 15,3 mm3/m for materials with 2 and 2,5 % Mg additions respectively. High wear rate values obtained in the as cast state is a results of high amount of intermetallics in the grain boundaries. These hard intermetallics tend to crack and break during wear tests and increase wear rate of material.

UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

676 IMMC 2016 | 18th International Metallurgy & Materials Congress

Figure 3. Wear rate values of 7050 alloys with different aging treatments

3.4. Worn Surface Investigations

Representetive SEM images of worn surfaces of 7050 alloys different with chemical compositions are given in Fig. 4. For both materials dominant wear mechanisms are abrassive wear and deformation. High difference in the hardness of aluminum and counterface causes formation of grooves and deformation on the worn surface of the specimen. This deformation results with trasnfer of material from center of the wear track to the edges. Also in some areas inside the wear track oxidation of aluminum was reported. The oxidation is mainly a result of the heat formed during sliding wear test as a result of friction between sample and counterface.

(a)

(b)

Figure 4. SEM images of worn surfaces of 24 hours aged (a) 6.7Zn-2Mg-2Cu and (b) 6.7Zn-2.5Mg-2Cu

alloys

Repeated plastic deformation causes work hardening in the deformed areas inside the wear track. Increased hardness of these local areas limits their ability to be deformed again. When counterface try to deform these areas during wear test because of limited toughness these local areas start to crack and break. In Fig. 5 an example of these areas is given. As can be seen on the image de edges of deformed layer is cracked.

Figure 5. SEM image of cracks formed on the worn surface of 24 hours aged 6.7Zn-2Mg-2Cu sample

4. Conclusion

In this study effect of heat treatment and Mg amount on the properties of 7050 alloy was investigated. The fallowing conclusions were drawn:

• Increasing Mg content increases hardness and wear resistance.

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67718. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2016

• Peak hardness was achieved at 24 hours of aging at 150 °C for both alloys.

• Maximum wear resistance was also obtained at 24 hours of aging.

• Main wear mechanisms were abrassion and deformation of aluminium matrix. Change of hardness with aging time also effects wear mechanisms that operates during wear tests.

• Some cracks caused by repeated deformation was also reported in the worn surface of specimens.

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