UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

240 IMMC 2018 | 19th International Metallurgy & Materials Congress

Eff ect of Chemical Composition, Solution Treatment and Artifi cial Aging on Microstructure and Mechanical Properties of AA 2011

O. Halil Çelik, Asım Zeybek, Görkem Özçelik, Mehmet Buğra Güner

ASAŞ Alüminyum Sanayi ve Ticaret A.Ş., Sakarya, Turkey

Abstract

In this study, the effects of chemical composition, solution treatment and artificial aging parameters on microstructure and mechanical properties of an Al-Cu alloy have been studied. Solidus temperature of the alloy were determined by DSC analysis. Optical microscopy, XRD and SEM/EDS methods were used for characterization. It has been showed that the mechanical properties of the alloy have significantly been improved by the increasing solution treatment temperature and artificial aging time depending on the chemical composition.

1. Introduction

Aluminum alloys are widely used in aerospace and automobile industries due to their low density, good mechanical properties and corrosion resistance [1,2]. 2000 series aluminum alloys in which copper is the principal alloying element, although other elements, notably magnesium, may be specified. 2xxx series alloys are widely used in aircraft where their high hardness, strength is valued. These alloys require solution heat treatment to obtain optimum properties; in the solution heat-treated condition, mechanical properties are similar to, and sometimes exceed, those of low-carbon steel [3,4].

The alloys in the 2xxx series do not have as good corrosion resistance as most other aluminum alloys, and under certain conditions they may be subject to intergranular corrosion. Besides, high hardness, strength,

good weldability, high fracture toughness are the other properties of 2xxx series aluminum alloys [3, 5, 6].

2011 aluminum alloy is a high mechanical strength alloy that machines exceptionally well because of the Bi and Pb content. It is often called free machining alloy (FMA) and well suited to use in automatic lathes. Machining at high speeds (1500 – 2000 RPM) produces fine chips that are easily removed. The excellent machining characteristics allow the production of complex and detailed parts. In some circumstances 2011 alloy can replace free machining brass without the need for alterations to tooling. It has poor corrosion resistance which means parts made from 2011 alloy tend to be anodized to provide additional surface protection [7].

This paper shows the effect of chemical composition, solution treatment temperature, aging parameters on mechanical properties of 2011 aluminum alloys.

2. Experimental Procedure

2011 alloy 1800 mm length and 355 mm diameter billets which had 4 different chemical composition were prepared by direct chill casting method in prototype casting facility. The chemical composition of the alloys are listed in Table 1. The billets were homogenized at 480 °C for 12 hours and cooled down with air. To obtain T6 hear treatment, Al-Cu alloys followed heat treatments of solution treatment at 510 and 525 °C for 2 hours, quenching in water at 20 °C. After solution treatment, artificial aging at different temperatures for various times were performed. Hardness were measured by Emco Test M4C and mechanical properties tests were performed by Zwick Roell Z250 universal testing machine. Metallographic microscope (Zeiss Axio Scope A1) was

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employed for metallographic microstructure analysis. SEM observations were performed on scanning electron microscope (Zeiss Evo MA15). Heat and process flow chart is given in Figure 1.

Table 1. Chemical composition of the billets (% wt.)

Alloy E* Si Fe Cu Bi S** 0.40 0.70 5.0-6.0 0.2-0.6 1 0.40 0.47 5.67 0.31 2 0.47 0.24 5.64 0.43 3 0.14 0.12 4.83 0.28 4 0.22 0.35 4.82 0.32 E* Pb Zn Mg Al S* 0.2-0.6 0.3 - Bal. 1 0.22 0.001 - Bal. 2 0.41 0.001 - Bal. 3 0.28 0.001 - Bal.

2011

4 0.24 0.001 - Bal. E*: Element S**: Standard

Figure 1. Heat and process flow chart of experiments

Figure 2. Schmetic view of extrusion process: a) press b) profile

55 MN (5500 tons) SMS direct extrusion press located at ASA Aluminum was used for profile manufacturing. Extrusion schematic view and profile are shown in

Figure 1. Each billet was heated to 200 °C in gas furnace. Each hot billet was then heated to 450- 460 °C separately in induction furnace before extrusion. Each billet then extruded to shape of 68 mm wide and 30 mm thick with profile speeds between 1 – 3.1 m/minutes. Extrusion parameters for each billet is shown in Table 2. Extruded profiles were cooled (quenched) with water pool at the press exit. Profile codes are come from billet codes.

Table 2. Extrusion parameters

BilletTemperature

(°C)

ProfileSpeed

(m/min.)

Press Exit Temperature

(°C)Billet-1 460 1.0 474 Billet-2 450 2.0, 2.5, 3.1 465, 472, 474 Billet-3 450 2.8 472 Billet-4 450 2.7 465

After extrusion, profiles were subjected to various heat treatments. Some profiles were left for natural aging for 10 days, some for 20 days and then solution treated and quenched then artificially aged. Solution heat treatment (ST) process was heating up the profiles to 510 and 525 °C and holding for 120 minutes then quenching into water at 20 °C. Heat treatments applied to profiles are shown in Table 3.

Table 3. Heat treatments after extrusion

Natural Aging (Days)

SolutionTreatment

Artificial Aging Condition

ST-1 10 510 °C 2 h 160 °C 12 hours ST-2 10 525 °C 2 h 160 °C 12 hours HT-1 20 525 °C 2 h 170 °C 12 hours HT-2 20 525 °C 2 h 180 °C 12 hours HT-3 20 525 °C 2 h 165 °C 16 hours HT-4 20 525 °C 2 h 170 °C 8 hours HT-5 20 525 °C 2 h 180 °C 6 hours HT-6 20 525 °C 2 h 180 °C 8 hours

3. Results and Discussion

Figure 3 demonstrates the microstructure of as cast and homogenized 2011 alloy. Dendritic phases can be seen as cast conditions. Homogenization temperature is not high enough for precipitate and intermetallic particles to dissolve inside matrix for Al-Cu alloy in homogenized images. Homogenization temperature is necessary to optimize with the changing chemical composition of

UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

242 IMMC 2018 | 19th International Metallurgy & Materials Congress

alloy. Higher temperature increases the possibility of local melting of the residual phases. As cast and homogenized SEM image of billet-3 is given in Figure 4. Billet-3 was homogenized better than other billets at 480 °C.

Figure 3. Optical microstructures of billets: a) billet-1, c) billet-2, e) billet-3 and g) billet-4 as cast, b) billet-1, d)

billet-2, f) billet-3 and h) billet-4 homogenized

EDS analysis of as cast condition billet-3 is given in Figure 5 which light grey particle is mostly Bi-Pb phase.

Figure 6 shows mechanical properties of the solution treated at 510 and 525 °C and than aged at 160 °C 12 hours. It is obvious that yield and tensile strength of the Al-Cu alloy increases up to 386 MPa with increasing the solution temperature from 510 to 525 °C.

Figure 4. SEM image of billet-3: left) as cast, right) homogenized

Figure 5. EDS analysis of as cast billet-3

Tensile strength results are shown in Figure 7 after aged at different temperatures and holding times.

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Artificial aging at 160 °C does not reveal an appreciable improvement in yield strength, probably because up to this aging temperature, the GP zones and/or precipitates formed may not be sufficient enough to reflect noticeable changes in the yield strength. It is observed that mechanical properties of profile 2 and 4 chemical composition are effecting with temperature change.

Figure 6. Yield and tensile strengths of profiles solution treated at 510 and 525 °C

Figure 7. Tensile strengths of profiles solution treated at 525 °C than agaed at different temperatures and times

4. Conclusion

4 different chemical composition of 2011 alloy billets were succesfully prepared by vertical semi continous direct chill casting method in prototype casting facility in ASA Aluminum. After casting process, 4 billet were homogenized at 480 °C for 12 hours. It was observed from optical and SEM images that homogenization temperature should be higher. Solid rectengular profiles were produced with extrusion technique in 5500 tons extrusion press with 2 cavities die. Profiles were cooled after press exit with water pool to room temperature. Profiles were natural aged for 10 and 20 days. 510 and 525 °C temperatures were used for solution treatment proces and it was found that 525 °C is more effective to get better mechanical properties. Aging experiments were carried out at different temperatures from 160 to 180 °C with different holding times like 6, 8, 12 and 16 hours. The ultimate tensile strength shows the maximum after solution treated at 525 °C and aging at 170 °C 8 hours.

Acknowledgement

It is a pleasure to thank Mr. Berk YÖRÜKER and Mr. Özcan YILDIZ for their help with the tensile testing part of the work. Mr. Mustafa Serkan ÖZCAN is gratefully acknowledged for his intensive support during production.

References

[1] Wilm, A., “Room Temperature Hardening of an Al-Cu Alloy,” Metallurgie, 1911, Vol. 8, 223

[2] Davies, J.R., Aluminum & Aluminum Alloys, ASM, Materials Park, Ohio, 1993, 121-142

[3] Bakavos D, Prangnell PB, :BES B, EBERL F., “The effect of silver on microstructural evolution in two 2xxx series Al Alloys with a high Cu:Mg ratio during aging to a T8 temper., Mater Sci Eng A, 2008, 491(1-2): 214-223

[4] Sofyan B. T., Raviprasad K., Effects of microalloying with Cd and Ag on the precipitation process of Al-4Cu-0.3Mg (wt.%), Micron, 2011, 32(8) 851-856

[5] Chang C. H., Lee S. L., Effect of Ag content and heat treatment on the stress corrosion cracking of Al-4.6Cu-0.3Mg alloy, Mater Chem Phys, 2005, 91(2-3): 454-462

[6] Xiao D. H., Song M., Superplastic deformation of an as rolled Al-Cu-Mg-Ag alloy, Mater Des, 2009, 30(2): 424-426

[7] Jang J., Nam G., Effect of solution treatment aand artificial aging on microstructure and mechanical properties of Al-Cu alloy, Trans. Nonferrous Met. Soc., 23(2013) 631-635