The 5th PSU-UNS International Conference on Engineering and Technology (ICET-2011), Phuket, May 2-3, 2011

Prince of Songkla University, Faculty of Engineering Hat Yai, Songkhla, Thailand 90112

Abstract: In this study the effect of as-welded and post welded aging on the microstructure and mechanical properties of friction stir welded SSM7075 aluminium alloy joints was investigated. Semi solid plates were applied for butt joint. FSW was produced using different welding speed and rotation speed. Microstructure and Transverse tensile strength of welds were studied in detail. It was found that the as-welded condition using 1110 rpm with 110 mm/min gives highest tensile strength. Artificial aging was applied from previously optimized welding parameters of as-welded condition. However, post-weld aging could not enhance the tensile properties of the FSW 7075 aluminium alloy joint. Key Words: Friction stir welding (FSW), SSM7075 aluminium alloy, post-welded aging

1. INTRODUCTION

Friction stir welding (FSW) was developed at The Welding Institute (TWI), UK in 1991 [1]. It is a solid state joining process, in which a cylindrical shouldered tool with a profiled pin is inserted into the joint line between two pieces of material. The basic principle of the FSW process is shown in Fig. 1. In FSW the work piece does not reach the melting point. In recent years, Semi-Solid Metal (SSM) has become popular for various applications such as SSM7075. This alloy is difficult to weld using fusion welding. This is due to solidification cracking and other weld defect [2-3]. The microstructure of FSW welded joint consists of four different zones as shown in Fig. 2. They are: a) base metal (BM), b) heat affected zone (HAZ), c) thermo-mechanical affected zone (TMAZ) and d) nugget zone (NZ). The nugget zone exhibited a recrystallized fine grain structure with grain sizes increasing from the weld region to base metal. The post-weld aging process can compensate the hardness decrease observed in as-welded joints [4]. In general, post-weld aging had very little influence on yield strength, increasing from 2 to 10% for

most heat treatment, while tensile strength decreased from 1.3 to 9.7% for all age treatment [5]. In this work SSM7075 in T6 condition have been post-weld aged after being joined by FSW. The effect of post-weld aging have been investigated for its microstructure, tensile strength and hardness.

2. EXPERIMENTAL

The base material used in this investigation was 4 mm. thick plate semi solid aluminium alloy of 7075-T6, have been cut into the required dimensions 100 x 50 mm. by hacksaw cutting and milling. Square butt joint configuration, as shown in Fig. 3 were prepared to fabricate FSW joints. The chemical composition and room temperature mechanical properties of the base metal are presented in Table 1. and Table 2. The cylindrical pin was used for friction stir welding as shown in Fig. 4. The diameter of the shoulder was 20 mm. The diameter and length of the pin were 5 mm and 3.6 mm, respectively. The welding tool was rotated in the clockwise direction and specimens, which were tightly fixed at the backing plate, were traveled, in this work, the welding speeds were 70, 90 and 110 mm/min. The tool rotation speed were 1110 and 1320 rpm and the tilt angle was 3˚. Three joints were fabricated using previously optimized FSW parameters of different rotation speed. The effect of post welded aging on tensile properties, was studied and compared with the as-welded condition. The post welded aging on artificial aging was carried out at 120 °C for a holding period of 36 h. After post welded aging, the welded parts were sliced using a hacksaw cutting and milling machine. Tensile properties of welded parts were determined by the use of transverse tensile specimens according to ASTM E-8M specification. Tensile test has been carried out in 10 kN, electronic universal testing machine. Tensile test were performed at room temperature, using a (Hounsfield,

Effect of post-weld aging on the microstructure and mechanical properties of

friction stir welded SSM7075 aluminium alloy

Jennarong Naktewan1, Abdul Binraheem2, Prapas Muangjunburee1* 1Prince of Songkla University, Faculty of Engineering, Thailand

2Prince of Naradhiwas University, Faculty of Engineering, Thailand * email: mprapas@eng.psu.ac.th

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UK; model: H25K-S). Results are compared with the base material and as-welded condition. Microstructure analysis was carried out using a light optical microscope (Olympus, Japan; model: BH2-UMA). The welded part were prepared by polished and finally etched with Keller’s reagent: 150 ml H2O, 3 ml HNO3, 6 ml HCl, and 6 ml HF [4]. The specimens were etched for about 5 s. in order to study the grain structure of the nugget zone and other areas for optical microscopy. Vickers hardness (Highwood, Japan; model: HWDM-3) measured on a cross-section at 2.5 mm from surface, with 100 gf load for 10 s.

Fig. 1. Illustration of the FSW process

a) Base Metal (BM) b) Heat Affected Zone (HAZ) c) Thermo Mechanical Affected Zone (TMAZ) d) Nugget Zone (NZ) Fig. 2. Different zones of FSW joint Table 1. Chemical composition of SSM7075 (wt%) Zn Mg Cu Fe Cr other Al

6.08 2.5 1.93 0.46 0.19 0.45 Balance

Fig. 3. Dimensions of square butt joint

Fig. 4. Tool pin profile Table 2. Mechanical properties of SSM7075

Material Yield

strength, MPa

Ultimate tensile

strength, MPa

Elongation, %

Vickers hardness at 100 gf load, Hv

7075-T6 361.84 452.3 14.32 102.4

3. RESULT AND DISCUSSION 3.1 Microstructure The joint constructed with a rotation speed of 1110 rpm shows defect-free for every welding speeds. On the other hand, the rotation speed of 1320 rpm reveals micro voids (300-900 µm) in the nugget zone as shown in Fig. 5a and b. The location of defect was occurred below the weld nugget on advancing and retreating side, which resulted due to excessive heat input. And insufficient of material flows around the advancing and retreating side of the pin [6], it may correspond to high rotation speed. For all of these reasons, this study was selected 1110 rpm condition for post-welded aging process.

Fig. 5. Showing macro and microstructure of nugget zone for 1320 rpm with 110 mm/min The investigated material, as it is well known, is an alloy of aluminium and zinc; it had undergone a T6 heat treatment, which is composed of a solution treatment, a quench in water and an artificial aging, In this way an MgZn2 precipitation hardening phenomenon occurs. The microstructure of base metal consists of acicular eutectic precipitates MgZn2 embedded in α–aluminium matrix [6-7]. Fig. 6a. exhibits a globular grains structure on the order of 30 to 70 µm in diameter is present in the parent SSM7075-T6 plate. Fig. 6b. shows the grains size increasing moving from the weld region to the base metal. The region adjacent to the nugget zone, TMAZ is characterized by a highly deformed structure at high temperature without recrystallization. In the HAZ, there is no plastic deformation occurring in this area.

Fig. 6. Four different zones associated with FSW: grobular grains in the BM, deformed grains in TMAZ, and fine recrystallized grains in the NZ and not deformed grains in HAZ Fig. 7. shows a recrystallized fine grain structure in the nugget zone. In the interior of the

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recrystallized grains, usually there is low dislocation density. However, some investigators reported that the small recrystallized grains of the nugget zone contain high density of sub-boundaries, subgrains and dislocations. The higher temperature and severe plastic deformation results in grains smaller than the base metal [8-9]. Fig. 7d. shows the coarse grains after a 120 °C for 36 h post-welded aging, a grain structure on the order of 5 to 20 µm, which larger than as-welded at 90 mm/min. Fig. 7e. reveals a fine equiaxed grain structure on the order of 3 to 8 µm compared to Fig. 7f. shows a fine equiaxed grain structure on the order of 5 to 15 µm in diameter is present in the nugget zone. In the as-welded condition, higher welding speeds were associated with low heat inputs, which result in faster cooling rates of FSW joint, this reasons, effect to grain sized was decreased, when the welding speed was increased.

a) As-welded b) Post-welded aging

c) As-welded d) Post-welded aging

e) As-welded f) Post-welded aging Fig. 7. Equiaxed grains of; a, b) 1110 rpm with 70 mm/min, c, d) 1110 rpm with 90 mm/min and e, f) 1110 rpm with 110 mm/min 3.2 Microhardness Microhardness measurements (HV) have been conducted for all joints, in order to determine the hardness properties across the weld region. The middle section (2.5 mm from the top surface) has been chosen for six types of joints is given in Fig. 8. That the hardness of all welds increases after a 120 °C for 36 h post-welded aging. The highest average hardness value across the all location (NZ, TMAZ and HAZ) is about 167.70 HV for 1110 rpm with 110 mm/min of aging. The 1110 rpm with 90 mm/min of as-welded shows lower

average hardness about 102.32 HV which is near that of the parent material. The post-welded aging seems to improve a hardness; as the figure depicts there is no significant decrease in hardness throughout the weld region. On the other hand, some investigators reported that the hardness was decreased in the nugget and TMAZ, which is due to the replacement of a fine precipitate distribution of ή/η-phase within the grains by a coarse particle distribution of η-phase [10].

Fig. 8. Hardness distribution along the weld region 3.3 Tensile strength The transverse tensile properties of friction stir-welded SSM7075 aluminium alloy joints are presented in Table 3. The values shown are average of four tests, in both as-welded and post-welded aging condition shown a reduction in yield and ultimate tensile strength compared base metal. The 1110 rpm with 110 mm/min of as-welded shown the higher yield strength and tensile strength are about 214.3 and 267.9 MPa, respectively. The 1110 rpm with 110 mm/min of aging joints showed a joint efficiency of 47.31%, while the as-welded joints exhibited a high joint efficiency of 59.23% Table 3. Transverse tensile properties of friction stir welded SSM7075 joints

Condition Welding Speed,

mm/min

Yield strength,

MPa

Ultimate Tensile

Strength, MPa

Elongation, %

Joint Efficiency,

%

1110 rpm

70 199.76 249.7 9.64 55.21

90 201.84 252.3 10.53 55.78

110 214.32 267.9 14.56 59.23

1110 rpm + Aging

70 187.36 234.2 10.4 51.78

90 194.48 243.1 10.24 53.75

110 171.52 214.4 7.12 47.31

In the as-welded condition, lower welding speeds were associated with high heat inputs, which result in slowly cooling rates of FSW joint, effected to higher temperature in the FSW processing zone causes grain growth and tensile strength decrease, welding speed also decrease at constant rotation speed. Some investigators reported that the reduce strength to reduction in pre-existing dislocations and the elimination of the very fine hardening precipitates. The tensile properties of the post-weld aging joints are far lower than those of as-welded joints. This resulted from abnormal

0

50

100

150

200

250

12.5

-11

-9.5 -8

-6.5 -5

-3.5 -2

-0.5 1

2.5 4

5.5 7

8.5 10

11.5

Ha

rdn

ess

(H

V)

Distance from weld center (mm)

1110 rpm 70 mm/min 1110 rpm 90 mm/min

1110 rpm 110 mm/min 1110 rpm 70 mm/min+Aging

1110 rpm 90 mm/min+Aging 1110 rpm 110 mm/min+Aging

Retreating side Advancing side

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grain growth throughout the friction stir welded zone [11-12]. Table 4. shows the location of fracture, in as-welded and post-welded aging condition, most failures occurred as shear fracture in the HAZ. The tensile specimens contained microstructure of all regions; BM, HAZ, TMAZ (Advancing and Retreating sides) and NZ. The different regions have different resistances to deformation due to differences in grain size and precipitate size and distribution. The HAZ has the lowest strength due to significantly coarsened precipitates and the development of the precipitate-free zones (PFZs). Thus, during tension, strain occurs mainly in the HAZ. Therefore, fracture always occurred in the HAZ, resulting in a low strength and ductility along transverse orientation of the weld [13].

Table 4. Location of fracture SSM7075 joints

Condition

Welding Speed,

mm/min Location of fracture

1110 rpm

70 In HAZ at advancing side

90 Weld center along the joint line

110 In HAZ at Retreating side

1110 rpm + Aging

70 In HAZ at advancing side

90 In HAZ at advancing side

110 In HAZ at advancing side

4. CONCLUSIONS

1). The nugget zone exhibited a recrystallized fine grain structure with grain size increasing moving from the weld region to the base metal.

2). The hardness of post-weld aging condition was increased after artificial aging at 120 °C under 36 h.

3). The post-welded aging improves the hardness; as the figure depicts there is no significant decrease in hardness throughout the weld region.

4). The maximum yield strength and tensile strength are 214.32 and 267.9 MPa, respectively, with 59.23 % of joint efficiency.

5). The tensile properties of the post-weld aging joints are far lower than those of as-welded joints. This is result of abnormal grain growth on nugget zone.

5. ACKNOWLEDGEMENTS

The authors are grateful to the Department Mining and Materials Engineering, Prince of Songkla University, Graduate School for scholarship and the Prince of Naradhiwas University for research fund. The helps from the Welding and Joining Research Team, especially, Mr. Jarun Tamjai and Mr. Chaiyoot Meengam for all kind support.

6. REFERENCES [1] T. J. Linert, W. L. Stellwag, JR., B. B. Grimmett, R. W. Warke, “ Friction Stir Welding Studies on Mild Steel” , The Welding Journal, 2003. [2] P. Cavaliere, A. Squillace, “ High temperature deformation of friction stir processed 7075 aluminium alloy”, Materials Characterization., 2005, Vol. 55, pp. 136-142. [3] Christian B. Fuller, Murray W. Mahoney, Mike Calabrese, Leanna Mcon,” Evolution of microstructure and mechanical properties in naturally aged 7050 and 7075 Al friction stir welds”, Materials Science and Engineering A., 2010, Vol. 527, pp. 2233-2240. [4] C. Yeni, S. Sayer, O. Ertugrul, M. Pakdil,” Effect of post-weld aging on the mechanical and microstructural properties of friction stir welded aluminum alloy 7075”, Archives of Materials Science and Engineering., 2008, Vol. 34, No. 2, pp. 105-109. [5] Rajiv S, Mishra, Murray W. Mahoney,” Friction Stir Welding and Processing”, 2007, ASM International., pp. 51-70. [6] S. Rajajumar, C. Muralidharan, V. Balasubramanian. ’’ Influence of friction stir welding process and tool parameters on strength properties of AA 7075-T6 aluminium alloy joints’’. 2011, Materials and Design., Vol. 32, pp. 535-549. [7] T. Azimzadegan, S. Serajzadeh,” An Investigation into Microstructures and Mechanical Properties of AA7075-T6 during Friction Stir Welding at Relatively High Rotational Speeds”. 2010, Journal of Materials Engineering and Performance. [8] Manoj Kumar Shivaraj, Vijay Dinakaran, Venkatesh Mahadevan.’’ Friction Stir Welding on Aluminum Alloys AA2024-T4 and AA7075-T6’’. 2nd ICCET, 2010, Vol. 5. [9] R.S. Mishra, Z.Y. Ma,’’ Friction Stir Welding and processing”. 2005, Materials Science and Engineering R., Vol. 50, pp. 1-78. [10] A. Sullivan, J.D. Robson,” Microstructural properties of friction stir welded and post-weld heat-treated 7449 aluminium alloy thick plate”. 2008, Materials Science and Engineering A., Vol. 478, pp. 351-360. [11] C.G. Rhodes, M.W. Mahoney, W.H. Bingel, R.A. Spurling, C.C. Bampton,” Effect of Friction Stir Welding on Microstructure of 7075 aluminum”. 1997, Scripta Materialia., Vol. 36, pp. 69-75. [12] Hakan Aydin, Ali Bayran, Ismail Durgun,” The effect of post-weld heat treatment on the mechanical properties of 2024-T4 friction stir-welded joints”. 2010, Materials and Design., Vol. 31, pp. 2568-2577. [13] M.W. Mahoney, C.G. Rhodes, J.G. Flintoff, R.A. Spurling, W.H. Bingel, Metall. Mater. Trans. A 29 (1998) 1955.

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