ScienceDirect Microstructure and Mechanical

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ScienceDirect Procedia Materials Science 6 (2014) 1600 – 1609

3rd International Conference on Materials Processing and Characterisation (ICMPC 2014)

Microstructure and Mechanical Properties of AZ31B Magnesium alloy by Friction stir Welding S.Ugendera, A.Kumar b , A. Somi Reddy a,b* a

b

Department of Mechanical Engineering, Research Scholar, JNTU, Hydearabad, India-500085 Department of Mechanical Engineering, National Institute of Technology, Warangal, India-506004 a,b * Department of Mechanical Engineering, VITS, Karimnagar, India-505468

Abstract

Friction stir welded Mg AZ31B alloy have been investigated. Friction stir welding (FSW) is carried out at different rotational speeds of 900 rpm, 1120 rpm, 1400 rpm and 1800 rpm and with change of tool materials such as High speed steel (HSS) and Stainless steel (SS) at a constant welding speed of 40 mm/min, tilt angle of 2.5 0 and axial force of 5 KN. It is observed In this study, the effect of tool material and rotational speed on microstructure and mechanical properties of that the joint fabricated using SS tool material at a rotational speed of 1120 rpm obtained higher mechanical properties as compared to those of 900 rpm, 1400 rpm and 1800 rpm and also to those of HSS material. © 2014 by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license 2014The TheAuthors. Authors.Published Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of the Gokaraju Rangaraju Institute of Engineering and Technology (GRIET). Selection and peer review under responsibility of the Gokaraju Rangaraju Institute of Engineering and Technology (GRIET) Key words:Friction stir Welding; Magnesium Alloy; Mechanical properties, Tool Material, Rotational speed

* Corresponding author. Tel.: +91 9949437892 FAX: 0870-2817456 E-mail address: [email protected]

2211-8128 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer review under responsibility of the Gokaraju Rangaraju Institute of Engineering and Technology (GRIET) doi:10.1016/j.mspro.2014.07.143

S. Ugender et al. / Procedia Materials Science 6 (2014) 1600 – 1609

1. Introduction Friction stir welding (FSW) is a significant manufacturing process for producing welded structures in solid state [1]. This process offers several advantages compared to the conventional welding methods including higher mechanical properties and lower residual stresses as well as reduced occurrence of defects [2]. In FSW process, a rotating tool having a shoulder moves along the center line. Rotational motion of the shoulder generates frictional Nomenclature A B C

Friction Stir Welding Magnesium Alloys Rotational speeds

heat leading to a softened region around the pin while the shoulder prevents deforming material from being expelled. In fact, a weld joint is produced by the extrusion of material from the leading side to the trailing side of the tool [3]. Magnesium alloys are constantly gaining importance as lightweight structural materials for automotive applications [4]. The Mg alloys are especially attractive due to their low density, high specific stiffness and strength and also the recycling ability [5]. Additionally Mg exhibits heat conduction and electromagnetic interference shielding, which makes it more attractive than polymeric materials to electronics industry for use in a variety of portable devices [6]. However, Mg is not amenable to satisfactory room temperature forming due to its hexagonal close-packed crystal structure and limited number of independent slip systems [7]. Deep drawing at elevated temperature is currently used for fabricating sheet parts while a majority of Mg products are fabricated in smaller and thicker geometries by die casting process .The lower formability of Mg alloys can be overcome by warm forming (stretch) processes. However, at elevated temperatures (>300ºC) oxidation problems complicate manufacturing process [8]. The use of magnesium alloy as the structural material has been generally increasing in automobile, electronics and other industries due to many advantages such as light weight, .high specific strength, and recyclability. Recently, casting process using the magnesium alloys has widespread usage in manufacturing the complex-shaped parts due to its good castability. However, the mechanical properties of the parts made by casting process may not meet industrial requirements. Thus, alternative process like forging is been employed to improve the mechanical characteristics. However, magnesium alloy has the hexagonal close-packed crystal structure whose dominant slip system at room temperature is basal slip. This is not sufficient for homogeneous deformation of polycrystalline material. This led to the friction stir welding of magnesium-base alloys has not been studied extensively as aluminum but some results have been reported [Nagasawa et al. [9] have friction stir welded 6-mm thick AZ31 plates with a rotational speed of 1750 rpm and a transfer speed of 88 mm/min and found out that the mechanical strength of the weld was comparable to the base material but with only half of the ductility. Park et al [10] also studied FSW on 6-mm AZ31 plates at 1230 rpm and 90 mm/min which showed a much lower yield strength and elongation, and slightly lower ultimate tensile strength of the weld from the transverse tensile test compared with the base material. Nakata et al [11] studied the optimal processing conditions for FSW of 2-mm AZ91D thixomoulded sheet. An increase of 38 to 50% of the tensile strength in the weld could be obtained over base material with a rotational speed between 1240 rpm to 1750rpm and transverse speed of 50mm/min. They contributed to the increase of strength due to the fine recrystallization structure of 2-5 om. Padmanabham et al. [12] studied the tool material effect on the friction stir welding of AZ31B magnesium alloy. Tool materials such as stainless steel, high speed steel, and armour steel, mild steel and high carbon steel were used. Hardness values of these materials are 40, 73, 58, 30 and 66 HRC respectively. It was concluded that the joints fabricated by high carbon steel tool with 66 HRC, threaded pin profile and a shoulder diameter of 18mm (D/d=3) exhibited superior tensile properties compared to their contradict parts. The tool material which possesses higher hardness may generate much higher heat due to higher coefficient of friction. If this is the case, high speed steel might have generated high heat than the stainless steel. Due to the high thermal conductivity of high speed steel much heat might have developed at the tool shank

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resulting in the poor mechanical properties compared to those of high carbon steel. Most of the published papers were focused on the effects of rotational speed and translational speed on the stirred zone properties of AZ31 magnesium-base alloy [13]. It has been shown that higher hardness values (72 Hv) can be obtained compared to base material hardness (68 Hv) at a certain combination of rotational and translational speeds. The hardness value decreased as the rotational speed increased in the range of 1200 rpm to 2000rpm and at 30 in/min (760 mm/min) translational speed. On the other hand the hardness value increased as the translational speeds increased in the range 15 to 30 in/min (381 mm/min to 760 mm/min) and at 1200 rpm rotational speed. However, there is no result reported in the literature related to combination of Tool materials and rotational speed, which influence on mechanical properties of FSWed AZ31B Mg alloy. In this present investigation, effect of rotational speed (i.e. 900 rpm, 1120 rpm, 1400 rpm and 1800 rpm) and tool material on (SS and HSS) mechanical properties of friction stir welded of AZ31B Magnesium alloy of the microhardness and mechanical properties are evaluated. 2. Experimental Rolled plates of 5 mm thickness AZ31B magnesium alloy were cut to the required dimensions (240mm×60 mm×5mm) by wire cut Electric Discharge Machine. The schematic diagram of AZ31B Mg alloy plates used for FSW is shown in Fig.1. The chemical composition of base metal is presented in Table 1.

Fig.1 The schematic diagram of AZ31B Mg alloy plates used for FSW

Table.1. Chemical composition (wt %) of base metal AZ31B magnesium alloy. Al

Mn

Zn

Cu

Ni

Si

Fe

Mg

3.0

2.0

1.0

0.05

0.005

0.1

0.005

Balance

The initial joint configuration was obtained by securing the plates in position using mechanical clamps. The direction of welding is normal to the rolling direction and single pass FSW used to fabricate the joints. The diameter of the tool shoulder (D) is 18 mm and that of the insert pin diameter (d) and pin length (L) are 6 mm and 4.8 mm respectively. The schematic diagram of Tool geometry is shown in Fig.2.


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Fig.2 The schematic diagram of Tool geometry The FSW parameters such as tool rotational speeds and travelling speed were 900 rpm, 1120 rpm, 1400rpm, and 1800 rpm with 40mm/min respectively. The tool onward tilted an angle of 2.5 0 and a vertical load of 5KN is applied. The FSW process parameters and tool nomenclature are presented in Table 2.The process is carried out on a vertical milling machine (VMM) (Make HMT FM-2, 10hp, 3000rpm). The macrographs of VMM and tool arbor are shown in Fig.3 and Fig.4 respectively. For various testing the required dimensions of the specimens were cut from the region under the tool shoulder (i.e. stir zone) by using wire EDM. Table. 2. FSW process parameters and tool nomenclature Rotational speed(rpm)

900,1120,1400,1800

Welding speed(mm/min)

40

Pin length(mm)

4.8

Tool shoulder diameter(mm)

18

Axial force(KN)

5

Tilt angle

2.50

Pin diameter(mm)

6

Shoulder diameter(mm)

18

D/d Ratio of tool

3.0

Tool materials

Stainless Steel, High Speed Steel

Tool Profile

Taper with Threaded

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Fig. 3. The macrographs of Vertical milling machine

Fig.4. The macrographs of tool arbor The specimens for metallographic examination were sectioned to the required size and then polished using different grades of emery papers. A standard reagent made of 4.2 g picric acid, 10 ml acetic acid, 10 ml diluted water, and 70 ml ethanol was used to reveal the microstructure of the welded joints. Micro structural analysis was carried out using a light optical microscope (Maker: Metzer-M, Binocular Microscope; model: METZ-57) incorporated with an image analysing at high magnification to estimate the weight percentage of elements. Microhardness properties were measured on the cross section of the FSWed joint perpendicular to the processing direction by using Vickers hardness tester utilizing a 100g load for 15 s. The schematic diagram of Microhardness survey is shown in Fig.5. The smooth tensile specimens were prepared as per ASTM: E8/E8M-11 standard to evaluate yield strength, tensile strength, and elongation of the joints. The schematic diagram of the tensile specimen is shown Fig.6. Tensile test was carried out in a 100 KN electromechanical-controlled universal


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testing machine (maker: FIE-Bluestar, India; model: TUE-600C). Tensile testes of as-received Mg alloy and the FSWed joint were determined at ambient temperature and three specimens were machined from each joint and the average was reported.

Fig.5 The schematic diagram of Microhardness survey

Fig.6 The schematic diagram of the tensile specimen The charpy impact specimens were prepared according to the ASTM: E23-06 standard and evaluate the impact toughness of the weld metal and stir zone, and hence the notch was placed (machined) at the weld metal (weld centre) as well as in the SZ. The schematic sketch of charpy impact specimen is shown in Fig.7. Impact testing was conducted at room temperature using a pendulum type impact testing machine with maximum capacity of 300 J.

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Fig. 7 The schematic sketch of charpy impact specimen The heat input for Friction stir welding process can be calculated as [14] Heat input, q=2π×μ×p×ω×Rs×η/3s Where p=normal force, KN w=Rotational speed, rps Rs=shoulder radius, meters s=welding speed, mm/sec 3. Results and Discussions 3.1. Effect of Rotational Speed on Mechanical Properties 3.1.1. Tensile properties The effect of tool rotational speed (i.e. SS tool material) on Mechanical properties such as tensile strength, yield strength and % of elongation of Friction Stir Welded AZ31B magnesium alloy joints are presented in Table 3. In FSW, tool rotation speed results in stirring and mixing of material around the rotating pin which in turn increase the temperature of the metal. It appears to be the most significant process variable since it is tends to influence the transitional velocity. It is known that the maximum temperature observed to be a strong function of tool rotation speed [15]. At lower rotational speed (900rpm), the ultimate tensile strength, yield strength and % of elongation of FSW joints is lower. When the rotational speed is increased from 900rpm, correspondingly the ultimate tensile strength also increases and reaches a maximum at 1120 rpm made of SS tool material. If the rotational speed is increased above 1120 rpm, the tensile strength of the joint decreased. Higher tool rotational speed (1800 rpm) usually resulting in higher heat input per unit length and slower cooling rate in the FSW zone causes excessive grain growth, which subsequently lead to lower tensile properties of the joints. A higher rotational speed also causes expensive release of stored materials to the upper surface, which produces micro-voids in the stir zone and this may be one of the reasons for lower tensile properties of the joints, even at lower rotational speed (900 rpm) results in lower tensile properties which is due to lack of stirring and lower heat input per unit length that leads to insufficient plasticization. It is observed that the joint fabricated at a tool rotational speed of 1120 rpm made of SS tool material exhibited higher tensile strength, yield strength and % of elongation and this may be due to optimum heat generation which is sufficient to cause free flow of plasticized material and adequate mechanical working [16].

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3.1.2. Hardness The hardness was measured across the weld in the nugget zone using Vicker’s microhardness testing machine, and the values are presented in Table 3. The hardness of base metal (unwelded parent metal) is 69 Hv. Vickers microhardness is measuring along the mid thickness line of cross section of the joint. The joint fabricated with the rotational speed of 1120 rpm, welding speed of 40 mm/min, recorded higher hardness (75Hv) in the stir zone, and this is also one of the reasons for superior tensile properties of these joints compared to other joints. These are two main reasons for the improved hardness of stir zone.Firstly, since the grain size of stir zone is much finer than that of base metal, grain refinement plays an important role in material strengthening, secondly the small particles of intermetallic compounds are also a benefit to hardness improvement [17]. Table.3. Effect of Rotational speed and tool material on mechanical properties of AZ31B Mg alloy using SS tool and HSS tool Joint no

Rotational Speed (rpm)

1

900

2

3

Ultimate Tensile strength (Mpa)

Yield strength (Mpa)

Elongation (%)

Hardness (Hv)

Impact Test, Joules

Heat Input KJ/mm

SS

96.18

71.52

1.39

70

5

0.28

HSS

129.77

94.51

2.48

66

5

0.38

SS

181.94

139.5

4.02

75

6

0.71

HSS

135.15

100.71

3.32

70

5

0.49

SS

171.12

129.86

2.91

69

5

0.78

HSS

186.76

139.1

5.00

71

6

0.85

SS

155.04

117.69

2.24

70

5

0.91

HSS

152.55

113.46

4.08

68

5

0.89

215

171

14.7

69

1120

1400 1800

4

5.

Base Metal

The joint fabricated with a rotational speed of 1120 rpm with SS tool material exhibited higher hardness 75 Hv in the stir zone compared to other rotational speeds and this is also one of the reasons for superior tensile properties of these joints. Higher tool rotational speed resulted in higher heat generation and this lead to the excessive release of stirred material to the upper surface which results in lower hardness. 3.1.3. Impact Toughness Charpy impact toughness of FSW joint was evaluated and presented in Table 3.The impact toughness of unwelded base metal is 8J.However, the impact toughness of FSW joint with notch placed at the SZ region and

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reached maximum 6 J at 1120 rpm, compared to the other rotational speeds. It is observed that the joint fabricated at a tool rotational speed of 1120 rpm made of SS tool material exhibited higher impact strength 6 joules and this may be due to optimum heat generation which is sufficient to cause free flow of plasticized material. 3.1.4. Microstructure The optical micrographs taken at stir zone of FSW of all the joints are displayed in Fig.8 (A-I).From the micrographs, it is understood that there is in appreciable variation in average grain diameter of weld region in AZ31B Magnesium alloy. Due to FSW, the coarse grains of base metal are changed in to fine grains in the stir zone. The joints fabricated with a rotational speed of 1120 rpm with a constant welding speed of 40 mm/min and SS tool contain finer grains in the weld region compared to other joints. This is one of the reasons for higher tensile properties of these joints compared to other joints. From the micrographs, it is inferred that there is an appreciable variation in grain size across the welds; this is because of in sufficient plastic flow and thermal exposure, It has been observed during this work that the total impact energy increased in the friction stir welding of (medium strength) AZ31B Mg alloy for both temper conditions especially at 1120 rpm and 40 mm/min with respect to the base metal while rotation and transverse speed have little effect on the impact value of (high strength) results were very close to each other. Finally it is important to mention that the relation between rotation speed, transverse speed and input heat which affect on the impact value seems to be compound and depend on the material properties being welded, Grains are relatively smaller in the retreading side of SZ compared to the advancing side, and this is caused by the greater straining in this location. The similar observation was made by Pareek etal in friction stir welding of AZ31B Magnesium alloy. This may be another reason for failure along the SZ region on the advancing side. [18].

Fig.8 (A-I): Effect of tool material on stir zone Microstructure with SS tool and HSS tool The heat input and material flow behavior decides the quality (defect free) of FSW joints. The heat input and material flow behavior are predominantly influenced by the FSW process parameters such as tool rotation speed,


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welding speed and axial force. The heat input increases with increase in rotation speed. At lower rotation speed, the heat input is not sufficient and also improper stirring causes a tunnel defect at the middle of the retreating side. Higher rotation speeds could raise the strain rate and turbulence (abnormal stirring) in the material flow caused a tunnel defect at the weld nugget. As the rotation speed increases, the strained region widens, and the location of the maximum strain finally moves to the retreating side from the advancing side of the joint. This implies that the fracture location of the joint is also affected by the rotation speed [19]. Conclusions The tool material and rotational speed have been identified as the important parameters that affect the stir zone microstructure and properties of FSW process. The following conclusions can be obtained. ‚ SS tool material provided fine grained microstructures and better mechanical properties as compared to HSS. ‚ The low rotational speeds provided high stirred zone micro hardness values compare to the base material. There exists a particular combination of tool rotational and tool material at which high strength properties may be achieved in the stir zone. ‚ The joint fabricated at a tool rotational speed of 900 rpm have shown lower ultimate tensile strength, yield strength, percentage of elongation, weld nugget hardness and impact test compared to the joints fabricated at a tool rotational speed of 1120 rpm. ‚ The joint fabricated at a rotational speed of 1400 rpm and 1800 rpm have also shown lower tensile strength properties compared to the joints fabricated at a rotational speed of 1120 rpm. ‚ Of the four joints fabricated using four different tool rotational speeds, the joint fabricated using with SS tool material with rotational speed of 1120 rpm exhibited superior tensile strength properties. Acknowledgements The authors would like to thank the authorities of JNTU Hyderabad, NIT Warangal and SR Engineering College, Warangal, AP, India for providing the facilities to carry out this work. References Thomas WM, Nicholas ED, Needham JC, Murch MG, Templesmith P and Dawes CJ. 1991. Friction stir welding, international patent application No.PCT/GB92102203 and Great Britain patent application No. 9125978.8. Salem HG, Reynolds AP and Lyons JS. 2002. Microstructure and retention of super plasticity of friction stir welded super plastic 2095 sheet. Scr. Mater. 46: 337-342. Nicholas ED and Thomas WM. 1998. A review of friction processes for aerospace applications. Int. J Mater Prod Technol. 13: 45-55. Q. Yang and A.K. Ghosh, Acta Materialia, 54 (2006), 5147-5158. S. Schumann and H. Friedrich. Mater. Sci. Forum, 419–422 (2003), p. 51. A. Takara, K. Higashi ,Materials Science Forum, v 475-479, pt.1, p 509-12, 2005 E.F. Emley, Principles of magnesium technology, Pergamon Press Ltd, 1966. F.W. Bach, M. Rodman, M. Schaper, A. Rossberg, E. Doege and G. Kurz,Magnesium, Wiley-VCH, Weinheim, Germany (2004), p. 285. Nagasawa T, Otsuka M, Yokota T and Ueki T, (2000), Magnesium Technology, TMS, Warrendale, PA 2000, 383-87. Park SHC, Sato YS and Kokawa H (2003) Proceedings of 4 th International Frictional Stir Welding Symposium, Park City, Utah, may 14-16. Nakata K, Inoki S, Nagaro T, Hashmito T Johgan S and Ushio M, (2001), Proceedings of 3 rd International Frictional Stir Welding Symposium Kobe, Japan, 27-28. Padmanaban G, V. Balasubramanian, Selection of FSW tool pin profile, shoulder diameter and material for joining AZ31B magnesium alloy, Materials and Design 30 (2009) 2647–2656. Darras B M, Khraisheh M K, Abu-Farha F K and Omar M A (2007),Friction stir processing of commercial AZ31 Magnesium alloy, J. Materials Processing Tech., 191, 77-81.F Heurtier P, Jones MJ, Desrayaud C, Driver JH , Montheillet F, Allehaux D (2006) Mechanical and thermal modeling of friction stir welding. J Mater Proecess Technol 171:348-357. Mishra R.S Ma ZY.FSW AND Processing, Mater.Sci.Eng R 2005; 50:1-78 Friction Stir Welding of AZ61A Magnesium Alloy: A Parametric Study (2011), ijamt, Springer Wang XH, Wang KS (2006) Micro structure and properties of friction butt –welded AZ31 magnesium alloy.Mater.Sci Eng A431:114-117. Pareek M, Polar A , Rumiche F, Inda cochea JE (2007), Metallurgical evaluation of AZ31B -24, Mg alloy Friction Stir Welds.Mater Eng Perform 16(5):655-662 G.Padmanaban, V.Balasubramaniam, IJAMT (2010) 49:111-121, An experimental investigation on friction stir welding of AZ31B magnesium alloy.