Effect of tool pin profile, shoulder diameter for joining on Friction stir

welding of AZ31B magnesium alloy

1 S.Ugender, 2 A. Somi Reddy

1Research scholar, JNTU, Hyderabad-506085

Email id:ugender014@yahoo.co.in

2Department of Mechanical Engineering, VITS, Karimnagar, India-506370

Email id:arumalle_reddy@yahoo.co.in

Correspanding Author: ugender014@yahoo.co.in

A B S T R A C T

In this investigation, an attempt was made to select proper tool pin profile, tool shoulder

diameter to friction stir weld AZ31B magnesium alloy. Three tool pin profiles and three tool

shoulder diameters were used to fabricate the joints. Tensile properties of the joints were

evaluated and correlated with the weld zone microstructure and hardness. From this

investigation, it is found that the joint fabricated using straight cylindrical profiled tool made

of high speed steel with 18 mm shoulder diameter produced mechanically sound and

metallurgical defect free welds compared to their counterparts. The absence of defects in

weld region, presence of very fine equiaxed grains in the weld region and higher hardness in

the weld region are the main reasons for superior tensile properties of these joints.

Key Words: FSW, Mg AZ31B alloy, tensile properties, hardness, Mechanical properties.

Metallurgical studies

1. Introduction

Magnesium alloys have many attractive properties, such as low density and high specific

strength. It is predicted that the application of magnesium alloys will grow rapidly in the near

future, especially in the transport industry [1]. With fast development and wide applications,

the welding of magnesium alloys becomes a main concern. The drawbacks associated with

the fusion welding include: (a) complex thermal stresses and severe deformation, (b) the

presence of porosity and crack in the fusion zone, and (c) the excess eutectic formation [2].

Friction stir welding (FSW) is capable of joining magnesium alloys without melting and thus

it can eliminate problems related to the solidification. As FSW does not require any filler

material, the metallurgical problems associated with it can also be eliminated and good

quality weld can be obtained [3]. FSW involves complex material movement and plastic

deformation. Welding parameters, tool geometry, and joint design exert significant effect on

the material flow pattern and temperature distribution, thereby influencing the micro

structural evolution of material. Of this tool geometry is the most influential aspect of process

development [4]. The design of the shoulder and of the pin is very important for the quality of

the weld. The pin of the tool generates the heat and stirs the material being welded but the

shoulder also plays an important part by providing additional frictional treatment as well as

preventing the plasticized material from escaping from the weld region. The plasticized

material is extruded from the leading to the trailing side of the tool but is trapped by the

shoulder which moves along the weld to produce a smooth surface finish. Clearly, different

materials and different thicknesses will require different pin profiles. The variations in tool

design are infinite and combinations of shoulder diameter, shoulder profile, pin length,

diameter and profile, are all important parameters in determining the speed of welding and

the quality of the finished weld. Another important parameter in the determination of the

suitability of a tool for a particular application is the tool material itself. Welding is carried

out around 70–90% of the material melting point so it is important that the tool material

should have sufficient strength at this temperature otherwise the tool can twist and break

[5]. several previous studies reported [5–10] the effect of tool geometry on mechanical and

micro structural properties. The effect of the tool shape on the mechanical and micro

structural properties of friction stir welded 1050-H24, 6061-T6, 5083-O aluminium plates

were investigated by Fuji et al. [6]. The simplest shape (column without threads), the

ordinary shape (column with threads) and the triangular prism shape probes were used for

their investigation. They found that, when the deformation resistance is relatively low, the

tool shape does not significantly affect the microstructures and mechanical properties of the

joints. At the same time, deformation resistance is relatively high; the weldablity is

significantly affected by the tool shape at high rotation speed. Scialpi et al. [7] studied the

effect of different shoulder geometries on the mechanical and micro structural properties of a

friction stir welded 6082-T6 aluminium alloy. They suggested that shoulder with combination

of fillet and cavity produced good joints for thin sheets. Effect of tool pin profile and tool

rotational speed on the formation of friction stir processing zone in AA2219 aluminium alloy

was studied by Elangovan et al. [8] and they concluded that, square pin profiled tool

produced defect free FSP region, irrespective of rotational speeds. The influence of pin

geometry on bonding and mechanical properties of friction stir welded 2014 aluminium alloy

was studied by Zhao et al. [9]. They opined that, the pin affects the flow of the plastic

material strongly and the best quality weld was acquired using the taper with screw thread.

Boz et al. [10] studied the influence of stirrer geometry on bonding and mechanical properties

of friction stir welded 1080 aluminium alloy. They suggested that high pitch threaded tools

acted like a drill rather than a stirrer and compelled the weld metal outward in the form of

chips and the best bond was obtained for low pitch threaded tools. Elangovan et al. [11]

investigated the effect of tool pin profile and tool shoulder diameter on the formation of

friction stir processing zone in AA6061 aluminium alloy. They found that the joint fabricated

using square pin profiled tool with shoulder diameter of 18 mm showed superior tensile

properties. The roll of friction stir welding tool on weld zone formation by material flow was

analyzed by Kumar et al. [12] and they found that, onion ring formation in the friction stir

welds are due to the combined effect of geometric nature of pin-driven material flow, and

vertical movement of the material due to shoulder interaction. Hence, the present

investigation was carried out to select proper FSW tool parameters to weld AZ31B

magnesium alloy and the details are reported in this paper.

2. Experimental work

The rolled plates of 5 mm thickness, AZ31B magnesium alloy were cut into the required size

(240mm×58.2mm) by machining process. Square butt joint configuration, was prepared to

fabricate FSW joints. The initial joint configuration was obtained by securing the plates in

position using mechanical clamps. The direction of welding was normal to the rolling

direction. Single pass welding procedure was used to fabricate the joints. Non-consumable

tools made of high speed steel are used to fabricate the joints. Three tool pin profiles (straight

cylindrical, triangular and square) and three shoulder diameters (15 mm, 18 mm, 21 mm)

were used to fabricate the joints. The chemical composition and mechanical properties of

base metal are presented in Tables 1a and 1b. The tool nomenclature is presented in Table 2.

A different type of tool pin profiles used in this Investigation is shown in Fig. 1. Various

nomenclature of tool is shown in Fig. 2. An indigenously designed and developed FSW

machine (15 hp; 3000 rpm; 25 KN) was used to fabricate the joints.

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

Table.2.Mechanical Properties of Base Metal

Yield strength

(MPa)

Ultimate tensile

strength

(MPa)

Elongation

(%)

Reduction in

C/S area (%)

Hardness at 0.05

kg load (Hv)

171 215 14.7 14.3 69.3

Table. 3 Details FSW tool parameters and tools

Rotational speed(rpm) 1800

Welding speed(mm/min) 40

Pin length(mm) 4.5

Tool shoulder diameter(mm) 15,18,21

Axial force(KN) 4

Tilt angle(degrees) 2.5

Pin diameter 6

D/d Ratio of tool 2.5, 3.0, 3.5

The welded joints were sliced and then machined to the required dimension, according to the

ASTM E8 M-04 standard for sheet type material (i.e., 50 mm gauge length and 12.5 mm

gauge width). The tensile specimens were prepared to evaluate yield strength, tensile strength

and elongation. Tensile test was carried out in 100 KN, electro-mechanical controlled

universal testing machine (Make: FIE-Blue star, India; Model: UNITEK-94100). The 0.2%

offset yield strength and the percentage of elongation were evaluated. Rockwell hardness

testing machine (Make: SHIMADZU,INDIA, Model: HMV-2T) was employed for

measuring the hardness of the weld region with 0.05 kg load for 20 s.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. Macro- and microstructuralanalysis was carried out using a light optical

microscope (Make: MEIJI, India; Model: MIL-7200) incorporated with an image analyzing

software (Metal Vision)

3. Results

3.1. Tensile properties

The tensile properties such as yield strength, tensile strength, percentage of elongation,

percentage of reduction in cross-sectional area and joint efficiency of friction stir welded

AZ31B magnesium alloy joints were evaluated. In each condition, three specimens were

tested and the average of three results is presented in Tables 1–3. The joint fabricated using

threaded pin profile exhibited superior tensile properties compared to the joints fabricated by

other pin profiles. Similarly, the joint fabricated with shoulder diameter of 18 mm exhibited

superior tensile properties compared to the joints fabricated by other two shoulder diameters.

In summary, the joint fabricated with straight cylindrical in profile tool made of high speed

steel with shoulder diameter of 18 mm (D/d = 3.0) exhibited superior tensile properties.

3.2. Macrographs

In fusion welding of magnesium alloys, the defects like porosity, hot crack etc. deteriorates

the weld quality and joint properties. Usually, friction stir welded joints are free from these

defects since there is no melting takes place during welding and the metals are joined in the

solid state itself due to the heat generated by the friction and flow of metal by the stirring

action. However, FSW joints are prone to other defects like pinhole, tunnel defect, piping

defect, kissing bond, cracks, etc. due to improper flow of metal and insufficient consolidation

of metal in the FSW region [8]. Hence, all the joints fabricated in this investigation were

analyzed at low magnification (10_) using optical microscope to reveal the quality of FSW

region. It is found that the joint fabricated with threaded pin profile is free from defects.

However, the joints fabricated by other pin profiles contain defects. Similarly, the joint

fabricated using a tool shoulder diameter of 18 mm (D/d = 3.0) is completely free from the

defects. The joint fabricated using high speed steel is completely free from the defects.

Invariably, all joints show wider upper surface than the lower surface because the upper

surface experienced an extreme deformation and frictional heat caused by contacting weld

specimens with a cylindrical tool shoulder.

Straight cylindrical Square tool Triangular tool

Fig.1 Tool geometry

3.3. Micro hardness and microstructure

The hardness was measured across the weld at mid thickness region using Rockwell

hardness testing machine. The hardness of base metal (unwelded parent metal) is 69 Hv. The

joint fabricated with straight cylindrical pin profile exhibited higher hardness (76 Hv) in the

nugget zone compared to their counterparts and this is also one of the reasons for superior

tensile properties of these joints. Similarly, the joint fabricated using a tool shoulder diameter

of 18 mm (D/d = 3.0) recorded higher nugget zone hardness (75 Hv) compared to other

joints. In defective joints the failure occurred along the defect but in defect free joints the

failure occurred along the TMAZ region. Very low hardness was recorded in the TMAZ

region (64 Hv). This is also one of the reasons for failure in TMAZ region of defect free

joints during tensile test. The optical micrographs taken at nugget zone of all the joints are

displayed in Fig. 2. From the micrographs, it is understood that there is an appreciable

variation in average grain diameter and the coarse grains (18 lm) of base metal are changed

into fine grains in the nugget region. Hence, an attempt was made to measure the average

grain diameter of the weld metal region (nugget zone) of all the joints by applying Hein’s line

intercept method and measured grain sizes. The joint fabricated with threaded pin profile

contains finer grains (6.3 lm) along with more number of sub grains in the nugget region

compared to other joints .Similarly, the joint fabricated with shoulder diameter of 18 mm and

the joint fabricated with high carbon steel tool contain very fine grains in the nugget region

compared to their counterparts. The presence of very fine equated grains in the nugget region,

more number of sub grains and very clear grain boundaries are the reasons for better tensile

properties of these joints compared to their counterparts. To identify, the reason for failure in

TMAZ region, the detailed microstructure was taken in TMAZ region of both advancing and

retreating side (Fig. 2). From the micrographs, it is understood that there is an appreciable

difference in grain size (average grain diameter) in the nugget and thermo mechanically

affected zone (TMAZ). The grain size of TMAZ is coarser than the nugget region, because of

insufficient deformation and thermal exposure. Also, there is an appreciable variation in grain

size is observed in the retreating side (13 lm) compared to advancing side (15 lm) and this is

caused by the greater straining expected in this location. The similar observation was made

by Pareek et alin friction stir welding of AZ31B magnesium alloy. This is also another reason

for failure along the TMAZ region in the advancing side of defect free joints.

(b) Square

© Base metal (d) Triangular

Fig.2.Effect of tool pin profile on stir zone microstructure

D/d=2.5 D/d=3.0

D/d=3.5 Base metal

Fig.3.Effect of tool shoulder diameter on stir zone microstructure

4. Discussion

4.1. Effect of tool pin profiles

In case of straight cylindrical and tapered cylindrical pin profiles much of the material

movement takes place by simple extrusion and it seems to have no vertical motion of the

material which is apparently necessary to stabilize the rotational zone and to provide

sufficient deformation of material to obtain sound weld. Welds made with theses pin profiles

typically showed tunnel at the bottom (Table 7). Similar observation was made by McClure

et al. on FSW of 6061 aluminium alloy [9]. In triangular profiled tool, the frictional area tool

is about 50% smaller than that of the cylindrical pin tools. Since larger frictional area will

generate larger amount of friction between the probe side and the welding material is limited

to near three sharp edges, which is very smaller than that of other tools. heat, the friction heat

generated by the triangular probe might be smaller than that by the column tools.

Consequently, for the triangular prism tool, the bottom surface temperature is always lower

than the others, which produce tunnel at the bottom, due to lack of stirring in the friction stir

processing zone (Table 7). Similar observation was reported by Fujii et al. on FSW of 1050-

H24 aluminium alloy [6]. The screw thread will be beneficial to the heat generation, under

the same weld parameter; the pin with screw thread will generate more heat than the pin

without screw thread. More heat input can improve the flow of the plastic material. On the

other hand, the screw thread on the pin exerts an extra downward force that will be beneficial

to accelerate the flow of the plastic material. Material transports from the advanced side to

the retreated side, and goes around the pin, back to the advanced side. Without the screw

thread, material transfers insufficiently, and as the result, insufficient material comes back to

the advanced side; void defection will be generated at the advanced side [9]. Threaded pins

are found to assist in ensuring the plastically deformed work piece material is fully delivered

around the pin, and from the upper parts of the joint to the lower parts and vise versa. This

enhanced mixing enables the use of higher speeds and results in better quality, void free

welds (Table 7). Similar observation was made by Colligan et al. on FSW of 2519-T87

aluminium armour plate [8]. During stirring, square profile tool sweeps a large amount of

metal from the plasticized zone and results in an inhomogeneous structure and produce a

tunnel at the bottom (Table 7). Similar observation was reported by Boz et al. on FSW of

1080 aluminium alloy [10].Of the five tool pin profiles used in this investigation, the joints

fabricated using the threaded pin profile yielded defect free and fine grained nugget region

which led to the enhancement of hardness and tensile properties of the welded joints

4.2. Effect of tool shoulder diameter

From the above discussion, it is clear that the joint fabricated by threaded pin profile showed

superior tensile properties. Hence, threaded pin profiled tools were fabricated with varying

tool shoulder diameters (15, 18 and 21 mm) to study the effect of tool shoulder diameter on

tensile properties of FSW AZ31B magnesium alloy joints. The tool shoulder diameter is

having directly proportional relationship with the heat generation due to friction. If the

shoulder diameter is larger, then heat generation due to friction will be higher due to large

contact area and vice versa. In this investigation, it is observed that the larger tool shoulder

diameter (21 mm) led to wider contact area and resulted in coarse grains in the nugget region

and subsequently the tensile properties of the joints are deteriorated. It is also observed that

the smaller tool shoulder diameter (15 mm) lead to narrow contact area and resulted in less

frictional heat generation and hence the weld metal consolidation is not so good in the nugget

region. Of the three tool shoulder diameters used in this investigation, the joint fabricated

using 18 mm shoulder diameter (D/d = 3.0) yielded defect free and fine grained nugget region

which subsequently showed higher hardness and superior tensile properties (Table 5). Similar

observations were made by Elangovan and Balasubramanian [10] on FSW of AA6061

aluminium alloy.

5. Conclusions

In this investigation, an attempt was made to select proper tool pin profile, tool shoulder

diameter and tool material to friction stir weld AZ31B magnesium alloy. From this

investigation, the following important conclusions are derived: (1) the joints fabricated by

high speed steel tool with straight cylindrical pin profile and shoulder diameter of 18 mm

(D/d = 3.0) exhibited superior tensile properties compared to their counterparts.(2) The

absence of defects in nugget region, presence of very fine equiaxed grains in the nugget

region and the formation of more number of sub grains in nugget region are the main reasons

for higher hardness and subsequently for the superior tensile properties of the above joints.

6. References

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