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Procedia Engineering 90 (2014) 637 – 642

Available online at www.sciencedirect.com

1877-7058 © 2014 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 Department of Mechanical Engineering, Bangladesh University of Engineering

and Technology (BUET)

doi:10.1016/j.proeng.2014.11.784

ScienceDirect

10th International Conference on Mechanical Engineering, ICME 2013

Effect of tool pin thread forms on friction stir weldability of

different aluminum alloys

Md. Reza-E-Rabbya, Anthony P. Reynolds

a*

a Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Room A224, Columbia, SC 29208, USA

Abstract

The primary objective of this study is to investigate parametric effects of pin features on material flow and friction stir

weldability of two different aluminum alloys. A series of bead on plate friction stir welds were made on two different aluminum

alloys (AA 7050 and AA6061) with cylindrical tool pin having four thread pitches (1.02 mm, 1.41 mm, 2.12 mm & 3.18 mm)

including smooth/unthreaded pin attached to an unvarying single scrolled shoulder geometry. Welds were performed under a

range of process parameters (welding & rotational speed). It was observed that thread forms are obviously beneficial for

improving tool performance and reducing in-plane reactions (X & Y forces) forces on tool. Wormhole defects in the weld nugget

were eliminated or minimized by employing threaded pins as a consequence of effective material transportation near the weld

root. Tool pins having intermediate thread pitches (1.41mm and 2.12 mm) perform better than either extreme over the range of

attempted parameters. Defects are far more prevalent in 7050 welds than in 6061. In-plane reaction forces on pin are significantly

larger in AA7050 welds than in AA 6061 and torque- peak temperature values are of similar order in both welded materials.

© 2014 The Authors. Published by Elsevier Ltd.

Selection and peer-review under responsibility of the Department of Mechanical Engineering, Bangladesh University of

Engineering and Technology (BUET).

Keywords: Friction stir welding pin thread form; 7xxx series and 6xxx series aluminum alloys; rocess response variables; n-plane reaction.

1. Introduction

Friction stir welding (FSW) [1] is solid state thermo-mechanical joining process in which bonding between two

parts occurs by severe plastic deformation of adjacent interfaces under conditions of hydrostatic pressure. Peak

* Corresponding author. Tel.: +1-803-777- 9548; fax: +1-803- 777- 0106.

E-mail address: [email protected]

© 2014 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 Department of Mechanical Engineering, Bangladesh University of Engineering

and Technology (BUET)

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638 Md. Reza-E-Rabby and Anthony P. Reynolds / Procedia Engineering 90 (2014) 637 – 642

temperature during FSW process may be well below the melting temperature of welded material, lessening typical

undesirable effect of welding processes. This joining technique is an excellent choice for aluminum alloys which are

used in some aerospace and automotive applications due to some of its outstanding qualities such as, better

dimensional stability, preservation of base material properties, resistance to hot cracking etc. [2-4].

In FSW, a non-consumable rotating tool moves along the weld seam with a pin immersed into the work-piece.

The conventional FSW tool is comprised of (a) shoulder and (b) pin, where the shoulder predominantly generatesfrictional heat and prevents the expulsion of material from the weld zone. The role of FSW tool pins is to provide

sufficient plastic deformation to cause bonding across any pre-existing interfaces while transporting material to

positions behind the tool. Right after the invention of friction stir welding by TWI in 1991, efforts have been made

towards designing FSW tool pins [1, 5-9] in order for the process to be flourished over a range of manufacturing

applications. However, tool geometry studies are necessarily limited as the potential variation in geometry is

essentially infinite. Since instinctive perceptions are employed for most of the tool designs which are typically based

on empirical knowledge [10], process parameters and tool parameters may vary greatly in FSW of aluminum alloys.

Consequently, numerous efforts have been devoted to understand the relationship between tool parameters

(including geometric shape, dimensions and thread features) and mechanical-microstructural properties of different

alloys within a wide range of welding conditions [11-16]. FSW tool pins are often featured with thread forms. A

complex pin is designed with thread forms having different thread interruptions such as flats or flutes [9, 15-17].

However, vertical movements of material during FSW were presumed to be predominated by helical features in tool pin [18-19]. Tool pin geometries and features are one of the important process control parameters in FSW that

influence material flow which in turn affect joint efficiency. Differences between base metal properties of different

aluminum alloys also contribute to different material flow and different process response variables for identical

friction stir welding control parameters including tool geometry. Considering medium strength 6xxx series and high

strength 7xxx series aluminum alloys, resistances to deformation of 6xxx alloys are less compared to 7xxx series

alloy while thermal conductivity is greater for the lower alloyed 6XXX series. These properties lead to a weldability

advantage of 6xxx over 7xxx series alloys.

In the present study, tool features, specifically pin thread forms, are systematically varied for friction stir welding

of two aluminum alloys: AA 7050-T7451 and AA 6061-T651. This methodical variation in pin thread forms was

performed to explore how the thread pitches and process parameters affect the process response variables (in-plane

reactions on tool pin, torque, temperature etc.) and the quality of welds in both alloys. Process response variables

obtained in this study provide important information and guidelines for more radical pin designs with the ability to

produce defect free welds as well as estimation of FSW tool life for welding different materials.

2. Materials and Experimental Details

Bead on plate welds were performed in 32 mm thick AA 7050-T7451 (nominal composition: 5.6%Zn, 2.5%Mg,

1.6%Cu, 0.23%Cr, balance Al) and 25 mm thick AA 6061-T651 (nominal composition: 1.2%Mg, 0.5%Si, 0.33%Fe,

0.28%Cu, 0.17%Cr and balance Al) plates. The tool shoulder dimensions were 25.4 mm diameter (1 inch), single

scroll with a scroll pitch of 2.54 mm/revolution (0.1 inch/revolution) and these dimensions were kept constant

throughout the experiments. The cylindrical tool pin dimensions were: 12.5 mm (0.5 inch) length and 15.9 mm

(0.625 inch) diameter. Four pins were produced with the insertion of threads having different pitch dimensions suchas: fine threaded (FT)-1.02 mm, normal threaded (NT)-1.41 mm, coarse threaded 1 (CT1)-2.12 mm, coarse threaded

2 (CT2)-3.18 mm. A smooth pin was also employed to compare the effect of pin with unthreaded and threaded

condition. Fig. 1(a-c) shows several views of a typical tool illustrating the manner in which the shoulder and pin are

assembled. The pin model with different thread forms are also presented in Fig.1 (d). The weld parameters utilized

for this study are summarized in Table 1. Overlapping parameter sets (weld parameters used for both alloys) are

bolded. Welding was performed on the USC FSW Process Development System (PDS) using forge force control

mode. For each weld parameter set, forge force (Z-force) was adjusted to maintain similar levels of tool plunge for

the different thread forms and parameters but some variation is inevitable. Temperature during welding was

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monitored and recorded using a k-type thermocouple spot welded into the tool pin at mid depth in between shoulder

and pin tip along the axis of rotation.

Fig. 1. (a) Side view of shank; (b) Shoulder geometry, (c) Assembled tool and (d) Tool pin thread forms.

All the welds were performed at 0° spindle tilt angle. In-plane reaction forces on FSW conventional X axis were

recorded from the signal produced from the piston pressure transducer. Y axis forces were obtained from the load

cell in the spindle carriage. Tool torque was obtained from a torque transducer attached to the spindle. Standard

metallographic procedures were followed for macro-structural investigation. Specimens were chemically etchedusing Keller’s reagent (2.5% HNO3, 1.5% HCl, 1% HF and balance distilled water) for macro and micro structural

observation.

Table 1. Weld Parameters for different aluminum alloys.Alloy Tool Rotational Speed & Welding Speed (RPM & mm/min)

AA 7050 120 & 51 150 & 51 180 & 51 160 & 102 200 & 102 240 & 102

AA 6061 160 & 102 200 & 102 240 & 102 240 & 203 320 & 203 400 & 203

3. Results and Discussions

3.1. Macro-cross sectional investigation of AA7050 and AA6061 welds

Figure 2 shows the transverse macro cross sections of the weld in both 7050 and 6061. In each image the

advancing side is on the left. Each row of images shows the cross sections for a particular pin thread form (U, FT

NT, CT1, and CT2) while each column is for a particular combination of RPM and welding speed.

Fig. 2. Transverse macro-sections of weld nugget for bead on plate weld on (a) AA7050 and (b) AA6061.

The macrographs of 6061 indicate a much lower incidence of defects than in 7050 which is consistent with the

general observation that 6061 is easy to weld relative to 7050. Defects are present in all of the 7050 welds with the

largest defects associated with the unthreaded pin (U). Welding with threaded pins in both 7050 and 6061 can lead

to formation of surface breaking defects under some welding conditions. The surface breaking defect formation may

be due to the action of the right hand threaded tool rotating in the counter clock wise direction (as viewed from

above) thereby pushing material downward toward the weld root. This down thrust helps to eliminate near root

wormhole defects but, if too much material escapes as flash; the surface breaking defect may be created. Effective

material flow was observed with intermediate thread forms (CT1) for both 7050 and 6061 welds within a subset of

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the attempted process windows. All tools produced the best results in 7050 with lower rotation speed. This is

consistent with welding experience in thick plate 7XXX alloy and is sometimes claimed to be related to prevention

of local melting [20]. This phenomenon is also evident in 6061 welds with the unthreaded pin. It is also observed

that, defects are significantly reduced while welding at lower rotational and travel speed in both alloy welds. This is

an interesting result in that it is not expected that local melting would be a problem in 6061 regardless of the RPM

used. Therefore, a more fundamental reason, not necessarily related to alloy composition and susceptibility to local

melting may be needed to explain the defect formation behavior with respect to RPM. Furthermore, the extent offully consolidated material transportation phenomena for a defect free weld may be associated with the tool pin

shape and geometries, features (thread forms and interruptions) and pressure on tool pin which is in turn governed

by process control parameters (rpm, welding speed and forge force).

3.2. Process Response variable for bead on Plate weld on AA7050:

In plane reaction forces, torque and pin peak temperature as a function of tool rotational speed are shown in Fig.

3(a-d) for the 7050 welds. The open symbols are for the 102 mm/min welding speed and the close symbols for the

51 mm/min. Each pin form has different symbol geometry.

Fig. 4. Reaction forces, torque and pin peak temperature as a function of tool rotation for bead on plate weld on AA7050.

The x-force plot (Fig. 3-a) exhibits several salient features such as: (1) the U pin results in the highest x-force

followed, generally, by the CT2 (the coarsest thread), (2) in almost all cases, the x-force increases with increasing

RPM and (3) increased welding speed leads to increased X-force. With respect to (1) it should be noted that the

threads on CT2 were only cut to the depth of the threads on CT1 as full depth would have resulted in a very small

minor diameter. So, this “abnormal” thread geometry may contribute to the high x-force associated with the CT2

pin. The increase in x-force associated with the increase in RPM (2) may in reality be an association with increased

defect size. Y force, on the other hand, shows very little correlation with either RPM or welding speed but is a

strong function of the pin geometry (see Fig. 3-b). Again, the U pin has the highest forces this time followed by the

FT and the CT1 has the lowest forces: the difference can be more than a factor of three. As might be expected,

torque is not a strong function of pin form. Torque declines with increasing RPM and is higher for the higher

welding speed at a given RPM. Temperature (measured in the pins) has an inverse relationship to the torque so

increases with increasing RPM. The U pin exhibits the highest temperature and the CT1 the lowest; however, it is

somewhat risky to draw too strong a conclusion from this as small differences in thermocouple placement may have

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substantial effects on measured temperature. It is probably best to use probe T to judge the effects of changing

control parameters on the temperature for a single pin and not too compare between pins. One effect which is

interesting is that the spread of temperature between the various pins is less for the higher welding speed than for the

lower.

3.3. Process Response variable for bead on Plate weld on AA6061:

Fig. 4 is the same as Fig. 3 except that the data are for the 6061 welds except that the open symbols are the 102

mm/min welding speed and the close symbols are the 203 mm/min welding speed. In 6061, a general declining x-

force with increasing rpm is observed. This is the opposite of what was seen in 7050. Significantly, the defect

content of these welds is much lower than that of the 7050 welds and this may be an important factor. However, if,

as has been observed for many aluminum alloys, including 6061 and 7050, that an X-force minimum is associated

with an intermediate RPM for a given welding speed [21], it may be that the minimum is at higher RPM for 6061

than for 7050 and that this part of the weld parameter envelope has not been encountered in this study. Interestingly

there does not appear to be a strong relationship between pin thread forms and X-forces in these 6061 welds. Y-

force exhibits a general, weak, trend to increase with increasing rpm and here, the U and FT pins generally produce

the highest forces. The torque and temperature relationships to rpm are “standard” and in-line with the 7050

although neither the temperature nor the torque exhibits an observable trend with pin form. It is also worthwhile to

note by comparison of figures 4 (a-b) and 5 (a-b) that the average x and y forces tend to be significantly larger in the

7050 welds (comparing only the welds at 102 mm/min welding speed: open symbols in both figures) and the torque

and temperature values are of similar order in the two alloys (again considering only the open symbols).

Fig. 5. Reaction forces, torque and pin peak temperature as a function of tool rotation for bead on plate weld on AA6061.

4. Conclusions

The effect of tool pin thread forms and the process control parameters on the friction stir weldability of two

different aluminum alloys along with some process response variables have been investigated. The following

concluding remarks can be made from present study:

Wormhole defects can be removed or minimized by inserting helical features such as threads in tool pin

during friction stir welding.

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Material flow around tool pin is a complex function of thread form and process parameters. Intermediate

threaded tool pins produced best quality welds with reduced defect production in both alloy systems.

Defect contents are reduced when welding was performed at lower rpm. Alloys 7050 and 6061 have

markedly different weldability:

► In – plane forces are much higher in 7050 than in 6061.

► 7050 exhibit more defected welds than 6061.

► Over the range of RPM/IPM examined, increasing RPM generally: (a) decreases in-plane forcein 6061 & (b) increases in-plane forces in 7050

Acknowledgements

This research is supported by Center for Friction Stir Processing which is a National Science Foundation I/UCRC

(Grant no. EEC- 0437341). The authors thank Dr. Wei Tang and Mr. Daniel Wilhelm, Department of Mechanical

Engineering, University of South Carolina, Columbia, SC, USA for their help in preparing the weld joints.

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