Title: Friction stir welding of commercially pure aluminium alloy using counter rotating twin tool

K.Kumari, Surjya K Pal

Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur, India

Abstract

An attempt has been made to study the effect of counter rotating twin tool on the commercially pure

aluminium alloy using friction stir welding. Further a comparative study is made between twin tool

(TT) and single tool using double pass (ST-DP).Twin tool helps in generating high heat caused

intense plastic deformation in the processed zone. Defect free zone not only depends on the

rotational speed but also on the combined effect of welding speed and rpm. Higher hardness profile

is observed with highest welding and rotational speed (1800 rpm with 63 mm/min).Joints fabricated

at higher rpm and higher welding speed are found to be free from defects. Further 31.5 mm/min

welding speeds is the critical point where a sudden change in mechanical properties was observed.

SEM results confirm the existence of ductile failure with microscopic voids of different shape and

sizes.

Keywords: Friction stir welding, counter rotating twin tool,

Introduction

Especially for aluminium alloy, Friction stir welding is an improved technique of joining compared

to fusion welding process. It is a solid state technique where the temperature is reached below

melting point of the welding material, invented by The Welding Institute, in 1991 (Mishra and Ma,

2005) and (Nandan et al., 2008).The process is advanced in joining materials which are difficult to

weld, require no filler and shielding gas, need less skilled workers.

However, careful selection of welding process parameters (such as weld speed, rotation

speed, plunge depth or plunge force, tool geometry) which are complex, is beneficial for preventing

the formation of defective weld (Leal and Loureiro, 2004).

Furthermore, some degradation in mechanical properties has been observed when there is a

deviation in the welding condition. Though it is a reliable technique for joining, complete

elimination of defect is not possible. So a lot of researchers focussed to repair the defective portion

by using multi-pass FSW/FSP using the nominal process parameters. That means, by using the re-

welding technique, defective portion of the weld is being repaired (Brown et al., 2009). Recent

literature reports several examples related to multi-pass FSP/FSW.

(Brown et al., 2009) performed five overlapping passes and found that there is no need of any

adjustment for multi-pass welding. Further there is a significant reduction in feed force when

welding is done over the previous weld. However grain size, hardness and temperature during

welding is unaffected with number of passes. Again there is a gradual reduction of residual stress

with increasing pass number.

(Nataka et al., 2006) reported an advancement of the mechanical properties in aluminium die

casting alloy using multi-pass FSP compared to the as-cast base metals. As compared to base metal,

hardness increased by 20HVand the tensile strength of the multi-pass specimens were significantly

increased to about 1.7 times of as-cast base metal. The main reason for improvement of mechanical

properties is due to the removal of cold flake, uniform distribution of the silicon particles over the

aluminium matrix which refines the grain. On the other hand (Ma et al., 2006) reported that there is

no effect of overlapping passes on size, aspect ratio or distribution of the Si particle while

performed five-pass with 50% overlap FSP on cast A365.

(Leal and Louriero 2008) investigated the effect of overlapping FSW passes using two Al alloys

(AA5083-O and AA 6063-T6).They found that the quality and strength of the welded joint is not

only dependent on the weld parameters, but also on the type of material and its heat treatment

conditions. Lastly they found that weld polishing improved the mechanical efficiency of the welded

joint.

As FSP is one of the most promising techniques for grain refinement, removing flaws, defects;

many researchers used multi-pass FSP to improve the properties of as-cast material. (Johannes and

Mishra 2007) used to demonstrate the effectiveness of multiple passes to create large area of super

plastic materials with properties. They conclude that for achieving the super plastic deformation

grain boundary sliding (GBS) is the most important mechanism. Similarly (Ma et al., 2009) noted

that two pass FSP shows enhanced super plastic elongation compared to single pass. Further the

temperature in central zone of second pass and transitional zone is more than the single pass.

(Surekha et al. 2008) reported that multi-pass FSP showed better corrosion resistance compared to

the base metal.

Using cast Al alloy, (Jana et al., 2010) reported that multiple passes helped in removal of abnormal

grain growth (AGG) occurred during single pass runs. They also examined and found that higher

rotational speed was found to be beneficial for controlling the AGG. (Barmouz and Givi, 2011)

used MPFSP to improve metallurgical and mechanical properties of cu/sic metal matrix

composites. Result shows Sic particle dispersed and fragmented to smaller size due to severe

stirring action in the nugget zone of the copper matrix. It also created strong interfacial bonding by

removing the porosity content.

Multi-pass overlapping FSP (MPO FSP) has been applied by (Ni et al., 2011) to transform the

coarse as-cast Nab alloy base metal to get defect free material with fine microstructure, which was

feasible to modify the large sized plates. Similar type of study had been conducted by (Izadi and

Gerich, 2012) to study the effect of multi-pass FSP on distribution and stability of carbon nano-tube

and to fabricate AL 5059 and MWCNTs metal matrix composite (MMC).

To avoid the use of multi-pass FSW/FSP, the two-tool-FSW concept is being developed at TWI in

several variations (Thomas, 1999). One of those techniques is named as Tandem twin-stir technique

(Thomas et al., 2005). Tandem Twin-Stir uses two FSW tools (with or without counter rotation)

positioned one in front of the other.

The aim of this investigation to determine the effect of two contra rotating FSW tool (Tandem

Twin-stir) on the friction stir processing/welding region of commercially pure aluminium alloys.

2. Experimental work

In order to demonstrate the characteristics of twin tool, a self designed twin tool setup is designed,

fabricated and used for friction stir welding is shown in Fig 1. The twin tool system is composed of

two tools which are rotating in opposite direction to each other. The primary tool is mounted on the

main spindle shaft. Therefore, the main tool rotates at the same rotational speed and in the same

direction as the spindle during the welding process. The secondary tool is connected with the

primary tool with the help of gear assembly. The power transmission from the primary to secondary

tool is similar to the transmission of power from driver to driven gear, So that the rotation of the

secondary tool is just opposite to the primary tool.

Fig.1: Twin tool attachment

Friction stir welding using twin tool and single tool with double pass were produced in 2.5 mm

thick plate of commercially pure 1100 aluminium alloy. In both the cases welds made with

complete overlapping passes. Both the plates were clamped using specially designed fixture as

shown in Fig 2. The nominal chemical composition of the plate is shown in Table 1.The FSW

carried out at knee type vertical milling machine (BFW, VF3.5), which has wide range of rotational

speed (45 to 1800 rpm) and welding speed/feed rate (16 to 800 mm/min).

Fig.2: Specially designed Fixture

Table 1.

Chemical composition of the work piece material

Chemical composition (weight %) of work piece material

Si Fe Cu Mn Mg Zn Ti Ga Na Others Remainder Aluminium

0.7055 .831 .00505 0.013 0.00465 0.0031 0.0048 0.0118 0.00245 Max. 0.05% 98.7

A non-consumable tool made of stainless steel SS316 with 16 mm shoulder diameter, a cylindrical

pin of 5 mm diameter and 2 mm length was used for welding. By using four rotational speeds (900,

1120, 1400, 1800 rpm) and three welding speeds (16, 31.5,63 mm/min), total 12 experiments were

performed using twin tool and single tool with double pass. Therefore, total 24 experiments have

been carried out at this stage. The coupled plate with simultaneous double pass using the twin tool

setup is shown in Fig 3.

Fig.3: Schematic diagram of welded plate using twin tool setup

To examine the superficial defects macroscopic analysis was done using Leica S6D Trinocular

stereo zoom microscope with Leica QWin-V3 image analysis software. Specimen for

metallographic analysis are sectioned comprising of welded zone, heat affected zone, thermo-

mechanical zone and unaffected base metal region. Samples are polished with a set of emery papers

with different grades. Further diamond paste is used for final polishing in variable speed grinder

polishing machine. Kellers reagent was used to examine the macroscopic view. The Buhler’s

Vickers hardness indentation machine with 200 gmf with 15 sec dwell time was used to get the

hardness profile of the welded sample on a cross section normal to the welding direction. To

evaluate the tensile strength of the welded samples, specimens were cut using electro discharge

machine transverse to the direction of the weld line. The tensile test was carried out at normal room

temperature using INSTRON-8862 machine with a ram speed of 1mm/min. Fig 4 shows the

dimension of the tensile test specimen. The fractured tensile surfaces were studied using scanning

electron microscope (JOEL-JSM 5800) to analyse the failure patterns.

Fig. 4: Shape of the tensile test samples

3. RESULTS

3.1 Macrostructural analysis

Porosity, solidification cracking, inclusions are some of the defects in fusion welding process

which degrades the quality of the weld and the property of the joint. Mainly these types of

defects are not generated in case of friction stir welding, in which there is no melting of metal

occurs. Joining takes place due to the stirring action of metal and heat generation by friction.

However due to improper selection of process parameters defects like pinhole, tunnels, piping

defect, kissing bond, cracks are generated in the friction stir welded joints. Stereo zoom

microscope with magnification of 10X was used to analyse the quality of the welded region.

Macrostructure of the welded regions are shown in Fig 5 using twin tool attachment with

different rotational and welding speed. At higher rpm and high welding speed joints using twin

tool shows defect free welds. Hence formation of defect free weld is both dependent on the

rotational speed and welding speed.

900-16 1120-16 1400-16 1800-16 900-31.5 1120-31.5

1400-31.5 1800-31.5 900-63 1120-63 1400-63 1800-63

Fig.5: Macrographs of welded samples using twin tool

3.2 Hardness Testing

Fig 5 shows the hardness profiles of the welds made with twin tool as well as single tool with

double pass. Welds made with twin tool shows higher value of hardness compared to the two pass

FSW joints for most of the welding parameters. This is due to the hardening effect caused by

intense plastic deformation in the processed zone. As materials in the nugget zone are subject to

two stirring actions so materials undergo severe plastic deformation due to which it gains a higher

cooling rate as compared to single pass material. With subsequent processing of one tool over the

other intense plastic deformation is occurred by which hardness caused due to second pass is more

than the first pass. Similarly in case of two pass using single tool, the material is subjected to two

stir effects but there is a time delay in between the two passes. Therefore, cooling rate is somewhat

less as compared to twin tool passes. From the Fig 5 (l), it is revealed that welds made with twin

tool shows higher hardness value compared to single tool with double pass corresponding to 1800

rpm with 63 mm/min welding speed. Further from Fig 5 (a-l) it is observed that at high rotational

speed of 1800 rpm with all constant welding speed, twin tool gives the higher value of hardness

profile in comparison to two pass using single tool. Fig 7 (a-d) shows the effect of welding speed

on average micro hardness of welded samples at constant rpm. It can be seen that 31.5 mm /min is

the critical welding speed where there is drastic change in the average micro hardness of a

particular sample with a definite welding parameter. So harness value is not only depends on the

rotational speed or welding speed it depends on the revolutionary pitch i.e. (welding

speed/rotational speed) in mm per rev.

Fig.5 (a-l): Effect of TT, ST-SP, and ST-DP on the nugget zone hardness

Fig.6 (a-c): Effect of rotational speed on average micro hardness of FSW zone using TT & ST-DP

Fig.7 (a-d): Effect of welding speed on average micro hardness of FSW zone using TT & ST-DP

3.3 Tensile testing

Fig 8 (a-l) shows all the comparative graphs corresponding to yield strength, ultimate tensile

strength, percentage of elongation and joint efficiency of the welded joints using twin tool

and single tool with double pass. It is seen that there is no significant variation in the yield

strength of the joints fabricated using twin tool and double pass.

Fig.8 (a-l): Effect of TT and ST-DP on YS, UTS, % age of elongation and joint efficiency of

the welded samples

Fig 9 shows the effect of rotational speed on yield strength at constant welding speed. It is

seen that at a constant welding speed 1800 rpm shows higher yield strength in all the cases

using single tool with double pass. Similarly from Fig.10 at a constant rotational speed 63

mm/min welding speed results higher yield strength using single tool with double pass. But

the variation in yield strength using both twin tool and single tool with double pass at higher

rpm (1800 rpm) and at higher welding speed (63 mm/min) is almost negligible. Further from

the comparative graphs it is observed that 31.5 mm/min welding speeds is the critical point

where there is a sudden change in the mechanical strength for all the cases. This similar

scenario is observed in case of ultimate strength, percentage of elongation and joint efficiency

also.

Fig. 9 (a-c): Effect of rotational speed on yield strength of welded joints using TT & ST-DP

Fig.10 (a-d): Effect of welding speed on yield strength of welded joints using TT & ST-DP

Fig.11 (a-c): Effect of rotational speed on UTS of welded joints using TT & ST-DP

Fig.12 (a-d): Effect of welding speed on UTS of welded joints using TT & ST-DP

Fig.13 (a-c): Effect of rotational speed on %age of elongation of welded joints using TT & ST-DP

Fig14 (a-d): Effect of welding speed on %age of elongation of welded joints using TT & ST-DP

Fig.15 (a-c): Effect of rotational speed on joint efficiency of welded joints using TT & ST-DP

Fig.16 (a-d): Effect of welding speed on joint efficiency of welded joints using TT & ST-DP

TT ST-DP900-16

1120-16

1400-16

1800-16

900-31.5

1120-31.5

1400-31.5

1800-31.5

900-63

1120-63

1400-63

1800-63

Fig.17: Appearance of the test pieces after tensile tests

Fig.17 shows the photographs of the test pieces after tensile testing using twin tool and single

tool with double pass. From the figure it is seen that the fractured position in the weld reflects

the location of minimum hardness zone. This implies that the joint strength can be correlated

with the micro hardness property.

3.4 Fractography

The fractured surface of the welded plate under tension is shown in the Table 3 for twin tool

and single tool with double passes. The presence of microscopic voids of different size and

shape confirms the existence of ductile failure using scanning electron microscope. The

fractured surfaces of the tensile sample were populated with a large number of fine dimples

revealing failure due to ductile behaviour. This type of situation arises due to optimal

material mixing with grain refinement. On the contrary due to complex process parameter a

less ductile failure occur resulted in a less ductile failure or combination of ductile with brittle

fracture or cleavage type. At 900 rpm and 16 mm per min welding speed the welded

specimen shows different nature of fractured surface. Using twin tool welded specimen is

fractured at the base metal zone but using single tool with double pass the specimen breaks at

the weld zone region which shows partly ductile and partly brittle fracture. Similarly with

twin tool the specimen breaks at the mid-zone of the weld region with 1400 rpm and 16 mm

per min welding speed. The fractured surface shows a combination of ductile and cleavage

type fracture. Therefore when the specimen breaks at the weld zone or nearby zone the

fractured surface is the combination of ductile and brittle fracture due to high heat generation

in that zone which causes intense plastic deformation.

Table 2.

Images of the fractured surface.

PARAMETER TT ST-DP

900-16

1400-16

1800-31.5

900-63

1400-63

Conclusions

In this investigation an attempt has been made to study the effect of twin tool and single tool

with double pass on the formation of friction stir welding zone in a commercially pure

aluminium alloy with different rotational speed and welding speed. From this, the following

conclusions are derived:

1. Formation of defect free weld is a function of both rotational speed and welding

speed. Joints fabricated at 1800 rpm and 63 mm/min welding speed shows the highest

hardness profile compared to the other welded joints.

2. Further from the comparative graphs it is observed that 31.5 mm/min welding speeds

is the critical point where there is a sudden change in the mechanical strength (yield

strength, ultimate tensile strength, percentage of elongation and joint efficiency) for

all the cases.

3. From the SEM analysis it is observed that the presence of microscopic voids of

different size and shape confirms the existence of ductile failure.

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