Effects of different backing plates and tool design on the flash defect developed during friction stir welding of AA6061 aluminium alloy

doi:10.21203/rs.3.rs-1968882/v1

The growing need for lightweight and higher strength materials, like aluminium alloys, in aircraft, ship construction, and railways has influenced manufacturers to adopt friction stir welding (FSW) due to the excellent mechanical properties of FSW joints as compared to other conventional welding processes. Friction stir welding (FSW) is defined by diverse interactions of thermal, mechanical, and tool geometry effects. The backing plate material is crucial for effective temperature control during the FSW process. Inappropriate FSW process parameters can result in flash defects. Due to extreme heat input, flash may be generated in FSW welds. This investigation aims to determine appropriate process parameters (backing plate thermal diffusivity, welding speed, and tool rotational speed) along with proper tool design to achieve flash defect-free joints. Visual observations indicate that the aluminium backing plate results in flash defect-free joints as compared to other backing plates.

Friction stir welding

backing plate thermal diffusivity

tool design

flash defect

The FSW process joins two workpieces without melting the material by using a non-consumable tool. In FSW, the side of the weld where the direction of the tool rotation is the same as the traversing direction is called the "advancing side" (AS). The other side, where the directions are opposite, is called the "retreating side" (RS).

In the FSW process, the material becomes soft due to extreme tool shoulder frictional heat, which results in the flash defect. Also, excess pressure exerted by the tool ejects material to be welded in the form of surface flash [1, 2]. During FSW, if tool rotational speed is high, weld material undergoes extremely hot conditions. Hence, extreme heat softens the material closer to the border of the tool shoulder, removes a significant part of the weld material, and creates the flash defect [1–3]. Inappropriate estimates of the length of the FSW tool pin compared to the base metal thickness to be joined, as well as variation in depth of penetration because of changes in the thickness of metals to be joined across the weld line, result in a flash defect [4]. When the tool pin plunge is too deep, the weld material near the tool pin is expelled, which results in a flash defect [4–8]. The flat tool shoulder results in a flash defect because the flat tool shoulder is not fully capable of transferring the weld material beneath the tool shoulder [9, 10]. The design of the FSW tool has a significant role in heat generation and material mixing during the FSW process [10]. A tool with a cylindrical pin results in a sound FSW joint [11]. A defect in a weld can cause material loss in the weld region and result in poor quality welds. The quality of the FSW weld depends on the selection of suitable process variables along with the appropriate interaction of the FSW tool and base metal [12].

The high diffusivity of the copper backing plate results in the void and root flaw defects in AA2024 FSW joints [13]. Appropriate selection of tool material, proper estimation of shoulder to pin diameter ratio, pin length, and geometry play a significant role in achieving defect-free FSW joints.

Although many researchers have studied the effects of process parameters such as tool rotational speed, welding speed, and tool geometry on flash defect formation, an investigation to study the effect of backing plate thermal diffusivity on the formation of flash defects is lacking.

Thus, the objective of the present work is to investigate the effect of the thermal diffusivity of different backing plate materials on the flash defect formation in FSW AA6061-T6 joints. Also, the effect of tool pin geometry and the direction of tool rotation on material flow and the formation of flash defects has been successfully studied.

2.1 Material

For the experimental study, the AA6061-T6 rolled plates of 6 mm thickness were selected as the parent metal. With the help of a power hacksaw, the plates were cut to the desired size of 200 mm x 60 mm.

The composition of chemical elements and mechanical properties of AA6061-T6 are given in Table 1 and Table 2, respectively.

Table 1

The composition of chemical elements of AA 6061-T6 (Wt%)
Si	Fe	Cu	Mn	Mg	Cr	Ti	Al
0.52	0.26	0.18	0.11	0.81	0.106	0.015	Balance

Table 2

Mechanical and thermal properties of AA6061-T6
Modulus of Elasticity (GPa)	Ultimate tensile strength (MPa)	Elongation (%)	Coefficient of Thermal Expansion [µm/m-⁰C]	Density (g/cm³)	Vicker’s Microhardness (HV)
68.9	311	18.23	23.6	2.7	98.63

2.2 Backing plates and thermal diffusivity

Single-pass welding was carried out in this work. The FSW process parameters such as backing plate, tool rotational speed, and welding speed are varied to study their effect on flash defect formation in AA 6061 FSW joints. As shown in Fig. 1, backing plates, monolithic asbestos (low thermal diffusivity), composite (asbestos with a width equal to tool shoulder diameter below the weld zone and aluminium on either side), and monolithic aluminium (high thermal diffusivity) were used.

Thermal diffusivity is defined as the rate at which temperature spreads through a material. It determines the rate of heat transfer from the hot side to the cold side of a material. Thermal diffusivities of the backing plate are tabulated in Table 3.

Table 3

Thermal diffusivities of backing plates
Backing plate	Thermal conductivity (W/mk)	Density (kg/mm³)	Specific heat capacity (J/KgK)	Thermal diffusivity (m²/s)	Thermal diffusivity (mm²/s)
Aluminium backing plate	218	2710	904	8.89854 x 10^− 5	88.99
Asbestos backing plate	0.14	1600	840	1.04167 x 10^− 7	0.10

2.3 FSW tool design

The FSW tool shoulder diameter was 18 mm, the tool pin diameter was 6 mm, and the pin length was 5.4 mm. The tool pin was characterized by left-handed threads, and the tool revolved in a counterclockwise direction so that plasticized weld material would flow towards the root side of the weld. The tool used for performing welds was made up of EN 31 alloy steel. The tool tilt angle was maintained at 0⁰, and the load was kept in the z-direction. The FSW tool and the diagrammatical representation of the FSW tool used for the present investigation are shown in Fig. 2 (a) and (b), respectively.

3.1 Visual Inspection

During the FSW, material flows from the retreating side to the advancing side. The weld material generally flows in an upward direction on the retreating side. Hence, a flash defect is generated on the retreating side. During experiment runs numbers 1, 2, 6, 9, 10, and 11, a severe flash defect is formed on the retreating side. In the FSW process, metals to be joined undergo severe hot processing conditions due to the low thermal diffusivity of the backing plate, high tool rotational speed, and low traverse speed. Hence, extremely hot conditions thermally soften the material close to the tool shoulder border and remove an immense portion of plasticized material in the form of surface flash. The flash defect has not been observed in experiment runs 5, 7, 12, and 13, indicating favourable conditions for a material to get transferred and settle successfully on the advancing side with the help of the tool shoulder. FSW welds with and without flash defects are as shown in Fig. 3 and Fig. 4 respectively.

3.2 Comparison of the effects of different backing plates on flash defect formation

When FSW welds with asbestos and aluminium backing plates are compared at the same tool rotational speed and welding speed for runs numbers 6 & 8 and 10 & 12 respectively, due to the high heat extraction rate, joints with aluminium backing plates result in defect-free joints or a small amount of flash. The asbestos backing plate maintains high heat input during FSW, which results in severe flash defect formation. In the case of welds with asbestos backing plate, 3 welds out of 4 have a flash defect. In the case of the composite backing plate, 4 welds out of 5 have flash defects, while in the case of welds with an aluminium backing plate, 1 weld out of 4 has severe flash defects. The high thermal diffusivity of the aluminium backing plate significantly reduces the formation of flash defects. Table 4 shows the reasons for flash defects for various combinations of process parameters.

Table 4

Reasons for the flash defect
Run No	Tool Rotational speed (RPM)	Welding speed (mm/min)	Backing plate	Flash defect
				Yes/No	Reasons
1	800	30	Composite	Yes	Low welding speed
2	1200	30	Composite	Yes	Low welding speed High tool rotational speed Low thermal diffusivity backing plate (asbestos) of width equal to tool shoulder diameter below metals to be joined
3	800	90	Composite	Yes but not severe	Low thermal diffusivity backing plate (asbestos) of width equal to tool shoulder diameter below metals to be joined
4	1200	90	Composite	Yes but not severe	High tool rotational speed Low thermal diffusivity backing plate of width equal to tool shoulder diameter below metals to be joined
5	800	60	Asbestos	No	Low/sufficient tool rotational speed Sufficient welding speed
6	1200	60	Asbestos	Yes	High tool rotational speed Low thermal diffusivity backing plate
7	800	60	Aluminium	No	Low tool rotational speed Sufficient welding speed High thermal diffusivity backing plate
8	1200	60	Aluminium	Yes but not severe	High tool rotational speed
9	1000	30	Asbestos	Yes	Low welding speed Low thermal diffusivity backing plate
10	1000	90	Asbestos	Yes	Low thermal diffusivity backing plate
11	1000	30	Aluminium	Yes	Low welding speed
12	1000	90	Aluminium	No	Sufficient/moderate tool rotational speed High welding speed High thermal diffusivity backing plate
13	1000	60	Composite	No	Sufficient/moderate tool rotational speed Sufficient/moderate welding speed

3.3 The effect of tool pin geometry and the direction of tool rotation on the flash defect

During the first run, the FSW tool was rotated in a clockwise direction, which resulted in a defective or incomplete weld, as shown in Fig. 5a. The reason behind this was that tool pin threads are left-handed and if a tool with left-handed threads rotates in a clockwise direction, all plasticized material will move along with the threads in an upward direction and form a flash defect on the retreating side, which will ultimately cause an incomplete weld as shown in Fig. 5 (a) and (c). When a tool with left-hand threads is rotated in a counterclockwise direction, it results in defect-free welds because the plasticized material will move towards the root side and there will be no flash formation as shown in Fig. 5 (b) and (d).

The following conclusions were drawn from the visual inspection and experimental work:

It is observed that the weld parameters strongly affect the flash defect formation.

Low thermal diffusivity backing plates, high tool rotational speed, and low traverse speed can cause flash defects on the retreating side of the FSW joints. These factors alone or in combination cause extreme heat generation, which results in a flash defect.

The flat tool shoulder causes a flash defect on the welded surface as the flat tool shoulder is not fully capable of confining the plasticized material below the tool shoulder.

During the friction stir welding, a tool with left-hand threads must be rotated in a counter-clockwise direction. If it is rotated clockwise, plasticized material will move along the tool pin threads in an upward direction, which results in a flash defect on the retreating side. This causes insufficient material on the advancing side.

In order to prevent a flash defect, proper FSW process parameters should be selected with the appropriate tool design.

Funding and/or competing Interest

All authors have no conflicts and/or funding to disclose.

Author Contributions

“All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by Bhardwaj Kulkarni and Sandeep Pankade. The draft of the manuscript was written by Bhardwaj Kulkarni, and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.”

P. Kah, R. Rajan, J. Martikainen, R. Suoranta (2015) Investigation of weld defects in friction stir welding and fusion welding of aluminium alloys. International Journal of Mechanical and Materials Engineering. DOI:10.1186/s40712-015-0053-8
N. Dialami, M. Cervera, M. Chiumenti (2020) Defect formation and material flow in friction stir welding. European Journal of Mechanics- A/Solids 80:103912. https://doi.org/10.1016/j.euromechsol.2019.103912
P.Podrzaj, B.Jerman, D.Klobcar (2015) Welding defects at friction stir welding. Metalurgija 54:387-389.
N. Soni, C. Sangam (2017) Defects formation during friction stir welding: A review. International Journal of Engineering and Management Research 7:121-125. DOI:10.13140/RG.2.2.19381.93921
Virendra Singh, Surendra Patel, Alok Ranjan, Basil Kuriachen (2020) Recent research progress in solid state friction stir welding of aluminium magnesium alloys: a critical review. Journal of Materials Research and Technology 6217-6256. https://doi.org/10.1016/j.jmrt.2020.01.008
K.P.Mehta, V.J.Bhadeka (2016) A review on dissimilar friction stir welding of copper to aluminum: process, properties, and variants. Materials and Manufacturing Processes 31(3):233-254. DOI:10.1080/10426914.2015.1025971
Kim, Y., H.Fujii, T.Tsumura, T.Komazaki, K.Nakata (2006) Three defect types in friction stir welding of aluminum die casting alloy. Material Science and Engineering A 415(1-2): 250-254. https://doi.org/10.1016/j.msea.2005.09.072
A.I.Albanani (2020) Review the common defects in friction stir welding. International Journal of Scientific and Technology Research 9 (11): 318-329.
M.M. Khalilabad, Yasser Zedan, Damien Texier, Mohammad Jahzi, Philippe Bocher (2018) Effect of tool geometry and welding speed on mechanical properties of dissimilar AA2198-AA2024 FSW joint. Journal of Manufacturing Processes 34:86-95. https://doi.org/10.1016/j.jmapro.2018.05.030
R.Rai, A De, H K D H Bhadeshia, T. DebRoy (2011) Friction stir welding tools. Science and Technology of welding and Joining 16(4):325-342. DOI:10.1179/1362171811Y.0000000023
Kush Mehta, Vishvesh Badheka (2016) Effects of tool pin design on formation of defects in dissimilar friction stir welding. Procedia Technology 23:513-518. DOI:10.1016/j.protcy.2016.03.057
Debtanay Das, Swarup Bag, Sukhomay Pal (2021) Investigating surface defect by tool-material interaction in friction stir welding using coupled Eulerian-Lagrangian approach. Manufacturing letters 30: 23-26. https://doi.org/10.1016/j.mfglet.2021.10.003
Zhian Zhang, Wenya Li, Junjun Shen, Y. J. Chao, Jinglong Li, Yu-E. Ma (2013) Effect of backplate diffusivity on microstructure and mechanical properties of friction stir welded joints. Materials and Design 50: 551-557. https://doi.org/10.1016/j.matdes.2013.03.034

Effects of different backing plates and tool design on the flash defect developed during friction stir welding of AA6061 aluminium alloy

Status:

Version 1

Abstract

Figures

1 Introduction

2 Material And Methods

2.1 Material

2.2 Backing plates and thermal diffusivity

2.3 FSW tool design

3 Results And Discussion

3.1 Visual Inspection

3.2 Comparison of the effects of different backing plates on flash defect formation

4 Conclusions

Declarations

References

Status:

Version 1