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Vol. 8. Num. 1.
Pages 1-1592 (January - March 2019)
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Vol. 8. Num. 1.
Pages 1-1592 (January - March 2019)
Original Article
DOI: 10.1016/j.jmrt.2018.05.011
Open Access
Friction Stir Spot Welding of Al6082-T6/HDPE/Al6082-T6/HDPE/Al6082-T6 sandwich sheets: hook formation and lap shear test performance
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Karn Kumar Ravi, R. Ganesh Narayanan
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ganu@iitg.ernet.in

Corresponding author.
, Pritam K. Rana
Department of Mechanical Engineering, IIT Guwahati, Guwahati 781039, India
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Tables (4)
Table 1. Tensile properties of Al6082-T6 and HDPE.
Table 2. Bond areas of tri-metallic and sandwich sheets.
Table 3. Mapping of bond area range for tri-metallic and sandwich sheets.
Table 4. Failure modes during lap shear test of FSSW joints.
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Abstract

In the present work, the effect of shoulder surface and pin profiles during friction stir spot welding of Al6082-T6/HDPE/Al6082-T6/HDPE/Al6082-T6 sandwich sheet has been attempted. Lap shear tests are performed to evaluate the mechanical performance. It is observed during lap shear tests that a tool with square pin produced stronger joints. The square pin produced joints with moderate hook distance, hook width and hook height and larger bond area resulting in stronger joints. The joints made from tri-metallic sheet perform better than sandwich sheet joints. The tool with square pin generated a joint withstanding 1.4kN fracture load in case of sandwich sheet, while it is 2.8kN in case of tri-metallic sheet. The tri-metallic joints are characterized by larger bond area and larger hook distance. Nugget pull out is the failure mode seen in sandwich and tri-metallic sheets during lap shear tests.

Keywords:
Friction stir
Spot welding
Sandwich sheet
Lap shear test
Hook
Fracture
Full Text
1Introduction

Sandwich sheet is part of light weight initiatives in many industry sectors. It is comprised of a polymer layer bond between two metal skins. The introduction of lightweight sheet materials posed problems in forming and joining. Hence novel forming and joining processes are proposed. In sandwich sheets, selection of core layer is delicate and it depends on its secondary purpose other than structural support. The following are some examples. Metallic foam core can act as acoustic damper and as cooling system [1], while sandwich sheet with polymer core can be used to fabricate large double curvature components [2]. SS316L/propylene–polyethylene/SS316L sandwich sheets are deep drawn successfully with and without local reinforcement [3].

The presence of polymer core makes the joining process complex. Hence selection of joining process and parameters is performed cautiously. It is shown by Salonitis et al. [4] that CO2 laser welding of sandwich steel sheets cause considerable damage to the core layer. Finite element simulations helped to make such analyses possible [5]. Tan et al. [6] work revealed the possibility of resistance spot welding (RSW) of sandwich sheets with stainless steel fibres as core layer. Self-pierced riveting of sandwich sheets caused defects like non-uniform deformation and tail buckling [7]. But solid state joining can be more sustainable as compared to fusion welding processes, especially for sandwich sheets as suggested by Narayanan and Das [8].

Friction stir spot welding (FSSW) is a solid state joining method that has been used to join metallic and polymeric sheets. The process has the potential to join sandwich sheets with much less quality problems. In metallic sheets, Gerlich et al. [9], Badrinarayanan et al. [10,11], Yuan et al. [12], Pathak et al. [13], and Nguyen et al. [14] carried out FSSW of variety of Al and Mg alloys for performance analyses and process optimization. Sung et al. [15], Hossain et al. [16], and Khan et al. [17] conducted FSSW and RSW of dual phase steel, and stainless steel, for joint formation studies and failure analyses. In friction stir welding (FSW), Ramulu et al. [18] proposed a new model based on axial force and torque to identify internal defects formation, and Das et al. [19] proposed a new indicator based on torque signals to identify the defective cases.

In the case of polymers and sandwich sheets, in the context of present work, the attempts made by Arici and Mert [20], Bilici et al. [21] and Oliveira et al. [22] are notable. FSSW of polypropylene sheet, high density polyethylene (HDPE) sheets, and polymethyl methacrylate were analyzed and process parameters like dwell time and plunge depth were optimized. Use of friction stir processing for fabricating aluminium foam sandwich sheets with dense steel face sheets was demonstrated by Hangai et al. [23]. The interface bond strength was acceptable.

The literature available on FSSW of sandwich sheet is scarce. There are no experimental analyses done on Al skin-polymer core sandwich sheets meant for automotive applications. Furthermore, the appropriate process that can be used for assembling/joining of two-layer sandwich sheets is unknown and there is no attempt made. In this context, the present work aims to analyze the effect of shoulder surface and pin geometries on the joint formation and joint strength during FSSW of double-layered, Al6082-T6/HDPE/Al6082-T6/HDPE/Al6082-T6, sandwich sheets. Lap shear tests are conducted to evaluate the joint mechanical performance after FSSW. Hook features are measured for analyses.

2FSSW experiments and joint testing

The skin material is Al6082-T6 of 1.5mm thickness and the core layer is HDPE of 0.8mm thickness. The chemical composition of Al6082-T6 is, Si: 1.3, Mn: 1.0, Mg: 1.0, Fe: 0.6, Ti: 0.4, Cr: 0.1, Cu: 0.3, Zn: 0.1, Al: Balance. The mechanical properties of Al6082-T6 and HDPE as obtained from standard tensile tests are given in Table 1. HDPE did not fail during tensile tests. The tensile sample and the properties evaluated were as per ASTM B557M-15 standard. The plastic strain ratio was evaluated as per ASTM E517-00. Few trial experiments were conducted for checking the repeatability of the tensile test data.

Table 1.

Tensile properties of Al6082-T6 and HDPE.

Material  Properties
  Yield strength (MPa) (at 0.2% offset strain)  Tensile strength (MPa)  Uniform elongation (%)  Total elongation (%)  Strain hardening exponent  Strength coefficient (MPa)  Plastic strain ratio 
Al6082-T6  314±358±12.2±16.2±0.5  0.11  505±1.2 
HDPE  –  29±0.5  17.4±2.4  –  –  –  – 

FSSW was conducted on two types of samples – (i) a double-layered sandwich sheet, and (ii) a tri-metallic sheet (Fig. 1). In sandwich, two sets of layers (Al/HDPE/Al and Al/HDPE/Al) were joined, and in tri-metallic, three Al sheets were joined. Only shoulder surface and pin geometries are identified for FSSW to study their effect on the joint formation and joint strength. The sandwich and tri-metallic samples are also compared for the analyses. Eight different tools were fabricated for FSSW experiments as depicted in Fig. 2 with the code names. These were decided based on thorough literature survey and fabrication accuracy. Other FSSW and tool parameters like tool rotation speed, plunge rate, shoulder diameter and pin length were maintained at 600rpm, 22mm/min, 12mm and 4.12±0.12mm, respectively. For cylindrical pin, an average diameter of 5.6mm, and for tapered pin, larger diameter of 3.74mm and smaller diameter of 1.98mm were maintained. Pin side of 4.4mm, 4.15mm and 2.85mm were maintained for triangular pin, square pin and hexagonal pin, respectively. A plunge depth of 1mm was used for FSSW of sandwich sheets and about 0.1mm for tri-metallic sheets. These were determined based on some initial trials aiming at good joint formation.

Fig. 1.

Schematic of (a) double-layered sandwich sheet, (b) tri-metallic sheet.

(0.04MB).
Fig. 2.

Tools fabricated for FSSW with tool code names. (a) Straight pin with flat shoulder (F tool), (b) straight pin with concave shoulder (CC tool), (c) straight pin with convex shoulder (CV tool), (d) tapered pin with flat shoulder (T tool), (e) threaded pin with flat shoulder (Thr tool), (f) triangular pin with flat shoulder (Tri tool), (g) square pin with flat shoulder (Sq tool), (h) hexagonal pin with flat shoulder (Hexa tool).

(0.12MB).

The Al skin and HDPE layer were cleaned thoroughly and FSSW was conducted on a milling machine as per the parameters mentioned above. The joint made was sectioned and polished to reveal the hook formation and hook dimensions like hook height, hook width and hook distance. These were measured using an optical microscope. In lap shear test, the dimensions of the samples were decided based on AWS B4.0:2007 standard. Two FSSW trials and lap shear tests were conducted for joint made by each tool. Third trial was conducted if the repeatability was not good. The fracture load was evaluated from the load-displacement data during lap shear tests. The separation force or fracture load was monitored by other authors as well in FSSW of sheets [10–14,17,20]. Separate set of welded samples were made for joint formation analyses and to obtain microscopic details. Fracture modes were identified after the lap shear tests.

3Results and discussion

In this section, the effect of shoulder surface and pin profiles on the lap shear test data of sandwich and tri-metallic sheets is explained. The relation between the lap shear test data, bond area, and hook morphology are revealed.

3.1Lap shear test results

Figs. 3 and 4 show the results of lap shear test at various shoulder and pin profiles for the sandwich and tri-metallic sheets, respectively. In both the cases, the tool with square pin (Sq tool) produced stronger joints. In the case of tri-metallic sheet, the square pin has produced a joint with larger displacement at failure as well. The joints made with square pin can withstand a fracture load of 1.4kN in case of sandwich sheet, and about 2.8kN in case of tri-metallic sheet. FSSW tool with threaded pin (thr tool) produced a weaker joint in both the cases. In sandwich sheet, CV tool (tool with convex shoulder) also produced weaker joint, almost the same as that of threaded pin (thr tool). There is about 65–75% reduction in load bearing ability when pin profiles are changed. All other shoulder and pin profiles fall in the intermediate range. It should be noted that CC tool did not produce a successful joint and lap shear test, in sandwich sheet, and hence result is not available in Fig. 3.

Fig. 3.

Lap shear test results of sandwich sheet after FSSW

(No data for concave (CC) shoulder).

(0.09MB).
Fig. 4.

Lap shear test results of tri-metallic sheet after FSSW.

(0.08MB).

Fig. 5 summarizes the fracture load of sandwich and tri-metallic sheets. In all the cases, except F tool, the joints made in tri-metallic sheet perform better than that produced in sandwich sheet. About 60% reduction in load bearing ability is seen in sandwich sheets for all the tool geometries. In the case of CV tool, both perform almost equally.

Fig. 5.

Comparison of fracture load achieved by sandwich and tri-metallic sheets for various tools (data variation: ±0.2kN).

(0.06MB).

It has been hypothesized and proposed that the joint strength after FSSW depends on the bond area, and hook morphology, in the following ways.

  • (i)

    The load bearing ability of the joint during lap shear test directly depends on the bond area. Larger the bond area, larger will be the fracture load.

  • (ii)

    Hook morphology also decides the joint strength [10,11]. Hook morphology can be quantified by hook height, hook width, hook distance and hook orientation (Fig. 6). Hook orientation towards the pinhole produces a weaker joint and hook orientation away from the pinhole produces a stronger joint. Greater the hook distance from the pinhole periphery, stronger will be the joint. At the same time, greater the hook height and hook width, weaker will be the joint.

    Fig. 6.

    Definition of hook morphology in a FSSW joint.

    (0.14MB).

3.2Bond area

Table 2 shows the summary of joint bond area. Using the same data, in Table 3, the tools are mapped to different bond area ranges. The joint bond area was calculated from joint bond diameter measured using an optical microscope from the fractured sample. The sq tool produced a larger bond area in the joint region in both the sandwich and tri-metallic sheets. Hence it delivers a larger fracture load during lap shear tests. On the other hand, in tri-metallic sheets, the CV tool and thr tool belong to the lower bond area ranges, with slight difference between them, and hence produced joints with lower fracture load. In the case of sandwich sheet, the thr tool belong to the second lowest bond area range (Table 3), producing a joint with the lowest fracture load during lap shear tests. It should be noted that in sandwich sheet, the T tool produced the lowest bond area (Table 2) and it has made a joint with second lowest fracture load in lap shear test (Fig. 5). All other tools produced bond area in the intermediate range and hence created joints with moderate load bearing ability. While comparing joints made by sandwich and tri-metallic sheets, the bond area is larger for tri-metallic sample, resulting in higher fracture load for tri-metallic samples (Figs. 4 and 5).

Table 2.

Bond areas of tri-metallic and sandwich sheets.

Tool  F tool  CC tool  CV tool  T tool  Thr tool  Tri tool  Sq tool  Hexa tool 
Bond area of tri-metallic sample (mm243.32  41.03  32.44  36.21  36.21  43.18  45.66  35.93 
Bond area of sandwich sample (mm241.46  Unsuccessful  39.32  18.24  23.41  22.74  40.14  30.71 
Table 3.

Mapping of bond area range for tri-metallic and sandwich sheets.

Bond area range (mm215–20  20–25  25–30  30–35  35–40  40–45  45–50 
Tri-metallic sample  –  –  –  CV tool  T tool, Thr tool, Hexa tool  F tool, CC tool, Tri tool  Sq tool 
Sandwich sample  T tool  Tri tool, Thr tool  –  Hexa tool  CV tool  F tool, Sq tool  – 
3.3Hook morphology and joint behaviour

For a stronger joint with larger fracture load, the joint should have (i) larger hook distance from the pinhole periphery, (ii) lower hook height and hook width, and (iii) hook orientation away from the pinhole. Figs. 7 and 8 show the individual hook dimensions for sandwich sheets and trimetallic sheets respectively. In general, three hooks are formed in sandwich sheets. Two hooks, h1 and h2, are formed at the interface of upper sheet and middle sheet, while the third hook, h3, is formed at the interface of middle sheet and lower sheet. Fig. 9 shows the summary of relation between the fracture load and the hook dimensions (for hook h3) for various tools.

Fig. 7.

Hook dimensions for FSSWed sandwich sheet.

(0.17MB).
Fig. 8.

Hook dimensions for FSSWed tri-metallic sheet.

(0.2MB).
Fig. 9.

Summary of relationship between hook dimensions and fracture load.

(0.24MB).

For sandwich sheets, it is seen that the joint made by sq tool show larger fracture load. The joint has moderate hook distance, moderate hook width and moderate hook height. The thr tool that generate weaker joint possesses the lowest hook distance. At the same time, it is characterized by lower hook width and height as well (Figs. 7–9). All other tools perform intermediately and hook dimensions are in between that of sq tool and thr tool. Moreover, the F tool, sq tool, hexa tool, generated hooks that are oriented away from the pinhole (or almost perpendicular to the pulling direction in lap shear test) (Fig. 10a) which has also contributed to attain larger fracture load during lap shear tests. The propagation of crack is delayed because of the normal orientation of the hook.

Fig. 10.

Hook formation during FSSW of (a) sandwich sheets, (b) tri-metallic sheets.

(1.1MB).

In case of tri-metallic sheets, for sq tool, three hooks are formed (Fig. 10b) and are well developed (Fig. 8). It has hook h3 oriented perpendicular to loading direction. Hook h1 has larger hook distance from the pinhole periphery, while hook h2 has lesser hook width and larger hook height and hook distance from the pinhole periphery. All the characteristics together generated a FSSWed joint with largest fracture load in lap shear test. The tri tool has generated second largest fracture load for tri-metallic sheets. In this case, only two hooks are formed (Fig. 10b). Hook h2 has lesser hook height and hook width and larger hook distance from the pinhole periphery which can resist fracture growth during lap shear test. Hook h3 has larger hook height and hook width, and lesser hook distance from the pinhole periphery which favours crack growth through interface I2. Moreover, hook h3 orientation is perpendicular to the loading direction in lap shear test resulting in a lap joint with the second largest fracture load.

It can also be understood that the hook dimensions and hook orientation contribute towards better performance of joints made on tri-metallic sheet as compared to sandwich sheet. In most of the joints on tri-metallic sheets, the hook distance is larger as compared to the joints made on sandwich sheets (Fig. 9). In some of the cases of tri-metallic sheets, the hook width and height are larger. Still the performance is better in these cases. This depicts the dominance of hook distance over the hook width and height.

Elangovan and Balasubramanian [24] supported the use of square pin for fabricating FSSW and FSW joints in case of Al 6xxx and Al 2xxx material grades respectively. They have shown that the swept volume and pulsating action during FSW are responsible for stronger joints in case of square pin. About 100 pulses/s are produced by square pin during friction stir processing which is much larger when compared to other tools.

Table 4 shows the failure modes during lap shear test of sandwich and tri-metallic sheets. Nugget pull out failure mode is observed in all the cases. In some cases, like sq tool, T tool and Tri tool of sandwich sheet, nugget pull out with some extension is observed. This may contribute to larger joint extension during lap shear tests.

Table 4.

Failure modes during lap shear test of FSSW joints.

S. No.  Tool profile  Sandwich sample  Tri-metallic sample  S. No.  Tool profile  Sandwich sample  Tri-metallic sample 
F tool 
 
 
Sq tool 
 
 
CC tool 
 
 
Hexa tool 
 
 
CV tool 
 
 
       
T tool 
 
 
       
Thr tool 
 
 
       
Tri tool 
 
 
       
4Conclusions

In the present work, FSSW of two-layered (Al6082-T6/HDPE/Al6082-T6/HDPE/Al6082-T6) sandwich sheets has been attempted and the influence of shoulder surface and pin profile are studied. The following conclusions are drawn from the results.

  • FSSW of two-layered sandwich sheets is possible with all the tools. A tool with square pin produced a stronger joint with maximum fracture load from lap shear test. The square pin is good for both sandwich and tri-metallic sheets.

  • It has been hypothesized and later demonstrated that the joint performance is mainly governed by bond area and hook geometry. The joint produced by square pin is characterized by larger bond area, moderate hook distance, moderate hook width and moderate hook height resulting in stronger joints among all the tools.

  • The tri-metallic sheet joints performed better than sandwich sheet joints in almost all the tools. About 60% reduction is observed in fracture load when sandwich sheets are used. The tri-metallic joints are characterized by larger bond area and larger hook distance yielding stronger joints. This means practically sandwich sheet applications are restricted in terms of joining efficiency. But both perform equally when straight pin with flat shoulder (F tool) and straight pin with convex shoulder (CV tool) are used. Hence, other than tool with square pin, a tool with straight pin and flat shoulder, and a tool with convex shoulder can also be used for joining sandwich sheets, but at the cost of joint performance.

  • Nugget pull out failure mode is seen during lap shear tests. Few tools (sq tool, tri tool, and T tool) witnessed nugget pull out with some extension.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgement

The present work is not funded by any funding agency. The authors thank CIF, IIT Guwahati for extending facility to perform mechanical tests.

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Copyright © 2018. Brazilian Metallurgical, Materials and Mining Association
Journal of Materials Research and Technology

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