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Effect of tool geometry on static strength of friction stir spot-welded aluminum alloy
ARTICLE IN PRESS International Journal of Machine Tools & Manufacture 49 (2009) 142–148
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Effect of tool geometry on static strength of friction stir spot-welded aluminum alloy H. Badarinarayan a, Q. Yang a,, S. Zhu a,b a b
Automotive Products Research Laboratory, Hitachi America Limited, Farmington Hills, MI 48335, USA Department of Materials Science and Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA
a r t i c l e in f o
a b s t r a c t
Article history: Received 24 July 2008 Received in revised form 12 September 2008 Accepted 26 September 2008 Available online 17 October 2008
Friction stir spot welding is performed on 5083 Al alloy using tools with a conventional cylindrical pin and the proposed triangular pin. Partial metallurgical bond (called as ‘hook’) is formed in the weld region between the overlapped metal sheets. The tool-pin geometry significantly affects the hook shape. Under the same process condition, welds made with the cylindrical pin have a continuous hook which bypasses the stir zone and points downward towards the weld bottom. By contrast, for welds made with the triangular pin, the hook is directed upwards and then arrested at the periphery of the stir zone. The difference in the hook shape could be attributed to the asymmetric rotation of the triangular pin that may cause the material in the vicinity of the pin to move back and forth in the radial direction resulting in the hook being broken-up (dispersed) in the stir zone. In addition, the triangular pin results in a finer grain structure in the stir zone compared to the cylindrical pin. Static strength of welds made with the triangular pin is twice that of welds made with the cylindrical pin, which is attributed to the finer grain size as well as tensile failure mode as a result of the arrested hook. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Friction stir spot welding Triangular pin Failure mode Weld strength Hook
1. Introduction Weight reduction is a critical challenge in the automotive industry in order to improve fuel economy and enhance vehicle performance. Light-weight design by replacing steel with Al and/or Mg alloys has been considered as a promising strategy to achieve this objective. Therefore, the joining of structures made of light-weight alloys become very important under the current scenario. In comparison with other conventional joining techniques, friction stir welding (FSW) has an attribute of providing higher joint efficiency associated with a fine recrystallized grain structure in the consolidated weld region [1]. Friction Stir Spot Welding (FSSW) is a derivative of FSW, and has been gaining momentum since the beginning of this decade. Unlike FSW, FSSW can be considered as a transient process due to its short cycle time (usually a few seconds). During FSSW, tool penetration and the dwell period essentially determine the heat generation, material plasticization around the pin, weld geometry and therefore mechanical properties of the welded joint. A characteristic feature of friction stir spot welds in lap configuration is the formation of a geometrical defect originating at the interface of the two welded sheets, sometimes called as ‘hook’. Metallic materials oftentimes have a thin oxide film present on the surface. During welding, hook is formed because
Corresponding author. Tel.: +1 248 474 2800x1814; fax: +1 248 473 8420.
E-mail address: [email protected] (Q. Yang). 0890-6955/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2008.09.004
of the upward bending of the sheet interface due to the penetration of the tool into the bottom sheet. The oxide film is broken up into particles by stirring of the tool. The particles could be dispersed into the weld region, thus causing partial or complete metallurgical bonding of the overlapped sheets in the weld region. The presence of the hook diminishes the integrity of spot welds since failure (crack propagation) can occur along the hook when the weld is subjected to external loading [2]. Many previous studies show that the strength of friction stir spot welds mainly depends on the size of the weld region which is further closely related to process condition and tool profile. Higher weld strength is attributed to a larger stir zone size attained by lowering rotational speed [3,4], and using a profiled tool [5,6]. Increasing tool plunge depth can increase weld strength [7]. However, a deep plunge depth may lead to decreased weld strength due to excessive thinning of the top sheet [7]. So far, investigations addressing the effect of the hook on weld strength have been far from satisfactory. In this work, the effect of pin shape on hook, weld strength and failure mode is examined. The purpose of this investigation is to arrest the hook formation and possibly improve the strength of spot welds.
2. Experiment Annealed 5083 Al sheets with two different thicknesses of 1.64 and 1.24 mm are chosen for the present study. The nominal
ARTICLE IN PRESS H. Badarinarayan et al. / International Journal of Machine Tools & Manufacture 49 (2009) 142–148
composition of this alloy is shown in Table 1. Welding is performed by plunging the tool from the thick sheet into the thin sheet. The tool shoulder diameter is 12 mm with a concave profile, and the pin length is 1.6 mm. Two different pin geometries are used: a conventional cylindrical pin and the proposed triangular pin, as schematically shown in Fig. 1. The cylindrical pin has a diameter of 5 mm with right hand threads. The triangular pin has an inscribed circle with a diameter of 5 mm with no features. Spot welds are made in cross-tension configuration with 50 mm 50 mm overlap. All welds are made under the following process condition: 1500 rpm tool rotational speed; 20 mm/min plunge speed; 2 s hold time; and 0.2 mm shoulder plunge depth. For simplicity of nomenclature, spot welds made with cylindrical and triangular pins are designated as FSSW-C and FSSW-T, respectively. Mechanical properties of the spot welds are characterized using hardness profile and cross-tension testing. Vickers hardness profile is measured 0.2 mm above the interface of the two overlapped sheets on the as-welded metallographic specimens with an indenting load of 100 g and a loading time of 10 s. Fig. 2 shows a schematic illustration of a cross-tension specimen. Specimens are tested on an Instron screw-driven machine (Model 1123) at a constant cross-head speed of 5 mm/min. The crosstension weld strength is obtained by averaging the strengths of 5 individual specimens. Crack initiation and propagation in the spot welds prior to failure is evaluated by partially pulling the samples to pre-determined loads. Macro- and micro-structure examinations are conducted on as-received, as-welded and mechanically tested specimens. Asreceived alloy is sectioned in the plane containing the sheet normal direction (ND) and the rolling direction (RD). As-welded and tested specimens are sectioned along the diameter of the weld keyhole in a plane containing the ND and the RD of the top sheet. All specimens are subjected to mechanical grinding and polishing with 0.05 mm silica suspension. For observation of weld macrostructures, specimens are etched with the Keller’s reagent (1 ml HF, 1.5 ml HCl, 2.5 ml HNO3, and 95 ml H2O). For observation of microstructures, metallographic specimens are electropolished at a voltage of 20 V by using Barker’s solution (2.5% Fluoroboric acid). Microstructures are examined using crosspolarized optical microscopy. Scanning electron microscope
143
(SEM, Hitachi S-2400) is used to observe the hook in as-welded specimens and the fractographic features of the failed spot welds.
3. Results Fig. 3 shows cross-sectional macrostructures of the welds. Three characteristic regions exist in sequence from the periphery of the weld keyhole towards the base material: completely bonded region, partially bonded region, and unbonded region, as shown in Fig. 3(A). Completely bonded region is formed due to severe plastic deformation by rotating tool that causes an elimination of the sheet interface. The initial layered surface oxide is broken into fine particles that are randomly dispersed. Next to this region, the surface oxide layer is broken into an array of discontinuous particles that profiles the upward bending of the sheet interface due to less deformation straining. Partial metallurgical bonding is therefore developed, and the array of particles is called ‘hook’. The hook in the FSSW-C weld runs gradually upward and then bypasses the stir zone and points downward towards the weld bottom, as shown in Fig. 3(A), (a1) and (a2). In the downward portion of the hook (Fig. 3(a2)), this partial bonding becomes less prominent. In contrast, the FSSW-T weld exhibits a hook geometry that is directed upward towards the stir zone and ends with a very short plateau, as shown in Fig. 3(B), (b1) and (b2). Fig. 4 shows cross-sectional microstructures in the stir zone of the welds (1 mm from the periphery of the weld keyhole). Compared to the base material having an elongated coarse grain structure (Fig. 4(a)), the stir zone of the welds possesses a uniform microstructure which is composed of fine equiaxed grains (Fig. 4(b) and (c)). Dynamic recrystallization during FSSW causes
Load
Weld Location
x
x
Table 1 Chemical composition of 5083-O Al alloy Element Mg Wt.%
Mn
Cr
Zn
Fe
Ti
Si
Cu
Al
4.6–5.0 0.8–1.0 0.08–0.5 o0.25 o0.25 o0.15 o0.15 o0.1 Bal.
φ 12 mm
Fig. 2. Schematic illustration of a weld in cross-tension configuration.
φ 12 mm
φ 5 mm
1.6 mm
1.6 mm
Fig. 1. Schematic illustration of FSSW tool geometries (a) cylindrical pin shape (threads not shown in illustration) and (b) triangular pin shape.
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Triangular
Cylindrical
I
A
II
B
a1
b1
a2
b2 partially
completely bonded
unbonded
bonded 200μm
a1
200μm
b1 plateau
partial metallurgical bond
partial metallurgical bond 20μ μm
a2
20μm
b2
complete metallurgical bond partial metallurgical bond
20μm
20μm
Fig. 3. Cross-sectional macrostructures of the welds: FSSW-C (left) and FSSW-T (right). (A) Magnified view of the hook geometry in region I for FSSW-C welds. (a1) and (a2) show partial metallurgical bonding within region I at the locations indicated. (B) Magnified view of the hook geometry in region II for FSSW-T welds. (b1) and (b2) show partial and complete metallurgical bonding in region II at the respective locations indicated.
grain refinement [1]. Under the same process condition, the triangular pin results in an even finer grain structure than the cylindrical pin. Fig. 5 shows the hardness profile of the welds. The welds have higher Vickers microhardness in the stir zone than the base material. Moreover, the FSSW-T weld has a slightly higher hardness in the stir zone than the FSSW-C weld. The hardness is relatively stable at a lower value after a distance of about 2 mm from the periphery of the weld keyhole. Fig. 6 shows the load versus displacement relationship for both FSSW-C and FSSW-T welds during cross tension. The accompanying cross-sectional macrographs show failure propagation in the welds in response to different levels of applied load. The strength for the FSSW-T weld is about 3630 N, twice as much as that of the FSSW-C weld at the same process condition used in the present study. For the FSSW-C weld, the crack initially passes around the stir zone and propagates downward towards the bottom of the weld, as dictated by the shape of the hook. When the external load
exceeds 1420 N, rather than following the potential crack path, the failure goes through the material near the weld bottom along the thickness direction due to plastic collapse. For the FSSW-T weld, the crack initially propagates upward along the hook at a lower level of applied load (1400 N). A further increase in crosstension load causes the crack to propagate through the stir zone. The slope of the load versus displacement curve significantly increases, when the crack propagates through the stir zone. Therefore, the different failure modes in cross tension determine the different weld strengths. Fractographs in Fig. 7 show that the welds fracture in the mode of dimple rupture. For the FSSW-C weld, in the downward portion of the hook (i.e., the location a1 in Fig. 6), small shallow dimples are primarily observed. The formation of these dimples is associated with the presence of discontinuous oxide particles that lowers the plasticity of the material in this local region and enhances the coalescence of adjacent microvoids nucleated at the particle surface. The equiaxed dimple shape suggests that the
ARTICLE IN PRESS H. Badarinarayan et al. / International Journal of Machine Tools & Manufacture 49 (2009) 142–148
145
110
a
FSSW-T FSSW-C
105
Vickers hardness Hv
100 95 90 85 80
50 µm
weld keyhole
75 70
b
0
2
4 6 8 10 Distance from the weld center, mm
12
Fig. 5. Hardness profile of the welds FSSW-C and FSSW-T, measured 0.2 mm above the interface between the two overlapped sheets.
has the characteristics that one end of the dimple is open. The presence of elongated dimples indicates that shear fracture occurs at the bottom of the weld at the final stage of failure. For the FSSW-T weld, fracture surface in the hook region is similar to that of the FSSW-C weld (not shown here). In the stir zone, through which the crack propagates (i.e., the location b in Fig. 6), intergranular fracture is observed due to the nucleation of microvoids at the grain boundaries.
4. Discussion
50 µm
c
50 µm Fig. 4. Cross-sectional microstructures of (a) the base material, (b) the FSSW-T weld and (c) the FSSW-C weld in the stir zone (1 mm from the periphery of the weld keyhole).
state of stress in this region is primarily tension. In the bottom of the weld (i.e., the location a2 in Fig. 6), the fracture surface exhibits elongated dimples of various sizes. The elongated dimple
As shown in Fig. 3, the hook geometry is primarily determined by the geometry of the tool pin. For the cylindrical pin, the rotation of the pin is symmetric in nature causing shear deformation of the material around the pin surface. The threads on the pin significantly enhance plastic flow along the thickness direction. As reported by Fujimoto et al. [8], immediately next to the pin the material driven by the threads moves downward from the upper sheet to the lower sheet, forming the major portion of the stir zone; the material originally from the lower sheet is pushed outward as well as upward towards the upper sheet. This explains the formation of hook geometry in welds made with cylindrical pin, wherein the hook bulges upward in the region away from the keyhole and runs downward towards the bottom of the weld near the keyhole. In contrast, the hook geometry is different for welds made with the triangular pin. Due to the asymmetric geometry of the triangular-shaped pin, successive rotation of the pin is believed to enhance the plastic flow of the material in the vicinity of the pin in the radial direction, compared to the cylindrical pin. This can be rationalized as follows. Considering only the effect of the tool-pin geometry on plastic flow of the material, it is assumed that (1) the material is only deformed in a closed chamber comprised of the tool shoulder, the pin, the material under the pin, and the wall of the outer bulk material (which undergoes marginal deformation); and (2) there is no-slip condition between the pin and the material in its immediate vicinity. Fig. 8(a) shows
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FSSW-C fractured
a1
1420 N
997 N
a2
4000 FSSW-T FSSW-C
3500
Load, N
3000 2500 2000 1500 1000 500 0 0
5
10 15 Displacement, mm
1409 N
3000 N
20
fractured b
FSSW-T Fig. 6. Load versus displacement relationship for the FSSW-C and FSSW-T welds during cross-tension. Cross-sectional macrographs show crack propagation during loading. The fracture surfaces ‘a1’ and ‘a2’ in the FSSW-C weld and ‘b’ in the FSSW-T weld show distinctive features that are characterized in Fig. 7.
b
a1
25 μm
25 μm
a2
25 μm Fig. 7. Fractographs at the selected locations (a1) and (a2) in the FSSW-C weld, and at the location (b) in the FSSW-T weld. The selected locations are indicated by arrows in Fig. 6.
the top view of the ‘stop action’ FSSW-T weld that is obtained by stopping the tool rotation just before the end of the dwell period and then gently separating the weld from the tool. This represents a snapshot of the keyhole geometry at an instance during the tool rotation. With the designated pin-rotation direction, the motion of
the material close to the pin face is shown in Fig. 8(b). Between location ‘a’ at the vertex and ‘b’ at the center of the pin face, the motion can be resolved into two components. The first component (Vs) causes the material to shear along the pin surface; the second component (Vp) pushes the material outward due to the pin
ARTICLE IN PRESS H. Badarinarayan et al. / International Journal of Machine Tools & Manufacture 49 (2009) 142–148
a
wall of outer bulk material (marginal deformation)
b pin
a V Vpp
Vss b V
c
Fig. 8. (a) top view of the ‘stop-action’ FSSW-T weld; (b) schematic illustration of the effect of the pin rotation on the material motion in the vicinity of the triangular pin (i.e., within the dotted circle region in (a)).
147
times the shear strength. In the FSSW-C weld, it is the shear strength of the material and the load-carrying area that determine the weld strength. Secondly, the material near the weld keyhole in the FSSW-T weld has higher strength than the material near the weld bottom in the FSSW-C weld due to a finer grain structure. In the FSSW-C weld, the material near the weld bottom has a microstructure similar to that of the base material; moreover, grain refinement in the stir zone does not appreciably affect the weld strength since the crack propagation passes around the stir zone. Thus, it can be said that the strength of friction stir spot welds is significantly affected by the hook geometry, which, in turn, is influenced by the tool-pin geometry.
5. Conclusions surface. From location ‘a’ to ‘b’, the latter component (Vp) gradually reduces until it becomes 0 at location ‘b’. Subsequently, the material (between location ‘a’ and ‘b’) that experiences an outward motion will be deflected inward by the wall of the outer bulk material towards the region between location ‘b’ and ‘c’. This is because the rotation of the pin has created a cavity in this region which is quickly filled by the inward-flowing material. Therefore, in addition to the rotational flow, successive rotation of the pin causes the material in its vicinity to move back and forth in the radial direction which may lead to high degree of material deformation, whereas, in contrast, successive rotation of the cylindrical pin primarily causes shearing of the material in its vicinity [9,10]. Furthermore, Fig. 4 clearly indicates the presence of finer grain structure in the stir zone for FSSW-T welds than FSSWC welds which substantiates the claim that the plastic deformation caused by the triangular pin is more severe than that by cylindrical pin. The finer grain structure could result in higher hardness in the stir zone of the FSSW-T weld as compared to the FSSW-C weld according to the Hall–Petch relationship. The effect of this increased plastic flow in the radial direction by the triangular pin may lead to the hook being broken-up (dispersed) in the weld region. Therefore, even though the hook begins to form when the pin plunges into the interface of the two work pieces to be welded, this radial back and forth motion of the material that is caused due to the rotation of the triangularshaped pin will inhibit the formation and further growth of a continuous hook. This is the primary reason for the hook arrest in the FSSW-T welds. The geometry of the hook can be used as a basis to explain the failure mechanism observed in the spot welds. The hook, which is a weak metallurgical bond, exists between the overlapped sheets. The crack first propagates along the hook at low levels of external load in both the welds (Fig. 6). A combination of stress concentration, mechanical property of the material, and even the load-carrying area in front of the hook tip determines further crack or failure propagation during which the weld strength is attained. The large difference in the cross-tension strength between these two types of welds could be caused by the following two factors. Firstly, as illustrated by the cross-sectional micrographs in Fig. 6 and the fracture surfaces in Fig. 7, during the final stage of failure, the FSSW-C weld experiences plastic collapse near the weld bottom in a shear mode, while the FSSW-T weld fractures by crack propagation through the stir zone under tension. The failure mode is different between these two welds when they are subjected to cross-tension loading. In the present study, the area of the fracture surface from the final stage of failure in the FSSW-T weld is comparable to that in the FSSW-C weld. Therefore,larger external load causing the failure of the FSSW-T weld might be associated with the tensile failure mode where the tensile strength of the material is approximately O3
The effect of pin geometry on the hook formation, crosstension strength, and failure mode of friction stir spot-welded 5083-O aluminum alloy is investigated. The following conclusions are made. 1. The tool pin geometry significantly affects the hook. In the FSSW-C weld, the hook runs gradually upward and then bypasses the stir zone and points downward towards the weld bottom. In the FSSW-T weld, the hook is directed upward towards the stir zone and ends with a very short plateau. 2. Due to asymmetric rotation of the triangular-shaped pin which greatly increases material deformation, a finer grain structure is formed in the vicinity of the weld keyhole as compared to the cylindrical pin. 3. The crack first propagates along the hook at a low level of load when a weld is subjected to external loading. With a further increase in load, the FSSW-C weld experiences plastic collapse near the weld bottom in a shear mode, while the FSSW-T weld fractures by crack propagation through the stir zone under tension. 4. The weld strength is determined by the failure mode and the microstructure-related strength of the material. The crosstension strength of welds made with the triangular pin is twice that of welds made with the cylindrical pin under the same process condition.
Acknowledgement This work was supported by internal funding from Hitachi America, Limited, R&D. The authors would like to express their gratitude to Dr. Kazutaka Okamoto of Hitachi Research Laboratory, Hitachi Limited, for helpful discussions. References [1] C.G. Rhodes, M.W. Mahoney, W.H. Bingel, R.A. Spurling, C.C. Bampton, Scripta Materialia 36 (1997) 69. [2] Y.S. Sato, H. Takauchi, S.H. Park, H. Kokawa, Materials Science and Engineering A 405 (2005) 333. [3] J.F. Henrichs, C.B. Smith, B.F. Orsini, R.J. DeGeorge, B.J. Smale, P.C. Ruehl, Friction stir welding for the 21st century automotive industry, in: Proceedings of the 5th International Symposium of Friction Stir Welding, September 14–16, 2004, Metz, France. [4] Y. Tozaki, Y. Uematsu, K. Tokaji, Effect of welding condition on tensile strength of dissimilar FS spot welds between different Al alloys, in: Proceedings of the 6th International Symposium of friction Stir Welding, October 10–13, 2006, Saint-Sauveur, Nr Montreal, Canada. [5] I. Ikegami, T. Tsumura, F. Ye, K. Nakata, Effect of tool geometry on joint strength of Mg alloy by spot friction welding, Preprints of the National Meeting of Japan Welding Society No. 78, Spring 2006, Paper #215.
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[6] A.C. Addison, A.J. Robelou, Friction stir spot welding: principal parameters and their effects, in: Proceedings of the 5th International Symposium of Friction Stir Welding, September 14–16, 2004, Metz, France. [7] Q. Yang, S. Mironov, Y.S. Sato, K. Okamoto, Microstructure and mechanical properties of friction stir spot welded AZ31 Mg alloy, in: Proceedings of the 7th International Symposium of Friction Stir Welding, May 20–22, 2008, Awaji Island, Japan
[8] M. Fujimoto, S. Koga, N. Abe, Y.S. Sato, H. Kokawa, Quarterly Journal of the Japan Welding Society 26 (2008) 67 (in Japanese). [9] M. Fujimoto, S. Koga, N. Abe, Y.S. Sato, H. Kokawa, Quarterly Journal of the Japan Welding Society 25 (2007) 553 (in Japanese). [10] Y.S. Sato, H. Kokawa, K. Ikeda, M. Enomoto, S. Jogan, T. Hashimoto, Metallurgical and Materials Transactions A 32A (2001) 941.
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Effect of tool geometry on static strength of friction stir spot-welded aluminum alloy H. Badarinarayan a, Q. Yang a,, S. Zhu a,b a b
Automotive Products Research Laboratory, Hitachi America Limited, Farmington Hills, MI 48335, USA Department of Materials Science and Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA
a r t i c l e in f o
a b s t r a c t
Article history: Received 24 July 2008 Received in revised form 12 September 2008 Accepted 26 September 2008 Available online 17 October 2008
Friction stir spot welding is performed on 5083 Al alloy using tools with a conventional cylindrical pin and the proposed triangular pin. Partial metallurgical bond (called as ‘hook’) is formed in the weld region between the overlapped metal sheets. The tool-pin geometry significantly affects the hook shape. Under the same process condition, welds made with the cylindrical pin have a continuous hook which bypasses the stir zone and points downward towards the weld bottom. By contrast, for welds made with the triangular pin, the hook is directed upwards and then arrested at the periphery of the stir zone. The difference in the hook shape could be attributed to the asymmetric rotation of the triangular pin that may cause the material in the vicinity of the pin to move back and forth in the radial direction resulting in the hook being broken-up (dispersed) in the stir zone. In addition, the triangular pin results in a finer grain structure in the stir zone compared to the cylindrical pin. Static strength of welds made with the triangular pin is twice that of welds made with the cylindrical pin, which is attributed to the finer grain size as well as tensile failure mode as a result of the arrested hook. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Friction stir spot welding Triangular pin Failure mode Weld strength Hook
1. Introduction Weight reduction is a critical challenge in the automotive industry in order to improve fuel economy and enhance vehicle performance. Light-weight design by replacing steel with Al and/or Mg alloys has been considered as a promising strategy to achieve this objective. Therefore, the joining of structures made of light-weight alloys become very important under the current scenario. In comparison with other conventional joining techniques, friction stir welding (FSW) has an attribute of providing higher joint efficiency associated with a fine recrystallized grain structure in the consolidated weld region [1]. Friction Stir Spot Welding (FSSW) is a derivative of FSW, and has been gaining momentum since the beginning of this decade. Unlike FSW, FSSW can be considered as a transient process due to its short cycle time (usually a few seconds). During FSSW, tool penetration and the dwell period essentially determine the heat generation, material plasticization around the pin, weld geometry and therefore mechanical properties of the welded joint. A characteristic feature of friction stir spot welds in lap configuration is the formation of a geometrical defect originating at the interface of the two welded sheets, sometimes called as ‘hook’. Metallic materials oftentimes have a thin oxide film present on the surface. During welding, hook is formed because
Corresponding author. Tel.: +1 248 474 2800x1814; fax: +1 248 473 8420.
E-mail address: [email protected] (Q. Yang). 0890-6955/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2008.09.004
of the upward bending of the sheet interface due to the penetration of the tool into the bottom sheet. The oxide film is broken up into particles by stirring of the tool. The particles could be dispersed into the weld region, thus causing partial or complete metallurgical bonding of the overlapped sheets in the weld region. The presence of the hook diminishes the integrity of spot welds since failure (crack propagation) can occur along the hook when the weld is subjected to external loading [2]. Many previous studies show that the strength of friction stir spot welds mainly depends on the size of the weld region which is further closely related to process condition and tool profile. Higher weld strength is attributed to a larger stir zone size attained by lowering rotational speed [3,4], and using a profiled tool [5,6]. Increasing tool plunge depth can increase weld strength [7]. However, a deep plunge depth may lead to decreased weld strength due to excessive thinning of the top sheet [7]. So far, investigations addressing the effect of the hook on weld strength have been far from satisfactory. In this work, the effect of pin shape on hook, weld strength and failure mode is examined. The purpose of this investigation is to arrest the hook formation and possibly improve the strength of spot welds.
2. Experiment Annealed 5083 Al sheets with two different thicknesses of 1.64 and 1.24 mm are chosen for the present study. The nominal
ARTICLE IN PRESS H. Badarinarayan et al. / International Journal of Machine Tools & Manufacture 49 (2009) 142–148
composition of this alloy is shown in Table 1. Welding is performed by plunging the tool from the thick sheet into the thin sheet. The tool shoulder diameter is 12 mm with a concave profile, and the pin length is 1.6 mm. Two different pin geometries are used: a conventional cylindrical pin and the proposed triangular pin, as schematically shown in Fig. 1. The cylindrical pin has a diameter of 5 mm with right hand threads. The triangular pin has an inscribed circle with a diameter of 5 mm with no features. Spot welds are made in cross-tension configuration with 50 mm 50 mm overlap. All welds are made under the following process condition: 1500 rpm tool rotational speed; 20 mm/min plunge speed; 2 s hold time; and 0.2 mm shoulder plunge depth. For simplicity of nomenclature, spot welds made with cylindrical and triangular pins are designated as FSSW-C and FSSW-T, respectively. Mechanical properties of the spot welds are characterized using hardness profile and cross-tension testing. Vickers hardness profile is measured 0.2 mm above the interface of the two overlapped sheets on the as-welded metallographic specimens with an indenting load of 100 g and a loading time of 10 s. Fig. 2 shows a schematic illustration of a cross-tension specimen. Specimens are tested on an Instron screw-driven machine (Model 1123) at a constant cross-head speed of 5 mm/min. The crosstension weld strength is obtained by averaging the strengths of 5 individual specimens. Crack initiation and propagation in the spot welds prior to failure is evaluated by partially pulling the samples to pre-determined loads. Macro- and micro-structure examinations are conducted on as-received, as-welded and mechanically tested specimens. Asreceived alloy is sectioned in the plane containing the sheet normal direction (ND) and the rolling direction (RD). As-welded and tested specimens are sectioned along the diameter of the weld keyhole in a plane containing the ND and the RD of the top sheet. All specimens are subjected to mechanical grinding and polishing with 0.05 mm silica suspension. For observation of weld macrostructures, specimens are etched with the Keller’s reagent (1 ml HF, 1.5 ml HCl, 2.5 ml HNO3, and 95 ml H2O). For observation of microstructures, metallographic specimens are electropolished at a voltage of 20 V by using Barker’s solution (2.5% Fluoroboric acid). Microstructures are examined using crosspolarized optical microscopy. Scanning electron microscope
143
(SEM, Hitachi S-2400) is used to observe the hook in as-welded specimens and the fractographic features of the failed spot welds.
3. Results Fig. 3 shows cross-sectional macrostructures of the welds. Three characteristic regions exist in sequence from the periphery of the weld keyhole towards the base material: completely bonded region, partially bonded region, and unbonded region, as shown in Fig. 3(A). Completely bonded region is formed due to severe plastic deformation by rotating tool that causes an elimination of the sheet interface. The initial layered surface oxide is broken into fine particles that are randomly dispersed. Next to this region, the surface oxide layer is broken into an array of discontinuous particles that profiles the upward bending of the sheet interface due to less deformation straining. Partial metallurgical bonding is therefore developed, and the array of particles is called ‘hook’. The hook in the FSSW-C weld runs gradually upward and then bypasses the stir zone and points downward towards the weld bottom, as shown in Fig. 3(A), (a1) and (a2). In the downward portion of the hook (Fig. 3(a2)), this partial bonding becomes less prominent. In contrast, the FSSW-T weld exhibits a hook geometry that is directed upward towards the stir zone and ends with a very short plateau, as shown in Fig. 3(B), (b1) and (b2). Fig. 4 shows cross-sectional microstructures in the stir zone of the welds (1 mm from the periphery of the weld keyhole). Compared to the base material having an elongated coarse grain structure (Fig. 4(a)), the stir zone of the welds possesses a uniform microstructure which is composed of fine equiaxed grains (Fig. 4(b) and (c)). Dynamic recrystallization during FSSW causes
Load
Weld Location
x
x
Table 1 Chemical composition of 5083-O Al alloy Element Mg Wt.%
Mn
Cr
Zn
Fe
Ti
Si
Cu
Al
4.6–5.0 0.8–1.0 0.08–0.5 o0.25 o0.25 o0.15 o0.15 o0.1 Bal.
φ 12 mm
Fig. 2. Schematic illustration of a weld in cross-tension configuration.
φ 12 mm
φ 5 mm
1.6 mm
1.6 mm
Fig. 1. Schematic illustration of FSSW tool geometries (a) cylindrical pin shape (threads not shown in illustration) and (b) triangular pin shape.
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Triangular
Cylindrical
I
A
II
B
a1
b1
a2
b2 partially
completely bonded
unbonded
bonded 200μm
a1
200μm
b1 plateau
partial metallurgical bond
partial metallurgical bond 20μ μm
a2
20μm
b2
complete metallurgical bond partial metallurgical bond
20μm
20μm
Fig. 3. Cross-sectional macrostructures of the welds: FSSW-C (left) and FSSW-T (right). (A) Magnified view of the hook geometry in region I for FSSW-C welds. (a1) and (a2) show partial metallurgical bonding within region I at the locations indicated. (B) Magnified view of the hook geometry in region II for FSSW-T welds. (b1) and (b2) show partial and complete metallurgical bonding in region II at the respective locations indicated.
grain refinement [1]. Under the same process condition, the triangular pin results in an even finer grain structure than the cylindrical pin. Fig. 5 shows the hardness profile of the welds. The welds have higher Vickers microhardness in the stir zone than the base material. Moreover, the FSSW-T weld has a slightly higher hardness in the stir zone than the FSSW-C weld. The hardness is relatively stable at a lower value after a distance of about 2 mm from the periphery of the weld keyhole. Fig. 6 shows the load versus displacement relationship for both FSSW-C and FSSW-T welds during cross tension. The accompanying cross-sectional macrographs show failure propagation in the welds in response to different levels of applied load. The strength for the FSSW-T weld is about 3630 N, twice as much as that of the FSSW-C weld at the same process condition used in the present study. For the FSSW-C weld, the crack initially passes around the stir zone and propagates downward towards the bottom of the weld, as dictated by the shape of the hook. When the external load
exceeds 1420 N, rather than following the potential crack path, the failure goes through the material near the weld bottom along the thickness direction due to plastic collapse. For the FSSW-T weld, the crack initially propagates upward along the hook at a lower level of applied load (1400 N). A further increase in crosstension load causes the crack to propagate through the stir zone. The slope of the load versus displacement curve significantly increases, when the crack propagates through the stir zone. Therefore, the different failure modes in cross tension determine the different weld strengths. Fractographs in Fig. 7 show that the welds fracture in the mode of dimple rupture. For the FSSW-C weld, in the downward portion of the hook (i.e., the location a1 in Fig. 6), small shallow dimples are primarily observed. The formation of these dimples is associated with the presence of discontinuous oxide particles that lowers the plasticity of the material in this local region and enhances the coalescence of adjacent microvoids nucleated at the particle surface. The equiaxed dimple shape suggests that the
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145
110
a
FSSW-T FSSW-C
105
Vickers hardness Hv
100 95 90 85 80
50 µm
weld keyhole
75 70
b
0
2
4 6 8 10 Distance from the weld center, mm
12
Fig. 5. Hardness profile of the welds FSSW-C and FSSW-T, measured 0.2 mm above the interface between the two overlapped sheets.
has the characteristics that one end of the dimple is open. The presence of elongated dimples indicates that shear fracture occurs at the bottom of the weld at the final stage of failure. For the FSSW-T weld, fracture surface in the hook region is similar to that of the FSSW-C weld (not shown here). In the stir zone, through which the crack propagates (i.e., the location b in Fig. 6), intergranular fracture is observed due to the nucleation of microvoids at the grain boundaries.
4. Discussion
50 µm
c
50 µm Fig. 4. Cross-sectional microstructures of (a) the base material, (b) the FSSW-T weld and (c) the FSSW-C weld in the stir zone (1 mm from the periphery of the weld keyhole).
state of stress in this region is primarily tension. In the bottom of the weld (i.e., the location a2 in Fig. 6), the fracture surface exhibits elongated dimples of various sizes. The elongated dimple
As shown in Fig. 3, the hook geometry is primarily determined by the geometry of the tool pin. For the cylindrical pin, the rotation of the pin is symmetric in nature causing shear deformation of the material around the pin surface. The threads on the pin significantly enhance plastic flow along the thickness direction. As reported by Fujimoto et al. [8], immediately next to the pin the material driven by the threads moves downward from the upper sheet to the lower sheet, forming the major portion of the stir zone; the material originally from the lower sheet is pushed outward as well as upward towards the upper sheet. This explains the formation of hook geometry in welds made with cylindrical pin, wherein the hook bulges upward in the region away from the keyhole and runs downward towards the bottom of the weld near the keyhole. In contrast, the hook geometry is different for welds made with the triangular pin. Due to the asymmetric geometry of the triangular-shaped pin, successive rotation of the pin is believed to enhance the plastic flow of the material in the vicinity of the pin in the radial direction, compared to the cylindrical pin. This can be rationalized as follows. Considering only the effect of the tool-pin geometry on plastic flow of the material, it is assumed that (1) the material is only deformed in a closed chamber comprised of the tool shoulder, the pin, the material under the pin, and the wall of the outer bulk material (which undergoes marginal deformation); and (2) there is no-slip condition between the pin and the material in its immediate vicinity. Fig. 8(a) shows
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FSSW-C fractured
a1
1420 N
997 N
a2
4000 FSSW-T FSSW-C
3500
Load, N
3000 2500 2000 1500 1000 500 0 0
5
10 15 Displacement, mm
1409 N
3000 N
20
fractured b
FSSW-T Fig. 6. Load versus displacement relationship for the FSSW-C and FSSW-T welds during cross-tension. Cross-sectional macrographs show crack propagation during loading. The fracture surfaces ‘a1’ and ‘a2’ in the FSSW-C weld and ‘b’ in the FSSW-T weld show distinctive features that are characterized in Fig. 7.
b
a1
25 μm
25 μm
a2
25 μm Fig. 7. Fractographs at the selected locations (a1) and (a2) in the FSSW-C weld, and at the location (b) in the FSSW-T weld. The selected locations are indicated by arrows in Fig. 6.
the top view of the ‘stop action’ FSSW-T weld that is obtained by stopping the tool rotation just before the end of the dwell period and then gently separating the weld from the tool. This represents a snapshot of the keyhole geometry at an instance during the tool rotation. With the designated pin-rotation direction, the motion of
the material close to the pin face is shown in Fig. 8(b). Between location ‘a’ at the vertex and ‘b’ at the center of the pin face, the motion can be resolved into two components. The first component (Vs) causes the material to shear along the pin surface; the second component (Vp) pushes the material outward due to the pin
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a
wall of outer bulk material (marginal deformation)
b pin
a V Vpp
Vss b V
c
Fig. 8. (a) top view of the ‘stop-action’ FSSW-T weld; (b) schematic illustration of the effect of the pin rotation on the material motion in the vicinity of the triangular pin (i.e., within the dotted circle region in (a)).
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times the shear strength. In the FSSW-C weld, it is the shear strength of the material and the load-carrying area that determine the weld strength. Secondly, the material near the weld keyhole in the FSSW-T weld has higher strength than the material near the weld bottom in the FSSW-C weld due to a finer grain structure. In the FSSW-C weld, the material near the weld bottom has a microstructure similar to that of the base material; moreover, grain refinement in the stir zone does not appreciably affect the weld strength since the crack propagation passes around the stir zone. Thus, it can be said that the strength of friction stir spot welds is significantly affected by the hook geometry, which, in turn, is influenced by the tool-pin geometry.
5. Conclusions surface. From location ‘a’ to ‘b’, the latter component (Vp) gradually reduces until it becomes 0 at location ‘b’. Subsequently, the material (between location ‘a’ and ‘b’) that experiences an outward motion will be deflected inward by the wall of the outer bulk material towards the region between location ‘b’ and ‘c’. This is because the rotation of the pin has created a cavity in this region which is quickly filled by the inward-flowing material. Therefore, in addition to the rotational flow, successive rotation of the pin causes the material in its vicinity to move back and forth in the radial direction which may lead to high degree of material deformation, whereas, in contrast, successive rotation of the cylindrical pin primarily causes shearing of the material in its vicinity [9,10]. Furthermore, Fig. 4 clearly indicates the presence of finer grain structure in the stir zone for FSSW-T welds than FSSWC welds which substantiates the claim that the plastic deformation caused by the triangular pin is more severe than that by cylindrical pin. The finer grain structure could result in higher hardness in the stir zone of the FSSW-T weld as compared to the FSSW-C weld according to the Hall–Petch relationship. The effect of this increased plastic flow in the radial direction by the triangular pin may lead to the hook being broken-up (dispersed) in the weld region. Therefore, even though the hook begins to form when the pin plunges into the interface of the two work pieces to be welded, this radial back and forth motion of the material that is caused due to the rotation of the triangularshaped pin will inhibit the formation and further growth of a continuous hook. This is the primary reason for the hook arrest in the FSSW-T welds. The geometry of the hook can be used as a basis to explain the failure mechanism observed in the spot welds. The hook, which is a weak metallurgical bond, exists between the overlapped sheets. The crack first propagates along the hook at low levels of external load in both the welds (Fig. 6). A combination of stress concentration, mechanical property of the material, and even the load-carrying area in front of the hook tip determines further crack or failure propagation during which the weld strength is attained. The large difference in the cross-tension strength between these two types of welds could be caused by the following two factors. Firstly, as illustrated by the cross-sectional micrographs in Fig. 6 and the fracture surfaces in Fig. 7, during the final stage of failure, the FSSW-C weld experiences plastic collapse near the weld bottom in a shear mode, while the FSSW-T weld fractures by crack propagation through the stir zone under tension. The failure mode is different between these two welds when they are subjected to cross-tension loading. In the present study, the area of the fracture surface from the final stage of failure in the FSSW-T weld is comparable to that in the FSSW-C weld. Therefore,larger external load causing the failure of the FSSW-T weld might be associated with the tensile failure mode where the tensile strength of the material is approximately O3
The effect of pin geometry on the hook formation, crosstension strength, and failure mode of friction stir spot-welded 5083-O aluminum alloy is investigated. The following conclusions are made. 1. The tool pin geometry significantly affects the hook. In the FSSW-C weld, the hook runs gradually upward and then bypasses the stir zone and points downward towards the weld bottom. In the FSSW-T weld, the hook is directed upward towards the stir zone and ends with a very short plateau. 2. Due to asymmetric rotation of the triangular-shaped pin which greatly increases material deformation, a finer grain structure is formed in the vicinity of the weld keyhole as compared to the cylindrical pin. 3. The crack first propagates along the hook at a low level of load when a weld is subjected to external loading. With a further increase in load, the FSSW-C weld experiences plastic collapse near the weld bottom in a shear mode, while the FSSW-T weld fractures by crack propagation through the stir zone under tension. 4. The weld strength is determined by the failure mode and the microstructure-related strength of the material. The crosstension strength of welds made with the triangular pin is twice that of welds made with the cylindrical pin under the same process condition.
Acknowledgement This work was supported by internal funding from Hitachi America, Limited, R&D. The authors would like to express their gratitude to Dr. Kazutaka Okamoto of Hitachi Research Laboratory, Hitachi Limited, for helpful discussions. References [1] C.G. Rhodes, M.W. Mahoney, W.H. Bingel, R.A. Spurling, C.C. Bampton, Scripta Materialia 36 (1997) 69. [2] Y.S. Sato, H. Takauchi, S.H. Park, H. Kokawa, Materials Science and Engineering A 405 (2005) 333. [3] J.F. Henrichs, C.B. Smith, B.F. Orsini, R.J. DeGeorge, B.J. Smale, P.C. Ruehl, Friction stir welding for the 21st century automotive industry, in: Proceedings of the 5th International Symposium of Friction Stir Welding, September 14–16, 2004, Metz, France. [4] Y. Tozaki, Y. Uematsu, K. Tokaji, Effect of welding condition on tensile strength of dissimilar FS spot welds between different Al alloys, in: Proceedings of the 6th International Symposium of friction Stir Welding, October 10–13, 2006, Saint-Sauveur, Nr Montreal, Canada. [5] I. Ikegami, T. Tsumura, F. Ye, K. Nakata, Effect of tool geometry on joint strength of Mg alloy by spot friction welding, Preprints of the National Meeting of Japan Welding Society No. 78, Spring 2006, Paper #215.
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[6] A.C. Addison, A.J. Robelou, Friction stir spot welding: principal parameters and their effects, in: Proceedings of the 5th International Symposium of Friction Stir Welding, September 14–16, 2004, Metz, France. [7] Q. Yang, S. Mironov, Y.S. Sato, K. Okamoto, Microstructure and mechanical properties of friction stir spot welded AZ31 Mg alloy, in: Proceedings of the 7th International Symposium of Friction Stir Welding, May 20–22, 2008, Awaji Island, Japan
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