Calculation of welding tool pin width for friction stir

The International Journal of Advanced Manufacturing Technology https://doi.org/10.1007/s00170-018-2350-x

ORIGINAL ARTICLE

Calculation of welding tool pin width for friction stir welding of thin overlapping sheets H. Zhao 1 & Z. Shen 2,3,4 & M. Booth 4 & J. Wen 4 & L. Fu 2,3,5 & A. P. Gerlich 4 Received: 24 February 2018 / Accepted: 14 June 2018 # Springer-Verlag London Ltd., part of Springer Nature 2018

Abstract The fabrication friction stir welded lap joints with Al 7075-T6 sheets is examined with a wide range of tool pin geometries and dimensions while comparing microstructures, bonded area geometry, hardness, and fracture load during overlap shear testing. The tool pin geometry and features significantly affected the hook geometry and thus weld strength and failure mode. A hook feature is prone to forming when a larger diameter or helical threaded tool pins are used and is shown to deteriorate overlap shear fracture load by reducing the bonded ligament in the upper sheet. This hook feature can be controlled by removing the helical threads on the tool pin and replacing these with concentric grooves. The overlap shear fracture load is controlled by a combination of the sheet thickness, the width of the bonded area, and extent of the hook feature. These factors are captured in a model which was shown to provide good predictions of the fracture load and can be used to select tool pin sizes for varying overlapping sheet thicknesses. Keywords Friction stir lap welding . Al 7075-T6 . Tooling design . Microstructure . Failure model

1 Introduction Friction stir welding (FSW) was invented by Thomas et al. at The Welding Institute (TWI) in Abington, UK, in 1991, as a solid-state joining technique. FSW is a green technology considering it offers energy efficiency, environment friendliness, and versatility. It also provides advantages such as joining materials that are difficult to fusion weld, low distortion, and excellent mechanical properties. FSW has been successfully used to join aluminum, magnesium, copper and titanium

* Z. Shen [email protected] 1

Shanghai Aerospace Equipments Manufacturer Co., Ltd, Shanghai 200240, China

2

School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China

3

Shanxi Key Laboratory of Friction Welding Technologies, Northwestern Polytechnical University, Xi’an 710072, China

4

Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, Canada

5

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China

alloys, steels, thermoplastics, and dissimilar combinations since its invention [1–8]. A unique feature of FSW is that the welding temperature is lower than the bulk solidus temperature of the base material, which can suppress liquation cracking issues and reduces thermal distortion resulting from high heat input. As shown in Fig. 1, the basic concept of FSW is remarkably simple, a rotating tool consists of a specially designed shoulder and pin is plunged between the abutting faces of the joint. When the tool shoulder contacts the workpiece top surface, relative motion between the rotating tool and the workpiece generates frictional heat that creates a plasticized region around the immersed portion of the tool [9]. FSW can be used to manufacture several joint configurations, including square butt, edge butt, T butt joint, lap joint, and T lap joint, although the most common configurations studied are butt and lap joints [9]. For each of these cases, the appropriate tool geometry, rotational speed, and travel speed are critical parameters to optimizing the FSW process. The primary functions of the tool are to induce rapid frictional and deformation heating, promote flow, and constrain the upward extrusion of plasticized material out of the weld zone. The tool geometry significantly affects the heat generation, material flow, weld integrity, macro/microstructure, and overall mechanical properties. For example, Badu et al. reported that the tool pin profile significantly influenced the hook

Int J Adv Manuf Technol

Tool Rotation

Weld RS

AS

Upper workpiece

Lower workpiece

Fig. 1 Schematic illustration of friction stir lap linear welding, indicating retreating side (RS) and advancing side (AS) relative to the tool rotation and travel directions

geometry, which in turn strongly influenced the joint strength and the failure mode [10]. An important feature of the weld specific to lap joints is the so-called hook feature which appears in the faying surfaces on the advancing or retreating side of lap welding, and this has been studied extensively. The presence of hook reduces the effective sheet thickness and thus weld strength. According to the available literature, the hooking is an inherent defect, which cannot be eliminated by the optimization of tool design or welding parameters, which can attribute to the material flow in the vertical direction [11–13]. Salari et al. [11] demonstrated the importance of controlling the geometry of the hook feature in terms of the mechanical properties of the joint. This emphasizes the importance of both welding parameters and tool geometry on controlling the material flow and interface geometry. In the simplest terms, a lap weld geometry needs to meet a minimum weld size or width in order to provide sufficient bonded area for lap shear tests to fail in the base material. However, no systematic study has been performed to provide a guideline for selection of the appropriate tool pin size. This is the focus of the present work in order to develop this guideline in terms of achieving a minimum weld bonded width and control of hook geometry. Typically, various features are incorporated in the tool pin geometry in order to enhance intermixing, material flow, and reduce tool forces [14]. In the case of lap welding, the flow and mixing in the vertical direction may be improved in order to enhance the bonding between the two sheets. However, the hook feature will extend from the faying interface and into the weld stir zone appear in the faying surface of lap weld. If the bonded

Fig. 3 Relationship between thickness of upper sheet and pin width [12, 13, 16–29]

area is sufficient in the stir zone, then during overlap shear testing of the joint the fracture may propagate through either the end of the hook and upwards through the stir zone to the surface of the top sheet [15], or from the faying interface on the advancing side and downward through the bottom sheet as shown in Fig. 2. If there is insufficient bonded area in the stir zone connecting the sheets, the fracture during overlap shear testing will propagate through the stir zone and prior sheet interface [11], and lead to inferior fracture loads. In order to develop a better understanding of the critical geometry and material properties which determine the fracture type shown in Fig. 2, one can compare the reported relationship between upper sheet thickness and pin width for various lap welding scenarios in the literature [12, 13, 16–29]. The general trend is shown for heat-treatable aluminum alloys in Fig. 3, which indicates a generally poor linear fit for both material types though a slightly better relationship with nonheat-treatable alloys. The main reason for this poor correlation is likely the variability in the hook feature geometry and widely varying material properties in the heat affected zone of the base metal. In order to resolve these issues, the present work will compare bonded widths, hook geometry, and material properties in a heat-treatable aluminum alloy in order to determine how these are inter-related to overlap shear joint strength.

Table 1

Fig. 2 Schematic illustration of FSW indicating retreating side (RS) and advancing side (AS) relative to the tool rotation and travel directions, in partially fractured lap shear specimens, exhibiting sheet fracture and interface fracture in stir zone

Base metal properties of AA7075-T6 sheet

Elastic modulus (GPa) 0.2% yield strength (MPa) strength of extension

Rolling direction

Transverse direction

45° to rolling direction

70.4 529.1

70.2 499.3

65.6 472.1

Int J Adv Manuf Technol

Fig. 4 Experimental stand and clamping configuration during FSW

2 Experimental procedure The material used in the present study is Al7075-T6 alloy with a thickness of 2.0 mm. The chemical composition of the alloy is as follows: Zn–5.41 wt%, Mg-2.38 wt%, Cu-1.51 wt%, Fe0.25 wt%, Cr-0.19 wt%, Si-0.07 wt%, Mn-0.02 wt%, Ti0.02 wt%, and a balance of Al. The mechanical properties of the sheet material are listed in Table 1. As shown in Fig. 4, two 610-mm-long × 76-mm-wide plates were positioned with a 30-mm overlap between the sheets. A displacement controlled manual milling machine was utilized to manufacture the FSW lap welds, with the spindle power of 7.5 kW. All of the welds are produced at a tool shoulder penetration of 0.15 mm, and the tool axis was tilted by 2.0° with respect to the vertical axis of the workpiece and kept constant during the process. Eight different tool designs were used, whose geometries and dimensions are described in Fig. 5 and Table 2, respectively. Tool 1 and tool 4 consist of a cylindrical pin tool, and tools 2 and 3, along with tools 5 through 8, incorporate a tapered pin tool. Tools 5 through tool 8 have a smaller pin root diameter in order to effectively vary bond width between the sheets. The welding parameters used in the present study are listed in Table 3. It should be noted some of the welding parameters were not attempted since they either led to tool pin breakage (such as tools 1 to 4 at travel speeds > 250 mm/min), or a large

Fig. 5 Schematic drawings of welding tools studied here

surface groove defect due to lack of material flow (mainly with tools 5 to 8 at travel speeds < 250 mm/min). After the welding process, the specimens were prepared using standard metallographic techniques with a final polishing using a 1-μm diamond. Optical microscopy was conducted on samples and etched by Keller’s reagent (2 mL HF, 3 mL HCl, 5 mL HNO3, 190 mL H2O) for 15 s; the microstructural features are observed using an Olympus optical microscope. The mechanical properties of the welds were measured during lap shear testing, as well as micro-hardness testing. The lap shear coupons were water-jet cut to a 30-mm width and lap shear tests were performed at a constant rate of 1.0 mm/min with an Instron 8874 tensile test machine. All of the lap shear values were obtained by averaging the loads of three individual specimens produced at the same welding condition. The micro-hardness at the location of advancing side, retreating side, and stir zone through the prior weld interface was measured using a micro-Vickers hardness tester at a load of 500 gf and a dwell time of 15 s, and the hardness values were also obtained by averaging three individual hardness data in adjacent indents (spaced at least three times the diagonal apart).

3 Results and discussion 3.1 Weld macro-structural feature and hook geometry In order to survey the range of characteristic microstructures and key features produced in the lap joints, selected weld cross-sections for different tools and travel speeds are shown in Fig. 6. The fine-grained recrystallized stir zone has a shape which highly depended on the tool pin geometry and dimension, and some displacement of the sheet interface upwards occurred on the retreating side in the welds produced by tool 1 through tool 4, forming the hook feature to varying degrees. When examining the microstructures in more detail, some key influences of different tool features can be generalized as follows. Based on Fig. 6b, c, a taper profile on the pin tool leads to formation of a smaller weld stir zone with a more basinshaped appearance compared to a pin with a constant diameter. Moreover, the taper pin tool produced smaller hook

Int J Adv Manuf Technol Table 2

Summary of geometry and dimensions for FSW tools studied

Tool

Shoulder diameters(mm)

Top pin diameters(mm)

Tool 1

15

6

2.60

6

M5

NA

Cylinder

Tool 2

15

5.1

2.60

6

M5

NA

Tapered

Too l3 Tool 4

15 15

5.1 4.46

2.60 2.60

6 4.46

NA 0.23 mm deep

0.6 mm NA

Tapered Cylinder

Tool 5

15

2.24

2.60

6.9

0.25 mm deep

NA

Tapered

Tool 6 Tool 7

15 15

1.84 1.46

2.60 2.60

6.9 6.9

0.25 mm deep 0.25 mm deep

NA NA

Tapered Tapered

Tool 8

15

1.06

2.60

6.9

0.25 mm deep

NA

Tapered

Table 3 Summary of welding parameters used in the present study

Pin length (mm)

Root pin diameters(mm)

3 flats on pins

Shapes of pin

Tool

Rotating speed (rpm)

Travel speeds (mm/min)

Tool 1 Tool 2 Tool 3

1120 1120 1120

125 125 125

180 – 180

250 – 250

– – –

– – –

Fracture type C (see Fig. 10c) Fracture type C Fracture type C

Tool 4 Tool 5 Tool 6 Tool 7 Tool 8

1120 1120 1120 1120 1120

125 125 – – –

180 – – – –

250 250 250 – 250

355 – 355 355 355

500 500 500 500 500

Fracture type A (see Fig. 10a) Fracture type B (see Fig. 10b) Fracture type B Fracture type B Fracture type B

compared to that of the straight cylindrical pin tool, which suggests the taper cylindrical pin tool is more appropriate for

Fig. 6 Macrostructures of the welds made using the different tool pin geometries

Thread parameters/groove depth diameters(mm)

AS

Fracture type

manufacturing lap joints [30]. The addition of three flats produced a comparable stir zone profile when comparing tool 2 to

AS

RS (a)Tool 1, 180mm/min

(b) Tool1, 250 mm/min

RS

AS

RS

AS

RS

(c)Tool 2, 125 mm/min

(d)Tool 3, 180 mm/min

RS

AS

AS

RS

(e)Tool 3, 250 mm/min

AS

(f)Tool 4, 125 mm/min

RS (g)Tool 4, 500 mm/min

AS

RS (h)Tool 5, 500mm/min

Int J Adv Manuf Technol Fig. 7 Weld microstructure and hook geometry on the advancing and retreating sides

(a) magnified view of region A in Fig. 6g.

(b)magnified view of region B in Fig. 6g. (c)magnified view of region C in Fig. 6g.

(d)magnified view of region D in Fig. 6h. (e) magnified view of region E in Fig. 6h.

(f) magnified view of region F in (b). (g)magnified view of region G in (c)

tool 3 due to the enhanced stirring effect (see Figs. 6c, d); however, increasing the travel speed results in a slight reduction in the bonded width (see Fig. 6a vs. b, and Fig. 6d vs. e). As indicated in Figs. 6a–e, tool 1, tool 2, and tool 3 produce a hook which points upwards on both the advancing side and retreating side, but the hook goes upwards more significantly on the retreating side due to the relevant material flow pattern. Moreover, the hook height increases with the increasing travel speed for tool 1 (see Figs. 6a, b), which is detrimental to the weld strength since the crack can propagate along the hook, since this effectively diminishes the top sheet thickness. However, the hook height significantly decreases with the increasing travel speed with tool 3

(see Figs. 6d, e), which indicates this is influenced by the reduction in flow stemming induced by the helical threads. One of the notable differences in the weld zone geometry can be observed when simple grooves are present on the pin as in tool 4, rather than a helical thread in tools 1 through 3. The simple grooves reduced the upward displacement of the hook interface, as observed in Figs. 6f, g, which appear to show the sheet interface remaining nearly horizontally close to the original location of the faying interface. However, these welds also contain small voids or tunnellike defects on the advancing side, which is a result of the limited intermixing and flow associated with the grooves (versus a helical thread). A drastic change can also be noted

Int J Adv Manuf Technol

Fig. 8 Relationship between shear layer thickness around pin, δ, and tool pitch during welding

in the shape of the weld zone when the stepped groove is used as shown in Fig. 6h, in which the stir zone boundary is outlined by red dashed lines. The boundary of the stir zone produced with tool 5 (as well as the similar stepped groove tools 6 through 8) all provided a similar microstructure which closely followed the tool pin geometry. Selected areas in the cross-sections labeled as regions A through E in Fig. 6g, h were examined at higher magnification to observe the actual grain structures in Fig. 7. As shown in Fig. 7a, the microstructural zones in the weld can be classified into three regions: (i) stir zone (SZ), (ii) a narrow thermomechanically affected zone (TMAZ), and (iii) the heat affected zone (HAZ). Also, it is worth noting that the weld geometry is asymmetrical because of the asymmetric flow on the advancing side and retreating side, which also affects the hook geometry. As shown in Figs. 6f, g, tool 4 produced voids or tunnel-like defects on the advancing side, which is shown in

more detail in Figs. 7b. As shown in Fig. 7b, the defect was formed at the location where the material which was displaced downward, yet there was insufficient material flow. It is interesting to note that the hook is curved downward on the advancing side, versus upward on the retreating side, which is rather different from geometry made by tools 1, 2, and 3 where the hook is displaced upward on both sides. Figure 7c shows the magnified view of region C in Fig. 6g and Fig. 7d, e shows the magnified views of regions D and E in Fig. 6h, respectively, which indicate the hook slightly propagated into the stir zone for a very short distance on the advancing side, while it propagated further into the stir zone on the retreating side, which is similar to the hook geometry produced by cylindrical and slightly tapered pin tool (see Figs. 7c, g). Figure 7f shows the magnified view of region F in Fig. 7b, which indicates that the hook is arrested within the thermo-mechanically affected zone at the boundary of the stir zone. Nevertheless, the hook propagates into the stir zone on the retreating side, and Fig. 7g illustrates a magnified view of region G in Fig. 7c, where a trail of remnant oxides or discontinuous particles remains along the hook path. It was reported the formation of these discontinuous particles can be attributed to the initial oxide layer on the base material surface, which are broken up into particles by stirring of the tool and dispersed into the weld region and caused partial metallurgical bonding [31]. The remnants of the surface oxide layer often adversely affect the mechanical properties in the weld, since they are prone to induce crack propagation. The hook height is < 0.1 mm at both the advancing and retreating sides when using tool 5 with the grooves on the tapered pin, since there is negligible vertical material flow. It was observed that based on the geometry of the recrystallized zones in Fig.6, the stir zone dimension is slightly larger than the pin diameters of the corresponding tools, and the weld bonded area is mainly correlated to the width of stir zone. Therefore, it is useful to evaluate the effects of tool design and travel speed on the final stir zone widths produced. Figure 8 presents the correlation between the thickness of the shear layer δ around the pin, which is essentially the difference between the measurement of the stir zone width and the tool diameter, versus the pitch which is the ratio of welding speed versus the rotating speed. The magnitude of δ decreases with pitch value for all the tool design used in the present study, which indicates that the heat input is the main factor to stir zone dimension, and the tools with the helical thread (tools 1, 2, and 3) had the highest value of shear layer thickness achievable here.

3.2 Mechanical properties of welds Fig. 9 Relationship between travel speed and tool design on FSW joint overlap shear loads for each tool design (designated as T1 through T8) considered here

The relationship between the tool travel speed and tool design on the overlap shear load is shown in Fig. 9. Overall, tool 1

Int J Adv Manuf Technol Fig. 10 Cross-sectional macrostructure of fractured overlap shear specimens

(a) Fracture type A: fractured at the heat affected zone on advancing side (T4-250 mm/min).

(b) Fracture type B: fractured through the weld interface (T5-500 mm/min)

(c) Fracture type C: fractured at the retreating side from the hook (T3, 125 mm/min).

and tool 3 produced much higher weld strength than other tool designs, although the weld strength obtained using tool 1 decreased with tool travel speed. In contrast, the weld strength obtained with tool 3 increased with tool travel speed, which is consistent with the variation in hook height (see Fig. 6). In other words, the hook height which occurs with tool 1 increased with increasing tool travel speed, and this deteriorated the effective top sheet thickness and resulting overlap shear strength by providing a stress localization point and short fracture path. The hook height observed with tool 3 decreased with increasing tool travel speed and this essentially increases the effective top sheet thickness. tool 2 and tools 4 through 8 produced weld strengths ranging from 4.58 to 11.36 kN, and there is no clear correlation between tool travel speed, tool design, and the weld strength. Further investigation was pursued to derive the relation between weld overlap shear load, fracture mode, effective failure area, and weld hardness.

Fig. 11 a Magnified view of region H in Fig. 9a indicating transgranular fracture in the heat affected zone. b Magnified view of region I in Fig. 9c, indicating intergranular fracture in the stir zone

As shown in Fig. 10, three different fracture modes were observed during overlap shear loading in the present work: fracture through the heat affected on the advancing side (referred as fracture type A), fracture through the stir zone along the prior interface (referred as fracture type B), and fracture through the top sheet material propagating from the hook (referred as fracture type C). To be more precise, the welds produced by tool 4 fractured at the heat affected zone of the bottom material on the advancing side (see Fig. 10a). As shown in Fig. 6f, g and Fig. 7b, the crack propagated along the hook for a short distance and then into the tunnel defects, and finally fractured through the bottom sheet material. Figure 11a presents the magnified view of region H in Fig. 10a, and it can be seen that the fracture path is transcrystalline through the heat affected zone. As shown in Fig. 10b, the weld produced using the grooved tapered pin (i.e., with tools 4 through 8) directly fractured through the

Int J Adv Manuf Technol

Fig. 12 a Effects of travel speed on the hardness of the weld made by tool 4. b Effects of tool design on the weld hardness (travel speed 500 mm/min)

interface since there is no hook formation and the bonded area is smaller compared to the other tools. As shown in Fig. 10c, the welds made by tools 1 through 3 which fractured through the top sheet material from the hook on the retreating side, since the remaining upper sheet thickness is much less than the advancing side (see Fig. 6). When fracture propagates through the stir zone material, the fracture path takes an intergranular path, as shown in Fig. 11b which illustrates the magnified view of region I in Fig. 10c. The hardness of Al 7075-T6 is mainly dependent on the fraction, size, and distribution of strengthening precipitates produced during heat treatment, and these are susceptible to coarsening which results in softening, as described by Shen et al. [32–34]. Figure 12 and Table 4 show the average hardness values observed in the key fracture locations typically observed (corresponding to the advancing side, retreating side, and the weld interface based on the three fractured locations in Fig. 10), when different tool designs and travel speeds are applied. For example, the effect of travel speed on the hardness of the weld made by tool 4 is shown in Fig. 12a, in which the hardness ranges from 140 to 152 HV, and slightly increases with travel speed. Figure 12b shows the effect of tool design in which the hardness at the three fracture locations is Table 4 Hardness of the weld at the fracture propagation location observed in overlap shear tests (near the AS, RS, or interface)

similar to the hardness values observed with tool 4, indicating that the tool design negligibly affects the hardness value in these locations (which range from 142 to 152 HV).

3.3 Model of overlap joint design In order to develop a model to predict the fracture loads, one can make some simplifications regarding the geometry of the FSW lap joint. The bonded width of the overlapping joined area can be assumed to be a distance w, as shown in Fig. 13. As the joint is loaded to a force F during overlapping shear testing, the stresses will cause failure through the paths shown in Fig. 10, where the effective sheet thickness h will be reduced by the presence of a hook feature with a height of hH as shown in Fig. 13. Based on the geometry in Fig. 13, one can determine the force required to fracture through the weld along the distance w versus through the sheets, assuming the weld is uniform and semi-infinite along the length, such that the tensile force of imposed on the hook side position (h-hH), that is the location of PQ, is balanced by the shearing force along the length of w, that is the location of QS: [35]

Tool travel speed (mm/min)`

Locations

Tested HV0.5

T1–250

AS

153.6

158.3

159.1

157.0

Interface RS AS Interface RS AS Interface RS

160.7 155.4 154.5 154.6 153.9 151.5 158.1 155.9

157.5 158.1 153.8 156.9 158.4 152 155.7 152.8

154.5 159.4 153.3 154.5 156.7 148.7 152.6 154.6

157.6 157.6 153.9 155.3 156.3 150.7 155.5 154.4

T2–125

T3–125

Average HV0.5

Int J Adv Manuf Technol Fig. 13 Schematic of lap weld geometry with sheets of thickness h, which includes a hook feature with a height of hH

1 ½τ  ¼ pffiffiffi ½σ 3

ð1Þ

w¼

where along the joined area w, τ sw ¼

F Aw

ð2Þ

such that: σsw Aw F ¼ τ sw Aw ¼ pffiffiffi : 3

ð3Þ

Where Aw represents the area of QS, τsw represents the shear stress along the joined area QS, σsw represents the normal stress perpendicular to the joined area QS. In the case of the sheet fracture location where(h-hH), which is the location of PQ, the stresses can be approximated by:

σsh ¼

F Ah

ð4Þ

If we re-arrange this expression, one finds that a balance between the forces to fracture along the sheet versus the bonded area along w occurs when: σsw Aw F ¼ σsh Ah ¼ pffiffiffi 3

Fig. 14 Comparison of measured versus predicted overlap shear fracture load

σsw lw σsh l ðh−hH Þ ¼ pffiffiffi 3 pffiffiffi σsh 3 ðh−hH Þ σsw

ð6Þ

ð7Þ

where Ah represents the area of PQ, σsh represents the normal stress perpendicular to the joined area PQ, and l represents the welding length. This type of shear loading is the main scenario for lap welds, and when the tensile strength of upper or lower sheet equals to the tensile shearing strength of the weld, the minimum weld width can be determined. For example, based on Eq. 7, when some of the measured values are compared for the hook heights measured, we find that if hH is 0.9 mm, the minimum effective weld width w, is 1.957. If the three types of fracture modes are considered (a) advancing hook originating fracture, (b) retreating side fracture, and (c) interface fracture through the stir zone, the overlap shear fracture loads can be expected if one can estimate the strength of the material through each corresponding fracture path. However, since it has been long known that the strength of a material can be roughly approximated by three times the Vickers hardness (in units of MPa) [36, 37], such that σsHAZ can be approximated by 3HVHAZ from the values in Table 4, then, for fracture type A:

ð5Þ F ¼ σsHAZ AhHAZ ≈3HV HAZ lh

ð8Þ

Int J Adv Manuf Technol Natural Sciences and Engineering Research Council of Canada is greatly appreciated.

for fracture type B: σsw Aw pffiffiffi F ¼ τ sw Aw ¼ pffiffiffi ≈ 3HV sw lw 3

ð9Þ

Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

for fracture type C:

References F ¼ σsw Aw ≈3HV sw l ðh−hh Þ

ð10Þ

where AhHAZ represents the area of HAZ in UT, σsHAZ represents the normal stress of HAZ in UT, and HVHAZ represents the Vickers hardness of HAZ in UT, HVsw represents the Vickers hardness of weld in QS. As shown in Figs. 6h and 7f, g, welds produced with the grooved tapered pin tools produced a hook that is extended upward into the top material or downward into the bottom material. Based on these equations, if the pin width of the welding tool is larger than the minimum effective weld width, the weld fractures as fracture type A or C failure mode. If the tool pin width is smaller than the minimum effective weld width, the weld will fracture as fracture type B failure mode. If one compares the predicted forces in Eqs. 8, 9, and 10, based on the minimum width equation, and compares these values to the actual force for each fracture scenario, one finds a rather good correlation as shown in Fig. 14. Future work may compare whether the fracture loads can be predicted even more accurately in non-heat-treatable alloys or other sheet thicknesses.

4 Conclusions The present work has demonstrated the vital role in selecting the appropriate tool pin geometry and dimension for FSW lap joining. This feature controls the weld macrostructure, hook geometry, fracture load, and bonded area of the joint. Increasing the height of the hook within the stir zone drastically decreases the overlap shear fracture load, by reducing the remaining bonded ligament. This height was found to increase in welds made using helical threaded tools; however, the hook height decreases when simple grooved tools are used with no helical pitch. Since the fracture load is mainly controlled by the sheet thickness, remaining upper sheet ligament, and width of the bonded area, a model was developed to predict the fracture load based on limiting fracture strength in these locations. The fracture loads calculated from the proposed model were in good agreement with measured loads for lap welds produced using heat-treated Al 7075-T6 alloy sheets produced with a wide range of tool geometries.

1.

2.

3.

4.

5.

6.

7.

8.

9. 10.

11.

12.

13.

14.

15.

16. Funding information The authors acknowledge the financial support provided to Huihui Zhao by the China Scholarship Council (CSC) during the present investigation. Further financial and material support from the

Shanavas S, Dhas JER, Murugan N (2018) Weldability of marine grade AA 5052 aluminum alloy by underwater friction stir welding. Int J Adv Manuf Technol 95(1–12):4535–4546 Mironov S, Onuma T, Sato Y, Yoneyama S, Kokawa H (2017) Tensile behavior of friction-stir welded AZ31 magnesium alloy. Mater Sci Eng A 679:272–281 Heidarzadeh A, Jabbari M, Esmaily M (2015) Prediction of grain size and mechanical properties in friction stir welded pure copper joints using a thermal model. Int J Adv Manuf Technol 77(9–12): 1819–1829 Brassington W, Colegrove PA (2017) Alternative friction stir welding technology for titanium–6Al–4V propellant tanks within the space industry. Sci Technol Weld Join 22(4):300–318 Manvatkar V, De A, Svensson L-E, DebRoy T (2015) Cooling rates and peak temperatures during friction stir welding of a high-carbon steel. Scr Mater 94:36–39 R. Kumar, R. Singh, I. Ahuja, R. Penna, L. Feo (2017) Weldability of thermoplastic materials for friction stir welding—a state of art review and future applications, Compos Part B Shen Z, Chen Y, Haghshenas M, Gerlich A (2015) Role of welding parameters on interfacial bonding in dissimilar steel/aluminum friction stir welds. Engineering Science and Technology, an International Journal 18(2):270–277 Shen Z, Chen Y, Haghshenas M, Nguyen T, Galloway J, Gerlich A (2015) Interfacial microstructure and properties of copper clad steel produced using friction stir welding versus gas metal arc welding. Mater Charact 104:1–9 R.S. Mishra, P.S. De, N. Kumar, Friction stir welding and processing: science and engineering, Springer2014 Babu S, Ram GJ, Venkitakrishnan P, Reddy GM, Rao KP (2012) Microstructure and mechanical properties of friction stir lap welded aluminum alloy AA2014. J Mater Sci Technol 28(5):414–426 Salari E, Jahazi M, Khodabandeh A, Ghasemi-Nanesa H (2014) Influence of tool geometry and rotational speed on mechanical properties and defect formation in friction stir lap welded 5456 aluminum alloy sheets. Mater Des 58:381–389 Yue Y, Li Z, Ji S, Huang Y, Zhou Z (2016) Effect of reversethreaded pin on mechanical properties of friction stir lap welded alclad 2024 aluminum alloy. J Mater Sci Technol 32(7):671–675 Liu H, Zhao Y, Hu Y, Chen S, Lin Z (2015) Microstructural characteristics and mechanical properties of friction stir lap welding joint of alclad 7B04-T74 aluminum alloy. Int J Adv Manuf Technol 78(9–12):1415–1425 Yang Q, Li X, Chen K, Shi Y (2011) Effect of tool geometry and process condition on static strength of a magnesium friction stir lap linear weld. Mater Sci Eng A 528(6):2463–2478 Bisadi H, Tour M, Tavakoli A (2011) The influence of process parameters on microstructure and mechanical properties of friction stir welded Al 5083 alloy lap joint. American journal of Materials science 1(2):93–97 Song Y, Yang X, Cui L, Hou X, Shen Z, Xu Y (2014) Defect features and mechanical properties of friction stir lap welded dissimilar AA2024–AA7075 aluminum alloy sheets. Mater Des 55:9– 18

Int J Adv Manuf Technol 17.

Chen H, Fu L, Liang P, Liu F (2017) Defect features, texture and mechanical properties of friction stir welded lap joints of 2A97 AlLi alloy thin sheets. Mater Charact 125:160–173 18. Imam M, Racherla V, Biswas K (2015) Effect of backing plate material in friction stir butt and lap welding of 6063-T4 aluminium alloy. Int J Adv Manuf Technol 77(9–12):2181–2195 19. Wang M, Zhang H, Zhang J, Zhang X, Yang L (2014) Effect of pin length on hook size and joint properties in friction stir lap welding of 7B04 aluminum alloy. J Mater Eng Perform 23(5):1881–1886 20. Fadaeifard F, Gharavi F, Matori KA, Daud AR, Ariffin M, Awang M (2014) Investigation of microstructure and mechanical properties of friction stir lap welded AA6061-T6 in various welding speeds. J Appl Sci 14(3):221–228 21. Zhou Z, Yue Y, Ji S, Li Z, Zhang L (2017) Effect of rotating speed on joint morphology and lap shear properties of stationary shoulder friction stir lap welded 6061-T6 aluminum alloy. Int J Adv Manuf Technol 88(5–8):2135–2141 22. Ji S, Li Z, Zhou Z, Wu B (2017) Effect of thread and rotating speed on material flow behavior and mechanical properties of friction stir lap welding joints. J Mater Eng Perform 26(10):5085–5096 23. Xu Z, Li Z, Lv Z, Zhang L (2017) Effect of welding speed on joint features and lap shear properties of stationary shoulder FSLWed Alclad 2024 Al alloy. J Mater Eng Perform 26(3):1358–1364 24. Rajendran C, Srinivasan K, Balasubramanian V, Balaji H, Selvaraj P (2017) Identifying combination of friction stir welding parameters to maximize strength of lap joints of AA2014-T6 aluminium alloy. Aust J Mech Eng:1–12 25. Yue Y, Zhou Z, Ji S, Zhang L, Li Z (2017) Improving joint features and tensile shear properties of friction stir lap welded joint by an optimized bottom-half-threaded pin tool. Int J Adv Manuf Technol 90(9–12):2597–2603 26. Balakrishnan M, Leitão C, Arruti E, Aldanondo E, Rodrigues D (2018) Influence of pin imperfections on the tensile and fatigue behaviour of AA 7075-T6 friction stir lap welds. Int J Adv Manuf Technol:1–11

27.

Zou S, Ma S, Liu C, Chen C, Ma L, Lu J, Guo J (2016) Multi-track friction stir lap welding of 2024 aluminum alloy: processing, microstructure and mechanical properties. Metals 7(1):1 28. Gao S, Wu C, Padhy G (2018) Process and joint quality of ultrasonic vibration enhanced friction stir lap welding. Sci Technol Weld Join:1–11 29. Liu H, Hu Y, Peng Y, Dou C, Wang Z (2016) The effect of interface defect on mechanical properties and its formation mechanism in friction stir lap welded joints of aluminum alloys. J Mater Process Technol 238:244–254 30. Buffa G, Campanile G, Fratini L, Prisco A (2009) Friction stir welding of lap joints: influence of process parameters on the metallurgical and mechanical properties. Mater Sci Eng A 519(1–2): 19–26 31. Mitlin D, Radmilovic V, Pan T, Chen J, Feng Z, Santella M (2006) Structure–properties relations in spot friction welded (also known as friction stir spot welded) 6111 aluminum. Mater Sci Eng A 441(1–2):79–96 32. Shen Z, Yang X, Zhang Z, Cui L, Li T (2013) Microstructure and failure mechanisms of refill friction stir spot welded 7075-T6 aluminum alloy joints. Mater Des 44:476–486 33. Shen Z, Chen Y, Hou J, Yang X, Gerlich A (2015) Influence of processing parameters on microstructure and mechanical performance of refill friction stir spot welded 7075-T6 aluminium alloy. Sci Technol Weld Join 20(1):48–57 34. Shen Z, Ding Y, Gopkalo O, Diak B, Gerlich A (2018) Effects of tool design on the microstructure and mechanical properties of refill friction stir spot welding of dissimilar Al alloys. J Mater Process Technol 252:751–759 35. R.H. Wagoner, J.-L. Chenot, Metal forming analysis, Cambridge University Press, 2001 36. Tabor D (1951) The hardness of metals. Clarendon, Oxford 37. D.R.H. Jones, M.F. Ashby, Engineering materials 2: an introduction to microstructures, processing and design, Elsevier 2005