GKSS

Fracture Toughness Behaviour of FSW Joints on Aluminium Alloys

Authors: A. v. Strombeck J. F. dos Santos F. Torster P. Laureano M. Kocak

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Fracture Toughness Behaviour of FSW Joints on Aluminium Alloys

Authors: A. v. Strombeck J.F. dos Santos F. Torster P. Laureano M. Kogak (Institutefor Materials Research)

First published in: P. L. Threadhill (Ed.): First InternationalSymposium on Friction Stir Welding - Proceedings, June 14-16, 1999, in Thousands Oaks, CA, USA. Cambridge, UK: The Welding institute Ltd, 1999. CD-ROM.

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Fracture Toughness Behaviour of FSW Joints on Aluminium Alloys

A. von Strombeck, J. F. dos Santos, F. Torster, P. Laureano, M. Kogak

13 pages with 7 figures and 2 tables *DE016017573*

Abstract —

The friction stir welding (FSW) process can be successfully used to achieve defect-free joints in Al-alloys. However, a thorough characterisation of the joints is needed in order to satisfy the stringent requirements of advanced applications such us aerospace, automotive and shipbuilding.

In this work, FSW was performed on four different aluminium alloys, namely 5005-H14, 2024-T351, 6061-T6, and 7020-T6 (plate thickness being 5 mm except alloy 5005 which is 3 mm thick). The main objective was to establish the local microstructure-property relationships and to determine the fracture toughness levels of welded plates with weld zone strength undermatching. The FSW welds were void and crack free in all of the investigated alloys. Tensile and fracture toughness properties (in terms of CTOD) of the FSW joints were determined at room temperature in addition to extensive hardness meas­ urements and tensile tests. The effects of strength mismatch and local microstructure on the fracture toughness of these joints were discussed.

Bruchmechanische Untersuchungen reibriihrgeschweiiBter Aluminiumlegierungen

Zusammenfassung

Das ReibriihrschweiBen (Friction Stir Welding - FSW) kann erfolgreich zum Erstellen von fehlerfreien SchweiBverbindungen in Al-Legierungen eingesetzt werden. Es ist allerdings eine vollstandige Charak- terisierung der SchweiBverbindungen erforderlich, um die strikten Vorgaben der Flugzeug-, Automobil- und Schiffbauindustrie zu erfiillen.

Im Rahmen dieser Arbeit wurden ReibriihrschweiBverbindungen in vier Al-Legierungen 5005-H14, 2024- T351, 6061-T6 und 7020-T6 (Plattenstarke jeweils 5 mm auBer AA5005 mjt 3 mm) hergestellt. Das vorrangige Ziel war es, den Zusammenhang zwischen Mikrostruktur und Eigenschaften der Verbindung sowie die Bruchzahigkeitseigenschaften der Verbindungen im SchweiBnahtbereich zu bestimmen. Die ReibriihrschweiBnahte waren in alien untersuchten Legierungen frei von Fehlstellen und Rissen. Die Zugfestigkeits- und Bruchzahigkeitseigenschaften (CTOD) wurden bei Raumtemperatur bestimmt und durch Harteverlaufsmessungen erganzt. Die Einflusse der lokalen Festigkeit und Mikrostruktur auf die Bruchzahigkeit der SchweiBnahte werden diskutiert.

Manuscript received / Manuskripteingang in der Redaktion: 6. Dezember 2000

5

1 INTRODUCTION

Friction stir welding (FSW) is a promising solid state joining process particularly suited to Al- alloys which is not associated with major weldability problems such as porosity formation or cracking. FSW, an alternative friction welding process, was specifically developed and patented for joining Al-alloys at TWI [1]. In principle, two pieces to be welded are brought into contact, placed on a backing plate and securely clamped. A specially designed cylindrical tool, consisting of a shoulder and a profiled pin is inserted into the joint. During welding, rotation of the shoulder (which is in intimate contact with the upper surfaces of the workpiece) and the pin produces fric­ tional heat, bringing the material to the plasticised state. As the tool translates along the joint line, plasticised material is stirred and forged behind the trailing face of the pin, where it consoli ­ dates and cools down to form the solid state weld [2]. In contrast to fusion welding processes, solid state FSW results in a much lower distortion and residual stresses owing to the low heat input characteristic of the process. Moreover, filler material is not required. Although there are several reports on the microstructural characterisation and determination of mechanical properties of friction stir welded Al-alloys [3-5], not much has been published on the fracture toughness be­ haviour of these joints [6],

In this work, solid solution strengthened alloy 5005 and precipitation hardened Al-alloys 2024, 6061, and 7020 were butt welded by FSW. Typically, a loss of strength was observed in the weld zone (strength undermatching), due to recrystallization and redissolution of hardening particles (formed from Mg and Si) in the nugget and overageing or annealing processes in the thermo- mechanically affected zone (TMAZ) and heat affected zone (HAZ).

The loss of strength in the weld zone will cause a strain concentration if such welds are exposed to an external loading. Confined plasticity development within the undermatched weld zone will therefore reduce the plastic straining capacity of the weld joint under tensile loading as well as increasing the constraint within the weld zone [7], An increase in constraint due to confined plas­ ticity may cause a reduction in fracture toughness of the sandwiched weld zone. Hence, there is a need to determine the level of fracture toughness and the effect of notch position (base material, nugget, and TMAZ/HAZ) on the fracture behaviour in the strength undermatched FSW weld zone of Al-alloys.

In the present study, the results of microstructural characterisation and mechanical tests of Al- alloys joined by FSW were conducted to establish the local microstructure-property relationships of these joints. Furthermore, the influence of the degree of strength loss (undermatching) in the weld zone (nugget and TMAZ/HAZ) and the local microstructures on the fracture behaviour of the joints has been investigated.

2 EXPERIMENTAL PROCEDURE

In this study, four different aluminium alloys 5005-HI4, 2024-T351, 6061-T6, and 7020-T6 (plate thickness being 5 mm except alloy 5005-H14 which is of 3 mm thickness), were friction stir welded by ESAB, Sweden, with optimised parameters for each alloy.

Optical microscopy was conducted to investigate the microstructural changes taking place in the nugget, TMAZ and FZ of the joints. Microhardness measurements were conducted across the joints in three locations (i.e. top, mid, and root) to determine the hardness profiles and the hard­ ness variations across the joints, Figure la.

The mechanical properties of the joints were determined by testing conventional flat transverse tensile specimens at room temperature. Micro flat tensile specimens were extracted across the — 6 - joints to determine the variation on tensile properties from the base metal to the weld region, Figure lb [8].

Standard compact tension (CT50) specimens (a/W=0.5) were extracted from the base material as well as from the welded joints with fatigue pre-cracks positioned at the nugget and TMAZ/HAZ locations in order to determine fracture toughness properties, Figure lc.

Fracture toughness tests were performed on these specimens as well as on the base material at room temperature to determine crack resistance curves (R-curves) via the multiple specimen method. The welded specimens have been tested in the as-welded condition without any further machining of the weld profile (i.e. no significant weld reinforcement present). Crack tip opening displacement (CTOD 55) values were directly measured as shown in Figure lc using a 85 clip-on- gage at the tip of the fatigue pre-crack as described by Schwalbe et al. [9].

(a)

e.g. fatigue crack tip in the middle of fusion zone (FZ)

CTOD (85)

thickness: 3 or 5 mm

(b) Figure 1: Schematic drawings showing: (a) friction stir welded joints; (b) extraction of micro flat tensile specimens from friction stir welded joints, and; (c) compact tension (CT) specimen in­ strumented with local crack tip opening displacement (CTOD) clip gauge (CTOD fracture toughness is directly measured by 85 clip-on-gauge).

3 RESULTS AND DISCUSSION

3.1 Microstructural Aspects

In contrast to fusion welded joints on Al-alloys, friction stir welded joints show a very flat and clean surface finish. The top surface of the joints appear like machined due to the rotating action of the shoulder. Optical microscopy revealed joints free of cracks and voids. -7-

Figure 2 shows typical macrographs of FSW joints. The microstructure of FSW joints can be di­ vided in four zones: unaffected base material, heat affected zone (HAZ), thermomechanically affected zone (TMAZ) and nugget [10]. In precipitation hardened alloys, such as the 6061-T6 investigated in this study, the nugget area shows a reduction in grain size by dynamic recrystalliza­ tion in an equiaxed structure (average grain size 10 pm). TEM studies have also shown a reduction in the dislocation density in this zone [11]. In the thermomecanically affected zone bending of the grains is clearly visible (Figure 5) with some degree of recrystallization. In this region, as re­ ported by Threadgill [10], the thermal cycle results in accelerated ageing and annealing processes. Furthermore, particularly in the region close to the nugget some work hardening should be ex­ pected. These observations are confirmed by hardness measurements (see item 3.2 below). Investigations by Liu et. al. [11] indicated that there is a complex variation in the precipitation phenomena taking place between the unaffected base material and the weld nugget. Variations in the precipitation have been also observed between the top and bottom regions of the weld.

Figure 2: Macrographs of the FSW welded joints, (a) Al-2024, (b) Al-5005, (c) Al-6061, (d) Al-7020.

The homogeneous distributed precipitates are generally smaller in the workpiece than in the weld zone with fewer larger precipitates being observed in the latter. Differences in composition of the precipitates in the weld zone have also been reported [11].

Figure 3: Micrographs showing FSW joints: a) 2024, b) 7020. -8

3.2 Hardness

A hardness decrease in the weld region was observed for the alloys 2024, 5005 and 6061, Figures 4a and 4b. In contrast to that, the weld region of the alloy 7020 did not show a significant hard­ ness decrease (Figure 4c). All joints exhibited a hardness minimum in the HAZ adjacent to the TMAZ indicating that this region has undergone an overageing/annealing process. Closer to the weld nugget an increase in hardness can be observed confirming that some degree of work hard­ ening has taken place. The hardness values in the nugget in all cases were lower than in the base material but higher than in the TMAZ. It has been reported [10, 11] that in the nugget some of the precipitates might have been taken into solution during welding. Upon cooling re­ precipitation and growth takes place resulting in the observed raise in hardness.

3robe2031-5mm 3robe6061-5mm 140 -

MitteSchweBnaht ' 120 - MitteSchweBnaft 5100 130 -

110 - -»-■ Rehe 1 fleite 1 Reihs 2 Rehe 3 -• —. Reihe 3

Abshndzu Mite SctiwetGnahfmm Abstindz uMMeSchweBnaH, mm

3robe7020-5mm

MhteSchweBnaht

Rehe 1 Rehe 2 "• Rehe 3

Absti ndz u M ite SchweBnaM, mm

Figure 4: Hardness profiles of the FSW joints: a) A12024, b) A16061, and c) A17020.

3.3 Tensile Properties

Global tensile properties were determined by conventional flat transverse tensile specimens at room temperature in the as welded condition. The results are summarised in Table 1, which also includes the base metal properties. All joints failed in the weld region due to the strength under­ matching condition (strength loss in the weld zone, see hardness profiles, Figure 4).

The FSW joints exhibited joint efficiencies in terms of tensile strength, from 74.7 % to 84.4% and joint efficiencies in terms of elongation from 26.8 % to 100 % (see Table 2, Figure 5). In the case of undermatching, observed in all specimens investigated in this work, the stress concentra­ tion and thus fracture takes place in the lower strength weld metal region of the joints. It should be noted that all of the deformation takes place within the lower strength weld zone width (2H). The width of the weld, which due to strength undermatch contains all the deformation, affects the measured ductility levels favouring therefore the FSW joints. -9

It can be concluded that the strength undermatched FSW joints can provide rather high loading capacity in terms of strength (due to confined plasticity) under tensile loading condition normal to the weld axis. But in terms of strain, the joint efficiency is significantly lower.

Table 1: Results of transverse tensile tests (values are average of 3 specimens).

Rp0.2 Rm. A Joint Efficiency Joint Effi­ Alloy Yield Tensile Elongation in terms of R^ ciency Strength Strength in terms of A (MPa) (MPa) (%) (%) (%) A15005 (BM) 147 158 7.0 - - A15005 (FSW) 73 118 7.0 74.7 100

A12024 (BM) 350 493 19.0 - -

A12024 (FSW) 268 410 5.1 83.2 26.8

A16061 (BM) 281 319 15.6 - - A16061 (FSW) 162 252 7.2 79.0 46.2

A17020 (BM) 326 385 13.6 - -

A17020 (FSW) 242 325 4.5 84.4 33.1

By testing micro flat tensile specimens extracted across the welded joint, the local mechanical property variations could be determined. All FSW joints displayed similar mechanical property profile across the weld region. Yield and tensile strengths decrease in the thermally affected zone reaching a minimum in the HAZ increasing again at the centre of the nugget as seen in Fig. 5, hence confirming the hardness results. In all alloys the specimens extracted from the nugget dis­ played higher elongation levels than the base metal which substantiates the microstructural features observed in this region (see item 3.1). For alloy 2024 the weld region exhibited a compa­ rable elongation level to that of the base material. The information obtained from these tests can provide a better understanding of the deformation, crack initiation and fracture process of strength mismatched weldments.

I

- Am. MPa —RP0.2, MPa Dehnung. %

(a) (b)

Figure 5: Results of micro flat tensile tests of FSW joints, a) A12024, and b) A16061. - 10­

3.4 Fracture Toughness

Table 2 summarises the CTOD values (85m), which have been obtained at the onset of maximum load levels. The specimens were unloaded after attainment of maximum load level and then fa­ tigue cracked to measure the extent of stable crack growth (Aa).

Table 2. Fracture toughness values of the joints and respective base materials.

CTOD(S,)m [mm] Base Material FSW Alloy Nugget TMAZ/HAZ A1 5005 HI 4 0.43 1.62 1.47 (3 mm) 0.34 1.68 1.52 0.29* 1.41 1.20 A1 2024 T351 0.31 0.23 0.21 (5 mm) 0.29 0.23 0.18 0.29 0.21 A1 6061 T6 0.28 1.01 0.62 (5 mm) 0.31 0.95 0.66 0.24 0.92 0.61 A1 7020 T6 0.41 0.52 Testing in progress (5 mm) 0.39 0.44 0.39 * bold values represent the minimum values of the three specimens tested

As seen from Table 2, the base material specimens of alloy A17020 exhibited the highest fracture toughness value of CTOD (85) = 0.39 mm. Specimens pre-cracked at the nugget, displayed frac­ ture toughness values higher than those of the respective base plates due to the intrinsically tougher microstructure formed in this region (Figure 7). The results obtained for the alloy A12024 were slightly lower than those obtained for the base material. Apparently, the modifications im­ posed on the inclusion and precipitates population in the nugget by the process counteracted the positive effects on toughness of smaller grain size and lower yield strength.

The crack resistance curves (R-curves) for the investigated joints are shown in Figure 6. As a re­ sult of a more favourable microstructure in the nugget region the welded joints displayed in most cases superior toughness as compared to that of the base material. As mentioned above, slightly lower values were only observed for alloy A12024. The positive microstructural development in the nugget region associated with the geometric features (higher 2H values) of the FSW joints resulted in the satisfactory toughness behaviour. The complex precipitation process occurring in this region, briefly described in item 3.1, has apparently not influenced toughness properties.

The toughness results obtained for the TMAZ/HAZ were placed between those of the nugget and base material (alloys A15005 and A16061) with the exception of A12024 which was lower than the both base material and nugget. In all alloys however, the toughness of the nugget was higher than the TMAZ/HAZ.

The TMAZ/HAZ can be described as a narrow strip of material (3 to 4 mm) surrounded by micro - structures with higher yield strength (see the tensile properties profile presented in Figures 5 and 6). Under external loading, strain concentration will take place on this narrow strip increasing the triaxiality at the crack tip. As mentioned above, an increase in constraint due to confined plas­ -11 - ticity may cause a reduction in fracture toughness of the sandwiched TMAZ/HAZ in comparison to the nugget.

Due to the reduction in yield strength in the welded zone of FSW joints, higher fracture toughness values were obtained at this test location than in the respective base materials (see Table 2). In the nugget the confined plasticity (increased constrain) within the fusion zone did not decrease toughness because apparently the inherent toughness of these zones is still higher than that of the base material. Hence, it can be concluded that the strength undermatching observed on FSW joints did not generally lead to a degradation of toughness properties.

Al 2024. 5mm Al 5005. 3mm

• Nugget

Metal 0.2 • « HAZ • Nugget

A a [mm] A a [mm]

Al 6061.5 mm Al 7020.5 mm

O Base Metal • Nugget Metal # Nugget ? 0.6 - » HAZ

O 0.4

A a [mm] A 3 [mm]

Figure 6: R-curves of FSW joints: a) A12024, b) A15005, c) A16061, and d) A17020.

4 CONCLUSIONS

The following conclusion have been drawn from the present work: • FSW produced defect-free welds in aluminium alloys 5005, 2024, 6061, and 7020. • Micro flat tensile testing revealed that in all tested alloys the yield and tensile strengths de­ crease in the thermally affected zone reaching a minimum in the HAZ increasing again at the centre of the nugget. Elongation decreased in the TMAZ/HAZ reaching in the nugget values comparable to (alloy A12024) or superior to the base material. • FSW welds exhibited joint efficiencies in terms of tensile strength, from 74.7 % to 84.4 % (strength undermatching) and joint efficiencies in terms of elongation from 26.8 % to 100 %. - 12 -

• The nugget and TMAZ/HAZ of alloys A15005, A16061, and A17020 exhibited significantly higher fracture toughness levels than the base material and thus displayed higher resistance to stable crack growth. On the other hand, the nugget of alloy A12024 joints exhibited similar or slightly lower fracture toughness values than the respective base plate. This has been attrib­ uted to changes in the characteristics of the inclusion and precipitates population.

7020

Figure 7: Fracture surfaces observed on specimens tested with fatigue pre-crack on the base material and nugget of alloys A12024, A16061 and A17020.

ACKNOWLEDGEMENTS

Authors wish to thank ESAB, Sweden for providing friction welded plates. Thanks are also due to Mr. V. Ventzke for conducting the mechanical tests and metallographic analysis. - 13 -

REFERENCES

[1] International Patent Classification B23K 20/12, B29C 65/06, Improvements Relating to Friction Welding, Applicant: The Welding Institute (TWI), Abington, Cambridge, UK.

[2] Midling, O.T., Oosterkamp, L.D. and Bersaas, J., “Friction Stir Welding Aluminium - Proc ­ ess and Applications”, Proceedings of the 7th International Conference Joints in Aluminium (INALCO ’98), Cambridge, England, Volume 2, 161-171.

[3] Dawes C., “Friction Stir Welding in Aluminium Joining”, Shipbuilding Congress, Odensee, 1995 [4] Dietzel, P. and Lippold, J.C., “Microstructural Evolution During Friction Stir Welding of Aluminium Alloy 6061-T6”, To be published in the Welding Journal. [5] Svensson, J. Karlsson, Karlsson, B., Larsson, H. and L. Karlsson “Microstructure and Proper ­ ties of Friction Stir Welded Aluminium Alloys”, Proceedings of the 7th International Conference Joints in Aluminium (INALCO ’98), Cambridge, England, Volume 2, 221-231. [6] von Strombeck, A., dos Santos, J.F., Torster, F. and Kogak, M., “Bruchmechanische Un- tersu-chungen an reibriihrgeschweiBten Aluminiumlegierungen”, Proceedings of the Werkstoffwoche 1998, Munich, Germany. [7] G. Cam, V. Ventzke, J.F. dos Santos, M. Kogak, G. Jennequin, and P. Gonthier-Maurin: Char­ acterization of Electron Beam Welded Al-Alloys 5005, 2024, and 6061, submitted to Science and Technology of Welding and Joining, October 1998. [8] Kogak M., Cam, G., Riekehr, S., Torster, F. and dos Santos, J.F. “Microtensile test Tech­ nique for Wedments”, IIW Doc. X-F-079-98. [9] Schwalbe, K.-H., Neale, B.K., and Heerens, J., “The GKSS Test Procedure for Determining the Fracture Behaviour of Materials: EFAM GTP 94”, GKSS Report 94/E/60, Geesthacht, Germany, 1994.

[10] Threadgill, P., “Friction Stir Welds in Aluminium Alloys - Preliminary Microstructural As­ sessment”, TWI Bulletin, March/April 1997, Reprint 513/2/97.

[11] Liu, G., Murr, L.E., Niou, C. S., McClure, J.C. and Vega, F.R., “Microstructural Aspects of the Friction Strir Welding of 6061-T6 Aluminium”, Scripta Materialia, Vol. 37, No. 3, pp. 355-361, 1997.