ORAL REFERENCE: FT 334 (SESSION 3: WELDS)

FATIGUE BEHAVIOR OF SHIPBUILDING ALUMINIUM ALLOY WELDED BY

FRICTION STIR WELDING

N. Pépe, P. Vilaça, L. Quintino, L. Reis and M. de Freitas

Department of Mechanical Engineering, Instituto Superior Técnico, Portugal

ABSTRACT Friction stir welding is definitely stepping into industrial applications especially in the joining of light alloys such as aluminium alloys. The increasing application of these materials in many industrial environments gives rise to significant development of this process in a world scale. One of the most important fields is the shipbuilding industry where the AA5083–H111 welded by synergic Gas Metal Arc Welding (GMAW) is nowadays the most typical solution. Demands for higher productivity and reliability in shipbuilding envisage the transfer of the significant advantages of Friction Stir Welding (FSW) into this industrial sector. Thus, the mechanical behavior of the FSW joints should be investigated and compared with the performance of the actual joints and used for development of new classification standards. Metallurgical features are investigated and the results emphasize the different joining mechanisms between fusion and solid state welding technologies. The metallographic analysis is then compared with hardness tests developed in all zones of the welded joints and along with the results from the static tensile tests enable to establish the static mechanical resistance efficiency. Fatigue tests were carried out on plate specimens welded by FSW in order to obtain S-N curves for several welding parameters. The fatigue behavior of both synergic GMAW and FSW joints as welded and smoothed are analysed and compared. Fractographic analysis was carried out in order to assess the failure characteristics of the FSW plates when subjected to fatigue. Results show the superior quality of FSW when compared to synergic GMAW. KEYWORDS Friction stir welding, Gas metal arc welding, Tensile efficiency, Fatigue behavior INTRODUCTION Friction Stir Welding (FSW) was a remarkable development in the recent history of the welding technology resulting in metallurgical, environmental and energy benefits, allowing high quality of both similar and dissimilar welds, between materials previously difficult or even impossible to join. Moreover FSW enabled the development of new and already existent products allowing new design concepts which

represented a significant opportunity for the improvement of several industrial products but most significantly for the ones produced by transport industry [1 to 5]. Aluminium alloys are increasingly used in many industries due to the combination of properties such as low ratio weight/mechanical resistance, good corrosion resistance enabling different surface finishing [6]. Nowadays the most significant industrial applications of FSW are in aluminium alloys. Among these the good combination of mechanical and corrosion resistance of the AA5XXX series determines that it is one of the mostly used aluminium alloys in the automotive and shipbuilding industries. In particular the AA5083-H111 is often applied in the shipbuilding industry [7]. Shipbuilding is one of the industries where the application of aluminium has been growing mainly in small and medium size ships. Moreover, shipbuilding has a great importance in the economy of many countries, and it has always had a great tradition in Portugal. The typical weld process applied in shipbuilding for joining aluminium alloys is the synergic Gas Metal Arc Welding (GMAW), thus the need of faster and reliable methods of production and lower maintenance costs are the main reasons to induce a rapid transfer of FSW to industry in general and shipbuilding in particular [8]. This paper envisages a contribution for further developments of shipbuilding standards for FSW. For shipbuilding and many other industrial applications, it is fundamental to understand the fatigue characteristics of the welds. Thus the aim of the present paper is to settle the fatigue performance in terms of stress-number of cycles to failure (S-N) behavior of the AA5083-H111 welded by FSW and synergic GMAW. The mechanical efficiency relative to Base Material (BM) is established under both static and fatigue load [9 to 11]. The paper starts with the presentation of the weld procedures and parameters implemented. The macrographs, micrographs and the hardness profile of the joints are then established. After addressing the visual appearance of the joints and non-destructive x-ray results, bending tests are carried out for the FSW trials and the mechanical static efficiency is evaluated for each of the weld processes. The fatigue test results for BM, FSW and synergic GMAW joints are presented and analysed in detail. Before the final conclusions the fractographic analysis of the FSW joints is presented. EXPERIMENTAL SET-UP Welding procedures and parameters The material investigated is the nonheat-treatable AA5083-H111 plate rolled to 4mm, with the composition presented in the Table 1. This aluminium alloy has a very good combination between the mechanical properties and corrosion resistance, therefore one of the most popular materials in shipbuilding. All the welds are performed along the rolling direction in a butt joint arrangement with straight edge preparation. Figure 1, shows the visual appearance of the top surface of the plates after welding.

TABLE 1 BASE MATERIAL (BM) AA5083-H111 CHEMICAL COMPOSITION [Wt.%]

Si Fe Cu Mn Mg Cr Zn Ti Al

0.12 0.33 0.03 0.51 4.39 0.08 0.01 0.02 remaining

FSW Synergic GMAW

Advancing Side

Retreating Side

Figure 1: Top surface appearance of the weld beads for both processes

FSW has been performed on a conventional milling machine with manual control of the position. The tool geometry is a cylindrical M5 threaded pin with 3 conical flats and a flat shoulder with 3 concentric striates as represented in Figure 2. The plates are strongly constrained with no gap between the plates. The welding parameters applied are presented in Table 2 and resulted from parameter development procedure [12]. The surface preparation of the plates consisted of cleaning with solvent immediately before the trials.

Pin Shoulder

Figure 2: FSW tool pin and shoulder geometry GMAW beads have been performed in a Portuguese shipyard by local certified welders following shipbuilding standards. The equipment is a Kemppi Pro 4000 power source with a Kemppi Pro MIG 500 synergic control unit using pulse arc on an automated installation. The shielding gas used is Argon and the filler wire AA5356 with the thickness of 1.2mm. The welded plates are strongly constrained during the welds with a gap of 2mm and a ceramic backing bar is used. The remaining welding parameters used are presented in Table 2 and resulted from the existing data at the shipyard for these conditions. The surface of the plates has been prepared using grinding and cleaning, with solvent, immediately before the trials.

TABLE 2 WELDING PARAMETERS

FSW Synergic GMAW

Rotation speed: 1120rpm Travel speed: 320mm/min Tilt angle: 1º Shoulder/Pin ∅: 17mm/M5 Pin length: 3.9mm

Travel speed: 300mm/min Wire feed speed: 6.5m/min Tension/Current: 21V/94A

Shielding gas flow rate: 20l/min

Experimental Conditions and Results For the x-ray inspection of the structural integrity of the welds a SCAN-RAY equipment, DOA 300/AC-103 model, calibrated and certified is used with a “source/film” distance of 700mm, current of 5mA, tension of 90kV and the exposure of 50s. The film is an AGFA D3 without the intensifier plumb film on the source side. Trials resulting from both welding processes do not show any defect during the stationary state of the welds. Although Figure 3 shows the x-ray results for the FSW trial where it is possible to

observe the excessive chip at the retreating side and a defect in the weld bead at the advancing side during the initial 30mm.

Figure 3: Sample of the x-ray results for the FSW trials For the macrographs the samples are mounted, polished down to 1µm and etched with Keller reagent. Then the different metallurgical zones are identified. The macrographs of the welded plates can be observed in Figure 4. They allow to assess the dimensions of the joining region, e.g., the location of entre and widthc at the root of the nugget for FSW and the reinforcement at the top and root of the ynergic GMAW bead. Figure 5 shows micrographs etched with Fluoridic Acid which allow the entification of weld bead characteristics.

sid

FSW Synergic GMAW

Figure 4: Macrographs of the transversal section of the weld beads (magnification factor: 5X).

FSW Synergic GMAW

Figure 5: Micrographs of 6 selected positions in the macrographs: FSW nugget (F1) ; FSW root defect (F2) ; GMAW Fusion Line (G1) ; GMAW Porosity (G2) ; GMAW Reinforcements (G3 and G4)

The characterisation of the mechanical properties of the welded joints is performed based on hardness field, bending tests, static and fatigue uniaxial tensile tests. The hardness field is established for both weld

1mm

1mm F1 G1

G2G3

G4F2

G1

G2G3

G4

F1

F2

20X

80X 20X80X

Advancin

20X20X

g SideRetreating Side

processes following the ISO 6507-2 with 1kg and about 30 measured points located at the mid-thickness of the joints cross section and the results can be observed in Figure 6. The hardness results are important

the assessment of the relative mechanical properties between the different zones resulting from the ermo-mechanical weld cycle.

inth

70

74

78

82

Har

dnes

s, H

V1

66

62-15 -10 -5 0 5 10 15

Distance to Weld Bead Center [mm]

FSW Synergic GMAW

Figure 6: Hardness profile measured at the mid-thickness of the weld joints cross section Bd

ending tests of 90º are carried out in Lloyd MX100 equipment, with a load cell of 100kN. The average istance between supports (distance between the centres of support rolls) is 50mm. Support rolls diameter

is 10mm and mandrel radius is 5mm. Mandrel velocity used thr l is 5mm/min. From each welded condition two specime n and bended in different directions. The specimens are carefully analysed after the tests. The results for the FSW trials are presented in Figure 7.

oughout the trians are take

Top Surface of FSW bead Root of the FSW bead

Figure 7: Bending tests of FSW trials with top surface an ad supporting tensile stress The uniaxial tensile tests are performed on an Instron 4507, with a load cell of 200kN and high resolution biaxial extensometers. Specimens were taken from each welded plate for tensile tests, with geometry according to the EN-895-2002. The BM properties are present in Table 3 and in Figure8 the mechanical efficiency of the welded joints relatively to BM is established for some of the most relevant static mechanical properties. The fracture location for the FSW s ns is well inside BM and for the synergic GMAW it starts at the root and propagates along the fusion line. All the specimens exhibit ductile fracture surfaces.

TABLE 3 BASE MATER BM) AA5083-H111 MEC A PROPERTIES

d root of weld be

pecime

IAL ( HANIC L

0.800

1.200

BM

1.000

0.000

0.200

0.400

0.600

Young's Yield Stress Ultimate Toughness Elonga

Wel

ded

Plat

e /

Modulus Strengthtion

FSW S

Figure 8: Mechanical static efficiency of the weld specimens relatively to Base Material (BM) Because a conclusive analysis about the best weld conditions based on the analysis of the 5 measured different static material properties is rather complex, an equivalent mechanical static efficiency weighted factor EGRET (1) was developed [2]. The EGRET factor (Eqn. 1) allows t establish in a scale from 0 to 100% the efficiency of the overall static behavior of each weld joint condition, i, relatively to the Base

)

ynergic GMAW

o

Ma

aterial (BM). The results obtained are presented in Figure 9. The weights considered in Eqn. 1 (Table 4im at balancing the elastic, plastic and ductility components , respectively: CE+CS02=CSmax=CA+CTen.

%100maxmax

0202

max02 ×⎟⎟⎠

⎞⎜⎜⎝

⎛×+×+×+×+×=

BM

iTen

BM

iA

BM

iS

BM

iS

BM

iE Ten

TenCAAC

SSC

SSC

EECEGRET (1)

TABLE 4 WEIGHTS CONSIDERED FOR EGRET FACTOR (EQN. 1)

C CE

(Yo Smung

ax’s (ulmotidul

us weight)

mate

strength weight)

0.05

0.33

95.57%

82.24%

75%

80%

85%

90%

95%

100%

FSW Synergic GMAW

EGR

ET [%

]

gure 9: Equivalent mechanical static efficiency factor (EGRET) obtained for both weld processeFi s

ing pplications, e.g., aeronautic. The fatigue tests are performed on an Instron 8502, with a load cell of 00kN. Stress ratio R is 0.1. Oscillation frequency is set to 20Hz. The S-N curve results obtai

In order to investigate the fatigue behavior of the AA5083-H111, specimens with the geometry represented in Figure 10 are prepared, for BM, FSW and synergic GMAW. For the friction stir welds specimens were prepared not only in the condition as welded but also in post weld smoothed root surface condition. The reason for this procedure is to overcome the typical root defect of the FSW welds, and is becoming nowadays a typical industrial post FSW procedure for the most fatigue resistance demanda1 ned are presented in Figure 11.

Figure 10: Specimen geometry for fatigue tests

0

20

40

60

80

100

120

1000 10000 100000 1000000 10000000

Number of Cycles

Stre

ss A

mpl

itude

[MPa

]

Base Material FSW "Smoothed" FSW "as Welded" Synergic GMAW

Figure 11: S-N curves (R=0.1) for Base Material (BM), FSW as welded and post weld smoothed root surface conditions and synergic GMAW specimens

The fracture location resulting from the fatigue tests for the synergic GMAW specimens always follow a similar pattern, starting in the stress concentration zone at the root and developing along the fusion line. The FSW specimens presented a different location as it is shown in Figure 12. All the fractures start at the root defect in the middle of the weld bead and develop in the direction to the opposite surface crossing the ugget. The fractographic analysis presented in Figure 13, emphasizes the fatigue fracture mechanism for e FSW specimens. Although the fatigue fracture mechanism is always similar for FSW specimens, the

surface condition of at the root plays an important role in the fatigue resistance, as can it be confirmed in Figure 11 and Figure 12.

nth

FSW as welded condition FSW post weld smoothed root surface condition

[ ]cycles ; 2059780 =≅ NMPaaσ [ ]cycles ; 17812480 =≅ NMPaaσ

4mm4mm

Figure 12: Fatigue fracture location and resulting surface for the FSW specimens (R=0.1)

I

II

III

zone I

Figure 13: SEM fractography’s illustrating the typical localization of different phases of fatigue fracture obtained for FSW specimens; cleavage planes (zone I), fatigue striates in fracture propagation (zone II)

and dimples at the final (zone III) A

isual Analysis

in the advancing side (Figure 3). This defect results from the insufficient lunging time until the start of the travel of the tool along the joint. When this time is insufficient, the

perature developed is low and does not allow the materia low in a perfect way around the pin, therefore a void is form et tail. This fact indicates the need of a longer indentation time for this tool geometry. In the stationary regime of both processes, the x-ray does not show any defect in the joint, although, e.g., light porosity is formed in GMAW and root defect in FSW (Figure 4 and Figure 5). Metallographic Characterization The metallurgical analysis in Figure 4 shows for the FSW bead a dynamically recovered zone (nugget) well defined with fine equiaxial grain presenting a homogeneous dispersion of the precipitates in the solid solution but not very regular onion rings (detail F1 in Figure 5), with a tail heading to the shoulder periphery, in the advancing side. Figure 4 also allocentre a ditions

r achieving minor root defects. The defects identified at the root (detail F2 in Figure 5) do not show any distinct kissing bond but show a layer, with 0.3mm in depth, of an aligned oxides/secondary phase particles, discontinuous, that develops from the bottom surface at the initial joint line towards the advancing side. This fact becomes very relevant when FSW structures are subjected to fatigue load. The most relevant metallurgical aspects of the synergic GMAW welds are the presence of small random distributed porosity inside the weld bead (detail G2 in Figure 5) and reinforcement at both surfaces. Details G3 and G4 in Figure 5, shows a significant concentration of precipitates in the reinforcement zones resulting from the dilution of the filler metal with the BM enabled by the high heat input of the GMAW process. The HAZ presents low grain coalescence and the distribution of precipitates in the grain boundaries is reduced due to dissolution of the secondary phase particles.

zone III zone II

NALYSIS OF THE RESULTS

VThe differences between the two welding processes are significant in terms of visual appearance (Figure 1). The synergic GMAW bead presents a significant reinforcement of material both at the top and at the bottom (Figure 4). In contrast the FSW beads show very regular surfaces with insignificant flash and very low depression at the top surface (Figure 4) due to the low tilt angle (Table 2) enabled by the shoulder geometry (Figure 2). X-Rays The x-ray results of the FSW plates show an initial defect, common in almost every experiment, haracterized by a void,c

ptem l to attain a correct level of viscosity to f

ed in the advancing side, under the nugg

ws to conclude about the low position of the nugget nd the significant width of the nugget at the root of the weld bead which are necessary con

fo

Hardness Tests Vickers hardness tests are performed in order to address initial information about the mechanical properties along the welded zone. From the results presented in Figure 6 the following comments can be drawn: i) BM presents an average hardness of 75HV1; ii) FSW bead presents an average hardness of 81HV1, about 8% higher than the BM. In opposition, synergic GMAW is 15% lower than the BM; iii) Along the TMAZ the FSW bead hardness decrease from the values of the nugget to the values of the HAZ, with a minimum value of 77HV1, about 5mm from the nugget centre; iv) The hardness values of FSW specimens are always higher than the ones resulting from synergic GMAW. Bending Tests In order to assess the critical level of the root defects in FSW beads, the pure bending with the root under tensile stress, is nowadays applied as one fast, low expensive test which returns the answer almost immediately. In this case the bending test show a small opening at the root of the FSW bead (Figure 7), that opened and propagated about 0.1mm at the first degrees of bending and do not propagate any further during the remaining bending action. Thus it is possible to conclude about the existence of root defects, as it was already discussed in the metallographic characterization and that the critical level is low. Tensile Tests Through the analysis of the EGRET factor in Figure 9 it is possible to confirm the higher mechanical resistance of the FSW welded plates when subjected to tensile static loads. The efficiency of friction stir welds is 14% higher than the one determined for synergic GMAW. Moreover the efficiency of the FSW specimens is about 96% thus close to the BM quality level FTd

ot defect always resulted in less n o post weld smoothed root surface ondition (Figure 11). This fact is emphasized in Figure 12, where it is shown that reduced in-depth size

thing increased the fatigue life in about 10X. Thus it is possible to anical resistance when smoothing the root of FSW beads. Moreover

oothed condition shows a fatigue life close to BM results, most significantly for the

aphic analysis, of Figure 13, and location of the fractures and resulting surfaces, presented in

.

atigue Tests he fracture mechanism of FSW specimens under fatigue load is mainly determined by the size of the efect at the root of weld bead. For the FSW beads in as welded condition the higher critical level of the

umber of cycles when compared trocof the root defect via surface smooonclude about the benefit of mechc

FSW beads in smlowest levels of stress amplitude. The results of FSW specimens in as welded condition are always better than the ones resulting from conventional welding solution: Synergic GMAW. The benefits of the fine equiaxial grain of the FSW nugget in fracture propagation phase plays an important role in this difference. FractogrFigure 12, illustrates the fatigue fracture mechanisms of FSW specimens. All the fractures started at the root of FSW bead, correspondent to zone I in Figure 13, and propagates along the middle of the nugget. The lack of mechanical resistance in zone I results in cleavage planes typical of fragile behavior. Figure 12 also emphasize different widths of the zone I for different surface finishing of FSW beads, where the smoothed root surface condition exhibit a much smaller extension with a consequent increase of the fatigue life addressed before. The zones II and III in Figure 13 represents the typical striates and dimples of the fracture propagation and ductile final rupture, respectively. CONCLUSIONS 1. Non-destructive X-ray tests does not reveal the root defects of the FSW beads which plays

fundamental role in the fatigue efficiency of the friction stir welded structures. Alternatively bending tests confirmed to be an efficient method to address critical level of the root defects in FSW.

2. FSW bead presents an average hardness of 81HV1, about 8% higher than the BM. In opposition, synergic GMAW is 15% lower than the BM. Thus, mechanical static efficiency of FSW is close to the BM quality level and 14% higher than the one determined for synergic GMAW.

3. The fatigue performance of the friction stir welds is always higher than the one obtained for the synergic GMAW.

4. Reduced in-depth size of the root defect from the initial 0.3mm, via surface smoothing, increased the fatigue life of the FSW specimens in about 10X for MPaa 80=σ and R=0.1. Moreover, FSW smoothed specimens shows a fatigue life very close to the BM results.

ACKNOWLEDGEMENTS The author would like to acknowledge:

• Financial support from the Fundação para Ciência e Tecnologia (FCT) via the project: POCTI/CTM/41152/01 (acronym: iSTIR).

• Material supply and performing of the synergic GMAW weld beads by Estaleiros Navais do Mondego S. A. – Portugal.

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