FRICTION STIR WELDED ALUMINIUM ALLOY 6063-T6: …meso2006/presentations/Meso_40.pdf · FRICTION STIR WELDED ALUMINIUM ALLOY 6063-T6: MECHANICAL CHARACTERIZATION, FATIGUE TESTS AND

8th MESOMECHANICS, July 19-22, 2006, Porto Portugal

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FRICTION STIR WELDED ALUMINIUM ALLOY 6063-T6: MECHANICAL CHARACTERIZATION,

FATIGUE TESTS AND DEFECTS IDENTIFICATION

P M G P Moreira, F M F de Oliveira,P M S T de Castro

IDMEC e Departamento de Engenharia Mecânica e Gestão Industrial,Faculdade de Engenharia da Universidade do Porto,

Rua Dr. Roberto Frias, 4200-465 Porto, Portugal


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OBJECTIVE

Quantify the influence of friction stir welding (FSW) on the fatigue life of aluminium alloy 6063-T6

Welding parameters: - 1000 rpm pin-tool rotation speed- 9,17 mm/s welding speed- 4,5 kN axial force


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SUMMARY

friction stir weld analysis

hardness tests

tensile tests

fatigue tests

non-linear finite element method (FEM) analysis of the fatigue test specimen

SEM analysis


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FRICTION STIR WELDING

Friction stir welding (FSW), a solid-state welding process developed and patented by the TWI in 1991

A rotating probe provides friction heat and pressure that joins the material

Superior weld quality joints:- low distortion- no porosity- no lack of fusion- no filler metal


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Friction stir weld visual analysis

Flaws or defects were not detected on the top and back surface of the weld

Since the sheet side faces perpendicular to the weld line were cut by guillotine it is possible to identify the different hard zones in each face

The darker zone indicates the material that was affected by the welding process. The increase in the darker area in the weld region is expected to be a characteristic of lower values of hardness when compared to those of the base material.

Specimens surface definition


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METALLOGRAHIC ANALYSIS

welded material specimen (MW)

transition between the zone affected by the welding process and the base material

microstructure MW2

The material affected by the welding process presents a fine stir grain structure, and the material near the HAZ presents regular grains. In the FS welded zone very fine recrystallized grains are present due to the high deformation and high temperature during the process.


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Microstructure MW3 shows larger grains near the plate free surface and smaller grains outside this layermicrostructure MW3

welded material specimen (MW)

grain diameter of 70 µm

grain diameter of 140 µm


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base material specimen (MB)

grain diameter of 73.1 µm

grain diameter of 219.2 µm

microstructure MB2


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Notes on the microstructural analysis:

The grain size diameter on top or back surface has similar average diameters, between 70.2µm and 76.5µm, in all microstructures.

Near the plate free surfaces grain diameters of 219.2µm and 144.0µmwere measured in the unwelded specimen, and in the base materialregion of the welded specimen, respectively.

This difference is explained by the different fabrication processes.


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Brinell Vickers (tube 3 mm thick) 72,9 77,2 (tube 4 mm thick) 70,2 73,8 Base material

(welded sheet) 78,2 76,2

HARDNESS TESTS

An increase of the hardness profile at the side surface was identified (zone not affected by the welding process).

This difference is due to the different grain size of these two regions.


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TENSILE TESTS- Unwelded specimens 3 and 4 mm thick- FS welded specimens 3 mm thick


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SPECIMENS

Fatigue test specimens

Fatigue specimen characterization:- Rectangular specimens containing notches;- Aluminium 6063-T6 sheet;- Specimen length: 100 mm;- Specimen width: 15 mm;- Specimen thickness: 3 mm.


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FATIGUE TESTSMTS 312.31

R ratio: 0.1test frequency: 8 Hz

Max stress levels:- 158,2 MPa (140% of σy)- 113 MPa (100% of σy)- 101,7 MPa (90% of σy)- 90,4 MPa (80% of σy)- 79,1 MPa (70% of σy)

[σy refers to material properties of tensile tests of FSW specimens (σy=113 MPa)]


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FATIGUE TESTS

specimens with and without welding were fatigue tested


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STRESS ANALYSIS OF WELDED AND UNWELDED SPECIMENS

Fatigue lives of FS welded specimens were found to be higher than those of unwelded material

A non-linear stress analysis using the finite elements software ABAQUS was carried out

mesh containing 4254 elements (C3D8 and C3D6 type) and 5916 nodes


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Unwelded specimens:

1st - the tensile test data obtained for the unwelded material was introduced as material properties

Welded specimens - FSW process zone width estimated as 14mm:

2nd - material properties data obtained in tensile tests measuring a 25 mm zone were used 3rd - material properties data obtained in tensiletests measuring only a 6 mm zone were used



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79,1 MPa (70 % of reference σy) 158,2 MPa (140 % of reference σy)

Stress distribution in the top surface of specimen mid plane

Specimen width [mm]

0 2 4 6 8 10 12 14 16

σ2 [MPa]

60

80

100

120

140

160

180

200

220

240Base material spec. 70%FSW spec. (mix mat behav) 70%FSW spec. 70%

Specimen width [mm]

0 2 4 6 8 10 12 14 16

σ2 [MPa]

120

140

160

180

200

220

240

260

280

Base material spec.140% FSW spec. (mix mat behav) 140%FSW spec. 140%


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Maximum values of total, plastic and elastic strain obtained when loading at several remote stresses a FS welded specimen using mix

material properties (25mm gauge length), and FS welded material properties (6mm gauge length)


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SEM ANALYSIS

SEM was used to identify microscopic features as:

- welding defects and - metallurgical details of fatigue cracks (fatigue striations).

Notwithstanding the good properties obtained in welded specimensfatigue tests, some defects were identified.

Inner defects, such as cavities, cannot be seen on the surface though it was revealed that a defect linearly exists along the joint line by SEM inspection.

Defects are formed outside the optimum FSW conditions.


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Defects regularly spaced; interval approximately similar to the advance per revolution

Probably the pin shape is not optimum for such a high advance per revolution, leading to a cavity or groove-like defect caused by insufficient heat input.

Defects are situated at near mid thickness so they are not a root flaw or lack of penetration

SEM analysis of a crack surface on a friction stir welded specimen

SEM ANALYSIS


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Fatigue striations were identified in both welded and unwelded specimens

Due to the more heterogenic fracture surface (stir effect), fatigue area and fatigue striations were difficult to be identified on the welded specimens.

SEM ANALYSIS

The fatigue crack growth area of the unwelded specimen

fatigue striations, welded specimen

fatigue striations, unwelded specimen


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CONCLUSIONS

Hardness drastically decreases in the weld-deformed zone; the average hardness of the nugget zone is significantly lower than the base alloy. Due to the different grain size diameter an increase of the hardness profile was identified at the side surface.

Yield and rupture stress of FS welded specimens have lower values than unwelded specimens.

The welding process lead to a decrease of tensile strength properties.


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CONCLUSIONS

Welded specimens presented longer fatigue lives for all stress levels

From the FEM analysis of the fatigue specimens it was found that the notch stress concentration factor is lower when the FS welded specimens are tested

Despite the good fatigue performance, defects were identified in the SEM inspection.These defects can be a cavity or groove-like defect caused by insufficient heat input

FRICTION STIR WELDED ALUMINIUM ALLOY 6063-T6: …meso2006/presentations/Meso_40.pdf · FRICTION STIR WELDED ALUMINIUM ALLOY 6063-T6: MECHANICAL CHARACTERIZATION, FATIGUE TESTS AND

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