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Details about aluminium 6063
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8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
IDMEC
1
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
8th MESOMECHANICS, July 19-22, 2006, 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
8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
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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
8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
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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
8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
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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
8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
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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.
8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
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METALLOGRAHIC ANALYSIS
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
8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
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METALLOGRAHIC ANALYSIS
base material specimen (MB)
grain diameter of 73.1 µm
grain diameter of 219.2 µm
microstructure MB2
8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
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METALLOGRAHIC ANALYSIS
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.
8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
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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.
8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
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TENSILE TESTS- Unwelded specimens 3 and 4 mm thick- FS welded specimens 3 mm thick
8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
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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.
8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
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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)]
8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
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FATIGUE TESTS
specimens with and without welding were fatigue tested
8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
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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
8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
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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
STRESS ANALYSIS OF WELDED AND UNWELDED SPECIMENS
8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
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STRESS ANALYSIS OF WELDED AND UNWELDED SPECIMENS
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%
8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
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STRESS ANALYSIS OF WELDED AND UNWELDED SPECIMENS
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)
8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
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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.
8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
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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
8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
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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
8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
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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.
8th MESOMECHANICS, July 19-22, 2006, Porto Portugal
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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