Fatigue failure investigation of pre-corroded and Laser-Welded Al-Cu-Mg-Ag alloy with different temper condition

Transcription

1 IIW 2010, Istanbul, July 2010 International Conference on Advances in Welding Science & Technology for Construction Energy & Transportation Systems Fatigue failure investigation of pre-corroded and Laser-Welded Al-Cu-Mg-Ag alloy with different temper condition A.D. Zervaki 1, A. T. Kermanidis 1, G. N. Haidemenopoulos 1 and Sp. G. Pantelakis 2 1 Department of Mechanical Engineering, University of Thessaly, GR , Volos, Greece 2 Laboratory of Technology and Strength of Materials (LTSM), Department of Mechanical Engineering & Aeronautics, University of Patras, Rion - Patras, Greece

2 Laser Welding Applications in Aeronautics ` Source: Airbus Deutschland GMBH AIM: Investigation of fatigue failure mechanisms

3 AIM Integral airframes offer improved weight and cost efficiency compared to differential (riveted) structures Advanced laser beam welded structures offer advantages important to the mechanical performance of the welded structure such as the production of narrow and low distortion heat affected zone (HAZ) and are now used in European civilian aircraft structures Increasing service life Corrosion Fatigue Performance? In this work the fatigue performance of new 2139 welds after exposure to corrosion environment has been investigated

4 OUTLINE Introduction Alloys 2139-T3 & 2139-T8 Experimental Conditions (LBW) Fatigue experiments 2139-T3 Fatigue behavior of the as received specimens Fatigue behavior of the pre-corroded specimens 2139-T8 Fatigue behavior of the as received specimens Fatigue behavior of the pre-corroded specimens Conclusions

5 alloy 2139 Chemical composition (%κ.β.) Si Fe Cu Mn Ag Mg Zn Ti Zr < Controlled Mg, Ag additions in Al-Cu alloys promote the Ω-phase which forms as very thin hexagonal shaped platelets on the {111} planes of the matrix Tensile properties TYS (Mpa) UTS (Mpa) Elongation (%) L LT Welding Conditions Nd:YAG laser 3,3kW (*) Power: 3000W, Speed: 1.8m/min, Focal Point: 0 (d=0.4mm) Filler: 4047, Ø 1.2, 2.5m/min Protection He 25 l/min (face) Ar 15 l/min (root) Microstructure Al-Cu-Mg-Ag-Mn (TEM) Τ3: solution,quenching,cold work, natural ageing Τ8: solution,quenching,cold work, artificial ageing (*) The butt-welded sheets were received from GKSS research centre

6 2139 Corrosion exposure ASTM B117 Corrosion environment: Salt Spray 720 hrs, in NaCl 5% 2139-T T8

7 Experimental, fatigue testing Machining of the fatigue specimens was performed according to the ASTM E 466. All specimens were cut in the longitudinal (L) orientation relative to the rolling direction. Fatigue tests were performed with a stress ratio R=0.1, and a frequency of 30 Hz on corroded and uncorroded welds for comparison.

8 2139 T3 -Fatigue behavior of uncorroded and corroded welds R=0.1, Frequency=25Hz 300 maximum stress σ max T3 uncorroded 2139 T3 corroded WM E7 1E8 cycles to failure

9 Weld Macro-Microstructure WM PMZ PMZ T3 BM WM HAZ 125 Microhardness, HV 0, Weld pool HAZ Base Metal PMZ Distance, mm Microhardness profile BM

10 2139-T3 Fractographic analysis uncorroded welds σ max =150, Nf= cycles

11 2139-T3 Fractographic analysis uncorroded welds Initiation site Fatigue striationς Microporosity in WM

12 2139- T3 Crack initiation and fracture path uncorroded welds

13 2139- T3 Crack initiation and fracture path corroded welds

14 2139 T8 -Fatigue behavior of uncorroded and corroded welds T8 uncorroded 2139 T8 corroded maximum stress max E7 1E8 cycles to failure WM

15 2139-Τ8 (uncorroded) face Initiation site root A Weld macrostructure (transverse section) Α:Fatigue initiation site σ max =150MPa cycles

16 2139-Τ8 (corroded) specimen surface σ max =125MPa cycles Fatigue crack initiatin nucleated at corrosion pit

17 Facture Surface fast fracture region Fatigue sectio Fatigue & Fast fracture regions. Fatigue initiation site. σ max =150 MPa cycles

18 Fracture surface SP.12 B A A: Fatigue crack initiation at the bottom of a corosion pit exfoliation corrosion B: Intergranular corrosion σ max =150 MPa Nf= cycles

19 Micro hardness profile, measured 1.5mm below specimen surface Microhardness, HV WM dissolution b e d c HAZ PMZ coarsening a BM Distance, mm

20 Conclusions In all cases examined corrosion exposure leads to degradation of fatigue perfomance of the welds. More specifically: 2139 T3 : Increased susceptibility of the weld area to corrosion damage Crack initiation occurs at geometrical discontinuities induced by weld reinforcement Corrosion pits at the WM/BM interface act as additional stress raiser sites reducing the crack initiation stage Fatigue crack initiates and propagates through the weaker material (PMZ zone) 2139 T8 : Corrosion was limited to the base metal possibly due to existing grain boundary phases Crack initiates at corrosion pits in the base metal and fracture appears at the base metal

21 Base Metal, 135HV

22 HAZ, 132 HV

23 HAZ, 120 HV

24 HAZ, 118 HV

25 HAZ, 115 HV (PDZ)

26 HAZ, 110 HV

27 PMZ, 100 HV