Advanced NDT to Monitor Friction Stir Welding Gerd DOBMANN Chair of NDT and Quality Assurance, Saarbrücken, Germany, Gerd.Dobmann@t-online.de
Outline: Introduction Process parameters Monitoring of process forces Early detection of irregularities Superposition of powered ultrasound Benefits & Conclusion Acknowledgements
Friction Stir Welding 1991 developed by TWI tool Schweißwerkzeug Vorschubrichtung Ende end piece 1 Werkstück 1 Anfang begin piece 2 Werkstück 2
Process in Detail Friction Reibungswärme heat plastification Plastifizierung Material Werkstofffluss transport
Tools Classic TWI 5651 Tri-flute TM Trivex TM Stir pins according to TWI
Process Parameters Rotation Drehrichtung direction v r ψ Work Werkzeug piece ψ δ v f v r -Inclination, Kippwinkeltilt -Penetration Eindringtiefedepth -Forward Vorschubgeschwindigkeit speed - Rotationsgeschwindigkeit speed F Work z Werkstückebene piece surface Fy δ v f, F x Work Werkstück piece
Process Schweißkraft force [kn] Process forces 1 F x -1-3 -5-7 1 2 3 Position1 pin on surface, starting of plastification Position 2- pin shoulder on surface Position 3 break for heat-up and start of welding Position 4 stop of welding 4 F y F z -9 0 10 20 30 40 50 60 70 80 90 time Zeit [s]
Material movement by rotation and translation Groove space Draufsicht, x-y-ebene RS AS Seitenansicht, x-z-ebene Querschliff, Micrograph y-z-ebene y-z-plane AS-advancing side RS-retreating side
Material movement visualized by 3D-CT according Dickerson 2003 AS RS
AlMg3Mn-AlMg3Mn FSW with nugget, TMAZ, HAZ and BM at RS and AS RS AS Nugget TMAZ TMAZ HAZ HAZ BM AM BM AM 2 mm
Joint Line Remnant JLR, Oxid-Hydroxid Particles 1 mm
Process monitoring and control Quality control with SPC Classical process (Statistical Process Control) monitoring and control with quality characteristics (dimensions, defects, material properties) Off-line, ex-line Random samples, discontinuous Observation of Quality degradation -> Process disturbances (delayed) -> Control cycle with process characteristics (force, pressure, temperature, etc.) On-line, in-line 100%, continuous Observation of Process disturbances -> Quality degradation? -> Control cycle Process integrated quality control with NDT
Process monitoring and control Disturbance variable Control Monitoring NDT sensors Control Monitoring NDT sensors Lead signals Open-loop Control Manipulated variable Actuator Input Process Output Closed-loop Control Control Monitoring NDT sensors Control variable
Welding parameters Wrought Al-alloy AA5454 (300x125 mm 2 ) with a thickness of 3.5 mm were friction stir welded. Milling machine from the type Deckel Maho DMU80T, DMG Germany. Tool shoulder possesses a 14 mm diameter and a M3.5 pin and was tilted by 2 during the process. The ratio of the welding parameters, feed rate and rotational speed per revolution, FPR, ranges from 50 µm/r to 200µm/R. The machine is equipped with a force measurement system which allows recording the welding forces in x-, y-, and z-direction emerging during the welding process. For microscopic investigations the cuts of the welded sheets were etched in hydrofluoric acid for 10 s.
Force measurement by piezoelectric sensors according to Kistler a b
ZigZag indications
Periodic welding forces
Feed per revolution and groove spacing Psm
Pattern recognition in the spectrogram of a sound weld Continuously high amplitudes in the F x -frequency range lower than that of tool rotation (contrary to the STFT of F y). Continuously high amplitudes in the frequency range of the tool rotation frequency. Continuously high amplitudes in the frequency range of the harmonics. Continuously low amplitudes at 100 Hz, independent of tool rotation frequency.
Development of a worm hole
Superposition of high powered (20 khz, 25-40µm, 3kW) ultrasound, 500N FSW-Werkzeug FSW tool Work piece 2 Werkstück 2 Sonotrode Work Werkstück piece 1 1
a [MPa] Benefit & conclusion more than a factor 2 in lifetime 130 120 AlMg3Mn R 0 110 100 90 D (2 10 6 ) = 85 MPa 80 10 4 10 5 10 6 10 7 1,E+04 1,E+05 1,E+06 1,E+07 N B Red=with oxide mit Linien particles, blue=oxide ohne Linien removed by grinding, US-FSW Trend: mit Linien Trend: ohne Linien Trend: US-FSW green=powered ultrasound
Conclusions The FSW-Process can be online monitored due to Force-Measurement and Analysis The early development of welding irregularities can be detected The Process can be controlled The superposition of High-Powered Ultrasound is enhancing the welding quality in terms of fatigue life time with at least a factor two.
Acknowledgements I have to acknowledge the PhD-thesis of Tobias JENE The fruitful and longtime cooperation with the Institute of Materials Science and Engineering, WKK, Technical University Kaiserslautern, Prof. Dietmar EIFLER and Prof. Guntram WAGNER