Marcel Hermans

In situ strain investigation during laser welding using digital image correlation and finite-element-based numerical simulation

ABSTRACT In situ strain evolution during laser welding has been measured by means of digital image correlation to assess the susceptibility of an advanced high strength automotive steel to solidification cracking. A novel method realised using auxiliary illumination and optical narrow bandpass filter allowed strain measurements as close as 1.5u2009mm from the fusion boundary with good spatial and temporal resolution. A finite-element thermomechanical model of the welding process supports the experimentally measured transverse strain. The validated finite-element numerical model can be used to assess the local strain and associated stress conditions which influences weldability and in particular, solidification cracking.

Observations on Droplet and Arc Behaviour during Pulsed GMAW

The influence of the oscillation behaviour of the weld pool and the pendant droplet on the electric arc in pulsed GMA welding has been studied. High speed video images are related to measurements of the voltage. It is found that the weld pool is triggered into motion by the impact of the transferred liquid droplet. The increase of the current in the peak pulse period hardly affects the weld pool. The remaining liquid at the electrode tip may start to oscillate by the action of surface tension and gravity after the droplet is detached. It appears that the oscillation pendant droplet changes the structure of the arc. A conceptual model is derived to explain the observed behaviour: visual observations and voltage measurements. Two different regions can be distinguished in the arc, a bright region and a normal region. The bright arc region surrounds the droplet and shrinks and expands with the up and down movement of the droplet. The behaviour of the bright arc region can be correlated with the measured arc voltage. The shape of the droplet has a negligible effect on the measured voltage, it only initiates the changes occurring in the arc. The periodical release of additional metal vapour in the arc affects the arc voltage considerably. The results of the experimental work presented here cannot yet explain the mechanism behind the alteration of the arc structure and further research is required to elucidate these phenomena. The results presented in this paper focus on bead on plate-pulsed GMA welding in partial penetration conditions.

Formation of Nanostructures in Severely Deformed High-Strength Steel Induced by High-Frequency Ultrasonic Impact Treatment

Surface modification by the generation of a nanostructured surface layer induced via ultrasonic impact treatment was performed at the weld toe of a welded high-strength quenched and tempered structural steel, S690QL1 (Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Moxa0(wtxa0pct)). Such high-frequency peening techniques are known to improve the fatigue life of welded components. The nanocrystallized structure as a function of depth from the top-treated surface was characterized via a recently developed automated crystal orientation mapping in transmission electron microscopy. Based on the experimental observations, a grain refinement mechanism induced by plastic deformation during the ultrasonic impact treatment is proposed. It involves the formation of low-angle misoriented lamellae displaying a high density of dislocations followed by the subdivision of microbands into blocks and the resulting formation of polygonal submicronic grains. These submicronic grains further breakdown into nano grains. The results show the presence of retained austenite even after severe surface plastic deformation. The average grain size of the retained austenite and martensite is

Investigation on hot cracking during laser welding by means of experimental and numerical methods

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Finite element simulation and experimental investigation of thermal tensioning during welding of DP600 steel

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Acoustic Emission as a Probe of the Kinetics of the Martensitic Transformation in a Shape Memory Alloy

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