M. J. M. Hermans

Grain refinement due to grain detachment in electromagnetically stirred AA7020 welds

Abstract For the first time evidence is presented that supports the role of the grain detachment mechanism during grain refinement of aluminium welds as a result of stirring. These grains are localised in the centre of the weld pool and have not been fully melted. The results of energy dispersive spectroscopy analysis of these grains indicate the similarity of their chemical composition to that of the grains in the base metal. This suggests that these grains are the partially melted grains, present at the fusion line, that are brought into the weld pool by the action of stirring.

Kinetics of the martensitic transformation in low-alloy steel studied by means of acoustic emission

Abstract The kinetics of the martensitic transformation in three carbon steels (C60, C70 and C80) have been studied using the acoustic emission (AE) technique. It is demonstrated that the volume fraction of martensite f as a function of time t during cooling can be derived from the measured AE power ℧ 2 , since ℧ 2 ∝ d f /d t . An analysis of results shows that the fraction data as a function of temperature T can be described by the Koistinen and Marburger (KM) equation with high accuracy. This indicates that the nucleation of martensite takes place heterogeneously and that the average volume of martensite crystals is constant over the extent of the transformation. The change in kinetics with carbon content is attributed to the amount of dislocations created in the neighboring austenite, which influences the degree of autocatalysis. Also, the acoustic emission energy generated per unit volume of martensite has been found to scale with the change in dislocation density in the formed martensite as the carbon content is varied.

Monitoring of martensite formation during welding by means of acoustic emission

The martensitic transformation during gas tungsten arc (GTA) welding of steel 42CrMo4 has been studied using the acoustic emission (AE) monitoring technique. Welds were produced under static conditions (spot welding) and under stationary conditions (travelling arc welding). After spot welding, the root mean square (RMS) value of the continuous acoustic emission was measured, revealing a peak that reflects the evolution of martensite formation during cooling of the spot weld. The RMS value was also measured during travelling arc welding at different heat inputs and corrected for the noise of the welding process to obtain the RMS value due to martensite formation. After welding, optical metallography was carried out to quantify the amount of martensite formed during cooling of the weld. An analysis of the results shows that the squared RMS value is proportional to the volume rate of martensite formation during welding, which is consistent with theory and in good agreement with the results obtained in the case of spot welding. The obtained results suggest that AE can be applied as a real time monitoring technique for the detection of martensite formation during steel welding.

Pendant droplet oscillation during GMAW

Abstract This paper deals with pendant droplet oscillation in pulsed current gas metal arc welding (GMAW). Using a constant current power source, bead on plate welds were made on mild steel plates. The frequency of the pendant droplet oscillation was monitored visually (using a high speed video camera) and electronically (by analysis of the voltage signals). The results obtained are compared with the results predicted by a model based on a mass spring system. It was found that the oscillation frequency of small droplets revealed by both monitoring techniques matches that predicted by the model. For larger droplets, the oscillation frequency measured by voltage signal analysis tends to be higher than that revealed by video analysis and that predicted by the model. This implies that during droplet oscillation in (GMAW) the arc behaviour may change and as a result the arc voltage is not any more proportionally related to the arc length.

Stress relaxation due to ultrasonic impact treatment on multi-pass welds

Abstract Ultrasonic impact treatment (UIT) is a relatively novel technique applied to the toe of welded joints to improve the fatigue life by changing the weld geometry and the residual stress state. In this study, the stress relaxation due to ultrasonic impact treatment is investigated on a six pass welded high strength quenched and tempered steel section. Stress measurements in two orthogonal directions were conducted by energy dispersive synchrotron X-ray diffraction. Results show that the application of only ultrasound to a welded component re-distributes the residual stresses more uniformly, while mechanical impacts in combination with ultrasound is an effective way to release the residual stresses. After welding, diffraction peak broadening due to the lattice distortion, characterised by the full width at half maximum (FWHM), is observed in the region of the weld toes. Ultrasonic impact treatment reduces the FWHM at these locations.

Influence of filler wire addition on weld pool oscillation during gas tungsten arc welding

Abstract The present paper reports on weld penetration control based on weld pool oscillation during gas tungsten arc welding (GTAW) with cold filler wire addition (cold wire GTAW). Experiments were carried out in which the weld pool was brought into oscillation by applying short pulses on the welding current. The frequency of the weld pool oscillation was obtained from the arc voltage variation via fast Fourier transform analysis. It was found that the weld pool oscillation approach is suitable for penetration control during cold wire GTAW when the metal transfer occurs in an uninterrupted bridging manner. Interrupted bridging transfer results in a disturbed oscillation signal due to agitation of the weld pool by mass transfer and/or oscillation of the pendant droplet.

Conceptual model for stress and strain development during welding with trailing heat sink

Abstract It has been proven by several research groups that welding with a trailing heat sink can reduce the buckling distortion of welded thin sheet. However, the transient mechanical behaviour during welding with a trailing heat sink was never fully understood. This paper presents a conceptual model that is capable of explaining both the longitudinal plastic strain and the longitudinal stress development during welding with an active cooling source. The model is based on an existing three bar model for stress and strain development during conventional welding. Comparison with three-dimensional finite element calculations shows that the model gives qualitatively realistic results. The model provides insight into the relevant mechanisms involved in stress development during welding with a trailing heat sink, which is essential for determining the optimum cooling source parameters.

A study of acoustic emission energy generated during bainite and martensite formation

Acoustic emission (AE) signals generated during bainite and martensite formation in steel C45 have been measured, and the AE energy has been correlated with the strain energy accompanying both displacive transformations. The gas tungsten arc welding process was used to vary the volume transformation rates of bainite and martensite formation. The root mean square (rms) voltage Urms of the continuous AE signals was measured during travelling arc welding and after spot welding. Depending on the cooling rate and the mean austenite grain size, martensite or bainite is formed in the weld. After spot welding with moderate arc currents, only martensite was formed during cooling, which was reflected by a peak in the Urms data: the martensite peak. An analysis of the results shows that the AE energy produced during the transformation ( ¯ U 2 m dt) is proportional to the volume Vm of martensite in the spot weld, with proportionality factor km. During travelling arc welding, bainite and martensite formation occur simultaneously and both displacive transformations contribute to the measured AE power at each moment. The AE power due to bainite formation ( ¯ U 2 b ) was calculated using the obtained proportionality factor km and was found to be proportional to the volume rate of bainite formation dVb/dt with proportionality factor kb.

Acoustic emission monitoring of bainitic and martensitic transformation in medium carbon steel during continuous cooling

Abstract This paper concerns acoustic emission (AE) measurements during continuous cooling of steel C45 using a Gleeble 1500 thermomechanical simulator. After austenising at a certain temperature, the studied specimen was cooled down and the root mean square (RMS) value of the continuous AE signal was measured. During cooling two distinct peaks in the RMS data were observed at temperatures of 200-300°C and 500-600°C, which have been attributed to martensite and bainite formation respectively. The observed bainite peak strongly indicated that the mechanism of bainite growth is displacive. The AE monitoring of bainite and martensite formation was supported by dilatation measurements, which were performed simultaneously. The effect of the austenite grain size on the evolution of the bainitic and martensitic transformation was studied by varying the austenising temperature T a. It was found that upon lowering T a, i.e. with decreasing austenite grain size, the bainite peak increases while the martensite peak decreases.

Elemental segregation during resistance spot welding of boron containing advanced high strength steels

The partitioning behaviour of carbon, phosphorous and boron during the solidification of a resistance spot weld pool was studied using experimental simulations and a phase field model. Steels with varying carbon, phosphorous and boron contents were designed and subjected to a range of resistant spot welding thermal cycles. Mechanical properties were evaluated by hardness and cross tension tests and correlated with the weld microstructure. Phase field modelling results and experimental predictions show that the phosphorus concentration in the last area in the weld pool to solidify can reach about 0.38 wt% for a steel with a bulk concentration of 0.08 wt%. Elemental analysis indicates that in the absence of boron, the grain boundaries of columnar grains in the weld pool are decorated with phosphorous. As a result, a complete interface failure occurs during cross tension testing. With the addition of boron, apart from an increase in weld strength and plug diameter, the failure mode switches to a complete plug mode, resulting from the phosphorous depletion at the grain/inter-phase boundaries.