The present study reports the results of a study examining the synergetic effects of hybrid laser/ arc welding. Experiments were carried out with a 500 W Nd:YAG laser in combination with standard gas tungsten arc welding equipment and attention was focused on two aspects: the heat transfer efficiency and the melting efficiency. The heat transfer efficiency was determined by calorimetric measurements, whereas the melting efficiency was obtained from the transverse cross-sections of welds produced under various conditions. In addition, analytic calculations of the melting efficiency were performed on the basis of a modified form of the Rosenthal equation. The results show that the interaction of the laser and the arc does not lead to a noticeable change in the heat transfer efficiency, but results in a significant increase in the melting efficiency. The observed synergic melting effect is caused by addition of the two heat sources (laser and arc) and the contraction of the arc by the laser beam.

This paper deals with the influence of laser radiation on the stability of the welding arc. Experiments were conducted using a low power (500 W) Nd:YAG laser in combination with a gas tungsten welding arc. The laser induced arc stabilising effect was measured under various experimental conditions. It was found that the stabilising effect can be explained in terms of two phenomena: the absorption of laser energy by the arc plasma and the change of the arc plasma composition caused by strong evaporation of workpiece material. Both phenomena lead to a reduction of the effective ionisation potential of the plasma and thus provide a more conductive, stable plasma channel for arc root and column that overcomes disturbance by external forces. The proposed stabilisation mechanism was validated by measuring the absorption of laser energy by the arc plasma using a laser energy meter and the changes of arc plasma composition caused by the laser radiation by means of emission spectroscopy.

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For the rst time evidence is presented that supports the role of the grain detachment mechanism during grain re nement 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. STWJ/352The authors are in the Netherlands

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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.

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 (∫Ū2m 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 (Ū2b) 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.

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. STWJ/378 Keywords: weld penetration control, weld pool oscillation, gas tungsten arc welding, arc voltage variation, metal transfer Mr Yudodibroto (b.y.b.yudodibroto@tnw.tudelft.nl), Dr Hermans (m.j.m.hermans@tnw.tudelft.nl) and Professor den Ouden are in the