G. den Ouden

Laser induced stabilisation of the welding arc

Abstract 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.

Synergetic effects of hybrid laser/arc welding

Abstract 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.

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.

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.

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.

Additive assisted through the arc sensing during gas tungsten arc welding

AbstractThis paper concerns a novel approach to through the arc position sensing, which is based on the influence that certain additives have on the physical behaviour of the welding arc. The changes in the physical behaviour are reflected by changes in arc parameters, in particular arc voltage, which can be monitored during the welding process. In the first part of the paper the results of bead on plate experiments aimed at determining the influence of different additives on arc voltage are presented. Of the different additives considered, SiO2 is found to have the most pronounced effect. It appears that addition of SiO2 results in a voltage increase in the range 1–3 V, depending on arc length, welding current, and travel speed. The observed effect is ascribed to arc contraction and arc trailing. The influence of SiO2 on the weld bead geometry and the mechanical properties of the weld was also determined. It appears that the addition of SiO2 results in enhanced weld penetration, presumably as a result of...

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.

Diffusion bonding of silicon nitride to austenitic stainless steel with metallic interlayers

This article deals with the diffusion bonding of hot-pressed silicon nitride (HPSN) to austenitic stainless steel using metallic interlayers. Experiments were carried out with Fe, Ni, invar, and kovar as the interlayer material. The best results were obtained with invar interlayers. Using these interlayers, joints having an average shear strength of 95 MPa were produced. The reaction layer formed at the ceramic-interlayer interface during the diffusion-bonding process consists of a porous zone anchored in the silicon nitride and a diffusion zone extending in the interlayer. No silicides were detected. The formation of the joint is governed by the decomposition of the silicon nitride. The free silicon generated by the decomposition reaction diffuses into the interlayer, whereas part of the free nitrogen disappears to the surface, the remainder being trapped in the form of pores. The mechanical strength of the joint depends strongly on the residual stress generated in the joint during cooling from diffusion-bonding temperature to room temperature due to the difference in thermal shrinkage between the ceramic and the metal. The residual stress level is directly related to the thickness of the reaction layer.