L. Q. Li

Experimental study on welding characteristics of CO2 laser TIG hybrid welding process

Abstract The experiments of CO2 laser TIG paraxial hybrid welding with 4 mm thick AISI 321 stainless steel sheet have been performed. The arc images and welding characteristics have been investigated with different energy ratios between laser and arc. The experimental results indicate that the hybrid welding is similar to laser welding and has also two welding mechanisms: deep penetration welding and heat conduction welding. Because of the effect of keyhole induced by laser, the arc root can be stabilised and compressed, and the current density and the penetration depth are all increased significantly, which show the characteristics of deep penetration welding. However, when the current is increased to a critical value, the laser induced keyhole disappears and the arc expands obviously, which decreases the penetration depth, so that the welding mechanism has been changed from deep penetration welding to heat conduction welding. Furthermore, the effects of distance between laser beam and electrode, pulsed laser and hybrid manners on hybrid welding characteristics have also been studied.

Joint strength and failure mechanism of laser spot weld of mild steel sheets under lap shear loading

Abstract Ultimate strength and failure mechanism of laser spot welds under lap shear loading were investigated. Optical micrographs of cross-section of spot welds before and after failure were examined to understand the failure behaviour. The experimental results indicate that laser spot welds can fail in two distinct modes, namely interfacial and pullout failure. A failure mechanism which was confirmed by SEM investigations was proposed to describe these two failure modes. According to the experimental observations, a simple stress solution related to the far field load was conducted and the critical weld nugget diameter to ensure pullout failure mode was estimated. The results were compared with the experimental data and also with the test data of resistance spot welds. It was observed that the critical nugget diameter of laser spot welding was larger than that of resistance spot welding due to the different failure location in pullout mode. Furthermore, the effect of welding parameters on joint strength and failure mode was studied.

Comparative study of microstructure and mechanical properties of laser welded–brazed Mg/steel joints with four different coating surfaces

Abstract This paper presents a comparative study of laser welding–brazing Mg to steel with four different coating surfaces, including (Zn+pre-existing Fe–Al phase), pure Zn coating, pre-existing Fe–Al phase and fresh steel without any coating. The presence of Zn coating was found to significantly improve the wettability of liquid filler on steel. However, Mg–Zn products enriching at the seam head tended to cause cracking. The weak bonding of Mg–Zn products and Fe–Al layer was mainly responsible for the decreased tensile strength and interfacial failure that occurred in joints with the first two coatings. For joints with the latter two coatings, the thickness of newly formed Fe–Al layer determined the mechanical properties. The reaction layer formed at the Mg/fresh steel was thin, inducing interfacial failure, whereas the joint with pre-existing Fe–Al phase fractured at the seam, indicating that the pre-existing Fe–Al phase was beneficial to formation and growth of the Fe–Al phase.

Arc characteristics of laser-TIG double-side welding

Abstract Based on the experiments of laser-TIG double-side welding (LTDSW) for aluminium alloys, the influence of laser radiation on the arc behaviours of the opposite side was investigated. Generally, with the variation of laser power, there are three typical arc shapes: arc column convergence, arc root constriction and arc expansion. An important point to notice is that the laser keyhole preheating will induce the arc column convergence in the LTDSW. The arc voltage in the LTDSW is lower than that in TIG welding over the entire range of the experimental currents. Moreover, with increasing welding current, the difference in arc voltage between TIG welding and LTDSW is diminished because of the self-stabilisation of the arc burning at high currents. The complex transformation of arc behaviours has a great effect on the arc current density and its stability. The laser generated hot spot or laser induced plasma will have a higher temperature and greater electron density than neighbouring regions, and will offer the line of least resistance or the lowest potential drop. Hence, it is very reasonable that the arc voltage should descend under the influence of laser radiation, and the arc electrons should compress and root to the hot spot or plasma zone.

Effects of weaving laser on scanning laser-MAG hybrid welding characteristics of high-strength steel

Scanning laser-MAG hybrid welding with weaving laser was developed for the bead-on-plate welding of high-strength steel. Weaving laser exerted little effect on the droplet transfer behaviour during the scanning laser-MAG hybrid welding process. In order to achieve the keyhole mode of laser welding, the weaving amplitude should be no more than 2 mm. The weld penetration decreased with the increasing weaving frequency. Compared to laser-MAG hybrid weld, the ratio of arc zone width to laser zone width decreased significantly. Scanning laser-MAG hybrid welding could suppress the porosity defects of weld efficiently. The optimal weaving frequency for the porosity defects suppression was 20 Hz. However, the porosity defects increased sharply with the weaving frequency of laser over 40 Hz. The impact-absorbed energy value of specimen fabricated by scanning laser-MAG hybrid welding was about 46 J, increased by about 31.4% compared to about 35 J of laser-MAG hybrid welding.