The effect of an air gap on the properties of high power Nd:YAG laser welds

Laser welding gives designers increased freedom in material selection and joint design, in comparison with many conventional joining techniques. Butt, lap and T -joints are typically used when laser welding sheet materials. The presence of an air gap between the sheets affects weld quality - it is recommended that no air gap is present in a butt joint, if an acceptable weld is to be produced. Although an air gap should be avoided in lap and T – joints, to avoid weld defects, an air gap is necessary in some cases, e.g. when welding zinc-coated sheets. Data for acceptable air gaps are also needed in order to be able to specify appropriate fixturing, based on economic and technical criteria. If a small air gap is acceptable, expensive fixturing that ensures no air gap is not necessary.The results of an investigation into high power Nd:YAG laser welding of low strength mild steel, in sheet thicknesses of 0.75 and 1 mm, using lap and T-joints (welded in a stake joint configuration), are reported and discussed. Some of the specimens were zinc-coated. The effect of an air gap on the weld properties was studied in both joint configurations, in order to establish the fixturing accuracy required in practical applications, as well as its effect on the formation of pores when welding zinc-coated sheets. Laser power levels up to 3 kW, and welding speeds up to 10 m/min were used. The focal plane position was varied in order to control the weld bead profile to ensure a satisfactory weld section in the joint. The following were characterized from transverse weld sections: penetration and bead profile; geometry of the bead root and face; effective weld area in the joint with regard to the accommodation of weld defects. Tensile tests and hardness profiles were also made on some of the welds.The results show that in the case of a stake-welded T-joint, the most important factors controlling weld quality are the size of the air gap between the web and the flange, and the alignment of the laser beam with the web sheet. An air gap of 0.1 mm can be tolerated with a sheet thickness of 1 mm, provided that the laser beam is aligned with sufficient accuracy. The same air gap can be tolerated in a lap joint configuration with a sheet thickness of 1 mm. A zinc coating density of 100 g/m2 was used. No pores were found from a macroscopic examination of sections.