Comparison of high power diode laser and Nd:YAG laser microwelding of k-type thermocouples

k-Type thermocouple (NiCr–NiAl) joints are currently manufactured by argon arc welding. One of the weaknesses of this joining method is that the weld bead diameters are about three times of the thermocouple wires. This slows the response time of these thermocouples. Furthermore, porosity exists in the joints affecting the conductivity and repeatability for temperature measurement. In this paper, the feasibility and the characteristics of welding 0.2 and 0.5 mm diameter k-type thermocouples using a 120 W diode laser and a 400 W Nd:YAG laser are reported. The energy density requirement and the effects of shroud gas are described and analysed quantitatively. The performances of the laser welded thermocouples are evaluated against the commercial ones. Reductions of 35 and 15% in response time have been achieved compared to the commercial and the Nd:YAG laser welded 0.2 mm diameter thermocouples, respectively, whereas for the 0.5 mm diameter thermocouples, the HPDL welded ones performed 5% better and the Nd:YAG welded ones performed 9% better. The tensile strength of the diode laser welded thermocouples was found to be around 1620 N mm−2 at 0.2 mm diameter and 480 N mm−2 at 0.5 mm diameter. This is well above the industrial standard of 400 N mm−2. The work shows that diode laser welding can generate beadless and porosity-free butt joints in the thermocouples superior to the ones produced by argon arc welding.