Pyrometry in laser surface treatment

Abstract The pyrometers specially developed for laser machining were applied to analyse temperature fields in laser cladding. It is shown that 2D temperature mapping is useful to optimise the cladding parameters: zone of powder injection in relation to laser beam, temperature gradients and their evolution versus cladding parameters. Multi-wavelength pyrometer was applied to restore the value of true temperature that is useful to control melting/solidification when complex powder blends are used and when it is necessary to minimise thermal decomposition of certain compounds. The developed multi-wavelength pyrometer and the applied notch filters are appropriate instruments to measure the evolution of surface temperature produced by Nd:YAG laser pulses of millisecond duration. The variations of several characteristics of the thermal cycle, such as the maximum peak temperature, T max , the instant when melting starts, t m , the melt lifetime, τ lt , the duration of the solidification stage, τ s , with various energy inputs (in the range 10–33 J) and pulse durations (10–20 ms), have been determined for rectangular laser pulses. By appropriate modification of the laser pulse shape (keeping the same energy input and pulse duration), it is possible to realise rather different temperature profiles to vary the melt lifetime and the instant when melting starts. In order to minimise surface temperature variation, to minimise thermal decomposition of certain melt compounds and to increase the melt lifetime, it is necessary to apply higher energy density flux at the beginning of the laser pulse. To obtain a higher peak of the surface temperature, for the given energy input and pulse duration, it is necessary to apply higher energy density flux at the pulse end. This will minimise the melt lifetime as well. In general, it is possible to impose the instant when melt cooling starts and thus to realise an intensive melt cooling during laser irradiation. The above results correspond to the action of laser pulses in the millisecond range with relatively low energy density flux (4 · 10 8 –10 9  W m − 2 ) on metallic materials, whose thickness is larger than the heat affected zone (i.e. semi-infinite body from the heat transfer point of view).