Laser cladding has many advantages compared to alternative non laser surface treatment processes like thermal spraying or deposition welding. It is possible to apply the laser energy into the base material in highly localized way. Therefore it is possible to build up claddings with a comparably high spatial accuracy. However, the efficiency of the process is generally low. For example the energy in terms of conducted heat into the base material beside the melt pool is lost. It can be shown that the so called heat conductivity losses can be reduced by optimizing the process parameters like scan velocity and beam intensity. Moreover, it was found that by coupling significantly more energy into the powder, both the energetic efficiency and the deposition efficiency were increased significantly. In this publication it is shown that it is possible with right parameter adaptation to reduce the processing time while simultaneously increasing the energetic process efficiency.
Laser cladding is an innovative surface treatment process which has several advantageous properties like a reduced material distortion compared to conventional techniques. In this technique the cladding material is fed as a powder through the laser beam to the melt pool. For an optimisation of this process with respect to treatment time and efficiency a characterisation of powder size, distribution and velocity is crucial.
Holographic particle image velocimetry is a powerful tool for characterisation of particle distributions with respect to size, 3D-position and velocity. Due to the holographic recording principle 3D-information can be evaluated from just one hologram. Its major drawback, the time-consuming development and repositioning of the hologram plates, can be avoided using the well-known technique of digital holography. In this case the hologram is recorded by a CCD-camera and reconstructed numerically.
Common digital holographic particle measurements are performed using an inline configuration in order to minimise the experimental effort. In this case the measurements are limited to low-density particle fields due to increased noise generated by an overlap of real and virtual image in the reconstruction process.
In this paper the application of off-axis digital holographic particle velocimetry to the characterisation of powder distributions in a laser cladding process is presented. Besides the experimental realisation special emphasis is given to the numerical reconstruction of the 3D-position and velocity of the particles. In extensive tests the suitability of the proposed technique is demonstrated. In the powder measurements up to 300 particles are detected with diameters of about 100μm and characterised with respect to position in a volume of about 1cm3 from just one hologram. In addition the speed of the particles is determined by double pulse measurements.