Powder-based Laser Metal Deposition offers opportunities for the build-up of complex structures with various materials, but improper process parameters can cause defects like lack of fusion and pores. Maintaining suitable melt pool characteristics is crucial, often requiring parameter adjustments due to local temperature and geometry variations.
Several developments have been made to ease the parameter selection. The numerical process simulation allows for the prediction of the melt pool evolution and can be used for the virtual optimisation of process parameters before the start of the actual deposition process. The results of the numerical simulation can be validated using in-situ monitoring systems, which are suitable for continuous examination of the melt pool size and offer a high potential to detect inappropriate melt pool characteristics. To assess the build-up process, the information gathered from monitoring systems and process simulation need to be linked with the formation of internal defects.
In this study, the complex time evolution of the melt pool characteristics, yielded by process monitoring and numerical simulation, is investigated regarding the detection of the internal defect formation. For this purpose, multi-layer sample parts of the material Ferro55 are manufactured and metallographically analysed. The variation of the melt pool properties in different layers is caused by the evolution of the accumulated sample temperature and the layer-wise adjustment of the laser power. The melt pool is monitored using a coaxial camera system. In some regions a steep decrease of the laser power leads to too excessive reduction of the melt pool size and to formation of lack of fusion defects.
The numerical analysis of the building process using a thermal FEM simulation predicts the transient temperature distributions and the progression of the melt pool size. The melt pool sizes in simulation and measurement show a very similar evolution. Through the comparison with the metallographic and computed tomography investigations of the sample part the critical size of the melt pool can be evaluated. The segment at which lack of fusion defects are visible coincides with the location at which the melt pool size is below a specific critical threshold value. To avoid defects, new process parameters can be designed via simulation.