Processo di Riparazione Per Crescita Epitassiale di Superleghe A Singolo Cristallo Impiegate nelle Parti Calde Delle Turbine A Gas Dell'ultima Generazione

摘要

First stage rotating blades of gas turbines are now made with single crystal Nickel based superalloys. To reduce maintenance costs big efforts are presently devoted to find repair techniques for such materials, able to produce the same crystallographic orientation of the substrate in the repaired region (epitaxial growth). For such a purpose the laser cladding process can be used, if operating parameters are adjusted to obtain very high values of temperature gradient and low solidification velocities at the liquid-solid interface (conditions required by the epitaxial growth to occur). Starting from literature models an user-friendly software for PC was developed, able to drive the operator in the choice of operating conditions during the experimental trials of laser cladding repair. As input such a software requires data relative to the material and the experimental configuration of the laser cladding workstation. For any laser power value the outputs are the temperature distribution in the work-piece, the melt pool size and shape, thermal gradients and solidification velocity along the fusion line, but mainly the laser head speed and powder feeding values required to produce a fixed height. The alloy CMSX-4 was used for trials with an atomised self-similar powder. With a Nd:YAG laser ( Lumonics 1kW) epitaxial growths were produced, with a misorientation less than 13° on respect the substrate orientation. This result and the absence of small disoriented grains were evaluated by the electron backscattered diffraction technique (EBSD) in a scanning electron microscope with an eucentric specimen holder. Using the Nd:YAG laser an epitaxial growth was produced about 1.5 times higher than that obtained in literature with the same number of passes by a CO_2 laser.