Abstract
The improvement of energy efficiency and power in turbomachines (used in aeronautical industry as well as power generation industry) is related to the increase of their maximum work temperature. The increase in temperature causes, depending on the material, severe degradation due to corrosion, oxidation, erosion by solid particles and creep.
In order to reduce this material deterioration and to increase components durability, thermal barrier coatings (TBCs) are introduced. TBCs provide thermal insulation and protection against corrosion and erosion at high temperatures.
Typical material for high temperature applications requiring high corrosion resistance are titanium alloys, particularly Ti-6Al-4V alloy, but its use is limited to temperatures below 400°C. Therefore, the purpose of this research is the development of laser cladding coatings to be used on Ti-6Al-4V in order to increase temperature operation of these components.
Laser cladding technology allows the deposition of metal coatings providing high accuracy and low thermal effect of the base material. The process involves the melting of powder with high power lasers on a substrate, so that the dilution of the fed material (over 5%) provides excellent metallurgical coating adhesion.
Titanium-Aluminum intermetallic alloys are materials for high temperature applications (their operating temperature is between 600-760°C). TiAl has properties such as high melting point (1440°C), low density, high elastic modulus and good structural stability. However, the laser processing of TiAl is difficult due to its brittleness behavior, suffering a sharp contraction on cooling, commonly causing cracking in the coatings.
The aim of this work is to obtain and to analyze laser cladding coatings of TiAl alloy on Ti6Al4V through the study of its cooling rate during the process. For this purpose, the process was monitored with a dual-color pyrometer which provides time-dependent temperature of laser tracks.
Laser cladding is a multiple-parameter-dependent process. Cooling rate of laser cladding layers depend on the laser processing parameters involved: laser power, powder feeding rate, scanning speed and preheating temperature. For process optimization tests, power was set in the range of 500-900W, scanning speeds between 100-600mm/min and powder feeding rate between 1-4 g/min. Heating before and during tests (350-450°C) was used.
For the observation of possible macroscopic defects, the coatings were evaluated by penetrating fluid (NDT). Further microstructure and geometrical quantities (clad area and dilution) of the coating were characterized by optical microscopy. The number of cracks significantly decreased for lower cooling rates.
