ABSTRACT
Titanium alloys have excellent specific mechanical properties combined with high resistance to corrosion and oxidation. However, their wear behavior is poor and this limits their use in many industrial applications for marine, aerospace and petrochemical industries.
This thesis proposes the use of laser coating or "laser cladding" to deposit layers of Ti6Al4V matrix composite reinforced with different contents in weight of hard ceramic particles of titanium carbide (TiC) in order to improve its performance against wear.
This is an experimental work since the titanium complex has an enormous chemical reactivity with the atmosphere that greatly hinders its laser processing. For this reason, a table of experiments has been designed to study the laser-processed in several stages of increasing complexity as well as to analyze the effect of variables separately.
The experimentation has been divided into four stages: first the effect of laser processing variables to coat Ti6Al4V with Ti6Al4V powder processing has been analyzed in order to obtain the processing window, then the effect of the addition of different fractions of TiC has been studied. Coatings were produced by overlapping strings, thirdly the wear behavior in dry conditions has been characterized to determine optimal processing conditions, finally the metallurgical transformations that occur in the coatings considering as optimal in order to propose improvements have been studied in depth.
The experimental results show that the laser processing of titanium requires a comprehensive study to optimize the processing conditions. Only through a proper experimental design has been possible to triple the productivity of the process and get free Ti6Al4V coating defects such as cracks, pores and oxidation. In this sense, the use of helium as a shielding gas is an absolute advantage compared to argon because of its higher ionization potential to prevent the formation of plasma.
The wear resistance is strictly dependent on two factors: the content of TiC and laser processing parameters. Not only is essential to maximize the content of hard particles but also to maintain the dissolution of TiC in a reasonable limit that allows good coating-substrate metallurgical bonding avoiding the massive dissolution of TiC.
The detailed analysis of the metallurgical transformations shows that TiC particles dissolve with the laser heating effect, then precipitated as non-stoichiometric titanium carbide (TiCx) with a lower carbon content. This new compound precipitates in the form of small globules of lower hardness and elastic modulus. Therefore, with the right combination of TiC and TiCx a composite coating can be achieved whose hardness and toughness is optimal for maintaining a high wear resistance combined with the absence of cracks.