# Effect of Laser Treatment on Intrinsic Mechanical Stresses in Titanium and Some of Its Alloys  
[PERSON]  
Department of Biomaterials Technology, Institute of Manufacturing and Materials Technology, Faculty of Mechanical Engineering and Ship Technology, Gdansk University of Technology, 80-233 Gdansk, Poland; [EMAIL_ADDRESS] (M.J.); [PERSON]@pg.edu.p1 (M.B. ); [PERSON]@pg.edu.p1 (D.B.K. );  
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[PERSON]  
Department of Biomaterials Technology, Institute of Manufacturing and Materials Technology, Faculty of Mechanical Engineering and Ship Technology, Gdansk University of Technology, 80-233 Gdansk, Poland; [EMAIL_ADDRESS] (M.J.); [PERSON]@pg.edu.p1 (M.B. ); [PERSON]@pg.edu.p1 (D.B.K. );  
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[PERSON]  
Correspondence: andrzej.zielinski@pg.edu.p1; Tel. : +48-501329368  
[PERSON]  
Department of Biomaterials Technology, Institute of Manufacturing and Materials Technology, Faculty of Mechanical Engineering and Ship Technology, Gdansk University of Technology, 80-233 Gdansk, Poland; [EMAIL_ADDRESS] (M.J.); [PERSON]@pg.edu.p1 (M.B. ); [PERSON]@pg.edu.p1 (D.B.K. );  
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Footnote 1: email: magdalena.jazdzewska@pg.edu.p1 (M.J.); michal.bartmanski@pg.edu.p1 (M.B. ); [PERSON]@pg.edu.p1 (D.B.K. );  
cessing parameters [9], simultaneously increasing the roughness. Using femtosecond laser line scanning, textured surfaces are created with some microcracks [10]. Cavities sized 20-30 nm were generated by laser irradiation [11]. The laser treatment changed the mechanical properties of the surface layer increasing the microhardness and toughness, and causing degradation under local loading [12; 13]. The laser texturing also influenced surface residual stresses [14] and laser-nitrided treatment caused compressive stress, as demonstrated by the cracks that developed in the nitrided layer [15], and also residual tensile stress [16]. The most important process parameters shaping the topography include laser power, the number of pulses, and laser spot [14]. The laser's lower speed and higher number of scans increase the roughness [2]. However, in [17], the increase in tensile strength, but not in Young's modulus and microhardness values, were observed. In contrast, in the other research [18], the surface microhardness was shown to increase following laser treatment from 351 to 445 HV. The fiber laser, when applied for remelting a Ti6 Al4V surface, created the appearance of a smooth, glossy, and crack-free surface. No distinct correlation between mechanical and tribological behaviors was observed in the nano-indentation tests [19]. Laser treatment can also affect wettability. A hydrophilic titanium surface was obtained using a femtosecond laser [9]. The hydrophilic surface was observed after modification with a femtosecond laser, with the water contact angle decreasing with the increasing laser fluence [10]. In the other research, all samples exhibited super-hydrophilic properties [16]. Two types of titanium surfaces were fabricated by femtosecond laser irradiation and the laser-treated surface was then subjected to either NaOH hydrothermal or H\({}_{2}\)O\({}_{2}\) oxidation treatments; the former surface was hydrophilic and the latter surface changed to superhydrophobic after 50 days of exposure. The changes in contact angle values were supposed to result from the adsorption of organic compounds present in the atmosphere [20]. For the nitride layer on the titanium alloy subjected to the action of a nanosecond laser, the wettability of microtextured samples was improved. The increasing hydrophilicity caused by laser irradiation was often observed and ascribed to the presence of a complex topography with numerous ripples, nanopillars, and microcolumns on the titanium surfaces [2; 3]. Corrosion was seldom investigated. In [21], the laser-textured titanium alloy showed improved corrosion resistance. The chemical composition was slightly related to the laser action. After laser processing, the oxygen content was higher in the micro-protrusion than in the micro-groove [9]. Excessive oxygen was appointed within the subsurface layer [16; 22].