# Effect of Laser Treatment on Intrinsic Mechanical Stresses in Titanium and Some of Its Alloys
## 3 Results and Discussion  
### Surface Topography and Composition  
The samples subjected to laser modification are presented in Figure 1. The surface of each sample was significantly modified creating distinct patterns composed of valleys and tops, as already observed [30]. The even textures were obtained by using the overlapping of a laser beam. In addition, blue or dark blue areas were visible on each material, resulting from local surface overheating and oxidation. The appearance of an oxide film on the titanium treated at a sufficient fluence is usual and often reported [9; 14; 23]. Such behavior can be explained by the oxygen adsorption by metal(s) melted by laser irradiation and eventually the formation of oxides [2]. Figure 2 shows the SEM-observed micrographs of laser-processed surfaces. The impacts of the laser beam on the surfaces are observed as the moderate destruction of the top layer and its partial porosity. Numerous short cracks in the surface layer appeared within the very thin subsurface layer. Such cracks are not often reported [31,32] as they are prevented by compressive stresses [33,34]. In [15], the formation of the cracks was attributed to the semi-brittle layer, which experienced high lateral tensile residual stresses. Figure 3 and Table 5 show the chemical compositions of samples after laser treatment. For sample LT2, the weight content of aluminum decreased by more than half, and a similar change was noted for niobium. After applying the laser treatment to the LT3 sample, the molybdenum-to-titanium ratio was significantly reduced. Thus, even qualitatively, all elements were present within the remelted zone; quantitatively, some of them could either evaporate or diffuse into a deeper part of the specimen. Moreover, the change in intensities of metallic elements was determined by the chemical and phase compositions of the laser-treated alloys. The small relative differences in EDS-measured contents of elements of the TC4 medical alloy showed a moderate decrease for all metals at the expense of oxygen [18], and this observed behavior can be attributed to the formation of oxides. The XRD spectra observed for each investigated surface are shown in Figure 3. The symbols (*) in the graphs indicate the reflections of the precipitates detected for individual materials, which do not result from the base material. In the case of sample LT2, the analysis allowed the identification of niobium oxide Nb\({}_{2}\)O\({}_{5}\) (_red_ *) appearing at 36.99\({}^{\circ}\) and 70.69\({}^{\circ}\) angles (according to JCPDS no. 30-0873); in sample LT1, titanium oxide TiO\({}_{2}\) (_black_ *) (anatase) was identified by peaks at 36.98\({}^{\circ}\) and 53.99\({}^{\circ}\) angles (JCPDS no. 21-1272), while in the LT3 sample, molybdenum oxides were identified MoO\({}_{2}\) (_green_ *) for 37.13\({}^{\circ}\) and 37.95\({}^{\circ}\) angles (JCPDS no. 86-0135) and MoO\({}_{3}\) for 39.23\({}^{\circ}\) (_red_ *) angle (JCPDS no. 05-0508). The appearance of oxides of alloying metals was then confirmed, as previously observed [14]. The microstructure and chemical composition created on the cross-sections of laser-remelted surface layers are shown in Figure 4. The EDS analysis of the chemical composition of the cross-sections of the remelted layers was conducted over a length of 150 \(\upmu\)m--starting from the near-surface zone, ending on the base material zone, and the averaged results from eight scanning lines were presented. These results confirm the presence of all significant alloying elements and their relative stability within the remelted zone. Thus, the applied pulse laser processing was relatively short and the fluence was small enough to avoid an excess diffusion of metals. Figure 1: The macrophotos of laser-processed samples: (**a**) Ti grade IV (LT1), (**b**) Ti7 Nb6 Al alloy (LT2), (**c**) Ti15 Mo alloy (LT3). Figure 2: Surface topographies of laser-processed samples (different magnifications for each pair of samples): (**a**,**a**_*_) LT1, (**b**,**b**_*_) LT2, (**c**,**c**_*_) LT3.