# Effect of Laser Treatment on Intrinsic Mechanical Stresses in Titanium and Some of Its Alloys
## 3 Results and Discussion
### Mechanical Properties
These discrepancies can be explained by different laser processing parameters and, only to some extent, different substrates, including the measurement technique, applied load, and indent depth, as discussed in detail below. As concerns the laser processing conditions, by increasing the number of laser pulses, the mechanical state of the adjacent area is more affected. The textured surfaces exhibited tensile residual stresses in the surface between the dimples and compressive residual stresses at the bottom of the dimples. The pulse period is very important for the width of the heat-affected zone and the level of residual stresses [34]. For example, for a femtosecond laser, such a zone is below 2 \(\upmu\)m; for a nanosecond laser, it increases to 40 \(\upmu\)m; and here, the applied pulse period was much longer. Thus, only the tensile stress induced by the femtosecond laser could be negligible if some special measures were not used. In the other research [40], compressive residual stress assessed at 350 MPa was observed in the near-surface zone. This maximum tensile stress of 100 MPa appeared at a depth of \(\approx\)2 mm, and at \(\approx\)6 mm, the residual stress again became compressive [36]. When increasing the laser energy from 6 to 8 J, the compressive residual stresses increased from 450 to 510 MPa and the affected depth from 500 to 600 \(\upmu\)m. In a similar laser treatment [41], a compressive \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multirow{2}{*}{\begin{tabular}{c} **Maximum Load** \\ **(mN)** \\ \end{tabular} } & \
\begin{tabular}{c} **Median of Forces at** \\ **the Assumed Depth** \\ **(mN)** \\ \end{tabular} &
\begin{tabular}{c} **Median of Resultant** \\ **Stress Value (GPa)** \\ \end{tabular} \\ \hline \multirow{2}{*}{BM1} & 50 & 4.48 & 2.03 \\ \cline{2-4}  & 200 & 5.16 & 2.34 \\ \hline \multirow{2}{*}{BM2} & 50 & 5.09 & 2.31 \\ \cline{2-4}  & 200 & 5.65 & 2.56 \\ \hline \multirow{2}{*}{BM3} & 50 & 11.36 & 5.15 \\ \cline{2-4}  & 200 & 13.89 & 6.30 \\ \hline \hline \end{tabular}
\end{table} 
Table 11: Differences between an imposed and residual force value and resultant residual stress values following stress relief annealing for different bare alloys and loadings. The medians are shown. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multirow{2}{*}{\begin{tabular}{c} **Maximum Load** \\ **(mN)** \\ \end{tabular} } & \
\begin{tabular}{c} **Median of Forces at** \\ **the Assumed Depth** \\ **(mN)** \\ \end{tabular} &
\begin{tabular}{c} **Median of Resultant** \\ **Stress Value (GPa)** \\ \end{tabular} \\ \hline \multirow{2}{*}{LT1} & 50 & 6.14 & 2.79 \\ \cline{2-4}  & 200 & 9.41 & 4.27 \\ \hline \multirow{2}{*}{LT2} & 50 & 5.35 & 2.42 \\ \cline{2-4}  & 200 & 8.77 & 3.98 \\ \hline \multirow{2}{*}{LT3} & 50 & 11.52 & 5.23 \\ \cline{2-4}  & 200 & 11.87 & 5.39 \\ \hline \hline \end{tabular}
\end{table} 
Table 12: Differences between an imposed and residual force value and resultant residual stress values following stress relief annealing for different laser-processed alloys and different loadings. The medians are shown. stress field was evident throughout the sample thickness at both locations, decreasing from a peak compression of 600 to 800 MPa at the surface to a minimum compressive stress of 200 MPa at the interior [31]. When increasing the laser power, a rapid increase followed by the decrease in residual compressive stress could be observed. This might be evidence that the laser energy can not only melt the metals, but it also can affect the dislocated structure and movement. We believe that the high values of our results, close to those already obtained [27], are specific to the test technique.