PSI - Issue 69

Carlo Alberto Biffi et al. / Procedia Structural Integrity 69 (2025) 121 – 126

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By raising the power level (P=55 W) the peaks are more evident and during the cooling phase there is a shift of the M peak towards that of the R phase. The peak of phase A in the heating phase is instead unique and well defined. A further increase in power (P=65 W) causes continuity between the R and M peaks. The latter appears jagged in shape, indicating a probable formation of different phases that transform at distinct temperatures. This inhomogeneity in the material could possibly be caused by a partial melting of the material; this hypothesis requires further validation, e.g. via direct temperature measurements. However, the two main phases M and R remain distinct. Peak A during heating still remains very intense, but no longer as well defined as the previous case. At power of 75 W a further rapprochement on the temperature axis of the R and M peaks, which are no longer distinct from each other, was detected. On the contrary, the two peaks of M and R appear to be part of a single phase transformation. Peak A noticeably loses intensity and appears blunt in shape, indicating that the phase transition between M+R→A is no longer as clear as previously observed. In the last step preceding the melting of the NiTi wire (P=85 W), the M peak increases considerably in intensity and appears perfectly defined. The material is now in a solubilized condition, in agreement with literature data [6]. High values of power emitted are therefore comparable with the high temperatures of conventional heat treatment performed in an oven. From a quantitative point of view, Figure 4-5 show the trends of the characteristic transformation temperatures and the related phase transformation enthalpies, respectively.

Figure 3: DSC scans of laser treated wires at varying the laser power; the laser scans were carried out at 45 mm/s. The quantitative analysis shows how during cooling (Figure 4.a), the rhombohedral phase (R) is not present for the entire power range investigated, but only up to a certain power value (P=65 W). Beyond this value, the R phase is no longer distinguishable as it is united and coexistent with the martensitic phase. By increasing the power level, we notice how the R phase appears at slightly lower temperatures, while the M phase shifts significantly towards higher temperatures and then stabilizes. During heating (Figure 4.b) the R phase is present and recognizable only for the lower power level (P=45 W). The austenitic peak, always present when the laser power of the heat treatment varies, shifts significantly towards higher temperatures as the power increases. A similar analysis can be done for the trend of the total transformation enthalpy which, in both the cooling and heating phases, increases in magnitude up to the same power value (P=65 W) and then decreases slightly again. This indicates how the quantity of material involved in the phase transformation can be maximized, which is directly proportional to the transformation enthalpy. Table 3-4 report the detected values of the transformation temperatures and heats exchanged during the direct and reverse phase transformation for the laser treated wires at a scanning speed of 45 mm/s. It can be concluded that the laser condition (P=55 W) can promote a promising MT with transformation