# Failure Response of Simultaneously Pre-Stressed and Laser Irradiated Aluminum Alloys
## 3 Results and Discussion
### Rupture Morphology  
The previous Section 3.2 provides the strength degradation trend of tested specimens subjected to tensile preload and laser irradiation. However, no clear information on the failure mode and physical changes of the fractured specimens can be obtained from the presented results. Optical microscopy investigation was carried out to get more clear analysis of the failure mode and to explore the laser heating effects on the irradiated surfaces. Optical microscopy images for the lowest and highest employed laser power densities are presented below. Figure 5a-d represents the surface rupture morphology for Al-6061 under the condition of the \(1.1\,\,\mathrm{kW}/\mathrm{cm}^{2}\) laser power density with various preloaded speeds. According to the rupture morphology, the distinct necking in the irradiated area is observed for all of the preloaded speeds. The extent of necking seems to be near similar for all of the applied preloaded speeds. Moreover, on the rupture surface or on the near-fracture sections, no plastic changes, e.g., heat affected zone or any other deformation, appeared. At the macro-level, this rupture pattern indicates the brittle fracture mode. For the laser power density of \(2.7\,\,\mathrm{kW}/\mathrm{cm}^{2}\) in Figure 6a-d, generally, the heat-affected zone expanded in area by increasing the preloaded speed. Local combustion, melting, necking, thermal expansion and cracks can be observed on the irradiated surface. Moreover, for lower preloaded speeds (Figure 6a,b) due to thermal expansion, a small dimple is developed. For the \(2\,\,\mathrm{mm}/\mathrm{min}\) preloaded speed (Figure 6c), comparatively less pronounced effects are found, and the specimen's burn through instead of thermal expansion is more obvious. This behavior may be explained as, for the combination of \(2.7\,\,\mathrm{kW}/\mathrm{cm}^{2}\) laser power density and \(2\,\,\mathrm{mm}/\mathrm{min}\) preloaded speed, the resulting temperature rise and heating rate did not allow the heat accumulation and localized effects. Meanwhile, it boosted the thermal activation, dynamic softening and failure process. Figure 5: Rupture morphology of the Al-6061 specimens fractured under the conditions of \(1.1\,\,\mathrm{kW}/\mathrm{cm}^{2}\) laser power density and various preloaded speeds of (**a**) \(0.5\,\,\mathrm{mm}/\mathrm{min}\); (**b**) \(1\,\,\mathrm{mm}/\mathrm{min}\); (**c**) \(2\,\,\mathrm{mm}/\mathrm{min}\); (**d**) \(4\,\,\mathrm{mm}/\mathrm{min}\). Figure 7a-d represents the surface rupture morphology for Al-7075 under the condition of 1.1 kW/cm\({}^{2}\) laser power density with various preloaded speeds. For 0.5 mm/min, the rupture pattern is in round or necked shape without any distinct surface deformation, except necking. Moreover, near the fracture sections, grains can be seen on the specimen's surface. For higher preloaded speeds, the common feature is hole formation before complete rupture. The hole formation is attributed to ablation of the melted part, which increased remarkably for higher preloaded speeds. In addition, with the increase of preloaded speeds, on the near fracture sections, grains reduced in size, become diffusive and finally completely vanished for the 4 mm/min preloaded speed. For the laser power density of 2.7 kW/cm\({}^{2}\) in Figure 8a-d, generally, the heat-affected zone expanded in depth and length by increasing the preloaded speed. Local damaging and combustion, melting, necking, ablation and cracks may be seen on the irradiated surfaces. Moreover, near the irradiated area/fractured surface, the development of grains can be found, which becomes more distinct with the increase of preloaded speeds up to 2 mm/min. For the highest preloaded speed, the presented results of Figure 8d showed the relatively high plastic surface deformation instead of distinct grains. The increasing preloading speed makes the local effects expand and severe.