Effect of porosities on brazed martensitic steel tensile properties: 2D and 3D pre-mortem vs post-mortem characterizations

A quantitative investigation of the porosity and its effects on mechanical properties was conducted on brazed materials presenting macroscopically brittle behavior with various defect levels. Three different techniques for evaluating the porosity and the fracture mechanisms were compared: (Opre-mortem X-ray computed tomography (XCT), (ii) post-mortem laser scanning confocal microscopy (LSCM), and (iii) post-mortem scanning electron microscopy (SEM). The apparent surface area of the pores was evaluated by segmenting the height maps produced by LSCM and the electron backscattered images produced by SEM. The results show good agreement for different porosity levels. The LSCM height maps for opposite fracture faces were virtually stitched together in order to reconstruct three-dimensional (3D) images of the whole brazed joints. These proved to be similar to the XCT 3D segmented images, yet with some discrepancies due to local plastic deformation. Quantitative volume fraction and spatial distribution of pores were assessed as well as the identification of fracture mechanisms. Interestingly, it was noted that two specimens with different ultimate tensile strengths presented the same porosity amount, but different distributions. Two distinctive features were identified in the broken specimens: plastically teared pillars and walls that were separating the pores before fracture, and plateaus of approximately constant height with micro-sized dimples showing interfacial fracture. Although the specimens exhibited a macroscopic brittle behavior, these features are characteristic of ductile fracture. The most probable scenario for fracture was conjectured as follows: (i) Final fracture occurs in the near-surface region of the samples and conversely early damage develops in the volume, (ii) Interfacial fracture within plateaus is responsible of the rupture/strength of the brazed specimen but detrimental effect is induced by the tearing if highly voided regions are present in the joint.