هادی قهرمان

In the last two decades, friction stir processing (FSP), as one of the prominent severe plastic deformation methods, has received much attention from researchers due to its unique advantages. However, so far there has been no study on the behavior of the FSPprocessed materials in the presence of cracks. So, this work investigated the mixed-mode (tensile-shear) failure, mechanical, and microstructure properties of the FSP-processed 6061 aluminum alloy, utilizing the essential work method of fracture. According to the tensile test, both the yield and ultimate tensile strengths of the processed sample showed an increase of 47% and 67%, respectively, due to the intensive grain refinement (57% reduction in grain size). Based on the fracture test, the essential work of fracture in the processed sample was 11 kJ/m2, which is 33% less than that of its initial annealed counterpart. Therefore, while the friction stir processing can substantially improve the mechanical strength of the 6061 aluminum alloy, its fracture toughness has significantly decreased.

Crack nucleation mechanisms, its growth, and the determination of critical failure parameters are of industrial importance. Therefore, it is inevitable to study the mechanical behavior of cracked materials under applied load during severe plastic deformation. The present study studied the mode I fracture behavior, mechanical properties, and microstructure characterization of the 6061 aluminum alloy fabricated by friction stir processing (FSP). For this purpose, a milling machine made a perfect stirring zone to perform the FSP method with the specific non-consumable tool. According to the tensile test results, the yield and ultimate tensile strength of the FSP-processed sample have increased by 39% and 37%, respectively. Based on the three-point bending test, the fracture toughness of the processed aluminum was calculated as 10.86 MPa√m, which shows a 14.3% improvement compared to the as-received annealed state. Eventually, the average grain size of the annealed and processed samples was measured as 35 and 15, respectively, which indicated a 57% reduction in the aluminum grain size after FSP. Note that this grain refinement is associated with improved strength and toughness.