The structure of water-submerged FSP samples was more homogenous than that of the non-cooled processed samples however, the ductility of the specimens after submerged FSP dramatically reduced. Their results show that after FSP on AZ91, the rough casting structure converted into coaxial graining, and the lattice Mg 17Al 12 eutectic phases converted into pin-like particles on the grain boundaries. investigated the microstructure and tensile properties of AZ91 after water-submerged FSP and non-cooled FSP. reported that after FSP, the β-Mg 17Al 12 networks broke into smaller particles, but the microstructure of AZ91 alloys was not finely equiaxed. reported that after single-pass FSP on AZ91 alloy, the coarse dendrites in the α-Mg field refined slightly, the network of secondary particles dissolved, and the mechanical properties (such as tensile and impact) at room temperature improved. In recent years, many researchers have investigated the effect of single-pass FSP on the mechanical properties of different alloys. Recently, several works attempted to develop the performance of this process and to increase the mechanical properties after performing FSP. Nevertheless, the improvement of properties after FSP is highly dependent on the process conditions. Also, after FSP, the casting defects such as cracks, cavities, interconnection, non-adhesion between the field, and intermetallic phase particles, which cause stress concentration and failure decrease. It is expected that after FSP, due to the dissolution of unstable Mg 17Al 12 eutectic phases and distribution of secondary particles on the grain boundaries, the mechanical properties improve at high temperatures. For this purpose, the severe plastic deformation approaches such as friction stir processing (FSP) is used for solving this problem. Hence, this alloy cannot be used for making heavier engine components that require thermal stability up to about 200 ☌. However, because of the unstable secondary particles with low melting point (120 ☌) located on the rough grain boundaries and in the dendritic regions, the creep and tensile strength of this alloy at high temperatures reduce. Among the cast Mg alloys, AZ91 is the most popular alloy because of its good casting properties. In recent years, magnesium alloys have attracted increasing attention due to their low density and high specific strength, which provide considerable weight saving potential in automobile and transport industries. Key words: multi-pass friction stir processing / numerical modeling / tensile / creep / AZ91 alloy The empirical results indicated the beneficial influence of the multi-pass friction stir processing on the microstructure and high-temperature mechanical properties of AZ91 alloy. Using the developed model, the peak temperature, thermal distribution, and residual stress field during multi-pass friction stir processing on AZ91 have been studied. To control the mesh distortion during consecutive passes, the Arbitrary Lagrangian-Eulerian technique was used. This model involved the Johnson-Cook models for defining material behavior during the process and identifying the fracture criterion. In addition, a three-dimensional model was developed to simulate two-pass friction stir processing using ABAQUS/Explicit software. Also, after friction stir processing, the tensile and creep strength of the samples at 210 ☌ increased by 31% and 47%. The experimental results indicated that at room temperature, the microhardness, tensile, and creep strength of the processed samples as compared to the unprocessed ones increased by 23%, 29%, and 38%, respectively. Optical microscopy and scanning electron micrograph were used to study the microstructure of the processed samples. For this purpose, the microhardness, tensile, and creep tests were conducted at several temperatures. * e-mail: this paper, the effect of multi-pass friction stir processing on mechanical properties of AZ91 alloy has been studied. Hoda Agha Amini Fashami 1, Nasrollah Bani Mostafa Arab 1 *, Mohammad Hoseinpour Gollo 1 and Bahram Nami 2ĭepartment of Mechanical Engineering, Shahid Rajaee Teacher Training University, Tehran, Iranĭepartment of Materials Engineering, Shahid Rajaee Teacher Training University, Tehran, Iran
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