The heat generated during friction stir processing greatly affects defects formation in the processed zone of workpieces. In this paper, numerical modeling of this process is performed to determine the influence of tool rotational and traverse speeds and hence their ratio on the thermal distribution attained during the process. The aim is to produce defect-free processed samples by selecting adequate tool speeds. The mechanisms of defects formation depending on the peak temperature are also investigated. Experiments to verify the simulation results were conducted with the same process parameters. Several traverse speeds of 20, 40, 60, and 80 mm/min and rotational speeds of 700, 1000, 1200, and 2000 rpm were used during modeling and conducting the experiments. From the numerical and experimental results, it was found that; the high-speed processing conditions (low-generated heat) can produce defects such as tunnels and grooves, and the low-speed processing conditions (high-generated heat) can cause defects such as flashes. The experimental results show that during friction stir processing with the rotational speed of 1200 rpm and the traverse speed of 60 mm/min (speed ratio of 20), no macro defects in the processed zone were observed. According to the numerical results, the peak temperature during friction stir processing with these speeds was 475 °C. At this temperature, the material softened, the structure finely equiaxed and no large scale melting zone appeared in the processed zone. The developed model can be useful to investigate the occurrence of defects associated with different tool rotational and traverse speeds. Graphic abstract

In this paper, the effect of multi-pass friction stir processing on mechanical properties of AZ91 alloy has been studied. For this purpose, the microhardness, tensile, and creep tests were conducted at several temperatures. Optical microscopy and scanning electron micrograph were used to study the microstructure of the processed samples. 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. Also, after friction stir processing, the tensile and creep strength of the samples at 210 °C increased by 31% and 47%. In addition, a three-dimensional model was developed to simulate two-pass friction stir processing using ABAQUS/Explicit software. This model involved the Johnson-Cook models for defining material behavior during the process and identifying the fracture criterion. To control the mesh distortion during consecutive passes, the Arbitrary Lagrangian-Eulerian technique was used. Using the developed model, the peak temperature, thermal distribution, and residual stress field during multi-pass friction stir processing on AZ91 have been studied. 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.

Demand for increasing strength to weight ratio, elimination of electromagnetic waves, and vibration damping has led to the wide application of magnesium-base alloys such as AZ91 in various industries like aerospace, military, vehicle, and shipbuilding. However, because of the unstable secondary particles and casting defects located on the rough grain boundaries and in the dendritic regions, due to sliding of the grain boundary, the creep resistance and tensile strength of Mg alloys at high temperatures reduce. To improve the high-temperature properties, rapid deformation processes such as friction stir processing can be employed. In this study, the influence of multi-pass friction stir processing on microhardness, tensile, and creep behavior of AZ91 at several temperatures from 25 to 210 °C has been studied. Optical microscopy and scanning electron micrograph were used to study the microstructure of the cast and processed samples and Clemex commercial software was used for grain size measurement. 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. In addition, after multi-pass friction stir processing, the tensile and creep strength of the samples at 210 °C increased by 31 and 47%, respectively. Also, the average grain size of the multi-passed friction stir processed AZ91 alloy decreased by 88%. The maximum ultimate tensile strength of 276 MPa was obtained at the tool rotational speed of 1200 r/min, the traverse speed of 60 mm/min, and the tool tilt angle of 3°. The empirical results indicated that this rapid deformation process can be useful in enhancing the mechanical properties of AZ91 alloy at high temperatures.