Search

Engineering stability control of microstructure evolution and precision forming is a key technical issue for the high-performance manufacturing of magnesium alloy components. In this paper, a fine-grain processing window was proposed for the workability optimization and microstructure control of AZ80 magnesium alloy. The hot activation energy map (AEM) and processing map (PM) were established to analyze the hot-working properties and microstructural mechanisms, respectively. The internal state variable (ISV) based finite element (FE) model was introduced to predict the evolutions of grain size and microstructure homogeneity with processing parameters during multi-angular extrusion process. The fine-grain processing window was used to form a magnesium alloy plate. Fine microstructure and good forming performance were obtained with the optimized processing parameters at the complete DRX domain. Forming at the processing instability domain exhibited flow defects and microstructure heterogeneity. The above research can be applied to the processing parameter optimization and microstructure regulation during hot forming processes of magnesium alloy.

The hot compression behavior of 316LN stainless steel for the supporting system in a magnet confinement fusion reactor was isothermally compressed at 1,050℃ and 0.1 s−1. Electron backscatter diffraction was used to study the microstructure and texture evolution during the deformation process. The results showed that the necklace structure is eventually formed by increasing compression strain due to dynamic recrystallization (DRX). The proportion of low-angle grain boundaries first increases and then decreases. The dominant DRX mechanism of 316LN is discontinuous DRX, which is characterized by the grain boundary bulging. Besides, twinning is found to be induced to accommodate the plastic strain, helping the development of DRX.

Different combinations of normalizing and tempering were carried out to optimize the microstructure and enhance the high-temperature mechanical properties of HRB400FR fire-resistant steel bars. The results showed that with the increasing of the tempering temperature from 400 to 600°C, the steel bar’s hardness decreases linearly, mainly due to the formation of quasi-polygon ferrite and granular bainite. Besides, the reduced width and the dissolution of the lath bainite also undermine the performance of the tempered steel bars. The highest Vickers hardness of 380 HV is achieved when the steel is normalized at 950°C and then tempered at 400°C, mainly due to precipitation strengthening and bainite strengthening. The hardness of the test steel tempered at 600°C gives the lowest value, only 230 HV since the least amount of bainite is obtained. When the tempering temperature reaches 650°C, the hardness rises to 260 HV due to the formation of the lath bainite. The emergence of needle bainite generally reduces the matrix grain size, and the appearance of lath martensite refines the precipitated carbides.

The precise forming process with fine-grain strengthening method of magnesium alloy is a common technical problem that needs to be urgently addressed for lightweight application. In this paper, a multi-angular extrusion process integrating severe plastic deformation (SPD) technique and hot forming process was proposed to manufacture the fine-grain plate of AZ80 magnesium alloy. Shear deformation path was introduced via the variable section and secondary angle of mold cavity. The unified internal state variable (ISV) based finite element (FE) model was established to simulate the multi-angular extrusion process. The interaction of viscoplastic flow and microstructure evolution was embedded into the user defined subroutine of DEFORM-3D software. The inhomogeneous strain distribution and microstructure characteristics during the extrusion process were investigated. The improvement of fine-grain homogeneity via the two-pass angular shear deformation was revealed. A fine-grain magnesium alloy plate with good forming performance and average grain size of 9 μm was manufactured. The yield strength is increased to 259 MPa by 23.3%, ultimate tensile strength is increased to 338 MPa by 27.5%, and elongation is increased to 12.9% by 122.4%. The developed multi-angular extrusion process can be applied to the integrated manufacturing of shape and performance of fine-grain Mg components.