Papers by Author: Jing Yuan Li

Abstract: The microstructure, texture and basic stamping properties of AZ31 magnesium sheet are studied in this paper with optical microscopy (OM), scan electron microscopy (SEM), X-ray diffraction (XRD) and MTS universal tester. The results show that the basal texture (0002) is enhanced and the grain size decreases as the thickness of sheet decreases. However the texture depresses in the 0.52mm sheet due to the complete recrystallization under deep rolling reduction. The strength and elongation decreases and the strain hardening index (n value) increases as the thickness increases. The precipitation decreases and the basal texture depresses after annealing treatment. The peak value ratio of crystal plane (0002) of 1.3mm sheet reduces from 67% to 53% when annealing treated at 300°C for 2h. In addition, the strain hardening index (n) value reduces from 0.2535 to 0.2053 and the anisotropic index (r) value increases from 1.966 to 2.108 when the sheet is treated at the same condition.

Abstract: In the present work, a two-dimensional finite difference model is built to realize the online control of the aluminum ingot tapered cooling process. The temperature evolution of aluminum ingot in the tapered cooling process is analyzed. According to the feature of the tapered cooling process, an efficient and simple algorithm of cooling time is developed. Online application shows that the efficiency and the accuracy of the control model can meet the production requirements.

Abstract: In the present work, a finite difference model (FDM) is built to predict the transient temperature field of the aluminum billet in the gradient cooling process. The billet is divided in 5 segments, each of which is corresponding with one water cooling ring, and all of which are water-cooled for different times respectively in order to obtain the target temperature gradient. On the following air cooling stage, the temperature of the outer layers, especially the surface, rises first, then falls slowly. With the rising of the surface temperature, the temperature deviation from the center to the surface is narrowed intensively. In comparison with the commercial finite element method (FEM) software, Deform 3D, the FDM model gets nearly the same prediction results as FEM simulation does.

Abstract: The hot plasticity of Mnl8Crl8 high nitrogen austenitic stainless steel is studied using the thermal simulated test in this paper. In addition, microstructural evolution and precipitate morphology are investigated through optical microscope(OM), scan electron microscope(SEM), X-ray diffraction(XRD) and Electron probe micro-analyzer(EPMA). The results show that the area reduction of Mnl8Crl8 steel varies in a “V” shape in the elevated temperature region. It means that plasticity of Mnl8Crl8 steel is relatively low when the deformation temperature is in 1000~1100°C, whereas an excellent plasticity can be obtained in 850~950°C and 1150~1200°C. The peak value of area reduction appears at 1050°C. The analysis of SEM on fracture morphology indicates that the inter-granular fracture and dimple fracture occur at 1050°C and 1150°C respectively. The results of OM exhibit that the dendrite precipitation occurs while the precipitate in grain boundary solutions as the temperature increasing and the grain coarsens immensely beyond 1000°C. However, dendrite precipitate solutions into the austenite matrix at further elevated temperatures. It causes the plasticity substantial improvement in 1150~1200°C. Moreover, the observed precipitation of the steel is confirmed to be Cr23C6 by XRD and EPMA.

Abstract: Two groups of magnesium alloys with various Al and Zn components are studied in this paper. One group of alloys are constant Al content of about 6% and various Zn content from 0 to 3%, another group are constant Zn content of about 0.4% and various Al content from 0 to 6%. The microstructures and mechanical properties of these alloys are investigated in as-cast and homogenized at 380°C for 15h. The results show that the tensile strength increases but yield strength decreases after homogenizing treatment. It can also be found that the morphology of second phrase and the size of grain exert the more effect on the mechanical properties than Zn content does. The alloys with uniform, fine and non-dendrite microstructure exhibit both high strength and elongation regardless of Zn content. On the other hand, the tensile strength and yield strength elevate significantly as Al content increases, and the elongation has a peak value in Al content of about 1.90%. The results show that the as-cast magnesium alloys with Al content of 5.6~6.0% and Zn content of 0.6~1.0% exhibit the best comprehensive mechanical properties.

Abstract: Thixoforming or Semi-Solid Metal Forming offers many advantages in comparison with casting and conventional forging. The purpose of the present study is to provide the basic microstructure and deformation data for austenitic and ferritic stainless steel under mushy state. As well known, the stainless steels solidify in different modes according to the different chemical compositions. In this paper, microstructural evolution of austenitic stainless steel type 304 which solidifies in FA mode ( L → L +δ → L +δ +γ →δ +γ →γ ),austenitic stainless steel type 310S which solidifies in A mode ( L → L +γ →γ ), and ferritic stainless steel type 430 which solidifies in F mode ( L → L +δ →δ )
are investigated during partial remelting by way of SIMA (Strain Induced Melted Activation). The results show that A and F mode of stainless steels melt directly at the grain boundary without phase transformation during reheating. A banded structure, originating from the primary dendritic segregation of the original ingots, is observed in type 310S steel during further heating. On the other hand, a perfect globular and insegregative two-phase semi-solid structure L +δ can be obtained while heated beyond the banded three-phase L +δ +γ semi-solid state in FA mode austenitic stainless steel type 304. This spheroidization can be attributed to the peritectic reaction occurred in the L +δ +γ semi-solid state. In addition, simple compression tests of these alloys in semi-solid state for varied combination of deformation rate and deformation temperature are conducted to examine the deformation behavior of stainless steel. Flow stress curves exhibit abrupt change in various alloys, even though in the same alloy such as type 304, various flow stresses are observed according to the difference in inner microstructure or morphology. Stress of type 310S steel shows the most reduction as the deformation temperature increasing at the same strain rate condition. The Liquid is centralized to periphery by the compression force in all deformed test pieces. Fracture, observed in all alloys except type 304 steel in globular L +δ semi-solid state, should be resulted from the lack of liquid in L +δ +γ state of type 304 steel and solidification crack in type 310S and type 430 steel. Deformation of solid particles occurs only in L +δ +γ state of type 304 steel. Last in this paper, various deformation mechanisms are proposed for various microstructures.