Advanced Materials Research Vols. 139-141

Abstract: In order to develop high density powder metallurgy forming technology, a new concept combining high velocity compaction and warm compaction called warm high velocity compaction (WHVC) was presented. A new warm high velocity compaction forming equipment which adopts gravitational potential energy instead of hydraulic cylinder as hammer driver was designed. By means of the newly developed equipment, a preliminary study on warm high velocity compaction was performed. 316L stainless powder compacts with green density of 7.47 g/cm3 were obtained; the density is much higher than those prepared by conventional high velocity compaction. These results demonstrate that the newly designed equipment can basically meet the demand of warm high velocity compaction and the new forming method is superior to the conventional high velocity compaction. In addition, Densification mechanism of WHVC was also discussed.

Abstract: Based on the finite element theory of dynamic explicit, the technology of multi-point forming was applied to tube forming. The process of tube multi-point forming was simulated by ABAQUS software. It is used to study the behavior of forming force under different wall thicknesses and materials for manufacturing tube. It has been found that when the curvature radius is set at 1.0m, the forming force gradually becomes larger as the wall thickness of the tube increases. The careful selection of materials is beneficial to controlling forming force. Due to the technology of mass scaling, the ultimate forming force is not the maximum forming force in the process of simulation forming. However, the ultimate forming force can be used to select the multi-point forming equipment.

Abstract: The glass forming ability, thermal stability and non-isothermal crystallization kinetics of Zr64Al10.1Cu11.7Ni14.2 glass forming alloy were investigated. Its maximum glass forming dimension is up to 5mm and its critical cooling rate is less than 40Ks-1. The apparent activation energies derived from the Kissinger for Eg, Ex, Ep1 and Ep2 are 244.97±12.90, 264.63±10.18, 268.75±40.10 and 222.34±24.12 KJmol-1, respectively. The fragility parameter m is about 20.27, indicating its better thermal stability and glass forming ability.

Abstract: The ribbon samples for Cu60Zr30Ti10 (at. %) with a thickness of 50 μm and a width of 13 μm were prepared by melt spinning at the wheel speed of 30 m/s. The glassy structure was confirmed by X-ray diffraction (XRD) using Cu Kα radiation. The corrosion property and corrosion mechanism in 1 M HCl, 1 M NaCl, and 0.5 M H2SO4 solutions were investigated by electrochemical polarization measurement. The corrosion mechanisms are dominated by the pitting corrosion in H2SO4 solution and the pitting and uniform corrosion in HCl and NaCl solutions. In addition, the corrosion resistance in H2SO4 solution is better than that in HCl and NaCl solutions. Cu60Zr30Ti10 glassy alloy in H2SO4 solution is nobler than in HCl and NaCl solutions.

Abstract: The rotary-draw bending process of thin-walled rectangular tube is a complex process with large deformation, large displacement and finite strain which makes the defects, such as wrinkling, distortion and crack be easily produced. To study the complex plastic deformation characteristics of thin-walled rectangular 3A21 aluminum tube during rotary-draw bending, the distributions of tangential, circumferential and normal strain have been studied by experiment using grid method. The results show that the maximum value of three-direction strain appears in the ridges of rectangular tube and tangential strain is the maximum principal strain. The value of maximum tangential tensile strain is lager than the maximum tangential compressive strain; it is the reason why crake is prone to occur during rotary-draw bending. The strain distribution curve along bending direction is not completely symmetrical with middle cross-section. With the increase of bending angle, three-direction strain of tube also increases and circumferential strain increases relatively smaller.

Abstract: During filling process of vacuum counter-pressure casting, the pressurizing velocity is one of the most important parameters, and it affects whole filling process of molten metal and the quality of castings, the pressurizing velocity must be reasonably controlled and chosen. Through analyzing the pressurizing velocity of vacuum counter-pressure casting filling process, the results show that pressurizing velocity is related with absolute temperature of down kettle gas, radius of rising tube, initial volume of down kettle, height from original position of molten metal to top of rising tube and gas flux. When other conditions are same during filling process of vacuum counter-pressure casting, pressurizing velocity is proportional to gas flux, and it is linear. Meanwhile, through experimental verification for relationship of pressurizing velocity with gas flux, their actual relationships and mathematical models are obtained, and correction coefficient is determined at the range from 0.95 to 0.98.

Abstract: In this paper, the hot forging behavior of Mn18Cr18N steel is studied based on the viewpoint of meso-damage. According to high temperature tensile stress - strain curve of Mn18Cr18N steel under different temperatures, the relationships of void initiation strain and fracture strain with deformation temperature are obtained. The temperature dependence of damage threshold is obtained at 950°C~1200°C for Mn18Cr18N steel by Cockcroft & Latham criterion, That is, Ci,f value increases monotonously with temperature increasing. The corresponding deformation amounts of void initiation and fracture appearing are predicted accurately by combining Cockcroft & Latham criterion with finite element analysis during hot forging of Mn18Cr18N steel, which is in good agreement with the actual production. Under these experimental conditions, once upsetting deformation amount should be controlled in 50% based on hot upsetting test and finite element simulation. These provide theoretical instruction for preventing forging cracking of Mn18Cr18N steel.

Abstract: Hot-compression experiments of 316LN stainless steel have been conducted on a Gleeble-1500D thermal-mechanical simulator. We have analyzed the flow stress-strain curve and acquired the constitutive equation of 316LN steel by calculating stress exponent, activation energy and Zemer-Hollomon parameter. Then, based on the material model theories and Prasad instability criterion, the iso-efficiency map at strain 0.6 of 316LN steel has been developed. The larger power dissipation rate is emerging at 1050~1200°C and lower strain rate. In addition, we have also analyzed the hot deformation microstructure mechanism of 316LN steel is discontinuous dynamic recrystallization by the observation of deformation microstructure. These results have been of great significance to understand microstructure evolution and to determine the optimum hot-working conditions in the production.

Abstract: Magnesium alloy sheets have poor formability at room temperature, and the hydraulic forming may be a choice to improve its formability. Experiments were conducted to investigate the deformation behaviors of AZ31B magnesium alloy sheet in radial pressure deep drawing, one of the hydraulic forming technologies. The influence of hydraulic pressure, die dimension, blank dimension on the limit drawing height (LDH) of the drawn workpieces is investigated, and the location, evolvement and pattern of the fractures are also analyzed. The research results show that the obtained LDH increases with the hydraulic pressure when the punch corner radius is large enough, the formability can be improved only under a proper moderate hydraulic pressure when the punch corner radius is small; moreover, the location of the fractures may be around the corner, on the flange and the sidewall of the drawn workpieces, which appears different evolvement directions and patterns through cross-section.

Abstract: The technological adaptability of the reclaimed sand applied in the chemical bonded sand was studied in this paper. The reclaimed sand can be obtained by the process of high-temperature baking followed by mechanical reclamation and separation of micro-powder. The reclaimed green sand which containing different types of chemical bonded core sand were applied in corresponding system of chemical bonded sand process, such as the process of cold-box sand, hot-box sand, precoated sand, sodium silicate sand and self-hardening sand. Through technological experiments, it can be found that the tensile strength value of the precoated sand and sodium silicate sand which were prepared using reclaimed sand can be equal or slightly higher than that of prepared using the base sand, the tensile strength value of the cold-box sand, the hot-box sand and the self-hardening sand which was prepared using the reclaimed sand are lower than that of prepared using the base sand. Compared with the chemical bonded sand requirements in the foundry practice, the cold-box sand, the hot-box sand, the precoated sand which was prepared using the reclaimed sand can still meet the casting process needs, but the self-hardening sand which was prepared using the reclaimed sand cannot meet the casting process needs.