Materials Science Forum Vol. 749

Abstract: The effects of key process parameters including initial lengths of tube blank, forming velocity and loading paths on hydroformability of stainless steel tubular components with rectangular section were systematically investigated. The results showed that sufficient axial feed must be given before the tube contacting the surface of die in order to prevent the excessive thickness thinning, especially for longer tube blank. The lower loading velocity led to enhanced formability. It is more important that pulsating loading path observably improved the formability of 304 stainless steel.

Abstract: Annular electromagnetic stirring (A-EMS) process, developed by General Research Institute for Non-Ferrous Metals, is an advanced processing technology for conditioning liquid metal prior to solidification processing. A-EMS can avoid skin effect effectively, impose a higher shear rate and a higher intensity of turbulence to the liquid metal, and increase the area of the melt heat dissipation by means of innovatively combining noncontact electromagnetic stirring and an annular chamber with specially designed profiles. The solidification structure, composition macro-segregation and property of 7075 alloy were investigated under the conditions of the conventional casting, the conventional electromagnetic stirring casting and A-EMS casting. The results indicated that A-EMS melt processing technology had significant effects on grain refinement, reduction of composition macro-segregation and improvement of mechanical properties.

Abstract: The traditional forming technologies of stainless steel tapered tube have some disadvantages, such as its complicated processes, few product specification, low production efficiency and high production cost. Dieless drawing is a new kind of near net-shaping process for fabricating tapered metallic tubes with large deformation per pass, high yield ratio of material and flexible production. Dieless drawing of stainless steel tapered tube was studied systematically in order to establish both temperature field model with respect to the changes of boundary condition and metal flow velocity during deformation, and a drawing speed controlling model with respect to the volume change of deformation zone. The models were verified by experiments in the present investigation. Finally, a continuous pickling process with variable-speed was proposed for tapered metallic tube, which was successfully applied to surface oxidation film removal of the stainless steel tapered tube.

Abstract: The hot deformation behavior of hypereutectic aluminium-silicon alloy was investigated by thermal simulation test at the deformation temperature of 330-480 and the strain rate of 0.1-10s^-1 using the Gleeble-1500 thermal mechanical simulator. The relationship of flow stress, temperature and strain rate was appropriately described by the deformation constitutive equation, and the deformation activation energy is 187.418 KJ/mol. In addition, the microstructures of these specimens were analyzed and the result showed that the inhomogeneous deformation enhances with increasing strain rate and decreasing deformation temperature, and the presence of primary silicon had a strong influence on the uneven deformation.

Abstract: The solidification microstructure and crystallization of Cr-Co-Mo-Ni bearing steel were simulated in solidification process based on CAFE methods in the electro-slag remelting process. The optimization of the intensity of cooling and molten pool temperature was simulated for Cr-Co-Mo-Ni bearing steel. The result shows that raising the cooling intensity to 7.5 m³/h and reducing the superheat temperature to 1600 can increase the grain nucleation number about 13.83%, and reduce the average columnar grain radius about 10.61%, to have a very good effect on refining the grains and get dense uniform solidification, which improve the homogeneity of the solidification structure, and reduce the performance differences of the materials.

Abstract: The microstructures and mechanical properties of the Cu-17Ni-3Al-X alloy extruded at different temperatures were investigated by hardness and tensile tests, optical microscope and scanning electronic microscope. The experimental results showed that dynamic re-crystallization occured during the hot extrusion at 1000 . The grain size of the extruded alloy was significantly refined and the mechanical properties increased remarkably compared to the as-cast alloy. The alloy extruded at 1075 exhibited good mechanical properties with tensile strength of 994 MPa, Brinell hardness of 296 and elongation of 8.0%, which are 30%, 9% and 285% higher than that of the as-cast alloy.

Abstract: Cu-12 wt.%Al (Cu-12Al) alloy fabricated by continuous unidirectional solidification (CUS) process has high elasticity, high strength, high conductivity, high plasticity and other good comprehensive properties, which has a potential to develop into a high-performance elastic material as the alternative to beryllium bronze. Especially, the room temperature tensile elongation of the columnar-grained structure (CGS) Cu-12Al alloy with high <001>β orientation along the axial direction exceeded 20%, which is six times more than that of the ordinary polycrystalline structure (OPS) Cu-12Al alloy (ε=2-4%) fabricated by traditional casting. The CGS Cu-12Al alloy shows lower flow stress and work-hardening rate during plastic deformation, which determines a dramatic improvement of workability comparing with the OPS alloy. In the present paper, both the effects of the rolling process and its parameters on the microstructure and properties of the CGS Cu-12Al alloy sheets were investigated. The results showed that the CGS sheets (σ_b=310 MPa, ε=19%) have the maximum reduction of 80% and remain columnar-grained structure after the first rolling pass at 700. After deformation the tensile strength was 401 MPa and the tensile elongation approached to 14%. Then, the hot rolled sheets were rolled again through the second pass at 325 and 700, respectively. The maximum reduction of CGS sheets at 325 warm rolling was 20%, the sheets remain columnar-grained structure and β₁'α₁' martensitic phase transformation occured; The second pass hot-rolling at 700 showed that the maximum reduction exceeded 60%, and the recrystallization occured in the sheets. After two passes of hot-rolling (total deformation was 92%), although the recrystallization occured, the CGS sheets remained columnar-grained structure (CGS) high tensile elongation (8.3%), which is two times more than that of the as-cast OPS sheets, and is five times as much as the OPS sheets with the same total deformation.

Abstract: Due to the excellent casting performance, good corrosion resistance, high strength and casting manufacturing costs, A356 casting aluminum alloy is widely used in automobile wheel industry of China. However, for the restrictions of the production equipment, technology and the production craft level, there are problems that product quality is not satisfied and the production efficiency is low in the mass manufacturing. In this paper, the effect of pouring temperature and mold temperature on the microstructure and mechanical properties were investigated on the basis of cooperation project with Bin Zhou wheel hub manufacturing company. The quantitative relationship between mechanical properties and microstructure was studied by statistical methods. The results indicate that pouring temperature is the main element to affect the mechanical properties of permanent mold casting A356 and the optimum pouring process parameter is about 744 .Mechanical properties have a liner relationship with secondary dendrite arm spacing in a certain extent.

Abstract: In the low pressure casting process of A356 aluminum alloy wheel hub, casting defects including shrinkage cavity, shrinkage porosity, impurity and pore usually occur inside the casting. These defects affect the mechanical properties of the casting. To solve this problem, we conducted a study based on a cooperation project with a well-known domestic automobile wheel manufacturer. In the present study, uniaxial tensile test of aluminum alloy casting containing defects was simulated and analysed, and the effect of types and number of defects on mechanical properties was studied by finite element analysis software. Statistical analysis of the data was provided by the manufacturer. It has been found that the degassing technology is effective by the quantitative analysis method. Based on the analyses of experimental data and the numerical simulation it is deduced that the tensile strength of casting increases with the increase of the defects due to the presence of impurity. This was confirmed in this research project, it has been observed that the defect rate of the casting sample is reduced from 5%-6% to less than 1%.

Abstract: A-TIG welding for Cu-Cr-Zr copper alloy was carried out on the eleven pure surface activating fluxes including oxides, fluorides and chlorides on the welding formation and the weld D/W ratio by the means of super depth of field and metallurgical microscope. It was found that the oxides and fluorides could improve welding penetration remarkably, especially with CaF₂,ZnF₂,SiO₂ andCr₂O₃. Four compound fluxes were chosen from the eleven pure surface activating fluxes, which could increase the penetration and modify the welding shape. An optimized formula composed of the four compounds (CaF₂,ZnF₂,SiO₂ and Cr₂O₃) was developed by using the formula of uniform design method. It showed that the optimized formula could reach a good weld ability and its weld penetration was 2.5 times more than that of TIG.