Materials Science Forum Vol. 850

Abstract: For the cold-rolled silicon steel strip lubricated with emulsion, the typical oil spot defects always can be found on the surface of rolled strip. The oil spot are parallel to the rolling direction. In the present study the micro-structure of oil spot defects was investigated by several surface analysis techniques, including LEXT ols4000 laser scanning confocal microscope, scanning electron microscopy (SEM), energy dispersive spectrometer (EDS). The chemical compositions of the oil spot defects were analyzed by EDS. The results showed that the surface quality of the rolled non-oriented silicon steel was affected strongly by emulsion stability. The emulsion stability decreased with the increasing content of CL^-. When the particle size of emulsion was larger, the lubricity of the emulsion deteriorated. Furthermore, there were some emulsions which contain miscella on the strip steel surface. The emulsions existed in the confined areas where the plate shape defects formed. The majority of its water was evaporated with the increasing of temperature and time. The oxides and residual emulsions could be found on the surface strip steel, which originated from the oxidation reaction between vapors and silicon steel surface. Eventually, the reaction produced some oxides (Fe3O4, FeO, Fe2O3, SiO2, CoCr2O4, NiCr2O4, Fe-Cr) and other by-products. In addition, the emulsion spots area were easier to suffer corrosion than the normal area under the same conditions.

Abstract: The effect of natural aging, pre-aging and paint-bake aging technology on properties of Al-Mg-Si-Cu-Zr-Er alloy for auto sheets was investigated by performance testing, microstructure observation, and differential thermal analysis. The results showed that the natural aging alloy before and after paint-bake aging presented the double peaks feature of both the mechanical properties and hardness, the maximum value of strength and hardness appeared on the second aging peak, and the paint-bake aging softening occurred. The result of differential thermal analysis showed that, compared with the artificial aging, the precipitation of β′′ phase was delayed after the natural aging; the precipitates were mainly β′′ nuclei at the paint-bake aging, and it is difficult to achieve paint-bake aging hardening. After pre-aging and nature aging treatment, the precipitates were mainly composed of β′′ phase at the paint-bake aging, and the paint-bake aging hardening was realized. The addition of Er and Zr elements accelerated the precipitation of β′′ phase, the precipitation temperature of β′′ phase reduced and the time of the first natural aging peak and pre-aging was shortened. The parameters of pre-aging treatment of alloy sheets was solution and water quenching, and then age at 170 °C for 5 min.

Abstract: Aluminum alloy sheets were asymmetrically rolled at room and cryogenic temperatures by imposing different velocity ratios of 1~1.5 between the upper and bottom rolls. After rolling, the stress-strain curves, microhardness as well as the microstructures of the rolled samples were characterized and analyzed. The experimental results showed that the asymmetric cryorolling could improve the grain refinement and offered (~12%) higher room temperature tensile strength than that processed by symmetrical rolling with velocity ration of 1.0 (~280 MPa). However, at cryogenic temperature, the strength of asymmetrically cryorolling sheet (with velocity ratio of 1.5) was 5.1%, which is less than that processed by symmetrical rolling.

Abstract: CaF₂ inorganic nanometer powder particles were used as sintering aid to sintering good conductive Fe-6.5Si alloy. By a physical method, CaF₂ inorganic nanopowder particles were made with a granularity of 15-30 nm assembled between micron-sized Fe-6.5Si powder particles prepared by gas atomization. 6.5 % Si high silicon steel were fabricated by spark plasma sintering (SPS) with varying contents of CaF₂. The discharge enhancement effect of CaF₂ inorganic nanospark plasma aid is confirmed. The initial sintering temperature and the final sintering temperature were decreased by 75 °C and 70 °C respectively with 0.5 % CaF₂ inorganic nanopowder aid. In the case of reduced 60 °C, the higher density for the particles with the addition of CaF₂ was observed compared with without CaF₂. When the nanopowder was 2%, sintering performance decreased. The study indicates that sintering pressure has an enormous effect on the Fe-6.5Si sintering effect.

Abstract: High-silicon aluminum alloys as light-weight structural materials are attractive because of their high wear resistance, low density and low thermal expansion. However, the Si phase in the high-silicon aluminum alloys always has big size, which deteriorates the mechanical properties and machinability of alloys. In this paper, alloy ribbon of high-silicon aluminum alloy with the composition of Al₇₀Si₃₀ was prepared by melt-spinning method. Then alloy ribbon was broken to powder by ball milling. The alloy powers were consolidated by spark plasma sintering (SPS) at different sintering parameters including time and temperature under vacuum atmosphere. The density, hardness and compression property of the compacts were investigated. The sintered samples were characterized by scanning electron microscopy (SEM) and X-ray diffractometry (XRD), and compared with their alloy power and original alloy ribbons. The results showed that the density of Al₇₀Si₃₀ alloy, prepared under the condition of 320 °C and 500 MPa, was above 98%, the grain size was refined to sub-micron, and the wear resistance was good, micro-hardness was 311.70 Hv. It is believed that the existence of local high temperature and discharge plasma can break down and disrupt oxidation film on powder surface, and therefore improving the efficiency of sintering. In the case of large current, short time and low temperature, sintering grain growth was not obvious, there was less supersaturated solid solution precipitation and the rapid solidification microstructure , and the properties can be preferably reserved.

Abstract: Al-6wt.%Si-3.5wt.%Cu alloy (A319) were prepared by casting and powder metallurgy (PM) methods to investigate the influence of processing procedure on properties. The casting samples were cast into a horizontal mold after degassing by argon. The PM samples were extruded after hot-press sintering before which Al-11wt.%Si master alloy powder, pure Al powder and 48 h ball-milled Al-40wt.%Cu powder were blended for 12 h at rotation speed of 250rpm. The samples underwent a T6 temper, i.e. solution, water quenching and artificial aging. Microstructures were observed by optical microscope (OM) and scanning electron microscope (SEM), and phases were determined by X-ray diffraction (XRD). The tensile testing was carried out on universal testing machine. The results showed that heat treatment could greatly improve the tensile strength of the samples, especially for the casting counterpart. Discussion was made to analyze the reason that the properties differed.

Abstract: For the problem that stress relief annealing has a great effect on corrosion resistance and mechanical properties of the C-276/Q235-B bimetallic composite plate, the microstructure and mechanical properties of the C-276/Q235-B bimetallic composite plate at different heat treatment processes were investigated in the present investigation. The results showed that after 560°C,580°C and 600°C heat treatment, the tensile strength of composite plate was in 515-530MPa and elongation rate was in 25.6-33%, which could meet the requirements for industries. Corrosion tests were conducted according to A and B methods of ASTM G28 standard, corrosion resistance of composite plate after 560°C, 580°C and 600°C heat treatment exhibited a small difference with supplied C-276 alloy. Microstructure of composite plate after 580°C heat treatment exhibited a single austenite without intermediate phase and after 650°C heat treatment with small amounts of intermediate phase. The optimal heat treatment process for the C-276/Q235-B bimetallic composite plate was air cooling after 4 h heat treatment at 580°C.

Abstract: The formation of metatitanic acid particles in the form of rutile TiO₂ is an important step in the sulfuric acid process used to manufacture white pigment. F value, temperature and stirring speed were investigated to study their influence on the particle size and distribution in this paper. The results showed that suitable size and narrow particle size distribution were obtained in metatitanic acid on the condition that F value was 2.02, the temperature was 95°C and the stirring speed was 300r/min. Metatitanic acid was calcined above 850°C to form rutile TiO₂. The crystal structure and micromorphology of TiO₂ were characterized by XRD and SEM, respectively. The results demonstrated that a rutile TiO₂ with high crystallinity was prepared.

Abstract: Cold rolling (CR) was conducted on coarse grained (CG) and ultrafine-grained (UFG) coppers, obtained by 1 and 8 passes in the equal channel angel pressing (ECAP), to investigate the effect of grain size on rolling texture. The microstructure was refined to UFG (~420 nm) with the ECAP pass increased to 8, while only band-like CG microstructure was observed in the 1 pass processed copper. The influence of the texture before CR could be excluded as the crystallographic texture kept similar for different ECAP pass. Pole figures (PFs) showed that the shear texture introduced by ECAP was replaced by rolling texture after CR. Furthermore, the rolling texture was a kind of classical copper-type for the CG copper, while a brass-type rolling texture was observed in the UFG copper. TEM results confirmed that the deformation nanotwins were only observed in the UFG copper, while the microstructure of CG copper was further compressed and subdivided. It indicated that the observed differences in rolling texture component and density might be contributed to the grain size effect which resulted in different deformation mechanism and grain subdivision behavior.

Abstract: The temperature rising of pure copper during equal channel angular pressing (ECAP) was investigated based on the soft DEFORM 3D. The simulation results were compared with test results, and the microstructure evolution and recrystallization process was analyzed. The experimental results showed that under the same deformation conditions, different initial states of materials had different deformation temperature raised, with which the strain state further affected the recrystallization process. In the certain scope of die channel angle, it will have an optimal grainrefinement effect. As strain increased, the material microstructure engendered a non-equilibrium structure at the macro level, and recrystallization became complicated. Control ECAP deformation parameters had an observable effect on grain refinement. After plastic deformation, the parameters of the material’s state, die structure and deformation temperature determined the material’s grain size limit. The simulated results agreed well with the experimental results.