Papers by Author: Guo Qing Chen

Abstract: The paper mainly focused on the two issues that restricted the practical application of superplastic ceramics, which were the low strain rate in superplastic forming as well as resulted severely cavitation in deformed materials. The alumina-based composites Al₂O₃-ZrO₂ (3Y) and Al₂O₃-30mol%ZrO₂(3Y)-30mol%MgAl₂O₄ (AZ30S30) were selected as research materials. The nano-sized composite powders were synthesized by heating of ethanol-aqueous salt solutions method. The superplastic forming tests under the compressive stress state were carried out to evaluate the superplastic formability of the as-sintered materials. The results demonstrate that the following conditions are the essentials for attaining high-strain-rate superplastic forming in alumina based ceramic composites: reduction in the initial grain size by second phase dispersion and insurance of a homogeneous microstructure, enhanced diffusivity by co-doped certain elements, suppressed dynamic grain growth in deformation, as well as provide new rate-controlling accommodation process in superplastic forming. The results also indicate during the superplastic forming the cavitation damage was eliminated because of compression stress state, which ensured the mechanical properties after deformation. Therefore, the postdeformation mechanical properties after superplastic forming were enhanced in some extent.

Abstract: High strain rate superplastic forming is the key issue of the industrial application of superplastic ceramic and is also an effective approach to achieve “near-net shaping” of ceramic parts with complicated shape. The sol-gel method was adapted to synthesize the nano-sized composite powders. A three-phase nanocomposite ceramic Al2O3-30mol%YSZ-30mol%MgAl2O4 (AZ30S30) was prepared by using hot-pressing sintering in vacuum. Then the superplastic forming of a ceramic part with relatively complicated shape was accomplished and some new characters in superplastic forming were studied. The results demonstrate that as-sintered ceramic is a typical inter/intra granular nano-structured composite. Because the dispersed second phase inhibited the growth of the alumina matrix grains in deformation as well as in sintering, the material behaves excellent superplastic formability. The extrusion forming at a high strain rate was achieved and the part in cup shape was obtained. The yttrium element segregated obviously at the grain boundary after deformation, which demonstrated that the yttrium element directionally diffused during the deformation. The mechanical properties of the ceramic did not change much after extrusion. The maximum hardness of as-deformed composite ceramic is 32GPa, a litter lower than that of 35GPa before deformation.

Abstract: The high pressure waterjet peening (WJP) was adopted to strengthen the surfaces of 7075-T651 aluminum alloy. The results demonstrate that the processing parameters such as shooting distance, holding time and water pressure affected the abrasive capacity of gas cavities significantly. Compared with non-peening and shot peening (SP) specimens, the fatigue life of as-WJPed specimens was improved by 22% and 6.6%, respectively. The maximum residual compressive stress generated by WJP and SP was approximately equivalent, which is −350 ≤ σ ≤ −400MPa. While the residual compressive stress near the strengthened surface introduced by WJP was higher than that of SP, hence the fatigue life of WJPed specimen increased much more. The surface roughness strengthened by WJP was also better than that of SP. Because the surface profile was well protected in WJP, the good surface qualities also insured a long fatigue life.

Abstract: ZrO2 (3Y)-CaO-SiO2-TiO2 powders with excellent chemical homogeneity were synthesized by heating the alcohol-aqueous salt solutions. The powders have good dispersion, with particle size of 11~15nm and no hard agglomeration. The nanocomposite powders were hot-pressed in vacuum. The obtained zirconia-based multi-phase nanocomposites were characterized by using XRD and SEM, as well as relative density testing. The results demonstrated that, even though the sintering temperature was as low as 1250°C, a high relative density of 97.5% was achieved. The superplastic compressive tests of the as-sintered specimens demonstrate that this material behaves good deformability at 1350~1500°C, at which the maximum extrusion pressure is lower than 35MPa.

Abstract: In this paper constrained extrusion of the zirconia dispersed alumina nanocomposite under superplastic conditions was conducted. The mechanical properties of deformed material were studied and its results were compared with those of the initial materials. The microstructure evolution during superplastic deformation was also analyzed. The results demonstrated that after superplastic extrusion the flexural strength, relative density, Vickers hardness as well as fracture toughness of the material increased noticeably. The flexural strength of the deformed composite even retained at a high value of 310MPa at 800°C. The fracture toughness of the material increased from 6.92 MPa·m1/2 to 8.87 MPa·m1/2 after deformation. After superplastic extrusion due to grain boundary sliding and the compressive stress state, the internal porosities in as-sintered materials were eliminated. During extrusion with grain coarsening the effect of t-ZrO2 to m-ZrO2 transformation toughening increased because more zirconia grains reached the critical dimension. Although grain coarsening may cause the decrease of the fracture toughness in some extent, the phase transformation toughening and strengthening dominated. As a result, the mechanical properties of the deformed material were improved.

Abstract: NiAl-based alloy is a promising material applied in the fields of aeronautic and astronautic instruments. In the paper the compression deformation behavior and microstructure evolution of NiAl-Cr(Mo)-Hf alloy at elevated temperature were studied. The results demonstrate that the alloy behaves good formability in the temperature ranging from 1320°C to 1360°C, in which the maximum initial strain rate is about 8.3×10-4s-1 and the maximum deformation resistance is lower than 40MPa. During compression at temperature between 1250°C and 1300°C the flow stress increased sharply with the increasing of the deformation degree. When compression deformation at 1320°C~1360°C, the flow stress decreased obviously and the flow stress decreased slightly after reached the maximum value. By analyzing the microstructure evolution during compression it can be concluded that as-casting microstructure was improved in deformation. The grains were refined and the brittle phases of lamellar Cr(Mo) existing at NiAl matrix were broken. The porosities in as-casting material were eliminated during compression and the density of the material increased. The fracture toughness of the alloy increased from 6.4MPa·m1/2 to 9.8MPa·m1/2 after compression.

Abstract: The superplasticity of LY12 alloy was reviewed in this paper. Complex component was extruded by taking advantage of the superplasticity of supplied LY12 alloy. The research demonstrates that in isothermal compression process dynamic recrystallization may occur in supplied LY12 alloy. In superplastic forming test grain refinement was combined with initial extrusion step utilizing dynamic recrystallization to complete grain refinement, which ensured the superplasticity in supplied LY12. In the early stage of extrusion, using high deformation speed and large amount of deformation can result in grain refinement, which primarily satisfied the demand of the superplasticity. In the final stage, the forming speed decreased sharply so that the optimum strain rate was satisfied and the complicated component can be extruded successfully. The resulted product has reasonable flowing traces, which improves its service performance.

Abstract: In this paper uniaxial tensile tests were carried out at temperature range of 650~810°C and initial strain rate range of 10-4~10-1s-1 to evaluate the superplasticity of as-casting aluminum-bronze QAl10-3-1.5 alloy. The superplastic forming technology (SPF) was adopted to produce solid-bearing cages made of aluminum-bronze QAl10-3-1.5 alloy. The results demonstrate that as-casting QAl10-3-1.5 copper alloy shows good superplasticity without prior-treatment. The maximum elongation of 545% is obtained at 790°C and initial strain rate of 1.0×10-2 s-1, while the maximum flow stress is only 12.4MPa. By using superplastic extrusion technology solid-bearing cages used in railway vehicles were obtained. As-extruded cages have good surface quality and the dimensional accuracy satisfies the design standards. Using this technology the production efficiency is greatly enhanced and the manufacturing cost is reduced, especially the expensive copper alloys are saved.

Abstract: Si3N4 ceramic bodies were prepared by liquid phase sintering (LPS) with the amorphous nano-sized Si3N4 powders. Nano-sized Al2O3 and Y2O3 powders were introduced as additives. XRD analysis showed that the sintered body consists of β-Si3N4 and Si2N2O which confirms that phase change temperature of β-Si3N4 is lower than traditional Si3N4. SEM examination showed that the grain size of sintered body is smaller than 300 nm. Superplastic forming can be undertaken at the low temperature of 1550°C in a nitrogen atmosphere when the forming velocity is less than 0.5 mm/min. The formed parts rupture when the forming velocity is 1 mm/min or the forming temperature is 1500°C. Only a few defects are observed in the blank before forming, but many cavity groups are present in the formed workpiece.

Abstract: Using Al2O3-YTZ(3mol% yttria stabilized tetragonal zirconia) nanocomposite, superplastic extrusion under different conditions was adopted to form blade models. The results demonstrate that desired microstructure is achieved through the addition of 20mol% YTZ which acts as a second-phase pinning agent. At temperature range of 1650°C to 1700°C the material shows good deformability. At this elevated temperature the maximum extrusion pressure is lower than 25MPa, and the maximum punch speed is about 0.35mm·min-1. In superplastic extrusion the dominating deformation mechanism is grain sliding and rotation, the accommodating mechanism is intergranular zirconia coordinated deformation. Meanwhile static and dynamic grain growth also plays an important role in deformation.