Papers by Author: Tie Tao Zhou

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: Magnesium alloys have a good application prospect in the fields of bone implants and cardiovascular stents due to their excellent properties, such as close density and elastic modulus to those of nature bone, high specific strength and rigidity, biodegradation and biocompatibility. In this paper, the feasibility of the use of the forged Mg-Li-X alloys (Mg-Li-Al-Zn-Ca-Sr) as biodegradable metals was investigated by immersion tests in the Hanks solution and skin-tissue implantation tests. The methods of SEM-EDS and XRD were used to research the corrosion morphology and corrosion products of the forged Mg-Li-X alloys after the in-vitro and in-vivo tests. At last, the in-vitro cytotoxicity was investigated by the MTT tests on L929 cells. Results showed that corrosion mechanism of the forged Mg-Li alloys was pitting and the surfaces were covered by Mg (OH)₂,CaCO₃ and Ca (H₂PO₄)₂ . During the 5 weeks implantation period, the rats survived from the operation, which indicates that the elements and the compounds formed are non-toxic to the rats. The forged Mg-Li alloy showed Grade 0~1 cytotoxicity, which manifests the forged Mg-Li alloy has good biocompatibility and eligible toxicity for implant applications.

Abstract: In this paper, a new biomedical Mg-Li alloy for the improvement of the comprehensive mechanical properties by micro-alloying and processing to meet the need of mechanical properties of biomedical materials. And the Mg-Li (Mg-Li-Al-Zn-Ca-Sr) alloy's processing and heat treatment were investigated in detail. The crystal texture of cast state, forged state and rolled state were observed and analyzed by OM, XRD and SEM. The mechanical properties of every stage were tested as well. The results showed that the grain size was refined obviously by the concentrating of Ca and Sr in the grain boundary. With the increase of rolling lane, the second phase's distribution was changed to a scattered state gradually from the reunion state. The tensile strength of the forged alloy was improved as well as its elongation after cold rolling and with rolled heat treatment process. The tensile strength reached 220MPa and the elongation reach 22%, which might meet the demand of cardiovascular stents mechanics.

Abstract: 0.3%Er is added to Al-5.6Zn-1.9Mg-1.6Cu-1.0Li alloy (in wt.%) to investigate the effect of erbium on microstructure of the alloy. Most of erbium exists in the primary Al₃Er phases which are incoherent with the alloy matrix unevenly. A few of fine Al₃Er phases, precipitated from supersaturated solid solution, are coherent with the matrix. η′-MgZn₂ precipitates in the single-aged alloy are sparser than that in the double-aged alloy. A few of fine Al₃Er phases act as the heterogeneous sites of η phases. There exist some precipitation free zones in the matrix around fine Al₃Er precipitates. Erbium is segregated to grain boundary seriously. A few of Al₃Er phases are present in grain boundary. The grain boundary precipitates are not continuous about the Al₃Er phases.

Abstract: 1.1%Li was added to 7075 alloys to obtain the Li-containing Al-Zn-Mg-Cu alloys. The microstructure and hardness of the alloys are investigated by transmission electron microscopy (TEM) and Vickers hardness. The hardness of the single-aged alloys is low. When the alloys were double-aged or multi-aged, the hardness is comparable to that of Al-Zn-Mg-Cu alloys at peak ageing. Two peaks were present in the hardness curves of the multi-aged Li-containing Al-Zn-Mg-Cu alloys. With the last-step temperature increases, two-peak phenomenon becomes prominent. The density and size of precipitates are influenced remarkedly by the ageing processes. Coarse grain boundary precipitates and PFZ (precipitate free zone)can be observed when the Li-containing Al-Zn-Mg-Cu alloys were multi-aged, and the higher the last-step ageing temperature, the wider the PFZ is.

Abstract: The effect of ageing processes on the toughness and the yield strength of the Li-containing Al-Zn-Mg-Cu alloys (Al-5.6%Zn-1.9%Mg-1.6%Cu-1.1%Li-0.24Cr) was investigated. The microstructure was observed by transmission electron microscopy, tensile test was performed at a rate of 1mm·s-1, fracture toughness was experimentally determined by the impact test, and the fracture modes had been assessed by image analysis of scanning electron microscopy micrographs. The strength of the Li-containing Al-Zn-Mg-Cu alloys treated at 120 °C is not more than 430MPa, which is very lower than that of 7075 alloys. The strength is comparable to that of 7075–T6 alloys after double-ageing or multi-ageing, however its ductility is lower than that of 7075-T6 alloys. The single-aged Li-containing alloys have high toughness. The multi-aged alloys become brittle. The fracture surface for the Li-containing Al-Zn-Mg-Cu alloys is intergranular, since the intergranular precipitation weakness the grain boundaries.

Abstract: Texture evolution in Mg-13wt%Li-X alloy cold-rolled from 1.35 mm to 0.34 mm thickness was investigated, by obtaining pole figures and orientation distribution functions (ODFs). Punching tests were conducted to reveal the effect of texture nature on formability. It was found that: (1) the textures of the as-received sheet are characterized by α fiber texture, a γ fiber texture and a cubic texture in both cold-rolled and annealed conditions; (2) with thickness reduction though rolling, the intensity of the γ fiber texture continuously increases and finally the γ fiber texture connects into {111} tube texture, the texture of <11 0> orientation flows towards {223}<11 0> along α fiber, the cubic texture of {001}<100> turns into {035}<100>, while some grains concentrate at {011}<41 1> orientation; (3) good punching behavior of the cold-rolled sheet corresponds to the appearance of a well-developed γ fiber texture.

Abstract: In the present work Mg-Li-X alloys with different alloying are investigated and the studies are focus on the stability of microstructures and properties of β base Mg-13Li-X alloys. The alloying elements concerned are aluminum, zinc, zirconium, rare earth. The microstructure and corresponding mechanical properties are examined with the aid of DSC analyses, X-ray diffraction and OM observation. The results showed that the hardening effect and the stability of the alloys varied with different addition of aluminum, zinc, zirconium, rare earth elements. The mechanical properties of the alloys greatly depend on the additional elements.

Abstract: The effects of Al, Zr additions on the microstructure and properties of a β matrix Mg-Li-Al-Zr alloy of hot forged, cold rolled and heat- treated states have been systematically investigated. The tensile properties of the alloy was tested, and microstructure was studied by X-ray diffraction and SEM methods. The results showed that the addition of zirconium in the aluminum containing Mg-Li alloy was effective for grain refinement. The alloy containing zirconium of rolled state exhibited higher strength and satisfied ductility compared with the alloy without zirconium. The existence of zirconium in the alloy was mainly in the form of Al-Zr compounds of Al2Zr andAl3Zr. The reason for less age-hardening effect may be caused by the capture of aluminum by zirconium and as a consequent by the lake of aluminum in the matrix.

Abstract: In the present work the deformation behavior of magnesium alloys with different lithium content has been investigated in terms of microstructures, crystal parameters, textures and mechanical properties. The lattice constants of a phase (hcp) determined by XRD varied with its lithium content and at the same time there are some changes in the texture composition. The plasticity would remarkably increase while b phase (bcc) becomes dominant in the microstructure of alloys.