The high intensity of thermal stress was generated during fabrication of 6063 aluminum/60SiC‐35Al‐5Si double‐layer material. In order to decrease the thermal stress, graded materials of Al/60SiC‐35Al‐5Si was fabr...
The high intensity of thermal stress was generated during fabrication of 6063 aluminum/60SiC‐35Al‐5Si double‐layer material. In order to decrease the thermal stress, graded materials of Al/60SiC‐35Al‐5Si was fabricated by spark plasma sintering. The CTE in the graded layer was controlled by SiC fraction and particle sizes. The calculation results indicate that the net stress of more than 800 MPa was generated in the double‐layer materials, but it decreased to 170 MPa or less in the graded materials. So the stability can be guaranteed in the graded composites. The graded layers also have the potential for high thermal conductivity, more than 180W/Mk for every layer, which satisfies the application.
The 5-axis milling processes were conducted with several uncoated WC-Co tools, which are with taper ball end mills with four flutes. There is a severe problem of high rate of tool failure, which has led to the high co...
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The 5-axis milling processes were conducted with several uncoated WC-Co tools, which are with taper ball end mills with four flutes. There is a severe problem of high rate of tool failure, which has led to the high cost and low productivity of the milling process. In order to find out the real reason, some former work, which included the tool geometry analysis, tool path and tool posture analysis, as well as the cutting force calculating, had been done. It has been found that most of the tool failures are resulted from cutting edge chipping or the workpiece material of titanium alloys attached to the tool, Meanwhile, a suddenly applied load or an abrupt unloading of the tool at the end of a cut is another important factor. Moreover, the tool materials had been analyzed, which included the element composition analysis with XRD, the microstructure analysis with SEM. XRD results indicate the cobalt at the tip of tool is less than at the shank of tool. SEM results indicate there are micro-cracks concentrated in the cutting edge area. The tool material failure mechanism will be set up based on the tool material analysis in this study.
A study of silicon nitride ceramic radomes, which includes preparation of the material and optimal design of the radome wall structure, is presented in this paper. Multilayer radome wall structure with high dielectric...
A study of silicon nitride ceramic radomes, which includes preparation of the material and optimal design of the radome wall structure, is presented in this paper. Multilayer radome wall structure with high dielectric constant skins and a low dielectric constant core layer is used for broadband application. As a candidate material for both the skins and core layer, silicon nitride ceramics of controlled dielectric constant in the range 3.0∼7.5 were prepared by adding different content of sintering aids such as magnesia, alumina, silica and zirconium phosphate binder and choosing suitable sintering methods. A computer aided design (CAD) for the wall structure of silicon nitride multilayer ceramic radome based on microwave equivalent network method is carried out according to design requirements. By optimizing the thickness of skins and core layer, the power transmission efficiency of such a multilayer Si3N4 ceramic radome is calculated. The calculated results suggest that when the dielectric constant of skins lies in the range 6∼7.5 and core layer in the range 3.5∼4, the power transmission efficiency is above 85% with frequency of 2∼18 GHz while the thickness of skins is less than 0.03λ and the thickness ratio of skins to core layer is less than 1:15.
In the present study, Si3N4 ceramics with pore gradient structure were prepared by spark plasma sintering (SPS) technique. Silicon nitride ceramics with different controlled porosities were prepared by using ZrP2O7 as...
In the present study, Si3N4 ceramics with pore gradient structure were prepared by spark plasma sintering (SPS) technique. Silicon nitride ceramics with different controlled porosities were prepared by using ZrP2O7 as a binder material and heat treated at 1100 °C in a pressureless nitrogen atmosphere. Si3N4 ceramics with controlled porosity of 34–47% were obtained. The distribution of porous structure was homogenous. Fully dense Si3N4 ceramics could be sintered at 1400∼1600 °C by using MgO and alumina Al2O3 as the sintering aids. Pore gradient structure was formed by laminating the Si3N4 porous ceramics and powder mixture was used to obtain fully dense ceramics, and then sintering at 1400–1600 °C. Microstructure of sintered samples was observed by scanning electronic microscope (SEM) and the change of phase compositions was analyzed by X‐ray diffraction (XRD). The results showed that these samples exhibited a good porous graded structure with a highly porous layer and a dense surface layer. The major phase of the Si3N4 ceramics was still α phase.
The temperature field of two types of porous graded ZrO2 subjected to thermal shock loading is analyzed in this paper using the finite‐element method. The numerical simulation results indicate that the structure of Z...
The temperature field of two types of porous graded ZrO2 subjected to thermal shock loading is analyzed in this paper using the finite‐element method. The numerical simulation results indicate that the structure of ZrO2 with an un‐symmetric porous microstructure is superior to symmetric one and that, ignoring the influence of temperature dependent properties, the effects of thermo‐mechanical coupling can be ignored if the imposed heat flux density is less than 4 WM/m2.
Laser cladding has become an extensively attractive technique in the field of biomaterials. Significant effort has been devoted to combine mechanical properties of metal with biologic properties of bioceramic by this ...
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ISBN:
(纸本)9780912035895
Laser cladding has become an extensively attractive technique in the field of biomaterials. Significant effort has been devoted to combine mechanical properties of metal with biologic properties of bioceramic by this technique. The paper described the gradient bioceramic coatings on titanium alloy substrate (Ti-6Al-4V) obtained by laser cladding. The surface and cross-sections morphology of the composites was observed by scanning electron microscope (SEM). The element distribution of calcium phosphate coatings were investigated by electron probe microscopic analyzer (EPMA). Phase composition of the coatings was analyzed by X-ray diffraction (XRD). Microhardness tests also were performed. These analyses showed that the obtained coatings surface was homogeneous with well phase composition, and the coatings structure was gradual variation as well as a good microhardness distribution.
In the present work, three kinds of alloys with nominal compositions (at. %) of Ni-25Al, Ni-15Al-10Si and Ni-5Al-20Si have been fabricated successfully by a 10 kW continuous wave CO2 laser under optimum processing con...
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ISBN:
(纸本)9780912035895
In the present work, three kinds of alloys with nominal compositions (at. %) of Ni-25Al, Ni-15Al-10Si and Ni-5Al-20Si have been fabricated successfully by a 10 kW continuous wave CO2 laser under optimum processing conditions. X-ray diffraction (XRD) results showed that the phases of all alloys consisted mainly of Ni-Al type intermetallic compounds. The microstructures of three alloys were characterized using scanning electron microscopy (SEM). The hardness was measured on a HX-1000TM Vickers microhardness tester. The average hardness of Ni-15Al-10Si alloy was the highest value of 914 HV compared with Ni-25Al and Ni-5Al-20Si alloys.
Sintering behavior, microstructure and microwave dielectric properties of Ba1+x(Mg1/3Nb2/3)O3 (-0.01 [less-than or equal to] x [less-than or equal to] 0.01) were investigated. An improvement of the degree of 1:2 order...
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Sintering behavior, microstructure and microwave dielectric properties of Ba1+x(Mg1/3Nb2/3)O3 (-0.01 [less-than or equal to] x [less-than or equal to] 0.01) were investigated. An improvement of the degree of 1:2 ordering and sinterability was found in Ba-deficient Ba(Mg1/3 Nb2/3)O3 because there existed Ba vacancies. Excessive Ba inhibits the sintering and the degree of 1:2 ordering. It was proposed that excessive Ba exist in grain boundary and inhibit the movement of grain boundary. Therefore, the sintering was restrained. Ba-deficiency had little effect on dielectric constant of Ba(Mg1/3Nb2/3)O3 ceramics. With Ba-deficiency increasing, Qf value decreased drastically.
In this paper the inflaming retarding effects of epoxy resins blended with basic magnesium carbonate [Mg(OH)2·4MgCO3·5H2O] were synthesized. When the add-on of basic magnesium carbonate reached 56.5%, the LO...
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In this paper the inflaming retarding effects of epoxy resins blended with basic magnesium carbonate [Mg(OH)2·4MgCO3·5H2O] were synthesized. When the add-on of basic magnesium carbonate reached 56.5%, the LOI of the epoxy resins increased to 29.7%. The flame retardant mechanisms of basic magnesium carbonate for epoxy resins was investigated by XRD, TG-DSC and SEM. Results showed that through the ways of diluting, cooling, isolating and their synergies, basic magnesium carbonate achieves the flame retarding target.
The laser cladding technique was used to form Ni3 (Si, Ti) composite coating reinforced by in-situ formation TiC particles on GH864 Ni-based alloy substrate. By means of SEM, EDAX and XRD, the effect of (Ti + C) addit...
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The laser cladding technique was used to form Ni3 (Si, Ti) composite coating reinforced by in-situ formation TiC particles on GH864 Ni-based alloy substrate. By means of SEM, EDAX and XRD, the effect of (Ti + C) addition on the microstructure and microhardness of the coating was investigated. The experimental results show that an excellent bonding between the coating and the substrate can be ensured by the strong metallurgical interface. The clad coating is very good and free from cracks and pores. The microstructure of the coating is mainly composed of Ni (Si), Ni3 (Si, Ti) and TiC. The average microhardness of the coating is improved in terms of addition quantity of TiC in coating structure. When the addition quantity of (Ti + C) is 20wt%, the microhardness of coating was up to 780Hv that is 2.4 times of the substrate.
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