| Metal porous material has become one of the hot research topics currently due to advantages such as low density, high specific surface area, high shock resistance, good permeability and high temperature resistance. It can work at a low temperature and be prepared easily. There are two methods to improve material performances: the first is preparation of composite particles, and second is improving the way of forming and sintering. In this study, the main research contents include:CuSnlO composite powder, spherical tungsten powder, different forming modes and sintering mechanisms, relationship between the microstructure and properties of materials. The main researches and contributions are as follows:1) Compounding and spheroidization of powderPCS (particle composite system) was used to prepare tin coated copper composite powder and spherical tungsten powder. For preparation of composite CuSn composite powder, the electrolytic copper powder, atomizing copper powder and tin powder were used as raw powder, and then raw powder was mixed in ball milling machine for20min and the ball to powder weight ratio is1:2. The mixed powder was modified in the PCS for15min at the rotational speed of3000r/min. For preparation of spherical tungsten powder, tungsten oxide and tungsten powder were chosen as raw materials. The optimum processing parameters were at speed of4000r/min for45min, and both powders had better process effect.Fractal analysis was introduced for characterization of particle shaping effect, especially spherical process of tungsten powder. Through fractal box-counting dimension analysis of pore outline, the value of fractal dimension reduced with the rising of processing time, and spherical degree and surface smoothness were also improved. Particle size distribution fractal dimension was also introduced, and we can derive modified model (ln(dr/d0)=kt0) and modified fractal model Both models had good consistency with the experimental data.2) The preparation of sintered metal porous materialsFor porous copper-based materials, uniform microstructure and pore distribution materials were prepared by moulding method and gelcasting method.The sintering temperature was set at800℃for moulding method. With the increase of spherical process speed and time, crushing strength increased, but sintering shrinkage rate, density and porosity had slightly different tendency. Porous materials prepared by choosing powder processed at speed of3000r/min for15min had best comprehensive performances. Radial and axial shrinkage rates were0.56%and0.98%respectively. And density and open porosity were6.68g/cm3and22.45%respectively. Crushing strength reached220.8MPa.For the porous copper base material prepared by gel-casting method, differential thermal-thermogravimetric-infrared-mass spectrometry were used to analyze curing, drying and degreasing process. Effective drying and degreasing mechanisms were built and the HEMA-DEDA gelcasting system was determined.For curing processing, Kissinger method and Flynn-Wall-Ozawa were used to character dynamic process. And average apparent activation energy value of curing reaction kinetics was2258KJ/mol. Extrapolation for curing temperature was61.95℃. The expression of curing reaction kinetics equation was α(t)=1-[1-7.79×104exp(-271.59/T)t]20.For degreasing process,model methods and free model methods were used and average apparent activation energy value of degreasing process was between188.75and217.49KJ/mol. Sintered shrinkage ratio and porosity reduced with the increase of solid content, but Brinell hardness and compressive strength increased. The results showed that maximum intensity of green bodies could achieve12.76MPa, and the porosity was between15.54%and28.17%, while value of Brinell hardness was between35and55. Maximum compressive strength of sintered bodies could reach237.8MPa.For preparation of porous tungsten materials, irregular and spherical tungsten powder were chosen and porous tungsten was prepared by reaction sintering method. First, the tungsten powder was oxidized at500℃for30min. Second, Al-W mixed powder was obtained for ball milling. The porous tungsten was obtained by reaction sintering process at reducing atmosphere (at1150℃for30min). The results showed that porous tungsten materials had total porosity of36.26%and their open porosity reached35.58%. The samples prepared by spherical tungsten powder had better comprehensive properties.3) Friction and wear properties and thermal conductivity of porous materialsPorous copper-based composite prepared by composite powder had good connected porosity and lubricant retention property. In the friction test, Dynamic wear phase could be achieved in a short time. Under the condition of oilless friction, with the increase of porosity and applied load, the value of friction coefficient increased. When value of open porosity was18.74%, friction coefficient of samples increased from0.257to0.331, and corresponding volume wear rate increased from 14.41×10-14to30.25×10-14m/J; but for samples with porosity of27.15%, friction coefficient of samples increased from0.423to0.479, and corresponding volume wear rate increased from52.41×10-14to75.54×10-14m/J.For oil and oilless friction tests, different rotational speeds led to different variation. For oilless friction, average friction coefficient and volume abrasion rate decreased with the increase of of rotational speed. When value of open porosity was18.74%, friction coefficient of samples decreased from0.271to0.252, and corresponding volume wear rate decreased from32.32×10-14to18.63×10-14m/J; but for samples with porosity of27.15%, friction coefficient of samples decreased from0.438to0.391, and corresponding volume wear rate decreased from68.25×10-14to51.60×10-14m/J. For oil friction, average friction coefficient of materials increased from0.091to0.102when porosity of samples was22.52%. Friction state showed adhesive wear at low speed and converted to shearing action at high speed.For porous material thermal properties, thermal conductivity had close relationship with pore distribution and shape. GEM method was used to character relationship between thermal conductivity and pore. With the increase of porosity, the value of thermal conductivity decreased from32.96W/m·k to12.84W/m·k. Structure factor n value is close to zero, which means sintered samples had uniform pore structure and pore distribution... |
PDF Full Text Request