液态金属浸渗纤维预制体的临界压力与浸渗行为

开发液态金属浸渗过程动态测量装置,以其深入研究液态金属在纤维预制体内部的渗流行为。利用该装置进行液态镁合金浸渗10%体积分数短碳纤维预制体的研究,测量浸渗过程中的临界浸渗压力,研究浸渗压力对浸渗速率的影响,观察不同浸渗压力下复合材料的微观形貌。结果表明：浸渗所需的临界压力为0.048 MPa,大于理论计算结果;随着浸渗压力的增大,液态金属的浸渗速率提高,但提高的幅度逐步降低;增大浸渗压力能够有效消除复合材料中的未浸渗缺陷;浸渗压力为0.6 MPa时获得组织致密的Cf/Mg复合材料。     

金属基复合材料液态成型中液流在预制型浸渗过程的行为

总结近年来关于液态成型液流在预制型浸渗过程中行为的研究 ;在描述金属液流在预制型孔隙内的流动时 ,讨论液流在预制型的浸渗速度、最小浸渗压力以及其他因素的影响。讨论结果表明 ,液态金属在纤维中的浸渗充填过程是各种阻力与浸渗压力共同作用的结果。增强体的体积分数愈大 ,直径愈小 ,需要施加的浸渗压力就愈大。     

Modelling of infiltration of molten metal in fibrous preform by centrifugal force

Infiltration kinetics of molten metal in fibrous preforms by centrifugal force is discussed theoretically. The relationship between the location of infiltration front and time is introduced for three major types of centrifugal casting processes. By the relationship, pressure distribution curves in the preform are also obtained. Some calculated examples are shown for the infiltration of molten aluminum into an alumina short fiber preform. If the speed of revolution is slightly higher than the minimum value determined by threshold pressure, infiltration is continuous. Since the second derivative of the pressure distribution curves in the infiltrated region is always positive and proportional to angular velocity in the centrifugal force field, the pressure curves are convex toward low pressure. When the surface pressure of the preform is low enough or the speed of revolution is high, the region having lower pressure than the threshold pressure appears in the infiltrated region. These results are compared with experimental data from references and by the authors.     

DETERMINATION OF THRESHOLD PRESSURE FOR INFILTRATION OF LIQUID ALUMINIUM INTO SHORT ALUMINA FIBER PREFORM

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浸渗法制备ZTA陶瓷/铁基复合材料研究进展

浸渗法制备ZTA陶瓷颗粒增强铁基复合材料的研究取得了很大进展。针对陶瓷预制体制备,铁水对陶瓷预制体的浸渗,陶瓷与铁水的润湿性,复合材料界面结合,复合材料耐磨性等方面的研究进行了论述。解决铁水对预制体的润湿性是实现浸渗的先决条件,常用的方法有在陶瓷预制体中添加活性元素,通过化学镀、气相沉积以及包覆等方法对陶瓷表面进行改性等;在陶瓷与金属基体间形成过渡层可以改善结合界面的组织结构,促进陶瓷与金属基体形成冶金结合;铁水对陶瓷预制体的浸渗机理,以及ZTA陶瓷复合材料的耐磨机理尚需要深入研究。     

Experimental determination of threshold pressure and permeability based on equation-solving method for liquid metal infiltration processes

A new method for determining two key parameters (threshold pressure and permeability) for fabricating metal matrix composites was proposed based on the equation-solving method. An infiltration experimental device was devised to measure the infiltration behavior precisely with controllable infiltration velocity. Two experiments with alloy Pb-Sn infiltrating into Al₂O₃ preform were conducted independently under two different pressures so as to get two different infiltration curves. Two sets of coefficients which are functions of threshold pressure and permeability can be obtained through curve fitting method. By solving the two-variable equation set, two unknown variables were determined. It is shown that the determined threshold pressure and permeability are very close to the calculated ones and are also verified by another independent infiltration experiment. The proposed method is also feasible to determine the key infiltration parameters for other metal matrix composite systems.     

低浸渗压力制备纤维增强铝基复合材料

利用液态浸渗技术制备了氧化铝纤维增强铝基复合材料。研究结果表明,在低压下使液态合金浸渗纤维预制件制备铝基复合材料是可行的。在浸渗过程中,液态合金的温度对浸渗压力有较大影响。所制备的复合材料具有均匀的显微组织,基体中的共晶组织可依附在纤维表面形核生长。     

离心渗铸中铝熔液在多孔介质内的流动

依靠离心力使金属熔液渗入到短纤维预制件中，凝固后得到金属基复合材料。在不考虑传热影响的基础上，通过分析离心力场中铝熔液在Al2O3短纤维方形预制件中的渗流情况，研究了铝熔液质量、预制件孔隙率和模具转速对渗透压力的影响，推导出了临界渗透参数关系式，分析了各种条件下满足临界渗透的晟小转速、最小渗铸质量以及最小孔隙率及其相互之间的影响规律。结果表明，针对本研究所用模型，温度确定，铝熔液质量、模具转速以及预制件孔隙率决定着渗透能否发生；随着渗透前沿界面的不断推进，渗铸总压力也在逐渐增大，若起始时刻初始界面能满足临界渗透条件，渗铸过程将维持下去，直至渗透过程结束。     

添加钛对炭/炭复合材料渗铜的影响

通过铺展实验和渗透实验考查添加钛对铜与C/C复合材料润湿性能的影响。采用真空熔渗的方法成功地将铜合金液渗入到C/C复合材料坯体中。对渗铜后形成的C/C-Cu复合材料进行X射线衍射、金相和扫描电镜分析。结果表明,加入12%~16%(质量分数)的钛元素粉末使铜在C/C复合材料表面有好的铺展性能;含钛铜合金渗入到C/C复合材料中有TiC形成。添加钛元素能改善C/C复合材料渗铜性能的主要原因是改善铜在C/C复合材料中的化学吸附和物理吸附特性;通过毛细管力作用,合金液渗入到C/C复合材料坯体中。     

Fly Ash Permeability During Liquid AA7075 Infiltration, as a Reutilization Method for Novel MMCs

The reutilization of waste materials into novel products is widely recognized as a challenge of high technologic interest. This investigation aims to sustain fly ash as a suitable ceramic preform for the production of MMCs by liquid metal infiltration of a high-alloyed aluminum (AA7075). The effect of a series of production parameters on the resulting microstructural homogeneity of the MMC is quantified. The influence of binder concentration preform dimension and orientation are evaluated among others. Starting materials, preform, and final product are examined by optical microscopy, SEM, EDX, and XRD indicating the developing structures and dominating phenomena throughout the manufacturing process.     

Infiltration mechanism and factors influencing carbon/carbon–Zr–Ti–C composites prepared by liquid metal infiltration

The effects of fibre architecture, reaction temperature and holding time on the infiltration performance of carbon/carbon (C/C)–Zr–Ti–C composites prepared by liquid metal infiltration were investigated. The results indicated that samples with a chopped-web needled preform and low initial density had a high final density. Increasing the reaction temperatures resulted in a decrease of the final density of samples. Additionally, increasing the initial holding time appeared to obviously result in a high final density, but its effectiveness was not obvious in later observations. An analysis of the infiltration kinetics and mechanisms indicated that the diffusivity of carbon in the carbide, the open-pore sizes and their distribution in C/C composites were the essential characteristics that controlled the height of infiltrating melts.     

Infiltration kinetics of pressureless infiltration in SiCp／Al composites

The pressureless infiltration kinetics was investigated by plotting the infiltration distance as function of the infiltration time. The effects of key process parameters such as time, temperature, Mg content on the pressureless infiltration of silicon carbide particle compacts were studied and quantified. The preform with high volume fraction SiC was obtained by mixing SiC particles with bimodal size distribution, whose diameters are 5 and 50 btm, respectively. The results show that an incubation period exists before infiltration, the influence of temperature on the incubation time exceeds that of Mg content, infiltration rate increases with the increasing temperature and Mg content, infiltration rate decreases as Mg consumes. A model of macroscopical infiltration and microscopical infiltration of liquid alloy in porous SiC preform was proposed.     

工艺参数对SHS-铸造钢基表面TiC-Fe梯度复合涂层结构的影响

将自蔓延高温合成(SHS)技术与铸造工艺结合起来，制备了铸造钢基表面TiC—Fe梯度复合材料，研究了钢液的浇注温度、TiC—Fe预制块的组成及其相对密实度等工艺参数对梯度复合涂层结构的影响。结果表明：在保证预制块能被浇注的钢液直接点燃的条件下，适当提高钢液的浇注温度、降低预制块的密实度均有利于梯度复合涂层的形成；而且在预制块中掺加适量的低熔点金属稀释剂不仅可降低合成TiC颗粒的尺寸，而且也有利于梯度复合涂层的形成。梯度复合涂层的形成与钢液向预制块中的渗透、预制块中Fe和低熔点稀释剂的熔化以及合成的TiC颗粒向钢液中的扩散等作用有关。     

Pressure-infiltration processing of reinforced aluminum

Pressure infiltration of liquid metal is one of the most important processing routes for the production of aluminum-matrix composites having a self-supporting reinforcement phase. This article briefly examines the physical phenomena governing infiltration processes, to present practical guidelines derived from their analysis for optimization of the process and the materials produced. Engineering aspects that are pertinent to infiltration techniques, including preform preparation, process configurations, flow control, and innovative processes, are summarized.     

Effect of specific pressure on fabrication of 2D-Cf/Al composite by vacuum and pressure infiltration

Carbon fiber reinforced aluminum matrix (C_f/Al) composite has many excellent properties, and it has received more and more attention. Two-dimensional (2D) C_f/Al composites were fabricated by vacuum and pressure infiltration, which was an integrated technique and could provide high vacuum and high infiltration pressure. The effect of specific pressure on the infiltration quality of the obtained composites was comparatively evaluated through microstructure observation. The experimental results show that satisfied C_f/Al composites could be fabricated at the specific pressure of 75 MPa. In this case, the preform was infiltrated much more completely by aluminum alloy liquid, and the residual porosity was seldom found. It is found that the ultimate tensile strength of the obtained C_f/Al composite reached maximum at the specific pressure of 75 MPa, which was improved by 138.9% compared with that of matrix alloy.     

Dynamic measurement on infiltration process and formation mechanism of infiltration front

An infiltration measurement device was developed to research the infiltration process of molten AZ91D magnesium alloy into the Al2O3 short fibre preform.The variation of relationship between the heights of measuring points and the time for molten alloy to reach the measuring points was illustrated.The effect of infiltration process parameters on the infiltration front was analyzed. It is found that pressure and pouring temperature are the most important factors which affect the infiltration velocity and composite quality.Furthermore,considering the influence of temperature field,an infiltration model of molten AZ91D into the short fibre preform was constructed on the basis of experimental results and Darcy’s Law.The analysis shows that the results predicted by this model are consistent with the experimental results.     

Capacitance monitoring of the infiltration of ceramic Particulate Preforms with Liquid Metals at 750 to 850 °C

The authors previously developed a capacitance technique to determine the position of the infiltration front as a function of time during the pressure infiltration of ceramic particulate preforms with liquid metals. In the present work, the technique was extended to higher temperatures and was used to monitor the infiltration of alumina particulate preforms with Al-Mg alloys and silicon carbide particulate pre-forms with Sn and an Al-Si alloy. The infiltration front position could be determined in the alumina pre-forms. However, it was not possible to clearly interpret the results obtained from the infiltration of semiconducting silicon carbide preforms. The observed range of capacitance circuit voltage to infiltra-tion distance ratios agreed with the predicted range for both simulated and alumina preform infiltration experiments. Formation of unstable infiltration fronts could also be detected.     

The effects of binders and heating temperatures on the properties of preforms

The use of binders in fabricating ceramic fiber preforms is essential when producing metal-matrix com-posites via squeeze casting or liquid metal infiltration. The binder is not only used to stabilize the shape of the preform, but also to increase its strength. A strong preform is needed to make a net shape or near-net shape casting so that many post-machining processes can be reduced or eliminated. However, very lit-tle attention has been paid to preform fabrication, subsequent processing, and its characterization. This work focuses on the relation between preform processing (selection of binder, heat treatment of the pre-form, etc.) and the properties of the resulting preforms. Silica and phosphate binders were used to make short alumina fiber preforms. The use of binders and the heat treatments used were correlated to the properties of the resulting preforms. The results showed that the heat treating temperature, as well as the amount of binder, is relevant to the resulting properties of the preform. Correlations among the use of binders, heat treatment condition, composition of the binders, and the properties of the preforms were investigated.     

浸渗法制备高铬铸铁/TiC-SiC复合材料及微观组织

采用小颗粒TiC包覆SiC陶瓷颗粒,在惰性气体保护下选用无压浸渗方法制备了高铬铸铁/TiC-SiC复合材料;利用SEM/EDX观察和分析了液态铸铁在SiC预制体中的浸渗情况、组织特征和成分分布;结合高铬铸铁/Ti-SiC复合材料的组织特点和浸渗行为特点,分析了TiC粉体对浸渗行为和复合材料组织的影响。观察结果表明,当TiC加入量≤10%(质量分数,下同)时,Fe/Cr合金无法润湿SiC颗粒,而当加入量≥20%时,Fe/Cr合金和预制体之间润湿性得到改善,增加TiC含量更有利于Fe/Cr合金浸渗;基体中大尺寸SiC颗粒消失,出现了尺寸接近毫米级的条状单质碳,这与高铬铸铁/Ti-SiC复合材料的组织差异较大。对比两种复合材料组织发现,添加Ti粉末在金属液中可与C结合生成TiC,而添加的TiC颗粒在组织中呈鹅卵石状,边缘圆润,出现金属液与陶瓷颗粒之间的互溶。在浸渗过程中,添加TiC和Ti与浸渗金属发生的反应不同,且高质量分数的TiC对金属液浸渗过程有明显的促进作用。     

Virtual analysis of influence of a filter on mould filling

Ceramic filters are used to avoid slag and impurities in foundry applications. When not properly applied, the presence of these filters may have a significant influence on mould filling. 3-D casting simulation has been applied to study the effects of the use of a ceramic filter on the metal flow in a gating system. Instead of using a pressure drop model to represent the behaviour of a fluid metal flow passing through a filter, a real exact filter geometry, which is created by a high resolution CT-scan and a non-destructive imaging technique, in the gating system is applied in the simulation. In this research, nodular cast iron is poured into a block casting. A depressurized gating system is used. After a choke, a filter with different orientations is placed in the system. Mould filling coupled with temperature is simulated. Geometries using different orientations of the filter, and without the filter have been researched. The simulated results show that the filter has no influence on the pouring time of the casting if the choke section is small enough compared to the effective section of the filter. Although the filter has no significant influence on the flow patterns in the block casting itself, the flow patterns in the filter zone are different. When the liquid metal passes a horizontal filter, it will be broken into many small streams and show a shower effect. After the part under the filter is full, the shower effect disappears. When the filter is located at the vertical position, due to the gravity, the shower effect is less. If no filter presents on the system, the liquid metal passes through the filter zone with a high speed and causes surface turbulence.