Aluminium-matrix silicon carbide whisker composites fabricated by pressureless infiltration

Al-matrix SiC whisker composites were fabricated by pressureless infiltration of liquid Al-Mg or Al-Si-Mg alloys at 830–950°C in the presence of N₂ into a preform of nickel coated SiC whiskers. The nickel coating on the whiskers was obtained by electroless plating and made pressureless infiltration possible. The composite made by pressureless infiltration exhibited slightly lower tensile strength and modulus and slightly higher coefficient of thermal expansion than the corresponding composite made by pressure infiltration. However, the differences were small in spite of the lack of prior evacuation in the pressureless infiltration case. On the other hand, the hardness decreased with increasing distance from the preform-melt interface much more significantly in composites made by pressureless infiltration than those made by pressure infiltration. The hardness decrease, which was attributed to a porosity increase, was larger for composites made by pressureless infiltration without prior evacuation than those made by pressureless infiltration with prior evacuation. The Al-SiC reactivity was larger for composites made by pressureless infiltration than those made by pressure infiltration, because the infiltration time was longer in pressureless infiltration.[/p]     

Fabrication and infiltration kinetics analysis of Ti-coated diamond/copper composites with near-net-shape by pressureless infiltration

The Ti-coated diamond/copper composites with near-net-shape are manufactured by pressurelessly infiltrating liquid copper into porous Ti-coated diamond preforms. The contact angle between diamond and copper, relative density, thermal conductivity (TC), coefficient of thermal expansion (CTE), leak rate and microstructure are evaluated and characterized. In addition, the numerical analysis of the pressureless infiltration kinetics is also discussed. The results indicate that the relative density, TC and CTE of composites are 99.3%, 385 Wm⁻¹ k⁻¹ and 3–8 × 10⁻⁶ K⁻¹, respectively. It can meet heat-sink package requirement of high-power electronic devices as LED, insulated gate bipolar transistor (IGBT), etc. The liquid copper exhibits a turbulent flow with the Reynolds number in the range of 27.83–49.7. The porosity ? and the pressure drop Δp are the main influence factors controlling the velocity of liquid copper. Moreover, under vacuum condition of 8.7 × 10⁻³ Pa, the maximum theoretical infiltration length L_max of Ti-coated diamond/copper composites is found to be 552 mm.     

自渗透制备SiCp／Al复合材料及其耐磨性

研究了以Ｋ２ＺｒＦ６为助渗剂，用自渗透法制备ＳｉＣｐ／Ａｌ复合材料的技术，全面考查了Ｋ２ＺｒＦ６的配比，ＳｉＣｐ粒度，温度，保温时间等工艺参数对自渗透过程的影响。借助于复合材料金相组织的分析，认为粉体预热温度６５０℃，铝合金液浇注和保温温度７８０－８００℃，保温２ｈ后随炉冷却是本试验的最佳工艺参数。     

无压浸渗法制备Al/70vol%Sip复合材料

采用无压浸渗工艺,成功制备出Al/70vol%Sip复合材料,对Al-Si体系进行了自发浸渗的热力学及动力学分析,并分析了组织中残留微细孔隙的形成机理.研究表明:由于存在冶金润湿,在毛细压力作用下,Al合金液能较好浸渗Si多孔预制体,浸渗深度与时间成抛物线关系;采用饱和成分的Al合金浸渗,可有效抑制对Si预制体的溶解浸蚀;对复合材料浸渗组织观察表明,Si颗粒发生钝化,相邻颗粒融合连接,呈连续三维网状.     

The fabrication of metal matrix composites by a pressureless infiltration technique

A novel technique for fabricating metal matrix composites by the spontaneous (pressureless) infiltration of filler preforms with molten aluminium alloys is described. Numerous reinforcing materials, including Al₂O₃ and SiC of various configurations, such as particles, agglomerates, and fibres, have been incorporated as fillers. The effects of processing variables, such as alloy chemistry, process temperature, and filler material, on the infiltration kinetics and resultant microstructures are discussed. Comparisons with existing infiltration technology and preliminary composite properties are presented.     

Low-volume-fraction particulate preforms for making metal-matrix composites by liquid metal infiltration

A preform technology for making particulate metal-matrix composites with a low particulate volume fraction (as low as 18%) by liquid metal infiltration is provided. This technology used a non-combustible reinforcement (SiC) as the primary particulate and combustible particles (carbon) as the secondary particulate in the preform. The secondary particulate was removed from the preform by oxidation prior to liquid metal infiltration.     

无压浸渗制备梯度Si3N4/Al复合材料

采用Al-Mg合金无压渗入具有梯度孔隙分布的Si3N4多孔预制体,制得大尺寸、梯度Si3N4/Al复合材料;对复合材料的显微组织,相组成,硬度进行了观察和测定,分析了层间铝合金薄层在释放梯度层间应力的作用,并采用SHPB装置对梯度复合材料进行动态压缩试验.试验结果表明梯度复合材料层间结合紧密,硬度从HRA80.5逐层过渡到HRA61.5,主要组成相为Al,Si3N4,AlN和Mg2Si;梯度复合材料在动态压缩过程中表现出较高韧性.     

Fabrication and thermal conductivity of near-net-shaped diamond/copper composites by pressureless infiltration

Near-net-shaped diamond/copper composites with a relative density of over 99% and thermal conductivity of over 350 Wm⁻¹ K⁻¹ are successfully fabricated by powder press-pressureless infiltration processing. The effects of infiltration temperature, infiltration time, interfacial thickness, and type of protective atmosphere on the thermal conductivity of the diamond/copper composites were investigated. The results showed that the diamond-copper composites with complicated shape exhibited better thermal properties, which can be widely used in electronic packaging field. It was found that the properties of diamond-copper composites infiltrated in high vacuum atmosphere were better than that of composites infiltrated in other atmospheres. The thickness of interface showed great effects on the properties of composites. The carbide interfaces were attributed to the decrease of interfacial thermal resistance and enhancement of wetting properties between the diamonds and copper.     

The production of Al-Mg alloy/SiC metal matrix composites by pressureless infiltration

Metal matrix composites have been produced by pressureless infiltration of Al-Mg alloys into SiC preforms at 900°C under N₂ for different infiltration times. The wettability of the ceramic reinforcement by the Al-Mg alloy is crucial in determining whether an MMC can be produced by pressureless infiltration. Sessile drop results show that Al alloys with Mg contents greater than 8 wt% had a contact angle lower than 90°C after 5 minutes contact time. This was in agreement with the pressureless infiltration results as MMCs have been produced after 30 minutes with these alloys. Sessile drop experiments also show that SiC is similarly wetted by Al-Mg alloys under both N₂ and Ar. It is concluded that the infiltration process does not involve the intermediate nitride phase suggested by other authors.     

无压渗透法制备Al／AlN复合材料及其性能

在常压下通过熔渗工艺将AlSi7Mg合金渗入由AlN粉末模压成形、预烧所获得的预烧结坯中,得到了不同Al含量的Al／AlN复合材料。采用X射线衍射仪对复合材料的相组成进行了测试,采用金相显微镜和SEM对其显微组织进行了观测,并对不同Al含量的Al／AlN复合材料的维氏硬度、抗弯强度、热膨胀系数及导热系数等进行了测试分析...     

Microstructure and thermo-mechanical properties of SiCp/Al composites prepared by pressureless infiltration

In this paper, SiCp/Al composites with high reinforcement content are fabricated by pressureless infiltration with aluminum alloy into porous SiC preforms obtained by cold press forming. Microstructures and particulate distributions are analyzed with scanning electron microscope, X-ray diffraction and energy dispersive spectrometer. The reinforcement volume fraction reaches 65 % by using bimodal particle distributions. The bending strength ranges from 320 to 342 MPa, depending on particle sizes. Due to the intrinsically low thermal conductivity of the matrix, the thermal conductivity of SiCp/Al composites are in the range of 121–143 W m^?1 K^?1.     

Fabrication and characteristics of AA6061/SiCp composites by pressureless infiltration technique

The tensile properties and microstructures of AA6061/SiC_p composites fabricated by the pressureless infiltration method under a nitrogen atmosphere were examined. Since the spontaneous infiltration of molten AA6061 into the powder bed containing SiC_p occurred at 800 °C for 1 hour under a nitrogen atmosphere, it was possible to fabricate composites reinforced with SiC_p. Reaction product (Al₄C₃) was formed at the interface between SiC_p and Al alloy matrix. In addition, the amount and size of the Al₄C₃ is increased significantly by increasing the infiltration temperature. The reaction product (AlN) was formed as a result of the in situ reaction in both the control alloy and the composite. A significant strengthening even in the control alloy occurred due to the formation of in situ AlN particle even without an addition of SiC_p. While a further strengthening of the composite was produced by the reinforced SiC_p, strain to failure of the composite fabricated at 800 °C showed the lowest value (1.3%) in the T6 condition due to the formation of the severe reaction product (Al₄C₃). The grain size of the control alloy significantly decreased to about 20 m compared to 50 m for the commercial alloy. In addition, the grain size in the composite reinforced with SiC_p further decreased to about 8.0 m. This grain refinement contributed to strengthening of the control alloy and composite.     

Longitudinal centrifugal casting of metal-matrix functionally graded composites: an assessment of modelling issues

This study constitutes an effort to assess the relevant issues which should be addressed in order to adapt models currently available in the literature to predict final particle distribution resulting from radial centrifugal casting (CC) of particle-reinforced metal-matrix functionally graded composites (FGM) to the conditions prevailing in longitudinal CC. As such, a comprehensive review of a set of those models is performed, in order to ascertain the methodologies, the physical assumptions made, and the inherent limitations in applicability, so as to clearly identify the main deviations from the physical conditions prevailing during longitudinal CC. That review is then complemented by evidence of phenomena observed in longitudinally centrifuged FGM composites which are not in accordance with the physical assumptions admitted by the examined models, thus illustrating some of the issues which must be catered for in any successful effort to establish an adequate model for longitudinal CC.     

Microstructure and infiltration kinetics of Si3N4/Al–Mg composites fabricated by pressureless infiltration

Si₃N₄/ Al–Mg composites reinforced by ceramic interpenetrating network structure had been fabricated via pressureless infiltration technology. The matrix and the reinforcement phase, form an interconnected interpenetrating network structure. The Al–Mg/Si₃N₄ system exhibits an excellent wettability under moderate conditions. The increasing of Mg content (2–10 wt%) resulted in an increased amount of infiltration, once Mg content beyond 10 wt% has an adverse effect. Light chemical reaction occurs in the interface of Al–Mg/Si₃N₄ system and the reaction productions reduce the surface tension of melt and impulse the advance of infiltration. Infiltration temperature and infiltration time were the key parameters, which turn into the infiltration impetus. The appropriate infiltration temperature is 1050 °C and the corresponding infiltration time is 15 min, prolonging the infiltration time continuously has no significance.     

Ultrasonic anisotropy of metal-matrix composites

In recent work, Spies and Salama have attributed the ultrasonic velocity anisotropy observed in A1/SiC metal-matrix composites to the preferential alignment (texture) of crystallites in the aluminum matrix. In this paper it is shown that a simple relation exists between the compressional and shear wave anisotropy for an orthotropic orientation distribution of crystallites. It is shown that the data of Spies and Salama violates this relation and therefore that the ultrasonic anisotropy observed by these authors cannot be explained in terms of texture. It is proposed instead that the observed anisotropy is due to an anisotropic microstructure which may develop in the composites as a result of thermomechanical processing (extrusion), and a theory for this is presented.     

Fabrication and tribological properties of carbon nanotubes reinforced Al composites prepared by pressureless infiltration technique

Aluminum composites reinforced with CNTs were fabricated by pressureless infiltration process and the tribological properties of the composites were investigated. Al has been infiltrated into CNTs–Mg–Al preform by pressureless infiltration in N₂ atmosphere at 800 °C. By means of scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDS), it was found that CNTs are well dispersed and embedded in the Al matrix. The friction and wear behaviors of the composite were investigated using a pin-on-disk wear tester under unlubricated condition. The tests were conducted at a sliding speed of 0.1571 m/s under an applied load of 30 N. The experimental results indicated that the friction coefficient of the composite decreased with increasing the volume fraction of CNTs due to the self-lubrication and unique topological structure of CNTs. Within the range of CNTs volume fraction from 0% to 20%, the wear rate of the composite decreased steadily with the increase of CNTs content in the composite. The favorable effects of CNTs on wear resistance are attributed to their excellent mechanical properties, being well dispersed in the composite and the efficiency of the reinforcement of CNTs.     

Effects of reinforcement content and shape on cavitation and failure in metal-matrix composites

Cavity development in alumina-reinforced aluminium composites during tensile loading at room temperature has been monitored using microstructural studies and precision density measurements. The materials examined were based on commercial purity aluminium, reinforced with 10 and 20 volume% of short fibres, conventional angular particles or spherical particles, produced by a thermal spraying process. The composites were made by a powder blending and extrusion route, involving no liquidation of the matrix and leaving arrays of fine oxide particles aligned parallel to the loading direction. In all cases, stable voids were found to form well before final failure. Significant void contents were developed earlier in the test for the higher reinforcement content and when fibres were present. However, extensive voiding, corresponding to approximately hemispherical voids being formed at most fibres or particles, occurred in all cases before final failure. Voids tend to form adjacent to the reinforcement, most readily when it is elongated in the direction of applied stress and when it has a relatively flat surface normal to the stress axis. Sharp corners do not themselves appear to be favoured sites for cavity formation. Consistent with this, cavities can form with spherical particles, although their incidence is somewhat less than with angular particles, presumably because of the absence of elongated shapes and surfaces which are actually flat. A simple model is proposed which allows prediction of the failure strain for a given reinforcement volume fraction and aspect ratio. This is based on the constraining effect of the reinforcement on plastic deformation in adjacent regions of matrix and the contribution of cavitation to the observed strain. Fairly good agreement is observed between the predictions of this model and the experimental data.     

Fatigue properties of particle-reinforced metal-matrix composites

Fatigue-lifetime behaviour has been examined for extruded 6061 aluminium alloy composites reinforced with 15 vol% SiC and 10 vol% Al₂O₃ particles. The peak particle sizes are at about 4.5 and 6 μm. Within measured S- N curves the fatigue lifetime at given stress amplitudes of SiC_p/AA6061 is superior to that of Al₂O_3p/AA6061 in the low-cycle fatigue region as well as in the high-cycle fatigue region. These results are discussed by consideration of theoretically evaluated crack propagation curves.     

Wear of ceramic particle-reinforced metal-matrix composites

Pin-on-disc dry sliding tests were carried out to study the wear mechanisms in a range of metal-matrix composites. 6061-aluminium alloys reinforced with 10 and 20 vol% SiC and Al₂O₃ particles were used as pin materials, and a mild steel disc was used as a counterface. A transition from mild wear to severe wear was found for the present composites; the wear rate increased by a factor of 10². The effects of the ceramic particles on the transition load and wear with varying normal pressure were thoroughly investigated. Three wear mechanisms were identified: abrasion in the running-in period, oxidation during steady wear at low load levels, and adhesion at high loads. A higher particle volume fraction raised the transition load but increased the wear rate in the abrasion and adhesion regimes. Increase of particle size was more effective than increase of volume fraction to prolong the transition from mild wear to adhesive wear. The reasons for different wear mechanisms were determined by analyses of the worn surfaces and wear debris.