Fabrication of carbide-particle-reinforced titanium aluminide-matrix composites by laser-engineered net shaping

TiAl-based titanium aluminide alloys and their composites reinforced with ceramic particles are considered to be important candidate materials for high-temperature structural applications. Laser-engineered net shaping (LENS) is a layered manufacturing process, which involves laser processing fine powders into three-dimensional components directly from a computer-aided design (CAD) model. In this work, the LENS process was employed to fabricate carbide-particle-reinforced titanium aluminide-matrix composites using Tic and gas-atomized Ti−48Al−2Cr−2Nb powders as the feedstock materials. The composites deposited by the LENS process were susceptible to solid-state cracking due to high thermal stresses. The microstructures of the laser-deposited monolithic and composite titanium aluminide materials were characterized using light optical microscopy (LOM), scanning electron microscopy (SEM), X-ray energy-dispersive spectroscopy (EDS) analysis, electron-probe microanalysis (EPMA), and X-ray diffraction (XRD) techniques. Effects of the LENS processing parameters on the cracking susceptibility and microstructure were studied. Crack-free deposits were fabricated by preheating the substrate to 450 °C to 500 °C during LENS processing. The fabricated composite deposits exhibit a hardness of more than twice the value of the Ti−6Al−4V alloy.     

<Emphasis Type="Italic">In-situ</Emphasis> reactive processing of nickel aluminides by laser-engineered net shaping

Nickel aluminide intermetallics (e.g., Ni₃Al and NiAl) are considered to be attractive materials for high-temperature structural applications. Laser-engineered net shaping (LENS) is a rapid prototyping process, which involves laser processing fine metal powders into three-dimensional shapes directly from a computer-aided design (CAD) model. In this work, an attempt has been made to fabricate aluminide intermetallic compounds via reactive in-situ alloying from elemental powders using the LENS process. In-situ reactive alloying was achieved by delivering elemental Ni and Al powders from two different powder feeders, eliminating segregation observed in the samples deposited by using the premixed elemental powders. Nickel aluminides of various compositions were obtained easily by regulating the ratio of their feed rates. The aluminide deposits exhibited a high solidification and subsolidus cracking susceptibility and porosity formation. The observed porosity resulted from a water-atomized Ni powder and can be minimized or eliminated by the use of a N₂-gas-atomized Ni powder of improved quality. Cracking was due to the combined effect of the high thermal stresses generated from the LENS processing and the brittleness of the intermetallics. Crack-free deposits were fabricated by preheating the substrate to a temperature of 450 °C to 500 °C during LENS processing. Compositionally graded Ni-Al deposits with a gradient microstructure were also produced by the in-situ reactive processing.     

Fabrication of fiber-reinforced metal-matrix composites by variable pressure infiltration

A model for variable pressure infiltration of fibrous preforms by molten metal has been developed. The mechanism of the infiltration and the effects of fiber distribution and wettability on infiltration resistance and composite microstructure have been studied. It is shown that the model is in good agreement with the experimental data on infiltration of carbon-fiber preforms by Al-Si eutectic. The solution of the resulting equation shows that the rate of infiltration is only a function of the rate of change of pressure Ф, by which the infiltration processing is controlled precisely. Two kinds of infiltration modes have been found. A critical fiber volume fractionV _c exists, which is the turning point of the infiltration modes as well as permeability. As fiber volume fraction exceedsV _c, the infiltration mode changes from nonuniform to uniform, resulting in a sharp decrease in permeability. The permeability and resistances of these two infiltration modes are well predicted by the variable pressure infiltration theory. If fibers are wetted by molten metal, the preforms can be completely infiltrated at low applied pressure. In the case of nonwetting, poor infiltration of the preforms up toV _cresults, though high pressure is applied, but quality composites are formed at a low applied pressure if the fibers in the bundles are fixed relative to each other. A novel process, variable pressure infiltration technique, has been generated, which offers the advantages of low applied pressure, easy control of the pro-cessing, and no requirement of wetting. Quality C/A356 composites have been fabricated by this technique with the investment precision casting molds at a pressure of 0.6 MPa. Also, the mechanical properties of the composites are studied. The composites have high strength with a special fracture mechanism.     

Fabrication and characterisation of microporous titanium

Abstract

Microporous titanium was fabricated by a special powder metallurgy process and the microstructure and the mechanical properties of the material were characterised by scanning electron microscopy and quasi-static compression. The pore sizes and porosities of the samples are ranged of 10–50 μm and 40–65%, respectively. Macroporous Ti samples with the pore sizes ranged from 500 to 800 μm were also prepared by the similar process for comparison. The microporous Ti exhibits not only a very different deformation behaviour from the macroporous Ti but also lower flow stress than the latter, completely different from those observed in common porous metals.     

宏观颗粒增强铁基复合材料的制备与性能

颗粒与基体之间难以均匀稳定的混合以及二者的界面结合强度较差是限制颗粒增强金属基复合材料制备以及推广应用的共性关键问题,而目前的主要解决措施\     

热等静压近净成形数值模拟的研究进展

热等静压近净成形工艺具有材料利用率高、坯料组织均匀性好的突出优势,是镍基粉末高温合金、粉末钛合金等复杂形状部件的重要成形工艺。介绍了热等静压近净成形的工艺特点和生产流程、热等静压近净成形技术在国内外的发展和应用以及粉末热等静压致密化的微观模型和宏观模型,通过对比分析不同模型的热等静压数值模拟和试验验证结果,总结了不同模型的优缺点,分析了影响数值模拟准确性的因素。     

Fabrication of metal matrix composites of

Fine fibrous titanium carbide (TiC) was processed through the self-propagating high-temperature synthesis (SHS) method and employed to fabricate aluminum matrix composites. Two consol-idation methods were investigated: (1) combustion synthesis of TiC fiber/Al composites directly using titanium powders and carbon fibers ignited simultaneously with varying amounts of the matrix metal powder and (2) combustion synthesis of TiC using titanium powders and carbon fibers followed by consolidation into different amounts of the metal matrix powder, Al,via hot isostatic pressing (HIP). In the former method, when the amount of the Al in the matrix was increased, the maximum temperature obtained by the combustion reaction decreased and the propagation of the synthesis reactions became difficult to maintain. Preheating was required for the mixture of reactants with more than approximately 5 mole pct aluminum matrix powders in order to ignite and maintain the propagation rate. Microstructural analysis of the products from the Al/C/Ti reaction without preheating shows that small amounts of an aluminum carbide phase (AI4C3) are present. In the second method, following separation of the individual fibers in the TiC product, dense composites containing the SHS products were obtained by HIP of a mixture of the TiC fibers and Al powders. No ternary phase was formed during this procedure. Formerly Graduate Research Assistant, Department of Chemical Engineering, Michigan Technological University, is with Particle Technology, Inc., Hanover, MD 21076. This paper is based on a presentation made in the symposium “Reaction Synthesis of Materials” presented during the TMS Annual Meeting, New Orleans, LA, February 17–21, 1991, under the auspices of the TMS Powder Metallurgy Committee.     

Fabrication and creep properties of superalloy-zirconia composites

Graded composite interfaces have been proposed as a means to reduce thermally induced stresses between dissimilar materials. This is expected to be useful in applications such as ceramic thermalbarrier coatings (TBCs) on superalloy substrates. The interfaces, in such cases, are metal-matrix composites containing the ceramic phase within the superalloy matrix, whose creep properties during elevated-temperature service become critically important. This study was carried out to assess the creep properties of a typical superalloy-ceramic combination, namely, a René 95 alloy containing partially stabilized zirconia. Composites of these materials were prepared via powder metallurgy. Microscopy and X-ray work revealed that the zirconia reacted with γ′ (Ni₃Al) to form Al₂O₃, which resulted in the depletion of γ′ from the matrix. The creep behavior of the composites was markedly different from that of the unreinforced matrix. In addition to showing different stress exponents, the composites were stronger than the unreinforced material at low strain rates and weaker at the higher strain rates. A composite load-transfer model is used to isolate the effect of particles on strengthening. It is found that strengthening by the ceramic particles is smaller than strengthening arising from the change in chemistry of the matrix due to the addition of ZrO₂.     

镀钨碳纳米管增强铜基复合材料的制备及性能

采用羰基热分解法对多壁碳纳米管表面进行镀钨处理,并以镀钨碳纳米管和电解铜粉为原料,进行机械球磨混粉和放电等离子体烧结,制备了镀钨碳纳米管/铜基复合材料.采用场发射扫描电镜观察了粉体和复合材料的组织形貌,并对复合材料物相进行了X射线衍射分析.探讨了镀钨碳纳米管含量和放电等离子体烧结温度对复合材料致密度、抗拉强度、延伸率和电导率的影响.结果表明,镀钨碳纳米管质量分数为1%和烧结温度为850℃时,复合材料的致密度、抗拉强度和电导率最高.与烧结纯铜相比,复合材料的抗拉强度提高了103.6%,电导率仅降低15.9%.     

Fatigue in selectively fiber-reinforced titanium matrix composites

Many applications of the Ti alloy matrix composites (TMCs) reinforced with SiC fibers are expected to use the selective reinforcement concept in order to optimize the processing and increase the cost-effectiveness. In this work, unnotched fatigue behavior of a Ti-6Al-4V matrix selectively reinforced with SCS-6 SiC fibers has been examined. Experiments have been conducted on two different model panels. Results show that the fatigue life of the selectively reinforced composites is far inferior to that of the all-TMC panel. The fatigue life decreases with the decreasing effective fiber volume fraction. Suppression of multiple matrix cracking in the selectively reinforced panels was identified as the reason for their lack of fatigue resistance. Fatigue endurance limit as a function of the clad thickness was calculated using the modified Smith-Watson-Topper (SWT) parameter and the effective fiber volume fraction approach. The regime over which multiple matrix cracking occurs is identified using the bridging fiber fracture criterion. A fatigue failure map for the selectively reinforced TMCs is constructed on the basis of the observed damage mechanisms. Possible applications of such maps are discussed.     

Kinetics of biaxial dome formation by transformation superplasticity of titanium alloys and composites

By thermally cycling through their transformation temperature range, coarse-grained polymorphic materials can be deformed superplastically, owing to the emergence of transformation mismatch plasticity (or transformation superplasticity) as a deformation mechanism. This mechanism is presently investigated under biaxial stress conditions during thermal cycling of unalloyed titanium, Ti-6Al-4V, and their composites (Ti/10 vol. pct TiC _p , Ti-6Al-4V/10 vol. pct TiC _p , and Ti-6Al-4V/5 vol. pct TiB _w ). During gas-pressure dome bulging experiments, the dome height was measured as a function of forming time. Adapting existing models of biaxial doming to the case of transformation superplasticity where the strain-rate sensitivity is unity, we verify the operation of this deformation mechanism in all experimental materials and compare the biaxial results directly to new uniaxial thermal cycling results on the same materials. Finally, existing thickness distribution models are compared with experimentally measured profiles.     

Aluminum matrix composites: Fabrication and properties

Aluminum alloy matrix composites containing 1 to 30 wt pct of fibrous and particulate nonmetals varying in size from 0.06 μm to 840 μm were fabricated. The composites were cast into cylindrical molds for friction and wear tests, hot extrusion and tensile tests. The distribution of the nonmetals in the cast ingots was homogeneous. Friction and wear tests were done on a pin (52100 bearing steel) and dish type machine without lubrication. It was found that composites containing ∼10 wt pct or more of SiC, TiC, Si₃N₄, Al₂O₃, glass, solid waste slag, and silica sand wear less than the pure matrix alloy, but have slightly higher average coefficients of friction. Wear in composites containing soft particles, especially MgO and boron nitride was higher than the pure matrix alloy. The average coefficient of friction of all the composites was in the range of 0.35 to 0.58. Increasing the sliding velocity reduced this range to ∼ 0.4 to 0.45. The longitudinal tensile properties of the extruded composites (with the exception of loss of ductility in some cases) are comparable to that of the matrix alloys. Improvements in strength or ductility were noted. For example, addition of 15 wt pct of 3 μm size Al₂O₃ particles raised the yield and ultimate strength of the Al-4 pct Cu-0.75 pct Mg alloy matrix from 227 to 302 MPa, and 356 to 403 MPa, respectively. The corresponding percent elongation decreased from 25.8 to 12.5. The fact that the various composites can be readily cast and hot formed suggests a variety of engineering applications. AKIRA SATO, formerly Visiting Scientist at Massachusetts Institute of Technology, Cambridge.