James A. Cornie

Infiltration of fibrous preforms by a pure metal. Part I. Theory

General expressions are derived to describe fluid flow and heat transfer during infiltration of fibrous preforms by a pure metal. Analytical solutions to the problem are given for the case of unidirectional infiltration into a uniform preform of aligned fibers under constant applied pressure. Calculations are carried out for infiltration kinetics (including total infiltrated length) and temperature distribution, using as an example alumina fiber/aluminum composites. Limiting cases leads to very simple expressions. Initial fiber temperatures both above and below the metal melting point are considered. In the case of fibers at a temperature significantly below the metal melting point, it is concluded that the factor most strongly influencing infiltration is the solidification of metal in the interfiber region. In the calculations, it is assumed that this solidification is in the form of a uniform solid metal sheath around the fibers. Metal superheat, when present, serves to progressively remelt the solidified sheath from the upstream end of the preform. Fiber volume fraction and initial temperature are predicted to have a major effect on infiltration kinetics, while metal superheat exerts a relatively minor influence. When no external heat extraction is present and a constant pressure is applied to the metal, flow through the preform continues indefinitely. For the case of external heat extraction, flow ceases when sufficient solidification occurs to block flow.

ON THE INFILTRATION OF METAL MATRIX COMPOSITES

Keywords: FIBER REINFORCEMENT ; INFILTRATION ; LIQUID METAL FRONT ; PRESSURE DIFFERENTIAL ; METALS AND ALLOYS Note: MIT, Cambridge, MA, USA, MIT, Cambridge, MA, USA03602133 (ISSN) Reference LMM-ARTICLE-1987-001View record in Scopus Record created on 2006-10-09, modified on 2017-05-10

Measurement of interface strength by a laser spallation technique

Abstract A laser spallation experiment has been developed to measure the strength of planar interfaces between a substrate and a thin coating (in the thickness range of 0.3–3 μm). In this technique a laser pulse of a high enough energy and a pre-determined duration is converted into a pressure pulse of a critical amplitude and width that is sent through the substrate toward the free surface with the coating. The reflected tensile wave from the free surface of the coating pries-off the coating. The critical stress amplitude that accomplishes the removal of the coating is determined from a computer simulation of the process. The simulation itself is verified by means of a piezo-electric crystal probe that is capable of mapping out the profile of the stress pulse generated by the laser pulse. Interface strength values ranging from 3.7 to 10.5 GPa were determined for the Si/SiC system. For the interfaces between pyrolytic graphite and SiC coatings an average strength of 7.2 GPA was measured, while the corresponding interface strength between a Pitch-55 type ribbon with a fiber-like morphology and SiC coatings was found to be 0.23 GPa. Intrinsic strengths of SiC coatings and Si crystal were also determined using this technique. These were, on the average, 8.6 GPa for Si crystals and 11.9 GPa for a SiC coating. Furthermore, the potential of the laser technique to determine the interface toughness was also demonstrated, provided well-characterizable flaws can be planted on the interface.

Three-Dimensional Printing: Rapid Tooling and Prototypes Directly from a CAD Model

Summary Three Dimensional Printing is a process for the manufacture of tooling and functional prototype parts directly from computer models. Three Dimensional Printing functions by the deposition of powdered material in layers and the selective binding of the powder by “ink-jet” printing of a binder material. Following the sequential application of layers, the unbound powder is removed, resulting in a complex three-dimensional part. The process may be applied to the production of metal, ceramic, and metal/ceramic composite parts. An experiment employing continuous-jet ink-jet printing technology has produced a three-dimensional part comprising eight intersecting planes spaced 0.375 inches apart. Future research will be directed toward the direct fabrication of cores and shells for metal casting, and toward the fabrication of porous ceramic preforms for metal-ceramic composite parts.

Wetting of ceramic particulates with liquid aluminum alloys: Part II. Study of wettability

Wetting phenomena in ceramic particulate/liquid Al-alloy systems were investigated experimentally using a new pressure infiltration technique developed by the authors. Studies were performed on two different ceramic particulates, SiC and B4C, with four different liquid aluminum alloy matrices, pure Al, Al-Cu, Al-Si, and Al-Mg. Five major variables tested to study wetting phenomena in ceramic/Al-alloy systems were holding time, melt temperature, alloying element, gas atmosphere, and particulate. Metal: ceramic interfaces were investigated with optical microscopy, SEM, EPMA, and Auger Electron Spectroscopy (AES) in order to understand better the wetting process. The threshold infiltration pressure decreased with, temperature as well as with pressurization time for all the ceramic/metal systems. A strong correlation was found between the alloying effect on the threshold pressure and the free energy of formation of oxide phase of the alloying element. More reactive alloying elements were more effective in improving wettability. In air atmospheres, the threshold pressure usually increased markedly as a result of a thick oxide layer formation on the liquid front. Compacts of B4C particulates showed lower threshold pressures than those of SiC, particulates. Fracture occurred in a generally brittle manner in infiltrated SiC, specimens. AES element profiles on the fracture surfaces showed fast diffusion of Si, and pile-up of C at the metal∶SiC boundaries which promoted fracture through the carbon-rich layer. The fracture surfaces of infiltrated B4C specimens indicated plastic deformation, hence a more ductile failure mode.

Wetting of ceramic particulates with liquid aluminum alloys: Part I. Experimental techniques

An experimental technique for evaluation of wettability of solid particulates with liquid metal was developed. Uniformly packed powder specimens were prepared with a tamping device specially made for the present experiment, and wetting tests were conducted by pressure infiltration of liquid Al-alloys into the powder specimens. The threshold pressure for infiltration was used as a measure of wettability. With this technique, wettabilities were measured for 10 μm diameter SiC and B4C particulates by several Al-based alloys. Threshold pressures obtained from this technique showed reproducibility to approximately ±7 kPa. Microstructures of infiltrated powder specimens indicated planar front infiltration with no disruption of powder compact during infiltration test and virtually no porosity.

Measurement of interface strength by laser-pulse-induced spallation

Abstract The strength of planar interfaces between a substrate and a thin coating can be measured quite effectively by a laser spallation technique. In this technique a laser pulse of a high enough energy and a pre-determined length is converted into a pressure pulse of a critical amplitude and width that is sent through the substrate toward the free surface with the coating. The reflected tensile wave then pries off the coating. The critical stress amplitude that accomplishes the removal of the coating is determined from a computer simulation of the process. The simulation itself is verified by means of a piezoelectric crystal probe which is capable of mapping out the profile of the stress pulse generated by the laser pulse. In this paper the basic methodology is described and some preliminary results are given of the actual spallation measurements.

Infiltration of fibrous preforms by a pure metal. Part II. Experiment

In a previous paper, a theory was developed to describe the flow of a pure metal into a fibrous preform. This paper presents experimental data to test the results of the theory for pure aluminum flowing into fibrous alumina preforms. An apparatus was designed and built for unidirectional infiltration under constant pressure and carefully controlled temperature parameters. A sensor was also developed to measure the position of the liquid metal in the fibrous preform during the experiment. This technique enabled quantitative comparison of theory and experiment. Experimental data are reported for the infiltration by 99.999 and 99.9 wt pct pure aluminum of SAFFIL alumina fibers fabricated into two-dimensionally random preforms. Fiber volume fraction was varied from 0.22 to 0.26, fiber preheat temperature was varied from approximately 483 to 743 K, and metal superheat was varied from 20 to 185 K. Infiltration pressure was varied from 1 to 4.5. MPa (145 to 650 psi). Agreement between theory and experiment was very good under all the experimental conditions studied for the 99.999 wt pct pure matrix. The impurity level of the metal was found to influence infiltration significantly. The measured perform permeability for 99.9 wt pct aluminum was much lower than that for 99.999 wt pct aluminum.

Columnar Dendritic Solidification In A Metal-Matrix Composite

Results are presented of a study on columnar dendritic solidification of the matrix of a fibrous metal-matrix composite, the fibers of which are aligned SiC fibers 140 m in diameter and the matrix of which is Al-4.5 wt pct Cu. Samples were produced by pressure infiltration of the metal-matrix into a preform of the fibers. The matrix was subsequently remelted and resolidified under controlled thermal gradient and growth rate. Dendrite growth begins in the center of the interstices left between the fibers. The dendrite tip temperature is not significantly influenced by the fibers, but the usual linear dependency of dendrite arm spacing ont1/3 (wheret is time during solidification) is altered significantly in the narrower interstices at long solidification times. The underlying mechanism is dendrite arm coalescence which takes place at a sufficiently rapid rate in the composite that the microstructure gradually becomes nondendritic. The solid/liquid interface then is parallel to the matrix/fiber interface. A model is presented for the kinetics of dendrite arm coalescence and compared with experimental results. The amount of microsegregation that was found in the matrix within interstices is significantly less than that found in the usual cast alloy, especially at long solidification times (low cooling rates). The mechanism responsible for the observed reduction in microsegregation is solid-state diffusion which is enhanced in the composite by the fact that the fibers place an upper limit on the dendrite arm spacing, and hence on the required diffusion distance.

Three-Dimensional Printing: The Physics and Implications of Additive Manufacturing

Abstract Three Dimensional Printing is a process for creating parts directly from a computer model. 3D Printing builds parts in layers by spreading a layer of powder and then selectively joining the powder in the layer by ink-jet printing of a binder material. After all layers are printed, the layer loose of powder is removed to reveal the finished part. Application areas include ceramic molds for metal castings, directly printed parts for end-use and for use as tooling, ceramic preforms for metal matrix composites, structural ceramic parts, and others. 3D Printing is a member of a group of layer manufacturing techniques which have the primary distinguishing feature of creating parts by the controlled addition (rather than subtraction) of material. The primitive building element in 3D Printing is a spherical ensemble of powder particles held together by one droplet of binder. Ballistic effects are important in the formation of primitives due to kinetic energy associated with the incoming droplet. Stitching together of droplets forms surfaces and hence determines surface finish. Vertical dimensional control is determined in pan by the compression of powder layers by subsequently applied powder. These physical mechanisms help to determine the dimensional control and surface finish of 3D Printed parts.