T. Dwayne McCay

Laser processing of a SiC/Al‐alloy metal matrix composite

Preliminary studies were conducted on the laser processing of SiC/A356‐Al alloy metal matrix composite (MMC) for such applications as welding/joining and cutting. The SiC/A356‐Al MMC was processed using several different laser specific energies. Microstructural observations after laser processing revealed that the extent of reinforced material (SiC)‐matrix (A356‐Al) reaction is directly proportional to the laser energy input. As energy input increased, SiC particle dissolution became greater and aluminum carbide formation increased in both size and quantity. It appears possible to control substantial change (physical and chemical) in SiC particles during processing by controlling the amount and mode of energy input.

Pulse laser processing of a SiC/Al-alloy metal matrix composite

The microstructural changes and the tensile behavior of laser processed A356-Al alloy matrix composites reinforced with 10 and 20 vol.% SiC particulates are characterized. The autogenous bead-on-plate welds were made using a pulsed CO 2 laser operating at a peak power level of 3.2 kW. The pulse on-time was constant at 20 ms and the off-time was varied from 20 to 2 ms (duty cycles of 50–91%). The microstructure of the laser melted region was investigated by optical, scanning, and transmission electron microscopy, and x-ray microchemical analysis techniques. The extent of microstructural changes varied directly with duty cycle, i.e., being a maximum for the longest (91%) duty cycles. Pulsed laser processing produced partial to complete dissolution of SiC particles and sometimes resulted in the formation of aluminum carbide. The associated rapid cooling also produced a fine distribution of nonequilibrium complex precipitates. In addition, the laser energy modified the SiC surface both physically and chemically. The results of tensile tests indicated that the modified SiC and the distribution of fine nonequilibrium precipitates enhance the mechanical properties of the laser processed composites. Optimum changes in microstructure and mechanical properties were obtained in the composites processed with intermediate (67 and 74%) duty cycles; therefore pulsed processing appears to be a strong candidate for successful joining of these MMCs.

The nature and influence of convection on the directional dendritic solidification of a metal alloy analog, NH4Cl, and H2O

Using optical techniques, the onset and development of convection was characterized in the liquid ahead of a (transient) solidifying dendritic interface during Bridgman growth of NH4C1-H2O. Mushy zone growth rates were measured and correlated to three regimes: quiescent (dif-fusion dominated growth), cellular flow, and pluming flow. The growth rates were found to increase an order of magnitude as the system transitioned through the three regimes. Critical Rayleigh numbers, which are shown to be mushy zone height dependant, are calculated for the onset of the latter two regimes.

Laser surface modification of zinc-base composites

The unique, ultrafine microstructure that evolves during laser surface modification of a zinc-base metal-matrix composite (MMC) directly affects surface properties such as corrosion and wear. Preliminary studies indicate that over the experimental range of conditions, the corrosion rate and the wear rate of the laser surface-treated MMC are lower than of that the untreated MMC. Further, x-ray diffractometry analysis of the laser surface-treated sample shows that unconventional, nonequilibrium phases are created in the process.

Diamond Materials for Electromagnetic Railguns

Abstract The use of diamond film insulator in electromagnetic railguns is currently being seriously considered since the state of the art in synthetic: diamond exceeds the requirements of electromagnetic railguns. The reasons for not using diamond, however, include the major difficulty in producing diamond insulators in the required shape and size that can survive the harsh railgun environment. This paper reviews railgun operation dynamics and potential materials including diamond for high performance of railguns. It further reviews the present status of scientific, technological and commercial developments; of diamond coatings. Alternate coatings and the properties that make them amenable for railgun applications are also discussed.

The measurement of transient dendrite tip interface supersaturation in NH4Cl−H2O using optical techniques

Abstract Using laser optical techniques, the time-dependent concentration field ahead of a (transient) solidifying dendritic interface was measured in the NH 4 Cl − H 2 O system. Dendrite tip interface supersaturation was calculated from the measured concentrations. It increases with time until convective effects disrupt diffusion layer growth. The rate of increase of the supersaturation is a strong function of cooling rate but has only a slight dependence on temperature gradient.

Solidification studies using a confocal optical signal processor

The development of a confocal optical processing system and its application to a solidifying metal model are reported. This system has been used to acquire image format data from which quantitative temperature and concentration profiles have been measured. Strong agreement is shown to exist between experimental results obtained in this manner and numerical simulations.

Convective flow effects on diffusion layers during NH4Cl−H2O dendritic solidification

Abstract Diffusion layer widths ahead of NH 4 Cl-71.5wt%H 2 O dendritic growth fronts are compared for microgravity and ground based experiments. It is argued from the data that both micro (dendritic stalk boundary layer) and macro (bulk liquid) convection significantly influence the size of the ground-based layer, enhancing the dendrites lateral growth but also limiting the tip growth. The use of the microgravity environment illustrates the significant aspects of convection free measurements in solidifying systems.

Fusion Zone Structures in Laser Welded Al‐SiC Composites

Metal matrix composites continue to be desirable structural materials but difficulties in joining limit their usefulness. This paper describes the results of a study on the laser welding of A356‐AI/SiC particulate composites. Three separate processing regimes are identified based upon analytical model predictions and metallographic analysis of the solidification structures.

Evaluation of nozzle geometries for laser surface alloying

Five general types of nozzles and nine configurations were tested in a water tunnel and using gas pressure analysis to determine their effectiveness at reducing entrained air from the outside environment during laser surface alloying. Two orientations with regard to the work piece were employed: The optics and nozzle being normal, the optics and nozzle being at a 15° oblique angle. Both argon and nitrogen were studied. In the impingement region, they produced roughly the same results regardless of the nozzle configuration. The change in orientation of the nozzle, however, had an effect. For 0° orientation the 7° simple nozzle with shroud is best and at 15° orientation the conic nozzle is the best when used in the reverse processing direction. A shroud surrounding the nozzles had minimal influence on the reduction of oxygen (approximately 1%) at normal and oblique incidence. The impingement region of the gas was found to be adequate in size for all of the cases.