Mary Helen 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 influence of metals and carbides during laser surface modification of low alloy steel

The addition of both elements (Cr and Ni) and carbides (SiC and WC) during laser surface alloying under different processing speeds produced surfaces with both enhanced hardness wear resistance and corrosion properties compared to the base AISI 4340 steel material. These effects were due to the evolution of unique microstructures within the laser-processed region, which includes austenite, ferrite, martensite, Fe- and Si-based carbides and the retention of the original carbides (SiC and WC) in various combinations. The chromium and nickel stabilized the austenite and ferrite but reduced the formation of martensite that is useful to increase the hardness and prevent cracking. Also, the substantial dissociation of the original carbides (SIC and WC) into elemental silicon and tungsten supplemented the stabilization of ferrite and reduction in the hardness. The presence of the undissociated carbides and some martensite formation provided substantial increases in the microhardness. The improvement of both the mechanical properties and corrosion resistance might be self-exclusive due to the reduction of the carbides and the subsequent inability of the matrix to prevent cracking.

Formation of a wear resistant surface on Al by laser aided in-situ synthesis of MoSi2

Abstract Reduction in the weight of engines forms a key element in improving the fuel efficiency of automobile engines. The use of Al as the engine material offers a viable option, but is limited by the poor wear resistance of the metal, especially in the cylinder bores. Laser induced surface modification of the cylinder bores, by the addition of second phase particles offers one such viable option. An attempt was made to produce in-situ nano/micron-sized particles of molybdenum disilicide (MoSi2) in the laser melted zone of the Al (A-356 alloy) substrate. The in-situ formation of MoSi2 was conducted by adding a mixture of Mo and Si precursor powder layer followed by melting the layer and substrate under a laser beam. A partial mixing of the intermetallic layer with the substrate Al was obtained. The partial mixing with Al resulted in a ternary non-stoichiometric phase that improved the hardness and the ductility as compared to that of the pure intermetallic phase. The addition of 5 wt.% titanium to the precursor powders resulted in an increase in the fluidity of the intermetallic layer, thereby promoting better intermixing with the substrate Al.

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.

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.

Refractive index of NH 4 Cl–H 2 O as a function of wavelength: the effect of temperature and concentration

The refractive index of NH(4)Cl-H(2)O solutions has been measured over a wavelength range from 496.5 to 690 nm. The NH(4)Cl concentration was varied from 15 to 30 wt. % over a temperature range from 10 to 35 °C. We obtained mathematical equations relating the refractive index to wavelength, temperature, and concentration using the least-squares method. A knowledge of these properties is important for analyzing and modeling the dendritic growth of this system.

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.

The influence of microgravity on the dendritic growth rates of NH4Cl-H2O: an international microgravity laboratory experiment

Abstract Ten NH 4 Cl - H 2 O Bridgman directional solidification experiments were run on an International Microgravity Laboratory Space Shuttle flight in January 1992 using a matrix of temperature gradients and cooling rates. Ground-based experiments using otherwise duplicate conditions were conducted on earth in the flight science modules. Dendritic front growth rates were obtained using reconstruction holograms taken periodically during the runs. The ground-based growth rates of the dendritic fronts were approximately one-half those of the flight growth rates. This is attributed primarily to changes in the concentration of the bulk fluid supplying NH 4 Cl to the tips as a result of the onset of convective flow and the concomitant dendritic coarsening. Reduced transparency of the overall mushy zone on earth is attributed to the increased ammonium chloride concentration within the interdendritic fluid. These results indicate that microgravity is a valid experimental platform for minimizing flow effects and therefore ennabling the optimum production of directional materials.

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.