William H. Hofmeister

UNDERSTANDING THERMAL BEHAVIOR IN THE LENS PROCESS

Abstract In direct laser metal deposition technologies, such as the laser engineered net shaping (LENS) process, it is important to understand and control the thermal behavior during fabrication. With this control, components can be reliably fabricated with desired material properties. This paper will describe the use of contact and imaging techniques to monitor the thermal signature during LENS processing. Development of an understanding of solidification behavior, residual stress, and microstructural evolution with respect to thermal behavior will be discussed.

Single-pulse ultrafast-laser machining of high aspect nano-holes at the surface of SiO 2

Use of high numerical aperture focusing with negative longitudinal spherical aberration is shown to enable deep (> microm), high aspect ratio, nano-scale-width holes to be machined into the surface of a fused-silica (SiO(2)) substrate with single pulses from a 200 fs, 4 microJ Ti-Sapphire laser source. The depths of the nano-holes are characterized by use of a non-destructive acetate replication technique and are confirmed by imaging of sectioned samples with a dual focused ion beam/scanning electron microscope.

Undercooling of pure metals in a containerless, microgravity environment

The 105‐m drop tube at NASA/Marshall Space Flight Center has been used in a series of undercooling experiments on pure metals. Ti, Zr, Nb, Mo, Rh, Hf, Ta, and Pt were undercooled 17–20% of the melting temperature in a containerless, microgravity environment. Ir and Ru were undercooled to 13% Tm. Sample sizes ranged from 175 to 880 mg.

SiO2-coated porous anodic alumina membranes for high flow rate electroosmotic pumping

Electroosmotic pumping has been extensively used in lab-on-a-chip devices and micropumps for microelectronic cooling. High flow rate per unit area with a low applied voltage is a key performance requirement to achieve compact design and efficient operation. In this paper, we report work on using SiO2-coated porous anodic alumina membranes for high flow rate electroosmotic pumping under low applied voltages. High quality porous alumina membranes of controllable pore diameters in the range of 30‐100 nm and pore lengths of 60‐100 μm were fabricated by electrochemical anodization. The pores are straight, uniform and hexagonally close-packed with a high porosity of up to 50% of the total area. The inner surface of the pore was coated conformally with a thin layer (∼ 5n m) of SiO 2 to achieve a high zeta potential. The electroosmotic pumping performance of the fabricated anodic alumina membranes, coated and uncoated, was investigated using standard relevant aqueous electrolyte buffer solutions. The high zeta potential of the SiO2 coating increases the pumping flow rate even though the coating reduces the porosity of the membrane. Results show that nanostructured SiO2-coated porous anodic alumina membranes can provide a normalized flow rate of 0.125 ml min −1 V −1 cm −2 under a low effective applied voltage of 3 V. This compares favourably with other microporous materials such as glass frits.

Poly(ε-caprolactone)–carbon nanotube composite scaffolds for enhanced cardiac differentiation of human mesenchymal stem cells

AIM To evaluate the efficacy of electrically conductive, biocompatible composite scaffolds in modulating the cardiomyogenic differentiation of human mesenchymal stem cells (hMSCs). MATERIALS & METHODS Electrospun scaffolds of poly(ε-caprolactone) with or without carbon nanotubes were developed to promote the in vitro cardiac differentiation of hMSCs. RESULTS Results indicate that hMSC differentiation can be enhanced by either culturing in electrically conductive, carbon nanotube-containing composite scaffolds without electrical stimulation in the presence of 5-azacytidine, or extrinsic electrical stimulation in nonconductive poly(ε-caprolactone) scaffolds without carbon nanotube and azacytidine. CONCLUSION This study suggests a first step towards improving hMSC cardiomyogenic differentiation for local delivery into the infarcted myocardium.

Dual purpose pyrometer for temperature and solidification velocity measurement

A dual purpose pyrometer is described that allows both accurate radiance temperature measurement and fast temporal response. The system uses two silicon photodiodes with separate optical paths derived from a common spot on the sample. The optical bandwidths and response times of each detection circuit are tailored to the function of each radiometer. The radiance temperature of electromagnetically levitated metallic samples is measured over a narrow optical bandwidth with a high‐gain silicon detector. The velocity of solidification of undercooled melts can be deduced from the rise time of the second silicon detector which samples a broad optical bandwidth and has a fast response time. Results from experiments on the undercooling and solidification behavior of electromagnetically levitated pure nickel show that the solidification velocity approaches 17 m/s at high undercooling.

Solidification kinetics and metastable phase formation in binary Ti-Al

Near-equiatomic alloys of Ti-Al were solidified at various bulk undercoolings using electromagnetic levitation. Detailed thermal histories were acquired during experiments using optical pyrometry with sampling rates as fast as 500 KHz. Solidification and other high-temperature transformation pathways were deduced from the thermal data and microstructural analysis. Re- calescence rise times were used to determine semiquantitative primary solidification kinetics for the different phases. Primary β solidification was observed at compositions well into the equi- librium α regime; this is presented as part of a near-equiatomic nucleation domain diagram mat shows the primary solidification phase (β, α, ordered γ, or disordered γ) that results for each combination of nucleation temperature and composition. Solidification kinetics are faster for primary β (Vmax ≈ 15 to 18 m s-1) than they are for primary α (Vmax ≈ 10 to 12 m s-1). For undercoolings less than about 150 K, the primary solidification kinetics are about an order of magnitude slower for γ than for α. However, at an undercooling of about 150 K, the solidi- fication kinetics for γ increase discontinuously. This discontinuity is associated with a change in the primary solidification phase from ordered γ (Vmax ≈ 0.5 m s-1) to disordered γ (Vmax ≈ 10 m s-1).

The velocity of solidification of highly undercooled nickel

At large undercoolings (τ;10 pctTM, present theories relating solidification velocity to degree of undercooling do not agree well with reported experimental data for the solidification velocity of nickel as a function of undercooling. The present work shows that this discrepancy is due to two factors. First, the majority of previously reported results overestimate the solidification velocity of nickel at large undercoolings. Second, the scatter in experimental data is so large that a functional relationship between undercooling and velocity is not evident. In this study, the solidification velocity of undercooled nickel was measured using a linear array of 38 photodiodes. The results indicate that the velocity of the thermal field generated by the solid/liquid interface approaches a maximum velocity of 20 m s−1 atΔT} ≈ 10 pctTM (173 K) and men remains constant with increasing undercooling. This suggests that the velocity of the solid/liquid interface, at undercoolings greater than 10 pctTM, could be limited by attachment kinetics at the interface.

Solidification velocity of undercooled Ni-Cu alloys

Abstract The solidification velocity of Ni–Cu alloys was measured as a function of bulk undercooling using high-speed thermal imaging of electromagnetically levitated samples. Two departures from power law growth (approximating plateaus) in the velocity versus undercooling data were observed: the first occurred at intermediate undercoolings and is attributed to copper solute, while the second occurred at high undercoolings and is hypothesized to be an effect of oxygen. The Ivantsov solution with marginal stability arguments (IMS model) is a widely used model that relates dendrite growth velocity to total undercooling for dilute alloy systems. However, the model does not predict a plateau at intermediate undercoolings for alloys with a large equilibrium partition coefficient, k E . Satisfactory agreement between the model and experimental results can be obtained by using a value of k E that is smaller than the alloy’s actual value, but this is physically unreasonable and causes disagreement with currently accepted kinetic models.

Containerless processing and rapid solidification of Nb-Si alloys in the niobium-rich eutectic range

Containerless processing and rapid solidification techniques were used to process Nb-Si alloys in the Nb-rich eutectic range. Electromagnetic ally levitated drops were melted and subsequently splat quenched from different temperatures. A variety of eutectic morphologies was obtained as a function of the degree of superheating or undercooling of the drops prior to splatting. Metallic glass was observed only in drops quenched from above the melting temperature. Micro-structures of splats deeply undercooled prior to quenching were very fine and uniform. These results are discussed in terms of classic nucleation theory concepts and the expected heat evolution at different regions of the splat during the rapid quenching process. The locations of the coupled-zone boundaries for the α-Nb + Nb3Si eutectic are also suggested.