Stephen D. Ridder

Correlation between ultrasonic and hardness measurements in aged aluminum alloy 2024

Abstract Sound wave velocity, ultrasonic attenuation, eddy current and hardness measurements have been carried out on precipitation-hardening aluminum alloy 2024 subjected to a series of different pre-aging heat treatments prior to processing to T4, T351 and T851 tempers. For each temper the maximum hardness was found to correspond to a particular value of sound velocity. These results were correlated with electron microscopy observations of the microstructure. Ultrasonic attenuation was found to decrease consistently as hardness increased. Pre-aging at 350°C was found to induce the most rapid initial reduction in hardness, and corresponding changes in sound velocity and ultrasonic attenuation. This investigation has demonstrated the feasibility of ultrasonic techniques for non-destructive evaluation and characterization of age-hardened aluminum alloys.

Process control during high pressure atomization

Abstract High pressure inert gas atomization (HPIGA) has been studied using various metal alloy systems. The high yield of ultrafine (particles less than 45 μm in size) powder produced using HPIGA makes it an ideal system for producing rapidly solidified metal powder. High speed photography and laser scattering techniques have been applied to study droplet formation and measure powder size with the intent of future feedback and control of particle size during atomization. Liquid metal droplet formation will be discussed as well as on-line particle size measurement and control.

The potential of rapid solidification in oxide-dispersion-strengthened copper alloy development

Abstract The feasibility of producing yttria (Y 2 O 3 )-dispersion-strengthened copper by internal oxidation of rapidly solidified CuY alloys was investigated. Alloys containing about 1 at.% Y were prepared via conventional solidification, supersonic gas atomization and splat-quenching techniques. The spacing of the CuY intermetallic phase was found to depend on the solidification velocity as λV ″ = constant, where n increased from about 0.4 to about 0.6 as the solidification rate increases. Y 2 O 3 particles were produced by the oxidation of the yttrium present in the intermetallic phase, which was tentatively identified as Cu 7 Y. Specimens internally oxidized at 873 or 1123 K revealed that a relatively uniform distribution of fine Y 2 O 3 dispersoids was produced in the splat-quenched material. On the contrary, the Y 2 O 3 particles in the conventionally cast and gas-atomized powders were largely confined to the prior interdendritic segregate, presumably because of insufficient refinement in the scale of the cast microstructure. Selected-area diffraction pattern analysis of Y 2 O 3 particles in the splats indicated an {100} Y 2 O 3 |{100} Cu and 〈100〉 Y 2 O 3 |〈100〉 Cu orientation relationship with the copper matrix.

Calibration of a two-color imaging pyrometer and its use for particle measurements in controlled air plasma spray experiments

Advances in digital imaging technology have enabled the development of sensors that can measure the temperature and velocity of individual thermal spray particles over a large volume of the spray plume simultaneously using imaging pyrometry (IP) and particle streak velocimetry (PSV). This paper describes calibration, uncertainty analysis, and particle measurements with a commercial IP-PSV particle sensor designed for measuring particles in an air plasma spray (APS) process. Yttria-stabilized zirconia (YSZ) and molybdenum powders were sprayed in the experiments. An energy balance model of the spray torch was used to manipulate the average particle velocity and temperature in desired ways to test the response of the sensor to changes in the spray characteristics. Time-resolved particle data were obtained by averaging particle streaks in each successive image acquired by the sensor. Frame average particle velocity and temperature were found to fluctuate by 10% during 6 s acquisition periods. These fluctuations, caused by some combination of arc instability, turbulence, and unsteady powder feeding, contribute substantially to the overall particle variability in the spray plume.

Particle size measurement of inert-gas-atomized powder

Abstract Metal powder produced by supersonic inert gas metal atomization (SiGMA) has been analyzed using several diagnostic methods. This analysis has brought to our attention several interesting and unexpected results. Some of these unexpected results concern the reliability of the various particle-measuring techniques, the procedure for proper (reproducible) particle size analysis, and the graphical representation of the data that best shows the powders characteristics. This study has also shown that gas-atomized powders produced in the SiGMA facility have distinct size distribution characteristics that do not follow the log-normal pattern. The fragmentation mechanisms leading to droplet formation which explain the SiGMA powder size distribution data are examined. We conclude that these new powder analysis procedures are applicable to all inert-gas-atomized powder and can lead to a better understanding of an atomizing systems operative liquid disruption mechanisms.

Real-time particle size analysis during inert gas atomization

Abstract Metal powder produced by inert gas atomization was analyzed with a non-intrusive particle sizing instrument. The in situ particle sizer operates on the principle of laser Fraunhofer diffraction and provides a line-of-sight measurement of the particle size distribution and mean size. The instrument has been successfully tested during several obtained powder atomization runs. The results obtained with the particle-sizing apparatus compare favorably with data obtained using a total weight-sieving technique. It is expected that the laser diffraction technique will be a suitable candidate for process feedback and control.

Beneficial effects of nitrogen atomization on an austenitic stainless steel

Fully dense nitrogenated austenitic stainless steels were produced by gas atomization and HIP consolidation. The base alloy, 304L, contained about 0.15 wt pct nitrogen when melted under a nitrogen atmosphere, and a modified version of 304L with 23 wt pct Cr contained 0.21 wt pct nitrogen. A series of experiments using various combinations of N2 and Ar as the melt chamber backfill gas and atomizing gas demonstrated that the nitrogen content of the powder was largely controlled by the backfill gas and that the fraction of hollow particles was determined by the atomization gas. The hollow powder particles, which are common in inert-gas atomized materials, were virtually eliminated in the nitrogen atomized powders. Additional atomizing experiments using copper and a nickel-base superalloy indicate that low gas solubility in the metal leads to gas entrapment. Hardness and compression behavior (yield strength and flow stress) are substantially improved with the addition of nitrogen. The results of this study suggest that the properties of nitrogenated stainless steels fabricated in this manner are comparable to other high nitrogen austenitic alloys.

Beneficial effects of nitrogen

]Fully dense nitrogenated austenitic stainless steels were produced by gas atomization and HIP consolidation. The base alloy, 304L, contained about 0.15 wt pct nitrogen when melted under a nitrogen atmosphere, and a modified version of 304L with 23 wt pct Cr contained 0.21 wt pct nitrogen. A series of experiments using various combinations of N2 and Ar as the melt chamber backfill gas and atomizing gas demonstrated that the nitrogen content of the powder was largely controlled by the backfill gas and that the fraction of hollow particles was deter- mined by the atomization gas. The hollow powder particles, which are common in inert-gas atomized materials, were virtually eliminated in the nitrogen atomized powders. Additional atomizing experiments using copper and a nickel-base superalloy indicate that low gas solubility in the metal leads to gas entrapment. Hardness and compression behavior (yield strength and flow stress) are substantially improved with the addition of nitrogen. The results of this study suggest that the properties of nitrogenated stainless steels fabricated in this manner are com- parable to other high nitrogen austenitic alloys.

The intelligent control of an inert-gas atomization process

Intelligent control is an attempt to specify the function of a controller in ways which mimic the decision-making capabilities of humans. Traditionally, issues relating to the emulation of human-like capabilities have fallen in the domain of artificial intelligence. Intelligent processing is a specific form of intelligent control in which the system to be controlled is a process rather than the more conventional mechanical or electrical system. The National Institute of Standards and Technology’s program on intelligent processing of metal powders is a multi-disciplinary research initiative investigating the application of intelligent control technologies to improve the state of the art of metal powder manufacturing. This paper reviews the design of the institute’s supersonic inert-gas metal-atomizer control system.

Microstructural characterization of atomized powder of Al5Mn5Fe2Si (Wt.%) alloy

Abstract Aluminum-based Al-TM-Si (TM = transition metal) when rapidly solidified (RS) and consolidated could show a microstructure of fine, slow coarsening dispersoids. One such alloy, Al5Mn5Fe2Si (wt.%), in its RS condition was studied in this work by scanning and transmission electron microscopy. The alloy powder was prepared by an inert gas atomization technique. The powder was separated to different particle size fractions in order to determine its microstructural dependence on particle size. The following microstructures (with increasing particle size) were observed: microcellular/cellular aluminum with an amorphous-type intercellular phase, eutectic and primary globular particles. The observed structures of the globular particles display various growth forms of the α(AlMnFeSi) intermetallic phase depending on the degree of undercooling. The icosahedral symmetry of naturally occurring polycrystalline aggregates or defective quasicrystals is related to the icosahedral symmetry of the α(AlMnFeSi) phase structural motif.