Iver E. Anderson

A viable tin-lead solder substitute: Sn-Ag-Cu

Rising concern over the use of lead in industry provides a driving force for the development of improved lead-free industrial materials. Therefore, a new lead-free base solder alloy Sn-4.7Agl.7Cu (wt.%) has been developed upon which a family of lead-free solders can be based. This solder alloy exhibits a ternary eutectic reaction at 216.8 ± 1°C (L ↠ η+ ϕ + β-Sn; η = Cu6Sn5, θ = Ag3Sn). Preliminary tests of solderability demonstrate intermetallic phase formation on model solder joint interfaces and good wettability in a fluxed condition suggest technological viability and motivates much more extensive study of this solder alloy.

Consolidation of amorphous copper based powder by equal channel angular extrusion

Cu50Ti32Zr12Ni5Si1 gas-atomized powder was consolidated by equal channel angular extrusion (ECAE). Powder was vacuum encapsulated in copper cans and extruded at a temperature above the glass transition temperature (Tg), but below the crystallization temperature (Tx). Five samples were subjected to one extrusion pass, each with a different temperature and extrusion rate. Microstructure, thermal stability, X-ray diffraction measurements and hardness of the ECAE consolidates were examined and compared with those of the initial powder and with a conventional extrusion (CE) consolidate. All consolidates exhibit a supercooled liquid region slightly narrower than that of the starting powder. No significant crystallization peaks are observed in XRD measurements; however, changes in peak shape and the total enthalpy of crystallization in differential scanning calorimetry measurements are attributed to nanocrystallization that is not easily detected by these methods. Greater microhardness values in ECAE consolidates in comparison with the starting powder also support the probability of nanocrystallization. The brittle behavior exhibited by all consolidates is attributed to an initial high oxygen contamination of the powder (∼2000 ppm) and the possibility of crystallization due to long exposure to temperatures above Tg during consolidation. Microstructural examination of the ECAE consolidates shows significant shear deformation of the particles with one ECAE pass. The results of the present study encourage further work on the fabrication of bulk metallic glass from powder by ECAE consolidation.

Dynamical evolution of microstructure in finely atomized droplets of Al-Si alloys

The complex time-dependent evolution of microstructure in rapidly solidified droplets of Al-Si eutectic alloys is quantitatively studied. Fine droplets of the alloy were obtained by using a high pressure gas atomization technique. The recalescence effect within each droplet gives rise to non-steady state growth conditions that lead to a systematic variation in microstructure across the droplet. This variation in microstructure is quantitatively characterized first by determining the nucleation undercooling and then by measuring the spatial variation in microstructural scales within the droplet. The results are compared with theoretical models to obtain the interface velocity and interface undercooling as a function of distance from the apparent nucleation site in different sized droplets. Eutectic, dendritic and cellular microstructures have been observed in droplets and the presence of these different morphologies is explained by constructing a microstructure map for the Al-Si system. The volume fractions of these three microstructures, as well as the length scale of each microstructure, are shown to determine the properties of the material. Specifically, microhardness measurements have been carried out for different diameter droplets, and a minimum in the microhardness is observed that is related to the presence of different microstructures. A microstructure map is developed for the Al-Si system to establish regimes of alloy composition and undercooling (as related to droplet diameter) required for the design of materials with optimum mechanical properties.

Microstructures and mechanical properties of pure Al matrix composites reinforced by Al;Cu;Fe alloy particles

Abstract A new type of composite material was produced from elemental Al matrix powders and 30 vol.% Al ;Cu ;Fe quasicrystal particles by a powder metallurgy technique. SEM examination shows that reinforcement particle cracking perpendicular to the loading axis is the dominant failure mechanism for the composites. Because of the fine (diameter

Particle size effects on chemistry and structure of Al-Cu-Fe quasicrystalline coatings

Gas atomized Al63Cu25Fe12 powders of varying size fractions were plasma sprayed onto hot (~600 °C) and cool (~25 °C) substrates using Mach I and subsonic plasma gun configurations. The chemical composition and phase contents of coatings were determined. Furthermore, coatings were annealed in vacuum at 700 °C for 2 h to observe phase changes. It was found that finer particles (e.g., <25 μm) tend to vaporize Al during spraying, which shifts the coating composition away from the quasicrystalline (ψ) single-phase region in the Al-Cu-Fe phase diagram. Coatings deposited on hot substrates were denser, richer in theψ phase, and harder than the corresponding coatings deposited onto cool substrates.

Sn-Ag-Cu solders and solder joints: Alloy development, microstructure, and properties

Slow cooling of Sn-Ag-Cu and Sn-Ag-Cu-X (X = Fe, Co) solder-joint specimens made by hand soldering simulated reflow in surface-mount assembly to achieve similar as-solidified joint microstructures for realistic shearstrength testing, using Sn-3.5Ag (wt.%) as a baseline. Minor substitutions of either cobalt or iron for copper in Sn-3.7Ag-0.9Cu refined the joint matrix microstructure, modified the Cu6Sn5 intermetallic phase at the copper substrate/solder interface, and increased the shear strength. At elevated (150°C) temperature, no significant difference in shear strength was found in all of the alloys studied. Ambient temperature shear strength was reduced by largescale tin dendrites in the joint microstructure, especially by the coarse dendrites in solute poor Sn-Ag-Cu.

Solid state sintering and consolidation of Al powders and Al matrix composites

Abstract As an attempt to depart from conventional transient liquid phase sintering practice, solid state vacuum sintering was studied in loose powder and in hot quasi-isostatically forged samples composed of commercial inert gas atomized (CIGA) or high purity Al powder. The high purity Al powder was generated by a gas atomization reaction synthesis (GARS) technique that results in spherical powder with a far thinner surface oxide. After vacuum sintering at 525 °C for up to 100 h, SEM results showed that the GARS Al powder achieved significantly advanced sintering stages, compared to the CIGA Al powder. Tensile results from the forged samples also showed that although its ultimate tensile strength is lower, 95 vs. 147 MPa, the ductility of the GARS pure Al sample is higher than the CIGA Al sample. Forging also consolidated a model powder-based composite system composed of an Al matrix reinforced with quasi-crystalline Al–Cu–Fe powders, where the same powder synthesis methods were compared. Auger surface analysis detected evidence of increased matrix/reinforcement interfacial bonding in the composite sample made from GARS powder by alloy interdiffusion layer measurements, consistent with earlier tensile property measurements. The overall results indicated the significant potential of using Al powders produced with a thin, high purity surface oxide for simplifying current Al powder consolidation processing methods.

Progress toward gas atomization processing with increased uniformity and control

Abstract Many advanced technologies based on particulate materials demand the availability of fine spherical powders or spherical powders of a narrow particle size class. Generally, high-pressure gas atomization (HPGA) is a close-coupled discrete jet atomization method and is one of the most effective methods of producing such powders. Development of HPGA nozzles with discrete jets resembling convergent–divergent (C–D) rocket nozzle designs, instead of the previous cylindrical jets, was conducted to increase atomization efficiency and uniformity and to reduce the required gas supply pressures. Results of compressible gas flow measurements on both types of HPGA nozzles revealed a steadily increasing trend of gas mass flow with gas supply pressure and a positive deviation from isentropic behavior that increases for increasing supply pressure. This has been attributed to an insufficient volume in the atomization nozzle gas manifold that experiences enhanced expansion cooling at increasing pressures. In experiments on 316L stainless steel, the atomization efficiency of the HPGA nozzle with C–D jets was higher than that of the HPGA nozzle with cylindrical jets, reflecting a lower gas/metal mass flow ratio. In other words, while the powder size distributions were nearly the same for all of the HPGA experiments, the HPGA nozzle with C–D jets utilized atomization gas with a significantly reduced operating pressure and mass flow rate.

Hydriding behavior of gas-atomized AB5 alloys

Abstract The hydriding characteristics of some AB 5 alloys produced by high pressure gas atomization (HPGA) have been examined during reactions with hydrogen gas, and in electrochemical cells. The hydrogen storage capacities and the equilibrium pressures for HPGA processed LaNi 5 , LaNi 4.75 Sn 0.25 , and MmNi 3.5 Co 0.8 Al 0.4 Mn 0.3 alloys (where Mm denotes Mischmetal) are found to be nearly identical to annealed alloys produced as ingots. The large discontinuous volume change across the α–β plateau region for gas-atomized LaNi 5 H x was seen to produce extensive fracturing in all but the smallest alloy spheres. However, only the largest spheres of the gas-atomized MmNi 3.5 Co 0.8 Al 0.4 Mn 0.3 and LaNi 4.75 Sn 0.25 H x alloys exhibited any discernible fracturing. The maximum electrochemical storage capacities of the gas-atomized LaNi 4.75 Sn 0.25 and MmNi 3.5 Co 0.8 Al 0.4 Mn 0.3 alloys were found to be smaller than the capacities of annealed alloys prepared from ingots.

Experimental Study on Viscosity and Phase Segregation of Al–Si Powders in Microsemisolid Powder Forming

Semisolid powder forming is a promising approach for near-net shape forming of features in macro-/microscale. In this paper, viscosity and phase segregation behavior of Al–Si powders in the semisolid state were studied with back extrusion experiments. The effects of process parameters including shear rate, extrusion ratio, heating time, and precompaction pressure were analyzed using the design of experiments method. The results showed that the effects of shear rate, extrusion, ratio and heating time were statistically significant factors influencing the viscosity. The semisolid state powders showed a shear thinning behavior. Moreover, microstructure analysis of extruded parts indicated severe phase segregation during the forming process. As the extrusion opening became small 400 m, the phase segregation increased. This study expanded the semisolid processing technology by exploring the use of powdered materials instead of typical bulk materials for applications in micro-/mesomanufacturing. Replacing bulk materials with powdered materials may add a new dimension to the technique by allowing tailoring of material properties. DOI: 10.1115/1.4000636