Michael L Santella

Advances in processing of Ni3Al-based intermetallics and applications

Abstract The intermetallic-based alloys for structural applications have been an active field of research around the world for the last 20 years. Several major breakthroughs have occurred in this field during this time period. These breakthroughs include: (1) the dramatic effects of boron on ductility improvement for Ni 3 Al at ambient and high temperatures, (2) effect of chromium addition for intermediate temperature ductility improvement of Ni 3 Al, and (3) identification of an environmental effect from hydrogen generated by the reduction of moisture in air by aluminum in the aluminides. The knowledge of the compositional effects has led to the development of Ni 3 Al-based alloys, which allowed them to be taken from laboratory-size melts to commercial applications. This paper will describe the advances in melting practice, casting practices, solidification modeling as it applies to static and centrifugal castings and weld repairs, and welding of castings. This paper will also describe various applications of Ni 3 Al-based alloys and their current status of commercialization.

Empirical model of effects of pressure and temperature on electrical contact resistance of metals

Abstract An important input property in the development of process models for resistance spot welding is electrical contact resistance. A model for the pressure and temperature dependence of electrical contact resistance was developed from established concepts of contact resistance. The key to developing the desired relationship is determining surface roughness characteristics, which is experimentally problematic. To overcome this difficulty the electrical resistance of contacting interfaces was measured as a function of the pressure applied across the interfaces. Using known information about the temperature dependence of bulk resistivity and mechanical properties, a curve fitting procedure was used to establish the desired relationship of contact resistance to pressure and temperature. This empirical model agrees well with experimental measurements in the regime of low applied pressure. At high pressures, predictions underestimate contact resistance, and this was attributed to strain hardening of asperities at the contacting interface. The model also predicts that the competing effects of bulk resistance and contact resistance will produce a peak in the variation of contact resistance with temperature. The model provides a suitable means for incorporating the pressure and temperature dependence of contact resistance into process models of the resistance spot welding process.

Fabrication and tensile properties of continuous-fiber reinforced Ni 3 Al–Al 2 O 3 composites

Continuous-fiber reinforced metal-matrix composites consisting of Ni{sub 3}Al alloys and Saphikon Al{sub 2}O{sub 3} single crystal fibers were fabricated by hot-pressing of fiber-foil lay-ups. Two matrix compositions were employed, namely, IC50 (Ni--22.5Al--0.5Zr--0.1B, at. %) and IC396M (Ni--15.9Al--8.0Cr--0.5Zr--1.7Mo--0.02B, at. %). Etching of the foils in aqueous FeCl{sub 3} solution prior to lay-up and hot-pressing tended to improve fiber-matrix bonding and the density-normalized room temperature yield stress. Whereas strength improvements for the IC50 matrix were only moderate, significant improvements were found for a IC396M composite reinforced with 10 vol. % of Saphikon fibers.

Microstructures and mechanical properties of Ni3Al alloyed with iron additions

The alloying behavior of iron in B-doped Ni3Al was studied using optical and transmission electron microscopy for microstructural analysis and tensile testing for mechanical property evaluation. The aluminide dissolves less than 15 at. pct iron. At iron levels of 15 pct, depending on the aluminum level, formation of both transformed B2 phase (β′) and disordered fee phase (γ) is observed. The phase relationships in the aluminide containing 10 pct iron were studied in detail by quenching from temperatures below 1370 °C. Tensile properties of nickel-iron aluminides were determined as functions of iron content and test temperature. The tensile results are discussed in terms of solid-solution hardening, atomic size misfit, and precipitation of γ, β′, and carbides.

Friction stir spot welding of DP780 carbon steel

Abstract Friction stir spot welds were made in uncoated and galvannealed DP780 sheets using polycrystalline boron nitride stir tools. The tools were plunged at either a single continuous rate or in two segments consisting of a relatively high rate followed by a slower rate of shorter depth. Welding times ranged from 1 to 10 s. Increasing tool rotation speed from 800 to 1600 rev min–1 increased strength values. The 2-segment welding procedures also produced higher strength joints. Average lap shear strengths exceeding 10·3 kN were consistently obtained in 4 s on both the uncoated and the galvannealed DP780. The likelihood of diffusion and mechanical interlocking contributing to bond formation was supported by metallographic examinations. A cost analysis based on spot welding in automobile assembly showed that for friction stir spot welding to be economically competitive with resistance spot welding the cost of stir tools must approach that of resistance spot welding electrode tips.

Processing and operating experience of Ni3Al-based intermetallic alloy IC–221M

The cast Ni{sub 3}Al-based intermetallic alloy IC-221M is the most advanced in its commercial applications. This paper presents progress made for this alloy in the areas of: (1) composition optimization; (2) melting process development; (3) casting process; (4) mechanical properties; (5) welding process, weld repairs, and thermal aging response; and (6) applications. This paper also reviews the operating experience with several of the components. The projection for future growth in the applications of nickel aluminide is also discussed.

Ultrasonic Spot Welding of AZ31B to Galvanized Mild Steel

Ultrasonic spot welds were made between sheets of 0.8-mm-thick hot-dip-galvanized mild steel and 1.6-mm-thick AZ31B-H24. Lap-shear strengths of 3.0-4.2 kN were achieved with weld times of 0.3-1.2 s. Failure to achieve strong bonding of joints where the Zn coating was removed from the steel surface indicate that Zn is essential to the bonding mechanism. Microstructure characterization and microchemical analysis indicated temperatures at the AZ31-steel interfaces reached at least 344 C in less than 0.3 s. The elevated temperature conditions promoted annealing of the AZ31-H24 metal and chemical reactions between it and the Zn coating.

PVD synthesis and high-throughput property characterization of Ni–Fe–Cr alloy libraries

Three methods of alloy library synthesis, thick-layer deposition followed by interdiffusion, composition-spread codeposition and electron-beam melting of thick deposited layers, have been applied to Ni–Fe–Cr ternary and Ni–Cr binary alloys. Structural XRD mapping and mechanical characterization by means of nanoindentation have been used to characterize the properties of the libraries. The library synthesis methods are compared from the point of view of the structural and mechanical information they can provide.

Details of Mg–Zn reactions in AZ31 to galvanised mild steel ultrasonic spot welds

Abstract For ultrasonic spot welds between sheets of 0·8 mm thick hot dip galvanised mild steel and 1·6 mm thick AZ31B-H24, two Mg–Zn interface reaction layers, totalling around 22 μm in thickness, formed in 0·3 s, replacing the original 9 μm thick galvanised coating on the steel. The layer contacting AZ31 had composition near 25 at-%Zn. The second layer, contacting the steel, had composition near 66 at-%Zn. Mg–Zn alloying formed liquid phases during welding that were progressively squeezed from the joints until they were no longer evident at 1·0 s welding time. At that point a less than 500 nm thick layer of Al5Fe2 was identified by electron diffraction and microchemical analysis at the AZ31/steel interfaces. For welding times greater than about 0·5 s lap shear failures propagated through the AZ31 near the welded interfaces. This indicated that the strength of the AZ31–mild steel bonds was greater than that of the AZ31 near the welded interfaces.

On the Loss of Protective Scale Formation in Creep-Resistant, Alumina-Forming Austenitic Stainless Steels at 900°C in Air

A family of creep-resistant, Al2O3-forming austenitic (AFA) stainless steels was recently developed. The alloys exhibit excellent oxidation resistance up to ∼800°C, but are susceptible to internal attack of Al at higher temperatures. In the present work, higher levels of Ni, Cr, Al, and Nb additions were found to correlate with improved oxidation behavior at 900°C in air. The alloys generally appeared to be initially capable of external Al2O3 scale formation, with a subsequent transition to internal attack of Al (internal oxidation and internal nitridation) that is dependent on alloy composition. Compositional profiles at the alloy/scale interface suggest that the transition to internal oxidation is preceded by subsurface depletion of Al in the lower-Al compositions. In higher Al-containing compositions, NiAl second-phase precipitates act as an Al reservoir, and Al depletion may not be a key factor. Alloy design directions to increase the upper-temperature limit of protective Al2O3 scale formation in these alloys are discussed.