The effect of trace additions of magnesium on the sintering of aluminum and its alloys is examined. Magnesium, especially at low concentrations, has a disproportionate effect on sintering because it disrupts the passivating Al2O3 layer through the formation of a spinel phase. Magnesium penetrates the sintering compact by solid-state diffusion, and the oxide is reduced at the metal-oxide interface. This facilitates solid-state sintering, as well as wetting of the underlying metal by sintering liquids, when these are present. The optimum magnesium concentration is approximately 0.1 to 1.0 wt pct, but this is dependent on the volume of oxide and, hence, the particle size, as well as the sintering conditions. Small particle-size fractions require proportionally more magnesium than large-size fractions do.

Surface oxide and the role of magnesium during the sintering of aluminum

Liquid phase sintering of aluminium alloys

A manufacturing technique for the production of aluminum components is described. A resin-bonded part is formed by a rapid prototyping technique and then debound and infiltrated by a second aluminum alloy under a nitrogen atmosphere. During thermal processing, the aluminum reacts with the nitrogen and is partially transformed into a rigid aluminum nitride skeleton, which provides the structural rigidity during infiltration. The simplicity and rapidity of this process in comparison to conventional production routes, combined with the ability to fabricate complicated parts of almost any geometry and with high dimensional precision, provide an additional means to manufacture aluminum components.

https://science.sciencemag.org/content/sci/301/5637/1225.full.pdf

Rapid Manufacturing of Aluminum Components

The trends in integrated circuit packaging technology are toward high speed, high density, reliability, and low cost These demand the improvement of material formulations and processing technology Among the thick-film materials systems, conductor materials generally represent an important and the most expensive element Therefore, attention has been centered on the performance of the fired metal film and its cost Silver and palladium (Ag/Pd) conductors are important components of thick-film paste technology Thick-film Ag/Pd conductors find applications in many aspects of electronics and electronic packaging, such as hybrid microcircuits, multichip modules, packaging for integrated microcircuits, and in passive electronic components such as multilayer capacitors, varistors, and inductors In this paper, the performance and properties of fired Ag/Pd films are discussed through their physical and chemical aspects The final film properties are correlated to a number of factors, including thermodynamics and kinetics of Pd oxidation during burnout and firing; chemical and physical reaction of the Ag/Pd with the ceramic substrate, organic vehicle, and solder; Ag diffusion and migration; inorganic and organic additives; powders characteristics; and paste properties

Silver-Palladium Thick-Film Conductors

Alloys of Al, Al-0.15Mg, and Al-12Sn made using air atomized aluminum powder and pressed to green densities of 75 to 98 pct were sintered under argon or nitrogen. Sintering in argon is only effective at high green densities when magnesium is present. In contrast, highly porous aluminum can be sintered in nitrogen without the need for magnesium. The oxygen concentration in the gas is reduced by the aluminum through a self-gettering process. The outer layers of the porous powder compact serve as a getter for the inner layers such that the oxygen partial pressure is reduced deep within the pore network. Aluminum nitride then forms, either by direct reaction with the metal or by reduction of the oxide layer, and sintering follows.

The influence of the atmosphere on the sintering of aluminum

On the basis of thermodynamic and kinetic considerations, the role played by surface oxide layers in the sintering of metallic powders is evaluated. For metals with stable oxides, the influence of the surface layer is determined by the relative sintering and diffusional fluxes x s/x d to the interparticle neck region. When x s/x d>1 sintering is retarded by the oxide layer. Conversely, unretarded sintering occurs when x s/x d <1. For metals with oxides which are unstable with respect to dissolution in the metal at the sintering temperatures, the sintering process is preceded by an incubation period. Calculated values of the relative neck growth rates and the incubation periods provide the bases for predicting the effects of these surface layers. Predictions of the present analysis are shown to be consistent with experimental observations. PM/0174

Z. A. Munir

Papers

Surface Oxides and Sintering of Metals