Ronald G. Iacocca

Microstructure quantification procedures in liquid-phase sintered materials

Abstract The quantification of sintered microstructures is an inherent part of powder metallurgy. Often a means to explain the effects of process variations with respect to sintered properties is needed. Quantification of grain size in the microstructure is an essential part of that effort. If satisfactory models exist, then the effort might be simple. Along those lines, three-dimensional grain size distributions can be obtained from two-dimensional grain section distributions by using a new forward transformation method. The technique is applied to sintered microstructures to verify the procedure, but the method is not limited to a particular material. A computer simulation program is developed. The initial three-dimensional grain size distributions are obtained from a backward method. The method can handle grains with different shapes, dihedral angles, and contact numbers. Both the Weibull and normal distribution functions are used to process the data. The results show that dihedral angle and contact number greatly affect the grain size distribution causing the three-dimensional grain size distribution to become narrower at high solid volume fractions.

Microstructural evolution during the supersolidus liquid phase sintering of nickel-based prealloyed powder mixtures

A novel concept for full-density sintering is described. Two prealloyed powders with slight compositional differences are tailored to separate the solidus temperatures into high-melt and low-melt compositions. A mixture of these two powder compositions allows full-density sintering at a temperature between the two solidus temperatures. For these experiments, the two powders were nickel-based alloys, where the low-melt powder contained boron. The mixed powders were sintered at temperatures above the solidus of the low-melt powder to form a transient liquid that promoted rapid densification of the mixture. Microstructure evolution during sintering was assisted using quenching experiments. Variables in this study included the heating rate, peak temperature, hold time, and powder ratio. Interdiffusion between the two powders controls microstructure evolution, with a dominant role associated with boron diffusion and reaction. The transient liquid phase responsible for densification is linked to boron diffusion and subsequent compound precipitation.

The effect of porosity on distortion of liquid phase sintered tungsten heavy alloys

Abstract Porosity effects on distortion during liquid phase sintering were investigated experimentally. W–Ni–Fe alloys with compositions ranging from 78 to 93 wt% tungsten were liquid phase sintered at 1500°C for 30 min. Surprisingly, the results show that porosity has no appreciable effect on distortion. After sintering, similar distortion profiles were obtained for compacts with both high and low green porosities. On the other hand, distortion reduced with increasing solid:liquid ratio. No distortion occurred for samples with 93 wt% tungsten. Quenching experiments showed the onset of slumping was delayed until densification neared completion. In high liquid content samples, this means slumping occurred within a few minutes after melt formation, corresponding to liquid exudation. The results are in agreement with model predictions by German. The sintered microstructures were quantitatively measured for solid volume fraction and contiguity. No apparent effects of green porosity on microstructural evolution were observed.

Microstructural Anomalies in a W-Ni Alloy Liquid Phase Sintered under Microgravity Conditions

The gravitational role in liquid phase sintering (LPS) is a problem of great interest in both materials science and engineering practice. Gravity-induced microstructural gradients in grain size, grain shape, and solid volume fraction have been well documented in liquid phase sintered tungsten heavy alloys and have been analyzed by a number of theoretical models. However, gravity may have many unknown effects on LPS, which can only be revealed by experiments conducted under microgravity conditions.

The effect of thermal cycle on the microstructural development of a powder metallurgy superalloy braze material

This investigation examines the effect of thermal cycle on the microstructural development in a powder metallurgy (P/M) superalloy braze material. Using a vertical quench furnace, samples were quenched at various stages within the heat treatment. Microstructures were analyzed using optical microscopy and an electron microprobe. The results show that borides having a blocky morphology are stable at all temperatures, both compositionally and morphologically. Script phase undergoes a drastic change in chemistry; however, it remains morphologically stable throughout. The chemical analysis of the microstructure supports the conclusion that the extended heat treatment, which is employed in industry to homogenize the microstructure and dissolve detrimental phases, does not have a significant effect in preventing these phases from forming. By using shorter times at elevated temperatures, similar microstructures can be produced.

The experimental evaluation of die compaction lubricants using deterministic chaos theory

Abstract The use of lubricants in die compaction is becoming an increasingly important issue in the P/M industry, both from an economic and environmental perspective. Until this time, the evaluation of lubricants has been limited to apparent density, tap density, and various elementary flow tests. This paper examines the application of deterministic chaos theory and fractal geometry to characterize the effects of lubricants on powder flowability. The lubricants that were investigated included zinc stearate, polyvinyl fluoride, Teflon™ (polytetrafluoroethylene—PTFE), and Acrawax C (ethylene bis-stearamide), ad-mixed with irregular iron powders. This paper evaluates the applicability of this new technique for characterizing powder flowability. In addition to distinguishing between the efficacy of various lubricants, data are presented on the statistical reproducibility, the effect of testing history on subsequent measurements, and the effect of machine operating parameters on the data. Parameters that can be used to describe the flow data produced by this new technique will be discussed.

Liquid-Phase sintering under microgravity conditions

Liquid-phase sintering routinely is used to form dense composite structures. The technique is generally restricted to high solid contents to ensure rigidity of the compact to achieve net shaping. Experimental conditions present during microgravity processing have allowed liquid-phase sintering over a wider range of liquid-solid ratios than possible on Earth. Early results from experiments performed on the space shuttle} Columbia during 1994 are used to show the novel behavior associated with microgravity conditions.

A Comparison of Particle Size Measuring Instruments using Fine Metal and Ceramic Powders

ABSTRACT Accurate and repeatable size measurement continues to be a problem in particulate materials processing. Eight different powders commonly employed for the production of high performance metal and ceramic components were used as a basis for examining the effects of powder characteristics on particle size measurement. Several different techniques were used to measure the particle size, including: laser diffraction with the powder dispersed both wet and dry, aerodynamic time-of-flight, electrical zone sensing, dynamic light scattering or photon correlation spectroscopy, and optical image analysis. After reviewing the size data obtained from these vastly different techniques, it is concluded that accuracy is strongly dependent on dispersion of the powder in the carrier fluid. Once adequately dispersed, one obtains analogous particle size information, independent of the instrument.

Powder Metallurgy Processing of Intermetallic Matrix Composites

Intermetallic compounds are similar to ceramics because they are stoichiometric, with limited compositional ranges and brittle behavior. The limited ductility forces a reliance on powder techniques for shaping and consolidation. The high temperature character of intermetallics is beneficial to high temperature service, but this same attribute contributes to difficulty in processing. This paper reviews the several powder approaches to forming intermetallic structures. Examples are given on powders, consolidation options, and properties. Densification maps are introduced for estimation of consolidation cycles. Unfortunately, many of the composites exhibit little strengthening benefit from incorporation of reinforcing phases.