A Review of Nondestructive Testing Methods and Their Applicability to Powder Metallurgy Processing

The problem of forming defects in green parts during compaction and ejection has become more prevalent as parts producers have started to use higher compaction pressures in an effort to achieve high density, high performance P/M steels. In this review, several nondestructive inspection methods are evaluated, with the aim of identifying those, which are practical for detecting defects as early in the production sequence as possible. The most promising NDT methods for P/M applications include electrical resistivity testing, eddy current and magnetic bridge testing, magnetic particle inspection, ultrasonic testing, X-ray radiography, gas permeability testing, and gamma ray density determination. The capabilities and limitations of each of the techniques are evaluated in this review. INTRODUCTION In the ceramics industry, the fraction of the finished part cost which arises from scrap due to flaws introduced during processing is estimated to average 50%, and can be as high as 75% (1). While the ceramics industry has been mobilized for the past 15 years towards use of nondestructive evaluation in processing (2), the P/M industry has so far built up only a scattered background of experience. To remain competitive, P/M parts producers have turned increasingly to simplify processing. It has been shown that the physical properties of P/M parts, especially the fatigue strength, are always improved by increasing the density (3). The need for densification by double pressing can, in many cases, be avoided by pressing to high density in a single step. However, the use of minor problems compared with the introduction of a step or second level in the part. Depending on the severity of the step, a separate, independently actuated punch can be required for each level of the part. During the very early stage of compaction, the powder redistributes itself by flowing between sections of the die cavity. However, when the pressure increases and the powder movement is restricted, shearing of the compact in planes parallel to the punch axis can only be avoided by proper coordination of punch motions. When such shear exists, a density gradient results. The density gradient is not always severe enough for an associated crack to form upon ejection. However, a low density area around an internal corner, as shown in Figure 2, can be a fatal flaw, since this corner is usually a point of stress concentration when the part is loaded in service. MICROLAMINATIONS In photomicrographs of unetched part cross sections, microlaminations such as those shown in Figure 3 appear as layers of unsintered interparticle boundaries which are oriented in planes normal to the punch axis. They can be the result of fine microcracks associated with shear stresses on ejection, which fail to heal during sintering. Because of their orientation parallel to the tensile axis of standard test bars, they have little influence on the measured tensile properties of the bars, but are presumed to be a cause of severe anisotropy of tensile properties. POOR SINTERING When unsintered particle boundaries result from a cause other than shear stresses, they are usually present because of insufficient sintering time or sintering temperature, a non-reducing atmosphere, poor lubricant burn-off, inhibition of graphite dissolution, or some combination of these. A severe example is shown in Figure 4. Unlike microlaminations, defects associated with a poor degree of sintering are not oriented in planes.