Metal Processing Institute
Center for Imaging and Sensing

Electric Resistivity NDE

Detection of density variations in powder metallurgy compacts

Producing P/M compacts is generally a low-cost, high-volume manufacturing method with very special quality assurance requirements. When considering the three basic P/M steps of mixing, compacting, and sintering, it is the compaction process producing the green-state parts that offer the highest pay-off for quality control through nondestructive evaluation techniques. Detection of compaction related problems in the green-state will permit early process intervention, and thus prevent the creation of potentially significant numbers of faulty parts prior to sintering.

Initial research targeting resistivity measurements of pre-sintered, green-state powder metal (P/M) compacts resulted in a surface-breaking and subsurface crack detection instrument. This instrument relies on an array sensor concept whereby direct current (DC) is injected into the sample and voltage distributions on the surface are processed in an effort to detect hairline cracks as small as 20 microns in size. The presence of flaws, which are due to conductivity contrasts, can be sensed through a local voltage perturbation against the unflawed baseline voltage response. A prototype crack detection instrument was developed at MPI.

Our current work extends the previously developed approach of evaluating large variations in conductivity over microscopic distances by evaluating small changes of conductivity over macroscopic distances. It is reasoned that the same electrostatic approach should be applicable for the detection of density gradients in green-state compacts. However, the electrostatic density prediction approach differs in its evaluation and processing methods. Specifically, key issues which need to be established for such an approach to be viable, include:

Accordingly, we carried out both a theoretical track, as well as an experimental one, to address these “pre-requisites”.

Measured Parts and Experimental Arrangement

Controlled cylindrical green state compacts have been manufactured specifically for the purpose of conductivity determination. The cylindrical shape with a large diameter to length ratio of 4:1 (diameter D = 6cm, length L = 1.5 cm) was chosen for several reasons:

  1. The simple cylindrical geometry allows the mechanism to be easily modeled, and permits a simple measurement setup.
  2. The disc-like shape with its short compacted length assures a uniform density distribution within the green-state compacts.
  3. The large diameter/length ratio forces the current to flow through the inside of the part rather than on the surface only.

The samples consist of pure iron powder (1000B) with nominal compaction densities ranging from 6.0g/cm3 to 7.4g/cm3. Each specimen density was replicated three times to take into account possible process variations. Additionally, the parts were divided into four sets, each differing in the amount of lubricant in the powder mixture in order to examine the effect of these lubricants on the conductivity versus density relationship. Tight specifications imposed on the manufacturing process of the P/M samples should reduce measurement uncertainties. In addition, the influence of the production process can be investigated by having samples from four different manufacturers, each producing a set of identical parts.

A semi-automated testing arrangement was developed to accommodate the large number of samples as well as to ensure measurement reproducibility. The parts were contacted through a computer-operated bench press, which guaranteed constant contact pressure and repeatable geometric positioning. A voltage controlled electric current source with three different pre-set settings allowed the injection of currents of 1A, 2A and 2.5A. The voltage is recorded on the circumference of the parts with a dual pin sensor of fixed distance between the measuring points. The basic testing setup is illustrated in Figure 1.

In addition to the dual contact voltage sensors, two different sensors have been developed to investigate the effects of surface conductivity, geometrical averaging or lubricant migration. One sensor used the same concept for current injection as described above, the voltage sensing, however, relied on two ring bands which contacted the green state compact along the entire circumference, as depicted in Figure 2. This eliminated the dependency of the recorded voltage on the geometrical positioning of the two-pin sensor. Another sensor employed an isolated center pin within the aluminum rod for voltage sensing.

Figure 1. Current excitation and voltage measurement for controlled green-state samples. The compacts receive the current excitation through blocks of aluminum rods covering the entire surface of the sample.

The current was injected through the outer ring and the resulting voltage was measured from top to bottom of the contacted part. This arrangement allowed for the highest reciprocity, since the contact placement and the contact pressure were assured to be identical for each measurement.

Results

Figure 2 shows various sensor configurations that have been developed to record the conductivity of the green-state compacts. In general, the measurement data are generated as a result of averaging the numerical values over 3 different current strengths and over three samples with the same nominal density. From these measurements it can be concluded that density versus conductivity for pure metal powders follows a linear correlation over the given density range. This linear relationship was found to be consistent for parts produced by all manufacturers and measured with all of the above-mentioned methods. However, the absolute values of conductivity found in the different batches differ significantly.

Figure 2. Measurement setup with current injection through ring contact and voltage sensing through center points from top to bottom of tested part (A), and with a ring contact for voltage sensing (B).

While the recordings for pure iron powder yielded the expected linear density-conductivity correlation, the measurements of parts from iron/lubricant mixtures resulted in a surprising outcome. As shown in Figure 3, the conductivity in this case exhibits a more complicated behavior, which can best be approximated by a parabolic function. Contrary to the non-lubricated case, we find a maximum in conductivity for a density of approximately 6.8 g/cm3. If the density is increased beyond this point, the conductivity begins to decrease. Furthermore it is interesting to note that the amount and type of lubricants in the green-state samples significantly influence the density versus conductivity correlation.

Figure 3. Comparison between the conductivity of green-state samples with different amounts of lubricant (AWX).

Crack Detection - Commercialization

The inspection system developed above relies on a novel array sensor concept consisting of up to 100 spring-loaded probes in contact with the green-state sample. Current is injected in various directions, and voltages are measured over the surface area of the part. Although the method is most sensitive to surface breaking flaws as small as 20 microns in size, subsurface flaws can be detected to a depth of approximately 7.5 mm. The developed inspection apparatus is inexpensive and allows for flexible sensor configurations for parts of different geometries.

The research program resulted in a prototype system that was tested with industrial green-state samples. Additional modifications and enhancements resulted in a stepper motor driven press with foot paddles and indicator lights to enable an operator to make go/no-go part evaluations within 1 to 2 seconds. Testing with controlled and industry supplied production samples have underscored the success of this testing methodology. A U.S. patent has been issued in 2001 (US 6,218,846 - Multi-Probe Impedance Measurement System and Method for Detection of Flaws in Conductive Articles) and commercialization efforts are proceeding.

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Last modified: September 10, 2007 11:19:33