Conductivity testing and material sorting Introduction

Eddy current testing can be used for material sorting on the basis that different materials, and even the samples of the same material in different heat treatment conditions, have different values of electrical conductivity. The conductivity of a conductor can be measured using a dedicated eddy current conductivity meter. Alternatively, most modern impedance display instruments include a digital conductivity measurement facility when used with a probe designed for this purpose. Neither of these instruments will give a conductivity reading if a ferromagnetic material is tested. For material sorting, a comparison of the phase of the lift-off signals obtained from the unknown samples with signals from known reference materials may suffice: it may not be necessary to determine the actual conductivity value. This approach may be used with both non-ferromagnetic and ferromagnetic materials. Conductivity testing or sorting by conductivity comparison can also be used to determine the density of parts produced by powder metallurgy.

Conductivity and its measurement

The SI unit of conductivity is the Siemens/metre (S/m), but because it is a very small unit, its multiple, the megaSiemens/metre (MS/m) is more commonly used. Eddy current conductivity meters usually give readouts in the practical unit of conductivity,% IACS (% International Annealed Copper Standard), which give the conductivity relative to annealed commercially pure copper. To convert% IACS to MS/m, multiply by 0.58, and to convert MS/m to% IACS, multiply by 1.724.

For instance, the conductivity of Type 304 stainless steel is 2.5% IACS or 1.45MS/m.

Resistivity is the inverse of conductivity, and some publications on eddy current testing refer to resistivity values rather than conductivity values. However, conductivity in% IACS is universally used in the aluminium and aerospace industries.

Factors which affect conductivity

electrons reduces the conductivity. This includes any factor which stresses the crystal lattice.

Generally, factors which harden and strengthen metals also reduce the conductivity. This is because most, though not all, hardening mechanisms achieve their hardening by introducing stress into the crystal lattice.

Factors which affect the conductivity include the following:

(a) Temperature.

Increasing the temperature increases the amplitude of vibration of the atoms, which in turn increases the impedance to the electron flow. For accurate readings, the reference standards and the test part should be at the same temperature. For greatest accuracy, measurement therefore have lower conductivity than the pure metal they are formed from. Some impurities in metals, even in quite small amounts, can significantly reduce the conductivity. For example, 0.1% of phosphorus in copper reduces its conductivity from 100% IACS to 50%

IACS. This is important, because phosphorus is sometimes used to deoxidize copper during its refining and, if so, some phosphorus remains in the material. This material is therefore not suitable for high conductivity applications.

(c) Heat treatment.

Many alloys can be hardened by heat treatment, which introduces stress in the lattice and so reduces the conductivity. On the other hand, annealing processes, which generally consist of heating followed by very slow cooling, relieve stresses in the crystal lattice, and so reduce the hardness of the metal to a minimum and increase the conductivity to a maximum.

(d) Cold work

Plastic deformation of metals at ambient temperatures introduces stress into the crystal lattice, and so increases hardness and reduces conductivity. However, the effect on conductivity is generally less than 10% and so can usually be ignored. An exception is austenitic stainless steel, which is normally non-ferromagnetic, because it can become ferromagnetic if cold worked. This has a major effect on the eddy current signal, and prevents a conductivity reading being taken on conductivity meters.

Factors which affect the conductivity reading (a) Lift-off

Conductivity meters and impedance display instruments which include a digital conductivity measurement facility normally provide lift-off compensation for lift-off values up to at least 0.08 mm, so the reading should not be affected by paint coatings up to this thickness or by minor surface roughness. If it is considered likely that the reading will be affected by a coating or surface roughness, conductivity readings should be made on a surface showing the

condition of the test part and on a flat, smooth surface to determine if a lift-off correction is required.

(b) Test part thickness

The conductivity reading will be affected if the test part thickness is less than the ‘effective depth of penetration, which, for the coil diameter normally used for conductivity testing (approximately 10 mm), can be taken as 3 times the standard depth of penetration. If there is doubt as to whether the test part is thick enough, it should be tested, if possible, with and without a block of copper held against the far surface. Any difference in the readings indicates that the thickness is less than the effective depth of penetration. Most eddy current conductivity meters operate at a fixed, common frequency of 60 kHz. This is low enough to give sufficient penetration to avoid any contamination of the test surface having a significant effect, and high enough to allow testing of reasonably thin materials. Some conductivity meters have one or more, higher alternative test frequencies, and if so, a higher frequency may be selected to avoid excessive penetration. However, they should be operated at the lowest frequency possible for the test part thickness in order to avoid any contamination of the test surface having a significant effect. The second ‘higher’ frequency is commonly used to determine cladding thickness.

If the test part thickness is less than the effective depth of penetration, parts may be tested by determining a correction, or for sheet material, by stacking.

(c) Edge effect

Probes dedicated to conductivity testing are manufactured so that there is no edge effect if the area being tested is equal to or larger than the probe area and the probe is centered on this area. That is, if the edge (or hole) lies outside the area covered by the probe, there should be no edge effect. This should be verified by taking readings on a sample as the probe approaches an edge. If a smaller area is required to be tested, a smaller conductivity testing probe must be used, if available, or comparative testing with a standard eddy current instrument and a small diameter or shielded probe to determine the lift-off signal and compare it with those from known test samples must be used.

(d) Curvature

Curved surfaces with small radii of curvature may give an incorrect reading, the deviation increasing with increasing conductivity. If it is considered likely that the curvature will affect the reading, a correction must be determined.

If lift-off compensation is important because of a rough surface or a non-conducting coating, the lift-off signal should be as close as possible to 90⁰ from the conductivity signal for change in conductivity, because signals are more easily suppressed if they are at 90⁰ to the signal of interest.

FIG. 5.19 shows the operating points for a number of materials at three different frequencies.

It can be seen that, as the frequency increases, the operating point moves clockwise, further down the impedance curve. Furthermore, the greatest separation of the operating points for materials of different conductivity, and therefore the greatest sensitivity to conductivity is obtained if the operating point is in the central region of the impedance curve. If possible, conductivity testing and material sorting should therefore be carried out at a frequency which brings the operating points of the materials being tested to this central portion of the impedance curve. In addition, if lift-off compensation is important the operating point should be in the lower portion of this region, below the ‘knee’ of the curve. To allow the determination of an appropriate frequency, a relationship between frequency and operating point is required.

FIG. 5.19. Impedance diagrams and the conductivity curve at three different frequencies, showing that, as frequency increases, the operating point moves down the conductivity curve.

It can also be seen that the angle θ between the conductivity and lift-off curve is quite small for operating points near the top of the conductivity curve, but greater in the middle and lower parts of the curve. The increased sensitivity to variations in conductivity towards the centre of the conductivity curve can also be seen.

The Characteristic Parameter (PC) is a means of determining the operating point on the impedance curve. The formula for Pc depends on the units used, but the following is recommended:

PC = 4.6 × 10^-3 f µ_rσ r² (5.3a)

Or

f = PC /(4.6 × 10^-3µrσ r²) (5.3b) where

P_C = characteristic parameter, f = frequency (kHz),

µr = relative permeability, σ = conductivity (% IACS), r = mean coil radius (mm)

Note that the unit of frequency used in this formula, as in the other ‘applied’ eddy current formulae (for f90 and f/fg) is kHz, unlike the basic theoretical formulae for depth of penetration and inductive reactance.

Calculations of PC are used in conjunction with FIG. 5.20, which relates the value of PC to the operating point on the impedance curve for surface probes.

FIG. 5.20. Impedance diagram showing conductivity curves at a number of different values of lift-off to coil radius ratio (LO/r) (the solid curves) and a number of lift-off curves (dashed) Values of PC are indicated for the zero lift-off conductivity curve. These values also apply to the corresponding locations on the other conductivity curves.

Example:

Calculate the optimum frequency for maximum sensitivity in sorting commercially pure titanium (conductivity approximately 2.5% IACS) from Ti-6AI-4Valloy (conductivity 1%

IACS), using a coil with outer diameter 10 mm wound on a 6 mm diameter plastic core.

Calculation:

For maximum sensitivity to conductivity, and minimum response to lift-off, the operating point should be at or somewhat below the knee of the impedance curve. That is, P_C should be between 10 and approximately 200. The P_C value will be different for the two alloys, but, since PC is proportional to conductivity, the two values will differ by a factor of 2.5/1 = I (the

That is:

f = PC /(4.6 × 10^-3µ_rσ r²)

P_C = 30 (for titanium),

r = (10 + 6)/2 = 8 mm, so r = 4 mm, µ_r = 1

σ = 1% IACS Inserting these values into equation gives:

f = 30/ (4.6 × 10^-3 × 1 × 1 X 4²)

= 407.6 kHz or, rounding to a convenient figure:

= approximately 400 kHz

When eddy current conductivity meters are used, this approach is normally not possible, because they operate at one or more fixed frequencies. Normally, these instruments are designed to give an operating point in the region of the knee of the curve at the lowest frequency, normally 60 kHz, and higher frequencies are used only if the material is too thin for testing at the lowest frequency.

Test procedure using an eddy current conductivity meter

(a) Locate the reference standards and the test part in the same area, and allow time for the temperature of both to equalize.

(b) Switch on the instrument and allow it to warm up for a period of at least 5 minutes.

(c) Calibrate the instrument in accordance with the instrument manufacturer's instructions.

If possible, the calibration should be carried out or checked using reference standards with conductivity values similar to those to be measured. For aluminium alloys, the calibration should be carried out or checked using two reference standards which differ by at least 10% IACS, one in the range 25 to 32% IACS and the other in the range 38 to 50% IACS.

(d) Perform conductivity measurements of the test part. Normally at least three readings should be taken on each test part to obtain a conductivity measurement, each reading being taken on a different area. The test areas should be flat, of sufficient area to avoid edge effects, of sufficient thickness that the thickness does not affect the reading, and, if a coating is present, it should be thin enough not to affect the reading. If any of these conditions cannot be met, with the instrument and frequency being used, alternative frequency, probe, or instrument which allows these conditions to be met should be used if this is not possible, a correction must be determined experimentally for the particular material and test conditions, and added to or subtracted from the instrument reading.

Test procedure using other eddy current instruments

If an impedance plane display instrument is being used to sort materials, lift-off signals should be obtained from suitable reference standards and the lift-off signals of the materials to be tested should be compared with these. For example, if it is required to sort material known to consist of two different known alloys or heat treatment conditions, lift-off signals should be

obtained from a number of samples of each material. It is important to have enough samples to assure that the range of conductivities of the two materials do not overlap. The instrument is adjusted so that the lift-off signals from one material are located towards one edge of the screen, and the lift-off signals from the other material are located towards the other edge of the screen.

Meter-indicating flaw detectors can be used for the same purpose, in this case adjusting the instrument so that the readings from one material lie towards one end of the scale, and the readings from the other material lie towards the other end of the scale.

Heat treatment of aluminium alloys and its verification

During age hardening of aluminium or titanium alloys, the hardness and conductivity of the material change simultaneously so that the degree of hardening may be obtained by measuring the conductivity of the test specimen and comparing it with a standard of that material with a known hardness. The operator must be aware of the effects of discontinuities and lift-off on the meter readings.

As can be seen from FIG.5.21 a typical conductivity of 35% IACS could have a Rockwell B hardness of either 20 or 85, from this we can see that conductivity measurements for heat damage must be accompanied with hardness testing to confirm true condition. In the manufacturing industry, conductivity alone can be used to confirm material Temper after heat treatment.

FIG. 5.21. Diagram showing the relation between hardness and conductivity at various stages in the heat treatment, aluminium-zinc alloys 7075. Other heat-treatable aluminium alloys show similar curves.

Sorting materials by magnetic permeability

In eddy current testing the test coil is sensitive to many test parameters, including magnetic permeability. Due to the amplification effect caused by magnetic permeability, measurement of permeability and high levels is generally not possible. However many paramagnetic alloys can exhibit permeability properties and almost any non-magnetic alloy can pick up magnetic

At normal eddy current test frequencies magnetic indications will often appear similar to discontinuities. Magnetic indications can be distinguished from discontinuities by re-testing at a reduced test frequency.

5.1.4 Thickness measurement of non-conductive coatings