International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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Eddy Current Testing at High Temperatures for Controlling Heat Treatment ProcessesJochen Vetterlein, Foundation Institute for Materials Science, Bremen, Germany
Heinrich Klümper-Westkamp, Foundation Institute for Materials Science, Bremen, Germany
Thomas Hirsch, Foundation Institute for Materials Science, Bremen, Germany
Peter Mayr, Foundation Institute for Materials Science, Bremen, Germany
Increasing quality demands have made a reliable quality control getting more and more important for manufacturing and processing industries. Especially if 100% testing is required, the advantages of the eddy current technique are used by a wide range of applications to ensure constant quality: eddy current measurements require minimal testing time, offer the possibility to measure without human intervention and are non-destructive .
For heat treating processes, the method is used in various applications. Aircraft industry for example uses the eddy current technique to measure the conductivity of heat treated aluminium components to check hardness and tensile strength . To ensure that aluminium plates have been properly heat treated and exhibit a satisfactory degree of homogeneity, systems are developed that automatically measure conductivity on the top and the bottom of the plate as sensors scan across . Multi-frequency systems make the control of ferromagnetic workpieces possible, for example to check material structure or cracks [4-6].
For all of these test procedures, the measurements are performed after the heat treatment has taken place, as there are no systems, which can be used at the heat treatment temperatures and atmospheres. Measurements during the treatment would offer much more information about the process, because not only the final state, but also all the changes during heat treatment could be monitored continuously. This additional information makes the process saver and can be used to adapt the process more easily, if changes become necessary.
To perform measurements under such difficult conditions at high temperatures the eddy current technique shows to be the most suitable NDT system because of its easy handling and the simple build-up of the sensor.
The article describes the development of an eddy current measuring system to characterise micro-structural changes during heat treatment. It can be used at temperatures up to 500°C. The construction of the sensor and the method of data analysis is illustrated. The system was used to characterise the aging process of the Al Zn Mg Cu alloy EN AW 7075. The correlation between micro-structural changes and the eddy current signal are discussed.
2 Eddy Current Measurements
2.1 How to detect the heat treatment state with eddy-current testing
2.2 Build up of the measuring system
The impedance analyser supplies a sinusoidal current to the transmitting coil. At the same time the voltage of the receiving coil is measured and the complex impedance (inductance and resistance) is computed.
The sensor is a radial symmetrical two-coil-system. The receiving coil is placed between the sample and the transmitting coil where the strength of the secondary field reaches the highest values. The sensor has to be made of heat resistant materials, because it will be used inside the furnace. The winding wire of the coils is a fibreglass-insulated cooper wire. The coil former is made of machinable ceramic. The coils are encapsulated in a ceramic tube filled with ceramic sealant (Fig. 1).
Simulations were performed  varying the sensor's geometry and frequency to maximise its sensibility towards conductivity changes. The properties that were varied are illustrated in Fig. 2.
The resolution of the measuring system is not only dependent on the sensor. The precision of the impedance analyser affects the resolution of the system as well. The calculations of the resolution of the sensor were made assuming an error of the absolute impedance and the phase shift of ± 0,1% and ± 0,1°, respectively. Using an analyser of a higher or lower quality class will change the absolute values of the calculated resolutions, but the general dependencies will stay the same.
Additionally, it has to be considered that the precision of an analyser is only achieved if the amplitude of the sensor signal exceeds a specific value. For example, the analyser used for the investigations demands a minimum absolute impedance of 5W .
The simulations were performed keeping the winding density of the coils constant as this parameter is fixed if a winding wire of a specific diameter is chosen. As well, the outer diameter of the sensor was not varied, because it should be chosen according to the geometry of the part to be evaluated. The results of the simulations are presented in Table 2.
The construction of the sensor is based on these dependencies. The amplitude of the sensor signal is maximised by choosing a transmitting coil of high length, a minimal inner radius of the transmitting and receiving coil and a minimal distance between transmitting and receiving coil. The length of the receiving coil is a compromise. On the one hand a minimal coil length results in the best resolution, on the other hand the coil has to be long enough so that the required signal amplitude is achieved.
As the penetration depth of the eddy currents decrease with increasing frequency this parameter is commonly used to choose the depth to be inspected. For example, the inspection of thin sheets or of coatings is usually done at high frequencies. Thus it is usually not possible to vary the frequency to optimise the sensor's resolution. The measurements presented where performed at a frequency of 10 kHz which is close to the optimum frequency of 5kHz. Simulations showed that this frequency is high enough. The finite thickness of the 10 mm samples will not interfere the measuring results.
2.3 Elimination of unwanted variables
2.4 Calculation of the conductivity
To overcome these difficulties, a method was found to calculate the conductivity from the sensor signal. This makes a correlation between the conductivity and the parameters of interest independent of the frequency and the sensor.
Dodd et al.  found, that the normalized complex sensor impedance Z can be written asa function of the product of the sensor's frequency w and the conductivity of the workpiece s :
Assume a reference sample of known conductivity s0 will provoke a sensor impedance Z(s0,w 0) at the sensor frequency w0. A workpiece of an unknown conductivity sw will cause a sensor impedance Z(sw,w) at a sensor frequency w. According to eq. 1 the frequency w can be varied in such a way that Z(sw, w )=Z(s0,w0). In this case the unknown conductivity it given by:
However, an adjustment of the frequency to get the conductivity would be an inconvenient practice, therefore another method was used performing two measurements at the frequencies w0 and w1 provoking the impedances Z(sw,w0) and Z(sw,w1), respectively. According to eq. 1 a hypothetical measurement at the frequency w 0 of a workpiece with a conductivity
will provoke the impedance
Assuming small conductivity changes sw-sw' and s0-sw the impedance change will be proportional to the conductivity change:
Using eq. 3 and 4 you will get:
By this way the absolute value of the unknown conductivity s w can be calculated with only a single calibration measurement Z(s0,w0).
3 Age-hardening aluminium alloys
Age-hardening aluminium alloys contain at least one element that is soluble in the aluminium matrix at elevated temperature, but only to a small degree at room temperature. Heat treatment of such alloys is a three-step process:
The commercial age-hardening alloys are the Al Cu alloys (2xxx series), the Al Mg Si alloys (6xxx series) and the Al Zn Mg alloys (7xxx series). The precipitating equilibrium phases of these alloys are Al2Cu, Mg2Si and MgZn2, respectively.
Highest strengths are achieved by age-hardening alloys. This is the reason why these systems are predominantly used in light constructions, for example in the aircraft industry.
The age hardening process was investigated using the Al Zn Mg Cu alloy EN AW 7075. The heat treatment state was T651 (solution heat treated, quenched, stress relieved by stretching, artificially aged). The base material was delivered as a plate with a thickness of 10 mm, which was cut to sample size (70 x 70 x 10 mm3). More details are given in .
The samples were solution heat treated for one hour at 456°C using an air circulation furnace and quenched in water at room temperature. Two typical aging treatments  were used:
Artificial aging at 120°C is used to achieve maximum strength . The two-step process was developed to improve the resistance to stress corrosion cracking . As an additional advantage of the two-step aging, mechanical properties can be improved, if the critical quenching rate of the alloy cannot be reached, for example for workpieces of large dimensions.
In contrast to the Al Cu alloys and the Al Mg Si alloys, naturally aging is commercially not used for Al Zn Mg alloys, because no stable mechanical properties can be achieved. Strengths increase over a period of many years . Nevertheless, a set of samples was aged at room temperature to get a better understanding for the metallurgical reactions.
The conductivity measurements were performed in-situ and continuously during the heat treatment using the sensor described in chapter 2. To correlate the conductivity with the heat treatment state a set of samples was made for each aging variant interrupting the heat treatment after a specific aging time by fast cooling to room temperature. The heat treatment state was then characterised by hardness testing and TEM micrographs.
The propagation of the conductivity is presented in Fig. 3. A detailed view of the first 5 hours of aging in shown in Fig. 4. During natural aging the conductivity decreases during the hole process, while artificial aging at 120°C is associated with an increase of the conductivity.
The conductivity of the two-step aging variant reaches a minimum after 40 min followed by a slight rise. The second aging step causes a fast increase of the conductivity.
The two-step aging variant shows a peak hardness after aging for 8 h (Fig. 5). After long aging times the TEM micrographs show the appearance of coarse particles (Fig. 7). Using single step aging, a similar hardness is reached after 24 h but there is no hardness peak. After reaching maximum hardness there is no significant change in the strength and the size of the particles (Fig. 6) differs only slightly.
The hardness of the naturally aged samples increases in a similar way as the single-step aged samples, but at a much slower rate. A constant hardness is not reached. Even after an aging time of 1000 h the hardness is still increasing.
The quenching of the aluminium alloy results in a supersaturated matrix. During aging the decomposition of the alloying elements will take place. The aging sequence is generally stated as [19-22]:
GP Zones (coherent)
h '-phase ("MgZn2, semi coherent)
h -phase (MgZn2, noncoherent).
The variation of the electrical conductivity results from a number of contributions [23, 24] that are summarised in Table 3. Depending on witch of these processes are dominating, a rise or a fall of the conductivity is observed during aging (compare Fig. 3 and 4).
Single-step aging at 120°C
After aging for about 10 hours the hardness keeps almost constant while the conductivity is still rising. During this period neither the processes that strengthen the alloy nor the processes that soften the material are dominating. The following strengthening processes may occur:
Simultaneously the following mechanisms cause a decrease in strength:
As it can be seen in Table 3 the strengthening processes as well as the softening processes will tend to increase the conductivity. This is the reason why the conductivity is still raising even after the hardness has reached a constant level.
Two-step aging at 100°C and 160°C
The second aging step causes a prompt increase of conductivity. It can be assumed that the mechanisms that causes this rising are the same as for the single step temper: the purification of the matrix due to the transformation of GP zone into h' und h particles and due to the growth of the particles. The fact that these processes are controlled by diffusion explains why the rise of conductivity occurs mach faster at 160°C than during the single-step aging at 120°C.
The peak hardness is reached after 8 hours. Contrary to the singe step aging the hardness does not keep constant but decreases during further aging. Obviously the softening mechanisms are dominating. A reason for this behaviour is seen in the coarsening being the dominant mechanism after reaching peak strength. This can be observed in the TEM micrographs. After long aging times large particles can be seen, while for the single-step aging the diameter of the particles increases only slightly (Fig. 6 and 7). The resitivity rises after 5.2 h with t (-1/3) (Fig. 8). This connectivity is expected, if a coarsening process is assumed as the dominant process and if nucleation of new precipitations can be neglected [25, 26].
The fact that the conductivity rises even after peak hardness is reached confirms the assumption that the strengthening processes as well as the softening mechanisms cause an increase of the conductivity. This is the reason why the hardness peak is not connected with an extremum in the conductivity progression.
The build-up of the measuring system shows that eddy current measurements can be performed at high resolution even at high temperatures of a heat treatment process. This made it possible to record the electrical conductivity of the aluminium alloy EN AW 7075 during the aging process.
The conductivity progression can be associated with micro-structural changes of the material. During natural aging the decomposition takes place mainly by the formation of GP zones resulting in a decrease of conductivity. For temperatures above the solvus temperature of the GP zones the precipitation of h' and h particles becomes the dominating decomposition mechanism resulting in an increase of conductivity due to the purification of the aluminium matrix and the dissolution of GP zones. For long aging times at 160°C the conductivity progression reflects that the process is governed by coarsening.
The investigations on the aging of aluminium are seen as an example how the eddy current technique can be used during a heat treatment process to get continuous information about the material properties. This information can be used to make heat treatments saver and to improve their reproducibility.
The authors thank the Bundesministeriums für Bildung und Forschung and the Deutsche Forschungsgemeinschaft for their financial support in this project.