What is the conductivity of metal?
The conductivity of metal is defined as the measure of metal’s ability to transfer electric charge, heat, or sound.
Imagine you are touching a metal piece in one hand, and a piece of wood on the other hand. Which material would feel hot? Which material would produce electricity? Obviously, it will be the metal piece, though both materials are at the same temperature. They might be having the same level of electric charge. Still, metal has high conductivity than wood.
Metals conduct electricity by allowing the transfer of free electrons between the atoms. Like charges always repel each other but unlike charges attract each other. That’s how the negatively charged electrons move in the direction of the current towards the positively charged end.
Pure metals provide the best conductivity. Only the impurities in most metals restrict the electron flow. That’s how alloys (a combination of alloying agents with pure metals) offer less conductivity compared to pure metal.
Depending upon the application, it is mandatory to understand a material’s tendency to conduct. For example, the wire can readily conduct electricity. Similarly, heat exchanger and radiator tubes readily transfer heat. To achieve this objective, we perform a conductivity test, a part of non-destructive testing (NDT).
Conductivity exists constantly as long as there are free charges (ions or electrons) inside the metal. Metals comprise of symmetrically distributed crystal grid structure that has ions. The conductivity also determines the properties of the material. If the crystal grid gets disturbed due to alloy addition or flaws, the conductivity changes according to the degree of grid distortion.
The conductivity testing of aerospace aluminum sheet not only detects the changes in strength and hardness of the object. But even after the heat treatment, it can also detect the thermal damage and weakness of the object. This is where digital eddy current conductivity meter can quickly and accurately calculate the conductivity value.
Why do we need to assess the conductivity of Metal?
There are a few requirements for why we should test the conductivity of the metal.
1. Conductivity test is usually performed to test the workpieces in aluminum, copper, and other non-ferrous metal processing industries.
2. Conductivity test helps in performing quality assurance of the raw materials.
3. Determine the purity grade of metals
4. Evaluate the homogeneity of alloys.
5. Determine the hardness and rigidity of the heat-treated target metal
6. Identify the properties of the target metal
7. Monitor the highly stressed portion easily.
8. Check the heat damage and material weakness of the metal
9. Sort out the waste
10. Sort magnetic materials from nonmagnetic materials easily, by comparing with the varied reference standards. But it is challenging to separate the conductivity from the magnetic permeability effects. Hence, conductivity measurements are limited to only nonmagnetic materials. Be aware that the conductivity of a specific object may vary with slight differences in the chemical composition. Hence, the range of conductivity is general and not specific.
11. Track the precipitation process. (For example: Copper Chromium alloys).
12. Estimate the phosphorus content in copper alloys.
13. Inspect the alloying in coinage manufacturing. (For example: euro coins)
14. Control the stability of the target metal which might be altered due to temperature swings.
15. Conductivity test helps in monitoring the heat treatment processes, strength, and hardness of the forged aluminum alloys in the aerospace and automotive industries.
16. Test the resistivity of the target metal
17. Detect flaws that occur due to the change in material conductivity, such as thermal damage in aircraft structures.
Traditional methods of conductivity testing for metals
Electric conductivity shows how well an object will allow electricity to travel through it.
There are two methods available to evaluate the electrical conductivity of metals:
- Traditional electric bridge method
This method involves pulling the metal into a wire, intercept a section, and bridging to test the electrical conductivity of the given object. But there are several disadvantages to assess metal conductivity using the traditional electric bridge method. This process is cumbersome to operate, time-consuming, and labor-intensive. This practice is not feasible, since the accuracy of the reading is also not guaranteed.
- Eddy current method
Eddy current is defined as the localized circular electric current induced in a conductor by a varying magnetic field. This non-destructive conductivity test using the eddy current method measures the conductivity of non-ferrous metals based on the electrical industry’s workpiece conductivity requirements. This method is very simple and comparatively easy to implement. It is used to check quality control for enterprises and research institutions in the production line. This test abides by the national standards.
When the intercepted alternating current of the coil (also known as the probe) comes in contact with the surface of the conductive material, it induces vortex-like current due to the action of the alternating magnetic field of the coil. The eddy current that develops in the material in turn produces its own magnetic field to react with the coil. The magnitude of this reaction is related to the conductivity of the material at and near the surface, as shown in the figure.
Advantages of electrical conductivity meter for metals
- Electrical conductivity meter for metals uses microprocessor data processing to ensure the accuracy of detection
- The metal conductivity meter has an LCD display count machine. So, one can visually view the readings without any error.
- The circuit used in the electrical conductivity meter is integrated in order to ensure small size and stable reading without any fluctuations.
- As the metal conductivity meter employs digital circuit data acquisition and processing, there is no need for the repetitive periodic instrument calibration.
- In order to ensure the accuracy of detection, this instrument is integrated with the digital signal acquisition for the first minute about 1000 times, with the subtle changes in temperature. This integration will ensure temperature compensation.
- The detection method is inherently durable and can be automated.
- This conductivity meter test for metals is nondestructive. The same target object can be intermittently or continuously tested over time.
- This method tests conductivity before aluminum anode oxidation
- The short duration of exposure to the applied voltage does not heat the object significantly. Hence, this method of testing avoids erroneously high measured conductivity results.
Importance considerations in eddy current conductivity meter testing
Here’s what you need to pay attention while performing a conductivity test:
- Ensure that the probe is positioned correctly. It’s the right calibration that makes all the difference.
- The temperature has a great effect on the metal conductivity. Conductivity is given only at the reference temperature of 20°C conventionally. If the ambient temperature is different while measuring, the conductivity can be converted into the conventional specification. Hence, conductivity probes should be equipped with a temperature sensor.
- Thickness plays a vital role in the measurement of conductivity. The material should be thick enough for the propagation of the current. Hence, the depth of the penetration of the current should be selected appropriately via the probe frequency.
- The way the measuring device is operated plays a vital role. Make sure that the probe is placed vertically on the surface without any pressure. A stand is used to lower the probe onto the sample automatically for exact accuracy.
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Post time: Apr-29-2021