Use of Eddy Current Conductivity and Hardness Testing to Evaluate Heat Damage in Aluminum Alloys
Aluminum alloys are heat treated to provide optimal material properties for use in a variety of applications. However, when exposed to abnormally high temperatures, an evaluation must be performed to determine if the aluminum component has been compromised. Nondestructive evaluation of aluminum alloys, by means of electrical conductivity and hardness tests, can assist in determining the condition of the part. These techniques require experience and engineering judgment to properly interpret the data produced in order to determine whether a part needs to be replaced. This article will elaborate on issues with these nondestructive techniques to help diagnose the condition of aluminum alloys exposed to high temperatures.
KeywordsAluminum Data interpretation Eddy current Fire damage Hardness testing Heat treated Nondestructive examination
Modern aircraft structures use a variety of aluminum alloys that are carefully heat treated to optimize mechanical properties and corrosion resistance [1, 2, 3]. These structures can be subjected to heat from engines, electrical fires, hot air duct leaks, or excessive braking. Exposure to elevated temperatures in heat-treated or cold-worked aluminum alloys typically results in a significant loss of yield and ultimate tensile strength .
Reductions in strength are reflected in marked softening and a general increase in conductivity, as reflected in low Rockwell hardness values (in HRB) and high conductivity (in % International Annealed Copper Standard, or % IACS) readings, respectively [5, 6]. These occur due to the change in one or more of the following: the shape, size, distribution, and coherency strains of the second phase precipitates in heat-treated alloys [7, 8], recrystallization and grain growth , and/or recovery processes involving relaxation of dislocation stress fields in cold-worked alloys [10, 11].
Knowledge of the specified hardness and conductivity values for various aluminum alloys and their tempers is instrumental in determining the degree of damage when exposed to elevated temperatures . However, there are challenges in interpreting the data when examining potentially damaged aluminum structures in actual applications. This paper will review these challenges and offer solutions to assist in determining what, if any, damage may have occurred.
For heat-treated aluminum alloys, damage in most cases begins near 150 °C (300 °F) . The degree of damage sustained is dependent upon the time and temperature to which the component is exposed (an Arrhenius relationship). A prolonged exposure time at a relatively low temperature can cause just as much damage as a short time exposure to a relatively high temperature.
When performing a field survey, the measurement equipment being used should be portable and compact, allowing an operator to obtain precise readings in tight quarters. The operator should be familiar with the use of, and artifacts produced by, the equipment prior to use in on-scene conditions. For evaluating heat damage of wrought aluminum alloys, an operator needs to measure electrical conductivity and hardness. Any equipment capable of measuring electrical conductivity by the eddy current method in % IACS units is acceptable, although equipment capable of variable frequencies is preferred. Use of a standard such as ASTM E1004 or equivalent is suggested .
To measure hardness, any portable C-clamp-type hardness tester indenter that can use the 1/16-inch (0.25 mm) ball accurate to ± 1.0 HRB, such as the Ames series of instruments, is acceptable. However, the authors have found that using the smallest cantilever for the part is best, since larger (heavier) cantilevers can allow operators to place more torque on the instrument during measurement, creating more variability when reading hardness values. In fact, deflecting the hardness equipment arm after placing the major load was found to induce over 5 HRB difference in reported readings. Again, using a reference standard for portable hardness testing, such as ASTM E110 or equivalent, is strongly recommended . The operator should also note the use of a steel or tungsten carbide indenter, as described in ASTM E18 .
When determining where on a structure to look for regions that may have heat damage, the first visual indication is usually scorching or burning of the polymer-based primers and paints that coat the underlying metals. A chemical reaction between the polymer coatings and oxygen causes significant degradation and accompanying changes in color. The threshold temperature for damage is usually greater than 163 °C (325 °F). Again, the degree of damage is dependent on a combination of time and temperature: long exposures to low temperature can look visually similar to short exposures to high temperature. The discoloration will tend towards brown, gray, and then black as the temperature and/or exposure time increases.
Paint and primer need to be removed to examine the electrical conductivity, unless lower frequencies are used (around 60 kHz) and the primer layer is less than 0.5 mm (0.020 inches) thick. Chemical methods of paint removal are best, but mechanical grinding may be necessary in certain applications. However, it is important not to damage the underlying aluminum surface, which could create measurement artifacts with both electrical conductivity and hardness, as well as permanent physical damage to the structure. Cladding, ion-vapor deposited (IVD), and anodized surface layers can produce incorrect hardness and conductivity values, if not accounted for. Additionally, parts that are too thin, particularly at lower eddy current frequencies, will produce incorrect data.
When evaluating aluminum parts, the eddy current conductivity equipment used by the authors was set to either 60 or 500 kHz. Other models may allow for frequencies between or outside these values. The higher frequency is more surface sensitive but produces more consistent conductivity data. Use of a lower frequency enables further penetration into a component, past some surface coatings and cladding, but should it not be used on parts thinner than 1.5 mm (0.060 inches) . When using a higher frequency for measuring conductivity, consistent results can only be obtained for uncoated, bare aluminum parts thicker than 0.5 mm (0.020 inches). Otherwise, cladding, IVD aluminum, and anodize coatings can significantly influence the results.
Eddy current measurements are temperature sensitive. Instruments must be acclimated to the working environment for a minimum of 15 min prior to standardization. The eddy current equipment should be standardized to two reference standards before and during an inspection. Many instruments carry the two standards attached to their frame.
Typically, eddy current equipment used to measure electrical conductivity will use a low and high % IACS standard. In addition to standardization prior to any inspection, the equipment needs to be restandardized over time, as ambient temperature and humidity changes occur. These environmental changes will alter the readings produced by the equipment. Some modern equipment will invoke an alarm or warning over time to instruct the operator to restandardize the equipment, typically every 5–10 min.
The conductivity reading for alloys covered by specifications will vary based on the alloy composition, specific heat treatment, and the presence or absence of an external cladding layer. A standard or specification providing aluminum conductivity and hardness properties for specific heat treatments, such as AMS 2658 , must be used. Claddings, which are almost pure aluminum, in sufficient thickness will exhibit a conductivity of 61% IACS . In general, increased alloying weight percent, more finely distributed precipitates, and increased cold work, which all factor in higher strengthening, will raise the resistivity (lower % IACS) of an aluminum alloy. Exposure to elevated temperatures over a longer time will reverse these effects, increasing the measured electrical conductivity.
However, the conductivity results produced are not absolute and should not be used in and of themselves to determine if a part is heat damaged. Rather, the results should be contrasted against those of an undamaged portion of the part, since individual aluminum structures can exhibit a variety of readings within the “acceptable” range. It is essential to determine a baseline conductivity reading. Deviations from the established baseline are more significant than the numbers in isolation. Note that in obtaining baseline numbers, similar areas should be compared against other similar areas. For example, in a structural former, flanges should be compared against flanges and webs against webs. Otherwise, natural differences in the heat treatment of thick components can also produce misleading heat damage evaluation results.
As an additional and more detailed example, consider the acceptable conductivity range listed in AMS 2658 for an AA 7175-T6 alloy is 30.5–36.0% IACS . An inspector who performed conductivity readings in a suspected damaged area averaging 35.0% IACS could be tempted to believe the part is acceptable. However, if the conductivity of an “acceptable” area averaged 31.5% IACS, the part would still likely be heat damaged. The authors determined that areas exhibiting an increase of 1.5% IACS from undamaged areas would be considered suspect and should be subjected to follow-up hardness testing.
It is best to obtain a minimum of three conductivity measurements in any one area. The values should be with 0.5% IACS of each other. The operator should make additional readings in and around the heat-damaged areas. This process should begin in the darker or most likely damaged areas, working outward in a single path. The operation is repeated as many times as necessary to establish values in several directions leading away from the damaged zone, noting any increasing or decreasing trends.
The operator should be aware of artifacts in performing conductivity readings. Avoiding inspecting the end or the corner of a part is important, as the eddy current edge effect will produce spurious readings. One artifact should be noted in trends of observed conductivity readings—above a heat exposure threshold, a trend of increasing conductivity readings can appear to decrease rapidly. This is not an area that is “acceptable” but a more serious region of heat damage. This is known as a conductivity inversion, which occurs due to the resolutioning of alloying elements that remain in solution when the part exposed to elevated temperatures cools rapidly [7, 18]. This conductivity inversion is a strong indicator of significant thermal damage and, when it is observed, tends to occur on approaching the center of the heat-damaged region. This inversion has also been noted in alloys with fluctuating interior residual stresses . Inspection by hardness readings will confirm this severe heat damage, as hardness readings approaching the annealed hardness will be obtained.
Heat damage evaluation by hardness on aluminum alloys typically employs the Rockwell B scale. Performing hardness testing in laboratory settings is relatively easy, producing sound and repeatable data. However, in the field, when using a cantilever hardness tester in precarious positions, it is easy for the operator to introduce error during a test. It is recommended that the operator become familiar with using the equipment, not just on a standard while upright on a table or bench, but also in various more difficult physical positions including crouching and bending.
Field hardness inspection requires removal of paint, primer, and sealant on both sides of the part. It is imperative that the anvil side of the part be prepared such that the anvil sits flat and steady without wobbling. The part should be accessible from both sides, without impacting adjacent components that might bend the hardness tester, thereby changing the loading results. Inconsistent readings (> 3 HRB) indicate that something in the test is not optimized. This could be debris on the anvil side, roughness on the indenter side, location errors, equipment anomalies, or other factors which do not permit a controlled test. The operator should have a good line of sight to the loading gage to read the indicator hand without introducing a parallax error.
The hardness equipment must be inspected for accuracy and precision prior to each inspection. If possible, the equipment should be inspected using a known standard block as close as possible to the expected hardness of the structural part (within 5 HRB is ideal). If consistent hardness readings cannot be obtained on a standard block, then it is likely that some part of the equipment is loose, some contamination is on the contact surfaces, or the equipment has become damaged. This daily verification should be performed after the anvil, indenter, and any applicable spacers needed to test the component have been installed in the arrangement necessary to test the subject component.
As hardness indents can themselves become stress risers that initiate fatigue cracks, it is best to make only as many indents as necessary to understand the local hardness of the heat-damaged part. If possible, taking measurements on flanges and areas less likely to be load-bearing is best practice.
Use of eddy current conductivity and hardness measurements are an excellent complement to visual observations in determining whether aluminum alloys have been damaged after heat exposure. The presence and extent of damage can assist the engineering staff in determining where a region transitions from repairable to total loss. In some cases, where accurate data exists, the properties of intermediate damage can be ascertained from hardness readings and used to develop repair procedures.
Given that the damage mechanisms at elevated temperature are kinetic, the effect of any property losses will be related to both the magnitude of the temperature and the time of the exposure. As in any thorough inspection, the operator should be familiar with the equipment, including knowing what artifacts can be created during an inspection. This experience will ensure that the recorded data are understood and interpreted correctly.
Using nondamaged portions of a structure as a baseline to determine whether damage exists is a better practice than employing the conductivity values available in industry standards alone. Data and experience show that reliance upon conductivity acceptance values in isolation can lead to erroneous conclusions. As shown in this paper, good engineering judgment and interpretation of the data are vital to providing a proper recommendation as to whether an aluminum part is heat damaged or not.
In accordance with Title 5 Code of Federal Regulations §2635.807(b)(2), the views expressed in this article do not necessarily represent the views of the National Transportation Safety Board, The US Navy, or the United States of America.
- 5.B.Y. Lattimer, J. Jabra, M. Romios, J. Lai, E. Lee, M. Setiawan, J.R. Ogren, R. Clark, T. Oppenheim, O.S. Es-Said, E.W. Lee, N. Abourialy, W.E. Frazier, J. Witters, The effect of thermal exposure on the mechanical properties of 2099-T6 die forgings, 2099-T83 extrusions, 7075-T7651 plate, 7085-T7452 die forgings, 7085-T7651 plate, and 2397-T87 plate aluminum alloys. J. Mater. Eng. Perform. 15(5), 601–607 (2006)CrossRefGoogle Scholar
- 6.R. Mueller. Relationships among the Metallurgical Condition, Hardness, and the Electrical Conductivity of Aluminum Alloys. Missouri University of Science and Technology, Rolla, MO, Master’s Thesis T 2010, (1967) p. 6808Google Scholar
- 7.D.J. Hagemaier, Evaluation of heat damage to aluminum aircraft structures. Mater. Eval. 40(9), 962–969 (1982)Google Scholar
- 9.G.E. Dieter, in Annealing of Cold-Worked Metal, ed. by S. Rao Mechanical Metallurgy, 3rd ed., (McGraw-Hill, New York, 1986), pp. 233–236Google Scholar
- 12.N.K. Langhelle, J. Amdahl, Experimental and numerical analysis of aluminium columns subjected to fire. in Proceedings of the Eleventh International Offshore and Polar Engineering Conference., June 17–22, 2001 (Stavanger, Norway), International Society of Offshore and Polar Engineers, (2001), pp. 406–413Google Scholar
- 13.Standard Test Method for Determining Electrical Conductivity Using the Electromagnetic (Eddy Current) Method, ASTM E1004, ASTM International, West Conshohocken, PA (2017)Google Scholar
- 14.Standard Test Method for Rockwell and Brinell Hardness of Metallic Materials by Portable Hardness Testers, ASTM E110-14, ASTM International, West Conshohocken, PA (2017)Google Scholar
- 15.Standard Test Methods for Rockwell Hardness of Metallic Materials, ASTM E18, ASTM International, West Conshohocken, PA (2017)Google Scholar
- 16.Eddy Current Inspection, Metals Handbook Ninth Edition Vol 17—Nondestructive Evaluation and Quality Control, ASM International: Metals Park, OH, (1989) pp. 164-179Google Scholar
- 17.AMS 2658—Hardness and Conductivity Inspection of Wrought Aluminum Alloy Parts, AMS D Nonferrous Alloys Committee, SAE International. Warrendale, PA. (2016)Google Scholar
- 18.J. Calero, S. Turk, Effects of thermal damage on the strength properties of 7050-T7451 and 7075-T7351 aluminium alloys, DSTO Defence Science and Technology Organisation: Victoria, Australia, (2008), DSTO-TR-2104, p. 15Google Scholar