|TABLE OF CONTENTS|
2.0 HEAT TREATMENTS OF ALUMINUM ALLOYS
.....3.2 NON-DESTRUCTIVE TESTING
......... IMMERSION ULTRASONICS, LASER-ULTRASONICS
3.3 MECHANICAL TESTING
3.4 MICROSTRUCTURE EVALUATION
4.1 MICROSTRUCTURE EVALUATION
4.2 PATTERN RECOGNITION
4.3 ULTRASONIC ATTENUATION AND MECHANICAL PROPERTIES
Figure 1. Precipitate phase transformations due to heat treatment are shown with the relationship between hardness and conductivity for AA7050.
Figure 2. Time-temperature transformation diagram for AA 7050 (from Reference ).
In 7000 series aluminum, the precipitation of the major phases occurs in the following sequence1:
Solid Solution® Guinier-Preston (G-P Zones)® ' (MgZn2)® (MgZn2)
This sequence can be followed with the help of Figure 1, which shows the relationship between hardness and conductivity for AA7050, as determined under controlled conditions using test coupons simulating various degrees of thermal exposure1. Open circles are data from the samples A-H used in this experiment. Solid circles are data from reference . Hardness was measured in the Rockwell B scale (HRB), and the electrical conductivity was measured using the International Annealed Copper Standard (%IACS) units. Figure 2 shows the time and temperature dependence of precipitate formation (time-temperature transformation (TTT) diagrams) as modeled by X.J. Wu et al. for the 7050 aluminum alloy. Together, the hardness-conductivity relationship and the TTT diagrams form the basis for understanding the temperature-driven behavior and properties of the 7050 alloy.
Guinier-Preston (G-P) Zones, which have high concentrations of magnesium and zinc, are coherent with the FCC structure of the matrix, thus contributing to a rise in hardness and strength and a reduction in conductivity compared to the solid solution state. The standard artificial aging treatment to obtain high G-P Zone content is the T6 temper, which consists of a 24 hour treatment at 120°C (Figures 1 and 2).
From the T6 temper to the T7451 temper, the formation of semi-coherent metastable ' phase results in an increase in electrical conductivity, and a slight decrease in hardness and strength .
From the T7451 temper, any subsequent exposure to temperatures significantly higher than the normal aging temperatures (160°C for this particular temper) progressively leads to a loss of hardness and strength as the strengthening precipitates first over-age and then go into solution.
Heat treatments applied to AA7050 samples.
|3092||A||485°C / 2hrs + 120°C / 20hrs|
|16003||B||485°C / 2hrs + 120°C / 21hrs+ 160°C / 3hrs|
|16010||C||485°C / 2hrs + 120°C / 16hrs+ 160°C / 10hrs|
|40902||D||T7451 °as received)|
|40901||E||T7451 °as received)|
|27230||F||272°C / 30min.|
|30045||G||300°C / 45min.|
|30060||H||300°C / 60min.|
Each sample was then heated in a furnace at a given temperature for a specific duration to simulate different degrees of heat damage or achieve a particular temper (Table 1).
Since the as-received material was already at a T7451 temper, three of the samples were solution treated prior to their specific heat treatments. This was done to achieve the T6 temper as well as two treatments between the T6 and T7451 tempers. The samples were water quenched rapidly after each heat-treatment stage.
Hardness measurements were made using a Clark Hardness Tester, and electrical conductivity measurements were performed by means of a Magnaflux FM-140 conductivity meter. The measured values were considered to be accurate to within ±0.5 %IACS.
Conventional immersion ultrasonic measurements showed that the ultrasonic attenuation at 10 MHz was weak and did not differ significantly between samples. Pattern recognition techniques were then used to attempt to distinguish between different extremes of heat treatments. Because ultrasonic attenuation generally increases with frequency, the samples were examined using high-frequency laser-ultrasonics.
3.2.1 IMMERSION ULTRASONICS
Immersion ultrasonic measurements were conducted at a 10 MHz excitation frequency, using a pulse-echo technique. The facility at IAR is an automated immersion system which uses Tektrend International's ARIUS® software for control and data acquisition. Further data analysis can be performed using Tektrend's ICEPAK® software or IAR's NDI Analysis software.
At 10 MHz, changes in attenuation and velocity due to heat treatments were not significant. Therefore, pattern recognition analysis was performed on the data as described in the following.
Pattern recognition (PR) algorithms can be used to classify unknown data into classes defined using known data. In this application, three classes were defined corresponding to different heat treatments, these were T6, T7451, and a highly damaged state labelled as HEAT DAMAGED. Training data is used by PR algorithms to create estimates of the probability that a measurement xi belongs to a certain class s (P(xi|s)). Different algorithms generate different estimates of P(xi|s), and use different methods of combining the measurements xi to classify the input vector X, the vector sum of the individual measurements. Obviously the training data must sufficiently represent the P(xi|s). Classification accuracy will suffer if the training data and its variance are not representative of the test situation.
A classifier based on a multilayer perceptron type artificial neural network was used to distinguish between samples which corresponded to the T6 temper (sample A ), the T7451 temper (samples D, E), and a HEAT DAMAGED state (sample H). Fifty signals from each class were used to train the classifier, and a different set of fifty signals from each class were then used to test the classifier.
Ultrasonic attenuation as a function of frequency was measured on each sample using a laser ultrasonic system at IMI/NRC[7,8] (see Figure 3). For this work, a frequency range of 20 MHz to 60 MHz was employed.
Above 50 MHz, the Fresnel parameter, s, was less than 0.15 so that the measurements were made in the near acoustic field and no diffraction correction was required. Measurement repeatability varies with frequency and is better than (0.01 Np/mm at 50 MHz.
Figure 3. A simplified block diagram of the laser ultrasonic system used in these experiments.
B - E
F - G
Figure 4. TEM micrographs for heat-damaged AA7050, from top left with precipitates changing from coherent GP zones to incoherent ' and phases.
Figure 5. Optical microscope pictures of grains in different samples: on the left, sample G, a heat damaged sample. On the right, sample E, at temper 7451. Black bars in centre of images are 200 mm long.
The grain sizes were examined under an optical microscope using polished and etched samples. Figure 5 shows an example of grains from a T7451 sample (E) and a heat-damaged sample (G). The grain sizes did not change significantly throughout the heat treatment process. This is important because it implies that the changes in attenuation are likely due to changes in the precipitates.
Classifier results using 10 MHz immersion ultrasonics.
|classifier results||T7451||T6||HEAT |
|actual - T7451||48||0||2||96%|
|actual - T6||0||46||4||92%|
|actual - HEAT DAMAGED||0||3||47||94%|
These results show that even at a relatively low frequency of 10 MHz, ultrasonic measurements can be used to distinguish between the T6, T7451, and heat damaged samples with over 90% accuracy.
|Figure 6. The relationship of attenuation to frequency for the different AA7050 samples.||Figure 7. The relationship of yield strength to hardness for AA7050.||Figure 8. The relationship between ultrasonic attenuation at 60 MHz to yield strength in AA7050.|
Sample H (57 HRB) underwent natural aging and hardening at room temperature following the over-temperature exposure. Microstructural changes in this specimen are due to partial dissolution of the ( precipitates during the late stages of the heat-damage. At room temperature, the dissolved elements precipitated in the form of G-P Zones among the remaining larger precipitates. This process therefore results in a very inhomogeneous microstructure that could be responsible for the lower attenuation in this sample.
Figure 7 shows the relationship between yield strength and hardness for the samples tested. The hardness of this material is closely correlated with its strength.
Ultrasonic attenuation decreased with increasing strength, as shown in Figure 8. The TEM micrographs of Figure 4 show that the microstructure of the high strength material, Figure 4 sample B, is characterized by very small precipitates coherent with the aluminum matrix. As the alloy progresses through further heat treatments or heat damage, in Figure 4 samples E, F, and G; the microstructure changes to larger and non-coherent precipitates, which are more effective scatterers of ultrasound resulting in higher attenuation.
Ultrasonic attenuation measurements using laser generation and detection at high (>20 MHz) frequencies are sensitive to precipitate phase changes in the AA7050 alloy, and therefore can provide some indication of material degradation due to high temperature exposure.
Overall, ultrasonic measurements provide valuable information that can compliment electrical conductivity tests in characterizing heat-affected zones of aluminum aircraft structures.
For more information see backup issue: NDTnet 11/97: NDT in Aerospace .