Nondestructive Evaluation of Thermal Aging in Al6061 Alloy by Measuring Acoustic Nonlinearity of Laser-Generated Surface Acoustic Waves

Jongkui Jun ,Kyung-Young Jhang,Hye‐Sook Seo

doi:10.3390/met10010038

Jongkui Jun , Kyung-Young Jhang + Show 1 more

Open Access

https://doi.org/10.3390/met10010038

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Abstract

The structures in high-temperature environments are prone to undergo hardening and embrittlement as a result of thermal aging; this can cause variations in their mechanical properties. Because these changes occur at the microstructural level, it is difficult to evaluate them through linear ultrasonic techniques. In this work, a surface acoustic wave (SAW) was used to measure and compare the acoustic nonlinearity and mechanical properties of Al6061 alloys heat-treated at 220 °C for different durations (0 min, 20 min, 40 min, 1 h, 2 h, 10 h, 100 h, 1000 h). The SAW was generated by a pulsed laser and then received by an interferometer. Moreover, the yield strength, ultimate strength, and elongation to failure were measured by tensile tests. The results demonstrate that the critical variations in the mechanical properties can be detected by monitoring the variation features in the acoustic nonlinearity. Transmission electron microscopy images were captured to observe the microstructural changes, which shows that the acoustic nonlinearity varied according to the change in the precipitation phase. This supports the acoustic nonlinearity measurement using the laser-generated SAW being an effective technique for the fully noncontact nondestructive evaluation of material degradations as well as changes in mechanical properties.

Highlights

Aluminum alloys have been widely employed for various purposes, including aircraft, robots, ships, space vehicles, and automotive parts [1,2], because they provide cost-effectiveness for stiff and lightweight designs [3]
These results show that the acoustic nonlinearity of the laser-generated surface acoustic wave (SAW) is sensitive to the critical microstructural changes that degrade the mechanical properties and is a useful measurement technique for the noncontact nondestructive evaluation (NDE)
The aluminum alloys were heat-treated for eight different durations to damage them with gradual thermal aging levels

Summary

Introduction

Aluminum alloys have been widely employed for various purposes, including aircraft, robots, ships, space vehicles, and automotive parts [1,2], because they provide cost-effectiveness for stiff and lightweight designs [3]. The measurement of acoustic nonlinearity has shown its effectiveness in the evaluation of microstructural changes in materials, including stress corrosion cracks [12], plastic deformation [13,14,15,16], residual stress [17], fatigue [5,6,14,16,18,19,20,21], thermal aging [4,8,9,22,23,24,25,26], creep [10], and so on [18,27,28]. For each thermal-aged specimen, the relative acoustic nonlinearity parameter was obtained via a frequency analysis of the received SAW signals, and by its linear regression The results of this fully noncontact method were compared to that of contact and semi noncontact methods, as well as to the yield strengths reported in our previous works [33]. H: (a) the picture of the specimens; (b) the gradual heat-treatment process for each specimen

Tensile Destructive Tests

Transmission

Laser-Generated

Discussion

Variation in acoustic nonlinearity properties according to thermal

Conclusions