Roll cladding is a cost-effective way to increase resistivity of aluminum-alloys against corrosion. To achieve a cladding, pre-processed sheets of the clad-alloy and a slab of the core-alloy are hot- and cold-rolled together, and then annealed. The desired result is a thin layer of a high-grade clad-alloy providing surface quality and chemical stability, bonded to a sheet of core-alloy providing structural strength. Examples for aluminum cladding products are various heat exchangers used in the automotive industry. For further processing steps, i.e. joining parts of cladded aluminum, precise control over the cladding layer thickness is of great importance, as a small deviation may result in the rejection of an entire assembly. The state-of-the-art technique used to monitor the cladding thickness is an offline analysis of micrographs revealing the cross section of the cladding structure. The method provides precise and unambiguous results but requires several time-consuming steps to prepare the samples, which must be cut from the product. Typically, these steps are performed manually, and several samples must be cut to obtain convincing statistics.We suggest a non-destructive approach to characterize the cladding thickness based on the measurement of elastic waves propagating along the surface of the material. The energy of a surface acoustic wave (SAW) is concentrated within a depth on the order of its wavelength. Thus, at high frequencies (low wavelengths) the wave interacts mainly with the cladding material, while at low frequencies (high wavelengths) its displacement field expands into the core material. Thus, if the two materials have different sound velocities, the SAW undergoes dispersion. The resulting frequency dependent SAW velocity c(f) can be described by mathematical models depending on the elastic properties of the materials and the cladding thickness.We use contact-free, laser-based ultrasonic measurements to experimentally obtain the SAW dispersion curve c(f) on a set of aluminum clad materials. The slight difference in elastic properties of cladding- and core-material is sufficient to result in significant SAW dispersion, which is dictated by the cladding thickness. We fit theoretic dispersion curves to the measured data, to obtain the cladding properties. Our results are compared to reference values obtained from micrographs. The presented method does not require cutting samples from the product. It can be fully automated and has the potential to work as an inline inspection tool.

Characterization of Roll-Cladded Aluminum Plates With Laser-Based Surface-Acoustic-Wave Measurements

Paper Proceedings: https://asmedigitalcollection.asme.org/qnde/QNDE2021/volume/85529

ASME 2021 48th Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE2021)

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