Metals

For the casting of metals it may be interesting to monitor the depth of melting, but also to find hot cracks (aka solidification crack) in the freshly cast and still hot bars. Both is possible with Laser-Ultrasonics and was already investigated at RECENDT. In the figure a reconstruction and the corresponding photo of the cross section through a circular ingot of aluminum can be seen, which has an obvious center crack.

Non-destructive testing of welds is possible with Laser-Ultrasound. Thanks to the good automation capacity also inline measurements at high speed can be carried out, depending on the required resolution. With a repetition rate of 10 Hz, 100 Hz or even higher, flaws, inclusions, hot cracks and pore concentrations can be found. 

Using Laser-Ultrasonics, the frequency dependent velocity of surface acoustic waves in a layer can be measured. The data can be evaluated to find the layer properties, for example its thickness. Our systems enable us to measure SAW dispersion curves up to 1 GHz, which allows us to characterize layers ranging from 500 nm up to several mm thickness. The illustrations show a core material (red) with an oscillating layer on top (blue) at increasing frequencies of a guided wave mode from case 1 to 4, whereas the penetration depth of the displacement field decreases until only the layer material influences the wave propagation (4). Thus, depending on the geometry and elastic properties of the layer material under investigation, the appropriate parameters of the measurement can be selected
The investigation of layered structures with higher complexity (e.g., multilayers or gradual layers) is possible by adapting the theoretical models for the calculation of dispersion curves.

As illustrated in this image of a coin, the topography or roughness (parameters cf. ISO 25178) of a sample can easily be determined by Optical Coherence Tomography (OCT). The advantage of using OCT, compared to other methods, is the additional possibility of measuring steep edges. The accuracy in the measurement of height profiles is below 1 micron.

By means of Laser-Ultrasound also spot welds can be scanned in a short time and zones of actual material connection (weld nugget) can be imaged and evaluated.

Since Laser-Ultrasonics can be operated from a safe distance, this technology can be used in very early stages of metal processing at high temperatures. For example, the thickness of plates and the wall thickness of pipes can be measured in the still glowing state (even with only one-sided access). Also changes of the microstructure (e.g. grain growth), that are interesting in the rolling process, can be characterized with LUS.

Different materials (steel, aluminum, copper, semiconductors) - a wide range of issues (sheet thickness, defects, elastic properties, grain growth, anisotropy): the LUS technology combined with innovative data evaluation is highly versatile.

The Laser-Ultrasonics technology is also applicable for the testing of adhesion and bonding layers, e.g., solder joints. As the high-frequency ultrasonic waves are influenced at the interfaces, an image of this critical inner zone can be generated.

Metallic components are often coated dielectrically in order to optimize the operating characteristics (such as tribology).
With Terahertz technology (THz) such coatings can be measured efficiently inline.

In recent years the rapid development and quality improvements in the field of 3D printing for plastic and metal components has enabled many applications, including the serial production of critical components. Using the OCT inspection technique (for plastics, ceramics) and Laser-Ultrasonics (for metals, hard plastics) allow the detection of certain defects offline or even inline and thus enable to monitor the process closely in order to set corrective actions if needed.

The determination of the hardness penetration depth in thermally hardened components is essential for quality control. State-of-the-art is to cut samples and do etching and hardness measurements. Laser-Ultrasonics opens a non-destructive alternative. Thereby, zones of different microstructures at arbitrary positions can be imaged up to a tomographic representation of the hardness penetration depth.

By offering the possibility to investigate glowing samples, Laser-Ultrasound also enables the determination of in-situ metallurgical information during thermo-mechanical cycles of new steel grades. Therefor, samples can be inductively heated and deformed in our modified quenching and deformation dilatometer (by Linseis Messgeraete). From the measured parameters, like speed of sound and acoustic damping, it is possible to conclude on grain growth, phase transitions and texture changes with appropriate calibration.