Process monitoring during heat treatment of metals

Heat treatment of metals is a standard process to bring controlled changes in the microstructure and thereby set the desired mechanical properties in the material.
However, currently used methods for microstructure analysis (micrographs, dilatometry, tensile tests, ...) have the disadvantage that they either cannot be used directly in the process or require special sample geometries. By measuring plate resonances on sheet metal specimens using laser ultrasound, Poisson's ratio and, if the thickness is known, also the longitudinal and transverse sound velocity can be determined without contact and with high time resolution during heat treatment (e.g., in a thermal simulator). The temperature variation of these parameters correlates with changes in the microstructure, allowing phase transitions to be monitored using this method.

Further information can be found in our publication:

Monitoring precipitation hardening after solution annealing

Many metals are homogenized by solution annealing as part of the manufacturing process.
After quenching, however, the material is in a supersaturated state, and dissolved alloying constituents deposit over time into precipitate particles, e. g. Guinier-Preston (GP) zones. These have a strong influence on the strength and formability of the material. Using laser ultrasound, the changes in elastic and shear modulus of plates can be measured over many hours with a precision <1%, allowing the progress of precipitation hardening to be monitored with high time resolution.
One possible application is the adjustment of springback compensation depending on the stage of precipitation hardening in which the bent workpiece is in.
For further information, please refer to our publication:

Analysis of plate structures: elastic properties and thickness simultaneously

The longitudinal and transverse sound velocities allow conclusions to be drawn about the microstructure, for example the degree of recrystallization of metals during thermal treatments.
The thickness of a plate or sheet is also often required. Previous acoustic methods for measuring the speed of sound and the thickness of a plate require knowledge of the other quantity or spatial scanning. The method we have developed can determine these properties in a single measurement.
A specific surface wave and several plate resonances are excited simultaneously in the plate sample by a laser pulse with a periodic line pattern and then analyzed. This allows (with known density) the complete elastic characterization and simultaneous thickness determination of isotropic plates. A patent has been filed in this regard.
Further information can be found in our publication:

Casting process

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.

Inline quality assurance in welding

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. 

Testing of surfaces and topographies

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.

Spot welding

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.

Bondings and Solder Joints

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.

Tribological coatings

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.

3D printing / generative manufacturing / additive manufacturing

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.

Hardness penetration depth in steel

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.

Microstructure of metals in-situ

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.