Acoustic process monitoring in laser beam welding

Structure-borne acoustic emission (AE) measurement shows major advantages regarding quality assurance and process control in industrial applications. In this paper, laser beam welding of steel and aluminum was carried out under varying process parameters (welding speed, focal position) in order to provide data by means of structure-borne AE and simultaneously high-speed video recordings. The analysis is based on conventionally (e.g. filtering, autocorrelation, spectrograms) as well as machine learning methods (convolutional neural nets) and showed promising results with respect to the use of structure-borne AE for process monitoring using the example of spatter formation

Monitoring the Process Quality of Press Hardened Steels (PHS) with Non-destructive Testing (NDT) Methods

An overview about new monitoring concepts of nondestructive testing (NDT) methods for inspection of PHS (press hardened steel) process quality control will be given. Inline testing systems based on electromagnetic electromagnetic acoustic transducer (EMAT) techniques (see Figure1-a) could be used to inspect the raw material (decoiled strip or sheets), semi-finished and final products for defects like cracks or inclusions. The EMAT technique is well known for inline- and in-process inspection of materials which are electrically conductive and / or ferromagnetic. If such defect detection methods are combined with so-called micromagnetic methods (see Figure1-b), it is possible to simultaneously determine isotropic and anisotropic material properties, like tensile and yield strength, vertical and planar anisotropy as well as residual stress. During stamping the monitoring of structure-borne sound with tool integrated sensors is a possibility to recognize the crack formation in the part. Stamped parts can be inspected for typical drawing defects, like cracks or neckings with ultrasonic NDT techniques and thermography as well. On the other hand the assembly process is increasingly important for the PHS. In case of Resistance Spot Welding (RSW) the strength of the welds is mainly defined by size and quality of the weld nuggets. Fraunhofer IZFP has developed an Ultrasonic Welding Monitor (USWM) allowing real-time observation of the RSW process with US transducers integrated in the welding gun. Using this NDT system the size of the welding can be determined during its formation. Moreover welds can be distinguished from so-called “Kissing Bonds”. RSW generates a local tempering and hence a weakening in PHS. Weld spot size as well as hardness profile in the welded and heat-affected zone and further more a quantification of the corresponding layer thicknesses can be determined with the micro-magnetic 3MA technique. It was shown, that a sufficient correlation between destructive and nondestructive measurement values can be achieved. Manual or automated testing systems have become common tools for nondestructive testing of mechanical properties, material irregularities and joining quality in PHS. Especially the in-line monitoring of appropriate NDT data (shown in Figure2) enables an optimization of the PHS production process itself via feed-forward and –backward control not only for steel producer but furthermore for component supplier and original equipment manufacturers (OEM). As a consequence less waste and efficient lightweight constructions with high specific strengths and a high product quality, reliability and efficiency can be assured

Production integrated nondestructive testing of composite materials and material compounds - an overview

Composite materials and material compounds are of increasing importance, because of the steadily rising relevance of resource saving lightweight constructions. Quality assurance with appropriate Nondestructive Testing (NDT) methods is a key aspect for reliable and efficient production. Quality changes have to be detected already in the manufacturing flow in order to take adequate corrective actions. For materials and compounds the classical NDT methods for defectoscopy, like X-ray and Ultrasound (US) are still predominant. Nevertheless, meanwhile fast, contactless NDT methods, like air-borne ultrasound, dynamic thermography and special Eddy-Current techniques are available in order to detect cracks, voids, pores and delaminations but also for characterizing fiber content, distribution and alignment. In Metal-Matrix Composites US back-scattering can be used for this purpose. US run-time measurements allow the detection of thermal stresses at the metal-matrix interface. Another important area is the necessity for NDT in joining. To achieve an optimum material utilization and product safety as well as the best possible production efficiency, there is a need for NDT methods for in-line inspection of the joint quality while joining or immediately afterwards. For this purpose EMAT (Electromagnetic Acoustic Transducer) technique or Acoustic Emission testing can be used

Monitoring of PHS Joining Quality with Non-Destructive Testing (NDT)

PHS is an ultra-high strength material which is difficult to join either by welding or through mechanical joining techniques. Joining PHS always bears the risk of irregularities affecting the static and dynamic strength and of the joint. Therefore, inspecting the joint quality (post-process) and / or monitoring the joining itself (in-process) will be appropriate. This paper gives an overview of Non-Destructive Testing (NDT) methods for this purpose. In case of Resistance Spot Welding (RSW) the strength of the welds are mainly defined by size and quality of the weld nuggets [1]. Fraunhofer IZFP has developed an UltraSonic (US) welding monitor allowing real-time observation of the RSW process with US transducers integrated in the welding gun [2]. With this NDT system nuggets size can be determined and welds and bonds can be distinguished. RSW generates a local tempering and hence a weakening in PHS [3]. Weld spot size as well as hardness profile in the welded and heat-affected zone can be determined with the micro-magnetic 3MA technique. If PHS should be laser welded, the aluminum-silicon coating could be a problem. Therefore, the coating is partially ablated by laser [4]. Again, the 3MA technique can be used in order to verify the success of the partial ablation by determining the thickness of the residual coating layer. The laser welding process itself can be monitored by modern techniques of Acoustic Emission (AE) testing. New transducers and microphones with sensitivity up to the MHz range allow reliable detection of weld seam irregularities, like insufficient penetration, holes and undercut. Also a mechanical joining process can be monitored by AE - for example high-speed tack-setting. Defects like head overhang and breakthrough can be identified by AE. As in case of RSW also tool-integrated US transducers can be used to monitor the mechanical joining. During screwing the pre-tension force can be observed as well as the residual bottom thickness and with it the size of the undercut during clinching. A relatively new approach is the low-heat joining of PHS components by Friction Stir Welding (FSW) or by the special variant of UltraSound Enhanced Friction Stir Welding (USE-FSW). This approach is especially suited for mixed joints (e.g. PHS + Al). The development of this joining method and adapted NDT methods is topic of ongoing research

Combination of electromagnetic measurements and FEM simulations for nondestructive determination of mechanical hardness

The paper considers the Rockwell hardness investigation by finite element simulation in inspection situation of press hardened parts using the 3MA non-destructive testing system. The FEM model is based on robust strategy calculation which manages the issues of geometry and the time multiscale, as well as the local nonlinear hysteresis behavior of ferromagnetic materials. 3MA simulations are performed at high level operating point in order to saturate the soft microscopic surface soft layer of press hardened steel and access mainly to the bulk properties. 3MA measurements are validated by comparison with numerical simulations. Based on the simulation outputs, a virtual calibration is run. This result constitutes the first validation; the simulated calibration is in agreement with the conventional experimental data. As an outstanding highlight a correlation between magnetic quantities and hardness can be described via FEM simulated signals and shows high accuracy to the measured results

Hybrid Al/steel-joints manufactured by ultrasound enhanced friction stir welding (USE-FSW): Process comparison, nondestructive testing and microscopic analysis

The process of friction stir welding (FSW) is an innovative joining technique, which proved its potential in joining dissimilar metals that are poorly fusion weldable. This ability opens a wide range for applications in industrial fields, where weight reduction by partial substitution of conventional materials through lightweight materials is a current central aim. As a consequence of this, the realization of aluminum / steel-joints is of great interest. For this material compound, several friction stir welds were carried out by different researchers for varying Al/steel-joints, whereas the definition of optimal process parameters as well as the increase of mechanical properties was in the focus of the studies. To achieve further improved properties for this dissimilar joint a newly developed hybrid process named "ultrasound enhanced friction stir welding (USE-FSW)" was applied. In this paper the resulting properties of Al/steel-joints using FSW and USE-FSW will be presented and compared. Furthermore, first results by using the nondestructive testing method "computer laminography" to analyze the developed joining area will be shown supplemented by detailed light-microscopic investigations, scanning electron microscopic analysis, and EDX

Nondestructive quality inspection, process monitoring and adaptation of joint properties in friction stir welding

As the industrial use of Friction Stir Welding (FSW) is steadily increasing, appropriate nondestructive testing (NDT) methods have to be identified allowing the reliable detection of existent nonconformities in the weld during a post-process inspection. Moreover, there is a strong need for process monitoring and control by in-process NDT methods. Ideally the emerging formation of a defect has to be detected in such an early state during welding that corrective actions can be implemented and a proper weld quality still can be ensured. Different approaches to inspect the joint quality in post-process testing as well as by in-process monitoring, for example using the MonStir® method, will be presented. Additionally an innovative process variant, called Ultrasound Enhanced Friction Stir Welding (USE-FSW) and its potential to improve the microstructural and mechanical properties of joints will be described in detail