## 4 Applications
### Applications in the Automotive Industry
#### 4.3.1 Car Body Parts
The Barkhausen Noise (BN) signal and, to some extent, the Incremental Permeability (IP) signal show a characteristic double-peak structure. When comparing the measurements on coated and uncoated sheets have shown that the outer peaks, appearing at higher magnetic fields H, are generated by the steel substrate, whereas the inner peaks at lower fields H result from the ferromagnetic diffusion layer. The height of the inner peak is dependent on the amount of this diffusion layer. Therefore, some of the 3 MA measuring parameters are correlated to the diffusion layer thickness (DLT). Moreover, there are also 3 MA measuring parameters that are dependent on the lift-off between the 3 MA probe and steel substrate. These parameters can be used to determine the total thickness (TLT) of the AlSi coating. The values of DLT and TLT are relevant for the further processing of the PHS parts, because they affect weldability and paintability. The AlSi coating is burned into the steel during a time-consuming furnace process. During this time, the diffusion layer is developing. Therefore, the values of DLT and TLT are important measures for controlling the proper processing in the furnace. For these reasons, PHS producers also are very interested in measuring these coating properties. This is possible with 3 MA, as is shown in the calibration diagrams of Figure 21. The accuracy (RMSE = 1.0 \(\upmu\)m and 1.7 \(\upmu\)m) is sufficient in order to evaluate the coating quality. This is shown in Figure 22a, which is a screenshot of the 3 MA results that re displayed after a measurement on a PHS part. The measuring time was only three seconds. Due to the fact that the field-of-view of the 3 MA probe is only a 1-2 mm in diameter, 3 MA can be used to inspect sharp transitions of material properties, as they are preferred in tailor tempered blanks (TTB). The development of hardness and other mechanical properties in such a sharp transition can be detected with 3 MA, as shown in Figure 22b. Such measuring profiles of hardness, Rm, Rp0.2, etc., cannot be determined by conventional destructive testing methods. Therefore, 3 MA offers additional information that is not available from destructive tests. 3 MA can be manually applied in PHS production. Meanwhile, automated 3 MA systems that are based on conventional or collaborative robots have been developed. Meanwhile, there is a comprehensive calibration database for the 3 MA application on PHS. Calibrations are available for all steel blank and coating types (see above) and usual sheet thicknesses. When calibration is completed, all of the calibrated target quantities can be simultaneously determined in a single measuring step. Joining of PHS always bears the risk of irregularities that affect the static and dynamic strength of the joint. Additionally, for PHS welding, 3 MA can provide valuable measuring information for quality assurance and process monitoring. In the case of Resistance Spot Welding (RSW), the strength of the welds is mainly defined by the size and quality of the weld nuggets. It was shown that 3 MA allows for the size of the weld nugget to be accurately determined [69]. Figure 21: Results of the calibration to coating layer thickness in AlSi-coated PHS: (**a**) total layer thickness (TLT) and (**b**) diffusion layer thickness (DLT). If the PHS should be laser welded, the aluminum-silicon coating could be a problem. Therefore, the coating is partially ablated by laser. Again, the 3 MA technique can be used in order to verify the success of the partial ablation by determining the thickness of the residual coating layer [70]. Meanwhile, PHS testing is the most successful 3 MA application by far. Almost all of the PHS-producing OEMs and suppliers from the automotive industry use 3 MA.