Some months ago at SALDAT, the welding and cutting expo held in Verona in the month of May, the company TerniTecnologie S.p.A. had introduced in this journal (Tubology of June 2007) the TRWOC, a fully innovative sensor able to check in real time the quality of laser welding.
Just few months following its introduction, this sensor is already installed at different industrial plants and good results have been achieved thanks to its use.

The TRWOC sensor
This device is able to prevent any systematic error that can arise along a production line. Those errors are thus detected and classified in real time by the sensor, which enables to stop the process and correct the cause of the error. The welded joints are checked in real time, which enables to get the highest possible speeds while keeping the process parameters unchanged and ensuring a high quality level.
This monitoring sensor engineered by TerniTecnologie S.p.A. is able to check all the variables affecting the quality of the laser welding: insufficient filler gas, decrease and discontinuance in the power of the laser beam, misalignment and lack of flatness of the joints, lack of penetration, cracks and inclusions.
As early as during its installation, this device proved flawless. What’s more, it proved quite flexible and could be customized thanks to some small expedients.
The fibre optic housing, where the light coming from the welding plasma is collected, has been therefore adjusted in a very simple and efficient way to the different situations thanks to different proposed solutions. The main solution which provides the fibre optic to be directly secured to the laser head by means of an adjustable support, has proven successful, though in some difficult applications has been replaced by a “portal frame” placed just beside the laser head equipped with an adjustable support for the fibre optic. Moreover, in those cases where the line layout hasn’t allowed the presence of the portal frame, a flexible mechanical arm was used to house the fibre optic.
In all of the above cases, the sensor could be installed without any change in the line layout nor in the laser head.
After the installation, in concordance with the companies, we carried out a long set of tests to check whether the system worked properly.
During those tests, different situations were purposely created to arise defects in welding. The device was also tested during the normal working shifts. In both cases, the TRWOC proved very satisfactory and was able to detect all types of defects both simulated and due to the normal running of the welding lines.
Understanding the messages from the TRWOC and using the sensor is very easy for the operators.
The commands, which can also be used through the TRWOC touch screen monitor are few and highly intuitive. Hence, they do not require any special training and in a very short time, everyone is able to use them properly.

TESTS OUTCOME

THROUGH HOLE
This defect was obtained by slightly damaging the coil edge. This defect was detected by the TRWOC which showed immediately a strong change both in the values of the welding plasma electron temperature and in the covariance graph.
LACK OF PENETRATION DUE TO A DECREASED LASER POWER
In this case, too, the electron temperature graph has shown the defect was due to an imperfect welding penetration along the whole coil thickness due to a momentary decrease in the laser beam power, as shown in the micrograph realized subsequently. (PICTURE). This is particularly interesting as this type of defect is the hardest to be detected by the most traditional control instruments based on induced current methods.

INTERMITTENT INTERNAL WELDING
This time the defect was not caused purposely. It was detected during the normal welding process. Pictures show that in some parts of the internal portion of the welding seam, penetration is not so accurate.
The TRWOC graphs show that both the electron temperature and the covariance signal have detected all the areas where the joint quality was out of the tolerances set by the operator.
EXTERNAL ETCHING
This time, too, while the welding line was normally running, the TRWOC detected the onset of a tiny etching along the welding seam at the very beginning of the test. The electron temperature graph shows that the operator was once again able to detect the defect immediately and to put remedy to it.
Considering the TRWOC success rate, it goes without saying that this sensor, which is naturally able to monitor soon all the parameters of the welding process, is very useful even during the process optimization. In this way the operators can prevent any defect from arising through a constant quality check of the welding. This quality check can be carried out at an early stage and meet all of the company and production quality standards.

By R.Rossi e C.Giancaspro