Sensor systems for top-face penetration control

TWI Bulletin, November/December 1995

Paul Anderson is a Senior Research Welding Engineer in the Arc Welding Section of the Arc, Laser and Sheet Processes Department at TWI. He joined TWI in 1990 and has been actively involved with the development of gas-shielded arc welding processes. His recent activities include the development of shielding gases for the TIG welding of duplex stainless steels and assembly of a prototype top face penetration control system.

Paul Anderson reviews the available literature on sensor systems for the top-face monitoring of weld pool penetration in arc and laser welding and identifies candidate sensors for the development of a top-face feedback system for controlling weld penetration.

Problems are frequently experienced in ensuring consistent penetration of a joint, due to minor variations in material composition, component dimensions, joint fit-up or the surface condition of the component. In manual welding the welder can adjust the welding parameters, or his technique, to compensate for some of the variations. In mechanised and automated welding, the welding parameters are normally fixed prior to welding. A sensor which observes the back of the weld and modifies the welding parameters to ensure consistent penetration is commercially available. ^[1] However, for many applications, access to the back face of the weld is not possible.

There is a need for a sensor system to control accurately the weld bead penetration from the top-face, in both mechanised and automated welding. For the system to be widely applicable in the fabrication industry, it should ideally be able to accommodate:

• Arc and laser welding processes
• Flat, vertical and overhead welding positions
• Butt and fillet welds
• A broad range of materials, including C-Mn steels, stainless steels and aluminium alloys
• Full and partial penetration welds

Of equal importance, the sensor should not make contact with the workpiece, or intrude into the welding area.

Sensor systems

Ultrasonic techniques

The depth of penetration of a weld pool can be measured by locating the fusion interface between the weld pool and parent metal using ultrasonic waves, generated by an angled piezoelectric transducer in contact with the surface, Fig.1. ^[2] Accurate measurements from the top face can only be obtained for plate thicknesses of greater than 10mm or less than 2mm. To ensure good coupling between the transducer and the workpiece, the workpiece must have a simple, uniform surface geometry with minimum irregularities. Distortion of the workpiece must be avoided. The transducers must track the weld pool, and component vibration or unexpected probe movement must be prevented.

Acoustic emission techniques

Weld pool sag sensing techniques

As full penetration is achieved in the flat position, the weld pool surface begins to sag towards the back of the plate. This movement can be detected through the increase in arc length, ^[5] Fig.2, or the change in profile of the weld pool. However, the magnitude of the movement is very small, which limits the accuracy attainable. The accuracy is reduced as the workpiece thickness increases, and by oscillations within the pool due to pulsed welding current, power source ripple, and the addition of filler wire to the weld pool.

Weld pool oscillation sensing

Weld pool viewing techniques

This technique assumes that if the weld pool size on the top-face remains constant, then the penetration is constant. ^[7] The weld pool may be viewed using a CCTV or infra-red camera, which can be incorporated into the welding torch, Fig.3. The performance in practice is promising, although the differences observed in the weld pool may not compensate for all combinations of variations due to joint fit-up, material composition or surface contamination. Accuracy is reduced by contributions from the arc light which necessitates the use of filters or selective viewing by blocking out the arc.

Thermal sensing techniques

If the thermal gradient around the weld is constant, this technique assumes that the weld pool penetration is constant.

Temperature may be measured using a contact thermocouple or an infra-red pyrometer, or a camera observing the workpiece. Thermal sensing is probably the most widely applied technology in practical use and can also provide seam tracking. ^[8] Unfortunately, these systems are particularly sensitive to variations in the surface condition and the emissivity of the material. The accuracy of camera-based systems is reduced by contributions from the arc light - this necessitates the use of filters or selective viewing by blocking out the arc.

Discussion

The range of applications within which the sensor systems still function is summarised in the Table. Although some sensors have been applied in specific applications, they all have limitations to their capabilities. None of the systems appeared to be capable of fulfilling all of the above criteria for the ideal sensor system.

Conclusions

There is currently no system commercially available which satisfies all the requirements of an ideal system. The range of application and accuracy of control of the reported systems can be summarised as follows:

• Ultrasonic techniques are only suitable for plate thicknesses of less than 2mm or greater than 10mm. Accuracy of control needs to be improved before they can be used for practical applications.
• Acoustic emission monitoring will function independently of welding position, process and joint type, but has still to be effectively demonstrated.
• Weld pool sag sensing is limited to butt welds in the flat position and requires a highly accurate power source.
• Weld pool oscillation frequency monitoring by optical systems can only be used for full penetration butt welds, and requires the back surface of the weld to be free of oxide and contamination.
• Direct weld pool viewing functions independently of position, process and joint type, but requires accurate control of component thickness and joint fit-up.
• Thermal sensing functions independently of position, process and joint type, but its accuracy is reduced by variations in the surface emissivity of the material.

Recommendations

Use of a combination of the generic sensor methods to overcome the limitations of individual methods should be evaluated. Possible combinations of weld pool viewing, thermal sensing and acoustic emission monitoring should be considered. These methods are not limited by the welding position, process and joint type, and the limitations of each method can be overcome using data generated by the other methods. As acoustic emission monitoring has yet to be effectively demonstrated in practice, the most promising option for monitoring and controlling weld penetration appears to be simultaneous pool viewing and thermal sensing, which can be carried out with a single infra-red camera.

Future work

A system based upon simultaneous pool viewing and thermal sensing with an infra-red camera, is at an advanced stage of development.

Capabilities of the reviewed sensors

Capabilities of the reviewed sensors continued

References