CLET

Laser welding process is a technique widely used in industry. The interesting properties of the welded seams obtained and the possibility of automating the welding process has contributed to its diffusion among several kinds of industries. Laser welding has become a common technique in automotive, aeronautical and shipbuilding industries.

Laser welding has some drawbacks that could discourage SME to implement it. First is the parameterisation of the laser welding process. The process has a set of parameters (laser power, welding speed, focal length, shielding gas flow, lateral position bias) that must be chosen in order to obtain a good quality weld. Besides this, the problem of defects in the welded seams is always present. Typical defects are holes, pores and lack of penetration. Defects lead to a poor quality final product and cost increment.

Both academia and industry have made research efforts in order to find method for detecting faults in laser welding. The desired method should be able to detect the defects in real time and inspecting the whole production without time increment. Methods based on acoustic emission, machine vision and ultrasonic inspection have been proposed. But the most promising methods are those based on the analysis of the electromagnetic radiation emitted during the laser welding process.

Methods based on analysing electromagnetic emission along ultraviolet, visible and near infrared spectrum ranges can be classified in two main categories: those that use photodiodes and make computations over the signals generated by them and those based on spectrometers by means of which some specific plasma related features are computed.

When a defect is detected, correction actions must be taken as soon as possible. This involves a human operator who must reparameterise the laser process according to his/her experience and knowledge. The sensors used for defect detection can be also used for generating a set of signals to be introduced in a feedforward control. The controller can change the process parameters in real time in such a way the quality-related signals always keep their values close to those values associated to good quality welds. In this way, defects are avoided in an automatic fashion and, moreover, the reparameterisation of the facility for new kinds of specimens becomes easier.

CLET project pursues two main objectives:

To develop a system able to detect the typical defects in laser welding. It will be designed for both Nd:YAG and CO2 laser and several kinds of materials, like steel, stainless steel, zinc-coated steel and aluminium.

To develop a feedforward controller able to set the process parameters according to the signals generated by the defect detection system. In this way the perturbations that can originate defects will be rejected and final product quality will be assured. Moreover, the adaptation to new specimens will be simplified, which is of great interest for the SME that manufacture small batches.

The project is being developed by a consortium of RTD and SME. The RTD partners have a strong background in researching and developing laser welding defect detection systems and control systems for laser welding. SME partners are a representative variety of processed materials and laser systems.