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On-Line Monitoring of Laser Welded Steel Tailored Blanks

 

TWI Industrial Member Report Summary 698/2000

A Alès

Background

Laser welding is becoming a widely used joining technique with many different applications, but as with all welding techniques, the relationship between process parameters and weld quality is complex. A primary concern in industry is the detection of weld imperfections using real-time monitoring methods, which must be reliable, flexible, and cost-effective in high capacity, non-clean environments. Unacceptable welds tend to increase production cost due to loss in time, material, and productivity. Monitoring normally involves a sensor which converts a process event into an electrical output; a signal conditioner which converts the electrical output to a measured variable; and a classifier which relates the measured variable to a process state variable. In the field of real-time laser weld monitoring, various sensors have shown promise in detecting weld states and are beginning to be used in production systems. Laser weld monitoring would therefore seem to be of benefit to industries that seek the goal of total quality control on their production lines.

Previous investigation of the capabilities of a commercially available weld monitor by TWI, showed that the system was capable of detecting a wide range of engineered imperfections, in particular surface defects. This work has now been extended to look at a particular, large volume application of laser welding, that of steel tailored blanks in the automotive industry.

The benefit of using a weld monitoring system for this application is to provide an input to total quality control in manufacturing routes, where on-line imperfection detection is paramount. It is also important that a production weld monitor control system can identify, store and compare the data from a weld imperfection with welds of acceptable quality, so that trends in the type of imperfections can be established, thus providing feedback for control of parameters earlier in the manufacturing cycle. The benefits would be increased quality of laser welds and improved weld quality assurance of the tailored laser welded blanks. It has been estimated that 3-4% of all laser welded tailored blanks produced are scrapped. Important savings are therefore expected through improved quality on existing laser welding applications and new laser welding applications, by the introduction of effective weld monitoring systems.

Objectives

The objectives of this work were:

  • To identify a range of imperfections and problems typical of those currently observed when laser welding tailored blanks, which are believed to contribute significantly to the amount of scrap produced.
  • To determine the effectiveness of a commercial laser weld monitor in identifying a range of engineered imperfections when welding typical linear tailored blanks.

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