By: Jan Karlsson1, Alexander Kaplan1
1Luleå University of Technology, SE-971 87 Luleå, Sweden; www.ltu.se/tfm/produktion
This project aims at weight reduction by laser welding of high strength steel for certain applications, and at the same time aims at initiating a knowledge platform for lightweight structures, including, besides other issues, the optimization of welding technology. To enable welding information to be transferred and used in a larger perspective, creating a broad knowledge platform may be a solution. A problem with most studies today is that the information presented by them is not directly transferable to different applications, although the same solutions for suppressing defects might be applicable. By producing a broad knowledge platform from a weld situation (even if not all is usable) other similar, but different, applications may use parts of it to facilitate an increase the weld quality.
When building a knowledge platform, a wide spectrum of methods can be applied to analyze the welding process and its result, ultimately linking them together by conclusions and making generalizations. Intermittent sampling and analysis are the main steps when analysing welding in the present project. By comparing different methods, more comprehensive information is obtained and a better understanding of defects may be achieved. The figure below shows the context between different stages of the weld “lifecycle” (horizontal axis) and different suitable analysis methods and resulting data (vertical axis), to be combined in a comprehensive knowledge platform.
Some suitable analysis methods for each stage are being developed and in the future are going to be used in context with each other at present department. Among the included methods are:
As part of this work, present paper has been produced in a line of applying the method. However, some stages have yet to bee applied. In the experiments, which are part of a weight reducing program, two different high strength steel grades have been welded by a fibre laser to create a fillet corner joint. To make the story short, the following conclusions could be drawn from the analysis of the experiments:
- Undercut and root sagging defects can be prevented or suppressed by
- Changing weld position from flat to horizontal
- Sufficiently large beam inclination – again against sagging
- Adjusting the focal point depth (below the surface)
- Adjusting the welding speed (too slow leads to wide melt and sagging)
- Adjusting the lateral beam position (particularly sensitive when having an inclination angle)
- Spatter becomes more frequent and more intense for lack of penetration
- The weld joint interface acts as an insulator, to be thermally overcome by suitable inclination angle, position and joint separation
- The combined use of analysis methods (e.g. high speed imaging) provides more information and facilitates analysis, understanding and parameter optimisation
- Rules for suppressing defects can be stated and documented, to be proven for other cases with respect to their limits of generalisation
The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here
Paper # P128