By:  Álvaro Prada

In the last few years, the use of Advanced High Strength Steels in the automotive sector has been increasing steadily for the manufacturing of structural and safety parts. Indeed these steels make possible an improvement of passengers´ safety maintaining a reasonable weight. Five years ago, it was predicted that TRIP steels would be one of the most popular family of AHSS. However nowadays its use is not so significant and generally high strength dual-phase steels or martensitic steels are preferred in spite of the very good forming property of TRIP steels. Probably one of the major reasons is the behaviour of TRIP welded parts, whatever the process. Figure 1 presents comparative forming results between laser welded TRIP and DP joints. In order to understand better the manufacturing process of TRIP steels as well as their behaviour when forming and welding, the Spanish government is funding a project integrated by 4 members, each one leading a specific part of the study: ITMA is designing and manufacturing the steels, CEIT is selecting the best thermo mechanical cycles, CTM is evaluating the forming properties as manufactured and also of the welded joints, and AIMEN is studying the behaviour of experimental steels when welding.

In this project, the main objectives are to design TRIP steels in order to enhance their weldability, to assess the behaviour of experimental TRIP steels versus commercial TRIP steels, to make a comparison between TRIP steels and DP steels behaviour and finally to study the weld behaviour using different processes such as LBW, PAW and RSW.

The consecution of the objectives of the project will make possible:

• To increase the knowledge about the behaviour of these steels versus welding processes.
  • To develop know-how for the production of TRIP steels and future new steels in Spain.
  • To seek innovative solutions in order to solve the actual limitations of the most common commercial grades.

In the work presented, the main action performed is to study the effect of laser radiation on the microstructure and the mechanical response and weldability of experimental and commercial TRIP steels mainly for two types of joints: the autogenous lap weld employed in reinforcements and in subsets of security and commitment, and the butt weld employed in Tailored Welded Blanks.

The main conclusions obtained are:

• It was possible to obtain, at laboratory scale, TRIP steels that present similar welding properties to commercial TRIP steels.
• The butt laser welds present higher tensile strength than the base material, but lower formability.
• When laser welding, the fusion zones present very high hardness values. No decrease of hardness respect to the base material was observed in the HAZ.
• Using LBW process, the formability of dissimilar TRIP-XES joint is higher than the DP-XES joint.

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