Laser welding of ultra high strength steels

Generation and validation of material properties in heat-affected zones (HAZ) for a closed simulation chain from temperature field simulation of the joining technology to structural load simulation

IGF-Nr. 19449 N

Research Association: Forschungsvereinigung Stahlanwendung e.V. FOSTA
Sohnstraße 65                         
D-40237 Düsseldorf
Phone: +49 211 6707-856
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Promotion Notice:

The IGF project "Laser welding of ultra high strength steels 22MnB5 and DP1000 - Generation and validation of material properties in heat-affected zones (HAZ) for a closed simulation chain from temperature field simulation of the joining technology to structural load simulation", IGF project no. 19449N, of the Forschungsvereinigung Stahlanwendung e.V. (FOSTA), Sohnstraße 65, 40237 Düsseldorf was funded by the Federal Ministry of Economic Affairs and Energy via the AiF within the framework of the program for the promotion of joint industrial research (IGF) on the basis of a resolution of the German Bundestag.

Project description:

The objective of the project is the closing of the simulation chain for laser welding of ultra high strength steels (UHSS), from temperature field simulation of the joining technology to structural load simulation. A deeper understanding of the changes of material properties due to the welding process and the temperature profile in the heat-affected zone (HAZ) shall be gained. This understanding shall then be transferred to simulation approaches.

For this purpose material samples will be created, in which the temperature profiles of different HAZ areas are reproduced. The mechanical material properties of the different areas are determined and implemented in a finite element method (FEM) model for structural load simulations. Physical testing on coupon and component level is the basis for the development of a simulation approach for the complete joint. This approach will be simplified iteratively in order to determine the necessary level of detail for later use in full vehicle simulations and will be realised by an exemplary component concept.

Aspired results are the correlation of heating rate and conversion temperatures as well as the determination of their influence on the structure formation. Furthermore mechanical material properties depending on the temperature profile shall be analysed and the necessary level of detail for the joint zone in the FEM model shall be determined in order to achieve a good predictability of the behaviour and failure of the joints in full vehicle crash simulations.

The results can be implemented in FEM models and can be used commercially in the short term. Small and medium-sized enterprises (SMEs) in the simulation sector can add a valuable working area to their portfolio. SMEs that manufacture and process UHSS components can use new lightweight construction potential due to better prediction of the load behaviour and can reduce the cost of expensive testing and prototypes. Moreover, SMEs working on material technology can adapt a methodology to open a new field of material data generation.

Summary of the project results:

With the help of extensive experimental investigations on welded and tempered samples of the materials 22MnB5 and HCT980X a database was created. In different test series process parameters for the generation of welded joints at different laser focus diameters were developed. On the basis of temperature and hardness profile measurements, different microstructure areas could be identified on welded samples. Thus, a detailed subdivision into different zones from the weld seam centre to the unaffected base material could be made. A correlation between the temperature history, resulting material hardness and the microstructure was detected. Different microstructural states corresponding to the heat-affected zone microstructure were simulated with a modified spot welding system by conductive tempering at tensile specimen level. To obtain an analysis of the resulting microstructure, a time-temperature diagram was created in a dilatometer. Thus, the resulting microstructures in the different areas of the HAZ up to the base material could be identified.

With this a data base for the material behaviour of different microstructures was created and transferred into material cards for the load simulation with the help of FEM. A scaling methodology developed within the project allows the interpolation of material maps for different hardness values and thus enables an automated adaptation of the material behaviour to local hardness values. With this methodology the deformation and failure behaviour of welded specimens could be reproduced in detailed load simulations. The use of this methodology enables the closing of the simulation chain from temperature field simulation to load simulation by transferring generated hardness distributions of the joining technology simulation to models of the load simulation. This simulation chain has already been carried out by an SME of the project-accompanying committee based on the project results available here and underlines the industrial relevance of the research project.

In addition, a substitute modelling, including the WEZ properties, was able to show the joining behaviour at component level and in the context of overall vehicle simulations.
In further research work, the transferability of the knowledge gained to other manufacturing processes such as alternative welding processes or tempering should be investigated. Based on the project results, investigations into the transferability of the joint behaviour to other material classes are also useful with regard to more comprehensive application possibilities.

A detailed compilation of the project results can be found in the corresponding final report, which is available from the Forschungsvereinigung Stahlanwendung e.V. (FOSTA).

Contact

Dr.-Ing. Dinesh Thirunavukkarasu
Manager Research Area
Vehicle Concepts & HMI
+49 241 80 25699
Email

Project duration

04/2017 – 09/2019

Project partner

ISF, ika

Supported by

[Logo: BM Wirtschaft und Klimaschutz]

Address

Institute for Automotive Engineering
RWTH Aachen University
Steinbachstraße 7
52074 Aachen · Germany

office@ika.rwth-aachen.de
+49 241 80 25600

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