Joining
FOM-M2i Physics of Failure Programme
Welding induced hot cracking in advanced and ultra-high strength steels
PhD student Mr. Gautam Agarwal
Postdoc Dr. He Gao
Supervisor Dr.ir. Marcel Hermans
Advanced high strength steels (AHSS) and ultra-high strength steels (UHSS) under development have in general a higher alloying content and are therefore more susceptible to hot cracking than conventional high strength steels. It is known that poor weldability limits the commercial exploitation of steels to a large extent. Automotive customers report hot cracking issues in assembling parts using laser welding. Pre-formed parts made from steel sheets are assembled with a flange upon which a joint is formed. To reduce the weight of a car body, the flange width is minimised. When the laser welds are placed too close to the edge of steel flanges, severe hot cracking occurs. Also, during the steel production process, coils are joined using laser welding to facilitate a continuous pickling or galvanising process. The weld integrity is essential as a weld break in the production line involves considerable down time and production losses. The approach proposed here is of a generic nature and should be relevant for all applications sensitive to weld induced hot cracking.
The initiation of hot cracks involves a complex interaction between metallurgical and mechanical factors, which are in general difficult to predict due to the lack of experimental data on material properties at high temperatures (1800 to 2400 K) and local conditions prevailing during solidification. Whilst the thermal-metallurgical interactions control the solidification microstructure, the thermal-mechanical interactions control the local and global stresses and strains. The experimental methodologies that will be employed in this project are designed to elucidate the physical mechanisms of hot cracking and will contribute to the validation of numerical models that generate insight into the physical basis of this phenomenon, which is crucial to be successful in reducing the occurrence of hot cracking.
FOM-M2i High Tech Materials
Resistance spot welding of advanced and ultra high strength steels
PhD student Mrs. Parisa Eftekharimilani
Supervisor Dr.ir. Marcel Hermans
Safety in the automotive sector is of paramount importance. The commercial application of advanced (AHSS) and ultra high strength (UHSS) steels requires that premature weld failure, due to adverse microstructure development during resistance spot welding, and hence reduced crashworthiness can be avoided. Assurance of intrinsically safe weld microstructures in AHSS/UHSS is possible with a fundamental understanding of the physical response of the materials; including segregation, solidification, solid-state phase transformation, stress and elemental partitioning between coexisting phases, both at elevated as well as ambient temperatures.
In this study the underlying physics of key aspects of microstructural evolution, starting from the solidification of the weld pool and the subsequent solid state phase transformations, that occur under extreme conditions during resistance spot welding (RSW), involving heating and cooling rates in the order of 103 K s-1 and applied forces ranging from 2 to 8 kN is investigated.
Based on physical understanding of metallurgical developments, alternative thermal-mechanical weld schemes are envisaged. These include multiple current pulses to control the cooling rate, which are currently identified on a case-by-case, trail and error basis or the novel use of ultrasonic vibrations during weld formation to modify the inhomogeneous solidifying dendritic structure.
A new simulation-based approach to welding process optimisation
Mr. Amin Ebrahimi
This research proposal is devoted to the stability analysis of fusion welding processes. The challenge addressed in this research is to develop a simulation-based approach to get a better insight into weld pool dynamic behaviour for welding stability optimisation. The stability of an oscillating melt pool will be evaluated for full penetration conditions, subject to variations in orientation and geometric boundary conditions. Therefore, the primary aim of this research is to construct a novel, physically-based numerical approach for assessing the stability of a melt pool suspended between solid side walls and subject to excitation and variations in orientation and geometric boundary conditions. The proposed approach is envisaged to be applicable to any fusion welding processes, since the process can be defined by a set of boundary conditions and be controlled by introducing perturbation terms to the applied boundary conditions.