Resistance spot welding of a thin 0.7 mm EN10130: DC04 material onto a thicker 2.4 mm 817M40 engineering steel

The effects of welding current, electrode force, and welding time in a resistance spot weld were studied to investigate the effectiveness of welded joints between a thin EN10130: DC04 material and a thicker 817M40 part, through analysis of the microstructural and mechanical properties. All welded specimens were subjected to tensile testing at room temperature (25°C) and sub-zero temperature (-46°C) to test the strength of the welded joints. No full button failure was observed at either room temperature or sub-zero temperature after optimization of the weldng parameters. The fusion zone was observed to consist mainly of martensitic phase, due to rapid quenching, while the HAZ was composed of clusters of martensite in a ferrite and bainite matrix. The base 817M40 metal remained fully ferritic after welding. The hardness was found to increase with increasing welding current. An increase in nugget size, indicating good fusion of the weld, was observed with an increase in the welding current.

Liquid Metal Embrittlement of Advanced High Strength Steel: Experiments and Damage Modeling

In the automotive industry, corrosion protected galvanized advanced high strength steels with high ductility (AHSS-HD) gain importance due to their good formability and their lightweight potential. Unfortunately, under specific thermomechanical loading conditions such as during resistance spot welding galvanized, AHSS-HD sheets tend to show liquid metal embrittlement (LME). LME is an intergranular decohesion phenomenon leading to a drastic loss of ductility of up to 95%. The occurrence of LME for a given galvanized material mainly depends on thermal and mechanical loading. These influences are investigated for a dual phase steel with an ultimate tensile strength of 1200 MPa, a fracture strain of 14% and high ductility (DP1200HD) by means of systematic isothermal hot tensile testing on a Gleeble® 3800 thermomechanical simulator. Based on the experimental findings, a machine learning procedure using symbolic regression is applied to calibrate an LME damage model that accounts for the governing quantities of temperature, plastic strain and strain rate. The finite element (FE) implementation of the damage model is validated based on the local damage distribution in the hot tensile tested samples and in an exemplary 2-sheet resistance spot weld. The developed LME damage model predicts the local position and the local intensity of liquid metal induced cracking in both cases very well.

Tensile stress-strain analysis of resistance spot weld using non-linear FEM with experimental verification

This research presents an investigation on stress-strain behavior induced by resistance spot weld followed by tensile shear test. The spot weld is modeled according to standardized dimension for tensile test with main material properties of Cu as electrode and low carbon steel S235 as plates with 1mm thickness which include electric conductivity, resistivity and heat transfer coefficient for solid body as well as a contact interface. The FEM simulation is conducted using the process parameter of current between 6,000 A to 15,000 A, force at 5,000 N and different stages of time following the welding process and tensile test which is carried out after releasing both of the electrodes and material reaches the initial temperature with contact clamp velocity of 5mm/min. To ensure the glued elements between the plates, subroutine in MSC Marc/Mentat is used in the simulation with defined temperature. The outcome of simulation results will be verified with series of experiments. It is expected that simulation will give good agreement compared to experimental analysis within acceptable range of error.