Liquid metal embrittlement(LME) is a phenomenon where certain metals like Al and steels undergo brittle failure when stressed in contact with liquid metals like Zn, Sn, Pb, etc. One of the challenges in the welding industry is cracking due to LME observed on spot welds of Zn-coated High Strength Steel. Liquid zinc penetration results in a drastic reduction in the strength and ductility of the steel. The most common location of a crack in spot weld has reported the edges of the sheet/electrode indentation region (weld shoulders).
Why LME happens?
Many researchers analyze the factors including mechanical conditions and chemical changes during welding. Here we list up part of the critical factors suggested by researchers: temperature, stress, and liquid Zn.
1) Critical temperature
The temperature plays important role in LME. It makes Zn coating melted and also reduce the fracture energy of steel. The SORPAS® simulation shows the development of nodal temperature at this area. Generated heat at weld shoulder cannot be cooled down due to a lack of contact with electrode, thus temperature goes over 700c. Many researchers use this temperature development to understand and predict the occurrence of LME crack. They pointed out the temperature at edges of the sheet/electrode indentation region increases within the ductility trough range, which explains why this area is LME sensitive region in normal spot welding case.
2) Tensile stress
The tensile stress also has effect on the liquid metal penetration into the solid metal. The SORPAS® simulation shows the concentration of stress at the weld periphery region. The tensile stresses at the weld surface increases with increasing heat input during welding process. Also fast heat release from the weld during cooling process can increase the tensile stresses.
3) Existence of Liquid metal on surface
Zinc coating on the steel melts during the weld time. This caused high availability liquid Zn at the sheets which is in near contact of electrodes.
How to minimize LME?
Based on understanding of LME during RSW process, many researchers suggested different measures for preventing cracks. Some suggested to use multi-pulses, so small cooling time between pulses can reduce the temperature and thermally-induced stress at critical area. Similarly, using sufficient hold time for cooling is one of ideas. Pre-pulse to melt Zn coating away, or post-pulse to prevent heat loss can also be an alternative. These variation of weld schedule can easily be simulated by SORPAS® with only a few steps.
Selection of proper electrode geometry also studied. Dome-type or sharp edge on electrode cap promotes high contact pressure and heat concentration compared to flat-type electrode cap. You can create different angle, size of working plane of electrode in SORPAS® by simply typing the number. You can easily replace your electrode in your simulation model and check the influence of the electrode geometry.
The basic method to prevent LME is to use correct welding parameters. Recent study from worldautosteel.org pointed out LME cracks only occurred when there are deviations from proper welding parameters and set-up conditions. SORPAS® has developed a specialized weld optimizing & planning function more than 20 years. These automated optimizing functions can help you find correct welding parameters even in challenging welding combinations.
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