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, liquid Zn, and material susceptibility.
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?
- Optimize welding process parameters to avoid excessive heat.
- Use advanced weld current settings with multi-pulses to reduce the stress concentration, or by pre-heating pulse to melt Zn coating away.
- Choose a better electrode form to avoid sharp edges around the electrode tip contact area.
- Use sufficient hold time to make sure the liquid zinc to resolidify before electrode ejection.
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.
You can easily optimize welding parameters by weld optimizing & planning function in SORPAS®. These automated optimizing functions can help you find correct welding parameters to avoid excessive heat even in challenging welding combinations.
Predict LME crack risk
We have now developed a new function in SORPAS® to predict LME crack risk. It is an integrated factor dependent on many dynamic factors including distributions and evolutions of stresses, residual stresses, strain rates, temperature history and metallurgical behaviors of coatings and sheet materials etc. All these factors calculated during SORPAS simulations are combined in the new LME model to predict the locations and intensity of LME cracks.
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