Resistance Spot Welded Joint for Suppressing Zinc-Induced LME Cracking
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Solution Overview
Problem
Resistance spot welding of high strength steel sheets with zinc-based plating often results in liquid metal embrittlement (LME) cracking due to molten zinc infiltration into grain boundaries, which reduces the strength of the welded joint and is not effectively addressed by existing methods.
Innovation Solution
A resistance spot welding method that involves forming a nugget with a specific current and force profile, including a prolonged second period during which the current is gradually decreased from 0.9 to 0.3 times the initial value, and maintaining a force 1.1 times the completion force, to reduce tensile stress and prevent LME cracking in the corona bond.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If resistance spot welding is performed on high strength steel sheet with zinc-based plating, then the corrosion resistance is improved, but LME cracking occurs due to zinc infiltration into grain boundaries
Solution Approach 1:
The patent applies preliminary action by extending the force hold time after nugget formation before releasing the electrode force. This prolonged holding period prevents LME cracking by maintaining compressive stress on the welded joint during the critical cooling period when liquid zinc could infiltrate grain boundaries, thereby preserving both corrosion resistance and welded joint strength
Solution Approach 2:
The patent changes the time parameter by specifying a force hold time of 5 milliseconds or more after nugget formation. This parameter modification allows the welded joint to cool sufficiently under sustained compressive stress, preventing zinc infiltration while maintaining the integrity of the zinc-based plating for corrosion protection
2Strength
If the force hold time is extended to prevent LME cracking, then the welded joint strength is improved, but the welding cycle time increases
Solution Approach 1:
The patent applies partial action by setting the force hold time to a minimum effective duration of 5 milliseconds or more, which is sufficient to prevent LME cracking without unnecessarily extending the welding cycle. This optimized duration provides the necessary protection against zinc infiltration while minimizing the time penalty in the welding process
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method effectively suppresses LME cracking in the corona bond by reducing tensile stress and promoting slow cooling of the nugget, thereby maintaining the strength of the welded joint.
Implementation Method 1
a current and a force are concentrated into a relatively small portion to locally heat and simultaneously apply force on the portion with the electrodes
Implementation Method 2
LME cracking is intergranular cracking caused by the infiltration of zinc, which is melted and liquified, into grain boundaries of a steel sheet
Implementation Method 3
melting a weld of the material to be welded by Joule heat by energizing the electrode tips, and thereafter stopping the energizing to allow the weld to cool and solidify and form a nugget
Data Source
AI summary
A resistance spot welded joint according to an aspect of the present invention includes: a plurality of overlapping steel sheets; and a weld having a nugget by which the steel sheets are joined, and having a corona bond and a heat-affected zone formed around the nugget, in which one or more of the plurality of steel sheets are high strength steel sheets having a tensile strength of 780 MPa or more, one or more of the plurality of steel sheets are plated steel sheets having a zinc-based plating, the high strength steel sheet and the zinc-based plating are adjacent to each other on a contact surface, a diameter of the heat-affected zone is 1.5 times or more a diameter of the nugget, in the heat-affected zone, carbides having a circle equivalent diameter of 0.1 or more are distributed at a number density of 40/100 μm2 or more, and in the corona bond, an amount of an η phase of the zinc-based plating is 20 area % or less.