Resistance Spot Welding Current Staging for Crack-Safe Nugget Growth
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Solution Overview
Problem
Resistance spot welding of surface-treated steel sheets faces challenges in stably forming large nuggets while suppressing weld cracking, particularly due to operational disturbances and the low melting point of metal coatings like zinc, which can lead to deformation and cracking.
Innovation Solution
A two-step current application method is employed, where a first current applying step forms a nugget with a controlled diameter to minimize deformation, followed by a second step with increased current and short-time current application and cooling to grow the nugget without significant deformation, reducing the risk of cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a large current value is set during welding to ensure a large nugget diameter, then the nugget diameter is increased, but the weld undergoes large deformation which increases the risk of cracking
Solution Approach 1:
The welding current is divided into multiple stages with different current values. The first stage uses a first current value to form an initial nugget, and the second stage uses a second current value (higher than the first) to grow the nugget to the target diameter. This segmentation allows the nugget to grow in controlled increments, reducing overall deformation while achieving the required final nugget size.
Solution Approach 2:
The welding process employs periodic current application with intermediate cooling periods. After the second current application stage, a cooling period is introduced before electrode opening. This periodic action allows heat dissipation during the process, controlling thermal deformation and preventing cracking while still achieving sufficient nugget growth.
2Manufacturing precision
If the weld time is extended to form a large nugget, then the nugget diameter increases, but the heat input increases causing large deformation and cracking
Solution Approach 1:
The total weld time is segmented into distinct stages: a first welding period with a first current value, followed by a second welding period with a second current value, and then a cooling period. This segmentation allows the nugget to grow efficiently through targeted current application at different stages, achieving the required nugget diameter without excessive total heat input that would cause deformation and cracking.
3Device complexity
If a single-stage welding process is used, then the process is simple, but it is difficult to stably form a nugget with large diameter while suppressing weld cracking under operational disturbances
Solution Approach 1:
The welding current is segmented into multiple stages with different current values and durations. The first stage forms an initial nugget, and the second stage grows it to the target size. This multi-stage approach provides stability against operational disturbances by ensuring that even if conditions vary, the controlled progression through stages maintains consistent nugget quality and reduces cracking risk.
Solution Approach 2:
The welding process changes current parameters (current value and duration) between stages. The first stage uses a first current value optimized for initial nugget formation, while the second stage uses a second current value optimized for nugget growth. This parameter optimization at each stage ensures stable weld quality and reduces sensitivity to operational disturbances.
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 allows for the stable formation of large nuggets with reduced deformation and cracking, even under operational disturbances, enhancing the reliability of the welding process for surface-treated steel sheets.
Implementation Method 1
The resistance heat generated by the passage of the welding current is used to form a spot-like weld 5
Implementation Method 2
the metal coating layer with a low melting point on the surface of the steel sheet melts during welding
Data Source
AI summary
A resistance spot welding method for joining two or more steel sheets including at least one zinc-coated steel sheet. The method includes a first current applying step and a second current applying step. The first current applying step involves forming a nugget having a nugget diameter of 3√t or more and 4.5√t or less by setting a current value I1 (kA) and a weld time, where t is a thickness of the thinnest steel sheet among the overlapping steel sheets. The second current applying step involves growing the nugget by repeating a cooling step for maintaining a zero-current state for 10 ms or more and less than 160 ms and a current applying step for applying a current for 20 ms or more and less than 200 ms at a current value I2 (kA) greater than or equal to the current value I1 (kA).