Resistance Spot Welding Two-Step Current Control
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Solution Overview
Problem
Resistance spot welding methods fail to consistently form a nugget of sufficient diameter without splashing, especially when using high tensile strength galvanized steel sheets or when unintended gaps exist between steel sheets, due to unstable electrode force and contact conditions.
Innovation Solution
A method involving a two-step current process where an initial high current is applied for a short time to ensure contact and heat generation, followed by a reduction in current or its cessation to prevent splashing, with specific current and time parameters optimized to maintain contact and form a nugget of appropriate diameter.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength steel sheets are used to reduce weight and improve reliability, then automotive body strength-to-weight ratio is improved, but weld quality decreases and splashing occurs more easily
Solution Approach 1:
The welding current is segmented into multiple stages: a first-stage current (I1) applied for a first time period (t1) to initially join the sheets, followed by a second-stage current (I2) applied for a second time period (t2) to complete the nugget formation. This segmentation allows optimized control of heat input at different welding phases, preventing splashing while ensuring weld quality on high-strength steels
Solution Approach 2:
The first-stage current is applied before the second-stage current to preliminarily join the steel sheets. This preliminary action establishes initial contact and heat distribution, creating favorable conditions for the subsequent second-stage current to form a complete nugget without causing splashing
2Strength
If the number of welding spots is increased to guarantee weld strength, then weld strength is improved, but productivity decreases due to longer operation time and increased shunt current effect
Solution Approach 1:
The welding process is divided into two current stages with different parameters. The first-stage current quickly establishes initial joining, and the second-stage current completes nugget formation. This segmentation reduces total welding time compared to conventional single-stage welding, thereby improving productivity while maintaining weld strength
Solution Approach 2:
The welding current parameters are changed between stages: the first-stage current (I1, t1) differs from the second-stage current (I2, t2). By optimizing these parameters, the process achieves sufficient weld strength in reduced time, counteracting the productivity loss from increased welding spot numbers
3Strength
If the nugget diameter is expanded to ensure weld strength, then weld strength is improved, but device complexity increases and base material properties are lost due to expanded heat-affected zone
Solution Approach 1:
Instead of increasing electrode size or force to expand the nugget, the invention changes the current parameters through two stages. The first-stage current (I1, t1) followed by the second-stage current (I2, t2) optimizes heat distribution to form an adequate nugget with conventional electrodes, reducing device complexity while preserving base material properties
4Reliability
If galvanized steel sheets are used to prevent rust, then corrosion resistance is improved, but splashing occurs more easily and nugget formation is adversely affected
Solution Approach 1:
The current is segmented into two stages with the first-stage current (I1, t1) controlling initial heating and the second-stage current (I2, t2) completing nugget formation. This segmentation prevents excessive localized heating that causes splashing on galvanized surfaces, while still achieving adequate nugget diameter and maintaining the protective galvanized coating
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 approach effectively prevents splashing and ensures a nugget of sufficient diameter is formed, even with sheet gaps, enhancing the reliability of resistance spot welding for high tensile strength and galvanized steel sheets.
Implementation Method 1
by applying electrode force and current, the contact portion is welded to form a nugget 6 of a required size
Data Source
Figure 1~3(b)
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Figure 5
AI summary
Resistance spot welding is performed on a combination of overlapping steel sheets including at least one steel sheet that has, on a surface thereof, a coated layer with zinc as a main component, by (1) starting electric current passage in a state satisfying 0.9 × t ≤ L ≤ 1.1 × t, where t is the total thickness of the overlapping steel sheets and L is the distance between tips of upper and lower electrodes; and (2) dividing electric current into main current and initial current that precedes the main current and is two-step current, setting a current value I1 during current in a first step of the initial current to satisfy Im × 1.1 ≤ I1 ≤ 15.0 kA with respect to a current value Im during the main current, and setting a current value I2 in the subsequent second step to no current or low current satisfying 0 ≤ I2 ≤ Im × 0.7, thereby stably forming a nugget of sufficient diameter, without the occurrence of splashing.