Resistance Spot Welding Sequence for Delayed Fracture Suppression
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Solution Overview
Problem
Resistance spot welding of high strength steel sheets faces issues with hydrogen embrittlement leading to delayed fracture, as existing methods do not effectively reduce residual hydrogen in the weld, and increasing welding force can decrease weld strength and affect appearance.
Innovation Solution
A two-step resistance spot welding process is implemented, where an initial welding step produces spatter to discharge hydrogen sources and an optional cooling step stabilizes contact, followed by a main welding step to form a large diameter nugget, with controlled current and force settings to minimize hydrogen entry and maximize nugget growth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If welding force is increased to prevent delayed fracture, then hydrogen entry into weld metal is reduced, but sheet thickness at weld decreases and weld strength is reduced
Solution Approach 1:
The welding process is divided into multiple stages with different current and force levels. The first stage uses high current and high force to discharge hydrogen sources, while the second stage uses appropriate current and force to form the nugget without excessive sheet thinning. This segmentation allows optimization of each stage for its specific purpose.
Solution Approach 2:
The first welding stage performs preliminary action by discharging hydrogen sources (rust preventive oil, moisture, coated layers) from the steel sheet surface before the main welding process. This preliminary hydrogen removal prevents delayed fracture without requiring excessive force during the main nugget formation stage.
2Reliability
If welding force is excessively increased immediately after welding current passage, then residual stress is controlled, but sheet thickness at weld decreases and appearance is affected
Solution Approach 1:
The welding process is segmented into a first stage for hydrogen source discharge and a second stage for nugget formation. The force application is also segmented, with high force in the first stage and appropriate force in the second stage, preventing excessive sheet thinning while maintaining delayed fracture resistance.
Solution Approach 2:
The welding process uses periodic action with distinct phases: initial high current and force for hydrogen discharge, followed by appropriate current and force for nugget formation. This periodic variation in parameters achieves both hydrogen removal and proper weld formation without excessive sheet thinning.
3Ease of manufacture
If conventional welding methods are used, then welding process is simple, but hydrogen enters weld metal causing delayed fracture
Solution Approach 1:
The welding process is divided into two stages: first stage with high current and high force to discharge hydrogen sources, and second stage with appropriate current and force to form the nugget. This segmentation effectively removes hydrogen without significantly complicating the manufacturing process.
Solution Approach 2:
The invention changes the welding parameters (current and force) at different stages of the welding process. The first stage uses high current and high force, while the second stage uses appropriate current and force, optimizing both hydrogen removal and weld formation while maintaining process simplicity.
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 stably forms large diameter nuggets while suppressing delayed fracture, effectively reducing residual hydrogen and maintaining weld joint strength and appearance.
Implementation Method 1
passing a current through, while applying pressure to, two or more steel sheets that are placed on top of each other... forms a nugget of a predetermined size at the interface of the steel sheets
Implementation Method 2
applying pressure to, two or more steel sheets that are placed on top of each other and clamped by a pair of welding electrodes
Data Source
Figure 1~2(b)
AI summary
An object is to provide a resistance spot welding method and a method for producing a resistance spot weld joint. The present invention provides a resistance spot welding method for joining two or more steel sheets including at least one steel sheet having a tensile strength of 980 MPa or higher. The resistance spot welding method involves placing the steel sheets on top of each other to form a set of steel sheets to be welded, clamping the set of steel sheets with a pair of electrodes, and passing a current through the steel sheets while applying pressure thereto to join the steel sheets together. The resistance spot welding method includes an initial welding step of welding by passing a current I1 (kA) satisfying 2 × √F1 < I1 ≤ 10 × √F1 while applying a welding force F1 (kN) satisfying 0.2 × √t1 < F1 ≤ 4 × √t1, and a main welding step of forming a nugget having a predetermined nugget diameter. Spatter is produced in the initial welding step.