Resistance Spot Welding with Initial Spatter for Hydrogen Control
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Solution Overview
Problem
Resistance spot welding of high strength steel sheets is prone to delayed fracture due to hydrogen embrittlement, as hydrogen enters the weld metal during the welding process, and existing methods either fail to reduce residual hydrogen or compromise the strength and appearance of the weld joint.
Innovation Solution
A two-step resistance spot welding method is employed, where an initial welding step produces spatter to discharge hydrogen sources and maintain clean contact, followed by a main welding step to form a large diameter nugget, with specific current and force conditions to minimize hydrogen entry and maximize nugget growth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If welding force is excessively increased immediately after passage of welding current to control residual stress and prevent delayed fracture, then delayed fracture is suppressed, but sheet thickness at the weld decreases and weld strength is reduced
Solution Approach 1:
The welding process is divided into two distinct stages: a first stage with initial welding force to form the nugget, and a second stage with increased welding force to control residual stress and prevent delayed fracture. This segmentation allows each stage to be optimized independently, avoiding the trade-off between preventing delayed fracture and maintaining weld strength.
Solution Approach 2:
The first welding stage performs the preliminary action of forming the nugget with appropriate welding force before the second stage increases force to control residual stress. By performing the nugget formation first, the subsequent increase in welding force does not compromise weld strength since the nugget is already formed.
2Reliability
If welding force is increased to control microstructure and hardness of welds to prevent delayed fracture, then delayed fracture is suppressed, but sheet thickness at the weld decreases and appearance is affected
Solution Approach 1:
The welding process is divided into two distinct stages: a first stage with initial welding force to form the nugget, and a second stage with increased welding force to control residual stress and prevent delayed fracture. This segmentation allows each stage to be optimized independently, avoiding the trade-off between preventing delayed fracture and maintaining weld strength.
Solution Approach 2:
The first welding stage performs the preliminary action of forming the nugget with appropriate welding force before the second stage increases force to control residual stress. By performing the nugget formation first, the subsequent increase in welding force does not compromise weld strength since the nugget is already formed.
3Ease of manufacture
If conventional resistance spot welding is used on high strength steel sheets, then welding process is simple, but hydrogen enters weld metal causing delayed fracture
Solution Approach 1:
The welding process is divided into two distinct stages: a first stage with initial welding force to form the nugget, and a second stage with increased welding force to control residual stress and prevent delayed fracture. This segmentation allows each stage to be optimized independently, avoiding the trade-off between preventing delayed fracture and maintaining weld strength.
Solution Approach 2:
The welding force parameter is changed between two stages: initial welding force during nugget formation, then increased welding force during the second stage to control residual stress and prevent hydrogen embrittlement. This parameter change enables delayed fracture prevention 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 effectively suppresses delayed fracture and stabilizes the formation of large diameter nuggets, improving the resistance spot weld joint's strength and appearance while reducing residual hydrogen.
Implementation Method 1
passing a current through, while applying pressure to, two or more steel sheets that are placed on top of each other and clamped by a pair of welding electrodes
Implementation Method 2
applying pressure to, two or more steel sheets
Implementation Method 3
the welding force is increased and the current is decreased to control residual stress in welds and thereby prevent delayed fracture
Implementation Method 4
the welding force is increased immediately after passage of welding current (main welding) and the current is passed after a cooling period during which there is no current passage
Data Source
AI summary
A resistance spot welding method is disclosed 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.