Resistance Spot Welding Cycles for High-Mn Steel Joint Strength
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Solution Overview
Problem
Resistance spot welding methods fail to achieve favorable fracture forms in cross tensile tests when using steel sheets with high Mn content, due to brittle interface failures caused by Mn condensation and P segregation.
Innovation Solution
A resistance spot welding method involving main and subsequent current passages with a controlled cooling time and current ratio, defined by specific formulas based on Mn and P content, to prevent brittle interface failures and ensure sufficient cross tension strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If steel sheets with high Mn content are used to increase strength, then the tensile strength of the base steel sheet increases, but brittle interface failure occurs at the nugget in cross tensile tests
Solution Approach 1:
The patent applies preliminary action by performing a first current passage to form the nugget, then allowing a cooling period before performing a second current passage. This preliminary cooling action prevents Mn condensation and P segregation that would otherwise cause brittle interface failure during subsequent welding operations.
Solution Approach 2:
The patent employs periodic action by alternating between current passage and cooling periods in a cyclic manner. The welding process consists of multiple cycles where current is applied to heat and form the nugget, then paused for cooling, repeated as needed to achieve proper nugget formation without brittle failure.
2Productivity
If current passage is performed continuously to form a nugget quickly, then productivity increases, but Mn condensation and P segregation occur causing interface failure
Solution Approach 1:
The patent uses periodic action by dividing continuous current passage into discrete cycles with cooling intervals. Each cycle consists of current application followed by a cooling period, repeated multiple times to gradually form the nugget while preventing Mn condensation and P segregation that would occur with continuous heating.
Solution Approach 2:
The patent applies preliminary action by performing initial current passages with cooling periods to prepare the material structure before final nugget completion. The preliminary cooling cycles establish a microstructure that prevents subsequent Mn condensation and P segregation during final welding stages.
3Strength
If the nugget is hardened to increase shear tensile strength, then shear tensile strength increases, but plastic deformation is suppressed and opening stress concentrates at nugget edges
Solution Approach 1:
The patent applies preliminary action by performing cooling cycles before final nugget completion. These preliminary cooling actions create a microstructure that maintains toughness while allowing controlled hardening during subsequent current passages, preventing excessive stress concentration at nugget edges.
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
The method ensures sufficient cross tension strength even with steel sheets containing high Mn content, facilitating the production of stronger and thinner automotive parts.
Implementation Method 1
a current is passed to melt a contact portion and form a nugget
Implementation Method 2
with a cooling time of at least predetermined duration between the main current passage and the subsequent current passage
Data Source
AI summary
Main current passage and subsequent current passage are performed, with a cooling time of 0.01 s or more being provided between the main current passage and the subsequent current passage. Ip/I which is a ratio of a current value of the subsequent current passage to a current value of the main current passage is controlled to satisfy a predetermined relationship depending on a constant A defined by Mn content and P content of a steel sheet as a part to be welded, in relation to a welding time Tp of the subsequent current passage, a cooling time T, and the constant A.