Resistance Spot Welding Reheating for Coated Steel Sheet Cracking
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Solution Overview
Problem
Resistance spot welding methods for surface-treated steel sheets often result in cracking due to liquid metal embrittlement, and existing solutions are limited in preventing cracking and maintaining joint strength, especially with high-strength steel sheets and varying electrode angles.
Innovation Solution
A resistance spot welding method that includes a primary energization step followed by a suspension period and a secondary reheating step, with specific current and time conditions based on the electrode misalignment angle to prevent cracking and enhance joint strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If high welding current is applied to join surface-treated steel sheets, then welding speed and productivity are improved, but cracking occurs due to liquid metal embrittlement
Solution Approach 1:
The welding process is divided into three distinct stages: primary energization for nugget formation, non-energization for cooling, and secondary energization for reheating. This segmentation allows each stage to optimize for its specific function, preventing cracking while maintaining productivity
Solution Approach 2:
The welding process uses periodic energization with alternating current application and suspension. The current is applied in pulses rather than continuously, creating a rhythmic heating-cooling-reheating cycle that prevents liquid metal embrittlement while maintaining weld integrity
2Reliability
If electrode force is increased to prevent cracking, then weld integrity is improved, but the risk of metal embrittlement and cracking increases
Solution Approach 1:
The process dynamically changes multiple parameters including current value, electrode force, and timing throughout the welding cycle. During primary energization, high current forms the nugget; during non-energization, cooling occurs; during secondary energization, reheating strengthens the weld without causing embrittlement
3Reliability
If multi-stage energization is used to prevent cracking, then weld quality is improved, but process complexity increases
Solution Approach 1:
The welding process incorporates feedback mechanisms where the control system monitors welding parameters and adjusts current, voltage, and timing in real-time based on the welding state, optimizing the multi-stage process automatically
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 effectively prevents cracking and achieves high joint strength across various steel types, including high-strength sheets, by controlling the electrode misalignment angle and reheating conditions, thereby improving the weld's integrity and stability.
Implementation Method 1
a high welding current is applied through the upper and lower electrodes for a short time to thereby join the steel sheets 1 and 2 together. A spot weld 5 is obtained by using heat generated by resistance heating due to the flow of the high welding current.
Implementation Method 2
When the electrode force and tensile stress due to thermal expansion and contraction of the steel sheets are applied to the weld, the molten low-melting point metal penetrates into grain boundaries of the base metal of the surface-treated steel sheet and causes a reduction in grain boundary strength
Implementation Method 3
tensile stress due to thermal expansion and contraction of the steel sheets
Data Source
AI summary
Provided is a resistance spot welding method. The resistance spot welding method for joining a sheet set including a plurality of lapped steel sheets includes: holding the sheet set between a pair of electrodes; and energizing the sheet set under application of electrode force to thereby join the steel sheets together. At least one of the plurality of lapped steel sheets is a surface-treated steel sheet including a metal coating layer on a surface thereof. The energizing includes: a primary energizing step of performing energization to form a nugget portion; a non-energizing step in which, after the primary energizing step, the energization is suspended for an energization suspension time Tc (cycles); and a secondary energizing step of, after the non-energizing step, performing energization for reheating while the nugget portion is prevented from growing. During the energizing, the relations of a particular formula are satisfied.