Resistance Spot Welding Holding Time Control for Crack Prevention
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Solution Overview
Problem
Existing resistance spot welding methods for high-strength steel sheets with surface treatments, such as galvanized layers, face issues with liquid metal embrittlement (LME) and cold cracking in the weld heat affected zone, particularly due to complex factors involving alloy elements, surface states, and welding operation disturbances, which are not adequately addressed by current techniques.
Innovation Solution
A resistance spot welding method that optimizes the electrode holding time based on the composition and surface state of the steel sheets, including the contents of alloy elements like C, Si, and Mn, as well as the thickness of the decarburized layer and internal oxidation, while considering welding operation disturbances, to prevent both LME and cold cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If resistance spot welding is applied to high-strength steel sheets with surface treatments, then welding productivity is improved, but LME cracking and cold cracking occur in the weld heat affected zone
Solution Approach 1:
The invention changes the temporal parameter of electrode holding time after current application to prevent cracking. By optimizing the holding time based on steel sheet composition and surface state, the method resolves the contradiction between maintaining high welding productivity and preventing LME/cold cracking in the heat affected zone.
Solution Approach 2:
The invention performs preliminary analysis of steel sheet composition (alloy elements C, Si, Mn) and surface state (decarburized layer thickness, internal oxidation) before welding to determine the appropriate electrode holding time. This preliminary characterization enables prevention of cracking before it occurs, while maintaining efficient welding productivity.
2Reliability
If electrode holding time is extended to prevent LME cracking, then weld reliability improves, but welding cycle time increases
Solution Approach 1:
The invention optimizes the electrode holding time parameter to the minimum necessary duration to prevent cracking, rather than using excessive holding times. By basing the holding time on specific steel sheet characteristics, it achieves crack prevention with minimal time extension, balancing reliability improvement with productivity maintenance.
Solution Approach 2:
The invention makes the electrode holding time dynamic rather than fixed, adjusting it based on the specific composition and surface state of each steel sheet. This dynamic adjustment ensures sufficient holding time for crack prevention while minimizing unnecessary time extension for each welding cycle.
3Strength
If welding current is increased to improve weld strength, then joint strength improves, but LME cracking risk increases due to molten coating penetration
Solution Approach 1:
The invention performs preliminary characterization of the steel sheet surface state including coating thickness and composition before welding. This allows determination of appropriate welding parameters that achieve sufficient joint strength while preventing molten coating penetration and LME cracking.
Solution Approach 2:
The optimized electrode holding time acts as an intermediary parameter that mediates between the need for high welding current (for joint strength) and the risk of LME cracking. By controlling the holding time after current application, it allows high current welding while preventing harmful molten metal penetration during the cooling phase.
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 reduces LME and cold cracking by controlling the pressure holding time after current application, ensuring stable welds in high-strength steel sheets with surface treatments, regardless of their strength or assembly conditions.
Implementation Method 1
passing a high welding current between the upper and lower electrodes for a short period of time while pressing the steel sheets with the welding electrodes, thereby joining the steel sheets together. This method utilizes resistance heat, generated by applying a high welding current to the steel sheets
Implementation Method 2
a portion where the two overlapping steel sheets have melted in an area of contact between the steel sheets upon application of current to the steel sheets, and the melt has then solidified
Implementation Method 3
tensile stress due to thermal expansion or contraction of steel sheets
Implementation Method 4
A metal coating layer having a low melting point on the surface of a steel sheet melts during welding. When the pressure of welding electrodes or tensile stress due to thermal expansion or contraction of steel sheets is applied to a weld, the molten low-melting metal penetrates into the grain boundaries of the base material of the surface-treated steel sheet
Implementation Method 5
the molten low-melting metal penetrates into the grain boundaries of the base material
Implementation Method 6
holding the welding electrodes after completion of the application of current, wherein in the holding step, a pressure holding time H satisfies
Implementation Method 7
the melt has then solidified
Data Source
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AI summary
The present invention is directed to the provision of a resistance spot welding method and a method for producing a welded member. The present invention provides a resistance spot welding method for joining a plurality of steel sheets together by clamping a sheet assembly, consisting of the plurality of overlapping steel sheets, between a pair of welding electrodes, and applying a current to the sheet assembly while applying pressure thereon, the method including a holding step of holding the welding electrodes after completion of the application of current. In the holding step, a pressure holding time is controlled within a predetermined time range defined by a particular relational expression using the C content, the Si content, and the Mn content of a steel sheet, the thickness of a decarburized layer per one surface of the steel sheet, and the amount of internal oxidation per one surface of the steel sheet.