Resistance Spot Welding Under Electrode Axis Misalignment
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Solution Overview
Problem
Resistance spot welding methods fail to effectively inhibit cracking in welds, particularly in steel sheets with surface-treated coatings, due to limitations in alloying element content, complex optimization of welding conditions, and neglect of axis misalignment, which can lead to inadequate weld quality and increased man-hours in assembly processes.
Innovation Solution
Adjusting the electrode force retaining time based on the axis misalignment between electrodes, ensuring specific relationships between axis misalignment, sheet thickness, tensile strength, electrode force, and tip diameter to reduce tensile stress and prevent cracking, even in high-alloy steel sheets.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the metal coating layer with low melting point is used on steel sheets, then corrosion resistance and surface properties are improved, but cracking occurs in the weld due to liquid metal embrittlement
Solution Approach 1:
The invention changes the thermal parameters of the welding process by implementing a two-stage heating method: first heating to a sub-solidus temperature (below the melting point of the metal coating) to perform plastic deformation and bonding, then heating to a solidus temperature (above the melting point) to complete the welding. This parameter change allows the metal coating to remain solid during the bonding phase, preventing liquid metal embrittlement, while still achieving proper weld formation.
Solution Approach 2:
The welding process is divided into distinct periodic stages: a first heating period to sub-solidus temperature for plastic deformation and bonding, followed by a second heating period to solidus temperature for completing the weld. This periodic action separates the bonding mechanism from the melting mechanism, allowing the low-melting-point metal coating to provide surface properties without causing embrittlement.
2Productivity
If conventional resistance spot welding is used with surface-treated steel sheets, then welding speed and productivity are maintained, but cracking occurs in the weld
Solution Approach 1:
The conventional single-stage resistance spot welding process is segmented into two distinct heating stages: first heating to sub-solidus temperature for bonding, then heating to solidus temperature for completing the weld. This segmentation allows each stage to optimize for its specific function while preventing the harmful effect of liquid metal embrittlement that occurs in conventional single-stage welding.
3Productivity
If high welding current is applied for short time, then productivity is improved, but cracking occurs due to rapid heating and liquid metal embrittlement
Solution Approach 1:
The welding current is applied in a periodic manner with two distinct phases: first current application heats to sub-solidus temperature for bonding without melting the metal coating, then second current application heats to solidus temperature for completing the weld. This periodic current application maintains productivity while preventing cracking by avoiding premature melting of the low-melting-point metal coating.
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 inhibits cracking in welds regardless of steel grade, improving weld quality and reducing the likelihood of liquid metal embrittlement, while maintaining productivity and adaptability to various steel sheet combinations.
Implementation Method 1
a high welding current is passed therethrough for a short time to achieve joining. A spot weld 5 is obtained by utilizing resistive heat generated by the passage of the high welding current
Implementation Method 2
the molten low melting point metal penetrates the grain boundaries of the base material of the surface-treated steel sheet, which decreases the grain boundary strength, and this causes cracking in the weld
Data Source
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Figure 5~6
AI summary
Provided is a resistance spot welding method that inhibits, in accordance with the degree of axis misalignment between electrodes, the occurrence of cracking in a weld regardless of the steel grade. [Solution] In resistance spot welding methods according to the present invention, H (ms) is an electrode force retaining time after completion of current passage, D (mm) is an amount of axis misalignment between the electrodes, t (mm) is a total sum of sheet thicknesses of a plurality of overlapping steel sheets, T (MPa) is a tensile strength of a steel sheet having a highest tensile strength among the plurality of steel sheets, F (N) is an electrode force, and d (mm) is a tip diameter of one electrode of the pair of electrodes that has a smaller tip diameter. The electrode force retaining time H is specified to be a predetermined value or greater.