Spot Weld Nugget Shaping to Prevent Surface Protrusions
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Solution Overview
Problem
Existing spot welding techniques face issues with protrusions on the surface of welded sheet materials, electrode deformation, and increased production costs due to frequent electrode exchange and compound formation, especially when welding high-tensile steel and aluminum alloy sheets.
Innovation Solution
A spot welding method using electrodes with a specific copper alloy composition and shape, featuring a bottomed and cylindrical tip portion with a convex pressure-contact surface, which forms a flattened nugget with a diameter of at least 4√t and a flattening level of 3.5 to 8, reducing electrode deformation and compound generation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a concave electrode is used to suppress expulsion, then joint strength is improved, but protrusions appear on the outer surface of the welded sheet materials
Solution Approach 1:
The electrode tip is designed with a convex pressure-contact surface that creates localized high pressure at the center of the weld spot. This concentrates the forming force on the nugget while minimizing impact on the surrounding sheet material, thereby preventing protrusions on the outer surface while still achieving sufficient joint strength.
2Loss of energy
If pure copper electrodes are used for high conductivity, then resistance heat generation is reduced, but electrode strength is low causing buckling and deformation
Solution Approach 1:
The electrode is constructed as a composite structure combining copper material (for high electrical conductivity and low resistance heat generation) with a convex geometric design (for mechanical strength and resistance to buckling). This allows the electrode to simultaneously achieve low energy loss and high structural integrity during spot welding operations.
3Strength
If copper alloy electrodes are used for high strength, then electrode durability is improved, but conductivity is low causing self-heating and softening
Solution Approach 1:
The convex pressure-contact surface design creates localized high pressure at the center of the electrode tip during welding. This concentrates the current density and heat generation at the contact point with the workpiece, while the bulk of the electrode material remains cooler and maintains its mechanical strength, preventing self-heating and softening issues.
4Manufacturing precision
If large pressing force is applied to suppress gap, then welding quality is improved, but electrode deformation increases
Solution Approach 1:
The convex pressure-contact surface concentrates the pressing force at the center of the electrode tip, creating localized high pressure that effectively closes gaps and ensures good welding quality. The force distribution is optimized to achieve sufficient contact pressure without requiring excessive overall pressing force, thereby reducing electrode deformation.
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 achieves a stable, aesthetically pleasing weldment with reduced electrode wear and frequency of exchange, maintaining joint strength and corrosion resistance while lowering production costs.
Implementation Method 1
both outer surfaces of workpieces to be joined (materials to be joined, such as sheet materials) are pressed (applied with pressing force) between a pair of electrodes arranged opposite to each other and, in this state, the workpieces to be joined are supplied with a large current via the electrodes for a short time. When performing the spot welding, the workpieces to be joined as such are melted and solidified to form a nugget inside the workpieces.
Implementation Method 2
this document proposes an electrode that is made of a material (pure copper such as tough pitch copper or oxygen-free copper) with a high conductivity (97% IACS or more) and little resistance heat generation and that is provided inside with a small cooling hole. This electrode has high cooling efficiency, and the temperature rise of the pressure-contact surface in contact with a workpiece to be joined can be suppressed.
Data Source
AI summary
A highly corrosion-resistant spot weldment can be produced at low cost without occurrence of prominent protrusions and the like on the surface. The spot weldment is joined by a nugget formed inside stacked sheet materials through bringing a pair of electrodes arranged opposite to each other into pressure contact with the stacked sheet materials from outside and energizing the stacked sheet materials from the electrodes. The nugget has a diameter that is ≥4√t (t: thickness of sheet material) and a flattening level of 3.5 to 8, which is a ratio of diameter to thickness. Both outer surface parts of the sheet materials are free from protrusions formed due to bulging of molten metal. Even when the electrodes are made of a copper alloy, the increased amount of Cu in the outer surface parts is 0.2 mass % or less with respect to the component composition before spot welding.