Spot Welding Timing for Zinc-Plated Steel Crack Suppression
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Solution Overview
Problem
Spot welding of zinc-plated steel sheets often results in cracking due to liquid metal embrittlement, particularly at the corona bond and nugget boundary, which compromises joint strength and tensile strength, especially when disturbance factors such as misalignment and gap presence are present.
Innovation Solution
A spot welding method that involves specific squeeze and hold time parameters, expressed as functions of the total sheet thickness, to control the solidification of molten zinc and reduce residual stress, thereby suppressing cracking and ensuring joint strength, even with disturbance factors like misalignment and gaps.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength zinc-plated steel sheets are spot-welded, then the strength and corrosion resistance of vehicle body members are improved, but cracking occurs from the outer surface to the nugget due to liquid metal embrittlement
Solution Approach 1:
The invention changes the temporal parameters of the welding process by introducing a specific squeeze time (0.015t²+0.020 ≤ St ≤ 0.040t²+0.060) before electric current flow starts. This parameter change allows the molten zinc to solidify before welding, preventing liquid metal embrittlement while maintaining the strength benefits of high strength zinc-plated steel sheets
Solution Approach 2:
The invention performs a preliminary action by applying squeeze pressure for a predetermined time before the welding current is applied. This preliminary squeezing action solidifies the molten zinc on the steel sheet surface before it can intrude into the grain boundaries during welding, thereby preventing cracking while preserving the material's strength properties
2Reliability
If the squeeze time is extended to solidify molten zinc before welding, then cracking is suppressed, but the productivity decreases
Solution Approach 1:
The invention optimizes the squeeze time parameter within a specific range (0.015t²+0.020 ≤ St ≤ 0.040t²+0.060) that is sufficient to solidify molten zinc but short enough to maintain productivity. This balanced parameter setting resolves the contradiction between cracking suppression and production efficiency
Solution Approach 2:
The invention applies a partial squeeze time that is just sufficient to achieve the critical function of solidifying molten zinc before welding, without extending it unnecessarily. This partial action approach maintains productivity while achieving the necessary cracking suppression effect
3Weight of moving object
If high strength steel sheets are used to reduce vehicle body weight, then fuel efficiency and collision safety are improved, but liquid metal embrittlement cracking occurs more readily
Solution Approach 1:
The invention applies preliminary squeeze pressure before welding to solidify the zinc coating in advance. This preliminary action prevents the liquid zinc from causing embrittlement in high strength steel sheets, allowing these sheets to be used for weight reduction without suffering from cracking issues
Solution Approach 2:
The invention introduces a specific squeeze time parameter range that adapts to the total sheet thickness (0.015t²+0.020 ≤ St ≤ 0.040t²+0.060), enabling the use of high strength steel sheets with varying thicknesses while preventing liquid metal embrittlement across different material configurations
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 suppresses cracking at the corona bond and nugget boundary, securing high joint strength and maintaining productivity by optimizing squeeze and hold times in relation to sheet thickness, enhancing the quality of spot-welded joints in zinc-plated steel sheets.
Implementation Method 1
electric current flow for welding between the welding electrodes
Implementation Method 2
welding pressure of the electrodes
Implementation Method 3
zinc or an alloy of zinc and copper of the electrodes melt on the steel sheet surface at the welding zone intrudes into the grain boundaries of the steel sheets
Implementation Method 4
tensile stress due to thermal expansion or contraction of the steel sheets
Data Source
AI summary
The present invention provides a spot welding method for a member to be welded constituted of a plurality of steel sheets that are overlapped with each other at at least a welding zone, in which at least an overlapped face of at least one of the plurality of steel sheets at the welding zone is coated with zinc plating, a total sheet thickness t (mm) of the plurality of steel sheets is 1.35 mm or more, a squeeze time St (seconds) from the time when welding electrodes are brought into contact with the member to be welded to the time when electric current flow for welding starts satisfies “0.020≤St”, and a hold time Ht( seconds) after welding from the time when electric current flow for welding between the welding electrodes ends to the time when the welding electrodes and the member to be welded are brought out of contact satisfies “0.015t2+0.020≤Ht”.