Resistance Spot Weld Composition Control Against LME Cracking
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Solution Overview
Problem
High-strength zinc-coated steel sheets used in the automotive industry are prone to Liquid Metal Embrittlement (LME) cracking during resistance spot welding, which reduces the mechanical performance of welds and is challenging to address with existing methods that either compromise tensile strength or increase production complexity.
Innovation Solution
A method for producing zinc or zinc-alloy coated steel sheets with a tensile strength greater than 900 MPa, involving specific composition and annealing processes to create a microstructure resistant to LME cracking, including controlled carbon, manganese, silicon, and chromium levels, and a modified composition zone under the coating to minimize LME susceptibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength steel sheets (TS>900 MPa) are used to meet automotive structural requirements, then tensile strength is improved, but susceptibility to Liquid Metal Embrittlement cracking during resistance spot welding increases
Solution Approach 1:
The patent applies local quality by creating a specific compositional gradient in the steel sheet, where the zone immediately under the zinc coating (0-100 micrometers depth) has controlled lower concentrations of carbon, silicon, and manganese compared to the bulk material. This localized compositional modification reduces LME susceptibility at the critical coating-substrate interface while preserving the high tensile strength of the overall steel sheet through its bulk composition and microstructure.
Solution Approach 2:
The patent employs parameter changes by precisely controlling the chemical composition parameters (C, Si, Mn, Cr, Al contents) and their spatial distribution, along with applying specific annealing temperature parameters (720-1000°C) to transform the microstructure. These parameter adjustments enable the steel to achieve both high tensile strength (>900 MPa) and reduced LME cracking susceptibility during resistance spot welding.
2Strength
If alloying elements are added to achieve high tensile strength (>900 MPa), then strength is improved, but the complexity of steel composition control and fabrication increases
Solution Approach 1:
The patent establishes specific parameter ranges for alloying elements (C: 0.15-0.40%, Si: 0.05-2.00%, Mn: 1.00-3.00%, Cr: 0.05-2.00%, Al: 0.05-1.00%) and their concentration gradients in the subsurface zone, providing a standardized composition framework that balances high strength achievement with manageable compositional control in industrial fabrication.
3Ease of manufacture
If conventional annealing processes are used on high strength steel, then manufacturing simplicity is maintained, but the steel does not achieve optimal resistance to LME cracking
Solution Approach 1:
The patent specifies optimized annealing parameters (temperature range: 720-1000°C, duration: 100-500 seconds, atmosphere: nitrogen with 2-15% hydrogen) that transform the microstructure to achieve both high tensile strength and enhanced LME resistance. These parameter changes maintain reasonable manufacturing simplicity while significantly improving weldability and reducing cracking susceptibility.
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 resistance spot welds with an average of not more than two LME cracks deeper than 100 microns, maintaining high tensile strength and formability, suitable for automotive applications.
Implementation Method 1
heating the cold-rolled steel sheet up to a temperature T1 comprised between 550° C. and Ac1+50° C. in a furnace zone with an atmosphere (A1) containing from 2 to 15% hydrogen by volume, the balance being nitrogen and unavoidable impurities, so that the iron is not oxidized
Implementation Method 2
heating the sheet from the temperature T1 up to a temperature T2 comprised between 720° C. and 1000° C. in a furnace zone under an atmosphere (A2) of nitrogen containing from 2 to 15% hydrogen and more than 0.1% CO by volume, wherein the duration tD of said heating of the sheet from temperature T1 up to the end of soaking at temperature T2 is comprised between 100 and 500 s, soaking the sheet at T2
Data Source
AI summary
A method for the fabrication of a resistance spot weld containing not more than two Liquid Metal Embrittlement cracks having a depth of 100 μm or more, the method includes the following successive steps of providing at least two first zinc or zinc-alloy coated sheets having a first steel substrate of a first steel, with TS>900 MPa, a thickness of the zinc or zinc-alloy coated sheets being between 0.5 and 2.5 mm; measuring C1av(100), Si1av(100), Mn1av(100), Al1av(100), Cr1av(100), designating respectively the average content of C, Si, Mn, Al, Cr in the zone D100 of the first steel substrate comprised between 0 and 100 micrometers under the zinc or zinc-alloy coating; then calculating a factor CSI1 of the first steel CSI1=C1av(100)+(Si1av(100)/32)+(Mn1av(100)/14)−(Al1av(100)/48)+(Cr1av(100)/11) then; performing resistance spot welding on at least 10 welds with a certain intensity. A second steel sheet can be provided depending on measurements.