Laser-Welded Lap Joint Geometry to Prevent End-Zone Cracking

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Solution Overview

Problem

Laser lap welding of automobile structural members often results in cracking at the terminal end of the fusion zone, leading to reduced static and fatigue strengths due to the propagation of cracks, which is a significant issue with the increasing use of high-tensile steel sheets in automobile body components.

Innovation Solution

The solution involves controlling specific parameters such as the full length, final weld zone length, crater dimensions, and steel sheet thickness to form a weld zone that satisfies specific geometric and dimensional formulas, thereby preventing crack occurrence and propagation, and optimizing laser welding conditions to reduce stress concentration.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If laser lap welding is used to join steel sheets, then welding speed and productivity are improved, but cracking occurs at the terminal end of the fusion zone reducing joint strength

Engineering Contradiction:

Improvewelding speedVSAvoidjoint strength

Core Design Contradiction:

ProductivityVSStrength

Solution Approach 1:

The patent applies parameter changes by modifying welding conditions (laser power, welding speed, focal position) to control the formation of a final weld zone with specific crater characteristics. By adjusting these parameters, the patent prevents cracking at the terminal end while maintaining high welding speed and productivity.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent applies local quality by creating a distinct final weld zone with different characteristics from the main weld zone. This final zone has specific crater depth and width ratios that differ from the main fusion zone, providing localized crack prevention at the terminal end where cracking is most likely to occur.

Inventive Principle:

Principle #3Local quality

2Strength

If high-tensile steel sheets are used to improve automobile body strength, then static strength is improved, but crack propagation in weld zones becomes more significant

Engineering Contradiction:

Improvestatic strengthVSAvoidfatigue strength

Core Design Contradiction:

StrengthVSReliability

Solution Approach 1:

The patent converts the harmful effect of high tensile strength (which increases susceptibility to crack propagation) into a benefit by creating a final weld zone with controlled crater characteristics. The specific crater depth and width ratios in the final weld zone reduce stress concentration, preventing crack initiation and propagation even in high-tensile steel sheets, thereby maintaining both static strength and fatigue reliability.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Adaptability or versatility

If conventional welding methods are used, then design flexibility is maintained, but welding time increases and spatial limitations occur

Engineering Contradiction:

Improvedesign flexibilityVSAvoidwelding time

Core Design Contradiction:

Adaptability or versatilityVSLoss of time

Solution Approach 1:

The patent replaces conventional resistance spot welding mechanisms with laser beam welding. This substitution eliminates the need for physical contact between welding tools and workpieces, removing spatial limitations imposed by welding guns and electrodes. The laser beam can access difficult-to-reach areas, improving design flexibility while significantly reducing welding time.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

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

This approach effectively prevents cracking in the weld zone, enhancing the peeling strength and durability of the laser-welded lap joints, making them suitable for structural components in automobiles, while allowing for design flexibility and high strength.

Implementation Method 1

a surface of a plurality of lapped steel sheets is irradiated with a laser beam to join the steel sheets together

Methodology Applied

Scientific EffectLaser beam welding: Laser Beam Welding

Implementation Method 2

The irradiated portion of the steel sheets irradiated with the laser beam is fused and solidified

Methodology Applied

Scientific EffectMelting: Melting

Data Source

PatentUS11648626B2Laser-welded lap joint, method for producing laser-welded lap joint, and automobile frame component

Publication Date: 2023.05.16 JFE STEEL CORP

AI summary

The present invention includes a laser-welded lap joint including a weld zone formed by laser lap welding in a lapped portion including a plurality of lapped steel sheets. The weld zone includes a main weld zone that penetrates the steel sheets in the lapped portion and a final weld zone formed at one end of the main weld zone and having a crater, and the weld zone satisfies formulas (1) to (4):L≥15.0; (1)10.0≥L2≥2lc; (2)t1≥2dc; (3)wc>dc (4).