Projection Weld Joint Tempering in Al-Plated Hot-Stamped Steel

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Solution Overview

Problem

Projection weld joints in high-strength steel sheets with Al-based plating face significant challenges due to high tensile strength, hydrogen concentration, and residual stress, leading to delayed fracture.

Innovation Solution

A method for manufacturing projection weld joints involves pressing a projection portion of a steel member onto a hot-stamping steel sheet with Al-based plating, applying first energization to form a projection weld, quenching it, and then applying second energization to temper the edges while maintaining the central part quenched, thereby controlling hardness and reducing stress.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Strength

If projection welding is performed on high-strength steel sheets with Al-based plating, then joint strength is improved, but delayed fracture resistance deteriorates due to high hardness and residual stress

Engineering Contradiction:

Improvejoint strengthVSAvoiddelayed fracture resistance

Core Design Contradiction:

StrengthVSReliability

Solution Approach 1:

The welding process is divided into two distinct stages: first energization for forming the projection weld and second energization for tempering. This segmentation allows the joint to achieve both high strength and improved delayed fracture resistance by addressing different requirements at different stages of the process.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The first energization step performs the primary welding function of forming the projection weld before the second tempering step is applied. This preliminary action ensures the joint achieves sufficient strength before the tempering process begins to reduce hardness and residual stress.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 3:

The invention changes the energization parameters between the two stages: the first energization uses parameters optimized for weld formation (higher current, shorter time), while the second energization uses parameters optimized for tempering (lower current, longer time). This parameter change allows the same welding process to achieve both strength and delayed fracture resistance.

Inventive Principle:

Principle #35Parameter changes

2Strength

If high-strength steel sheets with tensile strength of 1.5 GPa or more are used, then vehicle weight reduction and collision safety are improved, but delayed fracture becomes more likely due to high hardness, hydrogen concentration, and stress

Engineering Contradiction:

Improvetensile strengthVSAvoiddelayed fracture resistance

Core Design Contradiction:

StrengthVSReliability

Solution Approach 1:

The invention converts the harmful effect of high hardness and residual stress (which cause delayed fracture) into a beneficial outcome by applying the second tempering energization. This step specifically targets and reduces the hardness and residual stress in the heat-affected zone, transforming the initial harmful conditions into improved delayed fracture resistance while maintaining the high tensile strength of the base material.

Inventive Principle:

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

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 method effectively improves the delayed fracture resistance property of projection weld joints in high-strength Al-based hot-stamping steel sheets by reducing hardness and stress, resulting in enhanced joint reliability.

Implementation Method 1

these projections are energized to locally heat a contact interface between each projection and the steel sheet, thereby causing melting and solidification or solid-phase joining

Methodology Applied

Scientific EffectJoule heating: Joule Heating

Implementation Method 2

the steel sheet is subjected to press forming while keeping it at a high temperature, and further, the steel sheet is quenched to cause quenching and hardening

Methodology Applied

Scientific EffectQuenching: Heat Treatment

Implementation Method 3

applying second energization to the quenched projection weld to temper edges of the projection weld while keeping a central part of the projection weld quenched

Methodology Applied

Scientific EffectResistive heating: Joule Heating

Data Source

PatentUS20250033137A1Method for manufacturing projection weld joint, projection weld joint, and vehicle component

Publication Date: 2025.01.30 NIPPON STEEL CORPORATION

AI summary

A method for manufacturing a projection weld joint according to the present disclosure includes: pressing a projection portion of a steel member having the projection portion on a plane onto a surface of a hot-stamping steel sheet, wherein the hot-stamping steel sheet has an Al-based plating layer on the surface and has a tensile strength of 1.5 GPa or more; applying first energization to the hot-stamping steel sheet, the projection portion, and the steel member to make the projection portion serve as a projection weld; quenching the projection weld; and applying second energization to the quenched projection weld to temper edges of the projection weld while keeping a central part of the projection weld quenched.