Laser Remelting Weld Toe Fatigue Strength
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Solution Overview
Problem
The fatigue strength of high strength steel sheets is compromised due to the deterioration of the heat affected zone (HAZ) during fillet arc welding, leading to reduced fatigue life and crack progression in welded joints, particularly in automotive applications where high strength steel with tensile strength above 780 MPa is used.
Innovation Solution
A method involving local quenching by a laser beam to remelt and solidify the weld toe portion and the surrounding HAZ, narrowing the HAZ and enhancing the hardness of the affected area, thereby suppressing crack progression and improving fatigue strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If high strength steel sheets with tensile strength of 780 MPa or more are used to reduce car body weight, then weight reduction is achieved, but fatigue strength of welded parts deteriorates due to stress concentration and residual stress at weld toes
Solution Approach 1:
The invention changes the physical-chemical parameters of the weld toe region by applying laser heat treatment to heat the region to a temperature below the melting point of the weld metal (typically 500-800°C), causing phase transformation and hardening of the weld metal side. This parameter change reduces strain concentration at the weld toe, thereby improving fatigue strength while maintaining the use of high strength steel sheets for weight reduction
Solution Approach 2:
The invention applies laser heat treatment to the weld toe region before the weld metal fully solidifies or immediately after welding. This preliminary action hardens the weld metal side and reduces strain concentration before fatigue loading occurs, preventing crack initiation at the weld toe
2Shape
If TIG arc or plasma heat source is used to remelt the weld bead to reduce stress concentration, then bead shape is improved, but excessive heat input deteriorates HAZ quality and lowers fatigue strength in thin sheet members
Solution Approach 1:
The invention replaces the TIG arc or plasma heat source with a laser heat source. The laser provides more concentrated and controllable heat input, allowing selective heating of the weld toe region without excessive heat input to the HAZ. This substitution enables precise thermal processing that improves weld toe geometry and hardens the weld metal side while minimizing HAZ deterioration
Solution Approach 2:
The invention applies heat treatment locally to the weld toe region rather than remelting the entire weld bead. The laser heats only the necessary region (weld toe and adjacent HAZ) to a temperature below the melting point, creating local hardening and strain concentration reduction without the excessive heat input that would deteriorate overall HAZ quality
3Strength
If laser heat source is used to heat weld toe portion to temperature below melting point to harden weld metal, then strain concentration is reduced, but hardening becomes insufficient when heating temperature is too low
Solution Approach 1:
The invention optimizes the heating temperature parameter to a specific range (500-800°C, below the melting point but above the transformation temperature) that achieves sufficient phase transformation and hardening of the weld metal side. This parameter optimization ensures adequate hardening effect to reduce strain concentration while preventing excessive heat input that would deteriorate HAZ quality
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 technique significantly improves the fatigue strength of high strength steel welds by narrowing the HAZ and delaying crack formation, resulting in a substantial increase in fatigue life, with the optimal depth of remelting being between 1/5 to 1/3 of the sheet thickness for maximum effectiveness.
Implementation Method 1
using a laser heat source to heat a weld toe portion until a temperature of an extent where the weld metal does not melt so as to harden the weld metal side
Implementation Method 2
heat a weld toe portion to a temperature of an extent where the steel sheets do not melt so as to reduce the residual stress
Implementation Method 3
remelting by laser a region including a weld toe portion of the fillet arc welding of at least one metal member and a boundary of a heat affected zone caused by the fillet arc welding
Data Source
AI summary
A fillet arc welded joint formed by fillet arc welding at least two metal members, comprising a remelted and solidified portion obtained by irradiating a laser at a weld toe portion of the fillet arc welding of at least one metal member and a region including a boundary of the heat affected zone caused by the fillet arc welding at the surface of that metal member, the remelted and solidified portion being a range from a surface of the metal member to a depth of ½ or less of the thickness of that metal member, an average effective crystal grain diameter of prior austenite at a heat affected zone from a boundary of the remelted and solidified portion at the surface of the metal member to a depth of 0.1 mm in the thickness direction of the metal member being 20 μm or less.