Laser Welding High-Tensile Steel Bead Sequence
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Solution Overview
Problem
Conventional laser welding methods often result in insufficient strength of the weld zone, particularly when welding high-tensile steel sheets with tensile strength of 780 MPa or more and carbon content of 0.07 mass% or more, leading to inadequate joint strength and potential welding deformation.
Innovation Solution
A laser welding method where the second beads are formed after the first bead has cooled to a temperature equal to or lower than the Ms point - 50°C, with the first beads having a circular shape and the second beads being concentric with the first beads, forming an angle of 10° or more, and the second beads being formed within a temperature range of 400°C to Ac1 point + 50°C to enhance joint strength and reduce deformation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional laser welding methods are used on high-tensile steel sheets, then welding speed and thermal deformation are controlled, but the weld zone strength is insufficient
Solution Approach 1:
The welding process is divided into multiple passes creating sequential beads instead of a single continuous bead. The first bead is formed, cooled to specific temperature, then the second bead is formed to temper the first bead, creating a multi-layered weld structure that enhances strength through controlled heat treatment
Solution Approach 2:
The invention controls the temperature parameter by waiting for the first bead to cool to Ms point - 50°C before forming the second bead, and maintains the first bead temperature between 400°C and Ac1 point + 50°C during second bead formation. This precise temperature control enables tempering of the first bead to achieve optimal hardness and strength
2Strength
If high-tensile steel sheets with high carbon content are welded, then material strength is maintained, but weld zone strength becomes insufficient and deformation increases
Solution Approach 1:
The first bead is formed and allowed to cool to a specific temperature (Ms point - 50°C) before the second bead is applied. This preliminary cooling action prepares the first bead for tempering, ensuring it reaches the optimal temperature state for the tempering process that follows
Solution Approach 2:
The heat from the second bead, which could potentially cause excessive thermal deformation, is instead utilized as a beneficial tempering heat treatment for the first bead. By controlling the timing and temperature, the harmful thermal effect is converted into a beneficial hardening and strengthening process
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 significantly enhances the strength of the weld zone while minimizing welding deformation, achieving improved joint strength and shear strength by tempering the first bead and optimizing the heat transfer process.
Implementation Method 1
The laser welding uses a laser beam as a heat source
Implementation Method 2
by appropriately forming the beads and making average Vickers hardness of the first bead lower than average Vickers hardness of the second and subsequent beads
Implementation Method 3
the second beads are each formed for the first bead whose maximum temperature has become equal to or lower than an Ms point - 50°C
Data Source
Figure 1~2A
Figure 2B~3B
Figure 3C~3E
AI summary
At a plurality of welding positions in an overlap portion of a plurality of members including a high-tensile steel sheet whose carbon content is 0.07 weight% or more, first beads (31 to 36) in a closed loop shape or a closed loop-like shape and second beads (41 to 46) in a closed loop shape or a closed loop-like shape on inner sides of the first bead (31 to 36) are formed by remote laser welding for joining. At this time, there are a procedure for successively forming the plural first beads (31 to 36) and a procedure for successively forming the plural second beads (41 to 46) for the plural formed first beads (31 to 36), and in both of the cases, the beads are each formed at a position except the closest welding position among the plural welding positions. Consequently, it is possible to enhance strength of a weld zone and to suppress welding deformation.