Solid-Phase Spot Welding With Temperature-Controlled Interface Deformation
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Solution Overview
Problem
Conventional resistance spot-welding methods result in weakened mechanical properties and formation of melt-solidified structures in high-strength steel welds, limiting the utilization of high-tensile steel sheets and causing cracks due to melt solidification, especially in high-carbon steels.
Innovation Solution
A solid-phase spot-welding method that raises the temperature of the interface between overlapping metal plates using energization and applies external stress greater than the yield strength to deform the softened region, preventing melting and forming a new surface for welding, thus controlling the welding temperature accurately.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If resistance spot-welding is used to join high-strength steel sheets, then welding speed and productivity are improved, but the mechanical properties (strength and toughness) of the welded portion deteriorate due to melt-solidified structure formation
Solution Approach 1:
The invention transitions from melt welding to solid-phase welding by controlling the heating process. The welded portion is heated to a temperature below the melting point (A3 transformation point) and held in the austenite phase, then transformed to martensite through rapid cooling. This phase transition approach eliminates the harmful melt-solidified structure while maintaining high welding speed and achieving strength equal to or exceeding the base material.
Solution Approach 2:
The invention changes the critical parameter of welding temperature from above-melting-point (conventional resistance spot-welding) to below-melting-point (solid-phase welding). By precisely controlling the temperature to remain below the A3 transformation point during heating and then utilizing rapid cooling for martensitic transformation, the method achieves high-strength joints without the deterioration associated with conventional melting-based welding.
2Strength
If conventional resistance spot-welding is used on high-carbon steel, then welding of high-strength material is achieved, but cracks occur in the welded portion due to melt solidification
Solution Approach 1:
The invention avoids melting and subsequent solidification by heating only to the austenite phase (below melting point) and then applying rapid cooling to induce martensitic transformation. This phase transition approach eliminates crack formation entirely, as there is no melt-solidification process that creates vulnerable microstructures in high-carbon steels.
3Reliability
If welding temperature is raised to ensure proper welding, then welding reliability is improved, but heat-affected zones are formed that reduce the mechanical properties of the base material
Solution Approach 1:
The invention precisely controls the welding temperature parameter to remain below the A3 transformation point of the base material during the heating phase. This prevents excessive heating that would create large heat-affected zones, while still achieving adequate welding through the subsequent rapid cooling and martensitic transformation 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 suppresses the formation of molten solidified structures and heat-affected zones, enhancing the strength and toughness of welds, enabling efficient use of high-tensile steel sheets and dissimilar metals without forming fragile intermetallic compounds, and maintaining the mechanical properties of the base material.
Implementation Method 1
a temperature raising step in which a temperature of the interface to be welded is raised by energization by a pair of electrodes to form a softened region
Implementation Method 2
a stress application step in which an external stress greater than or equal to the yield strength of the metal plate materials at a desired welding temperature is applied to the softened region, wherein the metal plate materials are welded to each other by subjecting the softened region to local deformation
Data Source
AI summary
The present invention provides: a solid-phase spot-welding method with which the welding temperature can be controlled accurately and with which a reduction in the welding temperature can be achieved, regardless of the type of metal material being welded; and a solid-phase spot-welding device that can be used suitably in this solid-phase spot-welding method. This solid-phase welding method involves overlapping metal plate materials and carrying out spot-welding, and is characterized by having a welding preparation step in which two or more metal plate materials are held in a state in which same overlap one another, thereby forming an interface to be welded, a temperature-raising step in which a pair of electrodes are used and the interface to be welded is heated by supplying a current by a direct method, an indirect method, or a series method, thereby forming a softened region in the vicinity of the interface to be welded, and a stress application step in which an external stress greater than or equal to the yield strength of the metal plate materials at a desired welding temperature is applied to the softened region, wherein the metal plate materials are welded to each other by subjecting the softened region to local deformation.