Resistance Spot Welding Heat Cycle for Stronger Nugget Edges
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Solution Overview
Problem
Resistance spot welding methods fail to consistently improve cross tension strength in high-strength steel sheets with tensile strengths of 780 MPa or more, particularly in medium Mn steel sheets, due to embrittlement and segregation at the nugget edge, leading to brittle fractures.
Innovation Solution
A resistance spot welding method involving a main current application followed by a cooling process to induce martensitic transformation, a post-tempering heat treatment to achieve tempered martensite, and a post-segregation reduction heat treatment to reduce solidification segregation, optimizing current and time parameters to enhance nugget edge toughness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If postheating is performed for a short time with current application after main current application, then productivity is improved, but cross tension strength cannot be sufficiently enhanced due to incomplete martensitic transformation
Solution Approach 1:
The welding process is divided into three distinct current application steps: main current application, intermediate current application, and postheating current application. This segmentation allows each step to perform a specific function - the main current forms the nugget, the intermediate current promotes martensitic transformation, and the postheating current completes the transformation and tempering, thereby achieving both high productivity and sufficient cross tension strength
Solution Approach 2:
The welding process employs periodic current application with specific time intervals between steps. The intermediate current is applied for a predetermined time period after the main current, and the postheating current is applied after another predetermined time period. This periodic action ensures complete martensitic transformation while maintaining efficient welding cycle time
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 method effectively improves cross tension strength by preventing embrittlement and segregation, resulting in enhanced joint strength and reduced brittle fractures in high-strength steel sheets, particularly medium Mn steel sheets.
Implementation Method 1
a cooling process to induce martensitic transformation
Implementation Method 2
a post-tempering heat treatment to achieve tempered martensite
Data Source
AI summary
The present invention provides a resistance spot welding method. A resistance spot welding method according to the present invention in which a sheet combination of two or more overlapping steel sheets is sandwiched between a pair of electrodes and joined together by applying current while applying pressure, the method including a main current application step in which current application is performed with a current Iw (kA), and subsequently, a post-tempering heat treatment step in which after cooling is performed for a cooling time tct (ms) shown in formula (1) below, current application is performed with a current It (kA) shown in formula (2) below for a current application time tt (ms) shown in formula (3) below: 800≤tct . . . formula (1), 0.5×Iw≤It≤Iw . . . formula (2), and 500≤tt . . . formula (3).