Resistance Spot Weld Tempering for High-Strength Steel Joints
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Solution Overview
Problem
Resistance spot welding of high-strength steel sheets with tensile strength of 780 MPa or more, particularly medium Mn steel sheets, results in reduced cross-tension strength (CTS) due to brittle fracture at the edge portion of the nugget, which is attributed to hard microstructures formed by rapid cooling, leading to interface failure.
Innovation Solution
A resistance spot welding method involving primary and secondary energization steps, where the edge portion of the nugget is tempered at appropriate temperatures to form a tempered martensite microstructure, and the hardness of the edge region and heat-affected zone (HAZ) is controlled to improve toughness, using specific formulas to define the metal microstructure and hardness ranges.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the edge portion of the nugget is hardened by quenching, then the strength of the nugget is improved, but brittle fracture occurs reducing the cross-tension strength
Solution Approach 1:
The patent changes the thermal parameter by controlling the cooling rate and tempering temperature to transform the microstructure from brittle martensite to tempered martensite, achieving the optimal balance between strength and toughness through precise temperature and time parameter control during the welding process
Solution Approach 2:
The patent creates a composite microstructure within the nugget, combining tempered martensite in the edge region with other microstructural phases in the center region, achieving a combination of toughness at the edge and overall strength that neither microstructure could provide alone
2Reliability
If secondary energization is performed to temper the nugget edge, then the toughness is improved, but the temperature control must be precise to avoid excessive heating or insufficient tempering
Solution Approach 1:
The patent applies parameter changes by establishing specific ranges for the tempering current (0.5-2.0 times the primary welding current) and tempering time (5-500 ms), along with the cooling time (10-10000 ms), to achieve the desired tempering effect without requiring extremely precise temperature control, making the process robust and manufacturable
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 enhances the toughness of the nugget edge portion and HAZ, thereby improving the joint strength and preventing brittle fracture, ensuring sufficient cross-tension strength.
Implementation Method 1
resistance spot welding method involving primary and secondary energization steps, where the edge portion of the nugget is tempered at appropriate temperatures
Implementation Method 2
the mode of failure changes from plug failure in which ductile fracture occurs in the HAZ (heat-affected zone) or the base material around the resistance spot weld to interface failure or partial plug failure in which brittle fracture occurs in the nugget
Data Source
AI summary
A resistance spot weld, a resistance spot welding method, a resistance spot welded joint, and a method for manufacturing the resistance spot welded joint. In the resistance spot weld, at least one of steel sheets is a high-strength steel sheet having a prescribed chemical composition. In at least one of a nugget edge region corresponding to a faying interface, the nugget edge region has a metal microstructure including tempered martensite as a main phase. The hardness Hv of the nugget edge region and the hardness Hmw of martensite in the nugget as a whole satisfy formula (4). In at least one of a strong HAZ region corresponding to the faying interface, the hardness Hh of the strong HAZ region and the hardness Hmh of martensite in a prescribed steel sheet satisfy formula (8).Hv≤Hmw-40(4)Hh<Hmh-25(8)