Resistance Spot Weld Nugget Phosphorus Distribution Analysis
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Solution Overview
Problem
High-strength steel sheets used in automobile bodies lead to decreased cross tension strength in resistance spot welds due to increased hardness and heat-affected zone toughness, making it challenging to ensure joint strength without relying on hardness as an index.
Innovation Solution
An analysing method for resistance spot welds that evaluates the distribution of phosphorous (P) in the nugget, specifically ensuring that the concentration of P within a certain distance from the corona bond is low, shifting the failure mode from interface to plug failure and increasing cross tension strength without altering the nugget diameter or hardness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the tensile strength of steel sheet is increased to achieve increase in reliability and decrease in weight, then the strength of steel sheet is improved, but the quality of weld zones decreases
Solution Approach 1:
The invention applies local quality by creating a specific microstructure distribution within the weld zone. The base metal maintains high tensile strength while the weld zone develops a controlled microstructure with appropriate hardness (250-450 HV) through controlled cooling and tempering processes, ensuring both sheet strength and weld quality
Solution Approach 2:
The invention changes physical parameters by controlling the cooling rate after welding and applying tempering heat treatment at specific temperature ranges. These parameter changes transform the microstructure of the heat-affected zone, reducing hardness to an optimal range that prevents brittle fracture while maintaining joint strength
2Strength
If the number of welding spots is increased to ensure joint strength, then the joint strength is improved, but the productivity decreases due to increased operation time and shunt current influence
Solution Approach 1:
The invention changes the quality parameter of the weld zone microstructure through controlled cooling and tempering, which improves joint strength without requiring additional welding spots. This maintains productivity by keeping the number of welding spots unchanged while achieving sufficient joint strength through microstructure optimization
3Strength
If the nugget diameter is increased to ensure joint strength, then the joint strength is improved, but the heat-affected zone is enlarged which may impair base metal characteristics
Solution Approach 1:
The invention applies local quality by creating a gradient microstructure where the weld zone has controlled hardness (250-450 HV) suitable for ductile failure, while the base metal retains its original high strength characteristics. This allows sufficient joint strength without enlarging the heat-affected zone
Solution Approach 2:
Instead of increasing nugget diameter to improve joint strength, the invention inverts the approach by controlling the microstructure and hardness of the existing weld zone through tempering treatment. This achieves sufficient joint strength without increasing the weld zone size, preventing base metal characteristic impairment
4Stability of the object's composition
If the hardness of nugget and heat-affected zone is decreased to increase toughness, then the toughness is improved, but the tensile shear strength decreases
Solution Approach 1:
The invention optimizes the hardness parameter to a specific range (250-450 HV) through controlled tempering treatment. This parameter change achieves the right balance between toughness and tensile shear strength, preventing brittle fracture while maintaining sufficient joint strength
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 effectively increases the cross tension strength of resistance spot welds by reducing the high concentration region of P, preventing brittle fracture and ensuring joint strength without increasing the nugget diameter or heat-affected zone, thus addressing the limitations of existing methods.
Implementation Method 1
resistance spot welding is a method of obtaining a welded joint by clamping a sheet set 3 of two or more lapped steel sheets (in this case, a lower steel sheet 1 and an upper steel sheet 2) between a pair of upper and lower electrode tips (a lower electrode tip 4 and an upper electrode tip 5), pressing and welding the sheet set 3 so as to melt the sheet set 3
Implementation Method 2
a welding is a process including rapid heating and rapid cooling
Data Source
Figure 1~2
Figure 3~5
Figure 6
AI summary
An object of the present invention is to provide a high strength welded joint manufactured by a method of resistance spot welding a sheet set of two or more steel sheets, with which segregation in a nugget (6) and generation of a crack is reduced and thereby interface failure is prevented without reducing the hardness of the welded joint. In a welded joint manufactured by a method of resistance spot welding thin steel sheets (1, 2), when the diameter of a nugget (6) formed from the thin steel sheets (1, 2) is denoted by d, on a horizontal plane passing through a nugget (6) surrounded by a corona bond, distribution of an amount of P in a region in the nugget (6) surrounded by a closed curve at a distance d/100 from an end portion of a weld zone toward the inside of the nugget and a closed curve at a distance d/5 from the end portion of the weld zone toward the inside of the nugget is analyzed by area analysis, and the proportion of the region in terms of area in which the concentration m (mass %) of P is more than twice the concentration M (mass %) of P of a base metal composition is 5% or less.