Friction Stir Spot Welding of Steel Plates With Martensite Tempering
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Solution Overview
Problem
Friction stir spot welding of steel plates faces challenges in achieving high joint strength and productivity due to the formation of martensite, which is difficult to suppress without extending cooling times, especially in high hardenability steels like medium-high carbon or alloy steel.
Innovation Solution
A method involving friction stirring, followed by rapid cooling to induce martensitic transformation, and subsequent tempering using the same tool to reduce hardness and increase toughness, while maintaining high joint strength and reducing cooling time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the steel plates have high hardenability and rapid cooling is applied to improve productivity, then cooling time is reduced and productivity increases, but martensite formation occurs and joint strength decreases
Solution Approach 1:
The cooling process is segmented into two distinct stages: first, rapid cooling to induce martensitic transformation and achieve high strength, then controlled tempering to reduce brittleness. This segmentation allows the process to achieve both high productivity (through rapid initial cooling) and high joint strength (through controlled tempering that prevents excessive brittleness)
Solution Approach 2:
The invention changes the temperature parameters during the tempering stage by controlling the cooling rate after martensite formation. By adjusting the tempering temperature and holding time, the microstructure is optimized to achieve a balance between strength and toughness, allowing rapid cooling to be used without sacrificing joint strength
2Strength
If slow cooling is applied to suppress martensite formation and improve joint strength, then joint strength is maintained, but cooling time increases and productivity decreases
Solution Approach 1:
The cooling process is segmented into two distinct stages: first, rapid cooling to induce martensitic transformation and achieve high strength, then controlled tempering to reduce brittleness. This segmentation allows the process to achieve both high productivity (through rapid initial cooling) and high joint strength (through controlled tempering that prevents excessive brittleness)
Solution Approach 2:
The invention converts the harmful effect of martensite formation (brittleness) into a beneficial outcome by first intentionally forming martensite through rapid cooling to achieve high strength, then using controlled tempering to eliminate the brittleness. This approach transforms what was previously a disadvantage into an advantage, allowing both high productivity and high 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 approach stabilizes martensite formation, enhances joint strength, and improves productivity by reducing cooling time and preventing tool interference, thus achieving high toughness and reduced hardness in the spot welding portion.
Implementation Method 1
friction stirring a spot welding portion of a plurality of steel plates by pressing a tool against the spot welding portion while rotating the tool, to plasticize the spot welding portion by friction heat
Implementation Method 2
the temperature of a spot welding portion of the steel plates is raised to a temperature at which austenitic transformation occurs by friction heat
Implementation Method 3
When a tool is pulled out (drawn out) of the steel plates, the temperature of the spot welding portion is decreased. If martensitic transformation occurs while the temperature is decreased, the spot welding portion is cured (hardened)
Implementation Method 4
tempering the spot welding portion by the friction heat by re-pressing the tool against the spot welding portion while rotating the tool
Data Source
AI summary
A method of performing friction stir spot welding of a plurality of steel plates, includes the steps of: friction stirring a spot welding portion of the plurality of steel plates by pressing a tool against the spot welding portion while rotating the tool, to plasticize the spot welding portion by friction heat; cooling the spot welding portion to cause martensitic transformation to occur in the spot welding portion, after the step of friction stirring the spot welding portion; and tempering the spot welding portion by the friction heat by re-pressing the tool against the spot welding portion while rotating the tool, after the step of cooling the spot welding portion.