Steel Friction Stir Spot Welding With Post-Cooling 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
1Strength
If the cooling time is extended to prevent martensite formation, then joint strength is improved, but productivity deteriorates
Solution Approach 1:
The patent applies parameter changes by modifying the cooling rate and temperature control parameters during the welding process. By controlling the cooling rate to be within a specific range (10-100°C/s) and managing the temperature profile through tool rotation speed adjustment, the patent achieves suppression of martensite formation without requiring excessively long cooling times, thus resolving the contradiction between joint strength and productivity
Solution Approach 2:
The patent employs dynamics by making the tool rotation speed variable during the process. The tool rotation speed is dynamically adjusted: initially set at a first rotation speed during friction stirring, then reduced to a second rotation speed (lower than the first) during the cooling phase. This dynamic adjustment allows control over the cooling rate and temperature profile, enabling suppression of martensite formation while maintaining reasonable process time
2Productivity
If the cooling rate is increased to reduce cooling time, then productivity is improved, but martensite formation increases reducing joint strength
Solution Approach 1:
The patent applies parameter changes by precisely controlling the cooling rate within a specific range (10-100°C/s). This controlled parameter adjustment ensures that the cooling is fast enough to improve productivity but not so fast as to cause excessive martensite formation, thus resolving the contradiction between cooling time and joint strength
Solution Approach 2:
The patent employs feedback control by monitoring the temperature of the spot welding portion and adjusting the tool rotation speed accordingly. The temperature is measured during the process, and based on this feedback, the rotation speed is adjusted to maintain the cooling rate within the optimal range, ensuring both productivity and joint strength requirements are met
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 effectively addressing the limitations of existing methods.
Implementation Method 1
Frictional heat is generated by friction between the rotary tool and the surface of the first steel plate
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) and becomes brittle
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
Figure 1
Figure 2
Figure 3A~3B
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.