Friction Stir Welding of High-Strength Steel for Uniform Toughness
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Solution Overview
Problem
Friction stir welding of high-strength structural steel with tensile strength of 800 MPa or more faces challenges in achieving sufficient strength and uniform toughness due to non-uniform heating and plastic processing states within the stirred portion.
Innovation Solution
A friction stir welding method is developed, where the welding heat input is controlled by managing the tool rotational speed, torque, and welding speed to achieve a specific heat hysteresis, resulting in a microstructure mainly composed of fine bainite, which enhances joint strength and toughness. The method involves precise control of chemical composition and welding conditions to ensure uniformity and high toughness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If friction stir welding is applied to high-strength structural steel with tensile strength of 800 MPa or more, then joint strength can be achieved, but non-uniform heating and plastic processing states occur within the stirred portion resulting in non-uniform toughness
Solution Approach 1:
The patent applies parameter changes by optimizing the chemical composition parameters of the steel (C: 0.03-0.12%, Si: 0.6% or less, Mn: 1.5-3.0%, P: 0.015% or less, S: 0.002% or less, Al: 0.1% or less, Ti: 0.005-0.030%, Nb: 0.01-0.10%, N: 0.001-0.008%, O: 0.03% or less) and controlling the Pcm value (0.18 ≤ Pcm ≤ 0.30) to achieve uniform microstructure formation during friction stir welding, thereby resolving the non-uniform toughness issue while maintaining high joint strength
Solution Approach 2:
The patent applies local quality by creating a specific microstructure mainly composed of fine bainite in the stirred portion through controlled heating and cooling processes. This localized microstructure control ensures uniform toughness distribution within the weld zone while maintaining the required joint strength
2Productivity
If conventional arc welding method is used for low melting point metal materials, then welding can be performed, but satisfactory characteristics in the weld portion cannot be obtained
Solution Approach 1:
The patent replaces the thermal field-based arc welding method with a mechanical field-based friction stir welding method. By using a rotating tool to generate frictional heat and create plastic flow, the process achieves both high productivity and reliable weld quality for low melting point metal materials, eliminating the deficiencies of conventional arc welding
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 achieves sufficient strength and uniform toughness in the stirred portion, resolving the non-uniformity issues associated with high-strength structural steel, ensuring high-quality welds with improved mechanical properties.
Implementation Method 1
utilizing the softening of the working materials caused by the frictional heat generated between the rotational tool and the working materials
Implementation Method 2
the plastic flow created by stirring the softened portions with the rotational tool
Data Source
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AI summary
In a friction stir welding method, steel sheets or plates used are each made of high-strength welding structural steel having a chemical composition controlled to a predetermined range, a Pcm value calculated by formula (1) satisfying 0.18 ≤ Pcm ≤ 0.30, and the balance being Fe and incidental impurities, friction stir welding conditions fall under ranges of tool rotational speed: 100 rpm to 1000 rpm, tool rotational torque: 50 N·m to 500 N·m, and welding speed: 10 mm/min to 1000 mm/min, and HI defined by formula (2) is within a range of 1.5 to 20, and satisfies formula (3) in relation with Pcm, Pcm%=C+Si/30+Mn+Cr+Cu/20+Ni/60+Mo/15+V/10+5B HIkJ/mm=6.28×RT×RS/TS/1000 1.5×109×Pcm13.8≤HI≤2.1×108×Pcm10.6