Press-Hardened Laser-Welded Steel Blanks With Homogeneous Weld Joints
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Solution Overview
Problem
Existing methods for producing press-hardened laser welded steel parts with precoated steel sheets result in incomplete austenitization due to high aluminum content, leading to non-homogeneous microstructures and increased production costs, and fail to ensure a fully martensitic or bainitic structure in the weld joint.
Innovation Solution
A method involving precoated steel sheets with specific thickness and tensile strength ratios, controlled aluminum content in the weld joint, and a tailored heat treatment process to achieve a martensitic or bainitic microstructure, using laser welding and a filler material with minimal aluminum content.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If precoated steel sheets with aluminum-based precoating are welded without prior preparation, then corrosion protection is maintained, but the aluminum content in the weld joint increases causing incomplete austenitization and non-martensitic microstructure
Solution Approach 1:
The precoating is removed in advance from specific zones (weld joints and edges) before welding occurs. This preliminary removal action prevents aluminum contamination in the weld pool, enabling complete austenitization and martensitic transformation without compromising corrosion protection in non-welded areas.
Solution Approach 2:
The precoating is selectively removed only from specific locations (weld joints and edges) while preserving it in other areas. This creates local quality differentiation where the weld joint has no precoating (allowing proper microstructure) while other areas maintain precoating (providing corrosion protection).
2Manufacturing precision
If the superficial layer of metal alloy is removed from weld edges to decrease aluminum content, then complete austenitization is achieved, but production time increases
Solution Approach 1:
The precoating removal is performed as a preliminary step before welding, allowing the welding process itself to proceed without delays for aluminum content adjustment or additional heat treatment. The laser welding process then completes austenitization during normal welding parameters.
Solution Approach 2:
Traditional mechanical methods of precoating removal (grinding, machining) are replaced with laser-based removal. The laser provides precise, rapid removal of precoating from weld zones without the time-consuming mechanical operations, thereby maintaining productivity while achieving complete austenitization.
3Manufacturing precision
If filler wire with austenite-stabilizing elements is used to compensate for aluminum, then martensitic structure is obtained, but production cost increases and weld joint homogeneity deteriorates
Solution Approach 1:
The harmful element (aluminum from precoating) is extracted/removed from the weld joint zone before welding. This eliminates the need to add compensating elements through filler wire, reducing production cost and avoiding homogeneity issues that would result from mixing different materials.
Solution Approach 2:
Instead of using expensive filler wire containing austenite-stabilizing elements, the solution uses a simple, cost-effective approach: removing the precoating entirely from weld zones. This disposable removal of precoating achieves the desired martensitic structure without the cost and complexity of specialized filler materials.
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 produces steel parts with enhanced crash performance and cost-effectiveness by ensuring a homogeneous microstructure and controlled aluminum content, achieving a martensitic or bainitic weld joint with improved mechanical properties.
Implementation Method 1
butt welding the first precoated steel sheet and the second precoated steel sheet using laser welding
Implementation Method 2
heating the welded blank to a heat treatment temperature, the heat treatment temperature being at least 10° C. lower than the full austenitization temperature of the weld joint and at least 15° C. higher than a minimum temperature Tmin
Implementation Method 3
cooling the thus formed steel part with a cooling speed greater than or equal to the critical martensitic or bainitic cooling speed
Data Source
AI summary
A method for producing a part includes providing a first and a second precoated sheet (1,2), butt welding the first and second precoated sheets (1) to obtain a blank (15), and heating the blank (15) to a heat treatment temperature at least 10° C. lower than the full austenitization temperature of the weld joint (22) and at least 15° C. higher than a minimum temperature Tmin:Tmin(° C.)=AC3(WJ)-αICmax100 (Ac 3(WJ)-673-40×Al).whereAc3(WJ) is the full austenitization temperature of the weld joint (22)αICmax=(1-(1+ρ)(max(1;ρ)Ts2-350)(1-β)(ρTs2+Ts1)+β(1+ρ)(3130CFW+750)-350×(1+ρ))×100,whereTs1 and Ts2 are the ultimate tensile strengths of the strongest and the weakest substrate after press-hardeningCFW is the carbon content of the filler materialβ is the proportion of filler materialρ is the ratio between the thicknesses of the weakest and the strongest substrateThe method also includes holding the blank (15) at the heat treatment temperature for a time between 2 and 10 minutes; and press-forming the blank (15) into a part and cooling.