Vehicle Structural Component Soft Zones for Spot Weld Integrity
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Solution Overview
Problem
The use of high strength or ultra-high strength steel in vehicle structural components leads to issues with spot welds, where the hardness of the heat affected zone drastically decreases, causing stress concentration and potential breakage at this zone, rather than in the base material, due to significant differences in hardness between the weld and the rest of the part.
Innovation Solution
A method involving a silicon and aluminium-based coated ultra-high strength steel plate subjected to a diode laser heat treatment in localized zones to reduce martensite content, followed by controlled cooling, which decreases yield strength and increases elongation, making these zones energy absorption areas and improving spot weldability by minimizing hardness differences with the weld.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength or ultra-high strength steel is used in structural components, then the overall strength and rigidity of the vehicle structure is improved, but the hardness of the heat affected zone in spot welds drastically decreases causing stress concentration and potential breakage
Solution Approach 1:
The patent applies local quality by creating zones with different mechanical properties in specific locations. Laser treatment is applied locally to the steel plate to create a softened zone with reduced hardness and increased ductility, while the rest of the component maintains its high strength properties. This localized modification allows the heat affected zone to better accommodate welding stresses without catastrophic failure.
Solution Approach 2:
The patent employs preliminary action by performing laser treatment on the steel plate before spot welding. This pre-treatment creates a softened zone in advance, preparing the material to better withstand the thermal and mechanical stresses that will occur during subsequent welding operations, thereby preventing weld-related failures.
2Strength
If the cooling rate is reduced in selected zones to achieve non-martensitic microstructure, then localized zones with different hardness and ductility are obtained, but the cooling process becomes more complex and time-consuming
Solution Approach 1:
The patent replaces the conventional mechanical cooling control system with a laser-based thermal field approach. Instead of using complex mechanical means to control cooling rates in different zones, a laser beam is used to selectively heat and hold zones at temperatures that prevent martensitic transformation, allowing simpler and faster cooling afterward.
Solution Approach 2:
The patent changes the thermal parameters during processing by using laser heating to maintain elevated temperatures in selected zones during the cooling phase. This parameter change (maintaining temperature above martensitic transformation point) prevents the formation of hard martensitic microstructure in specific areas while allowing rapid cooling elsewhere.
3Strength
If laser treatment is applied to reduce martensite content in localized zones, then yield strength decreases and elongation increases in treated zones, but additional equipment and process steps are required
Solution Approach 1:
The patent applies universality by using the laser processing system to perform multiple functions: it acts as both a heating source for microstructure modification and a means for localized thermal treatment. The same laser equipment can be used for different zones and different treatment intensities, reducing the need for separate specialized equipment for each treatment type.
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 effectively reduces the likelihood of breakage in the heat affected zone by distributing stress to the base material, as evidenced by increased elongation and reduced tensile strength in treated zones, enhancing the structural component's performance in collisions and weld integrity.
Implementation Method 1
irradiating it with a diode laser beam of a power comprised between 500 W and 6 kW until reaching a temperature comprised between 400 °C and 900 °C
Implementation Method 2
leaving the first part to cool in order to change its microstructure, providing said localized zone with an intentionally lower martensite content
Data Source
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AI summary
The invention relates to a method for the manufacture of a part of a structural component of a vehicle, intended to be connected by spot welding to a second part, starting from a coated steel plate and of a thickness of 1-3 mm, which comprises subjecting the plate to hot stamping, subsequently subjecting at least one localized and previously selected zone of the first part to a heat treatment, irradiating it with a diode laser beam of a power comprised between 500 W and 6 kW until reaching a temperature comprised between 400-900 °C, then leaving it to cool to change its microstructure, providing said zone with an intentionally lower martensite content and accordingly with a lower strength and greater elongation in comparison with those of its adjacent zones that have not been heat treated.