Graphite-Coated Filler Wire for Harder Laser Weld Seams
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Solution Overview
Problem
Fusion welding of press-hardenable steel sheets, particularly those with aluminum coatings, faces challenges in achieving weld seam hardness comparable to the base material, with existing methods being energy-intensive, costly, and requiring additional processing steps, while also dealing with the issue of ferrite formation reducing weld strength.
Innovation Solution
A method involving a laser beam welding process where the filler wire is coated with graphite particles, which are mixed with a waxy or liquid vehicle and applied directly to the melt pool, enhancing the hardenability of the weld seam and allowing for higher welding speeds without the need for decoating the aluminum coating.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the aluminum surface coating is removed before welding using mechanical tools or laser ablation, then ferrite formation is prevented and weld strength is maintained, but additional process steps are required that increase cost and production time
Solution Approach 1:
The filler wire is pre-coated with graphite particles before welding. This preliminary action ensures that carbon is already present in the filler material, which will later compensate for aluminum contamination in the weld pool, preventing ferrite formation without requiring prior removal of the aluminum coating from the workpiece surfaces
Solution Approach 2:
Instead of treating the aluminum coating as a harmful contaminant that must be removed, the invention accepts that aluminum will enter the weld pool and forms aluminum-containing phases. The pre-coated graphite particles provide excess carbon that compensates for the aluminum contamination, converting the harmful effect of aluminum coating into a manageable condition that does not compromise weld strength
2Productivity
If laser beam welding is used to join press-hardenable steel sheets with aluminum coating, then welding speed is high and process is simple, but ferrite forms in the weld seam reducing weld strength below base material strength
Solution Approach 1:
The chemical composition parameters of the filler wire are changed by coating it with graphite particles, increasing the carbon content. This parameter change allows the filler wire to compensate for aluminum contamination in the weld pool, preventing ferrite formation while maintaining the high welding speed and simplicity of laser beam welding
3Strength
If filler wire with high carbon content is used to compensate for aluminum contamination, then ferrite formation is prevented, but excessive carbon may cause other microstructure issues
Solution Approach 1:
The filler wire is coated with graphite particles to provide excess carbon beyond what would normally be present. This excessive carbon action is controlled and localized to the weld zone, where it specifically compensates for aluminum contamination. The coating amount can be optimized to provide just enough carbon to prevent ferrite formation without causing excessive carbon-related microstructure issues
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 improves the hardenability of the weld seam, reduces production costs, and increases productivity by eliminating the need for decoating, while maintaining or exceeding the strength of the base material, with a more favorable seam geometry and lower energy consumption compared to hybrid laser-arc welding.
Implementation Method 1
the fusion welding is carried out by supplying filler wire into the melt pool produced exclusively by means of a laser beam
Implementation Method 2
the filler wire is coated with graphite particles, which are mixed with a waxy or liquid vehicle and applied directly to the melt pool, enhancing the hardenability of the weld seam
Data Source
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AI summary
The invention relates to a method and to a device for fusion-welding one or more steel sheets (1, 2; 2') made of press-hardenable steel, preferably manganese-boron steel, in which method the fusion welding is performed with supply of filler wire (11) into the weld pool (9) produced exclusively by means of a laser beam (6). In order to improve the hardenability of the weld seam, regardless of whether the steel sheets to be welded to one another are steel sheets of identical or different material quality, according to the invention the filler wire (11) is coated with graphite particles prior to the fusion welding and the filler wire (11) thus coated is introduced directly into the weld pool (9) in such a way that the tip of the filler wire (11) melts in the weld pool (9), the graphite particles being mixed with a waxy or liquid carrier means in order to be applied to the filler wire (11), and the mixture being applied in the form of a coating (11.1) to the filler wire (11). The method according to the invention and the corresponding device are distinguished by a high productivity and a relatively low energy consumption. The method can be implemented with a relatively low equipment outlay.