Differential Copper Coating for Multiwall Steel Tube Brazing
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Solution Overview
Problem
Conventional multiwall steel tubes face issues with excessive copper coating leading to voids, inadequate fillet formation, and surface contamination during brazing, resulting in reduced strength and appearance quality.
Innovation Solution
A multiwall steel tube is formed by winding a steel sheet with copper films of differential thicknesses, where the outer surface has a thinner copper film (0 < t1 ≤ 3 µm) and the inner surface has a thicker copper film (3 µm ≤ t2 ≤ 6 µm), ensuring proper copper supply for brazing and preventing surface solidification, with the ratio of t1 to t2 between 1/3 and 2/3, allowing for efficient bonding and surface treatment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If both surfaces of the steel sheet are coated with copper films of the same thickness, then the brazing process can be performed, but the amount of copper is excessively large causing solidification on the surface and adhesion in the furnace
Solution Approach 1:
The patent applies local quality by coating the inner surface of the steel sheet with a thicker copper film (3-6 µm) and the outer surface with a thinner copper film (0-3 µm). This differential coating provides sufficient copper for brazing at the inner surface while minimizing excess copper on the outer surface that would cause solidification and adhesion problems.
2Reliability
If both surfaces are coated with copper films, then brazing can be performed, but voids are formed in the multiwall steel tube due to excessive copper
Solution Approach 1:
By concentrating the thicker copper coating (3-6 µm) on the inner surface where it is needed for brazing, and using a thinner coating (0-3 µm) on the outer surface, the patent prevents excessive copper accumulation that would create voids between layers, thereby improving manufacturing precision.
3Object-affected harmful factors
If carbon is coated on the surface prior to brazing to prevent copper solidification, then surface contamination is reduced, but carbon remains on the surface reducing cleanliness and causing troubles in subsequent surface treatment
Solution Approach 1:
The patent changes the parameter of copper film thickness distribution, using a thinner outer coating (0-3 µm) that provides just enough copper for brazing without excessive accumulation. This eliminates the need for carbon coating and subsequent carbon removal, maintaining surface cleanliness and enabling proper surface treatment.
4Manufacturing precision
If a steel sheet with one surface coated with copper film is used, then cleanliness is improved and power consumption is reduced, but a copper fillet is not formed along the seam reducing strength
Solution Approach 1:
The patent applies local quality by concentrating the thicker copper coating (3-6 µm) on the inner surface where the seam is located, ensuring sufficient copper is available to form a strong copper fillet along the seam for brazing, while the outer surface has a thinner coating (0-3 µm) that maintains cleanliness.
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 ensures a satisfactory fillet formation along the seam, enhances the strength of the brazed part, and maintains surface cleanliness, enabling the use of the multiwall steel tube in high-pressure applications like automotive brake piping with improved inspection sensitivity and reduced rusting.
Implementation Method 1
bonding together the layers of the steel sheet by brazing
Implementation Method 2
heated for brazing at a temperature not lower than the melting point of copper
Implementation Method 3
heated for brazing at a temperature not lower than the melting point of copper in a reducing atmosphere
Data Source
Figure 1~2
Figure 3(a)~3(e)
AI summary
A proper amount of copper can be supplied for brazing in a multiwall steel tube manufacturing process such that copper does not resolidify on the surface of a steel tube and a satisfactory fillet is formed along a seam in the steel tube. A steel sheet has first and second surfaces coated respectively with a copper film of a thickness t1 and a copper film of a thickness t2 greater than the thickness t1 by plating. The steel sheet is formed in a multiwall steel tube with the first surface facing outside.