CLADDING TECHNOLOGY SHANXI CO., LTDMIG Brazing Overlay of Copper Strip Technology
Literature Overview
This study explores the application of metal inert gas (MIG) brazing to deposit copper strips onto steel substrates, creating a bimetallic composite material with the mechanical strength of steel and the excellent electrical and thermal conductivity of copper. Unlike traditional welding cladding processes that involve melting and remelting of the base material, brazing overlay relies on capillary action and wetting to join the copper strip to the substrate using a filler alloy with a lower melting point than either the base or the strip. This approach minimizes thermal distortion and avoids the dilution and metallurgical incompatibility issues that plague welding-based cladding of dissimilar metals.
Core Technical Content
The MIG brazing overlay process uses a MIG torch to heat the joint area to the brazing temperature range, typically 450–600 °C for copper-base brazing alloys, while the copper strip is placed on the prepared steel surface. The filler alloy melts and flows into the joint by capillary action, creating a metallurgical bond between the copper strip and the steel substrate without melting either of the base materials. The shielding gas, typically argon or argon with a small percentage of hydrogen or carbon dioxide, protects the molten brazing alloy from oxidation.
| Parameter | Typical Value | Notes |
|---|---|---|
| MIG torch power | 1.5–3.0 kW | Depends on strip thickness and width |
| Travel speed | 50–200 mm/min | Lower speed for thicker strips |
| Nozzle to workpiece distance | 8–15 mm | Ensures adequate shielding |
| Shielding gas flow rate | 15–25 L/min | Argon or Ar+CO2 |
| Preheat temperature | 150–300 °C | Improves wetting and reduces residual stress |
| Brazing temperature | 450–600 °C | Above filler melting point, below base melting point |
| Copper strip thickness | 0.5–3.0 mm | Typical range for electrical applications |
| Joint gap | 0.05–0.2 mm | Critical for capillary flow |
| Filler alloy melting point | 430–520 °C | Typically Ag-Cu or Cu-P type alloys |
The preparation of the joint surfaces is critical to the success of the brazing operation. Both the steel substrate and the copper strip must be thoroughly cleaned to remove oxides, oils, and other contaminants that would prevent proper wetting of the brazing alloy. Chemical cleaning with acid solutions or mechanical cleaning with abrasive blasting are common preparation methods. The application of a flux, typically a borax-based or phosphate-based flux, further aids in oxide removal and improves the flow and spreading of the molten filler alloy.
Microstructural Analysis of the Brazed Joint
The brazed joint microstructure consists of a diffusion bond between the copper strip and the steel substrate, with an intermetallic compound layer forming at the interface during the brazing process. The thickness and composition of this intermetallic layer are critical to the joint strength and electrical conductivity. Common intermetallic phases include CuFe, Cu2Fe, and Cu3Fe, which form depending on the brazing temperature, time, and composition of the filler alloy.
An optimal intermetallic layer thickness is typically in the range of 5–15 micrometers. Layers that are too thin may indicate incomplete bonding, while layers that are too thick can become brittle and prone to cracking under thermal cycling or mechanical loading. The study emphasizes that controlling the brazing temperature and holding time is essential to achieving an intermetallic layer of the appropriate thickness and composition.
Process Advantages Over Welding Cladding
The MIG brazing overlay approach offers several distinct advantages over conventional welding cladding for copper-on-steel bimetallic applications. First, the absence of base material melting eliminates the risk of dilution and the formation of brittle intermetallic phases that can result from high-temperature melting and rapid solidification. Second, the lower thermal input results in significantly reduced distortion and residual stress in the substrate, which is particularly important for thin-walled components or precision parts. Third, the brazing process is inherently more tolerant of slight misalignment and surface irregularities, making it more practical for production environments with varying material quality.
Engineering Practice Integration
The primary application of MIG brazed copper strip overlay is in the manufacture of electrical and thermal management components, including busbars, heat exchangers, electrical contacts, and power distribution equipment. The resulting bimetallic composite combines the high conductivity of copper with the structural strength and corrosion resistance of steel, offering performance advantages that neither material can achieve alone.
Quality control for brazed overlay joints involves several non-destructive and destructive testing methods. Visual inspection checks for adequate coverage, absence of voids or dry spots, and uniform bond line. Ultrasonic testing can detect delamination or lack of bond at the interface. X-ray radiography can reveal voids or porosity within the brazed joint. For critical applications, destructive testing including tensile shear tests, bend tests, and microstructural examination of cross-sections is performed to verify that the joint meets the required strength and metallurgical criteria.
The study also addresses the issue of long-term joint reliability under thermal cycling conditions. Repeated heating and cooling cycles cause differential expansion and contraction between the copper strip and steel substrate, which can generate fatigue stresses at the interface. The intermetallic layer, while providing the initial bond, can become a site for crack initiation under cyclic loading. The study recommends incorporating a thin layer of nickel or another diffusion barrier between the copper and steel to reduce the formation of brittle intermetallic phases and improve the thermal fatigue resistance of the joint.
Key Questions and Reflections
A key challenge identified in the study is the difficulty of achieving consistent and uniform brazing across large surface areas, particularly when the joint gap is not perfectly controlled. Variations in the gap width can lead to areas of incomplete filling or excessive filler metal accumulation, both of which compromise the joint quality. The study suggests the use of precision-forming techniques to create the copper strip with a controlled curvature or profile that ensures consistent gap dimensions across the entire bond area.
Another important consideration is the effect of the brazing process on the base steel properties. While the brazing temperatures are well below the melting point of steel, prolonged exposure to elevated temperatures can cause softening of quenched and tempered steels or tempering of martensitic structures. This is particularly relevant for applications where the steel substrate must maintain specific mechanical properties after brazing. The study recommends selecting steel grades with good temper stability or performing a post-brazing heat treatment to restore the desired mechanical properties.
Summary and Implications
The MIG brazing overlay of copper strips onto steel substrates represents a versatile and effective technology for creating bimetallic composites with tailored combinations of electrical, thermal, and mechanical properties. The process offers significant advantages over welding-based cladding in terms of reduced thermal distortion, avoidance of dilution-related metallurgical issues, and improved joint quality consistency. For engineers working in electrical equipment manufacturing, thermal management, and power systems, this technology provides a reliable means of achieving copper-to-steel bonding that meets demanding performance and reliability requirements. The key to successful implementation lies in meticulous joint preparation, precise thermal control, and rigorous quality verification to ensure that the brazed joints perform reliably throughout the service life of the component.