Vacuum Brazing Process Solutions for Purple Copper and Violet Copper
Abstract
Purple copper is widely used in electronics, refrigeration and automotive applications due to its excellent electrical and thermal conductivity and corrosion resistance. Vacuum brazing, as an oxidation-free and high-clean joining technology, is particularly suitable for precision soldering of purple copper. In this paper, the vacuum brazing process of purple copper (including oxygenated and oxygen-free copper) is systematically studied, and the process schemes of silver-copper eutectic brazing material (AgCu28), amorphous copper-phosphorus brazing material (Cu-Ni-Sn-P) and silver-copper-phosphorus brazing material are proposed to optimize the vacuum degree, heating rate, cooling gas selection and fixture design to ensure that the assembly gap is ≤ 0.02 mm. The experimental results show that the optimized process can significantly increase the The experimental results show that the optimised process can significantly improve the shear strength and wettability of the joints, which is suitable for high-precision application scenarios.
1. Introduction
Copper (pure copper) is important in cables, heat sinks and vacuum devices due to its high electrical conductivity (close to 100% IACS), thermal conductivity and ductility. Vacuum brazing is particularly suitable for violet copper connections as it avoids oxidation through a high vacuum environment and does not require fluxes. In this study, suitable brazing materials and process parameters are developed for the characteristics of purple copper, and special fixtures are designed to meet the needs of high precision and reliability.
2. Characteristics of base material
2.1 Aerobic copper
Ingredients: copper content ≥99.5%, oxygen content 0.06%-0.1% (such as T2).
Characteristics: good electrical and thermal conductivity, but high oxygen content, easy to hydrogen embrittlement in the hydrogen-containing atmosphere; brazing after the softening of the obvious.
Grade: T1 (≥99.95%), T2 (≥99.90%), T3 (≥99.70%).
Applications: cost-sensitive scenarios such as cables, pipes, radiators, etc.
2.2 Oxygen-free copper
Ingredients: copper content ≥99.99%, oxygen content ≤0.003% (e.g. TU1).
Characteristics: Strong resistance to hydrogen embrittlement, higher electrical conductivity, suitable for high purity requirements.
Grade: TU0, TU1, TU2.
Application: vacuum environment, high-frequency electronics, precision machining.
3. Brazing material selection and process parameters
3.1 Silver-copper eutectic brazing material (AgCu28)
Process parameters:
Temperature 810-850 ℃, holding time 5 minutes, vacuum degree 10-⁴ Pa, heating rate 5-10 ℃ / min.
Optimal temperature: 810-850 ℃, holding time 5 minutes.
Performance: Shear strength: Maximum 240MPa, strength decreases beyond 850℃. The joint is composed of copper-based solid solution and eutectic organisation.
Organisation: copper-based solid solution + silver-copper eutectic, fracture mostly at the junction.
Advantage: vacuum brazing to avoid oxidation and reduce defects.
Features: high strength, but narrow temperature window (820-950 ℃).
Cooling: argon below 600 ℃, cooling rate 8-15 ℃ / min.
3.2 Amorphous copper-phosphorus brazing material (Cu-Ni-Sn-P)
Amorphous brazing material is a new type of welding material, which is made from liquid metal by rapid cold solidification.
Amorphous brazing consumables have the following characteristics:
(1) Uniform chemical composition, low content of impurities and gases, and high purity. Brazing material components do not separate, can significantly improve the brazing joint strength.
(2) Does not contain binder, brazing heating rate is not restricted, brazing seam metal without slag, brazing joints of high quality.
(3) The brazing material can be punched and sheared into various shapes according to the structure of the workpiece, so that the dosage of brazing material can be controlled and the diffusion of liquid brazing material can be suppressed.
(4) As amorphous brazing material is usually pre-set in the brazing gap, it provides convenience and reliability for large-area brazed joints.
5) In contrast to tape brazing materials, they are not subject to storage time and storage conditions.
The wettability of amorphous brazing material is better than that of crystalline brazing material and adhesive tape brazing material. Amorphous brazing material due to the composition is very uniform, when heated to the liquid phase line is almost simultaneous homogeneous melting and spreading; while the crystalline brazing material there is a large melting precipitation phenomenon, due to the melting point of the phases of the different when heated to the liquid phase line, the first is the melting of the low melting point phase and spread, followed by the high melting point of the part of the spread of the slow pile up, resulting in delamination phenomenon. Secondly amorphous brazing material in the impurities and gas content is very low, while the content of crystalline brazing material is higher, especially the oxygen content and nitrogen content is dozens of times higher. Therefore, the melting surface of amorphous brazing material is very clean; while the melting surface of crystalline brazing material has obvious black and grey slag. It is analysed as metal oxides.
Process parameters:
Optimum temperature: 680°C, holding time 20 min. , vacuum 10-³ Pa, heating rate 8-12°C/min.
Wettability: spreading area up to 270 mm², low melting point,, brittle phase Cu₃P reduced, plasticity enhanced.
Characteristics: Excellent wettability, strong elemental diffusion, tight interfacial bonding. Suitable for precision welding.
Cooling: 600 ℃ below through nitrogen, cooling rate of 15 ℃ / min.
3.3 silver-copper-phosphorus brazing material: (such as BCuP-5, BCuP-8)
Process parameters: temperature 700 ℃, holding time of 10 minutes, the vacuum degree of 10 - ⁴ Pa, the rate of temperature increase of 10-15 ℃ / min.
Properties: good fluidity, low melting point. Strong self-fluxing properties, optimised tensile properties.
Applications: HVAC/R, automotive fuel lines, vacuum switches.
Cooling: below 500℃ through argon, cooling rate 12℃/min.
4. Process optimisation
4.1 Vacuum control
Range: 10-³ Pa to 10-⁵ Pa.
Function: Reduce oxidation and porosity, ensure joint cleanliness.
Suggestion: 10-⁴ Pa for oxygenated copper, up to 10-⁵ Pa for oxygen-free copper.
4.2 Rising temperature rate control
Segments: room temperature to 400℃ (10℃/min), 400℃ to brazing temperature (5-8℃/min).
Role: balance efficiency and softening control, to avoid coarse grain.
4.3 Cooling gas selection
Argon: for oxygen-free copper, purity ≥ 99.99%, cooling rate of 20-30 ℃ / min.
Nitrogen: for oxygenated copper, low cost, cooling rate of 15-25 ℃ / min.
Timing: brazing material solidification (<600 ℃) through the pressure of 0.2-0.3 bar.
4.4 Assembly clearance control
Range: 0.005-0.02mm.
Method: Finishing parts (tolerance ±0.005mm), fixed with locating pins and clamps.
Function: Enhance brazing material filling rate and joint strength.
5. parts assembly part of the refinement (assembly gap ≤ 0.02mm)
Assembly gap is the key factor affecting brazing material filling, wettability and joint strength in vacuum brazing. For copper vacuum brazing, too large or too small gap will lead to defects, especially in the high-precision needs, control at ≤ 0.02mm need special attention.
Control points
Ideal gap range: 0.005-0.02mm.
Less than 0.005mm: it is difficult for brazing material to flow in and easy to form imperfections.
Greater than 0.02mm: overfilling of brazing material, which may lead to porosity, loss of strength or wide brazing seam.
Assembly method:
Interference fit: Gap control is achieved by cold pressing or micro interference (0.002-0.005mm), but excessive stress should be avoided.
Positioning assembly: use precision positioning pins or fixtures to ensure a uniform gap.
Surface treatment: the brazing surface of the parts need to be polished or fine grinding (roughness Ra ≤ 0.8μm), to remove burrs and oxidation layer, to ensure a close fit.
Gap Measurement:
Use a plug ruler (accuracy 0.001mm) or optical microscope to check the assembly gap.
Sample testing after assembly to ensure that each joint gap meets the requirements.
Influence analysis
Copper characteristics: copper ductility is good, but easy to soften at high temperature, assembly gap is too small may be due to thermal expansion and extrusion deformation, too large will affect the capillary effect.
Brazing material characteristics:
silver-copper eutectic brazing material (AgCu28): liquidity medium, gap 0.01-0.02mm when the best wetting.
Amorphous copper-phosphorus brazing material: better fluidity, suitable for 0.005-0.015mm gap.
Silver copper phosphorus brazing material: self-fluxing flux characteristics, gap 0.01-0.02mm to ensure uniform filling.
Joint performance: when the gap is ≤0.02mm, the brazing seam is dense, and the shear strength is improved by about 10-15% (e.g. AgCu28 can reach more than 240MPa).
