Aluminum brazing has become increasingly important across automotive, aerospace, HVAC, and electronics industries as manufacturers seek reliable joining methods that preserve base metal properties while creating strong, leak-tight joints. Unlike welding, brazing does not melt the base aluminum, resulting in minimal distortion and preserved mechanical properties.
CoreMetal Steel supplies aluminum products and filler metals suitable for brazing applications across all major industrial sectors.
Fundamentals of Aluminum Brazing
Brazing vs. Welding vs. Soldering
- Soldering: Below 450C – uses tin, lead, or zinc-based solders
- Brazing: Above 450C but below base metal solidus – uses aluminum-silicon filler alloys
- Welding: Above base metal solidus – melts both filler and base metal
The Aluminum Oxide Challenge
Aluminum forms a tenacious oxide layer (Al2O3) that melts at 2072C, far above any brazing temperature. Successful brazing requires effective removal or disruption of this oxide layer through flux, inert atmosphere, or mechanical action.
Brazing Filler Metal Selection
Aluminum-Silicon Filler Alloys
- BAISi-3 (4343): 7.5% Si, general-purpose, good flow. Melting range: 577-607C
- BAISi-4 (4047): 10% Si, lower melting point, most widely used. Melting range: 577-585C
- BAISi-5 (4043): 5% Si, higher strength. Melting range: 574-631C
- BAISi-6 (4104): 10% Si, 1.5% Cu, lower melting point. Melting range: 565-580C
- BAISi-9 (4604): 10% Si, 4% Cu, lowest melting point. Melting range: 525-535C
Brazing Methods for Aluminum
Controlled Atmosphere Brazing (CAB)
The most common industrial method, especially for heat exchangers and radiators. Uses nitrogen atmosphere with controlled oxygen level (less than 100 ppm O2) and Nocolok flux at 600-620C. Excellent quality, high throughput, no post-braze cleaning required.
Vacuum Brazing
Used for aerospace and high-performance applications. High vacuum (10-3 to 10-5 torr) eliminates need for flux. Clean joints with excellent mechanical properties but higher equipment cost and slower cycle times.
Torch Brazing (Flame Brazing)
Manual or semi-automatic process using oxy-fuel torch with chloride-fluoride flux. Low equipment cost, flexible for prototype and repair work but requires skilled operator and post-braze cleaning.
Dip Brazing
Workpiece immersed in molten chloride salt bath at 600-650C. Uniform heating for complex assemblies but extensive post-braze cleaning and environmental concerns.
Induction Brazing
Uses electromagnetic induction to heat the joint locally. Rapid heating, localized heat, easily automated, but joint geometry must suit induction coil design.
Surface Preparation
- Degreasing: Remove oils with alkaline or solvent cleaners
- Mechanical cleaning: Stainless steel brushing immediately before brazing
- Chemical etching: Alkaline etch followed by deoxidation
- Joint clearance: Optimal 0.05-0.15 mm (0.002-0.006 inches)
Base Metal Considerations
- 1xxx series: Excellent brazeability
- 3xxx series: Very good brazeability, commonly brazed
- 5xxx series: Brazeable when Mg below 2%
- 6xxx series: Brazeable with careful temperature control
- 7xxx series: Generally NOT recommended for brazing
Quality Control and Inspection
Visual inspection of fillet formation, leak testing (helium mass spectrometer, air-under-water, pressure decay), and metallographic examination for joint penetration and filler distribution.
Why Choose CoreMetal Steel
CoreMetal Steel provides 1xxx, 3xxx, 5xxx, and 6xxx series aluminum sheet, plate, tube, and profile; pre-placed filler metal shims and clad materials; brazing filler wire in 4047, 4343, 4043, and 4604 alloys; with technical support and competitive pricing.
Contact CoreMetal Steel today for aluminum brazing materials and technical guidance.