| name | brazing-soldering |
| description | Brazing and soldering — joint design, filler selection (AWS BAg, BAu, BCu, BNi), flux, gap clearance (capillary), strength, thermal effects, AWS C3.6, furnace vs. torch, vacuum brazing. |
| metadata | {"priority":7,"promptSignals":{"phrases":["brazing","soldering","braze joint","filler metal","silver brazing","torch brazing","vacuum brazing"],"minScore":3}} |
Brazing and Soldering — Complete Skill
Definitions
Brazing: filler metal melts above 450°C; base metal does not melt; capillary action fills gap
Soldering: filler metal melts below 450°C; similar capillary mechanism; lower strength
Brazing vs. welding: lower heat → less distortion; dissimilar materials possible; but lower strength
Capillary Action — Gap Clearance
Capillary pressure: ΔP = 4σ cosθ / d (d = gap width; σ = surface tension; θ = contact angle)
Optimal clearance at brazing temperature (not room T):
- Silver brazing: 0.025–0.125 mm (0.001–0.005 in) clearance
- Copper brazing: 0.00–0.05 mm (near contact for furnace)
- Nickel brazing: 0.025–0.075 mm
- Aluminum brazing: 0.025–0.15 mm
Too small → filler won't flow; too large → meniscus breaks → incomplete fill
Thermal expansion: account for differential expansion between T_room and T_braze when setting gap
Filler Metal Classification (AWS A5.8)
Silver Brazing (BAg)
BAg-1: 45%Ag-24%Cu-16%Zn-15%Cd; solidus 618°C, liquidus 640°C; most common general purpose
BAg-7: 56%Ag-22%Cu-17%Zn-5%Sn; solidus 650°C; Cd-free
BAg-24: 50%Ag-20%Cu-18%Zn-12%Cd; lower temperature; excellent flow
BAg-18: 60%Ag-30%Cu-10%Sn; Cd-free; food equipment, medical
Applications: copper pipe, HVAC, carbide tooling, precision instruments
Copper Alloy Brazing (BCu, BCuP)
BCu-1: pure Cu; solidus/liquidus 1083°C; used in furnace brazing steel
BCuP-2: 93%Cu-7%P; liquidus 715°C; self-fluxing on copper; HVAC tube joining
BCuP-5: 80%Cu-15%Ag-5%P; better flow; still self-fluxing on copper
Applications: copper tube plumbing, refrigeration (torch brazing with no flux on Cu)
Nickel Brazing (BNi)
BNi-1: Ni-14Cr-4Fe-4Si-3B; liquidus 1038°C; stainless, superalloys
BNi-2: Ni-7Cr-5Fe-3Si-2B; liquidus 1038°C; most common nickel braze
BNi-7: Ni-14Cr-10P; 1040°C; no B; less erosion; printed circuit boards
Applications: aerospace turbine components, stainless steel honeycomb, heat exchangers (vacuum furnace)
Aluminum Brazing (BAISi)
BAISi-4 (4047): 88%Al-12%Si; liquidus 577°C; torch or furnace; tight gap
CAB (Controlled Atmosphere Brazing): fluoride flux; automotive heat exchangers (Nocolok process)
Applications: automotive condensers/radiators, aerospace coolers
Gold Alloys (BAu)
BAu-4: 82%Au-18%Ni; liquidus 960°C; aerospace electronics, hermetic sealing
High cost; excellent oxidation resistance; used in critical applications
Soldering (SnPb and lead-free)
Sn63Pb37 (eutectic): 183°C; well-controlled; standard electronics (restricted by RoHS)
SAC305 (96.5Sn-3.0Ag-0.5Cu): 217–220°C; primary lead-free; IPC J-STD-006
Sn100C (Sn-Cu-Ni): 227°C; plumbing; potable water approved
Bi58Sn42 (low temp): 138°C; temperature-sensitive electronics
Flux Function
Flux removes oxide from base and filler at temperature → allows wetting
Silver brazing flux (AWS Type 3B, 4A, 5):
Temperature range: 3B → 565–870°C; 4A → 760–1200°C (black flux for higher T)
Fluoride-based for aluminum brazing (Nocolok)
Self-fluxing: BCuP on copper (phosphorus dissolves Cu₂O); no external flux needed
Heating Methods
Torch brazing: propane-air, propane-O₂, oxy-acetylene; manual or automated
Flexible; lower capital; operator-dependent quality; not suitable for mass production
Furnace brazing: continuous conveyor or batch; controlled atmosphere (H₂, N₂, N₂-H₂, vacuum)
Excellent quality; repeatable; mass production; vacuum for superalloys, aerospace
Induction brazing: localized rapid heating; high frequency coil; minimal heat input; fast
Good for automated in-line; ferritic materials respond better; requires fixture
Vacuum furnace brazing:
No oxidation; no flux required → cleaner joints; nickel/cobalt superalloy, stainless, Ti
Pressure typically 10⁻⁴ to 10⁻⁶ torr; titanium must be ultra-clean
Cycle: ramp to braze temp, hold 5–30 min; cool slowly to prevent residual stress
Joint Design
Shear joint (lap): most common; area = D × L_overlap; strength = S_joint × A_shear
Minimum overlap = 3× thinner member thickness for optimum strength
Butt joint: limited area; weak in tension; use only when constrained
Tee joint: stress concentration at root; reinforce with fillet
Joint strength:
S_joint ≈ 0.5–1.0 × parent metal strength (well-made joint in shear)
Lap length ≥ 3t to achieve parent-strength failure in base metal rather than braze
Quality and Testing
Visual: complete fillet, consistent color, no voids visible
Dye-penetrant: surface-breaking defects
X-ray: internal voids, porosity (critical joints in aerospace)
Tensile/shear test per AWS C3.6 or ASME Section IX
ASME Section IX: qualifies brazing procedures; PQR + BPS required for pressure boundaries
AWS C3.6: specification for furnace brazing
AWS C3.7: copper brazed components
Output
Provide: filler metal AWS designation, liquidus temperature [°C], gap clearance [mm] (at braze temperature), flux type, heating method, atmosphere or vacuum level, shear joint strength estimate [MPa], overlap length [mm], applicable standard (AWS/ASME).
