Brazing - Problems & Remedies
|
| SYMPTOM |
CAUSE |
ACTION |
| Failure of filler metal to wet both surfaces |
Surface dirt or contamination
Unsatisfactory fluxing |
Improve cleaning procedures
Apply sufficient flux of
correct grade |
| Failure of filler metal to wet one surface |
1.One component contaminated
2.Unsatisfactory heating pattern
3.Preform only in contact with one side of the joint |
1.Check cleaning procedure
2.Apply heat to the heavier part
3.Ensure
that capillary gap is bridged at melting point by adjusting
tolerance to give spring-fit. |
| Failure of filler metal to flow smoothly (Joint is
rough and fillet is uneven) |
1.Poor fitting components
2.Uneven heating
3.Poor joint ventilation
4.Bad fluxing
5.Overheating Liquation
|
1.Check that capillary gaps are consistent throughout
the joint area
2.Make sure that whole joint area is brought up to brazing
temperature simultaneously
3.Make sure gases generated have
escape route
4.Check grade of flux and increase amount used
5.Reduce brazing temperature to 50°C above liquidus
Increase heating rate or change to SBA with narrower melting
range |
| Voids (obvious gaps) |
Variable or excessive joint clearances; uneven or insufficient
heating; poor venting; inadequate fluxing |
Correct the applicable faults. |
| Blowholes (localised holes with rounded and shiny interiors) |
Hydrogen pickup by brazing alloy |
Adjust burners to give neutral to slightly oxidising
flame, check that organic contaminants like oil, grease
and paint are not present; check that joint is vented.
Pinholes can also be caused by overheating resulting in
zinc or cadmium vapourizing. |
| Porosity (shrinkage observed externally in the joint) |
Excessive local tolerance, unbalanced heat pattern,
overheating, use of alloy with too wide a melting range |
Correct the applicable fault. |
| Cracking of filler metal |
Thermal stress set up by differential contraction |
Redesign joint with higher coefficient of expansion
material on outside of joint |
| Cracking adjacent to one of the parent metals |
1.Contamination of the surface concerned
2.Formation of embrittling intermetallic layer. |
1.Upgrade preparation techniques
2.Identify possible contaminations,
consider trifoil. |
