14.8 Brazing, Soldering & Adhesive Bonding

The Defining Distinction

The 450-degree line is the single most examined fact in this topic. It is an arbitrary but universally adopted convention, and in both cases the filler must melt below the melting point of the parent metal.

Why not melt the parent metal?

Because not melting it brings four real advantages:

1. Dissimilar materials can be joined — steel to copper, carbide to steel, metal to ceramic — combinations that fusion welding handles badly or not at all.
2. Thin sections are not burned through.
3. Distortion and residual stress are far lower, since the whole assembly is heated more uniformly and to a lower temperature.
4. Heat-treated or work-hardened properties in the parent metal are less affected.

The corresponding disadvantage is that joint strength is limited by the filler, not the parent metal, and service temperature is capped well below the filler's melting point.

Capillary Action and Joint Clearance

Both brazing and soldering rely on capillary action to draw molten filler into the joint. This has an immediate design consequence: the joint must be a close-fitting lap or sleeve, not a butt.

The optimum clearance is typically 0.05 to 0.20 mm, and the relationship is not monotonic:

• Too tight and the filler cannot flow in, or flux cannot escape.
• Too loose and capillary action is weakened, the joint fills incompletely, and the strength falls towards that of the bulk filler alone.

Strength is highest at small clearances because the thin filler layer is constrained by the much stiffer parent metal on both sides, which raises its effective yield strength — the same triaxial constraint effect met in notch mechanics.

Because the joint is loaded in shear over a large lap area rather than in tension over a small butt area, a well-designed brazed joint can be as strong as the parent metal even though the filler is much weaker. The rule of thumb is a lap length of at least three times the thinner member's thickness.

Wetting and Fluxes

Molten filler must wet the parent surface, which requires the surface to be clean and free of oxide. The flux performs three functions:

1. Dissolves and removes existing oxide films.
2. Prevents re-oxidation during heating by blanketing the surface.
3. Reduces surface tension, promoting flow and wetting.

The distinction between corrosive and non-corrosive flux matters practically. Rosin-based fluxes are mandatory in electronics because residues from zinc chloride would corrode the joint over time. Where corrosive flux is used, residues must be washed off.

An alternative to flux is a controlled atmosphere — hydrogen, dissociated ammonia or vacuum — which prevents oxidation without leaving any residue at all. Vacuum brazing is standard for aerospace and for heat exchangers where flux residue could not be removed from internal passages.

Brazing

Filler alloys

Silver brazing, often loosely called silver soldering, is genuinely brazing because the filler melts well above 450 degrees.

Heating methods

Braze welding

A distinct variant worth knowing: braze welding uses a brazing filler but deposits it in a groove like a weld, without relying on capillary action. It is used for repairing cast iron, where the low temperature avoids the cracking that fusion welding of cast iron so easily produces.

Soldering

Soldering joins at low temperature and is used chiefly for electrical connection and sealing rather than for structural strength.

Solder alloys

The classic tin-lead system has a eutectic at 63% tin and 37% lead, melting sharply at 183 degrees Celsius. The eutectic composition melts and freezes at a single temperature with no pasty range, which is why it was preferred for electronics.

Off-eutectic compositions such as 60/40 or 50/50 have a pasty range between solidus and liquidus, which is useful in plumbing where the joint must be wiped.

Lead-free solders have largely displaced tin-lead for health and environmental reasons, principally tin-silver-copper alloys melting near 217 degrees Celsius, along with tin-silver and tin-copper systems.

Methods

Soldering iron for hand work; wave soldering and reflow soldering for printed circuit board production; dip and induction soldering for volume work.

Adhesive Bonding

Adhesives join by chemical and physical attraction rather than by metallic bonding.

Types

Joint design

Adhesives are strong in shear and compression but weak in peel and cleavage. This governs joint design absolutely:

A simple lap joint loaded in shear is the standard configuration. Where peel cannot be avoided, it is resisted by adding rivets or spot welds at the ends of the joint, giving a weld-bonded or riveted-bonded hybrid.

Advantages and limitations

Adhesives distribute load uniformly rather than concentrating it at fastener holes, join dissimilar and non-metallic materials, seal as well as join, add negligible weight, and involve no heat so there is no distortion or metallurgical change.

Against that: surface preparation is critical and often the limiting factor in practice; cure time may be long; service temperature is limited, usually below 200 degrees Celsius; strength is lower than a welded joint; and non-destructive inspection of the bond quality is genuinely difficult, which restricts adhesives in safety-critical structures.

Selection Summary