4.6 Soldering, Welding & Weld Quality Inspection

4.6 Soldering, Welding & Weld Quality Inspection

Four knowledge elements of ACS Subject AM.I.E deal with joining metal by melting it: soldering preparation, types of solder, and flux usage (AM.I.E.K7); characteristics of acceptable welds (K12); characteristics of unacceptable welds (K13); and procedures for weld repairs (K14). Skill element AM.I.E.S3 requires you to inspect and check welds during the practical test. This section covers all of them.

---

1. Soldering: Metallurgy, Flux, and the Cardinal Rules

Soldering joins metals with a filler alloy that melts below the melting point of the base metals, which are not melted at all. The bond is a thin intermetallic layer plus mechanical adhesion — it is not a fusion weld, and it carries a fraction of the strength.

Soft solder alloys

Flux: the rule that gets people violated

Flux chemically removes surface oxide so molten solder can wet the base metal. The choice of flux is a safety decision, not a preference.

• Rosin flux (non-corrosive) is the ONLY flux permitted on electrical and electronic connections. Its residue is inert at room temperature.
• Acid flux (chloride-based, corrosive) must NEVER be used on electrical work. Its residue keeps attacking the joint and the surrounding wire strands long after the iron is put away, producing a green corroded connection and eventual open circuit. Acid flux is confined to sheet-metal and plumbing work where the residue can be fully washed away.
• Always clean the joint — mechanically abrade or chemically clean to bright metal — before fluxing.

Producing an acceptable joint

1. Make the mechanical connection first. Solder is not a mechanical fastener; a soldered joint must not rely on the solder to hold the parts in position.
2. Heat the work, not the solder. Apply the iron tip to the joint, then feed solder to the joint, letting the heated metal melt it. Solder melted on the iron and dripped onto a cold joint produces a cold joint every time.
3. Do not disturb the joint while it freezes.
4. Judge the result visually. An acceptable joint is bright, smooth, and shiny, with a thin concave fillet through which the outline of the underlying wire is still visible. A cold joint is dull, grainy, or frosted and often bulbous — it must be reheated or remade. An excess-solder joint is a shiny blob that hides the wire outline; it can conceal a poor connection.
5. Never solder a connection that carries primary structural load, and never solder a control cable terminal — swaged or wrapped-and-soldered cable terminals are governed by the specific approved procedure in AC 43.13-1B and the manufacturer's data.

---

2. Welding Processes Used on Aircraft

Welding fuses the base metals together, with or without filler, producing a joint as strong as the parent material when it is done correctly.

Flame settings for gas welding are an exam item. Adjusting the mixture at the torch produces three distinct flames:

• Neutral flame — balanced oxygen and acetylene, a clearly defined rounded white inner cone. This is the general-purpose flame for steel, including 4130.
• Carburizing (reducing) flame — excess acetylene, showing a feathery secondary cone around the inner cone. Used for high-carbon steel, nickel, and hard-surfacing; it adds carbon to the puddle.
• Oxidizing flame — excess oxygen, a short, sharp, noisy inner cone. Used for brass and bronze; on steel it burns out alloying elements and makes the weld brittle.

Shielding gases. GTAW and GMAW use argon, helium, or argon-helium mixtures to blanket the molten puddle and exclude atmospheric oxygen and nitrogen. Loss of shielding gas produces a grey, sooty, porous bead.

---

3. Characteristics of an Acceptable Weld (AM.I.E.K12)

FAA AC 43.13-1B gives an inspectable list. A good weld shows all of these:

1. The bead is of uniform width, and the ripples are evenly spaced along its length.
2. The bead height is uniform, with the crown neither excessively high nor sunken below the base metal.
3. The face is smooth, free of gas pockets, pits, and inclusions.
4. No oxide forms on the base metal more than about one-half inch from the weld.
5. The weld tapers off smoothly into the base metal — no sudden change in section.
6. No burned-through areas, and no undercut at the toes.
7. Complete penetration at the root, with the base metals visibly fused into one another rather than merely coated.
8. No cracks of any kind, in the weld or in the heat-affected zone.

---

4. Characteristics of an Unacceptable Weld (AM.I.E.K13)

---

5. Weld Repair Procedures (AM.I.E.K14)

The single rule most often missed on the test: a defective weld must not be repaired by simply welding over it. Re-melting an existing bead traps the original defect, adds heat cycles, and grows the heat-affected zone.

The correct sequence is:

1. Completely remove the original weld metal by grinding, filing, or machining down to clean parent metal.
2. Remove all traces of the defect — a crack must be chased to its ends and eliminated, not blended over.
3. Prepare the joint (bevel, fit-up, cleanliness) exactly as if it were an original weld.
4. Re-weld using approved data, appropriate filler, and appropriate process.
5. Stress-relieve as required by the applicable data. Welded 4130 steel tube structure is normally stress-relieved after welding to relieve residual stresses in the heat-affected zone.
6. Re-inspect — visually first, then by dye penetrant, magnetic particle, or radiography as the approved data requires.

Two structural limits worth memorizing: repairs to a cluster joint or a splice in a longeron are structural repairs governed by the manufacturer's structural repair manual or AC 43.13-1B, and a welded joint that has been repaired more than the number of times allowed by the applicable data must be replaced, not welded again. Finally, welding on any part that has been heat treated to a specific temper destroys that temper in the heat-affected zone — the part must be re-heat-treated or scrapped per the approved data.