Soldering and Brazing: Process and Joint Selection: How Metal Joining Actually Works

How Metal Joining Actually Works

Before you touch an electrode, a torch, or a soldering iron, you need to understand one fundamental principle: metal joining is controlled heat management.

Every joining method—from a 360°F solder joint to a 50,000°F plasma arc weld—operates on the same basic physics. You're applying thermal energy to create a metallurgical bond between two or more pieces of metal. The differences between soldering, brazing, and welding come down to three variables:

• Temperature — How hot do you go?
• Base metal involvement — Does the base metal melt, or only the filler?
• Joint strength — What structural load must the joint carry?

That 800°F threshold is not arbitrary. It is the internationally recognized dividing line between soldering and brazing, established by the American Welding Society (AWS). Memorize it. Every process selection decision starts here.

Soldering — The Precision of Low-Temperature Joining

When the practitioner Saved the Circuit Board

the practitioner was a production engineer at an electronics assembly plant. A new batch of circuit boards was failing quality control—cold solder joints, bridging, and intermittent connections were showing up at an alarming rate. The operators were using the same solder they'd always used: a 50/50 tin-lead alloy.

The problem? The application required 63/37 tin-lead—the eutectic alloy that melts and solidifies at a single temperature (361°F) rather than passing through a "mushy" semi-solid range. The 50/50 alloy had a solidus of 361°F but a liquidus of 421°F, meaning it spent 60 degrees in a partially molten state where any vibration or movement created weak, crystalline joints.

The right solder alloy isn't a preference. It's an engineering requirement.

What Soldering Is (and What It Isn't)

Soldering employs lead- or tin-base alloys with melting points below 800°F. It is commonly referred to as soft soldering to distinguish it from hard soldering (brazing) which uses silver, copper, or nickel-base filler metals above 800°F.

Soldering does not create high-strength structural joints. It is used to provide a convenient joint that does not require great mechanical strength. In practice, soldering is used extensively in combination with mechanical fastening—staking, crimping, or folding—with the solder serving only to seal against leakage or assure electrical contact.

Solder Alloy Properties: The Complete Reference

The following table is your master reference for soft solder selection. Every alloy listed here is standardized per ASTM B32-70.

Antimony-bearing alloys (tin-lead-antimony compositions) serve the same general purposes as their non-antimony equivalents but are not recommended for use on galvanized iron due to adverse chemical reactions with the zinc coating.

Engineering Design Note: For some engineering design purposes, alloys with 15% tin or less should be considered as having practically no mechanical strength above 360°F.

Forms Available

Soft solders can be obtained in the following forms, each suited to specific application methods:

• Bar — Commonly used for hand soldering
• Pig, Ingot, Slab — Used in operations employing melting kettles
• Wire (Solid) — Used in hand and automatic machine applications
• Wire (Cored) — Contains acid or rosin cores for integrated fluxing; used in hand and automatic machine applications
• Ribbon, Segment, Powder, Foil — Used for special applications
• Cake — Used specifically for wiping
• Prealloyed Powders — Suspended in a fluxing medium; applied by brush for consistent wetting

Fluxes for Soldering: The Invisible Make-or-Break

Here's a truth that separates professional soldering from amateur guesswork: the flux matters as much as the solder itself.

Surfaces of metals being joined must be clean to obtain an efficient joint. Fluxes perform three critical functions simultaneously:

1. Remove existing oxide coatings from the metal surface
2. Prevent formation of new oxide films during the heating cycle
3. Lower the surface tension of the solder, increasing its wetting properties

Critical: Corrosive flux residues will cause joint failure over time if not neutralized and removed. Use one of these cleaning methods after soldering with corrosive fluxes:

• For non-ferrous soldering: Wash with water containing approximately 5 ounces of sodium citrate per gallon, followed by a clear water rinse
• For ferrous and non-ferrous soldering: Wash with water containing 1 ounce of trisodium phosphate per gallon, followed by a clear water rinse
• Commercial alternative: Wash with commercial water-soluble detergents

Methods of Solder Application

Solder is applied through six primary methods:

• Soldering iron — Direct contact heat transfer; most common for hand work
• Torch — Open flame heating; used for larger joints and pipe work
• Solder bath (dip) — Immersion in molten solder; used for high-volume production
• Electric induction or resistance heating — Precisely controlled, repeatable heat application
• Hot neutral gas stream — Non-oxidizing atmosphere heating
• Wiping — Manual application technique for cable sheaths and lead pipe joints

In all methods, the fundamental requirement is the same: clean surfaces that are hot enough to melt the solder being applied or to accept molten solder. Parts must be free of oxides, dirt, oil, and scale.

Soldering Special Metals

Soldering Aluminum

Two properties of aluminum make it more difficult to solder than most metals:

1. High thermal conductivity — Heat dissipates rapidly, requiring higher working temperatures (550–770°F vs. 375–400°F for ordinary metals)
2. Tenacious oxide film — The ever-present aluminum oxide layer resists wetting

Two methods are used:

Flux Method (most widely used — "flow soldering"): The flux dissolves the aluminum oxide and prevents re-formation. The flux must be fluid at soldering temperatures so the solder can displace it in the joint.

Friction Method: The oxide film is mechanically abraded with a soldering iron, wire brush, or multi-toothed tool while covered with molten solder. The molten solder prevents atmospheric oxygen from reacting with the newly exposed aluminum surface, allowing wetting to take place.

Solder alloys for aluminum generally contain 50 to 75% tin with the remainder zinc.

Aluminum alloys ranked by ease of soldering (easiest to hardest):

1. Commercial and high-purity aluminum
2. Wrought alloys containing not more than 1% manganese or magnesium
3. Heat-treatable alloys (most difficult)

Note: Cast and forged aluminum parts are not generally soldered.

Soldering Magnesium

Magnesium is not ordinarily soldered to itself or other metals. Soldering is generally used only for filling small surface defects, voids, or dents in castings or sheets where the soldered area will not be subjected to any load.

Two solder compositions for magnesium:

Procedure: Clean surfaces to a bright metallic luster by abrasive methods. Preheat with a torch to the approximate melting temperature of the solder. Apply solder and vigorously rub the surface under the molten solder with a sharp pointed tool or wire brush. Flux is not recommended for magnesium soldering.

Soldering Stainless Steel

Stainless steel is somewhat more difficult to solder due to:

1. Tightly adhering oxide film on the surface
2. Low thermal conductivity — requires a large soldering iron to reach proper temperature

Surface preparation: Thorough cleaning by abrasion or clean white pickling with acid.

Acceptable fluxes for stainless steel:

• Muriatic (hydrochloric) acid saturated with zinc
• Above mixture + 25% additional muriatic acid
• Above mixture + 10% additional acetic acid
• Above mixture + 10-20% additional water solution of orthophosphoric acid

Tin-lead solder works successfully on stainless steel. The proper temperature is reached when solder flows freely into the joint area.

Critical: Removal of the corrosive flux residue is essential to prevent joint failure. Clean with soap and water or a suitable commercial detergent.

Ultrasonic Fluxless Soldering

This method uses ultrasonic vibrations to facilitate penetration of surface films by the molten solder, eliminating the need for flux entirely.

Equipment consists of: ultrasonic generator, ultrasonic soldering head (including transducer coupling, soldering tip, tip heater, and heating platen).

Metals that can be soldered ultrasonically: aluminum, copper, brass, silver, magnesium, germanium, and silicon.

Brazing — Where Capillary Action Meets Precision

What Brazing Is

Brazing is a metal joining process that uses a non-ferrous filler metal with a melting point below that of the base metals but above 800°F. The filler metal wets the base metal when molten, causing a slight diffusion of the filler into the hot, solid base metal (or surface alloying of the base and filler metals).

The key mechanism: Molten filler metal flows between close-fitting metal surfaces because of capillary forces. This is what makes brazing fundamentally different from both soldering (which relies primarily on adhesion) and welding (which relies on fusion).

Brazing Filler Metal Classifications

The commonly used brazing filler metals fall into seven standard classifications as defined by the American Welding Society:

Solidus vs. Liquidus: The solidus is the highest temperature at which the metal is completely solid (above which melting starts). The liquidus is the lowest temperature at which the metal is completely liquid (below which solidification starts).

Selected Brazing Filler Metal Specifications

Aluminum-Silicon Filler Metals

Joint design for aluminum brazing: Lap and tee joints are preferred over butt joints. Joint clearances: 0.006 to 0.025 inches.

Copper-Phosphorus Filler Metals

Critical Warning: Copper-phosphorus filler metals are NOT for use on ferrous or nickel-base alloys. They may be used for cupro-nickels, but exercise caution when nickel content exceeds 30%.

Copper and Copper-Zinc Filler Metals

Fluxes for Brazing

Surfaces in and adjacent to the joint must be free from dirt, oil, oxides, or other foreign matter at the time of brazing.

Mechanical cleaning methods: Filing, grinding, scratch brushing, machining

Chemical cleaning methods: Trisodium phosphate, carbon tetrachloride, trichloroethylene (for oils and greases)

Brazing fluxes perform three functions:

1. Prevent formation of oxides
2. Remove existing oxides from base and filler metals
3. Promote free flow of the filler metal

Available flux forms: Powders; pastes or solutions; gases or vapors; and as coatings on the brazing rods.

Methods of Supplying Heat for Brazing

• Torch brazing — Most common manual method; uses oxy-fuel gas flame
• Furnace brazing — Controlled atmosphere for high-volume production
• Induction brazing — Parts heated by placement near a coil carrying electric current; eddy current losses dissipate as heat. Quick and clean.
• Dip brazing — Immersion in molten flux or filler metal bath
• Resistance brazing — Heat from electrical resistance at the joint
• Infrared brazing — Radiant heating
• Vacuum furnace brazing — Cold-wall vacuum furnaces with electrical-resistance radiant heaters, capable of evacuating to moderate vacuum (~0.01 micron) in 5 minutes. Used for stainless steels, heat-resistant alloys, titanium, refractory metals, and aluminum. No flux required.

Vacuum brazing note: Filler metals containing alloying elements with low boiling points or high vapor pressure are not suitable for vacuum brazing.

Welding — The Science of Fusion

The Fundamentals of Fusion Welding

Welding of metals requires that they be heated to a molten state so that they fuse together. A filler wire or rod is held in the heated zone to add material that replaces metal consumed by the process and to produce a slightly raised area that can be dressed down to make a level surface if needed.

Three primary heat sources are used in modern welding:

1. Electric arc — Most common; low-voltage, high-current arc generates intense heat
2. Oxy-fuel gas torch — Burns a mixture of (usually) acetylene and oxygen; still used for certain work
3. Laser beam — High-energy density beam for precision applications

Welding Electrodes, Fluxes, and Their Effects

Electrodes may be:

• Nonconsumable — Made of tungsten or other high-melting-point alloy; does not melt at welding temperatures. Filler metal is added separately.
• Consumable — Made of an alloy similar to the workpiece; melts and acts as the filler wire itself.

Effects of alloying elements in welding filler wires and electrodes:

Fluxes are added to the weld zone in granular form, as coatings on filler wire, or as a core in tubular electrodes. They:

• Shield the arc from atmospheric oxygen
• Clean impurities from the molten metal
• Prevent grain growth during recrystallization

The Four Processes That Account for 90% of All Arc Welding

There are approximately 100 welding and allied welding processes, but four manual arc welding processes account for over 90% of all arc welding used in production, fabrication, structural, and repair applications:

Two groups of weld types exist: groove and fillet. Each may be made with the work at any angle from horizontal (flat) to inverted (overhead). In a vertical orientation, the electrode tip may move down (vertical down) or up (vertical up).

Key insight: In any weld other than flat, considerable skill is needed to prevent molten metal from falling from the weld area.

GMAW: Gas Metal Arc Welding (MIG)

Why GMAW Dominates Modern Fabrication

GMAW and FCAW together consume more than 50% of all arc welding consumable electrodes purchased. GMAW is the most-used welding process in the world, and understanding its transfer modes, electrode selection, and shielding gas optimization is essential for any professional welder or engineer.

GMAW Transfer Modes

GMAW operates in two primary transfer modes:

Short-Circuit Transfer (SCT):

• Electrode wire melts into the molten pool through rapid succession of short circuits
• Arc extinguishes momentarily during each short circuit
• Used for thin metals (24 gage to 11 gage)
• Current range: 50–200 amps
• Voltage range: 14–22 volts

Spray Transfer:

• Stream of fine drops and vaporized weld metal propelled across a continuous arc gap
• Electromagnetic forces in the arc drive the transfer
• Used for metals ≥ 1/8 in. (3.2 mm) thick
• Current range: 200–400+ amps
• Voltage range: 25–35 volts

GMAW Electrode Selection

The single most important welding decision is selecting the optimum electrode diameter. A wrong choice can increase welding costs by 20 to 60%.

The two most popular GMAW electrode sizes are 0.035 in. (1.0 mm) and 0.045 in. (1.2 mm).

GMAW Electrode Classifications

Galvanized steel warning: When GMAW welding galvanized steels, minute cracks may be caused by the reaction of the zinc coating with silicon in the electrode. Use E70S-3 (lowest possible silicon content).

GMAW Deposition Rates

Cost example: When welding 1/4-in. steel with 100% arc-on time and a labor rate of 15 units/h, using a 0.035-in. electrode deposits at 11 lb/h = 1.36 units/lb. Upgrading to a 0.045-in. electrode deposits at 16 lb/h = 0.93 units/lb—a 32% reduction in labor cost per unit of weld metal deposited, plus less shielding gas consumed and lower wire cost per pound.

Optimum Settings for GMAW (Argon + 15–20% CO₂)

If argon + oxygen gas mixtures are used, voltage should be lowered by 1 to 4 volts for spray transfer mode. The faster the weld travel speed, the lower the voltage required.

Setting the Optimum Voltage for GMAW Spray Transfer

This is a critical skill. Follow this three-step sequence:

Step 1 — Set voltage too HIGH (30–35 volts):

• You'll see a visible gap between the electrode tip and the weld
• Arc sound will be free from crackle — a quiet, spray sound

Step 2 — Reduce voltage gradually:

• Lower voltage until you hear a consistent smooth crackle
• This is the optimum setting

Step 3 — Recognize "too low":

• If voltage drops too much, the electrode runs into the weld
• You'll hear harsh crackling and see weld spatter

Shielding Gases: The Complete Selection Guide

With more than 40 GMAW gas mixtures available, selection can be confusing. Here's how to cut through the noise.

Fundamental principle: Reactive oxygen and CO₂ are added to argon to stabilize the arc and add energy to the weld. CO₂ provides more energy than oxygen. As CO₂ content increases, voltage requirements increase. Argon + oxygen mixtures require lower voltages than argon + CO₂ mixtures.

Shielding Gases for Carbon and Stainless Steels

(1st = preferred choice; 2nd = alternate choice; 3rd = usable)

The single best multipurpose gas mixture for carbon and low-alloy steels: Argon + 15–20% CO₂ (ideally Argon + 17% CO₂). This two-part mixture provides:

• Higher weld energy than low-CO₂ or argon + oxygen mixtures
• An arc slightly less sensitive to mill scale
• Sufficient energy for all GMAW short-circuit and spray transfer applications
• Compatibility with all-position FCAW electrodes on carbon, low-alloy, and stainless steels

For GMAW welding of aluminum: Argon + 25–35% helium. This mixture provides additional weld energy, increased penetration width, and reduced porosity potential.

For thin-gage stainless steel (<14 gage): Argon + 2–4% CO₂. This mixture allows use of lower voltages, reducing distortion, oxidation, and melt-through potential.

FCAW: Flux-Cored Arc Welding

When FCAW Beats GMAW

FCAW offers unique benefits over GMAW for specific applications, but flux-cored consumable electrodes cost more than solid GMAW electrodes. You need to understand exactly when the premium is justified.

Use FCAW when:

• Material surface is contaminated with mill scale, rust, oil, or paint
• Fillet weld size exceeds 3/8 in. (9.6 mm)
• Welding position is vertical up or overhead
• Required impact strengths and mechanical properties exceed normal levels
• Crack resistance must be high
• Increased penetration is required

FCAW Electrode Standards (International)

All-Position FCAW Electrodes: The Vertical Advantage

The most commonly used all-position electrode in the USA for vertical up welding on carbon steels is the E71T-1. International equivalents:

The cost advantage is dramatic. In contrast with short-circuit GMAW or pulsed GMAW, all-position FCAW electrodes used for vertical up welding are:

• Simpler to operate
• Capable of greater weld quality
• 2 to 3 times the deposition rate

FCAW Welding Parameters

Deposition Rate Comparison: FCAW vs. GMAW vs. SMAW

For vertical up welding, the difference is stark:

Rule of thumb: Use of an electrode at 60–80% of its welding current capability indicates the correct diameter has been selected. At maximum current capability, move to the next larger size. At the low end of the current range, the electrode is too large.

Contact Tip Recess: The Hidden Quality Factor

The contact tip recess dimension is critical for high-quality welds with all-position FCAW electrodes. For spray transfer (GMAW), the recess should be about 1/8 in. (3.2 mm). For FCAW welding, it should be about 1/2 in. (13 mm) with a minimum electrode extension of 3/4 in. (19 mm).

Why this matters: All-position FCAW electrodes have fast-freezing slag and operate with low to medium current and voltage. If the recess is less than optimum, voltage drops below the minimum recommended, and the fast-freezing slag solidifies too rapidly—causing excess porosity or worm tracks on the weld surface.

Recommended electrode extension for all-position FCAW E71T-1 electrodes: 3/4 to 1 in. (19 to 25 mm)