Soldering and Brazing: Process and Joint Selection: Porosity and Worm Track Prevention

Porosity and Worm Track Prevention

Porosity and worm tracks result from a combination of:

• Incorrect electrode extension
• Incorrect welding settings
• Humidity and electrode moisture
• Mill scale, rust, paint, oils
• Poor welding technique

Remedies (in order of priority):

1. Grind clean the surface to be welded
2. Use recommended electrode extensions
3. Increase current (wire feed rate)
4. Decrease voltage
5. Use backhand welding technique
6. Slow down travel speed
7. Consider a different electrode formulation with increased deoxidizers
8. Avoid weaving
9. Change from argon + CO₂ to straight CO₂
10. Protect the electrode spool — keep clean and dry

FCAW Shielding Gas Selection

E70T-X electrodes (flat and horizontal): Use CO₂ gas shielding

E71T-1 electrodes (all-position): Use either CO₂ or argon + 15–25% CO₂

The argon + CO₂ mixture is often preferred because it provides:

• Higher energy from a reactive gas mixture
• Lower smoke levels
• Better arc characteristics
• Lower voltage requirements

Warning: If argon mixtures with less than 13% CO₂ (or argon + oxygen) are used, the reduced plasma energy can significantly change mechanical properties, increase porosity, and raise the potential for worm tracks.

Deposition Efficiency: FCAW vs. GMAW

This means for every 100 lb of FCAW electrode used, 85 lb becomes weld material (15 lb is slag). For GMAW, virtually all the wire becomes weld metal.

SMAW: Shielded Metal Arc Welding (Stick Welding)

Reading the Electrode Code

Understanding the ANSI/AWS electrode classification system is essential for proper SMAW electrode selection.

For E60XX and E70XX electrodes:

• E = Low-carbon steel, metal arc welding electrode
• Next two digits (60 or 70) = Approximate tensile strength of the weld deposit in thousands of PSI
• Third digit = Position usability: 1 = all positions; 2 = flat or horizontal; 3 = flat only
• Fourth digit (combined with third) = Flux coating type

AWS E60XX Electrode Characteristics

These electrodes are designed for welding low-carbon steels and provide typical tensile strength of 58,000–65,000 lbf/in².

E60XX Electrode Diameters for Sheet Metal

Current Ranges for AWS E60XX Electrodes

AWS E70XX Electrode Characteristics

Current Ranges for AWS E70XX Electrodes

Critical for E7018 low-hydrogen electrodes: Manufacturers' instructions regarding storage requirements for keeping these electrodes free from moisture must be followed. Moisture absorption is the primary cause of hydrogen-induced cracking in welds made with low-hydrogen electrodes.

GTAW: Gas Tungsten Arc Welding (TIG)

The Precision Process

GTAW uses a nonconsumable tungsten electrode with an inert gas shield. Until the development of plasma arc welding (PAW), it was the most versatile of all common manual welding processes.

A major benefit of GTAW compared with GMAW, FCAW, or SMAW is the highly concentrated, spatter-free, inert heat from the tungsten arc.

When to Choose GTAW (and When Not To)

The logical approach: first examine whether the job can be welded by gas metal arc or flux-cored methods. GTAW is generally slower and more expensive. Choose it when:

• Weld quality requirements exceed what GMAW/FCAW can deliver
• Thin materials (down to 0.001 in. / 0.025 mm with low-current plasma variant)
• Exotic metals requiring precise heat control
• Root passes on pipe where quality is critical
• Repair work requiring precision

GTAW Current Types

GTAW can use three types of welding current:

1. Direct-Current Straight Polarity (DC−, DCEN)

• Most common GTAW current
• Electrode connected to negative terminal; ground to positive
• Electrons flow from tungsten tip to workpiece
• Maximum penetration, concentrated arc
• 70% of heat at the workpiece, 30% at the electrode

2. Direct-Current Reverse Polarity (DC+, DCEP)

• Electrode connected to positive terminal
• Electrons flow from workpiece to electrode
• Cleaning action on the workpiece surface (breaks up oxides)
• Shallow, wide weld pool
• Limited current capacity (overheats electrode)

3. Alternating Current with High Frequency (ACHF)

• Combines benefits of both DC− and DC+
• Half-cycle of straight polarity + half-cycle of reverse polarity
• Required for aluminum and magnesium — breaks up their oxide films
• High-frequency current assists arc ignition during each cycle

Tungsten Electrode Selection

GTAW Shielding Gases

Gas purity requirements:

• Welding-grade argon: minimum 99.996% purity
• Welding-grade helium: minimum 99.995% purity

Contamination test: With HF and power on, create an arc without welding and hold for about 30 seconds. Examine the electrode tip for unusual coloration, oxidation, or contamination — these indicate gas impurities.

GTAW Filler Metal Specifications (AWS)

Plasma Arc Welding (PAW)

How Plasma Welding Works

When an electric current passes between two electrodes through certain gases, the energy of gas molecules increases until electrons are released from nuclei. The gas becomes ionized plasma — capable of conducting electric current.

The sequence of events:

1. Inert gas passes through the welding torch nozzle
2. High-frequency current generates between the tungsten electrode (cathode) and the torch nozzle (anode)
3. A low-current pilot arc forms (nontransferred arc)
4. The ionized path transfers from the electrode to the work
5. Plasma current is generated between electrode and work
6. Arc temperatures reach 30,000–50,000°F (16,650–27,770°C)

PAW vs. GTAW: Key Advantages

PAW Gases

Plasma gas: Argon is preferred (easily ionized; low thermal conductivity produces a concentrated hot core)

For steel up to 1/8 in.: Pure argon plasma For thicker materials: Argon 25% + Helium 75% for hotter arc For keyhole welding <1/8 in.: Argon + up to 15% hydrogen For stainless and nickel steels >1/8 in.: Argon + 5% hydrogen

PAW Operating Modes

Fusion Welding:

• Main use for PAW
• Soft, less-restricted arc with low gas flows
• Current: ~25–200 amps
• Used for lap, flange, butt, and corner welds in all positions

Low-Current Plasma Fusion Welding:

• Ideal for metals down to 0.001 in. (0.025 mm) thickness
• Arcs start consistently at less than 1 amp
• More economical than GTAW for thin materials

Keyhole Mode Welding:

• Abutting edges melted simultaneously, forming a vapor capillary
• The "keyhole" moves along the joint line
• Tungsten electrode positioned well back inside the torch nozzle
• Plasma flow rates: 1–3 cu ft/h (0.5–1.4 l/min)
• Flushes trapped gases and contaminants from the weld pool

Variable Polarity Plasma Arc (VPPA) Welding

The VPPA process ensures extremely low levels of porosity in aluminum welds. Often used in the vertical up position for aluminum because it provides superior control of root reinforcement.

Electron-Beam (EB) Welding

Extreme Precision at Extreme Energy

Electron-beam welding generates electrons, concentrates them into a beam, and accelerates them to 30–70% of the speed of light using voltages between 25 and 200 kV.

Key specifications:

• Beam diameter: 0.01–0.03 in. (0.25–0.76 mm)
• Beam power: up to 100 kW
• Power density: up to 10⁷ W/in² (1.55 × 10⁴ W/mm²) — higher than most arc welding levels
• Steel penetration: up to 4 in. thick via keyhole mechanism
• Current: approximately 6.3 × 10¹⁵ electrons/s in a 1-mA current stream

EB Welding Environments

Weldable materials: Carbon, low-alloy, and stainless steels; high-temperature and refractory alloys; copper and aluminum alloys. Single-pass butt welds in materials up to 1 in. (25.4 mm) thick at good speeds with 60 kW equipment. Dissimilar metals usually may be welded without problems.

Caution: EB welds solidify and cool very rapidly due to the heat-sink effect, causing cracking in certain materials such as low-ferrite stainless steel.

Laser Welding

How Laser Welding Works

Conversion of absorbed laser energy into heat causes metals to undergo a phase change from solid to liquid and back to solid. This fusion welding process produces selective area spot welds or linear continuous seam welds.

Two Types of Laser Welding

Conduction Welding:

• Relies on thermal diffusivity to conduct heat into the joint
• Heat concentrated in focused beam diameter for short time periods
• More heat conducted into joint than radiated outward
• Used for spot welding and partial penetration seam welding

Deep Penetration (Keyhole) Welding:

• Beam energy creates a hole through the thickness of the metal
• Vapor pressure of evaporated metal holds a molten layer against the hole wall
• Hole movement causes molten metal to flow around and solidify behind
• Maximum practical penetration: approximately 25 mm (1 in.)

Laser Welding Joint Design

For optimum results, edges of parts should be in close contact.

Key Advantage: Low Total Heat Input

The focused high-energy density beam causes most conduction to be perpendicular to the direction of motion. With the beam moving faster than the speed of thermal conduction, significant heat flow occurs only perpendicular to travel direction — resulting in minimum thermal distortion.

Processing gases: Helium is the ideal gas for laser welding. CO₂ and argon have been used as alternatives but neither produces a perfectly clean, smooth weld.

Weld and Welding Symbols: The Engineering Language

Understanding the Welding Symbol System

The American National Standard ANSI/AWS A2.4-79 provides graphical symbols that convey complete welding information from designer to welder.

Critical distinction:

• Weld symbol = the ideograph indicating the type of weld desired
• Welding symbol = the complete symbol containing up to eight elements

The Eight Elements of a Welding Symbol

1. Reference line — Basis of the welding symbol; all other elements oriented to this
2. Arrow — Connects reference line to one side of the joint (the "arrow side")
3. Basic weld symbols — Designate the type of welding
4. Dimensions and other data — Size, length, spacing, groove angles
5. Supplementary symbols — Extent of welding, field weld indicator, contour
6. Finish symbols — C (chipping), G (grinding), M (machining), R (rolling), H (hammering)
7. Tail — Contains specification, process, or other reference
8. Specification, process, or other reference — Placed in the tail

Basic Weld Symbol Types

Groove Welds:

Other Welds:

Symbol Placement Rules

• Arrow side welds → Symbol placed on the lower side of the reference line (toward the reader)
• Other side welds → Symbol placed on the upper side of the reference line (away from the reader)
• Both sides → Symbols on both sides of the reference line

Supplementary Symbols

Pipe Welding: The Specialty Within the Craft

Pipe Welding Positions

Critical Pipe Welding Considerations

For satisfactory pipe welding, you must consider:

1. Chemical composition and thickness of the metal
2. Electrode material composition and size selection
3. Current, voltage, and wire feed rate determination
4. Joint preparation (edge preparation of the pipes)
5. Fixturing (holding pipes in position during welding)

Tack welds are used to hold the assembly in position. They should be:

• Approximately 1.5 inches (38 mm) long
• Projecting about 1/16 inch (1.6 mm) beyond the inner pipe wall

Flux-Cored Electrodes for Pipe Welding

Flux-cored E71T-1, 0.035-in. (1 mm) diameter wire provides:

• Continuous, medium energy, open arc
• Practical current range: 135–165 amps
• 25–30% less current than minimum MIG spray transfer for the same wire size
• Broader weld coverage than MIG
• Wider plasma arc, less focused, easier to control
• 30–50% faster fill passes than MIG short-circuit or SMAW

Best practice for pipe: Use MIG short-circuit welding for root welds to reduce the possibility of trapped slag, then switch to flux-cored for fill and cover passes where higher deposition rates are needed.

Nondestructive Testing (NDT) of Welds

Basic NDT Symbols (per ANSI/AWS 2.4-79)

The testing symbol uses the same reference line, arrow, and side-significance conventions as welding symbols.

AWS Letter Designations for Welding and Allied Processes

The following are standardized letter designations per ANSI/AWS A2.4-91 for all welding, cutting, brazing, soldering, and allied processes:

The Process Selection Decision Tree

How to Choose the Right Metal Joining Process

When you're standing in front of a joint that needs to be made, ask these questions in order:

Question 1: What joint strength do you need?

• Seal or electrical contact only → Soldering
• Moderate strength, capillary-filled joint → Brazing
• Full structural strength equal to base metal → Welding

Question 2: What base metals are you joining?

• Aluminum to aluminum → GMAW with Ar+He, GTAW with AC, or brazing with BAlSi fillers
• Copper to copper → Brazing with BCuP fillers, or GTAW
• Carbon steel → GMAW, FCAW, SMAW, or GTAW (based on thickness and quality requirements)
• Stainless steel → GMAW with Ar + 2–4% CO₂, GTAW, or PAW
• Dissimilar metals → Brazing (usually silver alloy) or EB welding

Question 3: What thickness are you joining?

• < 0.001 in. → Low-current PAW
• 24 gage to 11 gage → GMAW short-circuit or GTAW
• 10 gage to 1/4 in. → GMAW spray transfer (0.035 in. electrode)
• 1/4 in. to 1/2 in. → GMAW spray (0.045 in. electrode) or FCAW

• 1/2 in. → GMAW (0.062 in. electrode), FCAW, or multi-pass SMAW

• 1 in. → EB welding, SAW, or multi-pass arc welding

Question 4: What position will you weld in?

• Flat or horizontal → Any process; use flat/horizontal-specific electrodes for best deposition rates
• Vertical up → FCAW all-position electrodes (2–3× deposition rate vs. GMAW/SMAW)
• Overhead → FCAW all-position or SMAW
• Pipe (fixed position) → Root: GTAW or GMAW short-circuit; Fill: FCAW

Question 5: What is the production volume?

• One-off repair → SMAW or GTAW
• Moderate production → GMAW or FCAW
• High-volume production → Automated GMAW, SAW, PAW, LBW, or EBW

The Master Comparison: All Joining Processes at a Glance

What the practitioner Learned

Back on that factory floor, the practitioner made the call that saved the production line. He set down the SMAW electrode, reached for the GTAW torch, selected an argon + 2% CO₂ shielding gas, and used a low-carbon stainless filler rod matched to the base metal chemistry. The repair held. The line restarted.

But more importantly, the practitioner understood why it held. He understood that stainless steel's low thermal conductivity concentrates heat in the weld zone—requiring lower current and precise gas shielding to prevent warping and oxidation. He understood that the right electrode chemistry prevents chromium depletion at the grain boundaries. He understood that the process selection wasn't about what he was comfortable with—it was about what the metallurgy demanded.

That's the transformation this guide is designed to create in you.

Your Next Step

You now have the complete reference for metal joining processes—from the 361°F solidus of a eutectic tin-lead solder to the 50,000°F plasma column of a PAW torch.

Here's what to do with it:

1. Bookmark this guide. You'll return to the parameter tables, electrode selection charts, and shielding gas recommendations repeatedly throughout your career.

2. Pick one process you don't currently use and study its section in depth. If you're a GMAW welder, study FCAW vertical up—the deposition rate advantage alone could transform your productivity. If you're a stick welder, learn the GMAW spray transfer parameters that could cut your welding time by half.

3. Test the parameters. Set up scrap metal and dial in the exact settings from the tables in this guide. Listen to the arc. Watch the puddle. Feel the difference between optimal and sub-optimal parameters.

The welders, brazers, and solderers who build careers—the ones who never lack for work, who command premium rates, and who get called for the critical jobs—are the ones who understand all the joining processes, not just the one they use most often.

Which process will you master next?

This guide covers soldering, brazing, GMAW, FCAW, SMAW, GTAW, PAW, electron-beam welding, laser welding, pipe welding, weld symbols, nondestructive testing symbols, and the complete AWS process designation system. All technical data is referenced from ASTM, AWS, ANSI, BSI, CSA, DIN, and JIS standards for universal applicability across all regions and industries.

Context and scope

Every failed joint tells the same story: someone skipped the fundamentals.

an illustrative engineering practitioner once stared at the wreckage of a hydraulic manifold assembly — three days of precision work undone by a single brazed joint that cracked under thermal cycling. The filler metal was wrong. The flux was wrong. The clearance was wrong. the practitioner didn't lack skill. He lacked a system.

This guide is that system.

Whether you are an apprentice picking up a soldering iron for the first time, a seasoned fabricator selecting filler metals for aerospace assemblies, or an engineer specifying joints on a drawing, what follows is the most comprehensive breakdown of soldering, brazing, and their supporting sciences you will find in a single resource.

Read it once for understanding. Return to it for reference. It will serve you for the rest of your career.

The Architecture of Metal Joining

The Three Pillars

Metals can be joined without mechanical fasteners through three fundamental processes. Understanding where each one begins and ends is the first decision you will make on every project.

The 800°F Line: This is the defining boundary. Below it, you are soldering. Above it, you are brazing. This distinction is not arbitrary — it reflects a fundamental shift in how the filler metal interacts with the base metal, in the strength of the resulting joint, and in the equipment and techniques required.

The use of a filler metal and the application of pressure are considered optional in fusion welding. In soldering and brazing, the filler metal is essential — but the base metals are never melted.

This single fact changes everything about how you think about joint design.

Soldering — The Art of the Low-Temperature Bond

What Soldering Actually Is

Soldering employs lead- or tin-base alloys with melting points below 800°F. It is commonly referred to as soft soldering. The use of hard solders, silver solders, and spelter solders — which have silver, copper, or nickel bases and melting points above 800°F — crosses into brazing territory.

Here is the critical thing to understand: soldering is not designed for high mechanical strength. It provides a convenient joint, and in a great many instances it is used in combination with mechanical staking, crimping, or folding. The solder itself serves only to seal against leakage or to assure electrical contact.

If you need structural strength, you need brazing or welding. If you need a sealed, conductive, or leak-proof joint in a low-stress application, soldering is your tool.

The Story of the Leaking Heat Exchanger

Consider an illustrative engineering practitioner, tasked with assembling copper tube-to-header joints in a commercial heat exchanger. She had welding experience. She understood metallurgy. But she had never worked with solder before.

Her first instinct was to treat it like a weld — apply heat, feed material, let it cool. The joints looked acceptable visually. But within seventy-two hours under operating pressure, three of twelve joints were weeping.

The diagnosis was straightforward. the practitioner had not understood that soldering depends on three conditions working simultaneously:

1. Surface cleanliness — oxide-free metal
2. Proper temperature — hot enough to flow the solder, not so hot that the flux burns off
3. Capillary action — the solder must be drawn into the joint gap, not puddled on the surface

She had gotten the temperature right. But she had under-cleaned the surfaces and over-gapped the joints. Solder sat on the outside. The interior of the joint was starved.

When she rebuilt those joints with proper flux, proper cleaning, and proper fit-up, every joint held. Permanently.

The lesson: soldering is not a lesser form of welding. It is its own discipline, with its own physics.