M — Airframe (Cellule)Chapter 2 · 53 practice questions

Chapter 2: Aircraft Structures — Welding & Plastics

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Chapter 7: Aircraft Structures — Welding & Plastics

Overview

This chapter covers the principles, procedures, and safety considerations for welding and plastic repair in aircraft maintenance. Welding is a critical process for repairing metallic airframe components, while plastic and composite repairs are increasingly common in modern aircraft structures. The chapter addresses material selection, process parameters, defect identification, and quality assurance for both disciplines, following guidelines from AC 43.13-1B and manufacturer-specific documentation.

Diagram — Aircraft Structures — Welding & Plastics Welding & Plastic Repair — Aircraft Structures Weld Joint Cross-Section — Oxyacetylene Process Base Metal (Aluminum 6061-T6) Base Metal (Aluminum 6061-T6) Filler Rod 4043 HAZ HAZ Heat Heat Torch Filler Rod Weld Joint Preparation — Butt Joint Configuration 1-2 mm gap Plastic Repair — Layered Composite Patch Original Structure — Fiberglass/Epoxy Laminate Scarf angle 10:1 Ply 1 Ply 2 Ply 3 Ply 4 Vacuum Bag — 25 inHg Heat Lamp 250°F cure Quality Assurance — Inspection Methods Visual Inspection Cracks, porosity Dye Penetrant Surface defects Radiography Internal voids Ultrasonic Delamination Per AC 43.13-1B Chapter 4 — Welding & Chapter 6 — Plastics HAZ Filler Base Metal

Key Concepts Explained in Detail

Welding Fundamentals

Weld defects: causes and clues Weld Defects: Causes and Indications POROSITY Weld bead ● Trapped gas Causes: • Contamination (oil, grease, moisture) • Lack of shielding gas • Current too low • Travel speed too fast Indications: • Small cavities visible on surface • Detection by dye penetrant or radiography → Repair: complete removal and re-welding LACK OF PENETRATION Joint cross-section Bead ▲ Lack of root fusion Causes: • Current too low • Travel speed too fast • Incorrect electrode angle • Inadequate joint preparation Indications: • Narrow and high bead • Lack of fusion at the root → Detection by radiography CRACKS Bead with cracks Longitudinal cracks Causes: • Cooling too fast • High residual stresses • Sensitive alloys (e.g., 7075-T6) • Contamination of the weld pool Indications: • Visible lines on surface (dye penetrant) • Detection by magnetic particle inspection → Post-weld heat treatment required UNDERCUT (GROOVE) Bead profile Undercut Causes: • Current too high • Travel speed too slow • Incorrect electrode angle • Excessive heat input Indications: • Depression in the bead • Wide and flat bead → Reduce amperage References: AC 43.13-1B Ch.6 | CARs Standard 571 | AME Training Transport Canada
Welding method selection Welding Process Selection Selection based on material, thickness and application — AME Training Transport Canada PROCESS MATERIALS THICKNESS APPLICATIONS Oxyacetylene (OAW) Neutral flame required Neither carburizing nor oxidizing 4130 steel tubing Carbon steel (low alloy) Thin to medium walls 0.020 – 0.125 in (0.5 – 3.2 mm) 4130 fuselage tubes and frames Engine mounts Structural repairs TIG (GTAW) Tungsten electrode non-consumable Inert gas: argon AC: aluminum / DC: steel, titanium Aluminum alloys Stainless steel Magnesium Titanium Thin to medium sheets 0.020 – 0.250 in (0.5 – 6.4 mm) High precision Structural repairs Aluminum fuel tanks Hydraulic lines Magnesium housings Titanium parts MIG (GMAW) Consumable electrode Shielding gas Continuous deposition Steel Stainless steel Aluminum (less common) Medium to thick sections > 0.125 in (3.2 mm) Non-critical applications Secondary repairs Supports, brackets Spot welding Electrical resistance Pressure + current Aluminum sheets Steel sheets Light assemblies Thin sheets 0.010 – 0.063 in (0.25 – 1.6 mm) Skin panels Bulkheads Non-structural fastening ⚠ Aluminum 7075-T6: welding not recommended (hot cracking) — Magnesium: Class D extinguisher required — Titanium: strict argon shielding
Weld pool and heat-affected zone Weld Pool and Heat-Affected Zone 1. Torch/Arc Progression Base metal Progression 2. Weld Pool and Filler Rod Weld pool Filler rod The filler metal melts and flows into the weld pool to create the weld bead. 3. Cooling and Heat-Affected Zone (HAZ) Bead HAZ Buffer zone Cooling HAZ (Heat-Affected Zone) • Metal not melted, but structure altered by heat Rapid cooling → Martensite in steel (excessive hardening) Post-weld heat treatment (stress relieving) is often necessary to reduce residual stresses. Weld pool HAZ Buffer zone Unaffected base metal
Welding — Common Defects and Inspection Welding — Common Defects and Inspection GOOD WELD Uniform reinforcement Full penetration Smooth surface, no porosity. Smooth transition to base metal. POROSITY Trapped gas Cause: Contamination (oil, grease) or lack of shielding gas. CRACKS Structural failure Cause: Rapid cooling, excessive thermal stress. LACK OF PENETRATION Unfused root Cause: Current too low, excessive speed or gap. SLAG INCLUSION Non-metallic residue Cause: Insufficient cleaning between weld passes. UNDERCUT Edge groove Cause: Amperage too high, incorrect electrode angle. PREPARATION AND MATERIAL SELECTION Critical cleaning: Remove paint, oil, grease, and oxides. Filler rod: Must match base metal (e.g., 4130). Alignment: Max misalignment 25% of thickness for butt joints. Neutral flame (Oxy-acetylene) to avoid carburization/oxidation. ACCEPTANCE CRITERIA AND REPAIR NDT Inspection: Dye penetrant (surface) or Radiography (internal). Critical defects: Cracks, porosity, and inclusions are REJECTED. Repair: Complete removal of defective weld required. Heat treatment: Often required post-weld (stress relief).

Filler Rod Selection

Filler rod selection matrix Filler Rod Selection Matrix Base metal → filler metal compatibility for aerospace structural repairs Base Metal Recommended Filler Rod Notes / Precautions Aluminum 2024 Al-Cu alloy 4043 (Al-Si) Good fluidity, crack-resistant TIG with alternating current (AC) Oxide cleaning via "cathodic" effect Aluminum 6061 Al-Mg-Si alloy 4043 (Al-Si) Prevents hot cracking Post-weld heat treatment recommended to restore strength Aluminum 7075 Al-Zn alloy 5356 (Al-Mg) ⚠ Hot cracking Welding generally DISCOURAGED High risk of hot cracking Steel 4130 Chromium-molybdenum steel Matching rod Same composition as base metal Oxyacetylene welding: neutral flame Stress relieving required after welding Stainless Steel Series 300 / 400 Matching rod Same grade stainless steel TIG with direct current (DC) Shielding gas: argon Cast Iron Cast parts Nickel-based Pure nickel rod Preheating recommended Slow cooling to avoid hardening Magnesium and titanium: matching rods with special precautions (Mg flammability, Ti gas shielding)

The choice of filler rod is critical to weld integrity and must match the base metal's mechanical properties and corrosion resistance. Key rules include:

  • Aluminum Alloys:
  • 2024 aluminum: Use 4043 (Al-Si) filler rod. 2024 filler is rarely used due to cracking susceptibility. 4043 provides good crack resistance and strength.
  • 6061-T6 aluminum: 4043 filler is also suitable. The weld deposit is slightly stronger in tension than the overaged heat-affected zone (HAZ).
  • 7075-T6 aluminum: This high-strength alloy is prone to hot cracking. 4043 filler is not recommended; 5356 (Al-Mg) filler is preferred. In general, welding 7075 is discouraged due to cracking risk.
  • Steel Alloys:
  • 4130 steel: Use filler rod matching the base metal (4130) to ensure similar strength and corrosion resistance. Mild steel filler is too weak; stainless steel or aluminum fillers are incompatible.
  • Stainless steel: Use filler rod matching the base metal to maintain corrosion resistance and strength.
  • Cast Iron: Requires a nickel-based filler rod to match material properties and prevent cracking.
  • Magnesium Alloys: Use filler rod matching the base metal. Fire precautions are critical due to magnesium's flammability.
  • Titanium: Use filler rod matching the base metal. Inert gas shielding (argon) is mandatory to prevent oxidation.

Welding Processes and Parameters

GTAW AC vs DC current and oxide cleaning TIG Welding: Alternating Current vs Direct Current ALTERNATING CURRENT (AC) Waveform — electrode positive half-cycle + + + + − − − Positive half-cycle: cleaning action Negative half-cycle: moderate penetration Tungsten electrode Aluminum Oxide layer (Al₂O₃) ✓ Cleaning action: breaks up oxide layer Recommended for aluminum and magnesium DIRECT CURRENT (DC) Waveform — constant direct current Constant current (electrode negative) Deeper penetration into base metal No oxide layer cleaning action Tungsten electrode Steel / Titanium ✓ Deep penetration Steel, titanium, magnesium — with argon Shielding gas: pure argon for all TIG processes. Magnesium requires fire precautions (Class D extinguisher).
  • Gas Tungsten Arc Welding (GTAW/TIG):
  • Aluminum: Use AC (alternating current) to provide cleaning action that breaks up the tenacious oxide layer during the electrode-positive half-cycle. DC provides deeper penetration but no cleaning.
  • Steel, Titanium, Magnesium: Use DC with argon shielding gas.
  • Flame Type (Oxyacetylene): Use a neutral flame for aluminum to avoid contamination. Carburizing (reducing) flames add carbon; oxidizing flames add oxygen.
  • Filler Rod Diameter: For thin materials (e.g., 0.040-inch 2024-T3 aluminum), use 1/16 inch filler rod to avoid overheating and burn-through. Larger rods require excessive heat.
  • Shielding Gases:
  • Argon: Most common for GTAW of aluminum, steel, magnesium, and titanium. Provides good arc stability and cleaning action.
  • Helium: Can be used but less common; provides higher heat input.
  • Carbon dioxide/oxygen: Not suitable for GTAW.

Pre-Weld Preparation

Butt joint misalignment limit (25% thickness) Maximum Misalignment of a Butt Joint Rule: misalignment ≤ 25% of sheet thickness Reference Weld D = 25% e D = 25% e e = 0.063 in Sheet A Sheet B Perfect alignment Alignment Rule Maximum misalignment: 25% of thickness (e) Example: 0.063 in sheet: Max D = 0.063 × 0.25 Max D = 0.020 in ⚠ BEYOND: Local stress concentrations Risk of cracking and fatigue failure Transport Canada — AME Training (Aircraft Maintenance Engineer) — Chapter 2: Aircraft Structures D ✓ Acceptable: D ≤ 25% of e ✗ Unacceptable: D > 25% of e → stresses
  • Cleaning: Surfaces must be free of paint, oil, grease, corrosion products, and moisture. Contaminants cause porosity, lack of fusion, and hydrogen embrittlement. For aluminum, cleaning to bare metal is critical.
  • Joint Fit-Up: Butt joints in sheet metal must have maximum misalignment of 25% of sheet thickness (e.g., 0.020 inch for 0.063-inch 2024-T3). Greater misalignment causes stress concentrations.
  • Soldering: Surfaces must be clean and free of oxides, grease, and dirt. Use mechanical abrasion (emery cloth) and chemical cleaning (flux). Grease prevents bonding; heating alone is insufficient.

Welding Defects and Causes

Post-Weld Treatments and Inspection

Post-weld NDT method selection NDT Method Selection After Welding Non-destructive testing — Selection based on defect type and material WELDING Repair completed Cooling VISUAL INSPECTION Obvious surface defects Weld bead profile NDT SELECTION Based on defect type and material ACCEPT / REJECT Per criteria from manufacturer / SRM Comparison Matrix — Defect → NDT Method DEFECT TYPE NDT METHOD PRINCIPLE APPLICATION ADVANTAGE / LIMIT Surface cracks Open porosity Lack of fusion on surface Penetrant (PT) Dye penetrant Penetrant applied to surface, then developer — the crack appears by contrast Aluminum, steel, titanium, magnesium ✓ Simple, economical ✗ Surface only ✗ Surface must be clean Internal defects Closed porosity Tungsten inclusions Radiography (RT) X-rays or gamma rays Radiographic film placed behind the weld — defects appear as lighter areas Thick welds, heavy structures ✓ Detects internal defects ✗ Heavy equipment ✗ Safety measures Subsurface defects Lack of fusion Weld root undercut Ultrasonic (UT) High-frequency sound waves Probe emits ultrasound that reflects off discontinuities Composites, welds, multi-layer ✓ Detects internal voids ✓ Portable ✗ Requires smooth surface Non-ferrous metals Aluminum, titanium, alloys Eddy Current (ET) Electromagnetism Induced currents in the metal — defects disturb the flow Detection of fine cracks ✓ Fast, non-contact ✗ Surface must be accessible The choice of NDT method depends on the type of defect sought, the material, and accessibility — Reference: manufacturer's standards (SRM)
  • Post-Weld Heat Treatment: For 4130 steel engine mounts, stress relief reduces residual stresses that cause distortion or cracking. It does not increase hardness, improve corrosion resistance, or fix porosity.
  • NDT Methods:
  • Dye Penetrant (PT): Most appropriate for surface-breaking defects in non-porous materials (e.g., stainless steel exhaust stacks, weld toes). Minimum required NDT after weld repair for crack detection.
  • Radiography (RT): For internal defects; not minimum for visible weld areas.
  • Ultrasonic (UT): For subsurface defects; difficult on thin walls.
  • Eddy Current (ET): For non-ferrous metals or conductivity changes.
  • Corrective Action for Porosity: Remove defective weld completely and re-weld. Sealant is not a structural repair; increasing amperage will not eliminate porosity.

Special Material Precautions

  • Magnesium: Highly flammable — use Class D fire extinguisher (dry powder). Preheating increases fire risk.
  • Titanium: Reactive at high temperatures — requires inert gas shielding (argon) to prevent oxidation.
  • Cast Iron: Requires nickel-based filler; preheating may be needed but is not the primary requirement.
  • 7075 Aluminum: Generally not recommended for welding due to cracking risk.

Removing Welded Components

To remove a welded-on steel component from aluminum without damaging the aluminum: Drill out the weld and carefully chisel the remaining weld. Grinding removes base metal; heat distorts or weakens aluminum; chemical strippers do not affect steel welds.

Plastic and Composite Repair

Composite repair inspection path Composite Repair Inspection Composite panel repaired (laminate) Repair scarfed Structural ply Repair ply NDT Inspection Tests 1. Visual Look for: cracks, porosity, discoloration 2. Tap test Hammer or coin: dull sound = possible delamination 3. Ultrasonic (if required) Detects internal voids and non-visible delaminations ✓ ACCEPTANCE • No defects detected • Repair conforms • Barcol hardness OK • Return to service Document per CARs Std 571 ↻ REWORK • Minor localized defect • Surface porosity • Sand and re-inspect • New tap test required Respect 2:1 ratio ✗ ESCALATION • Extensive delamination • Major internal voids • Repair out of limits • Consult SRM Major repair or replacement re-inspection AC 43.13-1B Ch. 3 • CARs Std 571 • Limits per manufacturer SRM Acceptance Rework Escalation Note: Visual inspection alone cannot detect internal voids. Tap test is subjective — an experienced technician is required.
Plastic/composite bonding repair Plastic/composite repair by bonding 1. Preparation Cleaning and drying Moisture check (moisture meter) Scarf bevel 2:1 Roughen surface 2. Adhesive mixing Resin + hardener Exact ratio per specs Homogeneous mix Limited pot life 3. Patch stacking Oriented layup 4. Clamping Vacuum bagging Uniform pressure Eliminates voids 5. Curing Embedded thermocouple Temperature monitoring Heating blanket If ambient temp < 10 °C Barcol durometer (hardness) Ultrasonic inspection (voids) Vacuum bagging layers Vacuum ring Release film Breather fabric Peel ply Composite patch Original structure Key rules • Overlap ratio 2:1 • No. of plies = original structure • Limits: SRM manual • AC 43.13-1B Ch. 3 • CARs Standard 571 • Prepreg: storage -18 °C REGULATORY REFERENCES AC 43.13-1B Ch. 3 (composites) AC 43.13-1B Ch. 6 (welding) CARs Standard 571 (limits) AME Training — Transport Canada — Composite structural repair

Material Types and Adhesive Selection

  • Thermoplastics (e.g., ABS, polycarbonate, nylon, acrylic):
  • Solvent cementing: Chemically fuses thermoplastics by dissolving the surface. Critical step: surfaces must be clean and free of contaminants.
  • Two-part epoxy: Requires mechanical abrasion or chemical etching for low-surface-energy plastics like ABS. Failure under light load often indicates insufficient surface preparation.
  • Two-part acrylic adhesive: Cloudy bond line on polycarbonate is often due to moisture absorption during cure (polycarbonate is hygroscopic).
  • Thermosets (e.g., epoxy, polyester): Use epoxy or polyester resins with appropriate hardeners.

Surface Preparation

  • Nylon: Roughen with 80-grit sandpaper and clean with MEK (methyl ethyl ketone). Epoxy primer does not bond well; heat can distort.
  • Polycarbonate: Clean and dry thoroughly before adhesive application.
  • General: Cleaning is critical for all plastic repairs. Roughening may help but is not sufficient alone.

Composite Repair Procedures

  • Repair Limits: Specified in the manufacturer's Structural Repair Manual (SRM). AC 43.13-1B is a general guide; the SRM is authoritative.
  • Delamination Repair: First step is scarf repair — remove damaged area and restore laminate structure. Simply injecting epoxy or applying heat will not restore integrity.
  • Patch Design:
  • Overlap ratio: 2:1 (patch diameter twice damage diameter) for adequate load transfer.
  • Ply count: Same number of plies as original to restore strength.
  • Ply orientation: Oriented layup — match original ply orientation.
  • Peel Ply: Applied over wet layup; removed after cure to create a textured surface for subsequent bonding. Does not provide UV protection or reduce cure time.
  • Breather Cloth: Allows air to be drawn out from under the vacuum bag, ensuring proper vacuum application.
Vacuum bag layup cross-section Vacuum Bagging Stack-Up Cross-Section of Stack-Up Rigid tool / mold sealant sealant Composite laminate (repair) Release film — perforated Breather cloth — allows air extraction Peel ply — textured surface Vacuum Bag Uniform pressure (vacuum) V To vacuum pump vacuum port ~0.1 mm 1-2 mm Functions 1 Peel Ply Creates a textured surface for subsequent bonding. 2 Breather Cloth Allows air to be evacuated from under the vacuum bag. 3 Release Film Prevents resin from sticking to the vacuum bag. 4 Vacuum Bag Applies uniform pressure to compact the part and eliminate voids. Typical pressure: 1 bar (14.7 psi) Peel ply Breather cloth Release film Vacuum bag Laminate M-AIRFRAME ch2 — Aircraft Structures
  • Release Film: Prevents resin from bonding to the vacuum bag, allowing easy removal.
  • Vacuum Bag: Applies even pressure to compact layers and remove air voids.

Cure Monitoring and Defects

  • Cure Temperature: Use a thermocouple embedded in the repair for accurate monitoring. Thermometers measure ambient; infrared measures surface only.
  • Cure Hardness: Use a Barcol hardness tester for quantitative measurement. Visual inspection and touch are subjective.
  • Moisture Check: Use a moisture meter for quantitative measurement. Visual inspection, touch, or applying heat are unreliable.
  • Voids: Most likely caused by insufficient resin failing to fill gaps between fibers. Detect with ultrasonic inspection.
  • Incorrect Mix Ratio: Too much resin or too little hardener prevents curing. Old resin may cure slower; high temperature accelerates cure.
  • Prepreg Storage: Must be stored at low temperatures (typically -18°C or below) to prevent premature curing. Room temperature accelerates curing; dry environment is insufficient.

Cure Temperature Adjustments

Composite resin systems require specific cure temperatures for proper cross-linking. If ambient temperature is lower than specified (e.g., 10°C vs. 24-hour room temperature cure):

  • Use a heat blanket to maintain correct temperature.
  • Proceeding without heat results in incomplete cure and weak bond.
  • Lower temperatures require longer cure times, not shorter.
  • Adding extra adhesive does not fix the cure issue.

Important Formulas, Regulations, and Procedures

Regulatory References

  • AC 43.13-1B: Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair. Chapters 3 (Composite Repairs), 4 (Cleaning), and 6 (Welding).
  • CARs Standard 571: Maintenance and Repair of Aircraft Structures.
  • Manufacturer's Structural Repair Manual (SRM): Authoritative source for repair limits and procedures.

Key Formulas and Standards

  • Butt Joint Misalignment: Maximum 25% of sheet thickness.
  • Overlap Ratio: 2:1 (patch diameter : damage diameter).
  • Ply Count: Same as original structure.
  • Filler Rod Diameter: 1/16 inch for 0.040-inch aluminum.

Critical Procedures

  1. Pre-Weld Cleaning: Remove all contaminants to bare metal.
  2. Filler Rod Selection: Match base metal or use specified alloy (e.g., 4043 for 2024/6061, 5356 for 7075).
  3. Shielding Gas: Argon for GTAW of aluminum, steel, magnesium, titanium.
  4. Post-Weld NDT: Dye penetrant minimum for surface crack detection.
  5. Composite Cure Monitoring: Thermocouple for temperature, Barcol tester for hardness.
  6. Moisture Check: Moisture meter before adhesive application.

Common Relationships Between Concepts

  • Filler Rod and Base Metal Compatibility: Mismatch causes cracking (e.g., 4043 on 7075), galvanic corrosion (mild steel on stainless), or weak joints.
  • Heat Input and Bead Shape: Low heat → high, sharp bead; high heat → wide, flat bead; fast travel → narrow bead; slow travel → wide bead.
  • Contamination and Defects: Oil/grease → porosity; moisture → hydrogen embrittlement; inadequate shielding → spatter/dirty puddle (steel) or oxidation/discoloration (titanium).
  • Surface Preparation and Bond Strength: Low-surface-energy plastics (ABS, nylon, polycarbonate) require mechanical abrasion or chemical etching for epoxy adhesion. Solvent cementing works by dissolving the surface.
  • Cure Conditions and Quality: Incorrect temperature or mix ratio prevents proper cross-linking. Prepreg requires cold storage to prevent premature cure.
  • NDT Method and Defect Type: Dye penetrant for surface cracks; ultrasonic for internal voids; radiography for internal defects in thicker sections.
  • Material Properties and Weldability: High-strength alloys (7075) are prone to cracking; reactive metals (titanium, magnesium) require special shielding or fire precautions; cast iron needs nickel-based filler.

Diagram

Practice this chapter

Reinforce Aircraft Structures — Welding & Plastics with 53 Transport Canada–style practice questions, matched to your weak areas.

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