Aluminium Bonded Structures

1. Design Considerations for Aluminium Bonded Structures

When designing aluminium bonded structures, the adhesive bonding of aluminum extrusions is a common solution for achieving strong, lightweight structures without the localized heat input of welding or the stress concentrations of mechanical fasteners. This document details:

1. Aluminum Alloy Considerations: 6063 and 6082
2. T5 vs. T6 Tempers & Overaging
3. Hot-Cure vs. Cold-Cure Adhesives
4. Extrusion Geometry & Tolerances
5. Bond Gaps (Nominal Min/Max)
6. Key Design and Process Recommendations

The aim is to provide sufficient technical depth for engineers or designers responsible for specifying, analyzing, and manufacturing bonded aluminum structures or components.

2. Alloys for Aluminium Bonded Structures

2.1 Overview

6xxx-series aluminum alloys (Al-Mg-Si) develop strength through precipitation hardening, where controlled aging forms finely dispersed Mg2_22​Si precipitates. Two commonly used extrudable alloys for aluminium bonded structures are:

• 6063:
  • Noted for excellent extrudability, good surface finish, and moderate strength.
  • Widely utilized in architectural and light-to-medium duty structural applications.
• 6082:
  • Higher strength than 6063 and often comparable to 6061; typically used for more demanding structural members.
  • Good balance of extrudability and mechanical properties, suitable for frameworks, railings, or machine structures requiring higher load capacity.

2.2 Temper Selection (T5 vs. T6)

T5

• Processing: Cooled from extrusion temperature, then artificially aged for a shorter time or lower temperature than T6.
• Strength: Moderate yield (often 160–200 MPa for 6082-T5, around 130–170 MPa for 6063-T5).
• Advantages: Less risk of overaging if subjected to additional thermal cycles (e.g., hot-cure adhesives). In some cases, partial additional aging can move the alloy closer to T6 properties.

T6

• Processing: Solution heat-treated, quenched, and then peak-aged for higher strength.
• Strength: Yield can reach 250–300 MPa for 6082-T6, about 160–200 MPa for 6063-T6 (specific values depend on precise chemistry and cross section).
• Consideration: Extended heat exposure during hot-cure bonding can cause overaging and a subsequent decrease in yield/tensile strength. Testing or cycle optimization may be required.

2.3 Overaging

• Definition: Overaging happens when the alloy is exposed to temperatures or durations beyond those used for peak aging. The Mg2_22​Si precipitates grow larger and more widely spaced, reducing strength.
• Practical Impact:
  • 6063-T6 or 6082-T6 subjected to 160–180 °C for 1–2 hours (common for hot-cure adhesives) may experience a drop in yield strength.
  • Overaging is less of a concern with T5 extrusions, which are under-aged relative to T6, thus they can often tolerate an additional thermal cycle without a significant net strength loss.

3. Adhesive Bonding Fundamentals for Aluminium Bonded Structures

3.1 Hot-Cure vs. Cold-Cure Adhesives

Hot-Cure

• Typical Temperatures: ~120–180 °C.
• Advantages:
  1. Faster Cure: Typically completed in 30–120 minutes.
  2. Higher Glass Transition Temperature (Tg) for the bonded joint, beneficial for elevated service temperatures.
• Considerations:
  • Risk of Overaging in T6 extrusions if not carefully managed.
  • Requires ovens or heated tooling to maintain uniform temperature.

Cold-Cure

• Typical Temperatures: ~20–30 °C (room temperature) to mild heating (40–80 °C).
• Advantages:
  1. No Impact on Temper: Minimal risk of overaging or strength reduction in T6 extrusions.
  2. Simplicity: Does not require large ovens if parts are extremely large or numerous.
• Considerations:
  • Longer Cure Times: May take hours or days to reach full strength.
  • Lower Tg depending on adhesive chemistry, affecting high-temperature service capability.

3.2 Surface Preparation

Regardless of hot- or cold-cure adhesives, surface preparation is critical:

1. Cleaning & Degreasing: Removes contaminants (oils, dust).
2. Mechanical Abrasion: Light sanding or Scotch-Brite™ to increase surface roughness.
3. Chemical Treatments: Chromate or non-chromate conversion coatings, or phosphoric-acid anodizing (PAA) for higher-performance structural bonds.

Good surface prep maintains consistent bondline properties and long-term durability.

3.3 Bondline Control & Bond Gaps

• Bondline Thickness (or bond gap) significantly impacts adhesive strength in shear and peel:
  • Common Structural Epoxies: Often specified for bondlines of 0.1–0.3 mm.
  • Some structural adhesives can tolerate gaps up to 0.5 mm or more, though typical recommended maximum is around 0.3–0.4 mm to prevent voids or slump.
• Nominal Min/Max Gaps:
  • Minimum: ~0.05 mm (50 µm) for extremely tight-fitting components with controlled surfaces—though many adhesives prefer at least ~0.1 mm for consistent wet-out.
  • Nominal: ~0.2 mm is widely used for standard structural epoxies.
  • Maximum: ~0.3–0.5 mm for typical two-part epoxies before joint performance can degrade significantly. Some specialized, high-viscosity adhesives or gap-fill products can exceed this.

Note: Each adhesive manufacturer provides recommended bondline thickness to ensure consistent mechanical performance (shear, peel, and cleavage). Designers often incorporate bondline spacers (e.g., glass beads) or extruded features to maintain uniform thickness.

4. Extrusion Geometry & Tolerances

This section offers a deeper perspective on geometric design, tolerances, and the relationship to bonded joints. It addresses both open and closed profiles, wall thickness variation, and how these factors affect the bonding process.

4.1 Open vs. Closed Sections

4.1.1 Open Sections

• Examples: Channels, angles (L-sections), T-sections, I-beams.
• Properties:
  • Torsional Stiffness: Generally lower for the same weight than a closed shape.
  • Access for Bonding: Easier to apply adhesive and fixtures on internal surfaces.
• Typical Uses: Secondary framing, stiffeners, or areas not subject to high torsional loads.

4.1.2 Closed (or Semi-Closed) Sections

• Examples: Square/rectangular/round tubes, multi-void box sections, partially enclosed “C” shapes with small apertures.
• Properties:
  • Torsional Rigidity: Higher for a given mass.
  • Adhesive Application: Limited internal access; may require careful planning of injection or partial sealing if bonding internal walls.
• Typical Uses: Main load-bearing members (e.g., frames, chassis) where bending and torsion resistance are key.

4.2 Wall Thickness and Internal Features

• Uniform vs. Variable Thickness:
  • Uniform: Straightforward die design, consistent metal flow. Often used for standard tubes or channels.
  • Variable: Thicker walls in high-stress regions, thinner walls elsewhere. Improves weight efficiency but requires sophisticated die design.
• Internal Webs or Stiffeners:
  • Can reduce local buckling and increase bending or torsional capacity. Ensure adhesives can be reliably applied if internal seams or folds need to be bonded.

4.3 Dimensional Tolerances and Straightness

• Standards:
  • 6063 & 6082 Extrusions typically follow EN 755 (EU) or ASTM B221 (US).
  • These standards define permissible dimensional deviations (wall thickness, overall width/height), straightness (bow), and twist.
• Typical Ranges:
  • ±0.3–0.5 mm on critical dimensions for medium cross-sectional sizes.
  • Bow or twist can be a few millimeters over a 2–3 meter length.
• Post-Extrusion Stretching: Often used to reduce internal stresses and improve straightness. Nevertheless, final assembly fixtures or light machining may be needed for tighter tolerance fits before bonding.

4.4 Bonding-Related Geometry Considerations

1. Mating Surfaces:
   • Design flanges or overlapping surfaces that ensure a consistent bond gap (e.g., 0.2 mm).
   • Avoid extremely thin contact lips (<0.1 mm) that may deform under clamp force.
2. Corner Radii:
   • Die Radii typically match the extrusion’s maximum flow rate. Sharp corners impede flow and can induce stress concentrations.
   • For bonded joints, minimal corner radii (e.g., 1× wall thickness) facilitate uniform adhesive coverage and reduce peel stresses at edges.
3. Bondline Access:
   • For closed profiles where internal bonding is desired, consider adding small holes or ports for adhesive injection, or design the assembly to bond external flanges.
4. Gussets and Ribs:
   • Integrate gussets where loads concentrate, ensuring the bonding surfaces remain parallel and maintain the intended bond gap.
   • Overly complicated internal geometry can make surface preparation and adhesive application difficult.

5. Structural Analysis and Validation

5.1 Load Path and Section Modulus

• Closed vs. Open: Closed sections provide superior torsional/bending stiffness per unit mass. Designers typically reserve open sections for less critical or secondary members.
• Analysis Tools:
  • FEA (Finite Element Analysis) using beam, shell, or solid elements, depending on required fidelity.
  • Joint Modeling: Adhesive interfaces can be simulated with cohesive elements or contact surfaces that define shear and peel strengths.

5.2 Overaging Checks (Hot-Cure Only)

• Verification: If a T6 extrusion (6063-T6 or 6082-T6) undergoes hot curing at 160–180 °C, consider conducting post-cure tensile tests or hardness checks to ensure the final mechanical properties meet design assumptions.

5.3 Bondline Testing

• Lap Shear and Peel Tests: Standard coupon tests (ASTM D1002, D3167, etc.) confirm adhesive performance under the chosen cure schedule, bond gap, and surface prep routine.
• Environmental Durability: Include cyclic moisture, thermal cycling, or salt spray if relevant to the service environment.

6. Implementation Guidelines

1. Select Alloy & Temper
   • Choose 6082 where higher strength is required, especially in T6 condition (with caution for hot-cure overaging).
   • Consider 6063 for more intricate profiles or where moderate strength at T5/T6 is sufficient.
2. Decide on Hot vs. Cold Cure
   • Hot-Cure: Higher Tg, faster production, but potential overaging of T6.
   • Cold-Cure: Simpler equipment, no overaging risk, but slower production and possibly lower final Tg.
3. Define Bond Gap Requirements
   • Typically 0.1–0.3 mm for structural epoxies.
   • Incorporate geometric or mechanical spacers to achieve consistent gap.
   • Validate the maximum permissible gap from the adhesive manufacturer (often 0.3–0.5 mm maximum for standard epoxies).
4. Ensure Effective Surface Prep
   • Consistent cleaning, deoxidizing, and (if needed) conversion coating or PAA.
   • Control humidity and handling to avoid contamination prior to bonding.
5. Design Fixturing & Process
   • Maintain alignment under cure.
   • Accommodate thermal expansion if hot-cure adhesives are used (e.g., partially compliant fixtures).
   • Provide stable supports for cold-cure processes with prolonged clamp times.
6. Conduct Testing & Validation
   • Perform prototype or coupon-level tests for mechanical properties post-cure (including joint shear strength, peel, and final alloy hardness or yield checks if T6 is used).
   • Evaluate environmental or fatigue performance if the application involves cyclical loads or harsh conditions.

7. Summary

Designing aluminium bonded structures using 6063 or 6082 extrusions involves a coordinated approach to:

1. Alloy & Temper Selection: Balancing the need for high strength (6082) or superior extrudability/surface finish (6063), and deciding on T5 vs. T6 with respect to potential overaging.
2. Adhesive Cure Method: Weighing the faster, higher-temperature stability of hot-cure adhesives against the simpler but slower cold-cure approach, and its effect on final alloy properties.
3. Geometry & Tolerances: Ensuring extrusion design (open vs. closed sections, wall thickness, corner radii) aligns with required stiffness, strength, and consistent bondline thickness.
4. Bond Gaps: Typically 0.1–0.3 mm is advisable for structural epoxies; up to 0.5 mm may be acceptable in certain cases, subject to adhesive specifications.
5. Structural Analysis & Testing: Finite element simulations plus physical verification (coupon tests, environmental conditioning) confirm that the bonded structure meets mechanical and durability requirements.

By adhering to these guidelines, engineers can create lightweight, strong, and reliable aluminum bonded structures using 6063 or 6082 extrusions, with robust adhesive bonding that meets performance and manufacturing objectives.

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