Aluminium extrusions are precision-toleranced profiles — their straightness, dimensional accuracy, and temper condition are specified to tight tolerances that heat input can permanently compromise. Welding aluminium extrusions introduces heat-affected zones that reduce strength by 30% to 50% in the weld region, distort the profile, and require post-weld straightening that may still leave residual bow. Mechanical fasteners preserve the extrusion integrity but require drilled holes, add hardware weight, and concentrate load at the fastener points. Structural epoxy bonding joins aluminium extrusions with full-strength bonds at the extrusion surface — no heat, no holes, no strength reduction from thermal modification of the temper condition.
Why Heat Is the Enemy of Precision Aluminium Extrusions
Aluminium extrusions are produced in heat-treatable alloys — 6061-T6, 6063-T5, 7075-T6 — where the T-designation indicates the temper condition achieved through controlled age hardening. The temper provides the yield strength and hardness that make the alloy useful for structural applications. Welding heats the metal above the solid solution temperature in the weld zone and above the over-aging temperature in the heat-affected zone, destroying the precipitation-hardened microstructure. The result is a weld zone and HAZ with substantially reduced yield strength — as low as 50% of the T6 base material strength — that cannot be recovered without full re-solution treatment and aging, which would require removing the assembly and heat treating it as a unit.
Distortion from welding is a related problem. The thermal gradient in a welded aluminium profile creates differential thermal expansion and contraction that bows the extrusion. Straightening after welding introduces residual stress and may not fully recover the original geometry.
For precision frames, enclosures, structures, and assemblies built from aluminium extrusions, welding destroys the tolerance and strength properties that made the extrusion the right choice. Adhesive bonding eliminates both problems.
Bond Performance on Aluminium Extrusions
Structural epoxy on properly prepared 6061-T6 achieves lap shear strength of 20 to 25 MPa on the aluminium substrate — adhesive failure is not the governing mode with correct surface preparation; cohesive failure in the adhesive or yielding of the aluminium substrate occurs first. The bond does not reduce the extrusion’s T6 temper condition because no heat is applied. The cross-section through the bonded joint carries its design load without the strength reduction that welding would impose.
For common extrusion joint geometries — end-to-end splice joints, L-joints, T-joints — the bond area is defined by the overlap length and the contacting face width. Joint design must provide sufficient overlap to develop the required load at the allowable shear stress with the appropriate safety factor. Typical structural adhesive applications use safety factors of 3 to 5 on lap shear strength for non-redundant structural bonds.
The mating surface fit. Extruded profiles have dimensional tolerances that result in gaps between mating surfaces. Structural epoxy tolerates gaps up to 1 to 3 mm with negligible strength penalty — within this range, the adhesive fills the gap and the bond strength is controlled by adhesive shear strength, not contact area reduction. For gaps larger than 3 mm, gap-filling paste adhesive or a bond line shim maintains bond strength.
If you need lap shear data on specific aluminium alloy and temper combinations, long-term performance data, and joint design guidelines for structural bonding of aluminium extrusions, Email Us — Incure provides application engineering support for aluminium extrusion bonding programs.
Surface Preparation for Aluminium Extrusion Bonding
Solvent degrease first. Extrusions from the mill have release agent, extrusion die lubricant, and handling oil on the surface. All of these are bond inhibitors. Solvent degreasing with isopropanol or MEK removes volatile organics. Wipe-on, wipe-off with clean cloths in one direction; never wipe back over a cleaned area.
Abrade to expose fresh aluminium oxide. The native oxide on extruded aluminium is thick, weakly adherent, and contaminated from the extrusion process. Abrading with 80-grit abrasive cloth or a Scotch-Brite pad removes the surface oxide and exposes fresh, adherent oxide. Solvent degrease again after abrasion to remove the abrasive residue.
Etch primer for service in moisture. For assemblies that will see humidity, condensation, or outdoor exposure, etch primer or chromate conversion coating (Alodine) applied after abrasion provides corrosion-resistant adhesion promotion that dramatically improves long-term wet durability. Bare abraded aluminium oxide begins to hydrate within hours; primed surface is stable for days to weeks.
Joint Geometry Considerations
Extrusion joint geometries that maximize bond area and minimize peel:
Lap splice joints transfer axial load in shear and are the most efficient structural adhesive joint. Overlap length should be 10 to 20 times the extrusion wall thickness to develop the full adhesive shear capacity while keeping peel stress at the joint end manageable.
Corner joints (L and T) require adequate flange width for bond area and a gusset or corner bracket for additional load transfer. The adhesive bond alone in a corner joint is subject to peel when the load pulls the corner open; gussets address this.
Sleeve joints for end-to-end connections insert a machined sleeve into the extrusion bore and bond the sleeve in the bore. The bonded sleeve transfers axial and bending loads through the joint — a higher-performance joint than a simple butt bond.
Contact Our Team to discuss structural epoxy selection, surface preparation, and joint design for aluminium extrusion bonding in your frame and assembly application.
Visit www.incurelab.com for more information.