Pretreatments for the Optimised Adhesion of Aluminium in Aerospace Applications G.W.Critchlow Department of Materials, Loughborough University, UK

Greece, Daedalus and Icarus – 1300 to 1000 BC

Metal technology Development SPF Ti Chemical milling 7475 Al-alloy sheet/plate Interference bolting Laser beam welding LBW 6013/6056 Al-alloys New bonding technology Age forming High Speed Machining 2X24 Al-alloys LVER* riveting SPF Al Premium Al-castings 7349 Al-alloy extrusions Split mandrel cold working EB and extended LBW Al-Li alloys, Ti-alloys Al-alloys: 2024HDT, 7055HF Large die forging (7085) New coatings Larger panels New assembly concepts A300/A310 A318 A320 A330/340 A /-500 A380 Technology introduction High strength Al-castings 7150 Al-alloy plate (wing) Automated Lok bolting Split sleeve cold working

Durability or permanance of the bonded structure is a function of: Alloy  Al 7075-T6 clad/bare  Al 2024-T3 clad/bare  Adhesive  FM 73 Film Adhesive  Coupling agent/primer  BR 127 Primer  Surface condition

Ideally, surface should be:  free from contamination  wettable  rough  mechanically stable  hydrolytically stable

Pretreatment options  Mechanical (degrease, grit-blast, laser texturing, cryoblasting)  Chemical (acid etch, conversion coating)  Electrochemical (anodic oxidation)

Standard Processes – 40/50V CAA

CAA 40/50V Anodising Process Vapour Degrease Optional Seal Rinse- DI Water Acid Etch Rinse- DI Water Alkaline Clean Rinse- DI Water AnodiseOptional Rinse Hot Air Dry Isoprep 44 FPL Etch CAASeal Solvent Wipe MEK 60g/l Concentration (pH 9.3 –10.45) 10 minute Submersion 60°C 60g/l – Na 2 Cr 2 O 7 166ml/l – H 2 SO 4 1.5g/l – Al 30 minute Submersion 60°C 30.5 – 50.0g/l 35 – 45 minute Anodising 40°C DI Water 15 minute Submersion Min 96°C

J.L. Cotter and R. Kolhler, Int.J.Adhesion & Adhesives, 1981

Disadvantages of CAA  There are health & safety and environmental issues associated with the use of hexavalent chromium.  Process is highly complex and time consuming to completion – costly.

3. Alternatives to CAA

Alternatives to Hexavalent Chromium  Trivalent Chromate First and Second Generation Systems  Non-Chromated Conversion Coating Sol-Gel (BAC 5663) Silane Coupling Agents  Non-Chromated Anodising Phosphoric Acid Anodising, PAA (BAC5555) Boric Sulphuric Acid Anodising, BSAA (BAC 5632) Phosphoric Sulphuric Acid Anodising (Airbus, Germany)  Friction stir welding – alternative to bonding  Use of Composites – alternative to aluminium

3.5 ACDC process

AC offers:  Degrease only required  Open porous structure  Rapid processing DC offers:  Thick, compact film

Anodic reactions  2Al +6OH - → Al 2 O 3 + 3H 2 O + 3e -  Al → Al e -  4OH - → 2H 2 O + O 2 + 4e -  2SO 4 2- →S 2 O e - Cathodic reactions  2H + + 2e - →H 2  SO H + + 6e - → S + 4H 2 O  Al 2 O 3 + 6H + + 6e - → 2Al + 3H 2 O  Al e - → Al

Objective To produce an AC/DC film in a single electrolyte at a constant temperature with approx 300 nm of outer porous oxide and >1 micrometre of inner dense oxide.

Variable parameters ParameterACDC Electrolytemixed sulphuric-phosphoric Voltage (V)50Hz, 10-20<25 Time (s) Temperature (°C) RT-50

Surface after AC anodising 5s AC anodising 30s AC anodising degreased-only

120s AC anodising 240s AC anodising

AlPO 4 Al 2 O 3 AlOOHAl(OH) 3 Slow dissolution

Influence of DC voltage

10v AC, 120 s: 20v DC,