This lecture describes the important aspects of surface preparation and pretreatments for adhesive joining of aluminium parts; it illustrates the methods and equipment necessary to obtain good adhesive bonds. General background in production engineering and material science, some knowledge of mechanics and polymer science is assumed.

1. TALAT Lecture 4704
Surface Preparation and Application
Procedures
12 pages, 13 figures
Basic Level
prepared by
Lutz Dorn, Technische Universität, Berlin
Objectives:
− to describe the important aspects of surface preparation and pretreatments for
adjesive joining of aluminium parts
− to illustrate the methods and equipment necessary to obtain good adhesive bonds
Prerequisites:
− general background in production engineering and material science
− background in mechanics and polymer science
Date of Issue: 1994
 EAA - European Aluminium Association

2. 4704 Surface Preparation and Application Procedures
Table of Contents
4704 Surface Preparation and Application Procedures...............................2
4704.01 Surface Preparation of Metallic Parts to be Joined............................... 2
Surface layers of metallic parts to be joined ............................................................2
Surface treatment processes .....................................................................................3
Methods of surface pretreatment..............................................................................4
Surface pretreatments for aluminium.......................................................................4
Correlation between adhesive strength and surface roughness................................5
4704.02 Application Procedures ............................................................................ 6
Methods of applying adhesives................................................................................6
Equipment for working adhesives ...........................................................................7
Correlation between adhesive joint strength and adhesive layer .............................8
Methods of applying pressure ..................................................................................8
Correlation between adhesive joint strength and applied pressure ..........................9
Correlation between hardening temperature, hardening time and adhesive ..... joint
strength...................................................................................................................10
4704.03 Literature/References ............................................................................ 11
4704.04 List of Figures............................................................................................ 12
4704.01 Surface Preparation of Metallic Parts to be Joined
• Surface layers of metallic parts to be joined
• Surface treatment processes
• Methods of surface pretreatment
• Surface pretreatments for aluminium
• Correlation between adhesive strength and surface roughness
Surface layers of metallic parts to be joined
The surfaces of parts which have to be joint are treated prior to joining in order to have
an optimal adhesion force between joint part surface and the adhesive layer.
For this purpose it is absolutely necessary to first remove impurities in an undefined
layer thickness (i.e., dust, dirt, oil, grease, fat, water) and the inactive adsorption layer
created by foreign molecules (i.e., water, gases) (Figure 4704.01.01).
TALAT 4704 2

3. Surface Treatment
Impurities
Absorption Layer ( ~1·10-7 to 1·10-6 mm )
Reaction Layer ( ~ 1·10-6 to 1·10-5 mm )
Cold Worked Interface Layer ( ~ 2 to 5·10-3 mm )
Base Material
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Surface Layers of Metallic Parts to be Joined 4704.01.01
Training in Aluminium Application Technologies
Surface treatment processes
The whole surface treatment can be subdivided into a preparation, pretreatment and
after-treatment operation.
In general, an active surface for adhesives can be created by cleaning and increasing the
surface area, creation of lattice defects on the surface and changing the chemical
structure of the surface.
Surface Treatment Processes
Surface
Treatment
Surface Surface Surface
Preparation Pretreatment After-Treatment
Cleaning Mechanical Conditioning
Process
Chemical Application
Fitting
Process of Primer
Electrochemical
Degreasing Conservation
Process
Source : Kaliske
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Surface Treatment Process 4704.01.02
Training in Aluminium Application Technologies
In cases where a longer time-lapse is expected between the surface treatment and the
adhesive joining, the surface of the joining parts should be conserved, using for
example, a primer (Figure 4704.01.02).
TALAT 4704 3

4. Methods of surface pretreatment
Because of current ecological reasons, there is an increasing tendency to replace wet
chemical surface pretreatment processes by mechanical, electrical and dry chemical
pretreatment methods (Figure 4704.01.03).
Methods of Surface Pretreatment
Thermo
Grinding Corona
Brushing Plasma Gun
Mechanical Electrical
Pretreatment Pretreatment
Blasting Corona
Low-Pressure
Thinning * Plasma
Electrochemical
Flaming Pretreatment **
Thermal
Pretreatment Radiation
Silicoater Pretreatment
With Reactive Coating
Chemical Gases
Pretreatment * only for Plastics
With Chemical
Process
Source: Dorn, Wahono ** only for Metals
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Methods of Surface Pretreatment 4704.01.03
Training in Aluminium Application Technologies
Surface pretreatments for aluminium
The list in Figure 4704.01.04 and Figure 4704.01.05 depicts processes, including their
parameters, which have proven to be successful for use with aluminium materials.
Surface Pre-Treatments for Aluminium
1. Degrease and Roughen with Griding Cloth or Fine Blasting or Brushing
or
2. Degrease and CSA Etch ( Pickling Process )
Solution for approx. 1l: 650 g H2O ( Distilled )
75 g Na2Cr2O7
275 g H 2SO 4 ( Conc. )
Pre- Etching: Addition of 0.5 g/l Aluminium
+ 1.5 g/l Copper Sulphate
Etching: Temp. : 60 - 65 °C ( Replace Steam Loss )
Time: 30 min
Rinsing: Under Running Cold Water and Final Rinsing with Deionised
Water
Drying: At max. 65 °C
or
3. Degreasing and FPL Etching
Solution for 1l: 650 g H2O ( Distilled ) ; 50 g Na2Cr2O4 * 2 H2O ; 300 g H 2SO 4 ( Conc. )
Top up with Distilled Water to be Dissolved
Pre- Etching: 1.5 g AlCuMg 2pl/1l Etchant to be dissolved
Etching: Temp. : 65 °C ( Replace Water lost at Steam )
Time: 10 min
Rinsing and Drying: see CSA Etching
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Surface Petreatment for Aluminium I 4704.01.04
Training in Aluminium Application Technologies
TALAT 4704 4

5. Surface Pre-Treatments for Aluminium
or
4. Degrease and CSB Etching and Chromic Acid Anodising ( CAA )
Solution for Approx. 1 l: 40g CrO 3 Cathode: Stainless Steel ( V2A )
o o
Temp.: 40 C ( + 2 C)
- Time: from 0 to 40 V in 10 min
(replace water losses) 40 V 20 min
Rising: 10 min in distilled H2O from 40 to 50 V in 5 min
at 40 oC 50 V 5 min
Drying: at max. 40 oC
or
5. Degrease and FPL Etching and Phosphoric Acid Anodising ( PPA )
Solution for Approx. 1 l: 129g 85% H3PO 4 / 1 l H2O Cathode: Stainless Steel ( V2A )
Temp.: RT Time: from 0 to 40 V in 10 min
40 V 20 min
Rising: 22 min in distilled H2O from 40 to 50 V in 5 min
o
at 40 C 50 V 5 min
Drying: at max. 40 oC
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Surface Pretreatments for Aluminium II 4704.01.05
Training in Aluminium Application Technologies
When used as the only surface pretreatment method, FPL-etching is inferior to the CSA-
etching.
A CAA or PAA anodisation which follows, improves the long-term stability of the
adhesive joint. Compared to the oxide layer created by the CAA process, the PAA oxide
layer is thinner and more sensitive to mechanical damage, making it necessary to use a
protective primer.
It must be stressed here, that etching solutions must be handled and later deposited with
great care, especially since these may contain dangerous (partly cancerogenic)
substances and the used solution may be dumped only in so-called "special" dumps.
Correlation between adhesive strength and surface roughness
In view of the attainable strength of the adhesive joint and the notch sensitivity of most
aluminium materials, the mechanical surface pretreatment should deliver only a low
maximum roughness (Figure 4704.01.06).
TALAT 4704 5

6. Strength of Adhesive Metal Joints
Adhesive Joint Strength TB
50 100 150 !m
Max. Surface Roughness R
max
Correlation Between Adhesive Strength
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4704.01.06
Training in Aluminium Application Technologies and Surface Roughness
4704.02Application Procedures
• Methods of applying adhesives
• Equipment for working adhesives
• Correlation between adhesive joint strength and adhesive layer
• Methods of applying pressure
• Correlation between adhesive joint strength and applied pressure
• Correlation between hardening temperature, hardening time and adhesive
joint strength
Methods of applying adhesives
The possible application processes, depending on the consistency (solid or fluid) of the
adhesive and on the application method used (manual or mechanised), have been listed
in Figure 4704.02.01.
TALAT 4704 6

7. Working with Adhesives
Manual Application Application Methods MechanicalApplication
Solid Fluid Application Type Fluid Solid
Adhesives Adhesives Adhesives Adhesives
Coating
Immersion
Flowing
Rolling
Spraying
Sintering
Spreading
Melting
Placing
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Training in Aluminium Application Technologies
Methods of Applying Adhesives 4704.02.01
Equipment for working adhesives
A large variety of equipment (including robots) is available for working with the
adhesives. The working principle of a 2-component dosing and application machine is
illustrated in Figure 4704.02.02.
Pneumatic
drive
Comp. "A" Comp. "B"
Pump "A" Pump "B"
By-Pass By-Pass
Mixing Block
Mixing Pipe
Rinsing Medium Blowing Air Outlet
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Equipment for Working Adhesives 4704.02.02
Training in Aluminium Application Technologies
TALAT 4704 7

8. Correlation between adhesive joint strength and adhesive layer
Generally, an adhesive layer thickness of about 50 to 200 µm should be strived at
(exception: CA adhesives). It should be remembered that the adhesive layer thickness
depends, to a large extent, on the pressure applied during setting (curing)
(Figure 4704.02.03).
Strength of Metal Adhesive Joints
Adhesive Joint Strength τΒ
1 2 3
0 0.05 0.2 0.5 [mm]
Adhesive Layer Thickness d
alu Correlation Between Adhesive Joint Strength
Training in Aluminium Application Technologies and Adhesive Layer Thickness (Schematic) 4704.02.03
Methods of applying pressure
The possibilities of applying pressure during the hardening of the adhesive are
illustrated in Figure 4704.02.04 and Figure 4704.02.05.
Most adhesives require only a contact pressure (about 0.1 N/mm2). Adhesives, which
react chemically and produce by-products during the curing (setting) process (i.e., PF
adhesives), are an exception.
TALAT 4704 8

9. Working with Adhesives
Methods of Applying Pressure - I
Wrong Right
Pressure Application
Spot Weld, Weight-Loading Spring Loading
Screw, Rivet
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Methods of Applying Pressure - I 4704.02.04
Training in Aluminium Application Technologies
Working With Adhesives
Methods of Applying Pressure - II
Autoclave Method Tension Band
Pressure-Bag Method Vacuum Method
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Training in Aluminium Application Technologies
Methods of Applying Pressure - II 4704.02.05
Correlation between adhesive joint strength and applied pressure
The method of applying pressure from all sides, like in the autoclave method, should be
used preferably (Figure 4704.02.06).
TALAT 4704 9

10. Working with Adhesives
tB
tB
Adhesive Joint Strength
Adhesive Joint Strength
1
2
Pressure p Pressure p
Correlation Between Adhesive Joint Strength
and Applied Pressure
Curve 1: Pressure Applied from All Sides
Curve 2: Pressure Applied from Two Sides
alu Correlation Between Adhesive Joint Strength 4704.02.06
Training in Aluminium Application Technologies and Applied Pressure
Correlation between hardening temperature, hardening time and adhesive joint
strength
Usually a functional correlation exists between hardening time and hardening
temperature (Figure 4704.02.07).
Especially in the case of cold-hardening adhesives, the strength can be increased by
hardening at higher temperatures or by tempering (after the cold-hardening).
It should be noted that the temperature, pressure and time recommendations of the
manufacturer apply for the adhesive layer. Heating times for the joining parts and for the
adhesive equipment must be determined and considered.
TALAT 4704 10

11. Working With Adhesives
[h] [Nmm²]
40
30
B
20 25
T
22
Adhesive Joint Strength
Time t
10 20
8
6
15
4
3
2 10
1
20 40 60 80 [°C] 100
Temperature T
Source: Schlegel
Correlation Between Hardening Temperature,
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Hardening Time and Adhesive Joint Strength 4704.02.07
Training in Aluminium Application Technologies
4704.03 Literature/References
1. Brockmann, W., Hennemann, O.-D. und Kollik, H.: Surface properties and
adhesion in bonding aluminium alloys by adhesives. Int. J. Adhesion and Adhesives
2 (1982) pp. 33-40.
2. Kollek, H.: Adhäsionsmechanismen beim Kleben. Tagungsband Fertigungssystem
Kleben, S. 210-222, TU Berlin, 1984.
3. Comyn, J.: Kinetics and mechanism of environmental attack. In: Durability of
structural adhesives, Applied Science Publishers New York and London, 1983, pp.
85-131.
4. Kollek, H.: Die Beständigkeit von Metallklebverbindungen. Adhäsion 36 (1986) H.
6, S. 17-24.
5. Steffens, H.D. und Brockmann, W.: Die Alterungsgeschwindigkeit geklebter
Leichtmetallverbindungen unter Berücksichtigung neuer Oberflächen-
vorbehandlungsverfahren. Adhäsion 15 (1971) H. 10, S. 330-338.
6. Brockmann, W., Dorn, L. und Käufer, H.: Kleben von Kunststoff mit Metall.
Springer Verlag Berlin-Heidelberg - New York 1989
7. Schlegel, H.: Technologie der Metall-Verklebung. Plaste u. Kautsch. 5 (1958), S.
327-330
TALAT 4704 11

12. 8. VDI-Richtlinie 2229: Metallkleben. Ausgabe Juni 1979. VDI-Verlag Düsseldorf
1979.
9. Schmitz, B.H.: Auswirkungen der Feuchtigkeit auf das Atlerungs- und
Langzeitverhalten von Metallklebverbindungen. Dissertation RWTH Aachen, 1989.
Schweißtechnische Forschungsberichte 28, DVS Verlag Düsseldorf 1989.
10. Matz, C.: Alterungs- und Chemikalienbeständigkeit von strukturellen
Klebverbindungen im zivilen Flugzeugbau. TUB-Dokumentation (1984) H. 21, S.
307/318.
11. Kollek, H.: Untersuchungen der Einflüsse auf die Festigkewit von geklebten
Verbindungen. Maschinenmarkt 91 (1985) H. 75, S. 1479-1482.
4704.04 List of Figures
Figure No. Figure Title (Overhead)
4704.01.01 Surface Layers of Metallic Parts to be Joined
4704.01.02 Surface Treatment Process
4704.01.03 Methods of Surface Pretreatment
4704.01.04 Surface Pretreatments for Aluminium I
4704.01.05 Surface Pretreatments for Aluminium II
4704.01.06 Correlation between Adhesive Strength and Surface Roughness
4704.02.01 Methods of Applying Adhesives
4704.02.02 Equipment for Working Adhesives
4704.02.03 Correlation between Adhesive Joint Strength and Adhesive Layer
4704.02.04 Methods of Applying Pressure - I
4704.02.05 Methods of Applying Pressure - II
4704.02.06 Correlation between Adhesive Joint Strength and Applied Pressure
4704.02.07 Correlation between Hardening Temperature, Hardening Time and
Adhesive Joint Strength
TALAT 4704 12