SURFACES BARRIORS & CLEANING

Surface Preparation

Lesson ObjectivesWhen you finish this lesson you will understand:• Barriers to Surface Bonding• Overcoming the Barriers• Some Metallurgical Effects of Concern

Learning Activities1. View Slides; 2. Read Notes, 3. Listen to lecture4. Do on-line workbook

Keywords: Asperities, Oxides, Surface Contamination, Elastic, Plastic, Surface Cleaning, Galvanic Corrosion, Brittle Phases

• Intimate metal to metal contact is very important in solid state welding. Contact is hindered by three surface barriers:

– Asperities

– Oxides

– Surface contamination

Barriers to solid state welding

Barriers to Solid State Welding

Asperities• Asperities are high and low

areas of the metal surfaces.• Asperities are caused by

bends, warps, or machining or grinding marks.

• No common industrial processing can produce asperities less than 10A in size, so perfect contact is not achieved.

Two surfaces are in contact at their asperities.

Barriers to solid state welding

Asperities

Asperities - Elastic and Plastic effects.

• Surfaces make contact only at the asperities .

• Localized pressure at the asperities is high

• As a result, the asperities undergo elastic and (under higher loads) eventually plastic deformation.

An external force F is applied to increase contact area

F

F

Asperities - Elastic and Plastic Effects

• Asperities act like springs, storing elastic strain energy.

• Plastic deformation permanently increases the contact area.

• Even after plastic deformation there is some elastic strain energy stored within the asperities which can push apart the welded surfaces.

Magnified top view of the contactarea

Side view ofthe asperities

Elastic deformation

Plastic deformation

Asperities - Area of Contact• Initially, mechanical contact is

established at the asperities.• If n is the number of asperities

and a is the area occupied by each , the total area of contact (Ac) is given by Ac = n a .

• The area of contact also varies with the load imposed on the surface (F). Flattening of the asperities takes place as the load increases.

Schematic view of two surfaces making contact at the asperities

F

F

a. b.Flattening of theasperities.

Initial contact atthe asperities.

Area of Contact• For 100% contact, Ac= A, where A is the total

cross sectional area.

• Since the load is sustained by the yielding of asperities,

y n a = y Ac = F, where

y = the yield strength of the material.

• For 100% contact, F = ynA = yA. The load must be raised to the point where gross yielding occurs throughout the material.

Yield Strength

• The elastic strain energy stored in compressed asperities is proportional to the yield strength squared.

• Reduced yield strength is very helpful in producing solid state welds. Increased Temperature helps (This is warm welding - covered later)

• Intimate metal to metal contact is very important in solid state welding. Contact is hindered by three surface barriers:– Asperities– Oxides– Surface contamination

Barriers to solid state welding

Barriers to Solid State Welding

Oxides• Most metals react with atmospheric

oxygen to produce oxide films which form a layer upon the metallic surface.

• Oxide films are hard and brittle, as are oxide-oxide bonded surfaces.

• Sufficient deformation is needed to break the oxide films; once these are broken, nascent metal is exposed to help bonding.

Metal

Metal Oxide

Barriers to solid state welding

Oxides• Form on the metal surface

due to the metal’s reaction with atmospheric oxygen.

• Metal surfaces (except gold) are covered with oxide film.

• The thickness of oxide films increases with temperature and time (prior processing important).

• Usually oxides are hard and brittle.

+ + + ++ ++ +

- - - - - --

Oxygen ion Metal ion Oxide film

Metal surface

• Intimate metal to metal contact is very important in solid state welding. Contact is hindered by three surface barriers:– Asperities– Oxides– Surface contamination

Barriers to solid state welding

Barriers to Solid State Welding

Surface Contamination.• Apart from oxides, metal surfaces are often

covered with grease, gas molecules , water vapor, and other surface contaminants.

• Contaminants adhere to the surface by secondary bonding.

• Surface contaminants form a coating on the metal surface and reduce metal-to-metal contact.

• For good bonding these contaminants must be removed or minimized.

Barriers to solid state welding

The following are conditions employed to minimize the barriers to solid state welding:

• Surface preparation

• Stress

• Heat

• Plastic deformation

Overcoming the Barriers to Solid State Welding

Two primary methods:

• Chemical

• Mechanical

Surface Cleaning Method

Surface Cleaning and Preparation

• Solvent and chemical cleaning

• Abrading and metal brushing

• Lapping and polishing

• Ultraviolet radiation

• High Frequency

Surface Preparation

Chemical Cleaning Methods

• Dissolve contamination layers

• Etch away thick oxide layers.

• Abrading and metal brushing (scratch brushing).

• Lapping and polishing (either mechanically or electrochemically).

Surface Cleaning Method

Mechanical Cleaning Methods

The following are conditions employed to minimize the barriers to solid state welding:

• Surface preparation

• Stress

• Heat

• Plastic deformation

Overcoming the Barriers to Solid State Welding

• Plastic Deformation– At asperities - increases contact area.

• Nascent Surface– Clean, oxide and contamination free

surface is easily bonded.

Stress

• Stress causes:– Plastic deformation.– Increases surface contact and

asperity deformation.– Interfacial shear stresses (beneficial

to disrupt oxide films).– Upsetting, increase in interfacial

surface, and increased nascent surface.

Normal stress

Shear stress

Stress

The following are conditions employed to minimize the barriers to solid state welding:

• Surface preparation• Stress• Heat• Plastic deformation

Overcoming the Barriers to Solid State Welding

• Relieves elastic residual stresses

• Increases diffusion – Increase in the microscopic movements

– Dissolution of oxides and contaminants.

– Increase the interaction range of atoms

– Metallurgical effects can occur

Heating

Metallurgical Effects

Metallurgical effects can be classified according to the type of metal pair being welded

• Similar metal pairs. (Usually Minimal Effects)

• Dissimilar metal pairs. (Consider Further)

Dissimilar metal weldments may be subject to a number of negative effects as-welded or in service including:– Galvanic corrosion - occurs to the more chemically

active of the two metals when exposed to an electrolyte.– Thermal stress - occurs due to the different thermal

expansion coefficients of the welded metal pair subjected to temperature variation.

– Thermal fatigue - may be induced by fluctuating temperature causing fluctuating thermal stresses.

– High temperature effects - interdiffusion may cause porosity or brittle phase formation.

Metallurgical Effect

Diffusion Layers in Al-Cu Cold Bond after 500F for 60 days

Thicker Layers May become Brittle

AWS Welding Handbook