Post on 14-Dec-2015
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
• 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