Transcript Document 7331225

SURFACES BARRIORS & CLEANING
Surface Preparation
Lesson Objectives
When you finish this lesson you will
understand:
• Barriers to Surface Bonding
• Overcoming the Barriers
• Some Metallurgical Effects of
Concern
Learning Activities
1. View Slides;
2. Read Notes,
3. Listen to lecture
4. Do on-line workbook
Keywords: Asperities, Oxides, Surface Contamination, Elastic,
Plastic, Surface Cleaning, Galvanic Corrosion, Brittle Phases
Barriers to solid state welding
Barriers to Solid State Welding
• 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
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.
Asperities
Two surfaces are in
contact at their 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.
F
F
An external force F is applied
to increase contact area
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.
Side view of
the asperities
Elastic deformation
Plastic deformation
Magnified top view of the contact
area
Asperities - Area of Contact
F
• Initially, mechanical contact is
established at the asperities.
• If n is the number of asperities
and Da is the area occupied by
F
each , the total area of contact
a.
b.
(Ac) is given by Ac = n Da .
Initial contact at Flattening of the
asperities.
the asperities.
• The area of contact also varies Schematic view of two surfaces
with the load imposed on the making contact at the asperities
surface (F). Flattening of the
asperities takes place as the load
increases.
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,
sy n Da = sy Ac = F, where
sy = the yield strength of the material.
• For 100% contact, F = synDA = syA. 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)
Barriers to solid state welding
Barriers to Solid State Welding
• 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
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 Oxide
Metal
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
Barriers to solid state welding
Barriers to Solid State Welding
• 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
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.
Overcoming the Barriers to Solid
State Welding
The following are conditions employed to
minimize the barriers to solid state
welding:
•
•
•
•
Surface preparation
Stress
Heat
Plastic deformation
Surface Cleaning Method
Surface Cleaning and Preparation
Two primary methods:
• Chemical
• Mechanical
Surface Preparation
•
•
•
•
•
Solvent and chemical cleaning
Abrading and metal brushing
Lapping and polishing
Ultraviolet radiation
High Frequency
Chemical Cleaning Methods
• Dissolve contamination
layers
• Etch away thick oxide
layers.
Surface Cleaning Method
Mechanical Cleaning Methods
• Abrading and metal brushing
(scratch brushing).
• Lapping and polishing (either
mechanically or
electrochemically).
Overcoming the Barriers to Solid
State Welding
The following are conditions
employed to minimize the
barriers to solid state welding:
•
•
•
•
Surface preparation
Stress
Heat
Plastic deformation
Stress
• 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
Overcoming the Barriers to Solid
State Welding
The following are conditions
employed to minimize the
barriers to solid state
welding:
•
•
•
•
Surface preparation
Stress
Heat
Plastic deformation
Heating
• 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
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)
Metallurgical Effect
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.
AWS Welding Handbook
Diffusion Layers in Al-Cu
Cold Bond after 500F for 60
days
Thicker Layers May
become Brittle