SURFACES BARRIORS & CLEANING

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 • View Slides; • Read Notes, • Listen to lecture • 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 • 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.

Asperities - Elastic and Plastic effects. F • 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 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 each , the total area of contact (Ac) is given by Ac = n Da . • The area of contact also varies with the load imposed on the surface (F). Flattening of the asperities takes place as the load increases. F a. b. Flattening of the asperities. Initial contact at the asperities. Schematic view of two surfaces making contact at the 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, 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.

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) Yield Strength

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

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. - - - - - - - + + + + + + + + Oxides Oxygen ion Metal ion Oxide film Metal surface

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.

Questions

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).

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

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

Homework

SURFACES BARRIORS & CLEANING