SURFACES BARRIORS & CLEANING There are barriers to getting the atoms of one part close enough to the atoms of another part in order for the attractive forces considered earlier to take effect. In this module we will investigate some of these surface barriers and ways to clean them from the surface.

Surface Preparation Lesson Objectives Learning Activities 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 Intimate metal to metal contact is very important in solid state welding. Contact is hindered by three surface barriers: Asperities Oxides Surface contamination The most common barriers are listed above. Asperities, Oxides and surface contaminations.

Asperities Asperities are high and low areas of the metal surfaces. 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. Even the smoothest commercial surface available (obtained by lapping and polishing) when examined on an atomic level consists of surface ridges and valleys which when brought together touch at the peaks and thus result in something less than perfect contact between the mating surfaces. Thus, when bonding happens, it happens at the contact points of asperities. Smoother surfaces will have more number of contact points than rough surfaces, but the total contact efficiency is still less than 100%. We need to examine ways to force these asperities to “flow” into the opening or valleys between the asperity contacts. 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 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 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 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.

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 Intimate metal to metal contact is very important in solid state welding. Contact is hindered by three surface barriers: Asperities Oxides Surface contamination The most common barriers are listed above. Asperities, Oxides and surface contaminations.

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 In addition, most metals also react with the atmosphere to form oxide layers. These layers inhibit metal-to-metal contact and are a barrier to inter-diffusion of metallic atoms and thus also hinder the formation of metallic bonds as described previously. Some oxides are very tenacious and when a small layer is formed additional growth of oxide layer is inhibited as oxygen can not get through the layer to the metal below to cause further oxidation. The chromium oxide layer on stainless steels is an example of this. Other oxide layers are porous and allow the free passage of atmospheric oxygen to the metal substrate and continued growth of the oxide layer occurs (I.e. rust). Many oxide layers are hard an brittle and under deformation will fracture. This may open new paths for further oxidation, or the oxide layers may spall off exposing nascent metal which can facilitate bonding between tow parts. Reliable techniques for oxide remove are required for solid state bonding.

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 Intimate metal to metal contact is very important in solid state welding. Contact is hindered by three surface barriers: Asperities Oxides Surface contamination The most common barriers are listed above. Asperities, Oxides and surface contaminations.

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. In addition to asperities and oxide layers, metal surfaces often have other contaminants such as moisture, and grease etc. These foreign materials are usually bonded to the surface by the secondary bonding forces described earlier such as the dipole forces. These are not usually held with forces that are insurmountable, but they de require removal before good primary bonding between the metallic atoms can occur. Cleaning techniques need to be applied.

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 and Preparation 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 Surface preparation mainly deals with minimizing or removing the barriers to solid state welding prior to the welding operation. There are a number of methods used for surface preparation: (a) solvent and chemical cleaning to remove contaminants and thick oxide layers. (b) abrading and scratch brushing to remove contamination and oxide. (c) lapping and polishing to reduce the size of asperities. (d) ultraviolet radiation to remove the adsorbed hydrocarbons from the surface.

Chemical Cleaning Methods Dissolve contamination layers Etch away thick oxide layers.

Mechanical Cleaning Methods 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 Nascent Surface 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 Higher temperature can improve solid state bonding in two ways : (a) decreasing residual elastic stresses. (b) increasing diffusion to aid in the deformation and desolution of oxides and contaminants. Residual stress is harmful for solid state welding, breaking welds when external loads are removed. Residual stresses can be removed by applying heat.

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. Galvanic corrosion is a chemical reaction which occurs between two dissimilar metals when they are exposed to an electrolyte. Galvanic corrosion results in metal loss which gradually leads to weldment failure. For example, steel screws in brass are corroded in damp marine environments. Thermal stress may occur to the joint due to different thermal expansion coefficients between the two metals being joined. The metals expand at different rates under the influence of changing temperature. Thermal fatigue results when a joint is subjected to fluctuations in temperature over a period of time. Flaws can develop and propagate under cyclic thermal stress. At elevated temperatures the dissimilar metals in the joint may interdiffuse. This can result in porosity or brittle phase formation.

Diffusion Layers in Al-Cu Cold Bond after 500F for 60 days AWS Welding Handbook Diffusion Layers in Al-Cu Cold Bond after 500F for 60 days Thicker Layers May become Brittle

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