Level 43
Level 44

28 - SURFACE PROCESSING OPERATIONS


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Importance of cleaning
The reasons include (1) to prepare the surface for subsequent industrial processing, (2) to improve hygiene conditions, (3) to remove contaminants which might chemically react with the surface; and (4) to enhance product appearance and performance.
Reasons for cleaning
Reasons for mechanical surface treatments include deburring, improving smoothness, adding luster, and enhancing surface properties.
Type of contaminants
Basic contaminant types are (1) oil and grease, (2) solid particles, such as metal chips, abrasive grits, shop dirt, and dust, (3) buffing and polishing compounds, and (4) oxide films, rust, and scale.
Chemical cleaning methods
The chemical cleaning methods can be categorized as follows (1) alkaline cleaning. (2) emulsion cleaning - water mixed in with organic solvents (oils) removes dirt, and then, the remnants of oils are cleaned with alkaline cleaning to remove all the stains. (3) solvent cleaning - solvents capable of dissolving oils and other shit are applied to the part in order to clean it. (4) acid cleaning and pickling - acid removes dirt and oil and oxides. and (5) ultrasonic cleaning - high frequency vibration causes cavitation in alkaline solutions, thus removing dirt and contaminants with the use of pressure waves.
Blast finishing
Blast finishing uses the high-velocity impact of particulate media to clean and finish a surface. The most well known of these methods is sand blasting, which uses grits of sand as the blasting media. Various other media are also used in blast finishing, including hard abrasives such as aluminum oxide and silicon carbide, and soft media such as nylon beads and crushed nut shells. The media is propelled at the target surface by pressurized air or centrifugal force. In some applications, the process is performed wet, in which fine particles in a water slurry are directed under hydraulic pressure at the surface.
Shot peening
In shot peening, a high-velocity stream of small cast steel pellets (called shot) is directed at a metallic surface with the effect of cold working and inducing compressive stresses into the surface layers. Shot peening is used primarily to improve fatigue strength of metal parts. Its purpose is therefore different from blast finishing, although surface cleaning is accomplished as a by-product of the operation.
Functions of shot peening
Shot peening is primarily used to improve the fatigue strength of metals by introducing cold working the metallic surface.
Tumbling
Tumbling, vibratory finishing, and similar operations comprise a group of finishing processes known as mass finishing methods. Mass finishing involves the finishing of parts in bulk by a mixing action inside a container, usually in the presence of an abrasive media. The mixing causes the parts to rub against the media and each other to achieve the desired finishing action. Mass finishing methods are used for deburring, descaling, deflashing, polishing, radiusing, burnishing, and cleaning. The parts include stampings, castings, forgings, extrusions, and machined parts. Even plastic and ceramic parts are sometimes subjected to these mass finishing operations to achieve desired finishing results. The parts processed by these methods are usually small and are therefore uneconomical to finish individually.
Vibratory finishing
Vibratory finishing was introduced in the late 1950s as an alternative to tumbling. The vibrating vessel subjects all parts to agitation with the abrasive media, as opposed to only the top layer as in barrel finishing. Consequently, processing times for vibratory finishing are significantly reduced. The open tubs used in this method permit inspection of the parts during processing, and noise is reduced.
Mass finishing
In mass finishing, parts are mechanically cleaned and deburred in bulk, usually in a barrel by the mixing action of an abrasive media.
Diffusion and ion implantation
Diffusion is a process in which atoms or molecules move across a boundary between two contacting materials. Ion implantation produces a similar result, but the process involves penetration of high-velocity ions into the surface of a substrate material.
Calorizing
Calorizing is the diffusion of aluminum into carbon steel, alloy steels, and the alloys of nickel and cobalt. The process is also known as aluminizing.
Diffusion processes
There are other diffusion processes in which corrosion resistance and/or high temperature oxidation resistance are main objectives. Aluminizing and siliconizing are important examples. Aluminizing, also known as calorizing, involves diffusion of aluminum into carbon steel, alloy steels, and alloys of nickel and cobalt. The treatment is accomplished by either (1) pack diffusion, in which workparts are packed with Al powders and baked at high temperature to create the diffusion layer; or (2) a slurry method, in which the workparts are dipped or sprayed with a mixture of Al powders and binders, then dried and baked.
Coating of metals
Reasons for coating metals include (1) to provide corrosion protection, (2) to enhance appearance, (3) to provide a specific color, (4) to increase electrical conductivity, (5) to increase electrical resistance, (6) prepare surface for subsequent processing, and (7) to rebuild worn or eroded surfaces.
Coating processes types
The common coating processes are (1) plating - coating a thin metallic layer on the surface of the substrate material, (2) chemical conversion coatings, such as anodizing, (3) vapor deposition processes such as PVD and CVD, (4) organic coating, such as painting, (5) porcelain enameling, and (6) thermal and mechanical coating processes.
Electroplating
Electroplating, also known as electrochemical plating, is an electrolytic process in which metal ions in an electrolyte solution are deposited onto a cathode workpart. The anode is generally made of the metal being plated and thus serves as the source of the plate metal. Direct current from an external power supply is passed between the anode and the cathode. The electrolyte is an aqueous solution of acids, bases, or salts; it conducts electric current by the movement of plate metal ions in solution. For optimum results, parts must be chemically cleaned just prior to electroplating.
Corrosion protection mechanisms
The basic mechanisms of errosion protection are (1) barrier protection, in which the coating simply covers the substrate to protect it, and (2) sacrificial protection, in which the coating metal corrodes sacrificially to protect the substrate.
Electroforming mandrel
A solid electroforming mandrel has certain geometric features, such as a taper, that permit the part to be removed. Parts are also sometimes removed by taking advantage of a difference in coefficient of thermal expansion.
Electroless plating
Electroless plating uses only chemical reactions to form the plating; electroplating uses electrolysis.
Conversion coating
A conversion coating is a thin coating produced by chemical reaction of the metallic surface. The most common conversion coatings are phosphates, chromates, and oxides. Immersion and spraying are the two common methods of exposing the metal surface to the reacting chemicals.
Uses of conversion coating
The important reasons for using a conversion coating process are (1) to provide corrosion protection, (2) to prepare the surface for painting, (3) to increase wear resistance, (4) to permit the surface to better hold lubricants for metal forming processes, (5) to increase electrical resistance of surface, (6) to provide a decorative finish, and (7) for part identification.
Annodizing
Anodizing uses electrochemical processing methods to convert the metallic surface. The best example is aluminum anodizing - the anodized aluminium layer is grown by passing a direct current through an electrolytic solution, with the aluminium object serving as the anode (the positive electrode). The current releases hydrogen at the cathode (the negative electrode) and oxygen at the surface of the aluminium anode, creating a build-up of aluminium oxide.
Physical vapor deposition
Physical vapor deposition (PVD) refers to a family of processes in which a material is converted to its vapor phase in a vacuum chamber and condensed onto a substrate surface as a very thin film.
Physical and chemical vapor deposition
In physical vapor deposition, the coating vapors are synthesized by heating the coating material and allowing it to condense as a thin film on the surface of the workpart. In chemical vapor deposition a coating is formed on a heated substrate by the chemical reaction or dissociation of vapors and/or gases; the reaction product nucleates and grows on the substrate surface.
Application of physical vapor deposition
Physical vapor deposition applications include: decorative coatings on trophies and automotive trim, antireflection coatings on optical lenses, deposition of metal in electronic connections, and coatings on cutting tool coatings.
PVD of cutting tools
The common coating materials deposited by PVD onto cutting tools are titanium nitride , titanium carbide , and aluminum oxide. TiN is probably the most common.
Advantages of chemical vapor deposition
Advantages of chemical vapor deposition include (1) capability to deposit refractory materials at temperatures below their melting or sintering temperatures, (2) grain size control, (3) process is performed at atmospheric pressure, and (4) good bonding to substrate surface.
CVD of titanium compounds
The most common titanium compounds which are coated onto cutting tools by chemical vapor deposition are: [1] titanium nitride (TiN) and [2] titanium carbide (TiC)
Organic coating
Organic coatings are polymers and resins, produced either naturally or synthetically, usually formulated to be applied as liquids that dry or harden as thin surface films on substrate materials. possible, their capacity to protect the substrate surface, low cost, and ease with which they can be applied.
Ingredients of organic coatings
The major ingredients are (1) binder, which are polymers, (2) dyes or pigments, which provide color, (3) solvents, and (4) additives such as surfactants and plasticizers.
Transfer efficiency
Transfer efficiency indicates how much of the organic coating liquid reaches the target surface - if a lot of paint stays deposited at the surface: the transfer efficiency is high; if a small portion of the paint stays deposited at the surface, the transfer efficiency is low.
Application of organic coating
The main methods of organic coating application include brushing and rolling, spraying, immersion (dip coating), and flow coating.
Drying and curing
Drying means evaporation of solvents in the organic coating liquid. Curing involves a chemical change in the organic resin (polymerization and/or cross-linking) which hardens the coating.
Powder coatings
The organic coatings discussed above are liquid systems consisting of resins that are soluble (or at least miscible) in a suitable solvent. Powder coatings are different. They are applied as dry, finely pulverized, solid particles that are melted on the surface to form a uniform liquid film, after which they re-solidify into a dry coating. Powder coating systems have grown significantly in commercial importance among organic coatings since the mid-1970s.
Porcelain coatings
Porcelain coatings are valued for their beauty, color, smoothness, ease of cleaning, chemical inertness, and general durability. Porcelain enameling is used in a wide variety of products, including bathroom fixtures (e.g., sinks, bathtubs, lavatories), household appliances (e.g., ranges, water heaters, washing machines, dishwashers), kitchen ware, hospital utensils, jet engine components, automotive mufflers, and electronic circuit boards. As a process, porcelain enameling consists of (1) preparing the coating material, (2) applying to the surface, (3) drying, if needed, and (4) firing.
Thermal surfacing processes
Thermal surfacing processes use thermal energy in various forms to apply a coating whose function is to provide resistance to corrosion, erosion, wear, and high temperature oxidation. The processes include: [1] Thermal spraying - molten and semi-molten coating materials are sprayed onto a substrate, where they solidify and adhere to the surface. [2] Hard facing - alloys are applied as welded deposits to substrate metals, a process in which fusion occurs between the coating and the base metal, just like in fusion welding. And [3] the flexible overlay process - capable of depositing a very hard coating material, such as tungsten carbide. In the flexible overlay process, a cloth impregnated with hard ceramic or metal powders and another cloth impregnated with brazing alloy are laid onto a substrate and heated to fuse the powders to the surface.
Mechanical plating
In mechanical plating mechanical energy is used to build a metallic coating onto the surface. In mechanical plating, the parts to be coated, together with plating metal powders, glass beads, and special chemicals to promote the plating action, are tumbled in a barrel. The metallic powders are microscopic in size - 5 micro meters in diameter; while the glass beads are much larger - 2.5 mm in diameter. As the mixture is tumbled, the mechanical energy from the rotating barrel is transmitted through the glass beads to pound the metal powders against the part surface, causing a mechanical or metallurgical bond to result. The deposited metals must be malleable in order to achieve a satisfactory bond with the substrate. Plating metals include zinc, cadmium, tin, and lead. The term mechanical galvanizing is used for parts that are zinc coated. Ferrous metals are most commonly coated; other metals include brass and bronze. Typical applications include fasteners such as screws, bolts, nuts, and nails.