Transcript Slide 1
Mechanical Fastening Processes Brazing Brazing – joining process where just the filler material’s heat is raised to its melting temperature to join materials First used as far back as 3000 to 2000 B.C. Filler material has a lower melting temperature than components that you are joining. Mechanical Fastening Processes Brazing Brazing uses a flux in order to prevent oxidation during the joining process Factors that affect braze strength Joint clearance Joint area The bond formed between the filler material and the base metal Proper use of flux Brazing Various Methods Torch – uses oxyfuel thru a torch Furnace – parts preloaded with consumable inserts then placed in furnace for uniform heating Induction – Heats thru use of High freq AC Dipping – dips entire base material into molten filler metal bath (used for very small parts) Mechanical Fastening Processes Soldering Main difference between soldering and Brazing lies in Temperature. Uses primarily tin-lead as filler material Main use is for electronics which can’t withstand extreme heat of brazing. Uses same techniques as brazing as well as that of Wave soldering – very useful for circuit boards. (b) Adhesive Bonding Types of Adhesives Natural adhesives-starch, dextrin, soya flour, and animal products Inorganic Adhesives-sodium silicate and magnesium oxychloride Synthetic organic adhesives-thermoplastics, thermosetting polymers (most important in manufacturing) Adhesive Bonding Electronically Conducing Adhesives Developed to replace lead-based solder, added metal particles (40% to 70%) Used in electronics such as calculators, TVs, and LCDs Adhesive Bonding TABLE30.4 Type Comments Applications Acrylic Thermoplastic; quicksetting; toughbondat room temperature; twocomponent; goodsolvent chemical andimpact resistance; short worklife; odorous; ventilationrequired Fiberglass andsteel sandwichbonds, tennis racquets, metal parts, plastics. Anaearobic Thermoset; easytouse; slowcuring; bonds at room temperature; curingoccurs inabsenceof air, will not curewhereair contacts adherents; onecomponent; not goodonpermeablesurfaces Closefittingmachineparts suchas shafts andpulleys, nuts andbolts, bushings andpins. Epoxy Thermoset; oneor twocomponent; toughbond; Metal, ceramicandrigidplasticparts. strongest of engineeringadhesives; hightensileandlow peel strengths; resists moistureandhightemperature; difficult touse Cyanoacrylate Thermoplastic; quicksetting; toughbondat room temperature; easytouse; colorless. “Crazyglue.”™ Hot melt Thermoplastic; quicksetting; rigidor flexiblebonds; easytoapply; brittleat lowtemperatures; basedon ethylenevinyl acetate, polyolefins, polyamides and polyesters Bonds most materials. Packaging, bookbinding, metal canjoints. Pressuresensitive Thermoplastic; variablestrengthbonds. Primer anchors adhesivetoroll tapebackingmaterial, areleaseagent onthebackof webpermits unwinding. Madeof polyacrylateesters andvarious natural andsynthetic rubber Tapes, labels, stickers. Adhesive Bonding Surface Prep, Process Capabilities, and Applications Surface Preparation Surface must be free of dirt, dust, oil and any other contaminants Porous or rough surface is desirable, improve adhesion Process Capabilities Used to bond metallic and non-metallic material Do not subject to peeling Adhesive Bonding Surface Prep, Process Capabilities, and Applications Applications Used in most industries Allows bond between dissimilar metals and reduces vibration and noise Distributes the load eliminating local stresses that usually result from mechanical fasteners External appearance is unaffected Thin and fragile components can be bonded without weight increase Adhesive Bonding Design for Adhesive Bonding Ensure that joints are subjected only to compressive, tensile, and shear forces, not to peeling or cleavage Mechanical Fasteners Preferred over other methods because of the following: Ease of manufacturing Ease of assembly and transportation Ease of disassembly, maintenance, part replacement, or repair Ease in creating designs that have movable joints Lowers cost Mechanical Fasteners Hole Prep Using different methods to create the hole produces different characteristics Hole with some residual stress is desirable, improves fatigue life Threaded Fasteners Common bolts and screws Rivets Most common method of permanent and semipermanent mechanical joining Works by placing rivet through hole and deforming Mechanical Fasteners Other Fastening Methods Metal Stitching and stapling Seaming Folding two pieces of material together Crimping Similar to ordinary stapling Physically forcing on piece onto another Design Use fewer, but larger, rather than many small Fit between joined parts Use standard sizes and keep holes away from edges Mechanical Fasteners Joining Plastics, Ceramics, and Glasses Joining Thermoplastics Thermal Methods Using heat to soften or melt two pieces at the interface to ensure good bond Many way to apply the heat Adhesive Bonding Uses adhesives previously discussed to attach the plastics Joining Thermosets Joined using threads, mechanical fasteners, solvent bonding, co-curing and adhesive bonding Joining Plastics, Ceramics, and Glasses Joining Ceramics and Glasses Ceramics Treat surface with coating that is easily bonded Braze tungsten carbide and titanium carbide Join during their primary shaping process Glasses Are bonded easily Done by softening and pressing two pieces together and cooling Can be bonded to metals due to diffusion of metal ions into the glass sturcture. 32.7 Economics of Joining Ops Cost from highest to lowest of various Joining Operations Highest: Brazing, bolts, nuts, and other fasteners. Intermediate: Riveting and adhesive Low: Seaming and crimping Closer look at Brazing Costs Manual Brazing: Furnace Brazing: Basic equipment costs $300 $50,000+ for automated systems Wide range from $2,000 for batch furnaces to $300,000 for continuous vacuum furnaces. Induction Brazing: $10,000 for small units Closer look at Brazing Costs Resistance Brazing: Dip Brazing $2,000 to $200,000+, depends on equipment which may be computer controlled Infrared Brazing: $1,000 for simple units $10,000+ for larger units $500 to $30,000 Diffusion Brazing: $50,000-$300,000 Closer look at Brazing Costs High End Convection CAB Brazing Furnace High End Vacuum Furnace Surface Technology How do we react to surfaces? Touch Roughness Texture Scratches Sight Waviness Color Reflectivity Surface Technology Jargon: “Surface Integrity” Describes the physical, chemical and mechanical characteristics of surfaces Influences on surfaces Friction and wear of tools, molds, dies and of the products themselves. Effectiveness of lubricants during manufacturing and throughout the products life. Thermal and electrical conductivity of contacting bodies. Surface Technology Influences on surfaces (cont.) Appearance and geometric features of the part and their role in subsequent operations, such as welding, soldering, adhesive bonding, painting, coating and corrosion. Crack initiation as a result of surface defects like roughness, scratches, seams, and heat–affected zones. These can lead to weakening and premature failure of a part. Chapter 33 Surface Roughness and Measurements of Friction, Wear and Lubircation 33.2 Surface Structure and Integrity Metal surfaces generally contain several layers 1ST Layer: Bulk Metal (substrate) has a structure that depends on the composition and processing history. 2ND Layer: Above the bulk metal is “Surface Structure” that is plastically deformed and work hardened more so than the Bulk Metal. The depth & properties of this layer depend on processing method and friction. 33.2 Surface Structure and Integrity 3RD layer: Is the “Oxide Layer.” The only way to avoid this is to keep the metal in an oxygen free environment or work with a noble metal (gold, platinum). 4TH layer: is the “Absorbed Layer.” This layer absorbs gas and moisture. 5TH layer: Is the outermost layer that may be covered with contaminants such as dirt, dust, grease, various residues and other environmental pollutants. 33.2 Surface Structure and Integrity Surface Integrity Surface integrity describes topological, physical, and chemical as well as their mechanical, metallurgical properties and characteristics. Various surface defects can weaken the Sruface Integrity Cracks: internal, external and microscopic Craters: shallow depressions Heat-affected zones Surface Integrity Surface Defects (continued) Inclusions: small nonmetallic elements Intergranular attack: the weakening of grain boundaries Laps, Folds and Seams: overlapping of material during processes Metallurgical transformations: microstructural changes caused by temperature cycling of the material Pits: shallow surface depressions from chemical or physical attack Surface Integrity Surface Defects (continued) Residual stresses: Either by tension or compression and causes nonuniform deformation and temperature distribution. Splatter: Small resolidified molten metal particles on the surface. Surface Plastic deformation: Severe surface deformation caused by high stress from friction, tool and die geometry, worn tools, and processing methods. Surface Texture are characteristics that define a surface. These can be separated into four categories which are: •Flaws or defects: random irregularities, such as scratches, cracks, holes, depressions, seams, tears, or inclusions •Lay (directionality): direction of the predominant surface pattern •Roughness: closely spaced, irregular deviations on a small scale •Waviness: recurrent deviation from a flat surface Surface roughness is usually described in two ways: 1. Arithmetic mean value (R a) – the schematic illustration of a rough surface 2. Root-mean roughness height – was known as RMS 3. Maximum roughness height (R t) – the height from the deepest trough to the highest peak; indicates how much material has to be removed to obtain a smooth surface Coordinates used for surface-roughness measurements R a = ( a + b + c + d + ….)/ n R q = sq rt [(a2 + b2 + c2 + d2 + ….)/n] Standard terminology and symbols used to describe surface finish Some symbols used to represent surface finish When measuring surface roughness, instruments called surface profilometers are used. Profilometers are tipped with a diamond stylus which usually has a 10 micon diameter. The way profilometers are used is they travel over the surface in a straight line recording about 10 to 15 roughness irregularities. The distance travel maybe anywhere from .08 to 25 mm. The most common distance used is .8 mm. There are other ways of measuring surface measurement. This is done by using either an Optical-interference microscope or an atomic-force microscope. •Optical-interference microscopes: shine a light against a reflective surface and record the interference fringes that result from the incident and its reflective waves •Atomic-force microscopes (AMFs): it’s a very fine surface profilometer with a laser. There are several theories that explain friction. Two of the more commonly accepted theories are the adhesion theory and the abrasion theory. •Adhesion theory: theory that states that the contact of two items is actually only a fraction of their apparent contact area. •Abrasion Theory: theory that states that the asperities of a harder surface penetrates and plows through a softer surface. Another thing to take in to account is that all friction dissipates energy. This energy is converted into heat. Sometimes the heat may soften or even melt the material in use Plastics generally possess low frictional characteristics. This is better for creating items such as bearings, gears, seals, prosthetic joints. But, an important consideration is that plastics have a low melting point. So, any heat caused by friction must be taken into account. Ceramics on the other hand have generally the same frictional characteristics as metals. Some ways of reducing friction are : 1. Selection of materials used 2. Lubricants or solid films (such as graphite) 3. Ultrasonic vibrations The coefficient of friction is found during the manufacturing process or in laboratory tests using smaller versions of various sizes of the material. One of the common tests used is called the ring-compression test. This is where a flat ring is upset plastically between two flat platens. If the both the diameters of the ring expand outward then the friction is zero. If the inner diameter becomes smaller, then there’s an increase of friction. 33.5 Wear Wear changes the shapes of tools and dies, affects the tool life, tool size, and the quality of the parts produced. Importance of wear is evident in the number of parts and components that continually have to be replaced or repaired. i.e. dull drill bits, worn cutting tools and dies 33.5 Wear Running In period removes the peaks from asperities. Under controlled conditions, wear may be regarded as a type of smoothing or polishing process. Adhesive Wear A tangential force is applied and shearing takes place either at a) the original interface b) along a path below or above the interface *adhesive bonds often are stronger than the base metals. Schematic illustration of (a) two contacting asperities, (b) adhesion between two asperities, and (c) the formation of a wear particle. Abrasive Wear Caused by a hard rough surface sliding across another surface. Microchips and slivers are produced, leaving grooves or scratches on the softer surface. *processes: filing, grinding, ultrasonic machining and abrasive jet machining act in this manner Other types of Wear Corrosive. Fatigue-when the surface of a material is subjected to cyclic loading Erosion-caused by loose abrasive particles on a surface Fretting Impact 33.6 Lubrication 4-Regimes of Lubrication in Manufacturing Ops. 1)Thick-film: surfaces separated completely by a film of lubricant. Results in dull, grainy surface appearance after forming operations 2) Thin-film: Load between the die and work piece increases, or the speed and viscosity of fluid decrease, the lubricant becomes thinner raising friction and results in slight wear 33.6 Lubricants 3)Mixed: a significant portion of the load is carried by the physical contact of the two surfaces. The rest is carried by the fluid film trapped in the valleys (asperities). 4) Boundary: load supported by contacting surfaces covered with boundary film of lubricant. The lubricant is attracted physically to the metal surfaces, thus preventing direct metal-to-metal contact and reducing wear. 33.7 Metalworking Fluids Reduce friction Reduce wear Improve material flow Act as a thermal barrier between the workpiece and its tool and die surfaces Act as a release or parting agent Oils and Emulsions Oils are very effective in reduction of friction and wear and have low thermal conductivity but they do not conduct away the heat generated by friction effectively. *It is difficult and costly to remove oils from component surfaces that are to be painted or welded, and is difficult to dispose of. Emulsion: mixture of oil and water in various proportions along with additives. aka water-soluble oils or coolants 1)direct: mineral oil dispersed in water in very small droplets Important in metalworking b/c the presence of water gives them high cooling capacity. Synthetic solutions Soaps, greases, waxes Synthetic Solutions: chemical fluids that contain inorganic and other chemicals dissolved in water (contain NO mineral oils) Oil found @ Pepboys Soaps: reaction products of sodium or potassium salts with fatty acids. Effective boundary lubricants. Alkali soaps are soluble in water but metal soaps are generally insoluble Greases: solid or semisolid lubricant that consists of soaps, mineral oil, and additives. Highly viscous and adhere well to metal surfaces. Waxes: Less greasy and more brittle. Limited to metalworking operations Additives Metalworking fluids usually blend with various additives: • • • • • • Oxidation inhibitors Rust-preventing agents Foam inhibitors Wetting agents Odor-Controlling agents Antiseptics Solid Lubricants Graphite: effective at elevated temps, however friction is low only in the presence of air or moisture. Molybdenum disulfide: lamellar solid lube, similar appearance to graphite however has a high friction coefficient in an ambient environment. Oils commonly used with MoS2. Metallic & polymeric Films: Thin layers of soft metals and polymer coatings. Suitable metals include lead, indium, tin, silver, PTFE(Teflon). Selection of Metalworking Fluids Considerations of several factors: 1) Particular manufacturing process 2) Work piece material 3) Tool or die material 4) Processing parameters 5) Compatibility of the fluid with the tool and die materials and the work piece. 6) Required surface preparation 7) Method of fluid application 8) Removal of fluid and cleaning after processing 9) Contamination of the fluid by other lubricants 10) Storage and maintenance of fluids 11) Treatment of waste lubricant 12) Biological and Environmental considerations 13) Costs involved in all of the aspects listed Biological and Economical Considerations Potential hazards may be involved by contacting or inhaling some fluid. Improper disposal of fluids may cause adverse effects on the environment. Much progress has been made in developing environmentally safe fluids, technology, and equipment for their proper treatment, recycling, and disposal. 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