Presentation on theme: "Powder Methods of Change of Form"— Presentation transcript:
1Powder Methods of Change of Form Chapter 10Chapter 10IT 2081
2Powder Methods of Change of Form CompetenciesList the advantages of, and the products made from Powder MetalList and describe the order of operations in Powder MetallurgyChapter 10IT 2082
3Powder Methods of Change of Form Power metallurgy - the process of compacting metal powders in suitable dies and sintering them.Net shape parts of fairly complex shape can be produced economicallyCompetitive with casting, forging and machiningGood dimensional accuracy and size; from balls for ball point pens to parts weighing 200 lbs.Chapter 10IT 2083
4Powder Methods of Change of Form Parts such asSelf lubricating bearings impregnated with oilBreak pads with embedded ceramic fibersMachine tool cutting instruments; cermets (ceramic-metals) higher heat absorptionCommonly made out of iron, copper, aluminum, tin, and nickelOrder of operation: Powder production, blending, compacting, sintering, finishingChapter 10IT 2084
5Powder Methods of Change of Form Powder PreparationVirtually any metal can be made into powder form. There are three principal methods by which metallic powders are commercially produced, each of which involves energy input to increase the surface area of the metal.Chapter 10IT 2085
6Powder Methods of Change of Form Atomization – involves the conversion of molten metal into a spray of droplets that solidify into powder. It is the most versatile and popular methods for producing metal powders todayGas atomization – in which a high velocity gas stream is utilized to atomize the liquid metal.Water atomization – a high-velocity water stream is used instead of air.ChemicalChapter 10IT 2086
7Powder Methods of Change of Form Electrolytic – an electrolytic cell is set up in which the source of the desired metal is the anode. The anode is slowly dissolved under an applied voltage, transported through the electrolyte, and deposited on the cathode. The deposit is removed, washed, and dried to yield a metallic powder of very high purity.In addition, mechanical methods are occasionally used to reduce powder sizes; however, these methods are much more commonly associated with ceramic powder production.Chapter 10IT 2087
8Powder Methods of Change of Form Comminution, a term used for the techniques for reducing particles size in ceramics processing, deliver mechanical energy in various forms. Two general types of communition operations are distinguished: crushing and grindingChapter 10IT 2088
9Powder Methods of Change of Form Crushing – the reduction of large lumps from the mine to smaller sizes for subsequent further reduction. Several stages may be required (e.g. primary crushing, secondary crushing)Chapter 10IT 2089
10Powder Methods of Change of Form Grinding – refers to the operation of reducing the small pieces after crushing to a fine powder. Grinding is accomplished by abrasion and impact of the crushed mineral by the free motion of unconnected hard media such as balls, pebbles, or rods.Ball Mill – hard spheres mixed with the stock to be comminuted are rotated inside a large cylindrical container.Roller mill – stock is compressed against a flat horizontal grinding table by rollers riding over the table surfaceImpact grinding – particles of stock are thrown against a hard flat surface, either in high velocity air stream or in a high-speed slurry. The impact fractures the pieces into smaller particles.Chapter 10IT 20810
11Powder Methods of Change of Form BlendingMixing several powders of different sizes, a drylubricant or an antioxidant for uniform compaction. (done carefully to avoid explosions)Can add lubricants- do not stick to mold wallsCan add binders- so green strength is adequateCan add sintering aids- acceleration of densification upon heatingChapter 10IT 20811
12Powder Methods of Change of Form CompactionBringing the materials into required shape . The workpart after pressing is called a green compact, the word green meaning not yet fully processed.Briquetting - Compression of powder in the die cavity from both the top and the bottom.Roller Compaction - compacted between two rollers to produce sheet or plate stock.Extrusion Compacting - powder packed into a mild steel tube, then forced through a die.Chapter 10IT 20812
13Powder Methods of Change of Form SinteringProcess of heating compressed powdered metals to within 70 – 90 % of its melting point. Often called (solid-state sintering, or solid-phase sintering) because the metal remains unmeltedUsed for materials such as ceramics and cermets that cannot be melted and cast by other methods.Chapter 10IT 20813
14Powder Methods of Change of Form Sintering involves mass transport to create the necks and transform them into grain boundaries. The principal mechanism by which this occurs is diffusion; other possible mechanisms include plastic flow.Sintering – the heat treatment consists of three stepsPreheat, in which lubricants and binders are burned offSinterCool downChapter 10IT 20814
15Powder Methods of Change of Form Finishing of Sintered Parts(Secondary Operations)Densification and SizingRepressing – the part is squeezed in a closed die to increase density and improve physical properties.Sizing - forcing the part through a finish die to provide dimensional accuracyCoining – pressing details into its surfaceMachiningChapter 10IT 20815
16Powder Methods of Change of Form Infiltration and Impregnation - Because powder-formed parts can be very porous, other materials can be placed in the voids to enhance the properties of the product.Prevents moisture penetrationLowers frictional propertiesInfiltration – pores are filled with a molten metalImpregnation - Impregnating the sintered part with oil to create a “self lubricating” bearing.Chapter 10IT 20816
17Advantages Powder Methods of Change of Form Wide range of mech. & phys. propertiesParts made from high melting point metalsHigh production rates on relatively complex partsGood dimensional controlImpregnating and infiltrationChapter 10IT 20817
18Limitations Powder Methods of Change of Form Size of parts, complexity of shapes of partsHigh cost of powdered metal compared to other materialsHigh tooling cost for small production runsLower strength and ductility than forgingChapter 10IT 20818
19Mechanical and Other Methods of Change of Form Chapter 11Chapter 11IT 20819
20Describe the fundamental characteristics of extrusion CompetenciesDefine ForgingDescribe the fundamental characteristics of extrusionDescribe the process of Coining and HeadingDescribe the reasons for using lubrication in forgingDescribe the fundamental characteristics of rollingList the common material change of form mechanical methodsChapter 11IT 20820
21Overview of Metal Forming Can be classified asBulk deformation processes – generally characterized by significant deformations and massive shape changes; and the surface area-to- volume of to work is relatively small.ForgingExtrusionRollingWire and bar drawingSheet metalworking processBending operationsDeep or cup drawingShearing processesMiscellaneousChapter 11IT 20821
22Forging Forging - “plastic deformation by compressive forces” Hand Forging exactly what the blacksmiths did.Drop Forging – a drop forge raises a massive weight and lets it fall.The two basic types of forging machines are presses and hammers.Presses exert enormous forces, which are applied slowly enough that the metal has time to “flow.”The hammer machines are designed to raise a massive weight and let it drop.Power hammers add to gravity with pneumatic or hydraulic assistance.Counterblow hammers use two opposed hammersChapter 11IT 20822
23ForgingOpen Forging - Presses the billet between two flat plates to reduce its thickness.Cogging – is a forging process that reduces the thickness of a single BILLET by small increments.Closed forging - The billet is forced into the cavities of one or more dies.Flashing is the excess material squeezed out from a BILLET in a CLOSED FORGING or stamping process.Chapter 11IT 20823
24ForgingCoining - the process used to form faces on coin blanks. It is a very intricate process.Heading - is the process of “upsetting” metal to form heads on nails or screws.Swaging is the forging process by which a hollow cylindrical part is forced tightly around a rod or wire to permanently attach the two parts. It is also known as RADIAL FORGING.Chapter 11IT 20824
26Forging Lubricants for Forging improve the flow of the material into the diesto reduce die wearto control the cooling rateto serve as a parting agentChapter 11IT 20826
27Forging Pressures Involved in Forging The force needed to forge a part depends on:the compressive strength of the metalthe area including flashings of the metal being forgedthe temperature at which the forging is being donethe amount of deformation each compressive stroke of the ram or hammer performs.Chapter 11IT 20827
28ExtrusionExtrusion is the process of forcing a material through a DIE to produce a very long WORKPIECE of constant shape and cross section. Extrusion can be done “cold” (at room temperature) or “hot” so that the material is softened slightly.Chapter 11IT 20828
29Extrusion Direct or forward - The product moves though a die Indirect (reverse or backward) - product stationary, die movesHydrostatic Extrusion – In hydrostatic extrusion a fluid is placed between the ram and the metal being extruded. This produces two advantages:(1) The fluid presses radially inward on the billet, which helps guide it into the opening in the die(2) the fluid lubricates the walls of the cylinder, which reduces the friction forces in the extrusion process.Hollow Extrusion – Hollow pieces such as pipes and tubing can be made by extrusion if some “obstacle” is part of the die design.Chapter 11IT 20829
30RollingA compressive deformation process in which the thickness of a slab or plate is reduced by two opposing cylindrical tools called rolls. The rolls rotate so as to draw the work into the gap between them and squeeze it. Rollers are pressed together with enough force so that whatever passes between them must take the shape of the space between the rollers.Chapter 11IT 20830
31Rolling Bend rods or sheets into curved surfaces Change the grain structure of cast bars or sheetsForm billets into structural shapes such as flanges, channels, or railroad railsProduce tapers or threads on rodsStraighten bent sheets, rods, or tubingChapter 11IT 20831
32Bending by Rolling: Crimped by rolling. Tube forming by rolling Threaded parts by rolling - faster than machining the threads and leaves a harder grain structure.Forming ball bearingsStraightening flat stockChapter 11IT 20832
33Rolling ShapesPlate is defined as stock that is thicker than 0.25 inch (6 millimeters)Sheet runs from 0.25 inch down to about inch (0.008 millimeter)Foil is considered to be less than inch thick.Large flange beams (I-beams), channels, and even wire are made by rolling.Chapter 11IT 20833
34Hot Versus Cold Rolling Hot rolling – Billets heated to the red hot range rapidly form an oxide coating or scale.Cold rolling - Softer materials such as aluminum and copper are cold rolled.rolling material at room temperature provides better surface finish and closer tolerancescharacterized by fine grain size. The finer the grain, the harder and less malleable the metal becomes.Chapter 11IT 20834
35Factors Affecting Rolling The material being rolledThe material of the rollersThe shape being rolledThe size of the stock being rolledThe size of the rollersPower requirementsChapter 11IT 20835
36DrawingThe pulling of a bar through a Die to reduce the cross section.Used to make wireSeamless TubingChapter 11IT 20836
37Sheet metalworking Processes BendingBrake – general use device for bending sheet metal.Punch and Dies – shaping material by punching it into a die. Punch is the moving form, Die is the stationary form.Press brake - an extension of the punch-and-die set extended along one dimension to make complex bends in a long piece of sheet stock.Chapter 11IT 20837
38Sheet Metalworking Processes Drawing - in sheet metal working, drawing refers to the forming of a flat metal sheet into a hollow or concave shape, such as a cup, by stretching the metal.Spin forming - A forming process in which a sheet of metal is held to a mandrel, rotated, and forced onto the mandrel to shape the sheet.Miscellaneous – stretch forming, roll bending, spinning, and bending of tube stockChapter 11IT 20838
41TensileThe stress-strain relationship has two regions, indicating two distinct forms of behavior: elastic and plastic.In the elastic region, the relationship between stress and strain is linear, and the material exhibits elastic behavior by returning to its original length when the load is released. This relationship is defined by Hooke’s Law:σe = E еwhere E = modulus of elasticity (psi) which is the inherent stiffness of a material; e = engineering strainChapter 11IT 20841
42Tensile Stress – Strain Curve As stress increases, some point in the linear relationship is finally reached at which the material begins to yield (yield point; Y) Often referred to as the yield strength, yield stress and elastic limit.Beyond this point, Hooke’s Law does not apply. As the elongation increases at a much faster rate, this causes the slope of the curve to change dramatically.Finally, the applied load F reaches maximum value, and the engineering stress calculated at this point is called the tensile strength or ultimate tensile strength of the material.Chapter 11IT 20842
43Tensile Stress – Strain Curve The amount of strain that the material can endure before failure is also a mechanical property of interest in many manufacturing processes. The common measure of this property if ductility, the ability of a material to plastically strain without fracture.Chapter 11IT 20843
44Tensile Stress – Strain Curve This measure can be taken as either elongation or area reductionElongation often expressed as a percent.where Lf = specimen length after fracture and Lo = original specimen lengthChapter 11IT 20844
45Tensile Stress – Strain Curve Area reduction often expressed as a percentwhere Ao = original area and Af = area of the cross-section at the point of fractureChapter 11IT 20845
46True Stress-StrainThere is a small problem with using the original area of the material the calculate engineering stress, rather than the actual (instantaneous) area that becomes increasing smaller as the test proceeds.Chapter 11IT 20846
47True Stress-StrainIf the actual area were used, the calculated stress value would be higher. The stress value obtained by dividing the instantaneous value of area into the applied load is defined as the true stressWhere F = force (lb) and A = actual (instantaneous) area resisting the loadChapter 11IT 20847
48True Stress-StrainSimilarly, true strain provides a more realistic assessment of the instantaneous elongation per unit length of the material.Chapter 11IT 20848
49True Stress-StrainThe value of true stain in a tensile test can be estimated by dividing the total elongation into small increments, calculating the engineering strain for each increment on the basis of its starting length, and then adding up the strain values, in the limit, true strain is defined asWhere L = instantaneous length at any moment during elongationChapter 11IT 20849
50True Stress-StrainAt this point if the engineering stress-strain curve is replotted using the true stress-strain, then we would see very little difference in the elastic region.The difference occurs at the point in which the stress-strain exceeds the yield point and enters the plastic region.The true stress-strain values are high due to a smaller cross sectional area being used, which is continuously reduced during elongation.As in the engineering stress-strain curve, necking occurs and therefore a downturn leading to fracture.Chapter 11IT 20850
51True Stress-StrainUnlike engineering stress-strain, true stress values indicate that the material is actually becoming stronger as strain increases. This property is called strain hardening. Stain hardening (work hardening) is an important factor in certain manufacturing processes, particularly metal forming.Chapter 11IT 20851
52True Stress-StrainBy replotting the plastic region of the true stress curve on a Log/Log scale, the result is a linear relationship expressed asKnown as the flow curve which captures a good approximation of the behavior of metals in the plastic region, including their capacity for strain hardeningWhere K = strength coefficient (psi) it equals the value of true stress at a true strain value equal to one.n = strain hardening exponent, and is the slope of the line. Its value is directly related to a metal’s tendency to work hardenChapter 11IT 20852
53True Stress-StrainEmpirical evident reveals that necking begins for a particular metal when the true strain reaches a value equal to the strain hardening exponent.Therefore, a higher n value means that the metal can be strained further before the onset of neckingChapter 11IT 20853
54Types of Stress-Strain relationships Perfectly elasticthe behavior of this material is defined completely by its stiffness, indicated by the modulus of elasticity E. It fractures rather than yielding to plastic flow.Brittle material such as ceramics, many cast irons, and thermosetting polymers possess stress-strain curves that fall into this category.These material are not good candidates for forming operations.Chapter 11IT 20854
55Types of Stress-Strain relationships Elastic and perfectly plasticThis material has a stiffness defined by E. Once the yield strength Y is reached, the material deforms plastically at the same stress level.The flow curve is given by K = Y and n = 0. Metals behave in this fashion when they have been heated to sufficiently high temperatures that they recrystallize rather than strain harden during deformation.Lead exhibits this behavior at room temperature because room temperature is above the recrystallization point for lead.Chapter 11IT 20855
56Types of Stress-Strain relationships Elastic and strain hardeningThis material obeys Hooke’s Law in the elastic region.It begins to flow at its yield strength Y. Continued deformation requires an every-increasing stress, given by a flow curve whose strength coefficient K is greater that Y and whose strain hardening exponent n is greater than zero.The flow curve is generally represented as a linear function on a natural logarithmic plot.Most ductile metals behave this way when cold worked.Chapter 11IT 20856
57TensileManufacturing processes that deform materials through the application of tensile stresses include wire and bar drawing and stretch formingChapter 11IT 20857
58Compression Properties Applies a load that squeezes a cylindrical specimen between two platens. The specimen height is reduced and its cross-sectional area is increased.Engineering stress and strain are calculated much like that in tensile engineering stress and strain.The engineering stress strain curve is different in plastic portion of the curve. Since compression causes the cross section to increase, the load increases more rapidly than previously. The result is a higher calculated engineering stress.Chapter 11IT 20858
59Compression Properties Although differences exist between the engineering stress-strain curve in tension and compression, when the respective data are plotted as true stress-strain, the relationships are nearly identicalImportant compression processes in industry include rolling, forging, and extrusionChapter 11IT 20859
60Shearing PropertiesShear involves application of stresses in opposite directions on either side of a thin element to deflect it.Shear stress (psi) is defined by:Shear strain (in/in) is defined by:Where δ is the deflection of the element (in) and b = the orthogonal distance over which deflection occursChapter 11IT 20860
61Shearing PropertiesShear stress and strain are commonly tested in a torsion test, in which a thin-walled tubular specimen is subjected to a torque.As torque is increased, the tube deflects by twisting, which is a shear strain for this geometry.Chapter 11IT 20861
62Shearing PropertiesThe shear stress can be determined in the test by the equationWhere T = applied torque (lb-in); R = radius of the tube measured from the neutral axis of the wall (in); t = wall thickness (in)Chapter 11IT 20862
63Shearing PropertiesShear strain can be determined by measuring the amount of angular deflection of the tube, converting this into a distance, and dividing by the gauge length (L). Reducing this to a simple expression.The shear stress at fracture can be calculated, and this is used as the shear strength S of the material. Shear strength can be estimated from tensile strength data by approximation S = 0.7(TS)Where α = the angular deflection (radians)Chapter 11IT 20863
64Rolling, Forging, Bending, Beating, Bending and Crushing. Chapter 12 Fundamentals of Manufacturing Processes – Changes of ConditionMechanical ProcessesRolling, Forging, Bending, Beating, Bending and Crushing.Cold Working makes metal harder but more brittle.
65Fundamentals of Manufacturing Processes – Changes of Condition Thermal or Heat Treatment ProcessesPhase Diagram – is a graph showing the parameters in which the phases of a system exist. Parameters are usually temperature, percent composition or pressure of the system.Phases of a material include gas, liquid and solid.Two-Component phase diagrams systems describe results of combining materials at different levels and environmentsExample is physical properties of iron-carbon combinations.Information on the Phase diagram is used to identify and classify materials and their characteristics.
67Fundamentals of Manufacturing Processes – Changes of Condition The Iron Carbon Phase diagramFerrite a-Fe stable at low temperature can dissolve < 0.02% C at 727oCAustenite g-Fe the high temperature form of FeCan dissolve < 2.11 % C at 1148 oCCementite Iron Carbide Fe3C contains 6.7 % C
68Fundamentals of Manufacturing Processes – Changes of Condition Pearlite the eutectoid mixture of a-Fe and Fe3C formed by the eutectoidal decomposition of g-Fe containing 0.77% C (the eutectoid composition) at 727oC (the eutectoid temperature).d-Ferrite the bcc iron that exists between 1394 oC and 1538 oC
69Method of Softening Steels AnnealingStress reliefHeat to above 500°C where steel can re-crystallize and relieve stressDo not heat above 723°C as transformation to austenite can occurNormalizationHeat steel until complete transformation to fine-grained austenite has just occurred
70Normalization Normalization Heat steel until complete transformation to fine-grained austenite has just occurredAllow to cool in air to produce a fine pearliteStandard state for material delivery with good mechanical properties and surface finishEven grain gives steel workability
71Method of Hardening Steels Quenching is cooling with a controlled rate to achieve a given microstructureQuench a steel at a sufficient rate to cause complete transformation to martensiteInitially heat steel to temperature sufficient just to cause complete transformation to austenite.Critical cooling rate achieved when quench rate is just sufficient to prevent transformation to ferrite and pearlite
72Method of Hardening Steels Critical cooling rate achieved when quench rate is just sufficient to prevent transformation to ferrite and pearliteA range of quenching media can be used:-order of severity 5% caustic soda, 5-10% brine, cold water, warm water, mineral oil, animal oil, vegetable oilQuenching into oils may produce bainite rather than martensiteSteels with less than 0.25%C cannot be hardened by quenching as the nose of the TTT curve is too close to the temperature axis See Figure page 236.
73Surface HardeningHarden the surface only for applications such as gears, shafts, lathe beds and cams.Flame hardening heats the surface which quickly cooled to harden to depth of only one quarter inch. Only effective with medium to high carbon steels.Induction hardening surface is heated to the austenitic range using high frequency electric current. Surface is cooled quickly to harden depth.
74Surface Hardening Chemical Processes Case Hardening Primarily used for low carbon steelCarburizing part is exposed to a high carbon atmosphere at high temperature ° F Carbon will penetrate at .005 inch per hour. Steel must be quenched to obtain the hardened surface.
75Surface Hardening Primarily used for low carbon steel Carburizing part is exposed to a high carbon atmosphere at high temperature ° F Carbon will penetrate at .005 inch per hour. Steel must be quenched to obtain the hardened surface.
76Surface HardeningCarburizing using gas is more uniform and faster than packing carbon. Rate is .04 to .05 inch depth in 4 hoursNitriding uses nitrogen gas instead of carbon based gas. It requires lower temperatures and no quenching but takes much longer. Parts retain their characteristics much longer at higher temperatures.Carbonitriding or Cyaniding uses a bath of sodium cyanide with low carbon steels at 1500 to 1650°F temperatures to obtain .01 inch per hour. Very poisonous if not controlled.
77Other Methods of Modifying Properties of Steel Tempering is heating the quenched steel back to 200 to 1200°F and cooling it at air temperature to relieve stress to give the steel better toughnessSpherodising is heat the to 650 to 700°C to ball-up the cementite Used to soften some tool steels for subsequent working.Martempering and Austempering are methods to provide a more uniform grain structure between the surface and the inner layers of the quenched steel. Each method stops the cooling process at a predetermined temperature for a set period of time to allow the grain structure to develop uniformly. See Figure and 22.
78Other Methods of Modifying Properties of Steel Patenting is quenching steel in molten lead which acts like austempering at the lead melting point of 621°F.Jominy Test is used to determine the hardenability of the steel.
79Other Methods of Modifying Properties of Steel Steel is cut to a standard test specimen size, heated to 1650°F and quenched with water for 20 minutes. Rockwell hardness measurements are taken from end to end and graphed to determine hardenability. Results are compared to a plain carbon bar for calibration. Adding alloys increases the steels ability to be hardened.
80Other Methods of Modifying Properties of Steel Hardenability – the ability of a metal to be hardened. Maximum hardness obtainable by heat-treating in plain carbon steels increases with carbon content up to 0.80% (eutectoid steel). Hardenability increases rapidly with increased carbon content to 0.45%. After this, the hardenability tapers off gradually until the carbon content reaches 0.80%.
81Selection of Materials Decide on the properties needed for the core and surface.Select carbon and alloying element based on the needs.Set target at 3 to 5 points higher Rockwell hardness than final product.Anneal or soften the part to permit fabrication.Fabricate part to rough but oversized tolerances.
82Selection of Materials Select method for hardening.Harden the steel part.Temper the part.Finish the part by reducing the dimensions to the design limits and applying the desired finish.
83Selection of Materials Heat treatment of nonferrous metalsCrystal structure cannot be modified.Annealing is used to reform grain structures between steps in processing.Precipitation HardeningTwo metals are combined.Alloy is heated to temperature that causes each metal to be in separate phases.
84Selection of Materials Metal cools and separates or precipitates causing hardening effect.Age hardening is a resultSolution Heat TreatingSpeeding up Age hardening by holding the metals at an intermediate temperature to motivate a higher rate of precipitation.
86CompetenciesIdentify the different types Consumable and Nonconsumable electrode welding processesIdentify the flame characteristics associated with different types of gas weldingIdentify the unique characteristics for each type of arc weldingList the advantages and disadvantages of gas and arc weldingIT 208Chapter 1486
87WeldingSoldering and brazing are adhesive bonds, whereas welding is a cohesive bond.Joint PreparationButt joints, vee joints, double-vee joints, tee joints, which require a fillet weld, and lap joints.Butt joints are used on metal that has a thickness of one-quarter inch or less.Preparation for Weld JointsSurfaces to be joined must be ground to the weld specification.Any slag, corrosion, or other foreign material must be removed.IT 208Chapter 1487
88GAS WELDING Oxygen-Acetylene Welding Oxygen tank (green) Acetylene tank (red, or black with a red top)Oxygen pressure valves have a right-hand internal threadAcetylene pressure valve has an external left-hand thread.An oxygen-acetylene flame is very hot, approaching 3500°F.IT 208Chapter 1488
89GAS WELDINGFusion weld is to place the two pieces against each other and melt their surfaces together.Reducing flame is used to melt low-melting-point metals and alloys because it does not oxidize or corrode the metals.Neutral flame is the hottest one possible and is the proper adjustment for welding.Oxidizing flame that can cause corrosion in the metal. It is only used for cutting flames or burning pieces of metal from a piece of stock. (Fig 14-9)IT 208Chapter 1489
90GAS WELDING Advantages of an oxy-acetylene weld Disadvantages inexpensiverequires very little specialized equipment.Disadvantagesany traces of carbon left in the weld will weaken it.IT 208Chapter 1490
91GAS WELDING Oxygen-Hydrogen Welding The oxygen-hydrogen torch can reach temperatures much higher than the oxy-acetylene torch.More expensive than oxy-acetylene welding and involves the flammability risk with hydrogen.Plasma WeldingHydrogen plasma burns even hotter than hydrogen gas, permitting the welding of extremely high-melting-point metals.Very clean procedure that results in very little slag or foreign matter in the weld.IT 208Chapter 1491
92ELECTRICAL WELDINGResistance Welding – The two parts are pressed together and an alternating current (A/C) is passed through the contact zone.Spot welding – used extensively on sheet metals (holds handles on pots, car body together)Ribbon welding rollers. - parts to be welded are drawn between electrodes rollers while electricity is applied.IT 208Chapter 1492
94Arc WeldingA sustained arc generates the heat for melting the work piece and filler material.Consumable electrodesNon-consumable electrodesIT 208Chapter 1494
95Consumable electrodes Flux Core Arc Welding (FCAW) developed in the early 1950s as an adaptation to SMAW to overcome limitation imposed by the use of a stick electrodes. Uses a spool of filler wire fed through the hand-piece. A core of flux is inside the wireTwo versionsSelf-shielded flux-cored arc welding – includes not only fluxes but also ingredients that generate shielding gases for protecting the arc.Gas-shielded flux-cored arc welding – developed primarily for welding steels, obtains a shielding from externally supplied gases, similar to GMAWIT 208Chapter 1495
96Consumable electrodes Submerged Arc Welding (SAW) – uses a continuous, consumable bare wire electrode, and arc shielding is provided by a cover of granular flux. Low-carbon, low alloy, and stainless steels can be readily welded by SAW.Electrogas Welding (EGW) – uses a continuous consumable electrode (either flux-cored wire or bare wire with externally supplied shielding gases) and molding shoes to contain the molten metal.Shielded Metal Arc Welding (SMAW) (stick) – arc is struck between the rod (shielded metal covered by flux) and the work pieces to be joined, the impurities rise to the top of the weld in the form of slag (18-19a, handout pg. 40)IT 208Chapter 1496
97A sustained arc, shielded by molten slag, is maintained in consumable-electrode welding by the (a) shielded metal-arc, (b) submerged arc, and (c) electrogas methods.IT 208Chapter 1497
98Selection of Welding Rods Filler rod should have a tensile strength greater than the metal to be joined.Rod must also be compatible with the welded metalWelding positions requiredWelding current (ac or dc)Joint design (groove, butt, fillet, etc.)Thickness and shape of the base metalService conditions and specificationsProduction efficiency and job conditionsIT 208Chapter 1498
99Welding Rod Classification (ex. E-6010) The E- stands for electrode.The first two numbers indicate the tensile strengthThe next-to-last number gives the welding positionsThe last digit of the weld rod number indicates the type of current for which the rod may be used (ac, dc straight, dc reverse), the penetration, and the type of flux around the rod.Example: E-6010 would have a tensile strength of 60,000 psi, could be used in all positions, has a cellulose-sodium flux, could give deep penetration, and must be used with dc reverse current. (p )IT 208Chapter 1499
100Inert Gas Arc WeldingAn inert gas is used to keep oxygen away from the hot metal during welding to prevent corrosion both on the surface and within the weld metal.Gas metal arc welding (GMAW) – (metal + inert gas) electrode is continuously fed through the welding gun and is shielded by an inert gas (figure 18-18c).Easily converted forautomatic welding machines, computer controlled welding machines, and robotics control.IT 208Chapter 14100
101The arc is shielded by gas in the (a) gas tungsten-arc, (b) plasma-arc, and (c) gas metal-arc welding processes. Note that the depth of penetration increases with increasing arc temperature.IT 208Chapter 14101
102Non-consumable Electrodes Gas Tungsten ARC welding - GTAW (Tungsten inert gas, a.k.a. TIG) – Tungsten electrode not consumed, but surrounded by an inert gas and produces an arc.Filler material is usually applied.Gas tungsten arc welding does not produce as deep a penetration as stick or other types of welding.GTAW is a slow method of welding, which results in an expensive product.It can be used to weld aluminum, magnesium, titanium, and stainless steels.Plasma-Arc welding (PAW) – when an arc is created in a plasma (ionized) gas and a filler material may or may not be applied to the weld jointIT 208Chapter 14102
103Other Welding Techniques Electron beam welding (EBW)the electron gun melts the parent metal, and the molten metal flows to fill the gapheat affected zone is very narrowwelds can be several inches deep, and leaves a very clean weld.Welding must be done in a vacuum.Laser beam welding (LBW) - the heat from laser can be used to heat the surface of material or penetrate the entire depth of the joint (good for thin gauge metals). The major problems with the current lasers lie in the cost and bulk of the power source.IT 208Chapter 14103
104Other Welding Techniques Friction WeldingRubbing two pieces of metal or plastic together at a very high frequency.It is simple, clean, quick, inexpensive, and effective.Friction welds have thus far been used mainly for very small applications.Chemical WeldingSheets of Lucite, Plexiglas, or acrylic can be fused by acetone or methyl ethyl ketone (MEK).The chemical simply dissolves the surfaces of the plastic. When the solvent evaporates, the surfaces repolymerize to form a true weld.IT 208Chapter 14104
105BrazingA joining process in which filler metal is placed at or between the surfaces to be joined. The temperature is raised to melt the filler metal but not the workpiece.Braze melts between degrees FThe filler material is in thin layers compared to base metalThe filler penetrates the gap by capillary attractionCan connect dissimilar metalsMost common braze defect is lack of braze or a voidIT 208Chapter 14105
106Soldering( degrees F) joints are usually of lesser strength than brazed but parts can be joined without exposure to excessive heatUsed extensively in electronics industry because of heat sensitive componentsSurface preparation and the use of fluxes are most importantFluxes –prevents oxidation and removes slight oxide films from work piece surfacesIT 208Chapter 14106