Presentation on theme: "Chapter 10IT 2081 Powder Methods of Change of Form Chapter 10."— Presentation transcript:
Chapter 10IT 2081 Powder Methods of Change of Form Chapter 10
IT 208 2 Powder Methods of Change of Form Competencies List the advantages of, and the products made from Powder Metal List and describe the order of operations in Powder Metallurgy
Chapter 10 IT 208 3 Powder 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 economically – Competitive with casting, forging and machining – Good dimensional accuracy and size; from balls for ball point pens to parts weighing 200 lbs.
Chapter 10 IT 208 4 Powder Methods of Change of Form – Parts such as Self lubricating bearings impregnated with oil Break pads with embedded ceramic fibers Machine tool cutting instruments; cermets (ceramic-metals) higher heat absorption Commonly made out of iron, copper, aluminum, tin, and nickel Order of operation: Powder production, blending, compacting, sintering, finishing
Chapter 10 IT 208 5 Powder Methods of Change of Form Powder Preparation Virtually 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 10 IT 208 6 Powder 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 today Gas 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. Chemical
Chapter 10 IT 208 7 Powder 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 10 IT 208 8 Powder 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 grinding
Chapter 10 IT 208 9 Powder 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 10 IT 208 10 Powder 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 surface Impact 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 10 IT 208 11 Powder Methods of Change of Form 2.Blending Mixing several powders of different sizes, a dry lubricant or an antioxidant for uniform compaction. (done carefully to avoid explosions) Can add lubricants- do not stick to mold walls Can add binders- so green strength is adequate Can add sintering aids- acceleration of densification upon heating
Chapter 10 IT 208 12 Powder Methods of Change of Form 3.Compaction Bringing 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 10 IT 208 13 Powder Methods of Change of Form 4.Sintering Process 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 unmelted Used for materials such as ceramics and cermets that cannot be melted and cast by other methods.
Chapter 10 IT 208 14 Powder 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 steps – Preheat, in which lubricants and binders are burned off – Sinter – Cool down
Chapter 10 IT 208 15 Powder Methods of Change of Form 5. Finishing of Sintered Parts (Secondary Operations) Densification and Sizing – Repressing – 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 accuracy – Coining – pressing details into its surface – Machining
Chapter 10 IT 208 16 Powder 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 penetration Lowers frictional properties Infiltration – pores are filled with a molten metal Impregnation - Impregnating the sintered part with oil to create a “self lubricating” bearing.
Chapter 10 IT 208 17 Powder Methods of Change of Form Advantages – Wide range of mech. & phys. properties – Parts made from high melting point metals – High production rates on relatively complex parts – Good dimensional control – Impregnating and infiltration
Chapter 10 IT 208 18 Powder Methods of Change of Form Limitations – Size of parts, complexity of shapes of parts – High cost of powdered metal compared to other materials – High tooling cost for small production runs – Lower strength and ductility than forging
Chapter 11IT 20819 Mechanical and Other Methods of Change of Form Chapter 11
IT 208 20 Competencies Define Forging Describe the fundamental characteristics of extrusion Describe the process of Coining and Heading Describe the reasons for using lubrication in forging Describe the fundamental characteristics of rolling List the common material change of form mechanical methods
Chapter 11 IT 208 21 Overview of Metal Forming Can be classified as Bulk deformation processes – generally characterized by significant deformations and massive shape changes; and the surface area-to- volume of to work is relatively small. Forging Extrusion Rolling Wire and bar drawing Sheet metalworking process Bending operations Deep or cup drawing Shearing processes Miscellaneous
Chapter 11 IT 208 22 Forging 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 hammers
Chapter 11 IT 208 23 Forging Open 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 11 IT 208 24 Forging Coining - 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 11 IT 208 26 Forging Lubricants for Forging improve the flow of the material into the dies to reduce die wear to control the cooling rate to serve as a parting agent
Chapter 11 IT 208 27 Forging Pressures Involved in Forging The force needed to forge a part depends on: the compressive strength of the metal the area including flashings of the metal being forged the temperature at which the forging is being done the amount of deformation each compressive stroke of the ram or hammer performs.
Chapter 11 IT 208 28 Extrusion Extrusion 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 11 IT 208 29 Extrusion Direct or forward - The product moves though a die Indirect (reverse or backward) - product stationary, die moves Hydrostatic 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 11 IT 208 30 Rolling A 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 11 IT 208 31 Rolling – Bend rods or sheets into curved surfaces – Change the grain structure of cast bars or sheets – Form billets into structural shapes such as flanges, channels, or railroad rails – Produce tapers or threads on rods – Straighten bent sheets, rods, or tubing
Chapter 11 IT 208 32 Bending 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 bearings – Straightening flat stock
Chapter 11 IT 208 33 Rolling Shapes – Plate is defined as stock that is thicker than 0.25 inch (6 millimeters) – Sheet runs from 0.25 inch down to about 0.0003 inch (0.008 millimeter) – Foil is considered to be less than 0.0003 inch thick. Large flange beams (I-beams), channels, and even wire are made by rolling.
Chapter 11 IT 208 34 Hot 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 tolerances – characterized by fine grain size. The finer the grain, the harder and less malleable the metal becomes.
Chapter 11 IT 208 35 Factors Affecting Rolling The material being rolled The material of the rollers The shape being rolled The size of the stock being rolled The size of the rollers Power requirements
Chapter 11 IT 208 36 Drawing The pulling of a bar through a Die to reduce the cross section. – Used to make wire – Seamless Tubing
Chapter 11 IT 208 37 Sheet metalworking Processes Bending Brake – 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 11 IT 208 38 Sheet 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 stock
Chapter 11 IT 208 40 Material Properties Tensile Compression Shear
Chapter 11 IT 208 41 Tensile The 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: σ e = E е where E = modulus of elasticity (psi) which is the inherent stiffness of a material; e = engineering strain
Chapter 11 IT 208 42 Tensile 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 11 IT 208 43 Tensile 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 11 IT 208 44 Tensile Stress – Strain Curve This measure can be taken as either elongation or area reduction Elongation often expressed as a percent. where L f = specimen length after fracture and L o = original specimen length
Chapter 11 IT 208 45 Tensile Stress – Strain Curve Area reduction often expressed as a percent where A o = original area and A f = area of the cross-section at the point of fracture
Chapter 11 IT 208 46 True Stress-Strain There 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 11 IT 208 47 True Stress-Strain If 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 stress Where F = force (lb) and A = actual (instantaneous) area resisting the load
Chapter 11 IT 208 48 True Stress-Strain Similarly, true strain provides a more realistic assessment of the instantaneous elongation per unit length of the material.
Chapter 11 IT 208 49 True Stress-Strain The 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 as Where L = instantaneous length at any moment during elongation
Chapter 11 IT 208 50 True Stress-Strain At 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 11 IT 208 51 True Stress-Strain Unlike 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 11 IT 208 52 True Stress-Strain By replotting the plastic region of the true stress curve on a Log/Log scale, the result is a linear relationship expressed as Known as the flow curve which captures a good approximation of the behavior of metals in the plastic region, including their capacity for strain hardening Where 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 harden
Chapter 11 IT 208 53 True Stress-Strain Empirical 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 necking
Chapter 11 IT 208 54 Types of Stress-Strain relationships Perfectly elastic – the 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 11 IT 208 55 Types of Stress-Strain relationships Elastic and perfectly plastic This 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 11 IT 208 56 Types of Stress-Strain relationships Elastic and strain hardening This 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 11 IT 208 57 Tensile Manufacturing processes that deform materials through the application of tensile stresses include wire and bar drawing and stretch forming
Chapter 11 IT 208 58 Compression 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 11 IT 208 59 Compression 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 identical Important compression processes in industry include rolling, forging, and extrusion
Chapter 11 IT 208 60 Shearing Properties Shear 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 occurs
Chapter 11 IT 208 61 Shearing Properties Shear 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 11 IT 208 62 Shearing Properties The shear stress can be determined in the test by the equation Where T = applied torque (lb-in); R = radius of the tube measured from the neutral axis of the wall (in); t = wall thickness (in)
Chapter 11 IT 208 63 Shearing Properties Shear 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 12Fundamentals of Manufacturing Processes – Changes of Condition Mechanical Processes Rolling, Forging, Bending, Beating, Bending and Crushing. Cold Working makes metal harder but more brittle.
Fundamentals of Manufacturing Processes – Changes of Condition Thermal or Heat Treatment Processes Phase 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 environments Example is physical properties of iron-carbon combinations. Information on the Phase diagram is used to identify and classify materials and their characteristics.
Fundamentals of Manufacturing Processes – Changes of Condition The Iron Carbon Phase diagram Ferrite a-Fe stable at low temperature can dissolve < 0.02% C at 727oC Austenite g-Fe the high temperature form of Fe Can dissolve < 2.11 % C at 1148 oC Cementite Iron CarbideFe3C contains 6.7 % C
Fundamentals 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
Method of Softening Steels Annealing Stress relief –Heat to above 500°C where steel can re- crystallize and relieve stress –Do not heat above 723°C as transformation to austenite can occur Normalization –Heat steel until complete transformation to fine-grained austenite has just occurred
Normalization –Heat steel until complete transformation to fine-grained austenite has just occurred –Allow to cool in air to produce a fine pearlite –Standard state for material delivery with good mechanical properties and surface finish –Even grain gives steel workability
Method of Hardening Steels Quenching is cooling with a controlled rate to achieve a given microstructure Quench a steel at a sufficient rate to cause complete transformation to martensite Initially 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
Method of Hardening Steels Critical cooling rate achieved when quench rate is just sufficient to prevent transformation to ferrite and pearlite A 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 oil –Quenching into oils may produce bainite rather than martensite Steels 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 12-12 page 236.
Surface Hardening Harden 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.
Surface Hardening Chemical Processes Case Hardening Primarily used for low carbon steel Carburizing part is exposed to a high carbon atmosphere at high temperature.- 1700° F Carbon will penetrate at.005 inch per hour. Steel must be quenched to obtain the hardened surface.
Surface Hardening Primarily used for low carbon steel Carburizing part is exposed to a high carbon atmosphere at high temperature.- 1700° F Carbon will penetrate at.005 inch per hour. Steel must be quenched to obtain the hardened surface.
Surface Hardening Carburizing using gas is more uniform and faster than packing carbon. Rate is.04 to.05 inch depth in 4 hours Nitriding 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.
Other 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 toughness Spherodising 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 12-21 and 22.
Other 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.
Other 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.
Other 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%.
Selection 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.
Selection 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.
Selection of Materials Heat treatment of nonferrous metals Crystal structure cannot be modified. Annealing is used to reform grain structures between steps in processing. Precipitation Hardening Two metals are combined. Alloy is heated to temperature that causes each metal to be in separate phases.
Selection of Materials Metal cools and separates or precipitates causing hardening effect. Age hardening is a result Solution Heat Treating Speeding up Age hardening by holding the metals at an intermediate temperature to motivate a higher rate of precipitation.
IT 208 Chapter 14 86 Competencies Identify the different types Consumable and Nonconsumable electrode welding processes Identify the flame characteristics associated with different types of gas welding Identify the unique characteristics for each type of arc welding List the advantages and disadvantages of gas and arc welding
IT 208 Chapter 14 87 Welding Soldering and brazing are adhesive bonds, whereas welding is a cohesive bond. Joint Preparation Butt 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 Joints Surfaces to be joined must be ground to the weld specification. Any slag, corrosion, or other foreign material must be removed.
IT 208 Chapter 14 88 GAS WELDING Oxygen-Acetylene Welding Oxygen tank (green) Acetylene tank (red, or black with a red top) Oxygen pressure valves have a right-hand internal thread Acetylene pressure valve has an external left-hand thread. An oxygen-acetylene flame is very hot, approaching 3500°F.
IT 208 Chapter 14 89 GAS WELDING Fusion 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 208 Chapter 14 90 GAS WELDING Advantages of an oxy-acetylene weld – inexpensive – requires very little specialized equipment. Disadvantages – any traces of carbon left in the weld will weaken it.
IT 208 Chapter 14 91 GAS 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 Welding Hydrogen 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 208 Chapter 14 92 ELECTRICAL WELDING Resistance 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 208 Chapter 14 94 Arc Welding A sustained arc generates the heat for melting the work piece and filler material. – Consumable electrodes – Non-consumable electrodes
IT 208 Chapter 14 95 Consumable 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 wire Two versions – Self-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 GMAW
IT 208 Chapter 14 96 Consumable 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 208 Chapter 14 97 A 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 208 Chapter 14 98 Selection 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 metal Welding positions required Welding current (ac or dc) Joint design (groove, butt, fillet, etc.) Thickness and shape of the base metal Service conditions and specifications Production efficiency and job conditions
IT 208 Chapter 14 99 Welding Rod Classification (ex. E-6010) The E- stands for electrode. The first two numbers indicate the tensile strength The next-to-last number gives the welding positions The 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.270-272)
IT 208 Chapter 14 100 Inert Gas Arc Welding An 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 for – automatic welding machines, computer controlled welding machines, and robotics control.
IT 208 Chapter 14 101 The 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 208 Chapter 14 102 Non-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 joint
IT 208 Chapter 14 103 Other Welding Techniques Electron beam welding (EBW) – the electron gun melts the parent metal, and the molten metal flows to fill the gap – heat affected zone is very narrow – welds 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 208 Chapter 14 104 Other Welding Techniques Friction Welding Rubbing 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 Welding Sheets 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 208 Chapter 14 105 Brazing A 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 840-2400 degrees F – The filler material is in thin layers compared to base metal – The filler penetrates the gap by capillary attraction – Can connect dissimilar metals – Most common braze defect is lack of braze or a void
IT 208 Chapter 14 106 Soldering (400-840 degrees F) joints are usually of lesser strength than brazed but parts can be joined without exposure to excessive heat – Used extensively in electronics industry because of heat sensitive components – Surface preparation and the use of fluxes are most important Fluxes –prevents oxidation and removes slight oxide films from work piece surfaces