Manufacturing Process II. Welding Processes-1

Transcription

1 Manufacturing Process II Welding Processes-1 1. Introduction: The term joining is generally used for welding, brazing, soldering, and adhesive bonding, which form a permanent joint between the parts a joint that cannot easily be separated. The term assembly usually refers to mechanical methods of fastening parts together. Some of these methods allow for easy disassembly, while others do not. Welding is a materials joining process in which two or more parts are coalesced at their contacting surfaces by a suitable application of heat and/or pressure. Welding involves localized coalescence or joining together of two metallic parts at their faying surfaces. The faying surfaces are the part surfaces in contact or close proximity that are to be joined. Welding is usually performed on parts made of the same metal, but some welding operations can be used to join dissimilar metals. Many welding processes are accomplished by heat alone, with no pressure applied; others by a combination of heat and pressure; and still others by pressure alone, with no external heat supplied. In some welding processes a filler material is added to facilitate coalescence. The assemblage of parts that are joined by welding is called a weldment. Welding is most commonly associated with metal parts, but the process is also used for joining plastics. 2. Types of Welding Processes: Some 50 different types of welding operations have been cataloged by the American Welding Society. They use various types or combinations of energy to 1

2 provide the required power. We can divide the welding processes into two major groups: (A) fusion welding and (B) solid-state welding. A. Fusion-welding processes use heat to melt the base metals. In many fusion welding operations, a filler metal is added to the molten pool to facilitate the process and provide bulk and strength to the welded joint. Fusion welding processes can be organized into the following general groups: 1- Arc welding (AW). Arc welding refers to a group of welding processes in which heating of the metals is accomplished by an electric arc. Some arc welding operations also apply pressure during the process and most utilize a filler metal. 2- Resistance welding (RW). Resistance welding achieves coalescence using heat from electrical resistance to the flow of a current passing between the faying surfaces of two parts held together under pressure. 3- Oxyfuel gas welding (OFW). These joining processes use an oxyfuel gas, such as a mixture of oxygen and acetylene, to produce a hot flame for melting the base metal and filler metal, if one is used. 4- Other fusion-welding processes. Other welding processes that produce fusion of the metals joined include electron beam welding and laser beam welding. B. Solid-state welding refers to joining processes in which coalescence results from application of pressure alone or a combination of heat and pressure. If heat is used, the temperature in the process is below the melting point of the metals being welded. No filler metal is utilized. Representative welding processes in this group include: 2

3 1- Diffusion welding (DFW). Two surfaces are held together under pressure at an elevated temperature and the parts coalesce by solid-state diffusion. 2- Friction welding (FRW). Coalescence is achieved by the heat of friction between two surfaces, accompanied by an axial pressure force. 3- Ultrasonic welding (USW). Moderate pressure is applied between the two parts and an oscillating motion at ultrasonic frequencies is used in a direction parallel to the contacting surfaces. The combination of normal and vibratory forces results in shear stresses that remove surface films and achieve atomic bonding of the surfaces. 3. Welding Applications. The principal applications of welding are: 1- Construction, such as buildings and bridges. 2- Piping, pressure vessels, boilers, and storage tanks. 3- Shipbuilding. 4- Aircraft and aerospace. 5- Automotive and railroad. 4. The Weld Joint Welding produces a solid connection between two pieces, called a weld joint. A weld joint is the junction of the edges or surfaces of parts that have been joined by welding. 3

4 Types of Joints: There are five basic types of joints for bringing two parts together for joining (figure.1). The five joint types are not limited to welding; they apply to other joining and fastening techniques as well. The five types of joint are: 1- Butt joint. In this joint type, the parts lie in the same plane and are joined at their edges. 2- Corner joint. The parts in a corner joint form a right angle and are joined at the corner of the angle. 3- Lap joint. This joint consists of two overlapping parts. 4- Tee joint. In a tee joint, one part is perpendicular to the other in the approximate shape of the letter T. 5- Edge joint. The parts in an edge joint are parallel with at least one of their edges in common, and the joint is made at the common edge(s). Figure.1 the basic types of joints. Types of Welds: Each of the preceding joints can be made by welding. It is appropriate to distinguish between the joint type and the way in which it is welded the weld type. Differences among weld types are in geometry (joint type) and welding process. 4

5 a- Fillet weld, as in figure.2. b- Groove welds, as shown in figure.3. c- Plug welds and slot welds, as in figure.4. d- Spot welds and seam welds, as in figure.5. e- Flange welds and surfacing welds, as in figure.6. A surfacing weld is not used to join parts, but rather to deposit filler metal onto the surface of a base part in one or more weld beads Figure.2 fillet welds. Figure.3 groove welds. 5

6 Figure.4 plug welds and slot welds. Figure.5 spot welds and seam welds. Figure.6 flange welds and surfacing welds. 6

7 5. Physics of Welding 5.1 Power Density Heat density can be defined as the power transferred to the work per unit surface area, W/mm 2. The time to melt the metal is inversely proportional to the power density. At low power densities, a significant amount of time is required to cause melting. If power density is too low, the heat is conducted into the work as rapidly as it is added at the surface, and melting never occurs. It has been found that the minimum power density required to melt most metals in welding is about 10 W/mm 2. As heat density increases, melting time is reduced. If power density is too high -above around 10 5 W/mm 2 - the localized temperatures vaporize the metal in the affected region. Table.1 provides a comparison of power densities for the major fusion welding processes. For metallurgical reasons, it is desirable to melt the metal with minimum energy, and high power densities are generally preferable. Table.1 power densities for the major fusion welding processes. Power density can be computed as the power entering the surface divided by the corresponding surface area: 7

8 Where PD = power density, W/mm 2 ; P = power entering the surface, W; and A = surface area over which the energy is entering, mm Heat Balance in Fusion Welding The quantity of heat required to melt a given volume of metal depends on 1- The heat to raise the temperature of the solid metal to its melting point, which depends on the metal s volumetric specific heat. 2- The melting point of the metal. 3- The heat to transform the metal from solid to liquid phase at the melting point, which depends on the metal s heat of fusion. This quantity of heat required to melt a given volume of metal can be estimated as: Where U m = the unit energy for melting (i.e., the quantity of heat required to melt a unit volume of metal starting from room temperature), J/mm 3 ; T m = melting point of the metal on an absolute temperature scale, ºK; and K = constant whose value is 3.33*10-6, J mm -3 K -2. Not all of the energy generated at the heat source is used to melt the weld metal. There are two heat transfer mechanisms at work, both of which reduce the amount of generated heat that is used by the welding process, as in figure.7. The first mechanism involves the transfer of heat between the heat source and the surface of the work. This process has a certain heat transfer factor (f 1 ), defined as the ratio of the actual heat received by the work piece divided by the total heat 8

9 generated at the source. The second mechanism involves the conduction of heat away from the weld area to be dissipated throughout the work metal, so that only a portion of the heat transferred to the surface is available for melting. This melting factor (f 2 ) is the proportion of heat received at the work surface that can be used for melting. Figure.7 heat transfer mechanisms in fusion welding. The combined effect of these two factors is to reduce the heat energy available for welding as follows: H w = f 1 f 2 H Where H w = net heat available for welding, J; f 1 = heat transfer factor, f 2 = the melting factor, and H = the total heat generated by the welding process, J. The factors f 1 and f 2 range in value between zero and one. We can now write a balance equation between the net energy input and the volume of welded metal: 9

10 H w = U m V Where H w = net heat energy used by the welding operation, J; U m = unit melting energy required to melt the metal, J/mm 3 ; and V = the volume of metal melted, mm 3. Most welding operations are rate processes; that is, the net heat energy H w is delivered at a given rate, and the weld bead is made at a certain travel velocity. This is characteristic for example of most arc-welding, many oxyfuel gas-welding operations, and even some resistance welding operations. It is therefore appropriate to express above equation as a rate balance equation: R Hw = U m R WV Where R Hw = rate of net heat energy delivered to the operation for welding, J/s = W; and R WV = volume rate of metal welded, mm 3 /s. In the welding of a continuous bead, the volume rate of metal welded is the product of weld area A w and travel velocity v. Substituting these terms into the above equation, the rate balance equation can now be expressed as: R Hw = f 1 f 2 R H = U m A w v Where A w = weld cross-sectional area, mm 2 ; and v = the travel velocity of the welding operation, mm/s. 6. Fusion Welding Processes: Fusion welding processes can be organized into the following general groups: 10

11 6.1 Arc welding (AW) An electric arc is a discharge of electric current across a gap in a circuit. It is sustained by the presence of a thermally ionized column of gas (called a plasma) through which current flows. To initiate the arc in an AW process, the electrode is brought into contact with the work and then quickly separated from it by a short distance. The electric energy from the arc thus formed produces temperatures of 5500 ºC or higher, sufficiently hot to melt any metal. A pool of molten metal, consisting of base metal(s) and filler metal (if one is used) is formed near the tip of the electrode. In most arc welding processes, filler metal is added during the operation to increase the volume and strength of the weld joint. As the electrode is moved along the joint, the molten weld pool solidifies in its wake. The resulting power balance in arc welding is defined by: R Hw = f 1 f 2 I E = U m Aw v Where E = voltage, V; I = current, A. The units of R Hw are watts (current multiplied by voltage), which equal J/sec Shielded Metal Arc Welding Shielded metal arc welding (SMAW) is an AW process that uses a consumable electrode consisting of a filler metal rod coated with chemicals that provide flux and shielding as in figure.8. The welding stick (SMAW is sometimes called stick welding) is typically 225 to 450mm (9 18 in) long and 2.5 to 9.5mm (3/32 3/8 in) in diameter. The filler metal used in the rod must be compatible with the metal to be welded, the composition usually being very close to that of the base metal. The coating consists of powdered cellulose (i.e., cotton and wood powders) mixed with oxides, carbonates, and other ingredients, held together by a silicate binder. Metal 11

12 powders are also sometimes included in the coating to increase the amount of filler metal and to add alloying elements. The heat of the welding process melts the coating to provide a protective atmosphere and slag for the welding operation. It also helps to stabilize the arc and regulate the rate at which the electrode melts. Currents typically used in SMAW range between 30 and 300 A at voltages from 15 to 45 V. Selection of the proper power parameters depends on the metals being welded, electrode type and length, and depth of weld penetration required. Shielded metal arc welding is usually performed manually. Common applications include construction, pipelines, machinery structures, shipbuilding, job shop fabrication, and repair work. It is preferred over oxyfuel welding for thicker sections above 5 mm (3/16 in) because of its higher power density. The equipment is portable and low cost, making SMAW highly versatile and probably the most widely used of the AW processes. Base metals include steels, stainless steels, cast irons, and certain nonferrous alloys. It is not used or seldom used for aluminum and its alloys, copper alloys, and titanium. Figure.8 shielded metal arc welding (SMAW) process. 12

13 6.1.2 Gas Metal Arc Welding Gas metal arc welding (GMAW) is an AW process in which the electrode is a consumable bare metal wire, and shielding is accomplished by flooding the arc with a gas. The bare wire is fed continuously and automatically from a spool through the welding gun, as illustrated in Figure.9. Figure.9 gas metal arc welding (GMAW) process. Because GMAW uses continuous weld wire rather than welding sticks, it has a significant advantage over SMAW in terms of arc time when performed manually. For the same reason, it also lends itself to automation of arc welding. The electrode stubs remaining after stick welding also wastes filler metal, so the utilization of electrode material is higher with GMAW. Other features of GMAW include elimination of slag removal (since no flux is used), higher deposition rates than SMAW, and good versatility. 13

14 6.1.3 Gas Tungsten Arc Welding Gas tungsten arc welding (GTAW) is an AW process that uses a non-consumable tungsten electrode and an inert gas for arc shielding. The term TIG welding (tungsten inert gas welding) is often applied to this process. GTAW can be implemented with or without a filler metal. Figure.10 illustrates the latter case. Tungsten is a good electrode material due to its high melting point of 3410 ºC. Typical shielding gases include argon, helium, or a mixture of these gas elements. GTAW is applicable to nearly all metals in a wide range of stock thicknesses. It can also be used for joining various combinations of dissimilar metals. Its most common applications are for aluminum and stainless steel. Figure.10 gas tungsten arc welding (GTAW) process. GTAW Polarity: three different polarities in GTAW which are described next. A. Direct-Current Electrode Negative (DCEN) This, also called the straight polarity, is the most common polarity in GTAW. The electrode is connected to the negative terminal of the power supply. As shown in figure.11 (a), electrons are emitted from the tungsten electrode and accelerated while traveling through the arc. A significant amount of energy, called the work 14

15 function, is required for an electron to be emitted from the electrode. When the electron enters the workpiece, an amount of energy equivalent to the work function is released. This is why in GTAW with DCEN more power (about two-thirds) is located at the work end of the arc and less (about one-third) at the electrode end. Consequently, a relatively narrow and deep weld is produced. B. Direct-Current Electrode Positive (DCEP) This is also called the reverse polarity. The electrode is connected to the positive terminal of the power source, as shown figure.11 (b). The heating effect of electrons is now at the tungsten electrode rather than at the workpiece. Consequently, a shallow weld is produced. Furthermore, large-diameter, watercooled electrodes must be used in order to prevent the electrode tip from melting. The positive ions of the shielding gas bombard the workpiece, knocking off oxide films and producing a clean weld surface, as shown in figure.12. Therefore, DCEP can be used for welding thin sheets of strong oxide forming materials such as aluminum and magnesium, where deep penetration is not required. Figure.11 polarities in GTAW process. 15

16 C. Alternating Current (AC) Reasonably good penetration and oxide cleaning action can both be obtained, as in figure.11 (c). This is often used for welding aluminum alloys. Tungsten Electrodes: Tungsten electrodes with 2% cerium or thorium have better electron emissivity, current carrying capacity, and resistance to contamination than pure tungsten electrodes. As a result, arc starting is easier and the arc is more stable. Shielding Gases: Both argon and helium can be used. Since it is easier to ionize argon than helium, arc initiation is easier and the voltage drop across the arc is lower with argon. Also, since argon is heavier than helium, it offers more effective shielding and greater resistance to cross draft than helium. Argon also has a greater oxide cleaning action than helium. These advantages plus the lower cost of argon make it more attractive for GTAW than helium. Figure.12 the positive ions of the shielding gas bombard the workpiece. Advantages and Disadvantages: Gas tungsten arc welding is suitable for joining thin sections because of its limited heat inputs. The feeding rate of the filler metal is somewhat independent of the welding current, thus allowing a variation in the 16

17 relative amount of the fusion of the base metal and the fusion of the filler metal. Therefore, the control of dilution and energy input to the weld can be achieved without changing the size of the weld. It can also be used to weld butt joints of thin sheets by fusion alone, that is, without the addition of filler metals (autogenous welding). Since the GTAW process is a very clean welding process, it can be used to weld reactive metals, such as titanium and zirconium, aluminum, and magnesium. However, the deposition rate in GTAW is low. Excessive welding currents can cause melting of the tungsten electrode and results in brittle tungsten inclusions in the weld metal. However, by using preheated filler metals, the deposition rate can be improved Plasma Arc Welding Plasma arc welding (PAW) is a special form of gas tungsten arc welding in which a constricted plasma arc is directed at the weld area. In PAW, a tungsten electrode is contained in a specially designed nozzle that focuses a high-velocity stream of inert gas (e.g., argon or argon hydrogen mixtures) into the region of the arc to form a high velocity, intensely hot plasma arc stream, as in figure.13. Argon, argon hydrogen, and helium are also used as the arc-shielding gases. Temperatures in plasma arc welding reach 17,000 ºC or greater, hot enough to melt any known metal. The reason why temperatures are so high in PAW (significantly higher than those in GTAW) derives from the constriction of the arc. Although the typical power levels used in PAW are below those used in GTAW, the power is highly concentrated to produce a plasma jet of small diameter and very high power density. 17

18 Figure.13 plasma arc welding process. Plasma arc welding was introduced around 1960 but was slow to catch on. In recent years its use is increasing as a substitute for GTAW in applications such as automobile subassemblies, metal cabinets, door and window frames, and home appliances. Owing to the special features of PAW, its advantages in these applications include good arc stability, better penetration control than most other AW processes, high travel speeds, and excellent weld quality. The process can be used to weld almost any metal, including tungsten. Difficult-to-weld metals with PAW include bronze, cast irons, lead, and magnesium. Other limitations include high equipment cost and larger torch size than other AW operations, which tends to restrict access in some joint configurations. 18

19 Questions and Problems A. Review Questions 1- What is meant by the term faying surface? 2- Define the term fusion weld. 3- What is the fundamental difference between a fusion weld and a solid state weld? 4- What is an autogenous weld? 5- Name and sketch the five joint types. 6- What is the unit melting energy in welding, and what are the factors on which it depends? B. Problems 1- In a laser beam welding process, what is the quantity of heat per unit time (J/sec) that is transferred to the material if the heat is concentrated in circle with a diameter of 0.2 mm? Assume the power density provided in Table A heat source can transfer 3500 J/sec to a metal part surface. The heated area is circular, and the heat intensity decreases as the radius increases, as follows: 70% of the heat is concentrated in a circular area that is 3.75 mm in diameter. Is the resulting power density enough to melt metal? 3- A welding heat source is capable of transferring 2600 J/s to the surface of a metal part. The heated area is approximately circular, and the heat intensity decreases with increasing radius as follows: 50% of the power is transferred within a circle of diameter = 2.5 mm and 75% is transferred within a concentric circle of diameter = 6.25 mm. What are the power densities in (a) the 2.5 mm diameter inner circle and (b) the 6.25 mm diameter ring that lies 19

20 around the inner circle? (c) Are these power densities sufficient for melting metal? 4- A fillet weld has a cross-sectional area of 25.0 mm2 and is 300mmlong. (a)what quantity of heat (in J) is required to accomplish the weld, if the metal to be welded is low carbon steel (T m = 1760 K)? (b) How much heat must be generated at the welding source, if the heat transfer factor is 0.75 and the melting factor = 0.63? 5- AU-groove weld is used to butt weld 2 pieces of 7.0 mm - thick titanium plate. The U-groove is prepared using a milling cutter so the radius of the groove is 3.0 mm. During welding, the penetration of the weld causes an additional 1.5 mm of material to be melted. The final cross-sectional area of the weld can be approximated by a semicircle with a radius of 4.5 mm. The length of the weld is 200 mm. The melting factor of the setup is 0.57 and the heat transfer factor is (a) What is the quantity of heat (in J) required to melt the volume of metal in this weld (filler metal plus base metal)? Assume the resulting top surface of the weld bead is flush with the top surface of the plates. (b) What is the required heat generated at the welding source? 6- The welding power generated in a particular arc welding operation = 3000 W. This is transferred to the work surface with a heat transfer factor = 0.9. The metal to be welded is copper whose melting point is 1350 K. Assume that the melting factor = A continuous fillet weld is to be made with a cross-sectional area = 15.0 mm 2. Determine the travel speed at which the welding operation can be accomplished. 7- A welding operation on an aluminum alloy makes a groove weld. The crosssectional area of the weld is 30.0 mm 2. The welding velocity is 4.0 mm/sec. The heat transfer factor is 0.92 and the melting factor is The melting 20

21 temperature of the aluminum alloy is 650 C. Determine the rate of heat generation required at the welding source to accomplish this weld. 8- A fillet weld is used to join 2 medium carbon steel (T m = 1700 K) plates each having a thickness of 5.0 mm. The plates are joined at a 90 angle using an inside fillet corner joint. The velocity of the welding head is 6 mm/sec. Assume the cross section of the weld bead approximates a right isosceles triangle with a leg length of 4.5 mm, the heat transfer factor is 0.80, and the melting factor is Determine the rate of heat generation required at the welding source to accomplish the weld. 21