Welding Inspection Technology PDF

WELDING INSPECTION TECHNOLOGY CHAPTER 3—METAL JOINING AND CUTTING PROCESSES ing process because the heat for welding is generated by an arc between the stud and the base metal. The process is controlled by a mechanical gun which is attached to a power supply through a control panel. So, welding is...

WELDING INSPECTION TECHNOLOGY CHAPTER 3—METAL JOINING AND CUTTING PROCESSES ing process because the heat for welding is generated by an arc between the stud and the base metal. The process is controlled by a mechanical gun which is attached to a power supply through a control panel. So, welding is accomplished very easily and repetitively. The process is performed in four cycles which are timed and sequenced by the control box once the stud is positioned and the trigger is pulled. Figure 3.39 illustrates this sequence. Sketch (a) shows the stud gun with the stud and ferrule in position, and then in (b) being positioned against the workpiece. In (c) the trigger has been pulled to initiate the current flow, and the gun lifts the stud to maintain the arc. In (d) the arc quickly melts the stud end and a spot on the workpiece beneath the stud. A timer in the gun then cuts off the current and the guns’ mainspring plunges the stud into the workpiece (e). The finished stud weld is shown in (f). When properly done, the stud weld should exhibit complete fusion throughout the stud cross section as well as a reinforcing fillet, or “flash,” around the entire circumference of the stud base. (A) (B) (C) (D) (E) (F) Figure 3.39—Stud Welding Cycle Typical SW equipment is shown in Figure 3.40. Stud welding equipment consists of a DC power source, a control unit, and a stud welding gun. Variations can include automatic stud feeding apparatus as well as gas shielding for use in the welding of aluminum studs. The building and bridge industries use SW extensively as shear connectors to structural steel members. Once the concrete is poured, covering the studs attached to the beams, the mechanical connection obtained allows the steel and concrete to act as a composite unit to enhance the overall strength and rigidity of the structure. Due to the convenience and simplicity offered by SW, it has seen tremendous usage in many industries for a variety of metals. Figure 3.41 shows some of the wide variety of stud shapes and sizes available. Figure 3.40—Stud Welding Equipment, Including Power Console and Stud Gun 3-29 CHAPTER 3—METAL JOINING AND CUTTING PROCESSES WELDING INSPECTION TECHNOLOGY chromatic light impinging on the joint to be welded (see Figures 3.42 and 3.43). The high energy of the laser beam causes some of the metal at the joint to vaporize, producing a “keyhole,” which is surrounded by molten metal. As the beam is then advanced along the joint (or the part is moved under the beam), molten metal flows from the forward portion of the keyhole around its periphery and solidifies at the rear to form weld metal. The equipment is fairly complex, as seen in Figure 3.45. Figure 3.41—Some Typical Stud and Fastener Configurations Available for Stud Welding Its wide range of applications is due to the number of advantages which are offered. First, since the process is controlled essentially by the electrical control unit and attached gun, little operator skill is required once the control unit settings are made. Also, SW is a tremendously economical and effective method for welding various attachments to a surface. Its use eliminates the need for hole drilling, tapping or tedious manual welding using some other process. Once welded, a stud can be easily inspected. First a visual examination is made to assure the presence of a 360° flash. Then the stud can be either struck with a hammer or pulled to judge its acceptability. When struck with a hammer, a good stud weld will ring while a poor joint will result in a dull “thud.” If the stud is threaded it can be torque tested to determine its quality. Figure 3.42—Laser Beam Welding Gun Since the process is controlled electrically and mechanically, its predominant limitation relates to this equipment. An electrical or mechanical malfunction could produce poor weld quality. Also, stud shape is limited to some configuration which can be held in the gun’s chuck. SW has two possible discontinuities. They are lack of 360° flash and incomplete fusion at the interface. Both are caused by improper machine settings. Presence of water or heavy rust or mill scale on the base metal surface could also affect the resulting weld quality. Laser Beam Welding (LBW) Laser Beam Welding (LBW) is a fusion joining process that produces coalescence of materials with the heat obtained from a concentrated beam of coherent, mono- Figure 3.43—Laser Weld Being Made on 1/8 in [3.2 mm] Thick Type 304 Stainless Steel 3-30 WELDING INSPECTION TECHNOLOGY CHAPTER 3—METAL JOINING AND CUTTING PROCESSES The main element for laser welding and cutting equipment is the laser device. Inside the equipment, the laser device is placed between end mirrors. When the laser medium is excited or “pumped,” the atoms or molecules in the medium are put into a higher than normal energy state, which generates a source of coherent, monochromatic electromagnetic radiation in the form of light. This light reflects back and forth between the end mirrors, which increases the energy state even more. This results in the device emitting a beam of laser light. Laser is an acronym for “light amplification by stimulated emission of radiation.” The laser beam has a very small cross section and does not diverge, or broaden, much. Thus, it can be transported over relatively long distances through fiber optics and mirrors. The beam is then focused to a very small spot size at the workpiece through the use of either lenses or reflective-type focusing. This provides the high level of beam power density needed to do a variety of material processing tasks, such as welding, cutting, and heat treating. The small laser beam produces a very narrow and deep weld bead (see Figure 3.44). The lasers predominantly used for welding are either solid-state or gas lasers. In the solid-state laser, neodymium-doped, yttrium aluminum garnet (Nd-YAG) crystal rods are utilized to produce a continuous monochromatic laser beam output in the 1 kW to 10 kW power range. In gas lasers for welding, the typical type is the carbon dioxide (CO2) laser. These are electrically excited and can put out a continuous or a pulsed laser beam, with power levels of up to 25 kW. Such lasers are capable of producing full penetration, single-pass welds in steel up to 1-1/4 in [32 mm] thick. Figure 3.45—Production Welding System for Automotive Transmission Components LBW is a noncontact process, and thus requires that no pressure be applied. Inert gas shielding is often employed to prevent oxidation of the molten pool, and filler metal is occasionally used. Major advantages of laser beam welding include the following: • Low overall heat input results in less grain growth in the heat-affected zone and less workpiece distortion. • High depth-to-width ratios (on the order of 10:1) are attainable when the weld is made in the keyhole welding mode. • Single pass laser welds have been made in materials up to 1-1/4 in [32 mm] thick. • The laser beam can be focused on a small area, permitting the joining of thin, small, or closely spaced components. Figure 3.44—Cross Section of a Laser Beam Weld Joining a Boss to a Ring 3-31 CHAPTER 3—METAL JOINING AND CUTTING PROCESSES WELDING INSPECTION TECHNOLOGY • A wide variety of materials can be welded, including combinations of materials with dissimilar physical properties. • The laser beam can be readily focused, aligned, and directed by optical elements. Thus, the laser can be located away from the workpiece and the laser beam directed around tooling and obstacles to the workpiece. • The laser beam is not influenced by the presence of magnetic fields, as in arc and electron beam welding. • No vacuum or X-ray shielding is required, as in electron beam welding. Figure 3.46—Exterior View of an Electron Beam Vacuum Pump • The beam can be transmitted to more than one work station using beam switching optics. Some limitations of the laser beam welding process include: • Joints must be accurately positioned under the beam. • Square groove butt joints are required. • Workpieces must often be forced together. • The high reflectivity and high thermal conductivity of some materials, such as aluminum and copper alloys, can affect their weldability with lasers. • The fast cooling rates can produce cracking and embrittlement in the heat-affected zone and can trap porosity in the weld metal. • With higher power lasers, a plume of vapors is often produced above the weld joint, which interferes with the ability of the laser to reach the joint. A plasma control device is often required, which utilizes an inert gas to blow away the plume of vapors. Figure 3.47—Electron Beam Welding Control Panel • Equipment is expensive, typically in the $100,000 range. Electron Beam Welding (EBW) Since electron beam welding (EBW) was initially used as a commercial welding process in the late 1950s, the process has earned a broad acceptance by industry. The process was initially limited strictly to operation in a high vacuum chamber. However, a system was soon developed that required a high vacuum only in the beam generation portion. This permitted the option of welding in either a medium vacuum chamber or a nonvacuum environment. This advancement led to its acceptance by the commercial automotive and consumer product manufacturers. As a consequence, EBW has been employed in a broad range of industries worldwide (see Figure 3.46 through 3.48). Figure 3.48—Electron Beam Welding Machine Designed for Joining Bimetallic Strip 3-32 WELDING INSPECTION TECHNOLOGY CHAPTER 3—METAL JOINING AND CUTTING PROCESSES piece (see Figure 3.50). Once the beam exits from the gun, it will gradually broaden with distance. To counteract this inherent divergence effect, an electromagnetic lens system is used to converge the beam, which focuses it into a small spot on the workpiece. The beam divergence and convergence angles are relatively small, which gives the concentrated beam a usable focal range, or “depth-of-focus” extending over a distance of an inch or so. EBW is a fusion joining process that produces coalescence of materials with heat obtained by impinging a beam of high-energy electrons onto the joint to be welded. The heart of the electron beam welding process is the electron beam gun/column assembly. Electrons are generated by heating a negatively charged emitting cathode or “filament” to its thermionic emission temperature range, thus causing electrons to “boil off” and be attracted to the positively charged anode (see Figure 3.49). A precisely configured grid or bias cup surrounding the emitter helps accelerate and shape the electrons into the beam. The beam then exits the gun through an opening in the anode and continues on toward the work- There are four basic welding variables: beam accelerating voltage, beam current, beam focal spot size, and welding travel speed. The basic equipment includes a vacuum chamber, controls and an electron beam gun (see Figures 3.46 through 3.48). Typical power levels are 30 kV to 175 kV and 50 mA to 1000 mA. The electron beam produces even higher power densities than a laser beam. Like laser beam welding, electron beam welding is usually done in the “keyhole” mode, which produces very deep and narrow weld beads (see Figure 3.51). In most applications, the weld penetration Figure 3.49—Simplified Representation of a Triode Electron Beam Gun Column Figure 3.50—Electron Beam Welding a Gear in Medium Vacuum 3-33 CHAPTER 3—METAL JOINING AND CUTTING PROCESSES WELDING INSPECTION TECHNOLOGY • Single pass electron beam welds have been made in steels up to 4 in [102 mm] thick. • A high-purity environment (vacuum) for welding minimizes contamination of the metal by oxygen and nitrogen. • Rapid travel speeds are possible because of the high melting rates associated with this concentrated heat source. • Hermetic closures can be welded with the high- or medium-vacuum modes of operation while retaining a vacuum inside the component. • The beam of electrons can be magnetically deflected to produce various shaped welds and magnetically oscillated to improve weld quality or increase penetration. • The focused beam of electrons has a relatively long depth of focus, which will accommodate a broad range of work distances. • Full penetration, single-pass welds with nearly parallel sides, and exhibiting nearly symmetrical shrinkage, can be produced. Figure 3.51—Cross Section of a Nonvacuum Electron Beam Weld in 3/4 in [19 mm] Stainless Steel Plate • Dissimilar metals and metals with high thermal conductivity such as copper can be welded. Some of the limitations of electron beam welding are as follows: formed is much deeper than it is wide, and the heataffected zone produced is very narrow. For example, the width of a butt weld in 0.5 in [13 mm] thick steel plate may be as small as 0.030 in [0.8 mm] when made in a vacuum. This stands in remarkable contrast to the weld zone produced in arc and gas welded joints, where penetration is achieved primarily through conduction melting. • Joints must be accurately positioned under the beam. • Square groove butt joints are required. • Workpieces must often be forced together. • The fast cooling rates can produce cracking and embrittlement in the heat-affected zone and can trap porosity in the weld metal. An electron beam can be readily moved by electromagnetic deflection. In most instances, this deflection is used to adjust the beam-to-joint alignment, or to apply a deflection pattern of a circle, ellipse or other shape. This deflection modifies the average power density into the joint, and results in a change in the weld bead shape. The focal spot can also be adjusted, which will alter the weld shape. • Equipment is expensive. • For high and medium vacuum welding, work chamber size must be large enough to accommodate the assembly operation. The time needed to evacuate the chamber will influence production costs. • Because the electron beam can be deflected by magnetic fields, nonmagnetic or properly degaussed metals should be used for tooling and fixturing close to the beam path. Electron beam welding has unique performance capabilities. The high-power densities and outstanding control solve a wide range of joining problems. The following are advantages of electron beam welding: • Low overall heat input results in less grain growth in the heat-affected zone and less workpiece distortion. • With all modes of EBW, radiation shielding must be maintained to ensure that there is no exposure of personnel to the X-radiation generated by EB welding. • High depth-to-width ratio (on the order of greater than 10:1) are attainable when the weld is made in the keyhole welding mode. • Adequate ventilation is required with nonvacuum EBW, to ensure proper removal of ozone and other noxious gases formed during this mode of EB welding. 3-34 WELDING INSPECTION TECHNOLOGY CHAPTER 3—METAL JOINING AND CUTTING PROCESSES Resistance Welding (RW) Resistance welding (RW) is a group of welding processes that produces coalescence of the joining surfaces with heat obtained from resistance of the workpieces to the flow of welding current in a circuit of which the workpieces are a part, and by the application of pressure. It is typically used for sheet metal applications, up to about 1/8 in [3 mm] thick. No filler metals or fluxes are used. There are three major resistance welding processes: resistance spot welding (RSW), resistance seam welding (RSEW), and projection welding (PW). Electrodes are usually copper alloys, but many different types of electrode materials have been developed for specific purposes, such as for welding of galvanized steel. The most common of these processes is resistance spot welding (RSW), which is shown in Figure 3.52. The electrodes are typically cylindrical in shape, but can have various configurations. The two electrodes apply a force to hold the two pieces of sheet metal in intimate contact. Current is then passed through the electrodes and the workpieces. Resistance to the flow of current produces heat at the faying surfaces, forming a weld nugget (see Figure 3.52). The electrodes continue to apply pressure to hold the sheets in intimate contact during welding. WATER-COOLED COPPER ALLOY ELECTRODE The workpiece surfaces must be very clean to obtain consistent electrical contact and to produce a sound weld nugget. Typically, one spot weld is made at a time. With projection welding (PW), one sheet has projections or dimples formed in it. When the two sheets are placed together, the current is concentrated to pass through the projections at the faying surfaces. Large, flat electrodes are used on opposite sides of the sheets, and current passes through the projections while the electrodes force the sheets together. This allows several welds to be made during a single welding cycle. BASE METAL BASE METAL In resistance seam welding (RSEW) a continuous seam weld is made that is actually a series of overlapping spot welds. The electrodes are typically rotating wheels between which the two sheets pass. Current and pressure are applied in a timed manner to produce a continuous seam weld. WATER-COOLED COPPER ALLOY ELECTRODE Figure 3.52—Resistance Spot Welding Process Equipment ranges from semiautomatic to fully automatic equipment. With semiautomatic equipment, the operator places the sheets to be welded between the electrodes or places a hand-held gun around the pieces and pushes a switch or foot pedal. The weld is made in a preprogrammed sequence. In automatic equipment, parts are automatically loaded into the machine, welded, and then ejected. Robotic resistance spot welding is utilized extensively in the automotive industry. The main welding variables are welding current, welding time, electrode force, and electrode material and design. Typical welding times for a resistance weld are less than a second with current levels of hundreds to thousands of amperes. 3-35 CHAPTER 3—METAL JOINING AND CUTTING PROCESSES WELDING INSPECTION TECHNOLOGY Brazing and Soldering Processes To perform brazing, one of the most important steps is to thoroughly clean the joint surfaces. If the parts are not sufficiently clean, an inadequate joint will result. Once the parts are cleaned and fitted together, heat is applied in some manner. When the parts have been heated to a temperature above the melting point of the braze filler material, the filler metal will be drawn into the joint when placed in contact with the parts as a result of capillary action. Now that the welding processes have been discussed, we will turn our attention to brazing and soldering. Brazing differs from welding in that brazing is accomplished without any melting of the base metals. The heat applied is only sufficient for the melting of the filler metal. Another joining process, soldering, is similar in that it too only requires melting of the filler metal to create a bond. Brazing and soldering are differentiated by the temperature at which the filler metal melts. Filler metals melting above 840°F [450°C] are considered braze materials, while those melting at or below this temperature are used for soldering. Therefore, the common term “silver soldering” is actually incorrect, because silver brazing filler metals melt above 840°F. Capillary action is that phenomenon which causes a liquid to be pulled into a tight space between two surfaces. You could observe this occurrence if two pieces of plate glass were held tightly together and stood on edge in a shallow pan of water. Capillary action will cause the liquid between the pieces of glass to rise to a level above that of the water in the pan. Since capillary action is related to surface tension, it is drastically affected by the presence of surface contamination. Even though the base metals are not melted and there is no fusion between the filler metal and base metals, a bond is created which has substantial strength. When properly applied, the braze joint can develop a strength equal to or greater than the base metal even though the braze material may be much weaker than the base metal. This is possible because of two factors. So, if the surfaces of the braze joint are not properly cleaned, the ability of the capillary action to occur will be reduced to the point that the braze material will not be drawn sufficiently into the joint. When this occurs, an insufficient bond will result. First, the braze joint is designed to have a large surface area. Second, the clearance, or gap, between the two pieces to be joined is kept to a minimum. Gaps greater than about 0.010 in [0.25 mm] may result in a joint having substantially reduced strength. Some typical braze joint configurations are shown in Figure 3.53. As can be seen, all of these joints have relatively large surface areas and tight gaps between parts. The braze filler material is available in a number of configurations and alloy types. The configurations include wire, strip, foil, paste and preforms. Preforms are specially shaped pieces of braze alloy designed for a particular application so that they are preplaced in or near the braze joint during assembly of the parts. Figure 3.54 shows how these braze preforms can be preplaced within a joint prior to the application of the brazing heat. Figure 3.55 then shows how the braze filler metal flows into the joint leaving voids where the preform had been placed. As with the welding consumables, braze alloys also have American Welding Society designations. Braze alloy designations are preceded by a “B” followed by abbreviations of the most prominent chemical elements included (see Table 3.4). Within these general groups are types with slightly different properties which are differentiated by individual numbers. The brazing filler metals having an “R” in front of the “B” in their designations denotes their chemistry is identical with Copper and CopperAlloy Gas Welding Rods. To maintain the cleanliness of the joint during the application of heat, brazing fluxes are often used. They too carry American Welding Society classifications according to the types of base and filler metals being used. They are designated alphanumerically as shown in Table 3.5. Figure 3.53—Examples of Various Braze Joint Configurations (Red Shows Resulting Braze Zone) There are numerous methods of brazing, with the primary difference being the manner in which the joint is 3-36