Adhesive Joining
Fundamentals and Principles of Adhesive Joining
Adhesive bonding is a joining process for metals and non-metals that uses and adhesive or glue, typically in the form of a liquid or a paste. It offers the major advantage of being able to join a wide array of materials, but is limited by joint strength and applicable service conditions. Many of the fundamentals are similar to brazing and soldering, in particular the need for wetting and capillary action, and overlapping joint designs that rely on joint area for strength. Adhesives are categorized as thermosetting or thermoplastic. Thermosetting adhesives require a chemical reaction to cure and cannot be remelted once cured. They are the most common adhesives for structural applications. Thermoplastic adhesives soften when heated and harden when cooled, a process that can be continually repeated. They are generally not used for structural applications because of their poor mechanical properties (in particular, creep) and decomposition that can occur at relatively low temperatures.
Surface preparation is usually a very important and time-consuming step prior to applying the adhesive, and typically includes cleaning and deoxidizing procedures. Further steps may be taken such as the application of a coating. Adhesives may be applied a variety of ways, including caulking and spray guns, dipping, rollers, and brushes. Curing may be accelerated by heating or some other energy source. In addition to joining, adhesives may be used for many other reasons including sealing, electrical insulation, and sound suppression. Some of the larger users of adhesive bonding include the automotive, appliance, and electrical/electronics industry sectors.
Adhesive Joining Procedures
The bond strength of an adhesive is constant, and in design applications, is proportional to the area covered by the adhesive. The adhesive joint strength will be unchanged and analysis of the joint should be comprehensive. In general, the use of AHSS with high-strength structural adhesives will result in higher bond strength than for mild steel if the same sheet thickness is applied (Figure 1). Reduction of sheet thickness will decrease the strength because more peel load will occur. The true mechanical load in the component part must be considered. If higher joint strengths are needed, the overlapped area may be enlarged. Adhesives with even higher strength are under development.
Figure 1: Effect of material strength on bond strength (W is the integral of the force/elongation curve.B-2).
Joining of AHSS with adhesive bonding is a good method to improve stiffness and fatigue strength in comparison to other joining methods (spot welding, mechanical joining, arc welding, and laser welding). Due to the larger bonding area with adhesive bonding, the local stresses can be reduced and therefore the fatigue strength is increased. These improvements in stiffness and fatigue strength are important factors to consider at the design stage, especially in those cases when AHSS is used to decrease the weight of a component.
RSW Modelling and Performance
The advantages of numerical simulations for resistance welding are obvious for saving time and reducing costs in product developments and process optimizations. Today’s modeling techniques can predict temperature, microstructure, stress, and hardness distribution in the weld and Heat Affected Zone (HAZ) after welding. Commercial modeling software is available which considers material type, various current modes, machine characteristics, electrode geometry, etc. An example of process simulation results for spot welding of 0.8-mm DC 06 low-carbon steel to 1.2-mm DP 600 steel is shown in Figure 1. Obviously, this technique can apply to dissimilar thicknesses, material types, and geometries. Application of adhesives is also being used with these simulations. This simulation techniques are found to be very beneficial to predict vehicle crashworthiness as it can dramatically reduce the cost of crash evaluations.
You will find several articles in this section describing RSW modelling studies and procedures.
Figure 1: Simulation results with microstructures and hardness distribution for spot welding of 0.8-mm DC06 low-carbon steel to 1.2-mm DP 600 steel.Z-1
Arc Welding
Fundamentals and Principles of Arc Welding
This section serves as an introduction to all the arc welding processes. The common features and important concepts and terminology of this family of processes are reviewed, with more process-specific details provided in the sections covering the specific processes.
Arc welding refers to a family of processes that rely on the extreme heat of an electric arc to produce a weld. They may or may not rely on additional filler metal to create the weld. Although generally considered “low-tech”, arc welding continues to be very popular primarily due to its low equipment cost and high flexibility. Some of the key discoveries that led to modern arc welding include the discovery of the electric arc in the 1820s (Davies), the first welding patent using a carbon electrode in 1886, and the first covered electrode in 1900 (Kjellberg).
The most common arc welding processes today are illustrated in Figure 1. The abbreviations refer to American Welding Society (AWS)A-11 terminology as follows:
- EGW – Electroglas Welding
- FCAW – Flux-Cored Arc Welding
- GMAW – Gas Metal Arc Welding
- GTAW – Gas Tungsten Arc Welding
- PAW – Plasma Arc Welding
- SAW – Submerged Arc Welding
- SMAW – Shielded Metal Arc Welding
- SW – Arc Stud Welding
Figure 1: Common arc welding processes.
Whereas the welding engineer should always use proper AWS terminology during formal communications, in reality, the use of slang terminology for SMAW, GMAW, and GTAW processes is very common. Thus, where appropriate the “slang” terminology is included in italics.
With all arc welding processes, the initiation of an arc basically completes (or closes) an electrical circuit consisting of the ground and work cables, the welding torch, the workpiece or parts to be welded, and the secondary of the welding power supply. Voltages provided by the power supply are commonly either 60 or 80 V. Such voltages are high enough to establish and maintain an arc, but low enough to minimize the risk of electric shock. Once the arc is struck, actual arc voltages commonly range between 10 and 35 V. Direct Current (DC) is most common, but Alternating Current (AC) is sometimes used. Pulsed DC is becoming a common feature in modern welding power supplies. The electrical polarity used during arc welding is very important, but it has different effects with different processes. The effect of polarity on heat input is especially important with GTAW and GMAW, but the effects are opposite. With GTAW, direct-current electrode negative (DCEN) produces the greatest amount of heat into the part and is the most common polarity. However, with GMAW, direct-current electrode positive (DCEP) produces the greatest amount of heat into the part and is used almost exclusively with this process (Figure 2).
Figure 2: DCEP – common with GMAW.
The heat input during arc welding is primarily a function of weld travel speed and current, based on the following equation:
Although voltage appears to play a prominent role in the heat input equation, it is a parameter that is chosen primarily to create the most stable arc, not to affect heat input.
AWS filler metal classifications vary somewhat depending on the process. A common example is the classification system for SMAW electrodes, “EXXXX” where “E” stands for electrode, the two digits following the E give the minimum deposited weld metal tensile strength in kips per square inch (ksi) (there will be a third digit if the strength is 100 ksi or higher), the third “X” provides information on what welding positions that electrode can be used for, and the final “X” provides information about the coating type. The electrode and filler metal classification schemes will be covered in more detail in the subsequent chapters covering each of the arc welding processes.
Shielding
When metals are heated to high temperatures approaching or exceeding their melting point, diffusion rates are accelerated and the metals become very susceptible to contamination from the atmosphere. Elements that can be most damaging are oxygen, nitrogen, and hydrogen, and contamination from these elements can result in the formation of embrittling phases (such as oxides and nitrides), and porosity. To avoid this contamination, the metal must be shielded as it solidifies and begins to cool. The arc welding processes all rely on either a gas or a flux, or a combination of both for shielding. The way these processes are shielded is their main distinguishing feature from one another.
Processes such as GMAW, GTAW, and PAW rely solely on gas shielding. Shielding gasses protect by purging the susceptible metal from atmospheric gasses. The GMAW process commonly uses argon (Ar), carbon dioxide (CO2), or blends of Ar and CO2. CO2 gas produces more spatter and a rougher weld appearance. It can produce fast welding speeds, is readily available, and is cheap. Additions of CO2 or small amounts of O2 to Ar can improve puddle flow. The choice of shielding gas for GMAW plays a major role in the type of molten metal transfer mode from the electrode to the weld puddle.
Arc Welding Weld Joints and Types
The selection of a proper weld joint and weld type is a very important aspect of arc welding. The joint refers to how the workpiece or parts that are being welded are arranged relative to each other, and weld type refers to how the weld is formed in the joint. Specifically in arc welding, there are numerous joint types, but only two weld types, a fillet and a groove weld. A fillet weld offers the advantage of requiring no special joint preparation because the geometry of the joint provides the appropriate features to place the weld. Groove welds facilitate the creation of full-penetration welds which are often required in critical applications. The choice of weld and joint type is often dictated by the design of the component being welded but will play a major role in the properties of that joint. The thickness of the parts being welded, as well as the material and type of welding process being used may also affect the choice of weld or joint type. Some very common arc welding joints and weld types are shown in Figure 3. Basic welding positions are shown in Figure 4. It is recommended that all welding joints be positioned for welding in either the flat or horizontal position whenever possible. The horizontal or vertical plane of the flat and horizontal joint may vary up to a maximum of 10 degrees.
Figure 3: Typical arc welding joint and weld types for automotive sheet steel applications.
Figure 4: Basic arc welding positions.
Electrode Feed Rate
When using the semiautomatic arc welding processes such as GMAW electrode feed rate (or wire feed speed) determines both deposition rate and current. Higher feed rates increase weld metal deposition and increase current since melting rates at the end of the wire must increase as the wire feed speed increases. As a result, with the semiautomatic processes, current is typically adjusted by changing the wire feed speed since the two are relatively proportional. Typical wire feed speeds are between 100 and 500 inches per minute (ipm).
Welding Travel Speed
Travel speed refers to how fast the welding arc is moving relative to the workpiece. The heat input equation clearly shows that travel speed, like current, plays a direct role in the amount of heat into the part. Faster speeds produce less heat into the part and reduced weld metal deposit. The choice of travel speed is typically driven by productivity, with the obvious desire to usually weld as fast as possible. Travel speed is independent of current and voltage and may be controlled by the welder or mechanized. Typical travel speeds range between 5 and 100 ipm.
Arc Welding Safety
There are many hazards associated with arc welding that are not only important concerns for the welder, but for personnel working around any arc welding operations. This section will provide a very brief overview of the most common hazards of which the welding personnel should be aware. It is strongly recommended regarding safety in arc welding and other welding and associated processes to refer to the American National Standards Institute (ANSI) Document Z49.1, “Safety in Welding, Cutting, and Allied Processes”.
Ultraviolet radiation from the arc can damage the eyes and burn the skin in the same way skin is burned from the sun. This requires the use of proper shielding for the eyes and protective clothing to cover any exposed skin. Personnel working near arc welders should be careful not to glance at an open arc without proper shielding. Sparks and spatter during welding mandate the need for proper eye protection for anyone near the welding operation. Additional protective helmets are needed for the welder. Although the low voltages used in arc welding are relatively safe, proper electrical safety must be exercised at all times, including grounding of parts and equipment and avoiding damp conditions.
Welding fumes can be hazardous to the welder when inhaled over long periods of times so proper ventilation is paramount. Shielding gases can produce suffocation in enclosed spaces, such as when welding in tanks. Ar is heavier than air, and in the absence of proper ventilation, will displace oxygen when it fills a room. Helium is lighter than air producing a similar risk for overhead welding. Compressed shielding gas bottles can explode when mishandled or abused, or an arc strike can weaken the bottle, leading to an explosion. Hot metal is always a hazard with fusion welding processes like arc welding. When working around a welding operation, one should always assume that any piece of metal is hot. Welding arcs and associated hot metal spatter are ready sources for ignition of flammable materials in the vicinity of welding. Many fires have been started by careless welders who are not aware of any combustible material.A-11, P-6
Arc Welding Procedures
Conventional arc welding (for example GMAW, TIG, and plasma) can be used for AHSS in a similar way to mild steels. The same shielding gases can be used for both AHSS and mild steels. For automotive applications, a design gap tolerance (G) of 0-0.5 mm is allowed for all weld joints, as illustrated in Figure 5. An edge trim tolerance (Et) of ±0.5 mm is required where the edge is part of the weld joint, as shown in Figure 6. The variation in edge location causes variation in alignment of the electrode wire with the weld joint, as shown in Figure 6. Misalignment of the electrode may cause poor weld shape, improper fusion and burn-though. To control this variable, the trim tolerance at the weld joint must be held to ±0.5 mm and the electrode must maintain a root joint alignment tolerance of ±0.5 mm.
Figure 5: Joint design tolerance.A-12
Figure 6: Edge location tolerance for fillet weld in a lap joint.A-12
A tolerance stack-up review must be performed on all GMAW joints. The worst-case maximum designed gap including tolerance stack-up shall not exceed what is listed in Figure 7. It is preferable to target the smallest possible gap (the thickness of the thinnest sheet or 1.5 mm, whichever is smaller). High-stress areas defined by CAE analysis and/or functional testing should be reviewed for weld optimization. Figure 8 illustrates techniques used to reduce the fillet weld stress concentration and to improve weld performance. These techniques include placing the weld start/stop away from corners and other high-stress areas, avoiding abrupt weld line direction changes when possible, etc.
Figure 7: Maximum GMAW welding gap.A-12
Intermittent welds that are properly sequenced can help keep joints closed by reducing the heat input which reduces distortion. Meanwhile, intermittent welds also introduce weld starts and weld stops, both of which are stress risers. Similar to continuous welds, weld start/stops of intermittent welds should be placed away from high stress areas. Intermittent welds are specified by the center-to-center distance (i.e., pitch) and weld length, as shown in Figure 9.
Figure 8: Reducing weld stress concentrations.A-12
Figure 9: Intermittent fillet weld spacing.A-12
Despite the increased alloying content used for AHSS, there are no increased welding imperfections compared with mild steel arc welds. Changing from mild steel to AHSS may also result in a change of arc blow. The strength of the welds for AHSS increases with increasing base metal strength and sometimes with decreasing heat input. Depending on the chemical composition of AHSS [for example, mild Steels and DP steels with high martensite content and strength levels more than 800 MPa], the strength of the weld joint may be reduced in comparison to the base metal strength due to small soft zones in HAZ (Figure 10). For CP and TRIP grades, no soft zones occur in HAZ due to the higher alloying content for these steels in comparison to DP and mild steels.
Figure 10: Relationship between martensite content and reduction in true ultimate tensile strength (UTS) (Data obtained by thermomechanical simulation of high heat input GMAW HAZ.D-1).
Higher strength filler wires are recommended for welding of AHSS grades with strength levels higher than 800 MPa (Figure 11 for single-sided welded lap joint and Figure 12 for butt joints). It should be noted that higher strength fillers are more expensive and, more importantly, less tolerant to the presence of any weld imperfections. When welding AHSS to lower strength or mild steel, it is recommended that filler wire with 70 ksi (482 MPa) strength be used. Single-sided welded lap joints are normally used in the automotive industry. Due to the asymmetrical loading and the extra bending moment associated with this type of joint, the strength of this lap joint is lower than that of the butt joint.
Figure 11: Influence of filler metal strength in arc welding of DP and mild steels. (Tensile strength is 560 MPa for low strength and 890 MPa for high-strength fillers. Fracture position in HAZ for all cases except DP 700/1000 and MS 1200/1400 combination with low-strength filler where fracture occurred in weld metal. Tensile strength equals peak load divided by cross-sectional area of sample.C-3)
Figure 12: Influence of filler metal strength in GMAW (butt) welding on weld strength for MS steel. (Filler metal tensile strength range is 510-950 MPa.B-1)
Arc welds are normally used in local areas of vehicles where the loads are high. As required with all GMAW of any grade of steel, care should be taken to control heat input and the resulting weld metallurgy. The length of the GMA welds is often quite short. The reduction in strength for some of the AHSS GMA welds, in comparison to BM, can be compensated by increasing the length of the weld.
By adjusting the number and length (that is the total joined area) of welds, the fatigue strength of the joint can be improved. The fatigue strength of an arc-welded joint, in general, tends to be better than that of a spot-welded joint (Figure 13).
Figure 13: Fatigue strength of GMA-welded DP 340/600 compared to spot welding.L-2
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Solid State Welding
Magnetic Pulse Welding (MPW) is a solid-state process that uses electromagnetic pressure to accelerate one workpiece to produce an impact against another workpiece. The metallic bond created by this process is similar to the bond created by explosion welding. MPW, also known as electromagnetic pulse or magnetic impact welding, is highly regarded for the capability of joining dissimilar materials.
Physics of the Process
Electromagnetic metal processing was developed in the late 1800s, and in succeeding years most applications for this technology were in metal forming. It was not recognized as a viable welding process, but a substantial renewal of interest has occurred recently in the further development of this technology for welding.
Fundamentally, both metal forming and welding use the same underlying physics. The process is driven by the primary circuit. A significant amount of energy, usually between 5 and 200 kJ, (1,124- and 44,962-lb force) is stored in capacitors charged to a high voltage that may range between 3,000 and 30,000 V. The capacitors are then discharged through low-inductance and highly conductive bus bars into a coil, or actuator. The resulting current takes the form of a damped sine wave, characterized as a ringing inductance-resistance-capacitance circuit. Peak currents during this process range between tens of thousands and millions of Amperes (A), with pulse widths on the order of tens of microseconds. This creates an extremely intense transient magnetic field in the vicinity of the coil. The magnetic field induces eddy currents in any conductive materials nearby, in the opposite direction to the primary current. The opposing fields in the coil and workpiece result in a high repulsion force. This force drives the flyer, or driver, workpiece (the workpiece closest to the driving coil) at high velocity toward the target, the stationary workpiece, resulting in a high impact between the two metals.
The impact pressure drives away the surface contaminants and provides for the intimate contact of clean surfaces across the weld interface. Metallic bonding results from this contact. A schematic of the process is shown in Figure 1 and in the video which follows.
Figure 1: Basic diagram of the MPW process.
The following three elements are fundamental to achieving good magnetic pulse-welded joints:
- Correct welding machine parameters.
- Consideration of metal or material properties.
- Relative positioning of the flyer and the target workpieces.
Welding machine parameters determine the frequency and magnitude of the current waveform. High frequencies typically are favored for MPW. If the frequency is too low, the buildup of eddy currents in the flier workpiece will not be sufficient to achieve the velocities necessary for impact joining. The frequency is directly related to the electrical characteristics (LRC) of the circuit, including the capacitors and coil. Low system capacitances and inductances favor HF characteristics.
The properties of the workpiece metal, particularly of the flier, also contribute to determining the weldability of a given metal. Properties to be considered include electrical conductivity and strength. Metals with high electrical conductivity and low strength are most easily welded with the magnetic pulse process. Higher electrical conductivity facilitates greater induced currents in the flier workpiece, with correspondingly greater magnetic pressures. Lower YS facilitate displacements of the flier at lower magnetic pressures and are easier to accelerate to the required speed for welding. Carbon steel also can be welded when adjustments are made to the system power and frequency. Metals with relatively low electrical conductivity, such as austenitic stainless steels, are almost impossible to directly weld with the magnetic pulse process. They are readily welded, however, with the use of a driver plate. The driver plate is essentially a band of conductive material (typically Cu) wrapped around the low-conductivity flier. During welding, the driver reacts with the coil, pushing the actual flier to the necessary velocities for metallic bonding.A-11
Power Source
The essential component of an MPW system is a capacitor bank. The energy stored in the system can be determined from the size (capacitance) of the bank and the charge voltage using the following equation:
where:
E = Energy
C = Capacitance
V = Voltage
The energy is provided to the capacitors by a dedicated charging system. The capacity of the charging system largely controls the time required to charge the bank between subsequent welds. The charging circuit generally is actively cooled, allowing repeated use during production applications.
As previously mentioned, energy is transferred from the capacitors to the coil with an assembly of bus bars. Two considerations are key in the design of the bus bar assembly: it must have low inductance (in general, the majority of the system inductance should be at the coil), and low resistance contacts. When in use, the capacitors are charged relatively slowly to a predefined voltage. Once this voltage is reached, a fast-action switch is used to allow current flow to the coil. Switching typically is done using solid-state Silicon-Controlled Rectifiers (SCRs).A-11
Tubular Structures
MPW has great potential for joining tubular structures for automotive and aerospace applications and for fluid-carrying tubes. Examples of MPW tubular applications are shown in Figure 2. The process has several advantages that can significantly reduce manufacturing costs, summarized as follows:
1. HS joints can be produced that are stronger than the BM.
2. Leak-tight welds can be made.
3. High welding speeds, in the millisecond range make the process readily adapted to automation.
4. Dissimilar metals and difficult-to-weld materials, such as 303 stainless steel, can be joined.
5. Cold processing enables immediate handling.
6. Welds are made with no HAZ and minimum distortion.
7. Post-cleaning operations and Post Weld Heat Treatments (PWHT) are unnecessary.
8. The process is cost efficient because no filler metals or shielding gases are needed, and environmental costs are reduced.
Figure 2: Examples of MPW tubular applications.
Applications
MPW has been successfully applied to various similar and dissimilar metal combinations. Materials with high conductivity, such as Al and Cu, are the easiest to weld with the magnetic pulse process. Al has been successfully welded to steel and stainless steel. Cu has been successfully welded to steel and stainless steel.
MPW has been used to join fuel pipes, fuel filters, exhaust system components, power cables, and for the construction of automotive body parts. The development of new applications of the MPW process continues, with the goal of advancing these applications to mass production. The process is achieving increased recognition for applications across the industrial spectrum.A-11
Safe Practices
The potential hazards of MPW include mechanical and electrical risks, noise, flash, and fumes.
Mechanical
The welding machine should be equipped with appropriate safety devices to prevent injury to the operator’s hands or other parts of the body. Initiating devices, such as push buttons or foot switches, should be arranged and guarded to prevent inadvertent actuation.
Machine guards, fixtures, and operating controls must prevent the operator from coming in contact with the coil and workpiece and must block or deflect the weld jet associated with the process.
Electrical
All doors and access panels on machines and controls must be kept locked or interlocked to prevent access by unauthorized personnel. The interlocks should interrupt the power and discharge all the capacitors through a suitable resistive load when the panel door is open.
Personal Protection
Appropriate guards should be in place to isolate the operator from the process. Operating personnel should wear ear protection when the welding operations produce high noise levels.
Additional information on safe practices for welding is provided in the latest edition of ANSI Safety in Welding, Cutting, and Allied Processes, Z49.1 published by AWS.
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Solid State Welding
High Frequency Welding (HFW) is the main welding technology for manufacturing cold-formed welded steel tubes. Welded tubes are normally made from flat sheet material by continuous roll forming and the High Frequency Induction Welding process. The tubes are widely used for automotive applications, including seat structures, cross members, side-impact structures, bumpers, subframes, trailing arms, and twist beams. A welded tube can be viewed as a sheet of steel having the shape of a closed cross section.
Two features distinguish the welded tube from the original sheet material:
- The work hardening which takes place during the tube-forming process.
- The properties and metallurgy of the weld seam differ from those of the BM in the tubular cross section.
Good weldability is one precondition for successful HF welding. Most DP steels are applicable as feed material for manufacturing of AHSS tubes by continuous roll forming and the HFIW process. The quality and the characteristics of the weld depend on the actual steel sheet characteristics (such as chemistry, microstructure, and strength) and the set-up of the tube manufacturing process. Table 1 provides some characteristics of the HF welds in tubes made of DP 280/600 steel.
Table 1: Transverse tensile test data for HFIW DP 280/600 tube.R-1
For DP 280/600 the hardness of the weld area exceeds the hardness of the Base Metal (BM) (Figure 1). There is a limited or no soft zone in the transition from HAZ to BM. The nonexistent soft zone yields a HF weld that is stronger than the BM (Table 1). This is an essential feature in forming applications where the tube walls and weld seam are subject to transverse elongation, such as in radial expansion and in hydroforming.
Figure 1: Weld hardness of a HF weld in a DP 280/600 tube.R-1
Figures 2 and 3 contain additional examples of the hardness distribution across HF welds in different materials compared to mild steel.
Figure 2: Hardness variation across induction welds for various types of steel.M-1
Figure 3: Hardness variation across induction welds of DP 350/600 to mild steel.D-1
Safe Practices
The health and safety of the welding operators, maintenance personnel, and other persons in the welding operations must be considered when establishing operating practices. The design, construction, installation, operation, and maintenance of the equipment, controls, power sources, and tooling should conform to the requirements of the United States Department of Labor in Occupational Safety and Health Standards for General Industry, (29) CFR Part 1910, Subpart Q.1.
The HF power source also must conform to the requirements of the Federal Communication Commission (FCC) as stated in Title 47, Part 15 concerning the radio frequency emissions from industrial, scientific and medical sources. Responsibility for complying with FCC standards is undertaken by the power source manufacturer and does not pose a problem for the end user of the equipment, if the power source is installed following the manufacturer’s recommendations. Information manuals provided by the manufacturers of equipment must be consulted, and recommendations for safe practices must be strictly followed. State, local, and company safety regulations also must be followed. The American Welding Society (AWS) document Safety in Welding, Cutting, and Allied Processes, ANSI Z49.1: 2012 covers safe practices specifically for the welding industry.
Voltages produced by HF power sources with solid-state inverter power sources (as high as 3,000 V) and voltages produced vacuum-tube oscillators (as high as 30,000 V) can be lethal. Proper care and precautions must be taken to prevent injury while working on HF generators and related control systems.
Modern power sources are equipped with safety interlocks on access doors and automatic safety grounding devices that prevent operation of the equipment when access doors are open. The equipment should never be operated with panels or high- voltage covers removed or with interlocks and grounding devices bypassed.
HF currents are more difficult to ground than low- frequency currents, and ground lines should be as short as possible to minimize inductive reactance. All leads between the power source and the contacts or induction coil should be totally enclosed in an insulated or grounded structure and constructed in a way that minimizes Electromagnetic Interference (EMI). Also, care should be taken to prevent the HF magnetic field around the coil and leads from induction heating of the adjacent metal mill components.
The weld area should be protected so that operating personnel cannot come in contact with any exposed contacts or induction coils while these devices are energized. Injuries to personnel from direct contact with HF voltages, especially at the upper range of welding frequencies, may produce severe local tissue damage.
Solid State Welding
Fundamentals and Principles of HF Welding
High Frequency (HF) welding processes rely on the properties of HF electricity and thermal conduction, which determine the distribution of heat in the workpieces. HF contact welding and high-frequency induction welding are used to weld products made from coil, flat, or tubular stock with a constant joint symmetry throughout the length of the weld. Figure 1 illustrates basic joint designs used in HF welding. Figures 1 (A) and (B) are butt seam welds; Figure 1 (C) is a mash seam weld produced with a mandrel, or backside/inside bar. Figure 1 (D) is a butt joint design in strip metal; and Figure 1 (E) shows a T-joint. Figures 1 (F) and (G) are examples of helical pipe and spiral-fin tube joint designs. Figure 1 (J) illustrates a butt illustrates a butt joint in pipe, showing the placement of the coil. Figure 4.L-6 (K) shows a butt joint in bar stock.
Figure 1: Basic joint designs for HF welds in pipe, tube, sheet, and bar stock.
HF current in metal conductors tends to flow at the surface of the metal at a relatively shallow depth, which becomes shallower as the electrical frequency of the power source is increased. This commonly is called the skin effect. The depth of electrical current penetration into the surface of the conductor also is a function of electrical resistivity, and magnetic permeability, the values of which depend on temperature. Thus, the depth of penetration also is a function of the temperature of the material. In most metals, the electrical resistivity increases with temperature; as the temperature of the weld area increases, so does the depth of penetration. For example, the resistivity of low-carbon steel increases by a factor of five between room temperature to welding temperature. Metals that are magnetic at room temperature lose the magnetic properties above the Curie temperature. When this happens, the depth of penetration increases drastically in the portion of metal that is above the Curie temperature while remaining much shallower in the metal that is below the Curie temperature. When these effects are combined in steel heated at a frequency of 400 kHz, the depth of current penetration is 0.05 mm (0.002 in.) at room temperature, while it is 0.8 mm (0.03 in.) at 800°C (1470°F). The depth of current penetration for several metals as a function of frequency is shown in Figure 2.
Figure 2: Effect of frequency on depth of penetration into various metals at selected temperatures.
The second important physical effect governing the HF welding process is thermal conduction of the heat generated by the electric currents in the workpiece. Control of the thermal conduction and of the penetration depth provides control of the depth of heating in the metal. Because thermal conduction is a time-dependent process, the depth to which the heat will conduct depends on the welding speed and the length of the electrical current path in the workpiece. If the current path is shortened or the welding speed is increased, the heat generated by the electric current in the workpiece will be more concentrated and intense. However, if the current path is lengthened or the welding speed is reduced, the heat generated by the electric current will be dispersed and less intense. The effect of thermal conduction is especially important when welding metals with high thermal conductivity, such as Cu or Al. It is not possible to weld these materials if the current path is too long or the welding speed is too slow. Changing the electrical frequency of the HF current can compensate for changes in welding speed or the length of the weld path, and the choice of frequency, welding speed, and path length can adapt the shape of the HAZ to optimize the properties of the weld metal for a particular application.A-11, A-15
Advantages/Disadvantages of HF Welding
A wide range of commonly used metals can be welded, including low-carbon and alloy steels, ferritic and austenitic stainless steels, and many Al, Cu, Ti, and Ni alloys.
Because the concentrated HF current heats only a small volume of metal at the weld interface, the process can produce welds at very high welding speeds and with high energy efficiency. HF can be accomplished with a much lower current and less power than is required for low-frequency or direct-current resistance welding. Welds are produced with a very narrow and controllable HAZ and with no superfluous cast structures. This often eliminates the need for Post-Weld Heat Treatment (PWHT).
Oxidation and discoloration of the metal and distortion of the workpiece are minimal. Discoloration may be further reduced by the choice of welding frequency. Maximum speeds normally are limited by mechanical considerations of material handling, forming and cutting. Minimum speeds are limited by material properties, excessive thermal conduction such that the heat dissipates from the weld area before bringing it to sufficient temperature, and weld quality requirements. The high process speed may also become a disadvantage if process settings are incorrect, as scrap can be generated at very high rates.
Considering the high processing speeds, with the high equipment cost required for HF welding, it is important to understand the amount of product needs for economic justification.
The fit-up of the surfaces to be joined and the way they are brought together are important if high-quality welds are to be produced. However, HF welding is far more tolerant in this regard than some other processes.
Flux is almost never used but can be introduced into the weld area in an inert gas stream. Inert gas shielding of the weld area generally is needed only for joining highly reactive metals such as Ti or for certain grades of stainless steel.A-11, A-15
Induction Seam Welding of Pipe and Tubing
The welding of continuous-seam pipe and tubing is the predominant application of HF induction welding. The pipe or tube is formed from metal strip in a continuous-roll forming mill and enters the welding area with the edges to be welded slightly separated. In the weld area, the open edges of the pipe or tube are brought together by a set of forge pressure rolls in a vee shape until the edges touch at the apex of the vee, where the weld is formed. The weld point occurs at the center of the mill forge rolls, which apply the pressure necessary to achieve a forged weld.
An induction coil, typically made of Cu tubing or Cu sheet with attached water- cooling tubes, encircles the tube (the workpiece) at a distance equal to one to two tube diameters ahead of the weld point. This distance, measured from the weld point to the edge of the nearest induction coil, is called the vee length. The induction coil induces a circumferential current in the tube strip that closes by traveling down the edge of the vee through the weld point and back to the portion of the tube under the induction coil. This is illustrated in Figure 3.
Figure 3: HF induction seam welding of a tube.
The HF current flows along the edge of the weld vee due to the proximity effect (see Fundamentals), and the edges are resistance-heated to a shallow depth due to the skin effect.
The geometry of the weld vee is such that its length usually is between one and one half to two tube diameters long. The included angle of the vee generally is between 3 and 7 degrees. If this angle is too small, arcing between the edges may occur, and it will be difficult to maintain the weld point at a fixed location. If the vee angle is too wide, the proximity effect will be weakened causing dispersed heating of the vee edges, and the edges may tend to buckle. The best vee angle depends on the characteristics of the tooling design and the metal to be welded. Variations in vee length and vee angle will cause variations in weld quality.
The welding speed and power source level are adjusted so that the two edges are at the welding or forge temperature when they reach the weld point. The forge rolls press the hot edges together, applying an upset force to complete the weld. Hot metal containing impurities from the faying surfaces of the joint is squeezed out of the weld in both directions, inside and outside the tube. The upset metal normally is trimmed off flush with the BM on the outside of the tube, and sometimes is trimmed from the inside, depending on the application for the tube being produced.A-11, A-15
The HF contact welding process provides another means of welding continuous seams in pipe and tubing. The process essentially is the same as that described above for induction welding and is illustrated in Figure 4. The major difference is that sliding contacts are placed on the tube adjacent to the unwelded edges at the vee length. With the contact process, the vee length generally is shorter than that used with the induction process. This is because the contact tips normally can be placed within the confines of the forge rolls where the induction coil must be placed sufficiently behind the forge rolls, so that the forge rolls are not inductively heated by the magnetic field of the induction coil. Because of the shorter vee lengths achievable with the contact process, an impeder often is not necessary, particularly for large-diameter tubes where the impedance of the current path inside the tube has significant inductive reactance.
Figure 4: Joining a tube seam with HFRW using sliding contacts. Note: Vee length extends from point of welding to sliding contacts.
An impeder, which is made from a magnetic material such as ferrite, generally is required to be placed inside the tube. The impeder is positioned so that it extends about 1.5 to 3 mm (1/16 to 1/8 in.) beyond the apex of the vee and the equivalent of one to two workpiece diameters upstream of the induction coil. The purpose of the impeder is to increase the inductive reactance of the current path around the inside wall of the workpiece. This reduces the current that would otherwise flow around the inside of the tube and cause an unacceptable loss of efficiency. The impeder also decreases the magnetic path length between the induction coil and the tube, further improving the efficiency of power transfer to the weld point. The impeder must be cooled to prevent its temperature from rising above its Curie temperature, where it becomes nonmagnetic. For ferrite, the Curie temperature typically is between 170 and 340°C (340 and 650°F).A-11, A-15
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