Learn Solid State Welding Explained – Cold, Ultrasonic, Friction, Resistance, Explosion, Diffusion and Flash Welding – Principles, Quality Assurance, Defects, Inspection and Economics

Introduction:

Solid state welding represents a fundamentally distinct category of joining processes within manufacturing technology. In these processes joining takes place without fusion at the interface of the two parts to be welded. This sets them apart from fusion welding processes where a liquid or molten phase is present in the joint. The core principle of solid state welding is best demonstrated with a practical example: when two exceptionally clean surfaces are brought into extremely close contact with each other under sufficient pressure, they form atomic bonds and produce a permanent joint. For a strong and reliable bond to develop, it is absolutely essential that the interface be entirely free of oxide films, residual chemical compounds, metalworking fluids, particulate contaminants and even adsorbed layers of gas that may have accumulated on the surfaces.

The bonding mechanism in solid state welding involves one or more of three primary physical phenomena. The first is diffusion, which is the transfer of atoms across the interface between the two parts. Applying external heat significantly improves the strength of the bond by accelerating this atomic migration as is prominently observed in diffusion bonding. Heat for these processes may be generated internally by friction as utilized in friction welding through electrical resistance heating as in various resistance welding processes including spot welding or externally by induction heating as in butt welding tubes. The second critical factor is pressure. Generally, the higher the pressure applied to the interface, the stronger the resulting bond. This is evident in roll bonding and explosion welding where significant plastic deformation also occurs at the joint. Pressure and resistance heating may be combined effectively in processes such as flash welding, stud welding, and resistance projection welding. The third phenomenon is relative interfacial movement. When controlled movements of the contacting surfaces known as faying surfaces, occur as in ultrasonic welding even very small oscillating amplitudes effectively disturb the mating surfaces, break up any tenacious oxide films and generate new, atomically clean surfaces, thereby substantially improving the strength of the bond.

The practical implementation of most solid state joining processes has been transformed by modern automation. Robotics, advanced vision systems, sensitive sensors, and adaptive computer controls are now routinely integrated into these operations. This automation serves multiple purposes: it reduces overall production costs, ensures consistent weld quality across high volume runs, guarantees the reliability of the weld quality and drives higher productivity compared to manual operations. The economic aspects of these joining processes including equipment and labor costs are critical factors that influence their selection in industrial applications.

Cold Welding and Roll Bonding:

Cold Welding:

Cold welding is a process in which pressure is applied to the workpieces through specialized dies or rotating rolls. Because the mechanism relies heavily on plastic deformation at the interface, it is necessary that at least one of the mating parts be highly ductile and preferably both should possess this characteristic. Prior to performing the weld, the interface must undergo rigorous surface preparation. This typically involves degreasing to remove oils and organic films followed by wire brushing and wiping to eliminate any remaining oxide smudges that could hinder the formation of a metallurgical bond.

Cold welding finds practical applications in joining small workpieces made of soft, ductile metals. Common examples include splicing wire stock and making reliable electrical connections. However, an important limitation arises when joining two dissimilar metals that are mutually soluble. Under such conditions, brittle intermetallic compounds may form at the joint interface, which will inevitably produce a weak and brittle joint that is unsuitable for load bearing applications. A classic example of this problematic behavior occurs in the bonding of aluminum to steel where a brittle intermetallic compound readily forms at the interface. Consequently, the best bond strength in cold welding is consistently obtained when joining two similar materials that have compatible mechanical and metallurgical properties.

Roll Bonding:

The pressure required for welding can be applied through a pair of rolls in a specialized process called roll bonding or roll welding. This technology was developed in the 1960s and has found a unique application in the manufacturing of some U.S. coins. The process can be carried out at elevated temperatures, and when conducted under heated conditions, it is referred to as hot roll bonding. Surface preparation remains critically important for achieving high interfacial strength in roll bonding just as it is in cold welding.

Typical industrial examples of roll bonding include the cladding of pure aluminum over precipitation hardened aluminum alloy sheet, a product known commercially as Alclad which provides corrosion resistance while maintaining the mechanical strength of the underlying alloy. Another common application is the cladding of stainless steel over mild steel to provide a corrosion resistant outer layer on a structurally sound and economical base material. A widely recognized application of roll bonding is the production of bimetallic strips for thermostats and similar temperature sensing controls. These strips use two layers of materials that possess different thermal expansion coefficients causing the strip to bend in response to temperature changes. Furthermore bonding in only selected regions of the interface can be achieved by depositing a parting agent, such as graphite or ceramic powder known as a stop off material. This technique prevents bonding in specific areas allowing for precise control over the joined regions.

Principles and Applications of Ultrasonic Welding:

In ultrasonic welding, the faying surfaces of the two components are subjected to a combination of a static normal clamping force and oscillating shearing stresses applied tangentially. The shearing stresses are delivered by the tip of a transducer which is analogous to the equipment used in ultrasonic machining. The frequency of oscillation in these operations generally falls within the range of 10 to 75 kHz although lower or higher frequencies can be employed depending on the specific material and thickness requirements. Proper acoustic coupling between the transducer and the tip which is called a sonotrode by analogy with an electrode in arc welding is essential for efficient energy transfer and consistent operation.

The oscillating shearing stresses cause localized plastic deformation at the interface of the two components which effectively breaks up oxide films and contaminants present on the surfaces. This cleaning action allows for good metallic contact and produces a strong solid state bond. The temperature generated in the weld zone during ultrasonic welding is usually in the range from approximately one third to one half of the melting point of the metals joined, measured on the absolute temperature scale. Consequently, neither melting nor bulk fusion takes place in the joint for metallic materials. However, in certain situations, the temperature generated can be sufficiently high to cause metallurgical changes in the weld zone such as localized recrystallization or grain growth, which may affect the mechanical properties of the joint.

It is important to note that the mechanism responsible for the joining of thermoplastics by ultrasonic welding is fundamentally different from that for metals. For plastics, melting does take place at the interface because these materials have much lower melting temperatures compared to metals. The ultrasonic-welding process is versatile and reliable, capable of joining a wide variety of metallic and nonmetallic materials, including dissimilar metals such as those used in bimetallic strips. It is used extensively in the joining of plastics, for packaging applications with foils and in the automotive and consumer electronics industries for the lap welding of sheet, foil and thin wire components. To accommodate different geometries, the welding tip can be replaced with rotating disks for seam welding of structures where one component is sheet, foil or polymer woven material, a process similar in concept to resistance seam welding. Operating the equipment effectively requires moderate skill from the operator although many modern systems incorporate automated controls.

Friction Welding:

In the joining processes described thus far, the energy required for welding is typically supplied from external sources such as chemical, electrical or ultrasonic energy. In friction welding, however, the heat required for welding is generated directly through friction at the interface of the two components being joined. Developed in the 1940s, this process involves one of the workpiece components remaining stationary while the other is securely placed in a chuck or collet and rotated at a high constant speed. The two members to be joined are then brought into contact under an applied axial force. The surface speed of the rotating parts may be as high as 900 meters per minute. After sufficient contact and heating have been established, the rotating member is brought to a quick stop to ensure that the newly formed weld is not destroyed by shearing forces while simultaneously the axial force is increased to forge the joint.

During the process, oxides and other contaminants present at the original interface are removed by the radially outward movement of the hot plastically deformed metal at the interface. The rotating member must be clamped securely to the chuck or collet to resist both the torque generated by the rotation and the axial forces without any slipping. The pressure at the interface and the resulting friction produce sufficient heat for a strong joint to form. The weld zone is usually confined to a narrow region, and its size depends on several key parameters: the amount of heat generated, the thermal conductivity of the materials being joined, and the mechanical properties of those materials at elevated temperatures.

The shape of the welded joint depends critically on the rotational speed and on the axial pressure applied. These factors must be precisely controlled to obtain a uniform, strong joint. The radially outward movement of the hot metal at the interface effectively pushes oxides and other contaminants out of the interface, contributing to the high quality of the joint. Friction welding can be used to join a wide variety of materials provided that at least one of the components has some rotational symmetry. Solid or tubular parts can be joined by this method with good joint strength. Solid steel bars up to 100 millimeters in diameter and pipes up to 250 millimeters in outside diameter have been successfully friction welded in industrial applications. The surface speed of the rotating member may be as high as 15 meters per second. Because of the combined heat and pressure, the interface in frictional welding develops a flash by plastic deformation of the heated zone, a phenomenon known as upsetting. This flash if objectionable for aesthetic or functional reasons can easily be removed by machining or grinding operations. Friction welding machines are fully automated and the operator skill required is minimal once the individual cycle times for the complete operation are set properly.

Inertia Friction Welding:

Inertia friction welding is a significant modification of the basic friction welding process, although the two terms have sometimes been used interchangeably. The energy required for frictional heating in inertia friction welding is supplied by the kinetic energy stored in a flywheel. The flywheel is accelerated to the proper speed, the two members are brought into contact and an axial force is applied. As friction at the interface slows the flywheel down, the axial force is increased to consolidate the joint. The weld is completed when the flywheel has come to a complete stop. The precise timing of this sequence is crucial for achieving good weld quality. The rotating mass in inertia friction welding machines can be adjusted for applications requiring different levels of energy, with the specific energy levels depending on the workpiece size and material properties. In one notable application of inertia friction welding, 10 millimeter diameter shafts are welded to automotive turbocharger impellers at a production rate of one joint every 15 seconds demonstrating the process's suitability for high speed manufacturing.

Linear Friction Welding:

In a further development of friction welding, the interface of the two components to be joined is subjected to a linear reciprocating motion, as opposed to a continuous rotary motion. In linear friction welding, the components do not have to be circular or tubular in their cross section, which significantly expands the range of possible applications. The process is capable of welding square or rectangular components as well as round parts made of metals or plastics. In this method, one part is moved across the face of the other part by a balanced reciprocating mechanism. In one specific application, a rectangular titanium alloy part was friction welded at a linear frequency of 25 Hz with an amplitude of 2 millimeters under a pressure of 100 MPa acting on a 240 millimeter interface. Various other metal parts with rectangular cross sections as large as 50 by 20 millimeters have been welded successfully by this process.

Friction Stir Welding:

In conventional friction welding, heating of the interface is achieved through friction by directly rubbing the two contacting surfaces together. In the friction stir welding process which was developed in 1991, a third body is used to generate the heat and stir the material. A rotating nonconsumable probe, typically 5 to 6 millimeters in diameter and 5 millimeters high, is plunged into the joint between the two parts. The contact pressure between the probe and the workpieces causes frictional heating, raising the temperature of the material to between 230° and 260°C. The probe at the tip of the rotating tool forces mechanical mixing or stirring of the material in the joint creating a solid state bond.

Materials such as aluminum, copper, steel and titanium have been welded successfully using friction stir welding, and ongoing developments are taking place to extend its applications to polymers and composite materials. The process is now being applied extensively in the aerospace, automotive, shipbuilding and military vehicle sectors, using sheet or plate materials. With continuous developments in rotating tool design, other possible applications are emerging including inducing specific microstructural changes, refining grain size in materials and improving localized toughness in castings. The welding equipment used can be a conventional, vertical spindle milling machine and the process is relatively easy to implement compared to some other advanced joining techniques. The thickness of the material that can be welded ranges from as little as 1 millimeter to as much as 50 millimeters in a single pass. Welds produced by friction stir welding are characterized by high quality, minimal porosity and a uniform material structure throughout the joint. The welds are produced with low heat input resulting in low distortion and little microstructural change in the surrounding base material. No shielding gas or surface cleaning is required for the process which further simplifies its industrial implementation.

Resistance Welding:

The category of resistance welding covers a number of processes in which the heat required for welding is produced by means of electrical resistance across the two components to be joined. These processes offer major advantages, such as not requiring consumable electrodes, shielding gases or flux. The heat generated in resistance welding is given by the general expression involving the square of the current, the resistance of the joint and the time for which the current is applied.

Resistance Spot Welding:

In resistance spot welding, the tips of two opposing solid, cylindrical electrodes touch a lap joint of two sheet metals and the resulting resistance heating produces a spot weld at the interface. To obtain a strong bond in the weld nugget, pressure is applied by the electrodes and maintained until the current is turned off and the weld has solidified. Accurate control and precise timing of the alternating electric current and of the pressure are essential for producing consistent, high-quality welds. In the automotive industry, for example, the number of cycles ranges up to about 30 at a frequency of 60 Hz. The weld nugget formed is generally 6 to 10 millimeters in diameter, and the surface of the spot weld exhibits a slightly discolored indentation. Currents used in spot welding range from 3,000 to 40,000 A. The current level depends on the materials being welded and on their thicknesses. For example, the current is typically 10,000 A for steels and 13,000 A for aluminum. Electrodes generally are made of copper alloys and must have sufficient electrical conductivity and hot strength to maintain their shape and resist deformation at high temperatures.

Spot welding is the simplest and most commonly used resistance welding process. Welding may be performed by means of a single pair of electrodes which is the most common configuration or with multiple pairs of electrodes, with as many as a hundred or more used simultaneously. The required pressure is supplied through mechanical or pneumatic means. Rocker arm type spot welding machines are normally used for smaller parts while press type machines are employed for larger workpieces. The shape and surface condition of the electrode tip, as well as the accessibility of the weld site, are important factors in spot welding. A variety of electrode shapes are used to spot weld areas that are difficult to reach, allowing for flexibility in joint design. Spot welding is used widely for fabricating sheet metal parts. Examples range from attaching handles to stainless steel cookware to spot welding mufflers and large sheet metal structures. Modern spot welding equipment is computer controlled for optimum timing of current and pressure, and its spot welding guns are manipulated by programmable robots. Automobile bodies can have as many as 10,000 spot welds, they are welded at high rates with the use of multiple electrodes.

Testing Spot Welds:

Spot welded joints may be tested for weld nugget strength by means of several techniques, including tension shear, cross tension, twist and peel tests. Because they are easy to perform and are inexpensive, tension shear tests are commonly used in fabricating facilities. The cross tension and twist tests are capable of revealing flaws, cracks and porosity in the weld area that may not be apparent from visual inspection. The peel test is commonly used for thin sheets, after the joint has been bent and peeled, the shape and size of the torn out weld nugget are evaluated to assess the quality of the weld.

Resistance Seam Welding:

Resistance seam welding is a modification of spot welding wherein the solid electrodes are replaced by rotating wheels or rollers. Using a continuous AC power supply, the electrically conducting rollers produce a spot weld whenever the current reaches a sufficiently high level in the AC cycle. With a high enough frequency or slow enough traverse speed, these individual spot welds actually overlap into a continuous seam producing a joint that is liquid tight and gastight. In roll spot welding, current to the rollers is applied only intermittently resulting in a series of separate spot welds at specified intervals along the length of the seam. In mash seam welding, the overlapping welds are about one to two times the sheet thickness, and the welded seam thickness is only about 90% of the original sheet thickness. This process is also used in producing tailor welded sheet metal blanks which can also be made by laser welding. The resistance seam welding process is used to make the longitudinal side seam of cans for household products, mufflers, gasoline tanks and other containers. The typical welding speed is 1.5 meters per minute for thin sheets.

High frequency Resistance Welding:

High-frequency resistance welding is similar to seam welding except that a high frequency current up to 450 kHz is employed. A typical application is the production of butt welded tubing or pipe where the current is conducted through two sliding contacts to the edges of roll formed tubes. The heated edges then are pressed together by passing the tube through a pair of squeeze rolls. Any flash formed during this operation is then trimmed off. Structural sections such as I-beams can be fabricated by high frequency resistance welding by welding the webs and flanges made from long, flat pieces. Spiral pipe and tubing, finned tubes for heat exchangers, and wheel rims also may be made by this technique. In another method called high frequency induction welding, the roll formed tube is subjected to high frequency induction heating providing an alternative approach to generating the necessary heat.

Resistance Projection Welding:

In resistance projection welding, high electrical resistance at the joint is developed by embossing one or more projections known as dimples on one of the surfaces to be welded. The projections may be round or oval for design or strength purposes. High localized temperatures are generated at these projections which are in contact with the flat mating part. The electrodes typically made of copper based alloys are large and flat, and are water cooled to keep their temperature low. Weld nuggets similar to those in spot welding are formed as the electrodes exert pressure to soften and compress the projections. Spot welding equipment can be used for resistance projection welding by modifying the electrodes. Although the embossing of the workpieces adds expense to the process, the process produces a number of welds in one pass extends electrode life and is capable of welding metals of different thicknesses such as a sheet welded over a plate. Nuts and bolts can be welded to sheets and plates by this process with projections that are produced by machining or forging. Joining a network of rods and wires, such as those making up metal baskets, grills, oven racks and shopping carts also is considered resistance projection welding because of the many small contact areas between crossing wires.

Flash Welding:

In flash welding also called flash butt welding, heat is generated very rapidly from an arc as the ends of the two members begin to make contact and develop an electrical resistance at the joint. After the proper temperature is reached and the interface begins to soften, an axial force is applied at a controlled rate and a weld is formed by plastic deformation of the joint. This mechanism is called hot upsetting, and the term upset welding is also used for this process. Some molten metal is expelled from the joint as a shower of sparks during the process, hence the name flash welding. Because of the presence of an arc, the process can also be classified under arc welding. Impurities and contaminants are squeezed out during this operation, therefore the quality of the weld is good. However a significant amount of material may be burned off during the welding process and the joint may be machined later to improve its appearance.

The machines for flash welding usually are automated and large and have a variety of power supplies ranging from 10 to 1500 kVA. The flash welding process is suitable for end to end or edge to edge joining of sheets of similar or dissimilar metals ranging from 0.2 to 25 millimeters thick and for end joining bars from 1 to 75 millimeters in diameter. Thinner sections have a tendency to buckle under the axial force applied during welding. Rings made by forming processes also can be flash butt welded. In addition, the process is used to repair broken band saw blades with the use of fixtures that are mounted on the band saw frame. The flash welding process can be automated for reproducible welding operations. Typical applications include the joining of pipe and of tubular shapes for metal furniture and windows. The process is also used for welding the ends of sheets or coils of wire in continuously operating rolling mills and in the feeding of wire drawing equipment. Once the appropriate process parameters are established, the required operator skill is minimal. Some design guidelines for mating surfaces in flash welding include the importance of maintaining uniform cross sections at the joint.

Stud Welding:

Stud welding is also called stud arc welding and is similar to flash welding. The stud, which may be a small part or more commonly a threaded rod, hanger or handle serves as one of the electrodes while being joined to another component which is usually a flat plate. Polarity for aluminum is usually direct current electrode positive, and for steel it is direct current electrode negative. In order to concentrate the heat generated, prevent oxidation and retain the molten metal in the weld zone, a disposable ceramic ring called a ferrule is placed around the joint. The equipment for stud welding can be automated with various controls for arcing and for applying pressure. Portable stud welding equipment also is available. Typical applications of stud welding include automobile bodies, electrical panels and shipbuilding the process is also used in building construction. In capacitor discharge stud welding a DC arc is produced from a capacitor bank. No ferrule or flux is required for this variant because the welding time is very short, on the order of 1 to 6 milliseconds. The choice between this process and conventional stud arc welding depends on such factors as the types of metals to be joined, the workpiece thickness and cross section, the stud diameter and the shape of the joint.

Percussion Welding:

The resistance-welding processes already described usually employ an electrical transformer to meet the power requirements. Alternatively, the electrical energy for welding may be stored in a capacitor. Percussion welding utilizes this technique in which the stored power is discharged within 1 to 10 milliseconds to develop localized high heat at the joint. The process is particularly useful in applications where heating of the components adjacent to the joint is to be avoided as in electronic assemblies and electrical wires.

Explosion Welding:

In explosion welding, pressure is applied by detonating a layer of explosive that has been placed over one of the components being joined, which is called the flyer plate. The contact pressures developed during the detonation are extremely high and the kinetic energy of the plate striking the mating component causes a wavy interface to form. This impact mechanically interlocks the two surfaces, while simultaneously pressure welding by plastic deformation also takes place. The flyer plate is placed at an angle, and any oxide films present at the interface are broken up and propelled out of the interface ahead of the advancing explosion front. As a result, the bond strength achieved from explosion welding is very high.

The explosive used in the process may be a flexible plastic sheet or cord, or it may be in granulated or liquid form which is cast or pressed onto the flyer plate. The detonation speed is usually in the range from 2400 to 3600 meters per second, it depends on the type of explosive, the thickness of the explosive layer, and the packing density of the layer. There is a minimum detonation speed necessary for welding to occur in this process, and detonation is carried out with a standard commercial blasting cap. This process is suitable particularly for cladding a plate or a slab with a dissimilar metal. Plates as large as 6 by 2 meters have been clad explosively in commercial operations. These clad plates may then be rolled into thinner sections for further fabrication. Tubes and pipes can also be joined to the holes in the header plates of boilers and heat exchangers by placing the explosive inside the tube; the explosion expands the tube outward to create a strong mechanical and metallurgical joint. Explosion welding is inherently dangerous so it requires safe handling by well trained and experienced personnel.

Diffusion Bonding:

Diffusion bonding or diffusion welding is a process in which the strength of the joint results primarily from atomic diffusion across the interface and secondarily from plastic deformation of the faying surfaces. This process requires temperatures of about 0.5 Tm where Tm is the melting point of the metal on the absolute scale in order to achieve a sufficiently high diffusion rate between the parts being joined. The bonded interface in diffusion welding has essentially the same physical and mechanical properties as the base metal. Its strength depends on several critical factors: the applied pressure, the temperature, the time of contact and how clean the faying surfaces are. These requirements can be relaxed somewhat by using a filler metal at the interface to promote diffusion. Depending on the materials being joined, brittle intermetallic compounds may form at the interface, they may be avoided by electroplating the surfaces with suitable metal alloys.

In diffusion bonding, pressure may be applied by dead weights, a mechanical press, differential gas pressure or the thermal expansion of the parts to be joined. The parts usually are heated in a furnace or by electrical resistance. High pressure autoclaves are also used for bonding complex parts. Although this process was developed in the 1970s as a modern welding technology, the principle of diffusion bonding dates back centuries to when goldsmiths bonded gold over copper to create a product called filled gold. In that historic technique, a thin layer of gold foil is produced and placed over copper, a weight is placed on top of the foil, and the assembly is placed in a furnace and left until a strong bond is obtained, hence, the process is also called hot-pressure welding. Diffusion bonding generally is most suitable for joining dissimilar metals. It is also used for reactive metals such as titanium, beryllium, zirconium and refractory metal alloys and for composite materials such as metal matrix composites. Diffusion bonding is also an important mechanism of sintering in powder metallurgy. Because diffusion involves the migration of atoms across the joint, the process is slower than other welding processes. Although diffusion welding is used for fabricating complex parts in low quantities for the aerospace, nuclear and electronics industries, it has been automated to make it suitable and economical for moderate volume production. Unless the process is highly automated, considerable operator training and skill are required. Equipment cost is related approximately to the diffusion bonded area and is in the range of $3 to $6 per square millimeter.

General Considerations for Successful Solid State Welding:

In most solid state processes, a metallurgical bond is created with little or no melting of the base metals. To metallurgically bond two similar or dissimilar metals, the two metals must be brought into intimate contact so that their cohesive atomic forces attract each other across the interface. In normal physical contact between two surfaces such intimate contact is prohibited by the presence of chemical films, gases, oils and other contaminants. For atomic bonding to succeed, these films and other substances must be effectively removed. In fusion welding, as well as other joining processes such as brazing and soldering, the films are dissolved or burned away by the high temperatures and atomic bonding is established by the melting and solidification of the metals. But in solid-state welding, the films and other contaminants must be removed by other means to allow metallurgical bonding to take place. In some cases, a thorough cleaning of the surfaces is done just before the welding process while in other cases, the cleaning action is accomplished as an integral part of bringing the part surfaces together. To summarize, the essential ingredients for a successful solid state weld are that the two surfaces must be very clean, and they must be brought into very close physical contact with each other to permit atomic bonding.

Welding processes that do not involve melting have several distinct advantages over fusion welding processes. If no melting occurs then there is no heat affected zone in the classic sense and so the metal surrounding the joint retains its original properties without the microstructural alterations common in fusion welds. Many of these processes produce welded joints that comprise the entire contact interface between the two parts, rather than at distinct spots or seams as in most fusion welding operations. Also, some of these processes are quite applicable to bonding dissimilar metals without concerns about relative thermal expansions, conductivities and other problems that usually arise when dissimilar metals are melted and then solidified during joining. The solid state welding group includes the oldest joining process as well as some of the most modern. Forge welding is of historic significance in the development of manufacturing technology, the process dates from about 1000 BCE when blacksmiths of the ancient world learned to join two pieces of metal. Forge welding is a process in which the components to be joined are heated to hot working temperatures and then forged together by hammer or other means. Considerable skill was required by the craftsmen who practiced it to achieve a good weld by present day standards. The process may be of historic interest, however it is of minor commercial importance today except for its variants which have been discussed in the preceding sections.

Weld Quality, Defects and Inspection Techniques:

The physical integrity of any welded structure depends fundamentally on the quality of the weld. The discussion of weld quality deals primarily with arc welding, the most widely used welding process and the one for which the quality issue is the most critical and complex but the principles apply broadly to solid state processes as well.

Residual Stresses and Distortion:

The rapid heating and cooling in localized regions of the work during fusion welding especially arc welding result in thermal expansion and contraction that cause residual stresses in the weldment. These stresses, in turn can cause distortion and warping of the welded assembly. The situation in welding is complicated because heating is very localized, melting of the base metals occurs in these local regions and the location of heating and melting is in motion, at least in arc welding. Consider the butt welding of two plates by arc welding. The operation begins at one end and travels to the opposite end. As it proceeds, a molten pool is formed from the base metal and any filler metal that quickly solidifies behind the moving arc. The portions of the work immediately adjacent to the weld bead become extremely hot and expand while portions removed from the weld remain relatively cool. The weld pool quickly solidifies in the cavity between the two parts, and as it and the surrounding metal cool and contract, shrinkage occurs across the width of the weldment. The weld seam is left in residual tension and reactionary compressive stresses are set up in regions of the parts away from the weld.

Residual stresses and shrinkage also occur along the length of the weld bead. Because the outer regions of the base parts have remained relatively cool and dimensionally unchanged while the weld bead has solidified from very high temperatures and then contracted, residual tensile stresses remain longitudinally in the weld bead. The net result of these residual stresses, transversely and longitudinally is likely to cause warping in the welded assembly. Thermally induced residual stresses and the accompanying distortion are a potential problem in nearly all fusion welding processes and in certain solid state welding operations in which significant heating takes place. Several techniques are available to minimize warping in a weldment. Welding fixtures can be used to physically restrain movement of the parts during welding. Heat sinks can be used to rapidly remove heat from sections of the welded parts to reduce distortion. Tack welding at multiple points along the joint can create a rigid structure prior to continuous seam welding. Welding conditions such as speed and the amount of filler metal used can be selected to reduce warping. The base parts can be preheated to reduce the level of thermal stresses experienced by the parts. Stress relief heat treatment can be performed on the welded assembly, either in a furnace for small weldments or using methods that can be used in the field for large structures. Finally, proper design of the weldment itself can substantially reduce the degree of warping.

Common Welding Defects:

In addition to residual stresses and distortion, other defects can occur in welding. Cracks are fracture type interruptions either in the weld itself or in the base metal adjacent to the weld. This is perhaps the most serious welding defect because it constitutes a discontinuity in the metal that significantly reduces weld strength. Several forms of cracks are defined and they are caused by embrittlement or low ductility of the weld or base metal combined with high restraint during contraction. Generally this defect must be repaired. Cavities include various porosity and shrinkage voids. Porosity consists of small voids in the weld metal formed by gases entrapped during solidification. The shapes of the voids vary between spherical which are known as blow holes to elongated which are known as worm holes. Porosity usually results from inclusion of atmospheric gases, sulfur in the weld metal or contaminants on the surfaces. Shrinkage voids are cavities formed by volumetric shrinkage during solidification. Both of these cavity type defects are similar to defects found in castings and emphasize the close kinship between casting and welding.

Solid inclusions are nonmetallic solid materials trapped inside the weld metal. The most common form is slag inclusions generated during arc welding processes that use flux. Instead of floating to the top of the weld pool, globules of slag become encased during solidification of the metal. Another form of inclusion is metallic oxides that form during the welding of metals such as aluminum which normally has a surface coating of aluminum oxide. Incomplete fusion also known as lack of fusion is a defect in which the weld bead has not fused throughout the entire cross section of the joint. A related defect is lack of penetration which means that fusion has not penetrated deeply enough into the root of the joint. Imperfect shape or unacceptable contour occurs when the weld does not have the desired profile for maximum strength. A properly profiled weld maximizes the strength of the joint and avoids incomplete fusion and lack of penetration. Miscellaneous defects include arc strikes in which the welder accidentally allows the electrode to touch the base metal next to the joint leaving a scar on the surface and excessive spatter in which drops of molten weld metal splash onto the surface of the base parts.

Inspection and Testing Methods:

A variety of inspection and testing methods are available to check the quality of the welded joint. Standardized procedures have been developed and specified over the years by engineering and trade societies such as the American Welding Society. For purposes of discussion, these inspection and testing procedures can be divided into three categories: visual, nondestructive and destructive. Visual inspection is no doubt the most widely used welding inspection method. An inspector visually examines the weldment for conformance to dimensional specifications on the part drawing for warping and for cracks, cavities, incomplete fusion and other visible defects. The welding inspector also determines if additional tests are warranted, usually in the nondestructive category. The limitation of visual inspection is that only surface defects are detectable, internal defects cannot be discovered by visual methods.

Nondestructive evaluation includes various methods that do not damage the specimen being inspected. Dye penetrant and fluorescent penetrant tests are methods for detecting small defects such as cracks and cavities that are open to the surface. Fluorescent penetrants are highly visible when exposed to ultraviolet light and their use is therefore more sensitive than conventional dyes. Several other nondestructive evaluation methods should be mentioned. Magnetic particle testing is limited to ferromagnetic materials. A magnetic field is established in the subject part and magnetic particles such as iron filings are sprinkled on the surface. Subsurface defects such as cracks and inclusions reveal themselves by distorting the magnetic field causing the particles to be concentrated in certain regions on the surface. Ultrasonic testing involves the use of high frequency sound waves above 20 kHz directed through the specimen. Discontinuities such as cracks, inclusions and porosity are detected by losses in sound transmission. Radiographic testing uses X rays or gamma radiation to detect flaws internal to the weld metal. It provides a photographic film record of any defects that may be present.

Destructive testing methods are those in which the weld is destroyed either during the test or to prepare the test specimen. They include mechanical and metallurgical tests. Mechanical tests are similar in purpose to conventional testing methods such as tensile tests and shear tests, with the difference being that the test specimen is a weld joint. Metallurgical tests involve the preparation of metallurgical specimens of the weldment to examine such features as metallic structure, defects, extent and condition of the heat affected zone, presence of other elements, and similar phenomena.

Process Selection Criteria and Economic Considerations:

The selection of an appropriate welding process involves a careful evaluation of multiple factors beyond just the technical capabilities of the process. In addition to taking into account the process characteristics, capabilities and material considerations, the selection of a weld joint and an appropriate welding process involves the configuration of the parts or structure to be joined, joint design, thickness and size of the components and the number of joints required. The methods used in manufacturing the components to be joined also influence the choice. The types of materials involved, which may be metallic or nonmetallic are a primary driver of process compatibility. The location, accessibility, and ease of joining are practical constraints that must be considered. Application and service requirements such as the type of loading, any stresses generated and the environment, determine the necessary joint strength and durability. The effects of distortion, warping, discoloration of appearance and service conditions must be evaluated for aesthetic and functional reasons.

Costs involved in edge preparation, joining and postprocessing, including machining, grinding and finishing operations are significant economic factors. Additionally, the costs of equipment, materials, labor and skills required and the joining operation itself all contribute to the final economic decision. It is important to note that no single process has a high rating in all categories. For example, arc welding, bolts and riveting have high strength and reliability but generally are not suitable for joining small parts. Resistance welding has strength and applications for both small and large parts, however, it is not easy to inspect visually for reliability and resistance welding has lower tolerances and reliability than other processes. Fasteners are useful for large parts and can be easy to inspect visually, but they are costly and do not have much design variability. Adhesive bonding has high design variability but it has relatively low strength and is difficult to visually inspect for joint integrity.

Economics of Welding Operations:

The characteristics, advantages and limitations of the welding processes described have included brief discussions regarding welding costs. The relative costs of some selected processes can vary widely depending on such factors as the equipment capacity, level of automation, labor skill required, weld quality, production rate and preparation required as well as on various other considerations specific to a particular joining process. The general welding and joining costs for some common operations can be summarized as follows. High costs are associated with brazing and fasteners such as bolts and nuts as they require hole making operations and the cost of the fasteners themselves. Intermediate costs apply to arc welding, riveting and adhesive bonding. Low costs are associated with resistance welding, seaming and crimping, as these operations are relatively simple to perform and automate.

Equipment costs for welding can vary widely. High equipment costs, in the range of $100,000 to $200,000, apply to electron beam and laser beam welding. Intermediate equipment costs, from $5,000 to $50,000 and above, apply to spot, submerged arc, gas metal arc, gas tungsten arc, flux cored arc, electrogas, electroslag, plasma arc and ultrasonic welding. Low equipment costs, from $500 and above, apply to shielded metal arc and oxyfuel gas welding. Labor costs in welding generally are higher than in other metalworking operations because of the operator skill required, the welding time and the preparation required. However much depends on the automation of the equipment employed including the use of robotics and computer controls programmed to follow a prescribed path, known as seam tracking, during welding. It has been observed that in systems with robotic controls, the productive welding time reaches 80% of the overall time, whereas in manual welding operations, the actual welding time spent by the operator is only about 30% of the overall time. Labor costs may be summarized as follows: high to intermediate costs apply to oxyfuel gas welding and shielded metal arc welding, high to low costs apply to electron beam and laser beam welding and flux cored arc welding and intermediate to low costs apply to submerged arc welding. These economic factors, combined with the technical requirements and quality considerations discussed, provide a comprehensive framework for selecting the optimal solid state welding process for any given application.