resistance and solid-state welding

Resistance and solid state welding covers a broad range of overlapping technologies. Generally, the resistance welding technologies are considered those which use Joule, or resistance heating in order to accomplish joining. Solid state processes are those, which through a combination of deformation and diffusion, allow joining to be accomplished without molten and re-solidified material in the final bond area.

What Is Resistance Welding (RW)?

Resistance welding processes are defined by the ability to deliver large electrical currents to the weld area. These currents react with local resistances to generate heat. Typically the resistance across the joint in these processes is small (on the order of micro-ohms to milliohms) so the necessary currents are large (thousands of amps to hundreds of thousands of amps).

In order to manage heat generation, resistance welding systems require not only precise control of current flow, but in addition contact forces and cooling of system components. The forcing systems used allow efficient conduction of current into the workpieces (through welding electrodes) and proper metallurgical formation of the resulting weld. The cooling systems allow proper heat balance in the welds formed (centering heating at the area where the weld is desired) and protection of system components. Resistance welding processes have been adapted to sheet (spot, seam, projection) and structural (flash butt, resistance butt, projection) applications. Processes are capable of very high production rates, and are ideally suited for high volume manufacturing.

Resistance Spot Welding (RSW)

Resistance spot welding is a process typically used in high-volume, rapid welding applications, such as those found in the automotive, appliance and aerospace industries, to join sheet metal from foil to 0.250-in. (3mm) in thickness. Multiple sheets and dissimilar metals can also be simultaneously welded. In spot welding, the pieces to be joined are clamped between two electrodes under high force. A large electrical current is passed from electrode to electrode to generate heat at the sheet-sheet (faying) interface. Heating of the joint may occur from both the electrical resistance across microscopic asperities at the faying interface and from the inherent bulk resistivity of the metal. After the welding current is shut off, continued electrode force maintains pressure on the molten zone during weld nugget solidification to provide joint integrity.

The Benefits of Spot Welding

The advantages of this process are that it is non labor-intensive and can be easily automated. Additional advantages of spot welding include:

• Adaptable to a wide variety of electrically conductive materials
• Applicable to a variety of thicknesses
• Very short cycle times
• A robust process
• Tolerant to fit-up variations

Spot Welding Equipment at EWI

• 30 kVA Taylor-Winfield pedestal welder; Tru-Amp controller. Welder also equipped with 25 kHz MFDC 10KA max power supply; Matuschek Spatz controller
• 100 kVA Taylor-Winfield welder; Miyachi STA 200A Controller
• 100 kVA Taylor-Winfield welder; Medar 3000S controller
• 180 kVA Savair Servo-Gun with 1 kHz MFDC 32 kA max power supply; ARO controller
• 180 kVA Obara Servo Gun with 2 kHz MFDC 30 kA max power supply; Obara controller
• 200 kva Newcor welder; Medar 3000S controller. Welder also equipped with 1 kHz MFDC 60 kA max power supply; Miyachi ISA-500A controller
• 200 kVA 3-phase frequency converter Thompson welder with forge capability; Medar controller
• 50 kVA Taylor-Winfield ½ cycle AC welder, 100 kA max ; Roboton controller
• 350 kVA welder with forge capability; Medar 3000S controller
• Series welder with 3 transformer options; Tru-amp V controller
• Various portable guns, transformers, and controllers also available

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Projection Welding (PW)

Projection welding is a variation of resistance welding in which current flow is concentrated at the contact surfaces of interest by an embossed, cold headed, or machined projection. The projection(s) effectively localize the current, forcing the parts to heat predominately at the mating surfaces. This rapid interfacial heating allows for the application of resistance projection welding across a wide range of applications not feasible by conventional resistance spot welding.

The projection is designed to localize current flow at the contact surfaces where the joint is desired. This focuses heat at the mating surfaces of the pieces and minimizes bulk heating of the parts. This rapid thermal cycle allows for precision joining of detailed parts, joining of metallurgically challenging materials, and formation of multiple welds simultaneously. While the projection usually collapses early in the weld cycle, the localized heating raises the material resistance locally and promotes further heating and finally weld development at the initial contact point. The process can be developed to produce a recast fusion type weld or a solid state weld depending of the application.

Because the formation of the weld is highly localized, the process is considerably more energy efficient than other resistance welding processes.

Benefits of Projection Welding

• Ability to simultaneously perform multiple welds
• Ability to join sheets of widely dissimilar thicknesses
• Can be used to minimize surface marking when joining sheets
• Increased energy efficiency
• Ability to join metalurgically challenging and dissimilar materials combinations
• Minimal cycle time

Projection Welding Equipment at EWI

• 400 kVA Soudronic frequency converter system, 750 kA max current, 10 ton max force
• 600 kVA Newcor Single/Three Phase Secondary Rectified system, 125 kA max current, 20 kip max force
• 350 kVA Precision AC welding system, 80 kA max current, 5 ton max force.
• 6 kJ Impulsphysik capacitive discharge welding system, 100 kA max current, 5 ton max force
• 50 kVA Taylor-Winfield ½ cycle AC welder, 100 kA max ; Roboton controller
• Specially designed fast follow-up welding heads capable of forces from less than 100 lbs to over 25 ton.

Most equipment available for resistance spot welding can also be used for projection welding. This includes an array of welding frames, as well as virtually all types of resistance welding power supplies (AC, half-wave DC, full-wave DC, frequency inverter type, capacitive discharge, etc.).

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Resistance Seam Welding (RSEW)

Resistance seam welding is similar to spot welding except that rotating wheel electrodes are used. The process is used when leak-tight welds or long strings of spot welds are required. Three forms of seam welding exist: standard seam, mash seam, and roll spot welding.

In standard seam welding, a series of overlapping weld nuggets are formed by rotating the wheel electrodes along the workpieces and firing a continuous series of current pulses. This action forms a continuous, leak-tight joint.

In mash seam welding (RSEW-MS), there is a small overlap of sheets, typically about one to two times the sheet thickness. Sheets are then mashed together while current applied, making a solid state joint. The resulting welded joint is generally 110-150% of the original sheet thickness. This final joint thickness can be reduced by post-weld planishing.

In roll spot welding, the current pulses as wheels traverse the workpieces to form a line of separate spot welds (not a leak-tight joint).

Benefits of Seam Welding

• Adaptable to a variety of electrically conductive materials
• Economical
• Ability to produce leak-tight welds
• Fast processing times

Seam Welding Equipment at EWI

• 350 kVA single phase National seam welding system.
• 60 kVA single phase Sciaky seam welding system.
• Saiwa HCS-200TU micro-resistance seam welding system.
• Tolerant to a variety of thicknesses up to 3:1 and multiple stack-ups.
• The power supplies of these seam welding systems are interchangeable. All types of power supplies, including medium-frequency and high frequency DC, medium frequency AC, single-phase AC, three-phase AC, secondary rectified DC and frequency changer power supplies, can be connected to the systems for different applications.

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Flash and Resistance Butt Welding (FW and UW)

Flash and resistance butt welding are two solid state processes that take advantage of localized resistance heating to create structural welds. Welding is always done in a butt configuration. Resistance heating locally softens the materials at the interface, and forging consolidates the joint. The processes differ by how resistance heating is applied to the joint area. During flash butt welding (FW), voltage is first applied across a gaped set of workpieces. These workpieces are then brought together at a controlled rate. As the parts contact, asperity heating occurs, with subsequent expulsion (flashing) and heating of the adjoining substrate. As the parts are advanced together, the process is repeated, effectively heating the substrate materials to desired forging temperatures. The parts are then forged to either a desired distance or to a desired pressure. Resistance butt welding (UW) differs from flash butt welding in that parts are in full force contact prior to initiating the welding current. Resistance occurs predominantly at the contacting surface between the workpieces, and as sufficient temperature is achieved the parts forge together. Resistance butt welding uses higher welding currents over a shorter time, and there is no loss of metal associated as with the flashing process. Both processes are well suited for high volume manufacturing of butt type joints. Flash welding offers advantages in that large irregular sections can be welded in a wide range of materials. This includes dissimilar (e.g. aluminum to copper) applications. Resistance butt welding offers improved cycle times (compared to flash welding) and less material loss associated with the process.

Benefits of Flash and Resistance Butt Welding

Flash and resistance butt welding are both high productivity joining technologies that require neither filler materials nor shielding gasses for use. The processes also are applicable for dissimilar materials joining. The individual approaches do, however, offer distinct advantages. These include, for the processes separately:

Flash butt welding:

• Applicable to nearly all metallic materials
• Tolerant to fit-up variations
• Tolerant to surface preparation variations
• Applicable to very small to very large section sizes
• Applicable to complex and irregular sections
• Applicable to dissimilar thickness joints

• Very short cycle times
• Excellent joint consistency
• Minimal material loss

Flash and Resistance Butt Welding Equipment at EWI

• F3 welding system capable of 7 ton upset force
  • 150 kVA AC transformer
  • 180 kVA DC transformer
• F6 welding system capable of up to 38 ton upset force
  • 600 kVA AC transformer
  • 300 kVA DC transformer

Friction welding is a general term to describe a family of solid-state welding process accomplished by the use of relative mechanical motion between the two pieces (rotary or linear) under an applied force. This application of motion and force generate enough heat to plastically deform material at the interface of the two materials.

There are two main variants of friction welding: direct drive friction welding and inertia welding. For direct drive friction welding, one of the parts is clamped rigidly to a spindle. As the one part rotates, it is forced against the other part also rigidly clamped under a preset force. After a given amount of time or distance, the rotating part is stopped and the two parts forged together typically under a higher force.

For inertia welding, one of the parts is rigidly clamped in a spindle that has weights bolted to it; commonly referred to as flywheels. Once the spindle, with its mass is at speed, the driving mechanism is disengaged, and the two parts to be joined are forced together under a preset force. As the spindle decelerates to a stop, the applied force, in some cases, is increased.

Benefits of Friction Welding

• Adaptable to nearly all materials including dissimilar alloy combinations
• A process with short cycle times with very high quality
• A robust process commonly used in both critical and high volume applications
• Uses no fluxes, fillers, and in most cases no shielding gases

Friction Welding Equipment at EWI

• Newcor Direct drive friction welder (6, 20, or 50 ton configurations)
• MTI Model 120B Inertia Welder
• MTI Model 250A Inertia Welder
• Ramstud H3500 portable Direct drive/Inertia welder
• Ramstud R450 portable friction welder
• Ramstud R1004 portable friction welder

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Friction Stir Welding (FSW)

Friction stir welding (FSW) utilizes a non-consumable tool to create local friction heating to produce a continuous solid state weld. The process allows a variety of joint configurations to be created using single pass, full penetration approach without the use of filler metals (including butt, lap, corner, and T-joints). The solid-state, low distortion welds produced are achieved with relatively low cost using simple and energy-efficient mechanical equipment.

The friction stir welding process involves plasticizing the material at the joint interface using a rotating tool. The tool is comprised of two primary components: a pin and shoulder. The pin first contacts the material with a downward force and rotation. This produces frictional heating, which allows the material to plastically flow as it heats up and loses strength as a function of temperature. The pin is inserted until contact is made between the shoulder of the tool and the top surface of the material. The shoulder acts to produce additional frictional heat and provides constraint against the flow of plasticized material while applying a forging force to the top surface of the weld. The tool then continues to rotate while traveling along the joint to complete the weld. When the desired length has been achieved, the friction stir tool is removed. There are several ways to eliminate or relocate the exit hole of the tool. The process is entirely solid state.

Benefits of Friction Stir Welding

The FSW process has many advantages over conventional arc and resistance welding processes. The process can be used to readily join 2000 and 7000 series aluminum alloys considered non-arc weldable. A few of the most notable benefits include:

• Ability to join aluminum, titanium, copper, magnesium, steel, nickel alloys, and other various high strength alloys
• Single pass, full penetration welds with only a butt-joint weld preparation
• Thicknesses up to 2-in. for soft metals and approaching 1-in. for hard metals (penetration can be doubled with two sided welding approach)
• A robust process with low distortion
• Repeatable, machine controlled process requiring little operator input
• Excellent mechanical performance
• No shielding gas required for aluminum applications (including some copper and magnesium alloys)
• High processing speeds
• Green process with low power demands and zero to minimal fume generation

Friction Stir Welding Equipment at EWI

• GTC Friction Stir Welder (25-ft by 10-ft by 10-ft welding envelope). This equipment has a 5 axis, complex curvature welding capability with gimbaling head; 20 degrees in any direction and an integrated rotary tilt positioner to increase the 5-axis gimbal and tube welding capability.
• Nova-Tech Friction Stir Welder (40-in. by 24-in. by 14-in. welding envelope). This equipment has 5 axis, complex curvature welding capability with gimbaling head.
• EWI Custom Gantry Style Welder (52-in. by 52-in. by 8-in welding envelope). This equipment is a 3 axis machine with a fixed head.

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Magnetic Pulse Welding

Magnetic pulse welding (MPW) is a high-speed joining process that uses electromagnetic force to accelerate and impact one part onto another, resulting in a solid-state weld. Magnetic pulse welding is typically used for lap joints between tubular components, or between a tube and a solid section. During magnetic pulse welding, the components are placed inside a coil in a lapped configuration. An electrical current is then sent through the coil, generating an intense magnetic field and creating high eddy currents in the outer tube. These eddy currents create intense opposing magnetic fields, forcing the outer component onto the inner component. The resulting high velocity impact (combined with appropriate contact angles) causes formation of a pseudo solid-state bond. Thermal effects with this process are very highly localized, resulting in little macroscopic heating of the workpieces. The process can also form joints by expanding the inner (tubular) component against an outer component. MPW is a process typically used for tubular components in the automotive, aerospace, and fluid products industries. Magnetic pulse technology can also be used to form mechanical (crimp type) joints.

Benefits of Magnetic Pulse Welding

• Applicable to dissimilar materials (e.g., aluminum to steel, nickel alloys to aluminum, etc.)
• Applicable to difficult materials (e.g., free-machining grades, copper, brass, etc.)
• Capable of high welding speeds (welding in the microsecond range)
• Joining with minimal heat (parts can immediately be handled)
• Joining with minimal distortion
• Joining without filler materials and shielding gases
• Environmentally friendly
• Easily automated

Magnetic Pulse Welding Equipment at EWI

• 15- to 90-kJ Magneform power supplies
• Coil systems for various sizes
• External coil systems (compression)
• Internal coil systems (expansion)
• Split (open) coil systems
• Process parameter optimization/prediction system

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Resistance & Solid-State Welding Capabilities:

• Spot Welding
• Projection Welding
• Seam Welding
• Flash and Resistance Butt Welding
• Friction Welding
• Friction Stir Welding
• Magnetic Pulse Welding