1. Module 1 - Processes Review
This module will be a rather quick review of resistance welding processes. This is material that was presented earlier in WE601. You may want to go back to review material from that course and to examine the web page at www-iwse.eng.ohio-state.edu/we601.
OK, let’s start by reviewing the various types of resistance welding processes.

2. Process Review Lesson Objectives
When you finish this lesson you will understand:
The basic principles of the most common resistance welding processes
Learning Activities
Review the contents of WE602
Look up Keywords
View Slides;
Read Notes,
Listen to lecture
Do on-line workbook
Keywords
Resistance Spot Weld, Resistance Projection Weld, Resistance Seam Weld, Flash Weld, Upset Butt Weld, Electric Resistance Weld, High Frequency Weld, Electro-Brazing
The course is divided into modules, and the first slide in each module looks like this. It will contain lesson objectives for that module, a series of learning activities which you should do each while in each module, and a list of keywords which are searchable in the welding engineering encyclopedia.

3. Introduction to Resistance Spot Welding
Top Electrode
Water
Weld
Nugget
Resistance spot welding is the most common of the resistance welding processes. It is used extensively in the automotive, appliance, furniture and aircraft industries to join sheet materials. The configuration for resistance spot welding is shown schematically in this slide. The welding sequence is as follows:
Copper water-cooled electrodes are used to clamp the sheets to be welded into place. Then, the force applied to the electrodes ensures intimate contact between all the parts in the weld configuration. A current is passed across the electrodes through the sheets.
The contact resistances, which are relatively high compared to the bulk material resistance, cause heating at the contact surfaces. The combination of heat extraction by the chilled electrodes and rapid contact surface heating causes the maximum temperature to occur roughly around the faying surface. As the material near the faying surface heats, the bulk resistance rises rapidly while the contact resistance falls. Again, the peak resistance is near the faying surface, resulting in the highest temperatures. Eventually melting occurs at the faying surface, and a molten nugget develops. On termination of the welding current, the weld cools rapidly under the influence of the chilled electrodes and causes the nugget to resolidify, joining the two sheets.
Distance
Resistance
Bottom Electrode

4. Typical Equipment of Resistance Spot Welding
The machine shown in Figure (a) in this slide is typical of many resistance spot welding machines with a foot-operated control (D) which initiates both the pressure and current cycles. The type illustrated is a swinging arm machine, the top arm being pivoted. In other machines, the upper electrode assembly may be carried on a slide.
The workpieces shown in Figure (b) are placed between the electrodes which can be interchanged for different applications. The type illustrated is a typical method of joining stiffeners to thin sheet (0.5 mm) as shown here using 18 mm diameter electrodes with a tip diameter of 3.5 mm.
(a) (b)
[Reference: Welding Process Slides, The Welding Institute]

5. Process Operation of Resistance Spot Welding
The welding operation is automatic with preset controls, although the positioning of the workpiece may be manual. The electrodes are made of highly conducting material, so that resistance heating is concentrated at the joint interface. The area of concentration which determines the weld size is controlled by the area of the electrode faces. As the current is passed through the workpieces, pressure is applied to the electrode which keeps the faying surface in contact and excludes atmospheric contamination.
[Reference: Welding Process Slides, The Welding Institute]

6. Advantages of Resistance Spot Welding
Adaptability for Automation in High-Rate Production of Sheet Metal Assemblies
High Speed
Economical
Dimensional Accuracy
Resistance spot welding is used extensively because it is a simple, inexpensive, versatile and forgiving process. It has also been shown to be adaptable to some degree of feedback control. Its high production rate, high speed, economical systems coupled with high dimensional accuracy, make it well suited for high production environments.

7. Limitations of Resistance Spot Welding
Difficulty for maintenance or repair
Adds weight and material cost to the product, compared with a butt joint
Generally have higher cost than most arc welding equipment
Produces unfavorable line power demands
Low tensile and fatigue strength
The full strength of the sheet cannot prevail across a spot welded joint
Eccentric loading condition
There are, however, some limitations. Disassembly for maintenance or repair is very difficult.
A lap joint adds weight and material cost to the product when compared to a butt joint.
The equipment costs are generally higher than the costs of most arc welding equipment.
The short time, high-current power requirement produces unfavorable line power demands, particularly with single phase machines.
Spot welds have low tensile and fatigue strength due to the notch around the periphery of the nugget between the sheets.
The full strength of the sheet cannot prevail across a spot welded joint, because fusion is intermittent and loading is eccentric due to the overlap.

8. Questions?
Turn to the person sitting next to you and discuss (1 min.):
The heating in resistance spot welding is related to the resistance by the equation H=I2Rt. The resistance can be both bulk and surface resistance. If we change from welding steel to welding aluminum what changes in resistance do you expect?

9. Introduction to Projection Welding
Resistance projection welding is a variation on resistance spot welding. Basically, a protrusion is placed on one of the two materials to be welded. This projection is then brought into contact against the second material.
The welding sequence is similar to that for resistance spot welding. The welding electrodes are used to apply both force and current across the configuration. The point of contact acts to constrict current flow (and is a point of high resistance in the welding circuit), and heating occurs preferentially at this point. As the material heats it becomes soft, and the projection collapses under the force applied by the welding electrodes. Due to the amount of plastic flow involved, melting is not always necessary to form a sound joint.
Projection welding is not limited to sheets. Any joint whose contact area is small compared to the thickness of the parts being welded is a candidate for projection welding.
The sequence of events during the formation of a projection weld is shown in the above slide. In Figure (a), the projection is shown in contact with the mating sheet. In Figure (b), the current has started to heat the projection to welding temperature. The electrode force causes the heated projection to collapse rapidly and then fusion takes place as shown in Figure (c). The completed weld is shown in Figure (d).
(a) (b) (c) (d)
[Reference: Welding Handbook, Volume 2, p.566, AWS]

10. Advantages of Projection Welding
A number of welds can be made simultaneously in one welding cycle of the machine
Less overlap and closer weld spacing are possible
1 < Thickness ratio < 6
Smaller in size than spot welding
Better appearance on the side without projection
Less electrode wear than spot welding
Oil, rust, scale, and coatings are less of a problem than spot welding
In general, projection welding can be used instead of spot welding to join small parts to each other and to larger parts. Selection of one method over another depends upon the economics, advantages, and limitations of the two processes. The chief advantages of projection welding are listed in the above slide.
The limitation on the number of welds is the ability to apply uniform electrode force and welding current to each projection.
Less overlap and closer weld spacing are possible, because the current is concentrated by the projection, and shunting through adjacent welds is not a problem.
Thickness ratios of at least 6 to 1 are possible, because of the flexibility in projection size and location. The projections are normally placed on the thicker part.
Projection welds can be located with greater accuracy and consistency than spot welds, and the welds are generally more consistent because of the uniformity of the projections.
The most deformation and greatest temperature rise occur in the part with the projection, leaving the other part relatively cool and free of distortion, particularly on the exposed surface.
Because large, flat-faced electrodes are used, electrode wear is much less than that with spot welding, and maintenance costs are reduced.

11. Limitations of Projection Welding
Require an additional operation to form projections
With multiple welds, require accurate control of projection height and precise alignment of the welding dies
Thickness limitation for sheet metals
Require higher capacity equipment than spot welding
The most important limitations of projection welding are shown in the above slide.
The forming of projections may require an additional operation unless the parts are press-formed to design shape.
With multiple welds, accurate control of projection height and precise alignment of the welding dies are necessary to equalize the electrode force and welding current.
With sheet metal, the process is limited to thickness in which projections with acceptable characteristics can be formed and for which suitable welding equipment is available.
Multiple welds must be made simultaneously, which requires higher capacity equipment than does spot welding. This also limits the practical size of the component that contains the projections.

12. Examples of Various Projection Designs
(b)
The means of producing projections depends upon the material in which they are to be produced. Projections in sheet metal parts are generally made by embossing, as opposed to projections formed in solid metal pieces which are made by either machining or forging. In the case of stamped parts, projections are generally located on the edge of the stamping.
The purpose of a projection is to localized the heat and pressure at a specific location on the joint. The projection design determines the currency density. Various types of projection designs are shown in the above and the following slides.
(c) (d) (e)
[Reference: Welding Handbook, Volume 2, p.562, AWS]

13. Examples of Various Projection Designs (CONT.)
(f) (g) (h)
(i) (j)
[Reference: Welding Handbook, Volume 2, p.562, AWS]

14. Questions?
Turn to the person sitting next to you and discuss (1 min.):
As projections collapse, the welding electrode and head must move rapidly to keep up with the projection collapse. What things might hinder the heads rapid movement? What would you do to design a more rapid moving head?

15. Introduction to Resistance Seam Welding
Roll Spot Weld
Upper Electrode Wheel
Knurl or Friction
Drive Wheel
Overlapping Seam
Weld
Resistance seam welding is another variation on resistance spot welding. In this case, the welding electrodes are motor driven wheels rather than stationary caps. This results in a “rolling” resistance weld or seam weld. There are three independent parameters in configuring seam welding machines: power supplies and control, welding wheel configuration and sheet configuration.
The major concern with power supplies and control is the frequency with which current is applied to the workpiece. Depending on this frequency and the speed with which the material is being welded, the weld will be either a continuous seam weld, an overlapping seam weld or a roll spot weld.
Seam welds are typically used to produce continuous gas- or liquid-tight joints in sheet assemblies, such as automotive gasoline tanks. The process is also used to weld longitudinal seams in structural tubular sections that do not require leak-tight seams. In most applications, two wheel electrodes, or one translating wheel and a stationary mandrel, are used to provide the current and pressure for resistance seam welding. Seam welds can also be produced using spot welding electrodes. This requires the purposeful overlapping of the spot welds in order to obtain a leak-tight seam weld. Overlapping spot welding requires an increase in power after the first spot weld to offset the shunting effect in order to obtain adequate nugget formation as welding progresses.
Continuous Seam
Weld
Workpiece
Throat
Lower Electrode Wheel
[Reference: Welding Handbook,
Volume 2, p.553, AWS]

16. Mash Seam Weld Before welding After Welding Slightly Lapped Sheets
Wide, Flat
Electrodes
Mash seam welding is a resistance welding variation that makes a lap joint primarily by high-temperature plastic forming and diffusion, as opposed to melting and solidification. The joint thickness after welding is less than the original assembled thickness.
Mash seam welding requires considerably less overlap than the conventional lap joint. The overlap is about 1 to 1.5 times the sheet thickness with proper welding procedures. Wide, flat-faced wheel electrodes, which completely cover the overlap, are used. In order to obtain consistent welding characteristics, mash seam welding requires high electrode force, continuous welding current, and accurate control of force, current, welding speed, overlap, and joint thickness. Overlap is maintained at close tolerances, by rigidly clamping or tack welding the pieces.
The exposed or show side of the welded component is placed against a mandrel which acts as an electrode and supports the members to be joined. A welding wheel electrode is applied to the side of the joint that does not show. The show surface of the joint must be mashed as nearly flat as possible in order to present a good appearance. Proper positioning of the wheel with respect to the joint is required to obtain a smooth weld surface. Some polishing of the weld area may be required before painting or coating, when the appearance of the finished product is important.
Mash seam welding produces continuous seams which have good appearance and are free of crevices. Crevice-free joints are necessary in applications having strict contamination or cleanliness requirements, such as joints in food containers or refrigerator liners.
Weld Nuggets
Before welding After Welding
[Reference: Welding Handbook, Volume 2, p.554, AWS]

17. Introduction to Flash Welding
Flash welding (FW) is a resistance welding process that produces a weld at the faying surface of a butt joint by a flashing action and by the application of pressure after heating is substantially completed. The flashing action, caused by the very high current densities at small contact points between the workpieces, forcibly expels material from the joint as the workpieces are slowly moved together. The weld is completed by a rapid upsetting of the workpieces.
Two parts to be joined are clamped in dies (electrodes) connected to the secondary of a resistance welding transformer. Voltage is applied as one part is advanced slowly toward the other. When contact occurs at surface irregularities, resistance heating occurs at these locations. High amperage causes rapid melting and vaporization of the metal at the points of contact, and minute arcs form. This action is called “flashing”. As the parts are moved together at a suitable rate, flashing continues until the faying surfaces are covered with molten metal and a short length of each part reaches forging temperature. A weld is then created by the application of an upset force to bring the molten faying surfaces in full contact and forge the parts together. Flashing voltage is terminated at the start of upset. The solidified metal expelled from the interface is called “flash”.
[Reference: Welding Process Slides, The Welding Institute]

18. Basic Steps in Flash Welding
Electrodes
(a) (c)
(b) (d)
Position and Clamp the Parts
Flash
The basic steps in a flash welding sequence are as follows:
(1) Position the parts in the machine.
(2) Clamp the parts in the dies (electrodes).
(3) Apply the flashing voltage.
(4) Start platen motion to cause flashing.
(5) Flash the normal voltage.
(6) Terminate flashing.
(7) Upset the weld zone.
(8) Unclamp the weldment.
(9) Return the platen and unload.
The above slide illustrates these basic steps. Additional steps such as preheat, dual voltage flashing, postheat, and trimming of the flash may be added as the application dictates.
Upset and Terminate Current
Apply Flashing Voltage
and Start Platen Motion
[Reference: Welding Handbook, Volume 2, p.583, AWS]

19. Equipment Example of Flash Welding
[Reference: Welding Process
Slides, The Welding Institute]
The large industrial unit shown in the above slide has been purpose-built for welding chain links for ships. Note the molten metal expelled and section size of the workpiece in relation to the operator. The cycle time for welding this section would be about 1 min.
Typical applications:
(1) Butt welding of matching sections.
(2) Chain links.
(3) Railway lines.
(4) Window frames.
(5) Aero-engine rings.
(6) Car wheel rims.
(7) Metal strip in rolling mills.
(8) Others.

20. Advantages of Flash Welding
Flexible cross sectioned shapes
Flexible positioning for similar cross section parts
Impurities can be removed during upset acts
Faying surface preparation is not critical except for large parts
Can weld rings of various cross sections
Narrower heat-affected zones than those of upset welds
Butt joints between parts with similar cross section can be made by friction welding and upset welding, as well as by flash welding. The major difference between friction welding and upset and flash welding is that the heat for friction welding is developed by rubbing friction between the faying surfaces, rather than from electrical resistance. Upset welding is similar to flash welding except that no flashing action occurs.
Listed in the above slide are some advantages of flash welding.
Cross sectioned shapes other than circular (such as angle, H sections, and rectangles) can be flash welded. Rotation of parts is not required.
Within limits parts of similar cross section can be welded with their axes aligned or at an angle to each other.
The molten metal film on the faying surfaces and its ejection during upset acts to remove impurities from the interface.
Preparation of the faying surfaces is not critical except for large parts that may require a bevel to initiate flashing.
Rings of various cross sections can be welded.
The heat-affected zones of flash welds are much narrower than those of upset welds.

21. Limitations of Flash Welding
Produce unbalance on three-phase primary power lines
The ejected molten metal particles present a fire hazard
Require special equipment for removal of flash metal
Difficult alignment for workpieces with small cross sections
Require almost identical cross section parts
Some important limitations of flash welding are listed in the above slide.
The high single-phase power demand produces unbalance on three-phase primary power lines.
The molten metal particles ejected during flashing may cause a fire, an injure of the operator, or shafts and bearings damage. The operator should wear face and eye protection, and a barrier or shield should be used to block flying sparks.
Removal of flash metal is generally necessary and may require special equipment.
Alignment of workpieces with small cross sections is sometimes difficult.
The parts to be joined must have near-identical cross sections.

22. Common Types of Flash Welds
Axially Aligned Weld
Dies
Three common types of welds made by flash welding are shown in this and the following two slides.
The axially aligned weld is shown in the above slide.
Cross Section After Welding
Fixed Platen
Movable Platen
Transformer
[Reference: Welding Handbook, Volume 2, p.589, AWS]

23. Common Types of Flash Welds (CONT.)
Miter Weld
Fixed Platen
Movable Platen
The miter weld is shown in the above slide.
Cross Section After Welding
Transformer
[Reference: Welding Handbook, Volume 2, p.589, AWS]

24. Common Types of Flash Welds (CONT.)
Ring Weld
The ring weld is illustrated in the above slide.
Fixed Platen
Movable Platen
Cross Section After Welding
Transformer
[Reference: Welding Handbook, Volume 2, p.589, AWS]

25. Questions?
Turn to the person sitting next to you and discuss (1 min.):
In flash welding of steel, the iron burns to iron oxide during flashing and removes oxygen from the flashing interface. The free energy of formation of iron oxides at 1500K are about –40 kcal/gram-atom of oxygen. The free energy of formation of titanium oxides at 1500K is about –90 kcal/gram-atom oxygen. What effect do you suppose this would have on the flash welding operation?

26. Introduction to Upset Welding
To Welding Transformer
Clamping Die
Heated Zone
Clamping Die
Upsetting
Force
Upset welding (UW) is a resistance welding process that produces coalescence over the entire area of faying surfaces, or progressively along a butt joint, by the heat obtained from the resistance to the flow of welding current through the area where those surfaces are in contact. Pressure is used to complete the weld.
With this process, welding is essentially done in the solid state. The metal at the joint is resistance heated to a temperature where recrystallization can rapidly take place across the faying surfaces. A force is applied to the joint to being the faying surfaces into intimate contact and then upset the metal. Upset hastens recrystallization at the interface and, at the same time, some metal is forced outward from this location. This tends to purge the joint of oxidized metal.
Upset welding has two variations:
(1) Joining two sections of the same cross section end-to-end (butt joint).
(2) Continuous welding of butt joint seams in roll-formed products such as pipe and tubing.
The first variation can also be accomplished by flash welding and friction welding. The second variation is also done with high frequency welding.
The general arrangement for upset welding is shown in the above slide. One clamping die is stationary and the other is movable to accomplish upset. Upset force is applied through the movable clamping die or a mechanical backup, or both.
Movable Part
Stationary Part
Finished Upset Weld
[Reference: Welding Handbook, Volume 2, p.598, AWS]

27. Typical Mill Forms and Products of Upset Welding
Upset welding is used in wire mills and in the manufacture of products made from wire. In wire mill applications, the process is used to join wire coil to each other to facilitate continuous processing. The process also is used to fabricate a wide variety of products from bar, strip, and tubing. Typical examples of mill forms and products that have been upset welded are shown in the above slide. Wire and rod from 0.05 to 1.25-in. (1.27 to mm) diameter can be upset welded.
[Reference: Welding Handbook, Volume 2, p.600, AWS]

28. Resistance Tube Welding (ERW)
In the resistance seam welding process, a strip of sheet material is uncoiled and roll formed through a series of “hour glass shaped” rolls which form it into the shape of a tube. Rolling contacts on either side of the joint pass current to the strip which passes across the joint heating and consummating the weld while “pressure rolls on the side of the tube apply force across the joint.
W. Stanley, Resistance Welding
McGraw-Hill, 1950

29. High Frequency Welding Applications
Induction Coil HF HF HF
High frequency welding includes those processes in which the coalescence of metals is produced by the heat generated from the electrical resistance of the work to high frequency current, usually with the application of an upsetting force to produce a forged weld.
There are two processes that utilize high frequency current to produce the heat for welding: high frequency resistance welding (HFRW), and high frequency induction welding (HFIW), sometimes called induction resistance welding. The heating of the work in the weld area and the resulting weld are essentially identical with both processes. With HFRW, the current is conducted into the work through electrical contacts that physically tough the work. With HFIW, the current is induced in the work by coupling with an external induction coil. There is no physical electrical contact with the work.
The above and the following two slides show some basic applications of high frequency welding.
Tube Butt Seam Tube Butt Seam Tube Mash Seam
[Reference: Welding Handbook, Volume 2, p.653, AWS]

30. High Frequency Welding Applications (CONT.)
Strip Butt
T-Joint HF HF
With low frequency (50 Hz Hz), direct current or “square wave” resistance welding, much higher currents are required to heat the metal, and large electrical contacts must be placed very close to the desired weld area. The voltage drop across the weld is very low, and the current flows along the path of least resistance from one electrode to the other. With high-frequency welding, by contrast, the current is concentrated at the surface of the part. The location of this concentrated current path in the part can be controlled by the relative position of the surfaces to be welded and the location of the electrical contacts or induction coil. Heating to welding temperature can be accomplished with a much lower current than with low-frequency or direct current welding.
Spiral Tube Fin
Spiral Tube
[Reference: Welding Handbook, Volume 2, p.653, AWS]

31. High Frequency Welding Applications (CONT.)
Induction
Coil
Projection Seam HF
Although the welding process depends upon the heat generated by the resistance of the metal to high frequency current, other factors must also be considered for successful high-frequency welding. Because the concentrated high-frequency current heats only a small volume of metal just where the weld is to take place, the process is extremely energy efficient, and welding speeds can be very high. Maximum speeds are limited by materials handling, forming and cutting. Minimum speeds are limited by material properties and weld quality requirements.
The fit of the surfaces to be joined and the manner in which they are brought together is important if high-quality joints are to be produced. Flux is not usually used but can be introduced to the weld area in an inert gas stream. Inert gas shielding of the welding area is generally needed only for joining reactive metals such as titanium and certain stainless steel products. HF
Pipe Butt
Bar Butt
[Reference: Welding Handbook, Volume 2, p.653, AWS]

32. Advantages of High-Frequency Welding
Produce welds with very narrow heat-affected zones
High welding speed and low-power consumption
Able to weld very thin wall tubes
Adaptable to many metals
Minimize oxidation and discoloration as well as distortion
High efficiency
High-frequency welding processes offer several advantages over low frequency and direct current resistance welding processes. One characteristic of the high-frequency processes is that they can produce welds with very narrow heat-affected zones. The high-frequency welding current tends to flow only near the surface of the metal because of the “skin effect” and along a narrow controlled path because of the “proximity effect”. The heat for welding, therefore, is developed in a small volume of metal along the surfaces to be joined. A narrow heat-affected zones is generally desirable because it tends to give a stronger welded joint than with the welder zone produced by many other welding processes. With some alloys the narrow heat-affected zone and absence of cast structure may eliminate the need for postweld heat treatment to improve the metallurgical characteristics of the welded joint. The shallow and narrow current flow path results in extremely high heating rates and therefore high welding speeds and low-power consumption. A major advantage of the continuous high-frequency welding process is its ability to weld at very high speeds. High-frequency welding can also be used to weld very thin wall tubes. Wall thickness down to less than in. are presently being welded on continuous production mills. The process is adaptable to many metals including low carbon and alloy steels, ferric and austenitic stainless steels, and many aluminum, copper, titanium, and nickel alloys. Because the time at welding temperature is very short and the heat is localized, oxidation and discoloration of the metal as well as distortion of the part are minimal. High-frequency welding power sources have a balanced three-phase input power system. Using conventional vacuum tube welding power sources, as much as 60 percent of the energy is converted into useful heat in the work.

33. Limitations of High-Frequency Welding
Special care must be taken to avoid radiation interference in the plant’s vicinity
Uneconomical for products required in small quantities
Need the proper fit-up
Hazards of high-frequency current
As with any process, there are also limitations. Because the equipment operates in the radio frequency range, special care must be taken in its installation, operation, and maintenance to avoid radiation interference in the plant’s vicinity.
As a general rule, the minimum speed in carbon steel is about 25 feet/min. For products which are only required in small quantities, the process may be uneconomical unless the technical advantages justify the application.
Because the process utilizes localized heating in the joint area, proper fit-up is important. Equipment is usually incorporated into mill or line operation and must be fully automated. The process is limited to the use of coil, flat, or tubular stock with a constant joint symmetry throughout the length of the part. Any disruption in the current path or change in the shape of the vee can cause significant problems. Special precautions must be taken to protect the operations and plant personnel from the hazards of high-frequency current.

34. Some Products of High-Frequency Welding
Examples of a few products that can be fabricated by high-frequency welding are shown in the above slide.
[Reference: Welding Handbook, Volume 2, p.665, AWS]

35. Questions?
Turn to the person sitting next to you and discuss (1 min.):
If you were going to make tubing or pipe materials what might be the criteria when you would select ERW over High Frequency Welding?

36. Electro-brazing
Often materials to be welded have such low inherent resistance (as an example copper alloys) that they do not develop sufficient heating on their own to melt and make a weld under resistance welding conditions. In this case, a lower melting braze alloy can be placed interfacial between the sheets to be welded and a braze made at the joint. Often additional heating is provided by high resistance electrodes which heat externally to the sheet and allow additional heating to soak into the braze joint.
W. Stanley, Resistance Welding
McGraw-Hill, 1950

37. Questions?