1. Spot Welding

2. Spot Welding Learning Activities Lesson Objectives View Slides;
Read Notes,
Listen to lecture
View Demo
Do on-line workbook
Lesson Objectives
When you finish this lesson you will understand:
Basics of Resistance Welding Processes
Heat Generation & Control
Spot Welding Process and Applications
Keywords: Resistance Spot Welding, Heat Generation, Equipment Control, Contact Resistance, Upslope, Downslope, Hold Time, Temper, Squeeze Time, Electrode

3. Definition of Resistance Welding
Resistance welding is a fusion welding process in which coalescence of metals is produced at the faying surfaces by the heat generated at the joint by the resistance of the work to the flow of electricity.
Force is applied before, during, and after the application of current to prevent arcing at the work piece.
Melting occurs at the faying surfaces during welding.

4. Principal Types of Resistance Welds
Electrodes
or Welding
Tips
Electrodes
or Welding
Wheels
Electrodes
or Dies
Projection
Welds
Spot Weld Seam Weld Projection Weld
Electrodes or Dies
Spot, seam, and projection welding operations involve a coordinated application of electric current and mechanical pressure of the proper magnitudes and durations. The welding current must pass from the electrodes through the work. Its continuity is assured by forces applied to the electrodes, or by projections which are shaped to provide the necessary current density and pressure. The sequence of operation must first develop sufficient heat to raise a confined volume of metal to the molten state. This metal is then allowed to cool while under pressure until it has adequate strength to hold the parts together. The current density and pressure must be such that a nugget is formed, but not so high that molten metal is expelled from the weld zone. The duration of weld current must be sufficiently short to prevent excessive heating of the electrode faces. Such heating may bond the electrodes to the work and greatly reduce their life.
The heat required for these resistance welding processes is produced by the resistance of the workpiece to an electric current passing through the material. Due to the short electric current path in the work and limited weld time, relatively high welding currents are required to develop the necessary welding heat.
Upset Weld Flash Weld
After Welding After Welding
[Reference: Resistance Welding Manual, RWMA, p.1-3]

5. 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]

6. Advantages of Resistance Spot Welding
Adaptability for Automation in High-Rate Production of Sheet Metal Assemblies
High Speed
Economical
Dimensional Accuracy

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
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. Resistance Welding Resistance welding depends on three factors:
Time of current flow (T).
Resistance of the conductor (R)
Amperage (I).
Heat generation is expressed as
Q = I2R T, Q = Heat generated.

10. Heating Value of Current = RMS Current
Irms=0.707 Ipeak

11. Block Diagram of Single-Phase Spot Welder
Contactor
Main Power Line
The electrical power system of a single-phase spot welder can be thought of simply as a contactor and a transformer whose secondary current is used to make a weld as shown in the above slide. The contactor shown in this figure is used simply to initiate and terminate the welding current. The transformer is a device which, through an inductive coupling, can change one AC voltage to another. A transformer consists of three elements:
Primary windings - A series of windings in the transformer which convert an alternating current into a magnetic flux.
Secondary windings - These windings receive the magnetic flux generated by the primary windings and convert it back into a usable AC current.
Core - A piece of iron used to carry the magnetic flux from the primary to the secondary windings. Spot welding transformers use one of two types of cores: stacked and wound (or “Hypersil”), as shown in the next slide.
As explained in the next section, the ratio of primary windings to secondary windings determines the voltage reduction and the available welding current. The type of core influences the size of the transformer and its saturation characteristics. National Electrical Manufacturers’ Association standards prescribe delayed firing for Hypersil transformers. In this case, firing is delayed for an additional 90 degrees from the selected setting to prevent transformer damage. Hypersil transformers are smaller than stacked-core transformers with the same ratings.
Spot Weld

12. The voltage available at the secondary can be calculated knowing the turns ratio of the transformer and the primary voltage. The turns ratio can be calculated as:
N= np/ns
where N is the turns ratio, np is the number of windings in the primary, and ns is the number of windings in the secondary. The secondary voltage can be calculated as:
Vs= Vp/N
where Vs is the voltage in the secondary and Vp is the voltage in the primary. Similarly, using Ohm’s Law it can be shown that the relationship between the primary current (Ip) and secondary current (Is) is:
Is= IpN
Typically, the turns ratio in resistance welding transformers is about 100 to 1. Therefore, if 480 V, 200 A power is available at the primary, 4.8 V at 20kA will be available at the secondary (neglecting losses).

14. Contact-Resistance Measurement
Electrode Force
Rec
Small Current
Rec = contact resistance
between electrode
and sheet surface
Rsc = contact resistance
at the faying surface
Rv = volume resistance of
the sheets
Rec
Rsc Rv
Rtotal
Rec Rv
If the power is applied over a time interval of t seconds, the heat energy developed in the resistance is:
Q = I2Rt (2)
where,
Q = Watt-seconds or joules.
It is evident from equation (2) that the magnitude of the heat energy generated can be varied by changing the value of any of the three factors of the equation. The I and t can be readily varied by adjusting the welding control while R, the resistance of the workpieces being welded, is fixed for any one process. There are two types of electrical resistance present in the secondary circuit of a resistance welding machine: 1) the volume resistance of the material in the circuit, including the workpieces, and 2) the interfacial resistance of contacting surfaces. Each of these is discussed below.
All metals have some degree of resistance to the passage of current. The resistance offered to direct current is known as resistivity and is the resistance of a standard volume measured at a known temperature. It is a function of the material composition, and it varies with temperature. The resistance is greater to the passage of alternating current (AC) due to the presence of eddy current generated in the interior of the conductor. This is known as skin effect and increases with an increase in the frequency of the current and increase in the sectional area of the conductor. The [continued]
Contact
Area
Rec
Electrode Force

15. Factors Affecting Heat Generation (Q):
Welding pressure
as welding pressure increases both R and Q decrease.
Electrodes
deformation of electrodes increases contact area. As contact area increases, both R and Q decrease.

16. Link to electrode force demo

17. Surface Condition Resistivity Electrode Force (a) Pickled Conditions
(b) Rusted Conditions
Steel
Resistivity
Rusty
Steel
Oils/Dirt
Oxide
Polished
Contact resistances, at least during the early stages of welding, are considerably higher than bulk resistances. The explanation for the magnitude of these resistances is shown in the above slide. Contact resistances are high because the surface are irregular. If the material is clean, it is in intimate contact only at various “asperity contact points.” The sum of these contact points is only a small fraction of the presumed contact area. As such, much less conducting area is available and the contact resistance increases accordingly. Figure (b) details the effect of rusted or dirty sheet surfaces. In this case, in addition to limited sheet-to-sheet contact, the surfaces are covered with a high resistivity film consisting of oxides or dirt. The effect is a further increased contact resistance. The magnitudes of the contact resistances for the surface conditions described are shown in the above slide. The effect of pressure for each condition is also shown. The polished surface has the smallest contact resistance, undoubtedly because the polishing has minimized both surface contamination and asperity heights. The latter has the effect of increasing the number of contact points and effective contacting areas. As discussed in the previous paragraph, sheets that have been pickled and then rusted show progressively larger contact resistances. The above slide also suggests welding pressure has an effect on contact resistance. Increasing welding forces would tend to cause the asperities to collapse. For pickled sheet, the effective conducting area is increased, resulting in a decrease in the contact resistance. For the rusted or dirty sheet, increasing welding force acts both to collapse the asperities and penetrate the insulating dirt and oxide films. Both have the net effect of lowering the contact resistance.
Pickled
Electrode Force

18. Resistance Varies with Pressure
In Case (a), low pressure will cause high contact resistance, because only the points of the rough surface meet.
In Case (b), medium pressure will get medium contact resistance, because the surfaces are pushed into each other.
In Case (c), high pressure can cause low contact resistance, because the surfaces are pressed tightly together.
Low Pressure Medium Pressure High Pressure
(a) (b) (c)

20. Volume-Resistance Measurement
Electrode Force
Small Current
Rec = contact resistance
between electrode
and sheet surface
Rsc = contact resistance
at the faying surface
Rv = volume resistance of
the sheets
Rec
Rsc Rv
Rtotal
Rec Rv
If the power is applied over a time interval of t seconds, the heat energy developed in the resistance is:
Q = I2Rt (2)
where,
Q = Watt-seconds or joules.
It is evident from equation (2) that the magnitude of the heat energy generated can be varied by changing the value of any of the three factors of the equation. The I and t can be readily varied by adjusting the welding control while R, the resistance of the workpieces being welded, is fixed for any one process. There are two types of electrical resistance present in the secondary circuit of a resistance welding machine: 1) the volume resistance of the material in the circuit, including the workpieces, and 2) the interfacial resistance of contacting surfaces. Each of these is discussed below.
All metals have some degree of resistance to the passage of current. The resistance offered to direct current is known as resistivity and is the resistance of a standard volume measured at a known temperature. It is a function of the material composition, and it varies with temperature. The resistance is greater to the passage of alternating current (AC) due to the presence of eddy current generated in the interior of the conductor. This is known as skin effect and increases with an increase in the frequency of the current and increase in the sectional area of the conductor. The [continued]
Contact
Area Rv
Electrode Force

22. Resistivity as a Function of Temperature
130
120
110
HSLA
100 90 80
Resistivity, mW-cm 70 60
Low Carbon 50
actual resistance is termed effective resistance and is directly proportional to the conductor length and nearly inversely proportional to the sectional area, considering skin effect, if present. Of the common metals, copper has the lowest resistance and is used as a base for comparison with other metals. For comparison purposes, the reciprocal of resistivity is used. This is known as conductivity and copper has been assigned a value of 100 percent, with common steel, for example, at about 10 percent. When a given current is passed through equal volumes and sectional areas of copper and steel, the generation of heat energy approximately ten times greater in the steel than in the copper.
Interfacial resistance is the resistance to the passage of current across the contacting surfaces of two metals. The value varies with the metal composition, surface condition, contacting area and pressure. In general, this resistance follows the volume resistivity of the metals involved to some degree. For example, the same contact area at equal pressure of copper to copper surfaces usually has much less resistance than a steel to steel contacting surface, with a copper to steel surface somewhere between the two extremes. It is important to note that the interfacial resistance can vary greatly due to surface contamination and physical condition.
If good machine design and construction practice has been followed, and the proper welding electrodes are used, the interfacial resistance of the workpiece faying surfaces is usually greater than that of the effective volume resistance of any one section of the secondary circuit and greater than that of any other contacting surface.
[continued] 40 30 20 10
Temperature, °C
[Reference: Welding in the Automotive Industry, D.W. Dickinson, p.125]

23. Questions

25. Heating Value of Current = RMS Current
Irms=0.707 Ipeak

26. Upslope/Downslope, Hold Time, & Temper
Electrode
Pressure
Weld Current
Current
Temper Current
Hold time allows the heat to be extracted from the workpiece through the electrodes following welding. As the electrodes remain in contact with the workpiece, the weld region is effectively quenched. Some materials, particularly the high-strength steels, are sensitive to brittle nugget formation upon quenching. For high-carbon and high-alloy steels, the quenched hold time (30 to 60 cycles) is sufficient to produce a bainitic or martensitic microstructure within the weld nugget. These microstructures, possessing high residual stresses, can promote interfacial failure during testing.
Upsloping and downsloping have been promoted as methods to improve the overall consistency of weld quality. Upsloping is used in situations where there is considerable variation in contact resistance or contact resistances are very high. Since the current is initiated at a very low value, mating of the contact surfaces can occur at low power levels. This promotes a uniform weld resistance at the onset of the full weld current. It also prevents excessive heating of the faying surface and subsequent premature expulsion. Downsloping is often used as a method of controlling the weld cooling rate. Where hold time sensitivity is a concern, the cooling rate can be reduced by the addition of a downslope. Longer downslopes typically result in slower cooling rates.
Given the rapid cooling rates observed during spot welding, particular for high-strength steels hard microstructures (martensite and bainite) are typically produced within the weld nugget. If weld microstructures are excessively hard, a postweld temper treatment promotes a tempered martensite structure that is less brittle. This acts to minimize the incidence of interfacial failures.
Upslope Downslope Temper
Squeeze Time Weld Time Off Time Hold Time

28. Heat Dissipation Water-Cooled Copper Alloy Electrode Base Metal
Weld Nugget
During welding, heat is lost by conduction into the adjacent base metal and the electrodes, as shown in the above slide. This heat dissipation continues at varying rates during current application and afterward, until the weld has cooled to room temperature. It may be divided into two phases: (1) during the time of current application, and (2) after the cessation of current. The extent of the first phase depends upon the composition and mass of the workpieces, the welding time, and the external cooling means. The composition and mass of the workpieces are determined by the design. External cooling depends upon the welding setup and the welding cycle. The heat generated by a given amperage is inversely proportional to the electrical conductivity of the base metal. The thermal conductivity and temperature of the base metal determine the rate at which heat is dissipated or conducted from the weld zone. In most cases, the thermal and electrical conductivities of a metal are similar. In a high-conductivity metal, such as copper or silver, high amperage is needed to produce a weld and compensate for the heat that is dissipated rapidly into the adjacent base metal and the electrodes. Spot, seam, and projection welding of these metals is very difficult.
If the electrodes remain in contact with the work after weld current ceases, they rapidly cool the weld nugget. The rate of heat dissipation into the surrounding base metal decreases with longer welding times because a larger volume of base metal will have been heated. This reduces the temperature gradient between the base metal and the weld nugget. For thick sheets of metal where long welding times are generally required, the cooling rates will be slower than thin sheets and short weld time.

29. Questions

30. Let’s put it all together

31. Initial Resistance Through Weldment
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 the above 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.
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.
Distance
Resistance
Bottom Electrode

32. Temperature Readings of A Spot Welding Process
(Note: Temp at Electrode Sheet Interface Higher than Bulk)
Workpiece
In order to better understand the nature of resistance welding and how heat generation is accomplished, it is best to consider a simple spot weld. The basic principles apply to all other resistance welding and heating methods except flash welding.
A spot weld is made by pressing two or more overlapping pieces of metal together while an electrical current is passed through a localized contact area. The electrical current heats the metal forming the weld nugget to the proper welding temperature.
One of the principles of resistance welding is to generate the heat energy in the weld zone very rapidly so that the minimum amount of heat will be dissipated by conduction to the cooler adjacent material. This requires a high rate of heat generation and is accomplished by bypassing a large value of current through the weld zone resistance for a short time interval. Another principle is to generate much more heat in the weld zone than in any other portion of the welding machine secondary circuit.
The theory and practice for achieving these two principles is stated as follows: heat energy is generated whenever an electrical current passes through an electrical resistance. The rate at which the heat is generated is given by:
W = I2R (1)
where,
W = Electrical power in watts.
I = Current in amperes.
R = Resistance in ohms [continued]
This illustration was taken
about 4/60th of a second
after the welding current
starts.

33. Temperature Distribution
distribution at
various location
during welding.
At the end of
welding time
Electrode
After 20%
welding time
Workpiece
Electrode
Temperature

34. Link to nugget growth demo

35. Nugget Solidification

36. Questions