Seam Welding.

Seam Welding

Seam Welding Learning Activities View Slides; Lesson Objectives
Read Notes, Listen to lecture Do on-line workbook Homework Lesson Objectives When you finish this lesson you will understand: The various forms of seam welding Nugget formation mechanisms Quality testing of Seam Welds Keywords Roll Spot Weld, Overlapping Seam Weld, Continuous Seam Weld, Lap Seam Weld, Mash Seam Weld, Metal Finish Seam Weld, Butt Seam Weld, Foil Butt Seam Weld, Projection Seam Weld, Weld Current-Pulse Time, Welding Speed, Overlap Distance, Nugget Formation, Seam Weldability, Pillow Test

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]

Lap Seam Weld Front view Side View Overlapping Electrodes Weld Nuggets
Travel Lap joints can be seam welded using two wheel electrodes or one wheel and a mandrel. The minimum joint overlap is the same as for spot welding, i.e. twice the minimum edge distance (distance from the center of the weld nugget to the edge of the sheet). Front view Side View [Reference: Welding Handbook, Volume 2, p.554, AWS]

Effect of Weld Current and Pulse Timing
The effect of pulse timing and current for overlap seam welding are present in these figures. As the pulse timing produces pulses with less percent heat time, the absolute current level required increases. As current is increased around this current level, the amount of nugget penetration increases and the percentage overlap of the individual spots also increases. 3 on, 2 off (60%) 6 on, 5 off (54%) 4 on, 3 off (57%) Resistance Welder Manufacturers Association Bulletin # 23

Effect of Cool Time (Heat %) on Nugget Properties
@ 6 Cycle Heat 75% % % % % % Heat 6 on, 5 off (54%) Resistance Welder Manufacturers Association Bulletin # 23

Effect of Electrode Force
Increased Electrode Force increases nugget penetration Increased Electrode Force causes increase in nugget width while reducing penetration, more electrode cooling Resistance Welder Manufacturers Association Bulletin # 23

Welding Speed As welding speed is increased, more effective cooling occurs as the cooler welding electrode and more cool material is presented to the weld region per unit of time. The nugget penetration drops rapidly. In addition, the amount of overlap is reduced and the mechanical properties of a leak tight joint are compromised. This necessitates an increase in current in the attempt to maintain a large nugget growth and maintain joint mechanical properties. The higher currents eventually result in excessive electrode sheet surface heating and electrode sticking. More will be presented on this later as we consider the nugget growth and current flow characteristics. Resistance Welder Manufacturers Association Bulletin # 23

Seam Welding Multiple Thickness Sheets
Work was performed to develop recommended welding procedures for welding sheets with differing thickness in the overlapping spot seam welding process. This table list the results of these trials. Resistance Welder Manufacturers Association Bulletin # 23

Improper selection of Parameters Leads to Porosity in Weld
Resistance Welder Manufacturers Association Bulletin # 23

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]

Effect of Sheet Overlap on Mash Lap Joint properties
One of the primary variables in mash lap seam welding is the amont of initial overlap. After an initial critial amont of overlap of about 60 to 80 %, the weld strength maintains its full value. As the overlap gets greater than this amount, however, the joint thickness increases and the penetration also increases. Resistance Welder Manufacturers Association Bulletin # 21

Effect of Electrode Force on Mash Seam Weld
For an initial overlap of 150% it is seen that the electrode force has an effect on joint properties. With increaseing force, the joint thickness and penetration both drop slowly. When force gets to and excessive level, in this case 1500 pounds, the weld looses a nugget and the strength drops. Resistance Welder Manufacturers Association Bulletin # 21

Metal Finish Seam Weld Before Welding After Welding Finish Side
Chamfered Electrode Flash Broad, Flat Electrode Lap and mash seam welds differ with respect to the amount of forging, or, as the name implies, mash down. The lap weld has practically no mash down, while the thickness of a mash seam weld approaches that of one sheet thickness. In metal finish seam welding, mash down occurs on only one side of the joint and is a compromise between lap and mash seam welding. The amount of deformation, or mash, is affected by the geometry of one electrode wheel face and the position of the joint with respect to that face. Before Welding After Welding [Reference: Welding Handbook, Volume 2, p.554, AWS]

Effect of Weld Current and Electrode Force on Finish Seam Welds
The effect of Weld Current and Electrode Force on nugget penetration and edge finish are presented here. As current increases, penetration increases (strength of the seam weld also increases to a maximum and then holds constant for further current increase, see next slide) and the edge finish decreases. As electrode force increases, the penetration decreases. The surface condition generally improves with electrode force, but decreases with additional current. Resistance Welder Manufacturers Association Bulletin # 24

Effect of Current on Metal Finish Seam Weld
With increasing current, the nugget penetration increases and the strength rapidly increases. Currents between 17 and 21 kiloamps for this particular material made successful welds. At 22,000 amps, burning of the seam weld began to occur. Resistance Welder Manufacturers Association Bulletin # 21

Recommended Sheet Positioning
This figure indicates the effect of sheet position and overlap on the nugget penetration and the edge finish. Note that if the lower sheet is retracted to the point where the chamfer begins, the penetration begins to drop. Thus the recommended overlap is just short of this point. Resistance Welder Manufacturers Association Bulletin # 24

A joint in which two abutting edges are welded is classified as a butt seam joint. The thickness of the weld should be approximately the same, or slightly less than, the sheet thickness. Butt seam welding is typically reserved for applications in which other butt welding processes cannot be used (for example, for tube welding and for sheet metal in railroad cars). ASM Handbook Vol6, 1993

Distribute the welding current more evenly to both sheets
Butt seam welds can be classified as either autogenous or exogenous. In autogenous butt seam welding, the weld is made by butting the materials together and welding without the use of any filler material. This type of weld is usually low in strength because of a reduced cross sectional area of weld. The strength of the butt weld can be increased by the addition of filler metal. Filler metal in the shape of a foil or a shaped wire is introduced on one or both surfaces and melted into the weld. This technique produces a smooth, non-overlapping seam that is high in strength with good appearance. Highest quality exogenous butt seam welds show nugget penetration to the filler-workpiece interface, complete filler bonding, and smooth, regular surface contours. The only edge preparation required is to ensure that the sheared edges are clean and straight without excessive burr or gap when they are butted together. The filler acts as a bridge to: Distribute the welding current more evenly to both sheets Concentrate the current in the joint Help contain the molten nugget as it grows and then cools Provide metal for a slightly raised bead, which will add strength. The variables having the greatest effect on the tensile strength of filled butt seam welds are current, speed, size of the gap between the edges butted workpieces, and the work metal thickness. (ASM Handbook Vol ) Porter, Galvanizing and welding structural steel, Metal Construction, Nov 1983

This is an example of the foil butt seam weld producing a square hollow tube. Note the variation in the lower electrode. ASM Handbook Vol6, 1993

In this process developed by the Soudronic Company uses a wire for the electrode. This process is often found in the welding of cans in the food processing industry. ASM Handbook Vol6, 1993

Nugget Formation in Continuous Seam Weld
Let us now start to look at the physics of nugget formation in seam welding. We will start by looking at the nugget formation in continuous seam welding (as there is essentially one nugget that just gets dragged along the seam). We will then examine the nugget formation in pulsed seam welds. When the electrode wheel turns, it applies a force upon the sheets being welded over a region under the electrode. The force at the entry and the exit is less than that in the middle directly under the center of the wheel, but this entire region can be called the electrode force “footprint”.

Let us examine physically what is happening in the nugget growth
Let us examine physically what is happening in the nugget growth. We will look at the process when continuous current is supplied and later examine the conditions when the continuous current is replaced by pulsing. (Note that in this diagram the material is being fed from the right and exiting on the left). As the material comes in, the sheets are not in intimate contact before they hit the electrode footprint region. When the sheets come together, current just begins to flow through that region and the temperature begins to rise. The interface resistance is still high because no bonds are yet formed and the temperature is still quite low because of the short time of current flow through this particular region. As the material moves more under the electrode, the heating continues and some solid state bonds are made. This bonding reduces interface resistance but the higher temperature raises the bulk resistance. A little further along, melting occurs first in a rather mushy state and then to a full nugget size (penetration up to the solid red line extent). Progressing slightly further, cooling from the cooling water sprayed on the exit begins to solidify the nugget. For a pulse process, the duration of the pulse must cover the duration seen in the central portion of this diagram for each pulse duration. Generally several such pulses with several such nuggets within the electrode footprint occur. Waddell, Mechanism of weld formation…, Welding and Metal Fabrication, Aug 1995

Peak occurs earlier with thinner materials
Stage 1 Takes cycles, Increased resistance due to increased temp outweighs reduction in resistance due to better surface contact through asperity softening and surface oxide breakdown 2 1 3 4 Let us examine the shape of the resistance curve during the formation of this first part of a continuous nugget.. The resistance changes dynamically. At first the resistance increases in stage 1 do to the increased temperature (increased bulk resistance) outweighing the better surface contact. This happens in the first 1 to 4 cycles of current passage. With thinner materials, the peak occurs more rapidly. Peak occurs earlier with thinner materials Resistance Curve for Establishment of Leading Edge of a Continuous Nugget Waddell, Mechanism of weld formation…, Welding and Metal Fabrication, Aug 1995

2 Stage 2 Drop in resistance because of current shunting into the mushy and solid state bonded region 1 3 4 In stage 2, drops because of the shunting into the mushy and solid state bonded region. Waddell, Mechanism of weld formation…, Welding and Metal Fabrication, Aug 1995

Stage 3 Instability in current path, nugget approaching a steady state growth mode
2 1 3 4 In stage 3 as the full liquid nugget occurs, there is some instability in the current path as a steady state growth mode is approached. The current path instability is noted in the next slide. Waddell, Mechanism of weld formation…, Welding and Metal Fabrication, Aug 1995

Stage 1 Stage 2 Stage 3 Nugget Initiates Nugget Moves Approaching
In stage 1 the nugget initiates, in stage 2 the nugget moves toward the exit and in stage 3 a full nugget is formed some solidification occurs and the current path instability is evident just for the short time until steady state can be established. Stage 1 Stage 2 Stage 3 Nugget Initiates Nugget Moves Toward Exit Approaching Steady State

2 Stage 4 Steady State Growth of Nugget until either expulsion or current off time and new nugget formation 1 3 4 After steady state is established, the dynamic resistance remains fairly stable as illustrated in stage 4. Waddell, Mechanism of weld formation…, Welding and Metal Fabrication, Aug 1995

Actual Data Particular Interest in Steady State Region 2 m/min 4 m/mim
(effect of Speed) 2 m/min Here is some actual data showing continuous seam welds in the steady state condition as a function of travel speed (speed of electrode rotation). Note as the travel speed increases, the initial stages occur more rapidly and steady state is reached more rapidly. 4 m/mim Attorre, Heat Development in Resistance Seam Welding, Ironmaking and Steelmaking, 1996

Increased Current Ahead of nugget
At Lower Speeds At Higher Speed Increased Current Ahead of nugget Less Heat Extracted through Electrode Larger thru Sheet Nugget Growth At low speeds the heating and current path conditions are those as previously described. But at higher speeds, the nuggets tends to move back toward the exit region of the electrode footprint. This forces increased current ahead of the weld molten region. In addition, less heat is extracted through the electrodes as there is less electrode contact time because of the increased speed. This results in greater through thickness nugget growth. Thus the slower speeds tend to have narrower penetration which the faster welds tend to have deeper penetration. Waddell, Mechanism of weld formation…, Welding and Metal Fabrication, Aug 1995

Through Sheet Nugget Growth 2 m/min 6 m/min 16 m/min
To examine further the effect of speed, the nuggets were sectioned and etched to reveal the cross sections of the seam welds. Note with the slower speeds there was more electrode indentation and the weld cross section was wide but narrow penetration. With fast speeds, they were narrow with significant penetration. At excessively fast speed they actually penetrate trough the sheet surface and expulsion occurs. 16 m/min Waddell, Mechanism of weld formation…, Welding and Metal Fabrication, Aug 1995

Factors Promoting Thru Thickness Growth
High Welding Speeds Increased Sheet Metal Thickness (less electrode cooling) Increased Sheet Metal Resistivity Increased Force (increased Electrode Arc Contact-Current Length) Increased Current In summary, the factors which promote this through thickness growth are listed here.

Steady State Heat Flow Patterns
(Examine HAZ in Front of Nugget Formation) Enter Exit HAZ Type A Nugget Length of Heat Zone Increased Speed Increased Speed Type B HAZ Nugget Unstable Transition Pattern Some additional work was done examining the heat affected zone patterns as a function of speed as shown here. Consider first the conventional Type A pattern. As the speed increased, the length of the heat affected zone tended to increase in this type “A” HAZ pattern. With further speed increase, however, the shape of the weld and heat affected zone changed radically from the conventional pattern of type A to a two stage pattern represented by the shape of type B. The length of the molten weld region also increased in this type B pattern. The type B pattern represents an unstable, transitional pattern. Finally with still more rapid travel speed, a more stable inverted HAZ patterned represented by type C occurred. Also with the changes in the HAZ pattern, there was a movement of the nugget position with respect to the electrode centerline as indicated here. Type C Nugget HAZ

Sheet Thickness 0.7 mm 1.2 mm 2.0 mm For Type A Pattern
In summary, heat zone distance in the type A mode was measured as a function of travel speed and represented here, showing that as the speed increased the heat affected zone length increased. This was more pronounced for the thinner sheet thickness than when the sheet thickness increased.

Higher Speeds move from “A” to “C”
2 m/min 10 m/min Here is the photomicrograph of the two stable patterns, Type A and Type C, described in the last slides. At High Speeds, Current Flow and Heat Generation Extended to Sheet Surface Higher Speeds move from “A” to “C” Waddell, Mechanism of weld formation…, Welding and Metal Fabrication, Aug 1995

Exit End Enter End Turning now to the type C pattern we can make the following observations. At the very high speed when the Type C profile is evident, the nugget moved back toward the exit end of the electrode. Here shows both the longitudinal and transverse section of the weld. The transverse section was sectioned near the exit end of the profile. Note that the nugget penetration has grown so great that surface break through and expulsion has occurred, solidified only by the stream of cooling water at the exit. Longitudinal Transverse Higher Speeds Result in Nugget Moving toward Exit End and Expulsion on Exit

This is an example of the top surface of the weld.
Waddell, Mechanism of weld formation…, Welding and Metal Fabrication, Aug 1995

Increased Electrode Force Contact Area Nugget Moves Back
The electrode force was also studied. With increases in electrode force, the HAZ pattern was also changed from type A to B to C. The increased force cause more of a contact area or “electrode footprint” and caused more electrode indentation as noted. Nugget Moves Back Heat Zone Increases Increased Indentation

Nugget Formation in Pulsed Welds
Let us now look at the nugget formation which occurs during the production of welds using pulsed currents.

The pulsing cycle might look something like this with 3 cycles on and one cycle off. This is a typical cycle sequence which give nugget formation with sufficient solidification of the nuggets during the off time. ASM Handbook Vol6, 1993

Length of electrode footprint holds 3 welds
Here is the representation of the nuggets occurring with such a weld cycle. Note that the nuggets grow during the on current pulses. The diameter of the electrode and the speed are set so that length of the electrode footprint holds about 3 nuggets in various stages of formation. In the early part of the feed “footprint” the nugget starts to melt, as it progresses it grows, with finally overlapping the preceding nugget as it exits the footprint region. Schematic of seam weld. Length of electrode footprint holds 3 welds ASM Handbook Vol6, 1993

irregular pulses that does not give time for heat flow
Irregular nuggets irregular pulses that does not give time for heat flow ASM Handbook Vol6, 1993 An examination of the nuggets can be done to determine the machine and parameter functioning. Irregular pulsing or erratic machine operation produces irregular shaped nuggets as illustrated on the top. On the other hand, acceptable pulsing and machine operation will result in uniform nuggets with approximately 30% overlap. Lesser overlap is generally unacceptable for leak tight joints. RSW Certification Training Class, Boeing

Series of Overlapping Nuggets
Continuous Seam Weld Dynamic Resistance Now let us examine the resistance curve in pulse cycle seam welds and compare it with the continuous seam weld resistance curve. Note that the first weld looks just like the initiation of the continuous weld examined previously. Thereafter for each additional pulse there is a peak as the next nugget is formed. To get full nuggets which overlap, each new nugget must not be initiated until the previous pulse duration has been sufficient to reach the stage 4 steady state. Because of the heat flow from the previous nugget, the steady state region for the subsequent nuggets is reach more rapidly than for the first nugget. Time Attorre, The influence of welding parameters on formation of solute bands…, Welding In the World, Vol37, 1996

Parameter (Lobe) Curves for Seam Weld
In spot welding, we looked at lobe curves to give us some feeling of the weldability of various materials. The same concept can be used for seam welding plotting welding current vs. welding speed. Notice that as the welding speed goes up, the current needed to get good overlapping welds goes up. There is a range for any welding speed over which we get good welds in this case, the “crack free welds”. At some current below this good range, we get solid state bonding or “stuck” welds. And below some absolute current we don’t even get a stuck weld. On the other hand, if current is increase above the good range, we start to get cracking and at greater currents yet we get surface burning and surface expulsion. When continuous current is used, the whole curve is shifted down to lower currents because there is no cooling between pulses in continuous current modes. Pulsed Current Continuous Current No Cooling Between Pulses in Continuous Therefore Lower Current Porter, Galvanizing and welding structural steel, Metal Construction, Nov 1983

Speed Limited for Pulse or Continuous Current Options Expulsion or
Overheating Minimum Condition for Continuous Weld In summary then, the lobe curve for acceptable seam welding looks like this. Note that there is a limit to the weld speed above which no acceptable welds can be made. More on this later. Gould, Resistance Seam Weldabilit, Report MR9112, EWI, 1991

What About Coated Sheets?
What about coated steels? Do these have any effect on the seam weld lobe curves? What About Coated Sheets?

Resistance Seam Weldability of Low-Carbon
Steel Lower Speed Higher Current Surface Eruption, Cu Contamination Cracking Non-Continuous Seam Here is a comparison of the seam weld lobe curves as a function of speed and electrode force for both uncoated and coated steels. As seen before, the limits of the curve for uncoated steels border on non-continuous seams at the lower current ends and surface eruptions or electrode burning at the higher current surface. With hot dipped galvanized steels, the curves are moved to higher current levels and lower travel speeds. Hot-Dipped Galvanized Uncoated Steel AWS Welding Handbook , Vol 4, 1998

Entry Exit Weld High Speed Coating Melts and Extrudes Out Coating
Expulsion Weld Coating Physically, the extra presence of the coating on the surface at the interface causes some difficulties. This coating makes good electrical contact and thus lowers the resistive heating until the coating can be melted and squeezed out. If it does not get squeezed out it can accumulate within the weld melts either vaporizing and causing porosity or increased inclusions. Coating Melts and Extrudes Out High Speed Peterson, “High Speed Seam Welding..” AWS Sheet Metal Welding Conf. VI, 1994

Coated Steels have lower contact resistance
delaying heating until coating is removed thus slower speeds are required to get coating out This is another representation that shows that because the coated steels have lower contact resistance, heating is delayed until the coating can be melted and squeezed out thus slower travel speeds become inevitable. Peterson, “High Speed Seam Welding..” AWS Sheet Metal Welding Conf. VI, 1994

Factors Limiting Travel Speed
Bulk Resistivity The higher the bulk resistivity of the material, the higher the heating (H=I2Rt) and thus the higher the travel speed possible Current Density As travel speed increases, more material is presented to the rolls per half cycle of current thus reaching a limit to the current density in the part Conductive Coating Materials When conductive coatings are present at the faying surface, they offer an increased shut path for current and thus also reduce current density. Welding takes place when these coatings are melted and extruded out of the weld zone. Expulsion With increased travel speed, complete nugget formation is delayed until the weld forms outside the pressure influence of the rolls (see next slide) and expulsion is easier. Thus, the factors limiting travel speed are enumerated here.

Experimental Procedure to Maximize Current Density
Projection Seam Welds Experimental Procedure to Maximize Current Density A seam welding process utilizing a projection on one of the sheets has recently been developed. The projection concentrates the current to maximize current density thus increasing the travel speed available. As illustrated, several projections shapes have been tried. These can all be formed using a roller die right on a continuous seam welding line. The presence of the projection also makes it easier to melt and extrude coating on coated steels.

As with conventional projection welds, the design of the projection becomes very important. The projection size and shape must be designed to get the proper mashdown as illustrated. Peterson, “High Speed Seam Welding..” AWS Sheet Metal Welding Conf. VI, 1994

Some work on projection design has been done as illustrated here and available in this publication.
Peterson, “High Speed Seam Welding..” AWS Sheet Metal Welding Conf. VI, 1994

Seam Weld Quality Tests
Several seam weld quality tests are commonly used including the standard cross weld tensile test and a pillow test. In the pillow test, a seam is made completely around the sample and compressed gas is inserted into the pillow causing it to expand if there is no leak in the seam. Tensile Tests Pillow Tests Resistance Welder Manufacturers Association Bulletin # 21

Typical data from Pillow Testing
Here is typical data obtained from the pillow test. 6 x 6 inch Pillow 6 x 10 inch Pillow

A second type pressure test is applied to a seam placed in this fixture illustrated here with pressure applied to one side of the sheet and the seam in the middle. Douglas Process Standard DPS , McDonnell Douglas Corp, 1996

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