S A Westgate, TWI Ltd
Paper published in Welding and Cutting, vol.55, no.5. 2003. pp.256-260.
Resistance welding processes have a century of history and have developed from very humble beginnings into one of the most sophisticated automated welding processes of the present day. Despite the invention of a number of more exotic processes in recent decades, such as laser welding, which has replaced some traditional resistance welding applications, the unique characteristics of resistance welding have kept the process in the forefront of manufacturing industry.
Probably the two key factors in joining technology are cost and quality. In manufacturing, maintaining the quality of the joint to a level that is fit for purpose is vital and increasing demand is being placed on reliability. The reason is clear in the case of a safety critical part. However, the cost implication of failure, either at a later stage of manufacture or in service, can be significant both in terms of the remedial action needed and also the loss of customer confidence. Although no welding process can be guaranteed 100%, pressure is put increasingly on the supplier to provide low parts-per-million risk of failure. In some cases, an isolated weld failure is unlikely to compromise the performance of the product, such as in vehicle body assembly. However, when making electrical interconnections, for example, a single failure is unacceptable.
Manufacturing cost is the other key factor in today's highly competitive marketplace. The initial capital cost and the running cost must be balanced over the life of a product. As one of the major contributors to running cost is labour, particularly in Western Europe, automation plays a very important role.
This paper provides a view of developments in spot welding in a number of areas which impact on these cost and quality factors. Topics covered include equipment and power supplies, control and monitoring, electrodes and tooling, plus material challenges and innovations.
The most significant developments in welding equipment in recent years have been related to power supplies and the force application systems. Medium frequency power supplies now dominate new automotive installations and feature widely in miniature applications. [1,2] They have seen gradual improvements in efficiency and control capability over a period of about 15 years. The advantage of reduced transformer weight with medium frequency power supplies, of around 1000Hz, accounts for their widespread use in robotic equipment. However, this has been further exploited by systems operating at even higher frequencies, such as 20kHz. This enables the transformers to be even more compact, with particular benefit on long reach guns where additional weight savings can allow reduced robot size to be used and also to reduce gun inertia. Transistorised and high frequency inverter dc supplies have also been widely applied in miniature applications and the feedback capabilities provide exceptional control of the weld pulse. An example is shown in Fig.1.
Fig.1. High frequency inverter dc power supply with miniature welding head Courtesy MacGregor Welding Systems
Servo controlled force application systems ( Fig.2) are gaining increased interest as they provide much greater flexibility in terms of programmable force levels. [3,4] In addition, the ability to control electrode position and approach speeds provides potential time savings in the welding sequence. The time required from initial electrode contact to reaching the set force is also claimed to be reduced compared to pneumatic systems. Reducing the operating stroke and electrode impact speed can also have positive benefits in terms of electrode wear. Pneumatic systems are frequently set with high approach speeds to minimise cycle time but the consequent high impact forces can increase the mechanical wear of electrodes.
Fig.2. Typical servo controlled spot welding gun Courtesy ARO
Integrating the servo gun control with the welding robot can enable the robot to be moving while the gun is opening or closing, thus minimising the manipulation time between spots. However, when welding long flanges, there is no need to open the gun if roll spot welding is used, and this can save further time. Robot mounted seam/roll spot welders ( Fig.3) are available capable of around 120 spots per minute.
Fig.3. Robot mountable resistance seam welding gun Courtesy Nimak
Automotive assembly line builders are also endeavouring to produce more effective tooling to locate components prior to welding. This has allowed greater accuracy and reproducibility of the dimensions of the welded structure. This has the added benefit of improving the fit up of components for welding, provided that this is coupled with good quality presswork. The integration of laser measuring devices at key stages of the build allows further feedback of potential tooling problems if deviations are found in these dimensions. These improvements should allow better spot weld location and consequently more reliable weld quality. However, this can only be achieved if the position and alignment of the electrodes relative to the component is achieved by initially programming the robot well.
Maintenance of spot welding guns is also an important aspect of reliable production. In some plants, the ideal situation exists whereby duplicate guns stand ready to replace any gun on the line and then the fault rectified or programmed maintenance can be carried out without interrupting production. As a means of re-testing a gun efficiently, it can be coupled to a dedicated testing station, equivalent to its robot connection. The test station can then check electrode force, water flow and electrical functions.  Additionally, the dimensional accuracy of the gun assembly can be checked with a manually operated high accuracy measuring arm. This ensures positional accuracy of the weld and correct clearances once the gun is reinstated. A means of storing welding gun specific data and welding programs within the gun itself is available,  in which off-line programmed data and maintenance records can be stored and downloaded to the welding controller ( Fig.4).
Fig.4. Welding gun data storage system MASDAT Courtesy Matuschek
Substantial refinements have been made in recent years in equipment for small scale welds for joining wires, foils, etc. Power supplies such as the higher frequency inverter type up to about 25kHz and transistorised supplies can provide fine control of short duration current pulses with feedback control to provide a precise current, voltage or power pulse shape. Such capabilities are a particular advantage when welding more difficult material combinations or through partially insulating surfaces. Welding head characteristics can also be quite critical for such cases and very low inertia, fast follow-up heads are often essential. These are available with spring, pneumatic or, more recently, electrical force application. In this latter case, variable force has been used within the welding pulse to provide active force control.
Control and monitoring
In order to permit continuous production of automated systems, much effort has been put into the computer control systems, so that faults may be immediately flagged up and rectified. This could include the sensing of failures such as tooling or gun problems, displaying the location of the fault, through the control computer, and paging the appropriate maintenance staff for immediate action. Other capabilities of the control systems allow remote interrogation of specific welding guns to check or adjust settings (with the appropriate password authority), in response to feedback from quality checks. The status of guns can be supervised as necessary, in terms of the number of welds made, relative to electrode maintenance intervals or current stepping sequences.
In many cases, current measurement is built into the welding equipment and used for the process control, linking to features such as constant current or stepping functions. A range of commercial meters is available to provide the means of verification of current values on a routine basis. Force is a factor less likely to be continuously monitored but again a range of tools is available. Newer force measuring devices provide force/time information enabling squeeze time to be optimised. In addition, guns have been produced which have built in force monitoring. 
Displacement monitoring has been shown to provide the most reliable indication of spot weld quality in the past, through weld expansion measurements. Weld quality monitors have been produced on this basis. However, this approach is less applicable to flexible equipment such as welding guns in production. Displacement measurement is particularly beneficial in some miniature applications where the measurement of set-down in projection welding applications, such as fine wire welding, can confirm process control and weld quality.
Electrical measurements of current and voltage provide the least risk of sensor damage and have been extensively studied. Simple measurements of dynamic resistance or power have some limitations in their ability to predict weld quality accurately under the wide range of production variables encountered in the process. However, newer systems and development studies are based on more complex algorithms or the application of fuzzy logic to the recorded data. [8,9]
While process control forms the basis of good weld quality control, the use of adaptive control has been implemented in a number of cases. In-weld adjustment of the welding current, and if necessary weld time has been shown to compensate successfully for variation in material, component and electrode variables, as well as the welding variables themselves.  However, there is little openly published data on production case history experience with such equipment.
The use of in-process weld quality control is relatively limited, although it is an ideal approach, provided that it is not expected to compensate for poor maintenance and process control. In the meantime, testing of spot welds in sheet steel is moving increasingly towards ultrasonic inspection. The PC based units now available ( Fig.5) often display some automatic evaluation of the ultrasonic signal to assist the operator. However, there is still a strong reliance on the skill of the operator to achieve a suitable signal and provide the final interpretation. Modern systems also provide a comprehensive data logging capability and the weld test data can be linked directly to the part drawings. 
Fig.5. PC based ultrasonic testing equipment Courtesy AGFA NDT
A new approach,  to incorporate the ultrasonic sensor into the welding equipment is also being tried for in-process weld quality monitoring. While this has been attempted previously with a sensor bonded to the inside of the electrode cap, the new approach has the sensor mounted within the electrode adaptor, or in the elbow of the electrode arm with the signal transmitted through the cooling water. The latter approach has a number of disadvantages but with the transducer in the adaptor, reasonable correlation to weld quality is claimed. Other techniques, such as global resonance testing  are considered to show promise and potential cost savings.
A number of systems have been devised and are being continually improved for modelling resistance weld formation. [13,14,15] ( Fig.6) This enables potential optimisation of weld designs and reduces the number of practical trials required to set up a particular application. The predictive modelling has been taken further to incorporate it into monitoring devices and provide weld quality monitoring or feedback control of the welding pulse as a means of avoiding weld splash.
Fig.6. Example of a spot weld model showing temperature distribution Courtesy Swantec
Electrodes and tooling
There has been long discussion on the merits of the available electrode materials and tip shapes, and there are different preferences between users. The choice depends on cost and overall electrode life under the conditions used. While the copper/zirconium and alumina dispersion strengthened copper types are frequently used for coated steels and to cope with hot welding conditions, electrode life can be controlled by the dressing procedures used.
Dressing can be conducted frequently, with little material removal, as a means of maintaining the tips in perfect condition, or by allowing substantial wear within the life of the tip, then dressing substantially back to the original shape. In the latter case, the procedure may be accompanied by a current stepping operation. As a means of minimising the time required for electrode changes, automatic tip changers are now available ( Fig.7). In addition to the dressing stations providing the cutting tool or tools to redress the electrode, additional features may be built in such as tip size and alignment checking and electrode force and current measuring devices. Such accessories offer the possibility of providing routine quality checks.
Fig.7. Electrode cap changing unit Courtesy Semtorq
Many applications incorporate designs that are not ideal in terms of spot welding assembly, because they limit access and require the use of non-preferred electrode configurations. This might include forward angled electrodes with small diameter adaptors that increase the flexibility of the electrode assembly under load and this leads to electrode skidding or misalignment.
Materials challenges and innovations
In the competition for lightweight materials, particularly for automotive applications, ultra-high strength (UHS) steels and aluminium alloys are receiving much attention. In general, steels with a tensile strength up to 600N/mm 2 are well established and do not cause problems in resistance spot welding. However, there has been limited spot welded application of steels over 600N/mm 2 and there is a still a need for weldability and performance data for these steels.
Plug failure is a standard weld quality criterion and allows the weld size to be verified after routine testing. This avoids the difficulty of interpreting the size and strength of the weld from an interface or part interface fracture. The occurrence of interface fractures can indicate excessive hardening and potential embrittlement of such welds. However, the fracture behaviour is dependent on sheet thickness, material type and weld size. Work at TWI on steels up to 1200MPa tensile strength, has shown that good weldability with plug failure can be achieved in some grades in thin sheet, despite relatively high hardness 400 to 450HV. Other types that harden excessively remain a problem, such as TRIP steels with a higher carbon content, which gives a weld hardness up to 600HV. Special welding procedures incorporating in-process heat treatment are then needed, which may not be compatible with high rate production. Long weld times and pulsed conditions, controlled cooling, in-process tempering, or weld dilution methods are possible means of modifying weld hardness and fracture behaviour in UHS steels.
In the highest strength steels, softening of the heat affected zone can occur and this can limit joint strength. In other cases, an interface fracture is not always associated with weld brittleness and high cross tension strength can be achieved. The potential range of material and thickness combinations likely to be found in automotive applications complicates the picture.
Aluminium alloys have more recently been self-piercing riveted instead of being spot welded in a number of major applications. However, spot welding provides a lower cost alternative, with matching properties if coupled with structural adhesives. The key factors in making reliable spot welds in aluminium are good control of the material surface and the electrode tip. Successful production applications are achieved where sufficient care is taken over these factors. A novel approach to tip dressing,  reinforces the benefit of such control. Frequent buffing of the tips was shown to prevent progressive tip contamination completely and reproducible weld quality was achieved over tip lives of several thousand welds.
The other material types gaining interest are the organic coated, pre-primed steels designed to eliminate the initial stages of the painting process, with consequent cost and environmental savings.  The very thin organic coatings around 1 to 2(m are readily weldable but some of the newer, much thicker coatings make spot welding more difficult. In addition, the underlying chromium free pre-treatments add to the electrical resistance of the coating. The coating suppliers are working to refine the coatings and conductive particle filling to make spot welding more reliable. This is again a case where good electrode dressing practice, programmed pulse shapes, including up slope, and advanced controller types may contribute to the successful welding of such materials.
There is still scope for novel approaches to resistance welding and recent developments such as conductive seam welding of aluminium alloys  ( Fig.8) and innovative resistance brazing methods  demonstrate this. In the former case, a butt seam weld in aluminium can be achieved by using consumable steel foils each side of the joint, which help generate the heat and contain the fused seam. The resistance brazing approach allows brazed spots to be made with similar static and higher fatigue strength than an equivalent spot weld.
Fig.8. Conductive heat resistance seam welding of 2mm thick 7075-T6 aluminium alloy sheet Fig.8a) Welded sample
Fig.8b) Transverse section
The computing power available in integrated systems allows an increasing amount of data to be handled regarding the welds made and enables monitoring and test data to be linked. Statistical handling of the information allows trends to be identified and action taken as appropriate. The refinement of modelling and monitoring algorithms offers the potential for increasingly reliable weld quality monitoring and adaptive control. In support of this, verification of quality by non-destructive testing is increasingly being implemented, but more reliable automatic interpretation is needed to reduce the demand on skilled operators.
The resistance welding processes provide relatively low cost, high productivity solutions to sheet metal joining and a range of other application areas. The tools are available to set up procedures and control the process to meet stringent customer demands and also to meet the European standards on weld quality and procedures, currently being finalised. It will be increasingly important to ensure that job knowledge and training are provided to enable the processes to be operated effectively.
The prospects for resistance welding processes seem secure in the foreseeable future but will be influenced to some extent by materials choices in the automotive industry. However, joining technology and materials development must go hand in hand, as cost effective processes are as important in manufacturing industry as the design and materials used.
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