Spot welding refers to several joining applications that have one feature in common. This feature is the joining of two or more workpieces at a single point by melting or mixing the metal locally. In laser spot welding [1], the incident laser beam penetrates and melts, successively or simultaneously, the thin metal workpieces rapidly and with high precision. The same goes for the gas tungsten arc spot welding [2], which, however, may employ a filler material and may not be as precise. The big advantage of these methods is that one-sided access is required for welding. Friction stir spot welding [3] is another application mainly used in research and development applications which, unlike the previous methods, does not require the involved materials to be melted. Similarly, ultrasonic spot welding [4] creates solid-state welds and is mainly used for joining thin metal components. Resistance spot welding (RSW) is by far the most used and researched spot welding method (Figure 1). Although it is considered conventional, several factors such as (i) the non-consumable electrodes, (ii) the absence of shielding gases and/or flux, (iii) its simple design, (iv) its simple operation, (v) the high welding speeds and (vi) the adaptable characteristics of the process make RSW suitable for automating welding in high-volume and/or high-rate production scenarios such as the “body in white” manufacturing stage in the automotive industry [5].

Controlling the RSW process in real time is not a trivial task, as the power control takes place in the primary coil of the transformer and the power delivery to the spot takes place in the secondary coil. This means that even if dynamic information of the joint is available during the process, such as the nugget evolution, fine adjustments on the secondary coil would induce huge fluctuations to the primary coil [6]. Two types of power sources are most commonly used in RSW: (i) the single-phase AC and (ii) the three-phase medium-frequency DC power. The main drawback of the first power source type is the low control frequency, while the main drawback of the second power source type is the negative phenomena that are induced in the welding transformer due to the frequent switching of the welding current direction [7]. According to that, even if dynamic information about nugget growth and formation is available, fine adjustments during the process are not guaranteed. Beyond that, external disturbances and errors related to the involved materials (surface roughness, contamination, poor fitting) and the welding machine’s conditions (electrode wear, misalignment) may also affect the process variability. This means that welds made with the same process parameters and with materials from the same batch may present different quality features [6].

To overcome these challenges, quality assurance programs (Figure 2) describe the steps that need to be implemented in order to ensure that a weld will perform according to its design or intended use. The implementation of these steps is called quality control and includes procedures and methods that can be utilized to execute a quality assurance plan for which the weld is compared to the design requirements (applicable code), specifications, standards, drawings, etc. While for different applications different quality assurance programs exist, the steps for executing quality control are the same. As an example, measuring the seam width, regardless of application, can be achieved using the same measuring equipment. The most common way of performing quality control is either by preventing a defect, correcting an out-of-spec joint feature (process control), or identifying the parts that are defective or not aligned with the design requirements. The industry norm is to keep the interference between inspection (if it is performed offline) and production to a minimum. Thus, for joined parts, samples are collected during production according to a specific standard, i.e., MIL-STD 105D, in order to ensure that the statistical significance of measurements is maintained with respect to production characteristics (e.g., batch size) [8].

Typically, depending on the application, legacy destructive inspection methods such as chemical, metallographic, and mechanical tests or offline non-destructive testing methods based on vision, radiography, and mechanical signals are used on samples or specimens for inspecting the joint’s features. These tests are carried out by certified personnel according to standardized procedures for defining whether a joint is defective, fulfills the design requirements, or is finally acceptable.

2. Quality Indicators

The main key performance indicators (KPIs) or otherwise the measurable values used to determine the quality of a joint without considering a specific application can be classified into three main categories. The 1st one concerns the joint’s fusion zone features, the 2nd one concerns the joint’s mechanical performance, and the 3rd one concerns the joint’s failure modes. Of course, depending on the application, dedicated KPIs may be used, e.g., the electrical resistance of the joint. The above are briefly described in the following subsections.

2.1. Joining Zone’s Geometrical Features

One of the most important aspects of the joint is the nugget diameter and the nugget thickness. These two have been linked to the mechanical strength ^[9][14] and fatigue ^[10][15] of the joint. In addition, some studies have indicated that the heat-affected zone (HAZ) is also another aspect that could be linked to the mechanical performance of the joint ^[11][16]. Finally, indentation features, caused at the electrode–workpiece interface, have been correlated to the strength of the joint ^[12][13][17,18].

2.2. Mechanical Performance

The tensile–shear load–displacement curves are the most widely used metric for evaluating the spot weld’s mechanical performance ^[14][20]. Similar metrics include the load–displacement curves resulting from cross-tension load tests ^[15][19]. Fatigue life is also another metric for evaluating the mechanical performance of the joint, and it is very important in the characterization of dissimilar joints ^[16][21]. Finally, the microhardness of the weld zone is another important metric, as it typically reveals hardness differences between the base metal, the heat-affected zone, and the nugget ^[17][18][22,23].

2.3. Failure Modes

Unlike the measurable aspects described previously, failure modes are qualitative metrics for classifying the mechanical properties of the joint. They describe the different modes upon which the weld zone can fracture under load. The most widely recognizable failure modes are interfacial and pullout. In the interfacial mode, failure occurs through the nugget, while in pullout mode, failure occurs as the complete (or partial) withdrawal of the nugget from one workpiece. In between these modes, several more have been identified in different studies, although without any consistency ^[19][20][24,25]. Finally, it has been stated that the load-carrying capacity and the energy absorption capability of those joints that fail under the interfacial mode are much less than those that fail under the pullout mode ^[21][26].

2.4. Application-Specific KPIs

While the majority of KPIs aim to provide a metric for the mechanical performance of a joint, this is not always the case. RSW is widely adopted for the assembly of batteries in e-mobility applications. This fact makes the measurement of the electrical resistance of the joints a requirement in order to assess their electrical performance ^[22][27]. However, standardization of these procedures is still under development.

3. Defects in RSW

In welding, defects are imperfections that can be caused by a bad process design but also by uncontrollable external factors. A defect does not always imply that a joint must be rejected. In the following paragraphs, the most common ones concerning the RSW process are described.

3.1. Expulsion

Expulsion can be described as the ejection of molten metal from the weldment area. It can be located at the electrode–workpiece interface or at the faying surface of the involved materials. Expulsion is a common problem during the BIW stage with a rate of occurrence up to 60%, requiring the car bodies to be cleaned and polished before reaching the paint shop due to stuck metal on the chassis part surfaces ^[23][28]. The root cause of expulsion is the result of the force applied by the liquid nugget onto the solid containment being equal to or greater than the force applied by the electrode ^[24][29]. The above condition can be true when excessive heat is induced at the weld spot (isovolumetric heating), which can happen due to the selection of non-optimal process parameters, such as low electrode force, high amplitude current, and short welding duration. Working close to the expulsion limit is very common to the industry in order to achieve a large nugget and achieve high production volume. In special cases of welding aluminum and copper alloys, the risk of expulsion is significantly higher compared to steel due to the abrupt change in the electrical conductivity when the materials change from solid to liquid ^[25][26][30,31]. While expulsion at the interface between the electrode and workpiece may only affect the electrode life and surface quality, expulsion at the workpiece’s interface can potentially affect the mechanical quality of the joint. The ejected material can cause liquid metal deficiencies in the fusion zone, which in turn can create discontinuities such as cavities during solidification. Severe expulsion at both interfaces can lead also to weld thinning, which means forming a significantly undersized nugget ^[23][28]. Expulsion can also cause damage or wear the equipment such as the welding machine, fixtures, sensors, and safety fences, especially in big automotive plants with a lot of expulsion incidences. This can decrease the service life of the equipment and increase the production costs due to prolonged downtimes and additional maintenance efforts.

3.2. Shrinkage Void

The way that the welded spot is cooled can favor the creation of voids. In more detail, the solidification of the nugget starts from the periphery and moves toward the center. This happens at different rates in different directions, which are parallel and perpendicular to the imaginary axis of the electrodes. More specifically, the cooling rate in the direction parallel to the electrode axis is more significant compared to the cooling in the direction parallel to the workpieces’ interfaces due to the high thermal conductivity of the copper electrodes and the fact that they are actively cooled. As a result, the growth of dendrites, in the direction of the electrodes, move faster, thus obstructing the inter-dendritic feeding during the final stages of solidification owing to dendrite coherency. This causes a shortage in liquid feeding to the nugget which along with the metal contraction is speculated to cause the so-called shrinkage voids typically near the center of the fusion zone (cavity) ^[27][28][32,33]. While the main cause of shrinkage voids is the immediate cut-off of the power delivery to the spot, their probability of occurrence can be increased if a number of conditions are true ^[29][34]. Thus, the tendency to form shrinkage voids is highly favored by the use of advanced high-strength steels along with inappropriate welding programs that do not aim to increase the electrode force but the welding current and time instead ^[28][33]. In addition, an increase in the sheet thickness, nugget size, and the number of sheets results in an increase in the cavity volume ^[27][32]. The void (cavity) size depends on the geometrical feature of the nugget such as its diameter and thickness. The tensile shear strength of the joint decreases drastically with an increase in cavity size as the local stresses are increased; however, if the cavity is located away from the pulling interface, it has minimal effect on the tensile shear strength ^[27][32].

3.3. Cracking

Typically, we can identify two types of hot cracking that are most commonly encountered in RSW. These are solidification cracking and cracking due to liquid metal embrittlement. Solidification cracking is due to the inhomogeneous deformation occurring during welding, which creates non-uniform stresses and strain fields. During the process, the resulting stresses at the fusion zone in their majority are compressive; however, due to the rapid cooling, tensile stresses evolve in the direction parallel to the faying surface. This along with the fact that the grain boundaries are perpendicular to this direction causes cracks along the thickness direction at these points ^[29][34]. Liquation cracking, as with the solidification cracking, is intergranular. It occurred in the partially melted zone, which is right outside the fusion zone. It is caused by the presence of low-melting eutectics, which causes continuous intergranular liquation. These liquid films have no strength to resist thermal stresses during solidification ^[30][36]. Liquid metal embrittlement is a phenomenon that can occur in specific solid/metal systems and usually under the action of tensile stresses. In RSW, the involvement of zinc-coated advanced high-strength steel induces the risk of surface cracking. These cracks most commonly appear at the periphery of the weld zone during cooling and are favored by short hold time and misaligned electrodes ^[31][37].

4. Practices and Norms in Manufacturing

In the industry, quality assurance for RSW is typically achieved using offline destructive inspection methods on samples and by utilizing empirical rules to select the process parameters depending on the involved materials, machinery, and design requirements. Destructive methods/techniques are carried out to qualify a welding process, a welder, or a welding operator in the context of quality control (QC), which is based on quality assurance (QA) programs and standards managed by organizations such for example the American Welding Society ^[32][39] and European Welding Federation ^[33][40]. In their majority, destructive inspection methods incorporate statistical process control by using control charts to detect and track abnormalities on product batches and to help identify non-conforming welding trends so appropriate attention is to be applied to process design and achieving stability ^[34][41]. Destructive examination is carried out offline either before the process, on the parts that are going to be welded, or after the process. Destructive inspection methods for RSW are applied to directly measure the mechanical strength, fatigue, hardness characteristics, and failure modes of the joint. The techniques used for doing that include tensile shear tests and cross-tension tests ^[15][19]. Beyond mechanical testing, metallographic tests are also conducted in the context of process optimization and analysis and not for tracking-quality purposes (e.g., identifying defects and determining the fusion zone’s geometrical features or grain boundaries) ^[35][43]. Destructive testing means that the joints or the corresponding assemblies that undergoing inspection have to be scrapped. However, this is not always an option for inspecting production batches, as it is time-consuming and requires special machinery that is not portable and easy to use. Initially, the research community and then the standardization institutions established a set of methodologies to correlate the fusion zone geometrical features (e.g., nugget diameter) and defects with the mechanical performance of the joint, creating also rejection and acceptance rules. Non-destructive methods and techniques for many applications are the norm to determine the geometrical features of the fusion zone and identify defects such as cavities pores and cracks. In RSW except for the visual inspection, which can be utilized for identifying surface defects (expulsion, indentation, cracks) and for measuring features such as the HAZ, ultrasonic testing is probably the most used one, especially in the automotive industry. Ultrasonic testing is carried out and interpreted using A-scan modules or advanced phased ultrasonic arrays ^[36][11] to depict the partial or complete morphology of the nugget. However, ultrasonic testing is used on a limited number of samples as it requires additional preparation and physical access to the part. Other methods such as radiographic testing ^[37][44] are also used for inspecting RSW joints, although these methods cannot be easily validated outside the research community, which utilizes them in the context of process model validation and the development of quality assessment approaches or process optimization approaches. On the other hand, ultrasonic testing has been integrated into robot systems for carrying out quality assessment tasks. These tasks are realized as an additional step in the production line ^[38][39][45,46]. Its utilization is limited to applications where access to the joint by the robot end-effector is possible and when short times between production steps are not required. In the same way, thermal vision monitoring has been also utilized for implementing active lock-in thermography ^[40][41][47,48]. The utilization of this system concerns battery welding applications only. As regards process control, the majority of the available welding systems are implementing two types of control strategies: namely, constant current control (CCC) or constant power control (CPC) ^[42][49]. In the first type of control, the current variations are controlled during welding. In the second type, both the current and voltage variations are controlled, or otherwise, the power that is delivered to the spot. The CCC is the most common one, and it is the simplest one to implement with the selected welding current being used as a setpoint for the closed-loop controller of the welding system. The quality of the joints is typically controlled by correlating these setpoint values, which are presented as process parameters, with the quality features of the joints depending on the welding design and the application’s standards and requirements. A common case that makes use of the above-mentioned strategies concerns the creation of joints with predetermined desired geometrical features, such as the nugget diameter ^[43][13]. In several studies, the nominal value of the welding current has been linked to the diameter of the nugget, which in turn has been linked to the mechanical strength of the joint ^[44][45][46][50,51,52]. Thus, by ensuring that the welding current is properly regulated to this value, the mechanical quality of the joint can be regulated within a predetermined range. To conclude, the practices and norms for quality assurance in resistance spot welding still use, in the majority of cases, destructive testing to determine the quality features of the joints. This is especially true when the mechanical performance of the joints needs to be assessed. Recently, in high-volume industrial applications, non-destructive methods such as ultrasonic testing have been established for measuring the geometrical features of joints on samples. Finally, quality assurance through process control is achieved by regulating a dynamic value, such as the welding current and in turn linking this value to a quality feature.