The continuous evolution of industry, especially of the automotive industry, has led to an interest in the continuous development of state-of-the-art materials, and in particular that of advanced high-strength steels. These steel types provide notable benefits, allowing the manufacture of lighter vehicles, with lower fuel consumption, lower emissions and greater autonomy, in the case of electric vehicles, while maintaining high safety standards. In order to reduce costs in the final product, the aim is to reduce the thickness while maintaining the formability of the material to facilitate stamping and die-cutting operations. These aspects are achieved through an adequate compromise between strength and ductility [1
The industrial use of AHSS, which include dual phase steels, complex phase steels, transformation-induced plasticity steels and martensitic steels, is conditioned by the adaptability of the processes and the conventional equipment to these new materials. From this point of view, it should be noted that the RSW process is one of the most widely used in the automotive world due to its low cost and high cycle speed. Each car can include about of around 4500 resistance welding spots, making it the main joining method in the automotive industry [6
The resistance welding process has been studied by many authors [10
]. It is a process used to join metals thanks to the heat generated by the resistance offered by the pieces to be joined to the flow of the electric current and the application of pressure through the electrodes or caps. The size and geometry of the electrode are important factors for the welding quality, and the manufacturers’ recommendations vary depending on the material [13
]. The heat generated is based on Joule’s law:
where, Q is the heat, I the welding current, t the welding time and R the resistance. The latter can be decomposed into seven partial resistances, which depend on various parameters such as the material, the pressure or cooling [6
]. The higher the resistance, the higher the temperature reached at that point and therefore the larger the size of the melting zone.
The combination of phases with very different thermomechanical behaviors gives rise to a high microstructural complexity of the CP, bainite, ferrite and martensite [15
], so the failure mechanisms of the RSW-welded joints of all these steels have not been studied in depth [16
]. In addition, due to their high carbon equivalent content, problems related to the formation of martensite in the weld bead can occur during welding of these steels, therefore the search for the weldability lobe is of clear interest. In order to have an efficient industrial welding process it is necessary to properly choose the welding parameters. The main parameters that influence the quality of the welding are: the welding current, I; the welding time, t; and the welding force, F, which is inversely proportional to the resistance [6
]. Other important parameters are: the diameter of the active face of the electrode; the material to be welded and its coating; the type of power supply; the design of the machine and many other factors that make this apparently simple process one of great complexity [10
]. The weldability lobe or weldability range defines the tolerances to produce quality welds, and facilitates the parameterization of welding equipment in the industry.
Recent publications [18
] have sought to optimize the welding parameters of some AHSS based on the metallographic study, the tensile strength and the type of failure, concluding that the key characteristics are controlled by phase transformations. In general the increase of welding current gives rise to a greater force to the shear, and there is a great variation in hardness along the cross section in the steels of high resistance attributed to rapid heating and cooling during welding that generate phase transformation. AHSS undergo complex microstructural transformations during the RSW process, the failure mode is related to hardness and microstructure in the fusion zone (FZ) and in the heat affected zone (HAZ). Regarding the AHSS complex phase, the predominant failure is the interface failure when there is an absence of Boron [21
]. The tendency to interfacial failure of AHSSs means that a large button diameter size does not always result in button failure. A study of the cycle of the welding process can improve the mechanical characteristics. Therefore, the need to define new quality criteria is established [8
The RSW equipment varies depending on the final application for which it has been designed, being of great importance its correct design [23
], structure and dimensioning to optimize the functionality, repeatability, quality and efficiency of the process. In RSW equipment, the power blocks have progressed in their technology, going from the most common single-phase power conversion stage at mains frequency to the power converter of medium frequency to direct current (MFDC) that work at frequencies in the kHz range. Different studies [24
], have demonstrated the advantages of MFDC technology over alternating current (AC). Among others, the weight reduction in transformers by using MFDC technology with a frequency of 1000 Hz or higher, allows this element to be placed in a robotized gripper, avoiding an extra cost in the acquisition of a robot of greater weight and volume. In this way, the robotization and automation of welding processes is facilitated, helping their incorporation into assembly lines [12
] and therefore achieving a reduction in production costs. The high currents required in the RSW process give rise to high temperatures, such that the greatest energy loss is in the transformer and in the interconnection and electrodes of the secondary circuit, which makes necessary an efficient cooling system. One way to increase power density while reducing cooling requirements is to reduce power loss in these elements.
Regarding the switching frequencies in MFDC equipment, in [29
] it is established that the switching efficiency improves as an inverse function of the frequency. Increasing the switching frequency allows the use of smaller and lighter transformers [12
]. The type of transformer and its design are fundamental factors in the efficiency of the welding process. The most significant losses are due to the rectifier located between the secondary of the transformer and the electrodes. An improved distribution of the windings results in a decrease in saturation and consumption [30
At present, industrial processes are highly automated and penetrated by robotic systems, in this sense the use of RSW equipment based on MFDC is justified. In addition, the frequency of work of the MFDCs has been gradually increased in recent years in order to have lighter, less bulky, more precise and faster welding equipment.
Despite studies conducted in the past in relation to these technologies, it has not carried out a detailed study on how it affects the switching frequency of the inverter to the quality of the welding of AHSS, the lobes of weldability, the parameterization of the welding process and to the electrical performance of the assembly.
This article presents a study of the influence of the inverter working at frequencies of 1 kHz and 7 kHz on the quality and efficiency of the RSW process in CP1000 AHSS. In order to verify the performance of the process, two quality metrics are established: the efficiency of the electronics conversion stage and the quality of the welded joint.
In Section 2
the materials, the equipment used and the methodology are described, in Section 3
the results are presented and the discussion of these is carried out and finally in Section 4
the conclusions of the article are presented.