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Fundamentals and Principles of Resistance Welding
Figure 1: Resistance Spot Welding
Resistance welding processes represent a family of industrial welding processes that produce the heat required for welding through what is known as joule (J = I Rt) heating. Much in the way a piece of wire will heat up when current is passed through it, a resistance weld is based on the heating that occurs due to the resistance of current passing through the parts being welded. Since steel is not a very good conductor of electricity, it is easily heated by the flow of current and is an ideal metal for resistance welding processes. There are many resistance welding processes, but the most common is Resistance Spot Welding (RSW) (Figure 1). All resistance welding processes use three primary process variables – current, time, and pressure (or force). The automotive industry makes extensive use of resistance welding, but it is also used in a variety of other industry sectors including aerospace, medical, light manufacturing, tubing, appliances, and electrical.
In addition to RSW, three other common resistance welding processes are Resistance Seam (RSEW), Projection (RPW), and Flash Welding (RFW) (Figure 2). The RSEW process uses two rolling electrodes to produce a continuous-welded seam between two sheets. It is often the process of choice for welding leak tight seams needed for automotive fuel tanks. RPW relies on geometrical features machined or formed on the part known as projections to create the required weld current density. RFW is very different from the other processes in that it relies on a rapid succession of high-current-density short current pulses which create what is known as flashing. During flashing, molten metal is violently expelled as the parts are moved together. The flashing action heats the surrounding material which allows a weld to be created when the parts are later brought together with significant pressure. Other important resistance welding processes which are not shown include High-Frequency Resistance Welding (HFRW) (used for producing the seams in welded pipe), and Resistance Upset Welding (RUW).
Figure 2: Common resistance welding processes.
In summary, most resistance welding processes offer the following advantages and limitations:
With the rise of electric vehicles, evaluating the environmental impact of each manufacturing process is essential. This article presents an EV battery enclosure welding LCA to compare the sustainability of different joining methods. As automotive manufacturers strive to reduce their carbon footprints, understanding the impact of production processes becomes crucial—especially the joining techniques used in car body assembly. Using Life Cycle Assessment, these impacts can be analyzed in terms of their reduction potential. This article focuses on the example of an all-steel EV battery case joined using several different welding methods.
Life Cycle Assessment is a systematic method for evaluating the environmental impacts of a product or process throughout its life cycle—from raw material extraction to production, use, and disposal. According to DIN EN ISO 14040 standards, LCA is structured into four main components:
Figure 1: Considered system boundaries of the LCA comprising supply of materials and energy, the joining process and disposal of consumables.
The assessed joining processes include:
In the context of joining processes for EV battery cases, this LCA aims to compare the environmental impacts of different welding methods on a meter of weld length. The assessment uses both primary and secondary data from literature and established LCA databases. As not all data is available in-detail for welding processes, several simplifications and assumptions are required:
For example, filler wires aresimplified to pure steel or copper wire rather than taking their complex chemical composition into account. Therefore, it is likely that the actual impact of the filler material production is higher than assumed here. For electricity emissions, the German grid mix is used and to compare resistance spot welding and laser beam welding per weld length, 20 resistance spot welds per meter are assumed.
The emissions from the structural materials are not considered, as the same design is used for all welding processes. Second-order effects due to the switching a joining technology, i.e. the material savings due to reduced flange-widths for laser welding, are also not considered. The second-order material saving effects are known to be larger in comparison to the environmental impact of the joining processes and should be optimized together with the choice of joining process. Further information on these effects can be found in this study.
Figure 2 All-steel EV battery case with zones marked for laser beam and resistance spot welding
The results of the LCA are shown in Figure 3 for the two impact categories Global Warming Potential (i.e. CO2 equivalent emissions) and Acidification Potential (i.e. emission of SO2 equivalent). Main driving factors of emissions are electrical energy, compressed air as well as filler material in the form of steel or copper wire (for laser wire and laser brazing respectively) and adhesive for RSW bonding.
Using the German electrical grid mix, RSW bonding shows the lowest GWP impact. As it does not use any filler material, laser remote welding has the largest potential for CO2 emission reduction, if the electricity generation incorporates more renewable energy .
When analyzing the laser processes, a high idle energy consumption of the laser systems is determined. This is due to the electricity demand of the laser source, cooling and control systems. The consumption only rises slightly, when the lasers are operating. This leads to the conclusion that the overall energy efficiency of laser welding systems can be significantly improved by optimizing “beam-on times” in relation to “idle times”.
In terms of the acidification potential, laser brazing stands out with a far larger impact compared to the other processes, because of the emissions associated with mining and extraction of its copper-based filler material. The wear of copper electrode caps also contributes to this impact category.
Figure 3: Environmental impact of the different welding processes in Global Warming Potential (left) and Acidification (right) per meter of weld length.
Life Cycle Assessment provides invaluable insights into the environmental impacts of joining processes in the automotive industry. By understanding the implications of material choices and energy consumption, manufacturers can make informed decisions that promote sustainability. Both the effect of electricity consumption and filler materials on the environmental impact of automotive joining processes is discussed in this article. Joining processes are one of the major drivers of an OEM’s emissions with ample potential for optimization through LCA analysis.
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Thanks go to Dr.-Ing Max Biegler, Group Lead, Joining & Coating Technology Fraunhofer Institute for Production Systems and Design Technology IPK |
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