RWMA Q&A BY JERRY GOULD

Transcription

1 RWMA Q&A BY JERRY GOULD Q: Reducing energy consumption is a constant theme in our production facility. Our industrial engineering staff look at the instantaneous power demands of our resistance welding systems and see this as an area where improvements can be made. How is energy consumed during resistance welding, and how do system changes affect these energy requirements? A: Resistance welding has historically been considered a high-productivity, low-energy-input, low-cost joining technology. Resistance welding processes are typically characterized by very short processing times (from milliseconds to seconds), and to achieve necessary energy inputs typically do have high instantaneous power draws. The theoretical energy Fig. 1 Theoretical energy required to heat and melt spot weld nuggets of a nominal size for aluminium, titanium, and steel at various sheet thicknesses. required to form a resistance weld is quite low. Figure 1 shows the energy required to create spot welds for aluminum, titanium, and steel. This plot is based on a simple calculation of the energy to heat and melt spot weld nuggets of nominal size for the given sheet thickness and material. You can see that for any of these metals, energy ranges from hundreds to thousands of joules. These numbers, however, differ drastically from energy measurements taken from real spot welds. An example of actual energy consumption for a real spot weld in 0.8-mm-thick steel is presented in Fig. 2. Here it can be seen that energy can easily achieve several thousand joules (instead of several hundred) depending on the process conditions used. 26

2 Fig. 2 Measured energies to create minimum, nominal, and expulsion spot welds in 0.8 mm bare steel stackups. Fig. 3 Influence of on time and sheet thickness on the energy efficiency of resistance spot welding steel sheet metal. Heat balance considerations: A major factor affecting energy consumption during resistance spot welding is heat extraction to the electrodes themselves. Resistance welding is implicitly a balance between heat generation in the workpieces and extraction through the electrodes. Heat generation in the workpieces effectively facilitates local melting of the weld nugget and creation of an effective joint. Heat extraction into the electrodes centers the spot weld nugget in the workpiece stackup as well as provides thermal protection for the sheet surfaces and electrodes themselves. A dynamic analysis of this heat balance shows how energy consumption is affected by weld time. This relationship is shown graphically for two different steel sheet thicknesses in Fig. 3. For short-duration processing, the energy retained in the weld (and TECHNOLOGY FOR THE WELDER S WORLD. RAB Grip & FES-200 W3 Mobile, powerful, and easy to handle! The fume extraction unit FES-200 combined with fume extraction torches provides the best possible welding fume extraction directly at the source. W3 version is licensed for all CrNi steels! SEPTEMBER 2014 / WELDING JOURNAL 27

3 COR-MET COR-MET QWP Stainless Steel Flux Coated TIG Wire. Eliminate the need for gas purging and backing for TIG pipe welding. The QWP Flux Coated TIG rod forms a slag on the backside of the pipe protecting the weld from oxidation (sugaring). Grades Available 308H, 308L, 309L, 316L, 347, 2209, , 625. Available in 3/32 and 1/8 diameters. Call for special chemistry and diameter requests. Manufactured by COR-MET INC., Brighton, Michigan USA / sales@cor-met.com therefore used for creating weld nuggets) and subsequent energy efficiency is relatively high. This is because at these short times relatively little heat is lost to the electrodes (through the workpiece material). At longer durations, however, more heat is extracted to the electrodes, and energy efficency decreases. Finally, increasing weld times can be achieved where the rate of heat extraction (through the electrodes) matches heat generation. In this case, the process is effectively steady state, and energy is wasted with no benefit to the spot weld itself. This is seen as energy efficiencies continuously decrease with longer weld times. Clearly, there is balance between stable nugget growth, protection of the electrodes and the sheet surfaces, and minimum energy consumption. These are the conditions that define best practice resistance spot welding schedules. Energy consumption in AC spot welding systems: AC spot welding systems were dominant for sheet metal joining for decades. These systems provide single-phase, 60-Hz (or 50 Hz in some countries) electrical current to the welding machine secondary for heat generation. Energy delivery through AC current is affected by both resistive and inductive loads. Resistive loads for these systems are dominated by the workpiece itself. For spot welding, steady-state resistance loading is typically less than 50 μω. Inductive loading is affected by the size of the secondary loop. Increases in both the loop circumference and conductor size can influence secondary inductance. A plot showing the relationship between secondary inductance and loop perimeter is shown in Fig. 4. It can be seen that for typical secondary loop circumferences (2 up to about 10 m) the implied inductance varies from about 2 up to about 10 μh. Increases in loop size can have a dramatic effect on energy efficiency. Essentially, that portion of the secondary load represented by inductance is lost energy, required to provide current at the workpiece. These inductive and reactive loads can then be used to calculate the influence of the welding machine s secondary geometry on energy efficiency. This relationship is also shown in Fig. 4. Here it is clear 28

4 that increasing loop sizes can have a dramatic influence on energy consumption for the process. Energy consumption in secondary rectified DC systems: Secondary rectified DC systems are those that utilize diodes at the transformer output. These diodes are configured to provide direct current to the welding machine secondary. Common power supplies using secondary rectification include medium-frequency direct current (MFDC), single-phase direct current (1øDC), and three-phase direct current (3øDC) systems. Direct current systems have the advantage of being considerably less sensitive to the secondary loop configuration. At steady-state current flow, there is no implicit load associated with secondary circuit inductance. There are, however, some losses occurring during current rise at the initiation of the weld cycle. The resulting relationship between secondary loop size and energy-efficiency of DC systems is shown on the right-hand side of Fig. 5. For the application shown, it can be seen that an increase of an order of magnitude in secondary loop perimeter results in only a 20% loss in energy efficiency. A larger influence for these systems is that of the impedance drop across the diodes. These diodes operate at full secondary current, with impedances ranging from 300 to 1200 μω. When considering these impedances to a typical workpiece load of roughly 50 μω, the effect is evident. The effect of increasing such system impedance (with a constant 50-μΩ workpiece load) is also shown in Fig. 5. Here the influence of the diode impedances is evident, dramatically reducing system efficiency. Balancing overall efficiency with system functionality: It is clear from examining the limited data presented here that system configuration and design can have a dramatic influence on the energy efficiency for resistance welding systems. It would be tempting to take these observations at face value. However, to some degree the causes for these losses in energy efficiency have their roots in other aspects of system functionality. For example, changing the electrodes to minimize heat extraction would SEPTEMBER 2014 / WELDING JOURNAL 29

5 / Perfect Welding / Solar Energy / Perfect Charging TIG VERSION AVAILABLE SEPTEMBER 2014! / Lugging around long mains leads or a bulky generator to do repair welding isn t always practical. We know that. That s why Fig. 4 Relationship between system inductance, power efficiency, and secondary loop size for AC resistance spot welding. Fig. 5 Influence of system resistance and secondary loop size on the efficiency of secondary rectified DC welding systems. Calculations are done assuming a constant 50 μω workpiece resistance. certainly improve energy efficiency, but would be catastrophic to electrode wear and workpiece surface quality. Limiting loop sizes for AC systems is always good practice, but can reduce system accessability and subsequent utility of the process. Finally, DC power has a range of benefits for specific applications. Most notably, MFDC systems are seen as enablers for lightweight portable gun configurations, greatly contributing to the overall cost competitiveness of the technology. In short, resistance welding system design must be seen as a balance between these competitive engineering benefits, where energy efficiency is only one component. WJ JERRY GOULD is technology leader, Resistance & Solid State Welding, EWI, Columbus, Ohio. Send your comments and questions to Jerry Gould c/o Welding Journal, 8669 NW 36 St., # 130, Miami, FL 33166, or via e mail at jgould@ewi.org. 30