Resistance welding Principle of resistance welding (Joule heating + Pressure) The metals are melted by Joule heat (i.e., ). Pressure is imposed to confine the molten nugget between the interfaces. Therefore, the process parameters are the current, time and pressure. Resistance Spot Welding (RSW) Is most widely used. In the figure of RSW, the R at the interface is larger than that of the bulk metals. The resistance welding is the fusion welding or SSW? RSW: fusion ? Butt & Flash W: SSW ? Fig. 9.2 Schematics of resistance spot welding

Resistance welding processes Various processes have been developed: RSW, Seam welding, Projection welding, Butt welding, Flash welding, HFRW, HFIW, etc. Feature of each process is how to concentrate the resistance heat (or current) Fig. 9.1 Resistance welding processes

Determination of process parameters in RSW: Weldability Lobe Two issues of RSW: Welding condition and Electrode life (You cannot see the molten nugget) To determine welding parameters, the weldability lobe is used. Use a fixed pressure (step 1) Use a fixed weld time (step 2) Change current and conduct welding Measure the nugget diameter Change weld time and repeat step 2 Make weldability lobe (Weld current vs. time) Change pressure and repeat step 1. To measure the nugget size, a simple peel test is used in Fig. 9.5. The nugget diameter and behavior of tearing indicate the bond strength & quality of welds. Fig. 9.3 Construction of weldability lobe Fig. 9.5 Peel test coupon

Determination of electrode life in RSW: Oscillating weldability lobe Electrode life is important in weld quality and productivity. (Note: electrode itself is cheap) Prediction of electrode life is done experimentally using the oscillating weldability lobe. To determine oscillating weldability lobe in Fig. 9.4, Use fixed parameters (I, t and P) Conduct RSW Conduct the peel test Measure the nugget size Proper nugget diameter is The electrode should be replaced after the determined life. Electrode lives of coated steel & Al are very short… Al & Cu are difficult to weld. Why? Fig. 9.4 Electrode life test showing the relationship between nugget size and number of welds

Monitoring nugget size - Dynamic resistance It is difficult to monitor the nugget size in real-time because you cannot see the nugget. The resistance changes during the weld time depending on the contact area, resistivity and indentation in Fig. 9.6. Thus, nugget size can be measured indirectly by measuring the resistance in real time. The idea is very nice… But for coated metals, the resistance is affected by the coating in Fig. 9.7. Therefore, its application is limited! Fig. 9.6 Dynamic resistance curve of plain carbon steel Fig. 9.7 Comparison between dynamic resistance curves of plain carbon, galvanized and stainless steel

Monitoring nugget size – Electrode displacement Another way of monitoring the nugget size is the Electrode displacement. - When the metals are heated, they are expanded. - When the nugget becomes larger, indentation occurs due to softened metals. Therefore, the nugget size can be monitored indirectly by measuring the displacement of the electrodes. The principle is very nice, and it is not affected by the coating. The disadvantage is the proper sensor and its attachment. Fig. 9.8 Electrode displacement with upper and lower tolerance limits

Inclined Spin Electrode Process developed by Nippon Steel Co. Good and Simple idea to elongate the electrode life. Mathematically, electrode life is extended by 6 times. In practice, the life is longer by 20 times. due to self-dressing effects. Cons Since rotating the electrode is difficult, it is not used in practice. Note: You can propose such nice ideas if you are interested in and focus on the subject continuously!! Fig. 9.9 Inclined spin electrode system to increase electrode life

Resistance seam welding Electrode of the seam welding is a roller or wheel. Continuous welds are made by seam welding. Mash seam welding uses Joule heating and high pressure (forging) so that the overlapped seam has flat surface -> Solid state welding.. It is applied to TBW (Taylored Blank Welding). Fig. 9.10 Results of various seam welds Fig. 9.11 Mash seam weld

HFRW & HFIW Principle: Concentrate the resistance heat by HF current (Skin and Proximity effects) Forge the molten parts by suqeeze roll At final stage, welds are made with SSW! HFRW & HFIW are widely used to fabricate the pipe. For small diameter: HFIW or HFRW For medium diameter: HFRW For large diameter (>1m): Arc welding Fig. 9.12 High frequency resistance and high frequency induction welding

Parallel gap welding Parallel gap welding is used for micro-joining and involves accessing the weld area from the same direction with both electrodes. - In parallel gap welding, the gap between the electrodes is typically less than 1 mm with both electrodes contacting the same part. - The current flow through the weld materials is accomplished through the variation in material and interface resistances as the weld occurs. - When the weld starts, the majority of the current flows though the material in contact with the electrodes. Because current always follows the path of least resistance between the electrodes, the current flow will initially be through the upper material. - As this material heats, the material resistance increases. Fig.9.13 Parallel gap welding

Summary of Resistance Welding Pros Fast (within 1/6s or 10 cycles) and high productivity. No consumables are needed. Cons Difficult to monitor & control the nugget size. (The automotive industry is now replacing RSW to laser welding) High hardness due to quenching