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High-FrequencyResistance Welding at high Speed and Efficiency.SOLUTIONS with Effective, Practical Advice
High-Frequency Resistance Welding (HFRW) describes a group of processes that use high frequency electric current to concentrate the welding heat at the needed location. A short note on this subject was published (7) in Issue 20 of Practical Welding Letter for April 2005. Click on PWL#020 to read it. Typical frequencies may range from 80 to 800 kHz. Solid state inverter power sources for these applications may be found with output ranging from 50 to 1800 kW. High-Frequency current in metal conductors tends to flow at high densities along surfaces at a shallow depth (the higher the frequency, the shallower the depth) in what is known as skin effect. Due to the fact that only a small amount of metal is heated at the welding interface, the processes produce welds at high speeds at very high energy efficiency. The advantages of High-Frequency are the suitability for high speed welding of a large range of sizes and materials. Steels, Stainless, Aluminum, Copper, Brass and Titanium are successfully welded by High-Frequency processes. Weld quality is generally not sensitive to the presence of air around the weld, except possibly for reactive metals like titanium, in which case argon may be used as a shield. The limitations are that it cannot be used at slow speeds or as a manual process, and that any stop and restart will cause visual objectionable discontinuities, generally to be discarded, not repaired.
The predominant application of High-Frequency Resistance Welding is for continuous manufacturing of pipe and tubing. These are generally prepared for welding in a continuous roll forming strip mill where the flat strip is gradually shaped to a round form. In the weld area the open edges of the formed strip are brought together by pressure rolls, to form a vee with the apex at the weld point. Here a set of pressure forge rolls squeezes the edges, forge welding them together with expulsion of excessive metal and impurities. The main two versions differ in the way of application of High-Frequency welding current. In the first, called Induction Welding, the current is applied through a water cooled induction coil. The secondary current, induced in the strip, concentrates at the welding point, heating the metal to the correct temperature for forge welding. In the second version the current is applied by the direct contact of sliding electrodes on the opposite edges surface of the moving strip.
The comparison of both versions shows advantages and limitations High-Frequency contact welding is the more efficient of the two, because the distance between the point of weld and the sliding contacts is shorter and there are no induction losses. However its major limitation is contact wear, that requires maintenance and replacements. Induction Welding is suited for long production runs where coils of strip are butt welded end to end to avoid the need to stop and restart the line. Although it is less energy efficient than contact welding, its major advantage is the absence of contact wear with attendant surface blemishes or arcing that must be repaired. While the equipment lines may be capable of offering both types of current supply, in practice, for smaller tubes up to about 305 mm or At larger diameters contact welding is preferred for the greater efficiency. Shapes other than round tubes can also be produced with High-Frequency Resistance Welding if there is economic justification for setting up the equipment needed. See the following Research Paper on: A special Issue of Practical Welding Letter, the Mid June Bulletin No.74, full of Resources on Resistance Welding was published on June 14, 2012. An introduction to a new ASM Handbook Volume 4C on An Article on Induction Heating Assisted Underwater Wet Welding was published (11) in Issue 138 of Practical Welding Letter for February 2015. An Article on HTPro - October 2015 (including report of an article on building advanced induction coils) was published (11) in Issue 147 of Practical Welding Letter for November 2015. An Article on HTPro - December 2016 (including report of an article on Computer Modeling for Induction Hardening) was published (3) in Issue 161 of Practical Welding Letter for January 2017.
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