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Projection-welding:simultaneous spots exactly located.SOLUTIONS with Effective, Powerful Advice
the optimal solution, in certain cases.
The choice of the preferred process depends on availability of But... how to To answer this question one has first to agree on the meaning of the word "best". The best welding process is that which satisfies:
The final selection, however may be determined, even at higher cost if necessary, by constraining circumstances.
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It helps to know the characteristics of as many processes as practical. That permits to appreciate the differences among different ways of welding. And also to evaluate advantages and limitations of available processes.
Projection-welding is an electrical resistance welding process producing welds by the localized heat generated by the flow of current. Specially designed projections, embossments or intersections, formed in at least one of the items, determine the points of contact. The parts to be welded together are pressed by force between simple electrodes, generally flat. Projection-welding produces solid state bonding or forge welding without fusion, with plastic deformation at the interface. On embossed sheet metal parts, melting results at the welds, similar to resistance spot welds. Advantages Electrode life is generally longer because larger contact areas are used. One remarks the fact that in Projection-welding the fixtures used are simpler and need less precision than for regular resistance welding. That is because the points of weld are localized in the parts themselves, and that currents and forces involved are much smaller than for conventional spot welding. Projection welds are smaller than spot welds. Uniformity of projections permits accurate and consistent location and the final products results more satisfactory. As projections concentrate heat where needed, this type of resistance welding results suitable to a larger variety of joints than normally considered. It is possible, with flat electrodes, to weld simultaneously many projections because they concentrate the current where needed. Closer weld spacing and less overlap is possible, as current is concentrated by projections. In common with other semi-automatic resistance welding processes the skill required from the workforce is minimal. Projection-welding is capable of accepting mating elements of widely different thickness. That is because heating is concentrated at the point of contact, irrespective of members' thickness. Therefore the possible thickness ratio (typical 6:1 and more) is quite larger than for regular spot welding (3:1 max.). Raised protrusions of proper dimensions can be formed on metal elements which would have been too thick for regular spot welding. Occasional surface presence of light rust is less critical, because current breaks through at the protrusions. Limitations The limitations point to the need for additional forming operations to prepare the projections. Furthermore the equipment needs some form of rapid response of the loading system, because the projection collapse brings a loss of force. To take care of the collapse, the electrodes have to close the gap rapidly to prevent overheating and weld defects. However production of projections may be more costly than the alternative unless the quantity of parts produced reduces the cost of welding per part. Close tolerances must be held to obtain acceptable quality, and weld size is limited by projection size Equipment Equipment consists in modified resistance welding machines provided with fast follow up head of low inertia capable of exerting a constant force. Applications Applications of Projection-welding include sheet metal construction with embossed projections, welding of annular items with machined projections like bushings and nuts onto flat or curved sheets or plates, and cross wire welding. Materials Mild and low alloy steel, nickel base alloys, copper and copper alloys are materials suitable for Projection-welding. Stainless steels and high alloy nickel base materials are difficult to weld by Projection-welding because of the presence of surface oxides that interfere. Aluminum alloys cannot be joined by Projection-welding because of their tenacious oxides and because these materials soften at low temperature. Also titanium alloys are excluded because, although their oxides could readily be dissolved, high resistivity and low forging temperature cause early collapse of projections before the weld is established. Projections The suggested exact form of projections, the shape of punch and die and the recommended starting schedules can be found in handbooks. See for example at page 232 Alternatives When the purpose of Projection-welding is to attach an annular body to a surface, other processes could be competitive. Friction welding may offer definite advantages over Projection-welding, especially regarding the materials weldable, if the production volume is adequate. See Friction Welding Processes. Otherwise Stud welding, an arc process, may be preferable in certain situations. See Stud Welding.
For orientation, specific Requests for Quotation (RFQ) for each one of the three processes considered (Projection-welding, Friction-welding, Stud-welding) could be issued to specialized manufacturing facilities in view of obtaining and rating accordingly comparable offers for outsourced operations. An Article on Projection welding of steel nuts was published (3) in Issue 86 of Practical Welding Letter for October 2010. 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 article on Adaptive Controls for Resistance Seam Welders was published (7) in Issue 139 of Practical Welldib=ng Letter for March 2015. An article on Resistance Projection Welding Revisited was published (3) in Issue 150 of Practical Welding Letter for February 2016. An article on Follow-up on VFAW was published (2) in Issue 151 of Practical Welding Letter for March 2016. An article on Resistance Welding Nuts to High-Strength Low Alloy (HSLA) Steel was published (4) in Issue 168 of Practical Welding Letter for August 2017.
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Projection-welding may be the most economic solution for certain applications if the conditions are just correct. See all advantages, limitations and alternatives...
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