Projection-welding:simultaneous spots exactly located.SOLUTIONS with Effective, Powerful AdviceProjection-welding can be To answer this question one has first to agree on the meaning of the word "best". It is generally recognized that, in this context, the best welding process is the one that satisfies all minimum quality requirements at the least cost. The final selection however may be determined, even at higher cost if necessary, by availability of lead time, materials, equipment, workforce and by the quantities to be supplied.
In order to appreciate the differences among different ways of welding and to evaluate advantages and limitations of available processes it would help to know the characteristics of as many processes as practical. See the following as an example. What is this? Projection-welding is a resistance welding process generating localized heat in two items to be welded together, pressed between flat electrodes, by the flow of current at their points of contact, possibly through specially designed projections. Advantages Among the quoted advantages of Projection-welding one remarks the fact that the fixtures used are simpler and need less precision than for regular resistance welding, 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. As projections concentrate heat where needed, this type of resistance welding results suitable to a larger variety of joints than normally considered. Another advantage is that it is possible, with flat electrodes, to weld simultaneously many projections because they concentrate the current where needed. In common with other semi-automatic resistance welding processes the skill required from the workforce is minimal. Also the thickness ratio can be made more favorable than for the case where projections are not used, because raised protrusions of proper dimensions can be made on metal elements which would have been too thick for regular spot welding. Projections can be located more accurately in place and the final products results more satisfactory. 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 with consequent overheating and weld defects, if not taken care of. 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 tolerance must be held to obtain acceptable quality, and weld size is limited by projection size
Equipment Equipment consists in a modified resistance welding machine 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. A characteristic of Projection-welding is that the joint itself is made by solid state bonding without fusion, also called strain assisted diffusion bonding, in contrast with classic resistance spot welding. An additional difference is that Projection-welding is capable of accepting mating elements of widely different thickness, because heating is concentrated at the point of contact, irrespective of members' thickness. 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 accordongly comparable offers for outsourced operations.
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