Projection Welding

Projection Welding Learning Activities View Slides; Lesson Objectives Read Notes, Listen to lecture Do on-line workbook Lesson Objectives When you finish this lesson you will understand: The advantages and limitation of projection welding Projection design for various thickness materials Typical Applications of the process Keywords Projection Welding, Projection Design, Thin Material Projections, Thick Material Projections

Introduction to Projection Welding Resistance projection welding is a variation on resistance spot welding. Basically, a protrusion is placed on one of the two materials to be welded. This projection is then brought into contact against the second material. The welding sequence is similar to that for resistance spot welding. The welding electrodes are used to apply both force and current across the configuration. The point of contact acts to constrict current flow (and is a point of high resistance in the welding circuit), and heating occurs preferentially at this point. As the material heats it becomes soft, and the projection collapses under the force applied by the welding electrodes. Due to the amount of plastic flow involved, melting is not always necessary to form a sound joint. Projection welding is not limited to sheets. Any joint whose contact area is small compared to the thickness of the parts being welded is a candidate for projection welding. The sequence of events during the formation of a projection weld is shown in the above slide. In Figure (a), the projection is shown in contact with the mating sheet. In Figure (b), the current has started to heat the projection to welding temperature. The electrode force causes the heated projection to collapse rapidly and then fusion takes place as shown in Figure (c). The completed weld is shown in Figure (d). (a) (b) (c) (d) [Reference: Welding Handbook, Volume 2, p.566, AWS]

Examples of Various Projection Designs (b) The means of producing projections depends upon the material in which they are to be produced. Projections in sheet metal parts are generally made by embossing, as opposed to projections formed in solid metal pieces which are made by either machining or forging. In the case of stamped parts, projections are generally located on the edge of the stamping. The purpose of a projection is to localized the heat and pressure at a specific location on the joint. The projection design determines the currency density. Various types of projection designs are shown in the above and the following slides. (c) (d) (e) [Reference: Welding Handbook, Volume 2, p.562, AWS]

Examples of Various Projection Designs (CONT.) (f) (g) (h) (i) (j) [Reference: Welding Handbook, Volume 2, p.562, AWS]

Considerations for Various Materials Mild and HSLA Steels: Both are considered readily projection weldable. Both can adequately retain projection welding shape until adequate heating has occurred and are weldable using either embossed or solid projections. The HSLA steels may, depending on the particular composition, suffer an array of metallurgical problems. Galvanized Steels: Projection welding can offer some major advantages in resistance welding galvanized steel. The relatively low contact resistance is a major concern. The use of a projection can put contact resistance back into the welding circuit directly at the faying surface. This, in turn, results in lower welding currents and possibly better electrode-life characteristics as compared to resistance spot welding. Effective projection welding is, to a large degree, a function of the properties of the materials being welded. Important characteristics include the strength of the material (for maintaining projection shape), the strength of the material at temperature (to maintain projection shape on heating), and the material oxidation resistance. The metallurgical characteristics of the material being welded are also of concern. These considerations, however, are similar to other resistance welding processes. Listed in this and the following two slides are some characteristics for projection welding a range of materials.

Considerations for Various Materials (CONT.) Aluminum and Aluminum Alloys: They are considered not projection weldable. Most aluminum alloys are of too low a strength to allow the projection to survive under the necessary welding forces. The oxide formed appears to prevent the solid-state bond necessary to form the type of joint. High Alloy Steels: Projection welding is also quite readily applicable to the higher alloy steels. The major concern here is material hardenability. Adequate precautions must be taken to prevent the development of brittle microstructures. Not all metals can be welded by projection welding. The metal must be strong enough to support the projection. Brass does not lend itself to this welding method because the projections would collapse under force. Copper and the red brasses also are considered unweldable by this method. Aluminum has been welded to a limited extent, and the best results have been obtained from extruded parts. Coated stock, such as galvanized steel, terne plate, tin plate, etc., is being successfully welded. In some cases it is possible to weld dissimilar metals, such as steel to brass, or steel to bronze. These latter cases require more current and shorter "weld time" for successful welding than that required for steel. Due to the difficulty of maintaining small projections in this material, thin steel sheets are more readily spot welded than projection welded.

Considerations for Various Materials (CONT.) Copper Alloys: Projection welding has definite implied advantages for resistance welding copper and its alloys. Just as for the galvanized steels, the weld circuit resistance can be localized at the faying surface. Effective projection welding is largely a function of the specific copper alloy used. With respect to embossed projection welding, the suitability for welding appears to vary with the material strength level. Higher-strength copper alloys are relatively projection weldable. However, lower-strength alloys appear to have difficulty retaining projection shape under the applied welding force. Most copper alloys appear to be weldable with one or more forms of solid projection welding

Advantages of Projection Welding Ease of obtaining satisfactory heat balance for welding difficult combinations More uniform results in many applications Increased output per machine because several welds are being made simultaneously Longer electrode life During the welding operation, the major portions of the heat tends to develop in the part bearing the projections. Weldments composed of thickness ratios of 4 (or more) to 1 are sometimes difficult to spot weld. The flexibility of selecting the projection size and its location allow ratios of 6 (or more) to 1 to be readily projection welded. For this reason, the projections should be produced on the heavier of the two pieces of the same metal or, if possible, on the piece of higher conductivity if dissimilar metals are being joined. The reverse can be used under some conditions. Uniform projections may be readily obtained on a punch press. The effect of variables such as the contact area between the parts, and electrode mushrooming are reduced. The surface condition of the parts has less effect than in spot welding. Since multiple welds can be made simultaneously, the shunting effect can be avoided. The number of welds that can be simultaneously made is only limited by the ability to control force and current so that they are equally divided between the welds, and the capacity of the equipment. It is much easier to achieve the proper division of force and current with projections than with flat surfaces. In many cases the areas to be joined are flat except for the projections. The electrodes are flat and large enough to contact a large area in those cases. When the contact surface is irregular in shape, the electrode is fitted to the surface so that a high force may be applied without distorting the part. A large current may be introduced without damaging the surface. Electrodes and dies with large contact surfaces show little wear and, consequently, require less attention or maintenance.

Advantages of Projection Welding (CONT.) Welds may be placed more closely together Parts are more easily welded in an assembly fixture Finish, or surface appearance, is often improved Parts may be projection welded that could not be otherwise resistance welded If two spot welds are located too closely together, current from the second weld is shunted through the preceding weld. Since projection welds are made simultaneously, there is less trouble due to shunting. However, if they are placed too closely together irregularities in forming the projections may lead to poor distribution of the current. The arrangement or spacing of projections is limited by the ease of equalizing the distribution of force and current and the size of the machine. If, however, more than three projections are welded simultaneously, the height of the projections must be uniform within close limits to avoid having some of the projections fused before others have made contact. Multiple-impulse welding or the use of upslope may be helpful in this case. In a conventional spot welding machine, parts may be located by an assembly fixture and moved to make a second or third spot weld. When using projections the fixture may be mounted solidly on the machine. The parts are simply placed in a nest and, with one operation of the machine, all the welds are made at once. One part may be located in relation to the other by punching holes in one and matching them with semi-punchings from the other. When small parts, such as brackets or handles, are to be welded to large pieces, they are difficult to locate in a spot welding machine, which results in misplaced spots or extruded metal. In these cases neat embossing would be less unsightly and a fitted electrode would not mark the exposed surface. Thoughtful design with ingenious preparation of metal parts can make the use of resistance welding practical and can generate great savings in quantity production of parts for which it was previously considered unsuitable.

Limitations of Projection Welding Requires an additional operation to form projections Requires accurate control of projection height and precise alignment of the welding dies with multiple welds Requires thickness limitation for sheet metals Requires higher capacity equipment than spot welding The most important limitations of projection welding are shown above. The formation of projections may require an additional operation unless the parts are press-formed to design shape. With multiple welds, accurate control of projection height and precise alignment of the welding dies are necessary to equalize the electrode force and welding current. With sheet metal, the process is limited to the thickness in which projections with acceptable characteristics can be formed and the availability suitable welding equipment. Multiple welds must be made simultaneously, which requires higher capacity equipment than does spot welding. Multiple welds also limit the practical size of the component that contains the projections.

Requirements for A Projection in Sheet Material Rigid enough to support the initial weld force before current is applied. Sufficient mass to raise a spot or weld nugget in the plane surface to welding temperature. If it is too small it will collapse before the other surface is heated. Collapse without extruding between the parts. Surfaces should be in intimate contact after welding. Not be partially sheared. Such projections are weak, tear out easily and are of low shear strength. Easy to form, so that the punch and die require little maintenance. Cause minimal distortion of the part during forming. So far we have considered only projections in two flat sheets or plates with the application of welding force normal to the surface of the stock. There are many applications where the welding force is at an angle other than 90° from the stock. Projection welds may be made down to an angle of about 45°. In this case, elongated projections should be used. The minor diameter should be the same as that required for 90° welds, and the major diameter should be equal to the minor diameter times one plus the tangent of the angle at which the stock is inclined from the horizontal plane. Those requirements which a projection in sheet material should meet are listed above.

Basic Projection Design in Steel Sheet Punch Die Spherical Radius A D 45° T H 15° Projection Wall Thickness Should Be at Least 70% of Sheet Thickness D B The general design of a projection suitable for steel sheet is shown in the above slide. This design avoids the tendency for the forming operation to shear the sheet or to significantly thin the projection wall. The designs of the punch and die that form this projection shape are also illustrated in the above slide. Point Radius “R” Projection Should Blend into Stock Surface without Shouldering [Reference: Welding Handbook, Volume 2, p.563, AWS]

Bubble - Button Type Projections (a) (c) (b) (d) The simplest and most commonly used projection is the bubble or button type. In all four illustrations in the above slide ‘P’ is the punch diameter; ‘D’ is the die diameter, which is also the projection diameter; ‘H’ is the projection height; and ‘T’ is the stock thickness. Figure (a) shows a spherical radius on the punch. It should be noted in all cases that the metal extruded by the punch is equal to the metal extruded into the die. The radius of the punch has the same center as the radius of the extruded projection. In a balanced projection there is no change in stock thickness. Figure (b) shows a conical punch and a larger projection diameter. In this and the succeeding types, the stock is thinned out at the center of the projection. Figure (c) shows a spherical punch with a large projection diameter. Figure (d) shows a truncated punch with a large projection diameter. In all cases, there should be no sharp edges in the die, at ‘Y’ in Figure (a), as this introduces a notch effect which reduces the shear strength. There should be no thinning of stock thickness in area ‘X’ in figure (a), as this will reduce both the shear and tensile strength. In projection welding parts of unequal thicknesses, the projections are usually placed in the thicker piece. Projection dimensions should be proportioned to the thinner piece. If the thicker piece is not over 1.5 to 2 times greater than the thinner piece, the projections should be in the thinner piece. Projection welds of the type shown in this slide are satisfactory for stock thicknesses up to about 0.125-in. [continued] <T [Reference: Resistance Welding Manual, p.3-3, RWMA]

Projections for 0.500-in & 0.250-in Stock 120° 60° 90° 0.52” 45° 0.15” 0.45” 0.094” 0.50” 0.25” When the thickness is greater than 0.125-in, the projection will not be completely forged back. This results in a ridge around the projection area which, in turn, produces an undesirable sheet separation. Note ‘Y’ in Figure (a) in the preceding slide. To overcome this, projections in stock greater than 0.125-in thick are made with an annular cavity, as shown in the above slide. The annular cavity acts as a reservoir for excess metal squeezed out from the projection and allows the two pieces to be set down tightly. The above slide shows details of the type of projection recommended for 0.250-in and 0.500-in stock. The volume of the cavity below the stock surface should be about 70 percent of the stock projecting above the stock surface. When the thickness is greater than 0.125-in, the projection will not be completely forged back [Reference: Resistance Welding Manual, p.3-4, RWMA]

Embossed Annular Projection The methods of preparation which make projection welding possible are frequently specialized to suit particular conditions. Therefore, only general directions can be indicated. Annular or ring projections are frequently used for screw machine parts and for pressure-tight joints around a hole between two parts. Such preparation also gives high strength when a large stud or boss is to be applied to thin sheet material. In some applications, it is preferable to form a ring type projection in the sheet, as shown in the above slide. [Reference: Resistance Welding Manual, p.3-5, RWMA]

Punch & Die Dimensions for Spherical Dome Projections The projection sizes recommended for various sheet thicknesses and the punch and die dimensions to produce the projections are given in the above table. Projections may be elongated to increase nugget size and to strengthen the weld. In this case, the contact between the projection and the mating section is linear. Elongated projections are generally used for the thicker sheet gage. On thin sheet, an annular projection of small diameter may be used instead of a round projection. The annular projection has greater stiffness to resist collapse when the electrode is applied. [Reference: Welding Handbook, Volume 2, p.563, AWS]

Projection Types for Sheet and Solid Applications Spherical Projections Projection welding is applicable to a wide range of joint configurations. As such, the type of projection used varies widely and is usually adaptable to the particular application. A range of typical projection types is illustrated in this and the following slides. Sheet-to-sheet joining applications generally employ embossed types of projections. These are projections which are stamped into the surface of one of the two sheets to be joined. Embossed types of projections can be spherical, elongated, or annular, and match the requirements of the parts to be joined. Elongated Projections [Reference: Metals Handbook, Volume 6 (Welding, Brazing and Soldering), p.503-524, ASM]

Projection Types for Sheet and Solid Applications (CONT.) Annular Projections Annular Projection on Pin-and-Tenon Joint Solid projections are those machined onto the surface of a part. In practice, these are most commonly found as either annular or pyramidal types of projections. Annular projections are common for tube-to-plate applications. Solid projections, however, cover a range of nonconventional projection types which include interference fit projection, such as shown in Figure (j), and cross-wire projections, as shown in Figure (n). Though different in configuration, these nonconventional projection types perform as conventional solid projections. Pyramidal Projections Cross-Wire Weld [Reference: Metals Handbook, Volume 6 (Welding, Brazing and Soldering), p.503-524, ASM]

Projection Welded Front Axle and Radiator Support for Tractors The above slide shows a representative production assembly of heavy stampings assembled with the type of projection as discussed in the preceding slide. The four stampings forming the complete assembly range from 0.250-in to 0.500-in. [Reference: Resistance Welding Manual, p.3-4, RWMA]