Projection Welding Process and Method-Projection Welding Electrode Mould and Fixture

Projection Welding Process and Method-Projection Welding Electrode Mould and Fixture

Projection Welding Process and Method--Projection Welding Electrode Mould and Fixture

Projection welding is mainly used for welding low-carbon steel and low-alloy steel stamping parts. There are many types of projection welding. In addition to plate projection welding, there are also projection welding of nuts and screws, wire cross projection welding, tube projection welding and plate T projection welding.

The most suitable thickness for plate projection welding is 0.5-4mm. When welding thinner plates, the bump design requirements are strict, and a welder with excellent followability is required. Therefore, it is easier to spot welding for plates with a thickness of less than 0.25mm.

Compared with spot welding, projection welding also has the following advantages:

1) Multiple welding points can be welded at the same time in one welding cycle. Not only the productivity is high, but there is no diversion effect. Therefore, the solder joints can be arranged on narrow parts without being restricted by the dot pitch.

2) Since the current density is concentrated on the bumps, the current density is large, so a smaller current can be used for welding, and a smaller nugget can be formed reliably. In spot welding, it is very difficult to form a nugget smaller than a certain size corresponding to a certain plate thickness.

3) The position of the bump is accurate, the size is consistent, and the strength of each point is relatively uniform. Therefore, for a given strength, the size of the projection welding spot can be smaller than that of the spot welding.

4) As the large flat electrode is used and the bumps are set on one workpiece, the indentation on the exposed surface of the other workpiece can be minimized. At the same time, the large plane electrode has low current density and good heat dissipation. The wear of the electrode is much smaller than that of spot welding, which greatly reduces the maintenance and repair costs of the electrode.

5) Compared with spot welding, oil, rust, oxide scale, plating and other coatings on the surface of the workpiece have less influence on projection welding, but a clean surface can still obtain a more stable quality.

The disadvantage of projection welding is that it requires an additional process of punching projection welding; the electrode is more complicated; because multiple solder joints need to be welded at a time, a high-power welding machine with high electrode pressure and high mechanical precision is required.

Because of the above-mentioned advantages of projection welding, it has been widely used.

Projection welding electrodes, molds and fixtures

1. Electrode material

Projection welding electrodes are usually made of two types of electrode alloys, because this type of electrode alloy has the best overall performance in terms of electrical conductivity, strength, hardness and heat resistance. Class 3 electrode alloys can also meet the requirements.

2. Electrode design

There are three basic types of projection welding electrodes:

1) Round flat electrode for spot welding

2) Large flat-head rod electrode

3) An electrode with a set of partial contact surfaces, that is, the electrode is processed into a convex contact surface at the contact part, or a harder copper alloy insert is fixed to the contact part of the electrode by brazing or fastening.

Standard spot welding electrodes are used for single-point projection welding. In order to reduce the indentation on the surface of the workpiece, the diameter of the electrode contact surface should not be less than twice the diameter of the bump.

The large flat-head rod electrode is used for local multi-point projection welding. For example, projection welding of reinforced gaskets can weld 4-6 points at a time. The contact surface of this electrode must be large enough to exceed the boundary of all bumps, and the excess should generally be equivalent to the diameter of a bump. This kind of electrode can generally be installed on a high-power spot welding machine.

3. Welding molds and fixtures

The welding mold is used to hold and clamp the workpiece in place, and it is also used as an electrode. The clamp is a non-conductive auxiliary positioning device. For small workpieces, the electrode and the positioning fixture are usually integrated. The function of the horizontal spring clamp is to fix the bolt in the upper electrode. In order to prevent shunting, the fixing pin inserted into the electrode must be made of non-metallic material.

Other types of workpieces to be welded can also be fixed on the upper electrode with spring clips. When conditions permit, use vacuum suction to keep the workpiece in the upper electrode. Sometimes a moving device can be used to clamp small workpieces and send them to the part to be welded.

Large projection welding components require much more complex welding molds and fixtures to meet the needs of positioning, tightening and electrical conductivity.

Process characteristics and process parameters of projection welding

1. Process characteristics of projection welding

Projection welding is a deformation of spot welding, usually by punching a bump on one of the two plates, and then welding. Due to the current concentration, the defect of nugget deviation during spot welding is overcome, so the thickness ratio of the workpiece during projection welding can exceed 6:1.

During projection welding, the electrode must drop rapidly as the bump is pressed and fed, otherwise it will cause spatter due to the loss of voltage, so a projection welder with good electrode followability should be used.

In the case of multi-point projection welding, if the welding conditions are not appropriate, it will cause the bumps to shift and cause the joint strength to decrease. Experiments have proved that the displacement is caused by the electromagnetic force when the current passes.

In actual welding, due to the inconsistent height of the bumps and the poor parallelism of the upper and lower electrodes, one fixed point moves more than two points move at the same time.

In order to prevent the bumps from shifting, in addition to selecting a larger electrode pressure and a smaller welding current under the condition of ensuring the normal nugget, the followability of the pressurizing system should be improved as much as possible. The main method to improve the followability is to reduce the mass of the movable part of the pressurizing system, and to use rolling friction in the guiding part.

In multi-point projection welding, in order to overcome the pressure imbalance between the bumps, you can

Adopt additional preheat pulse or adopt the method of rotatable electrode head.
2. Process parameters of projection welding

The main process parameters of projection welding are: electrode pressure, welding time and welding current.

1. Electrode pressure

The electrode pressure of projection welding depends on the properties of the metal to be welded, the size of the bumps and the number of bumps welded at one time. The electrode pressure should be sufficient to fully feed the bumps when they reach the welding temperature and make the two workpieces fit tightly. Excessive electrode pressure will prematurely feed the bumps and lose the function of projection welding. At the same time, the joint strength will be reduced due to the decrease in current density. Too little pressure will cause serious splashing.

2. Welding time

For a given workpiece material and thickness, the welding time is determined by the welding current and bump stiffness. In projection welding of low carbon steel and low alloy steel, the welding time is of secondary importance compared with the electrode pressure and welding current. After determining the appropriate electrode pressure and welding current, adjust the welding time to obtain a satisfactory welding point. If you want to shorten the welding time, you must increase the welding current, but excessively increasing the welding current may cause metal overheating and spatter. Generally, the welding time of projection welding is longer than that of spot welding, and the current is smaller than that of spot welding.

3. Welding current

The current required for each spot of projection welding is smaller than that of the same spot of spot welding. But before the bumps are completely crushed, the current must be able to melt the bumps. The recommended current should be the maximum current that will not squeeze out too much metal under the appropriate electrode pressure. For a certain bump size, the amount of extruded metal increases as the current increases. The use of incremental amplitude modulation current can reduce the extruded metal. Like spot welding, the properties and thickness of the welded metal are still the main basis for selecting welding current.

In multi-point projection welding, the total welding current is approximately the current required for each bump multiplied by the number of bumps. However, considering the tolerance of the bumps and the shape of the workpiece. As well as the impedance of the welder's secondary circuit, some adjustments may be needed.

During projection welding, the heat balance between the two plates to be welded should also be achieved. Otherwise, the plate will melt before the welding temperature. Therefore, when welding the same metal, the bumps should be punched on the thicker workpiece to weld dissimilar metals. At the time, the bump should be punched on the workpiece with higher conductivity. But when it is difficult to punch bumps on a thick plate, bumps can also be punched on a thin plate.

The electrode material also affects the heat balance between the two workpieces. When welding thin plates with a thickness of less than 0.5mm, in order to reduce heat dissipation on one side of the plate, tungsten-copper sintered materials or tungsten are often used as the electrode inserts.

Projection welding joint and bump design

1. Projection welding joint design

The design of projection welding lap joints is similar to that of spot welding. Usually the amount of overlap of projection welded joints is smaller than that of spot welded joints. The spacing between the bumps is not strictly limited.

When the surface quality of a workpiece is required to be high, the bumps should be punched on the other workpiece. When the workpiece is projected to weld nuts, bolts and other fasteners, the number of bumps must be sufficient to withstand the design load.

2. Bump design

The function of the bump is to confine the current and pressure to a specific position of the workpiece, and its shape and size depend on the application and the strength of the solder joint. The solder joint sizes recommended by different materials are often very different. Generally, it is recommended to use the bump size specified in the table below. Compared with the thickness of the plate with bumps, small bumps are used when the plate is thin, and large bumps are used when the plate is thick. There are two types of bumps: spherical and conical. The latter can improve the rigidity of the bumps and prevent premature crushing when the electrode pressure is high; it can also reduce the splash caused by excessive current density. However, ball-shaped bumps are usually used. In order to prevent the extruded metal from remaining around the bumps and forming gaps between the plates, bumps with annular overflow grooves are sometimes used. In multi-point projection welding, the inconsistent height of the bumps will cause the imbalance of the current at each point and make the joint strength unstable. Therefore, the height error of the bump should not exceed ±0.12mm. If the preheat current is used, the error can be increased.

The bumps can also be elongated (approximately elliptical) to increase the size of the nugget and increase the strength of the solder joints. At this time, the bumps and the plate will be in line contact.

In projection welding, in addition to using the above-mentioned types of bumps to form joints, there are also many types of joints according to different types of projection welding workpieces.

The convex points and convex rings on the bolts and nuts used for projection welding are mostly formed at one time when the parts are forged.