Wire Welding

GB Innomech has considerable expertise in a variety of welding technologies, including resistance welding of wires and studs.

Wire Welding

Welded fine wires are used in products ranging from light bulbs to fuses, from semi-conductors to airbags. The quality of the weld is critically influenced by factors including physical conditions, weld parameters and design of weld electrodes. During the development of a wire welder, GB Innomech have addressed these aspects through use of servo controls and a sophisticated control system.

The GB Innomech high speed wire welder is able to weld a variety of wire diameters to a tight quality specification, with up to 10 verified welds per second on wires from a few microns in diameter to 10mm diameter. The welder setup across this wire diameter range can be completed by entering product codes on a screen without manual mechanical adjustment or compromise of the resulting welds

The Wire Welding Process

The basic resistance weld process joins two parts, or multiple layered parts such as laminations, by using a current to heat the materials at the joint so that they melt. The welding is then completed by holding the joint together as the materials melt and cool (weld forging). The parts to be welded are placed in the correct location and a pair of electrodes are placed on the part to form a current path. In many cases the return electrode is applied to the opposite side of the joint to the weld point and the welding electrode is brought into contact. However, if rear access is not possible e.g. a track on a PCB, the return electrode can be placed close to the weld area and from the same side as the welding electrode. In this case the part must be suitably supported to take the light weld forge loads.

Electrode Design

The substrate may vary in thickness from bulk metal such as a stainless steel machined component to a thin sheet such as a PCB track, and the position size and form of the return electrode must be chosen to minimise distortion or damage to the substrate. It is important to place the return electrode as near to the joint as possible to reduce the current path and hence the resistance to a minimum. Witness marks can occur at the return electrode location if local heating occurs because the resistance is too high. This can be minimised by ensuring that the return electrode makes good contact and that it is larger than the actual weld. Water cooling can also be considered to prevent overheating.

Welding Physical Conditions

Only rarely will the parts to be welded be in perfect contact at the weld point even when clamped nearby, so some force has to be applied by the weld electrode to bring the materials into contact at the weld location. This force can be high if the parts are rigid, which can cause a problem as the material melts during the weld. Most standard welders use springs to apply the force; these have to be manually set and do not monitor the weld motion. GB Innomech found that a servo motor can be controlled to produce a high clamp force and reduce the follow-through force during the weld as the materials soften and flow. If insufficient force is applied as the material melts the electrodes can lose contact with the surface and pull out an arc, damaging the components and creating a poor weld. By applying the correct follow-though force, the electrode will continue to clamp the parts and at the end of the current pulse forge the two parts together.

Of course if the follow-though force were too high, there is a risk of flattening the joint or pinching, which would create a local thinning and weakness in the wire. Only a servo based control system can provide a controlled force profile through the weld period. With a programmable servo, the force can be ramped down or stepped part way through the weld and also during the subsequent few milliseconds of cooling to forge the parts together. Servo feedback also gives the ability to control the clamping force during cooling, based on the ramp down of the weld current provided by a precision DC resistance weld power source. Controlling the cooling in this way reduces the risk of embrittlement of the finished weld,

A servo system is able to control both force and motion. This can be used to reduce cycle times, as the motion and force of the electrode during approach to the parts can be monitored. The weld electrode can rapidly approach the material using motion control, and then as the polished electrode face contacts the wire the control system can ramp up to a pre-specified clamping force, holding it in contact with the substrate. A further development is to ramp the force up (within boundaries) until no further motion is observed, indicating sound contact).

During a weld, the electrode can be programmed to follow a particular motion profile to prevent collapse of the weld or limit the travel, and monitoring the motion against limits to allow pass/fail evaluation of the weld.

Weld Quality Control

In order to avoid weld splatter and electrode damage, a brief current pulse is passed thought the joint for a few milliseconds prior to the actual weld. If the parts are correctly clamped the resistance will be low and hence the voltage required to achieve the current will be low. If this is within tolerance, the weld is commenced, applying controlled current profiles of up to 3000A. The current flows through the joint melting the wire and substrate at the interface. The cooled tungsten weld electrode and the return electrode absorb the heat and once a melt occurs the resistance at the joint drops, focussing the current though the melt pool.

Once the weld is complete, the weld electrode can be rapidly lifted clear by the servo drive, allowing very fast re-positioning of parts. The compact nature of the voice coil technology and flexure support allows multiple electrodes to be positioned close to each, other each individually controlled.

GB Innomech believe that the experience gained through working with tightly controlled weld processes for a variety of industries gives a great foundation to work on, and we would be happy to discuss your resistance welding requirements.