Downtimes due to wear parts during robot welding

In our first blog on how to reduce downtimes with robotic welding equipment, the focus was on robotic torches. We looked at the right choice of robot welding torch, torch power, torch geometry and highlighted that without “clean”' programming none of this will work.

1. Selection of the robot welding torch according to the application ✓
2. Selection of wear parts such as contact tip, gas nozzle, gas distributor and liner
3. Cable assembly and cable assembly guide
4. Robot torch mount and switch-off protection
5. Torch cleaning station

This blog article will focus on the important subject of wear parts. The contact tip, gas nozzle, gas diffusor and liner are right at the front of the process, exactly where the heat generation is greatest, weld spatter flies around and inferior, difficult to process wear part material provides plenty of potential for downtime. Regular maintenance and replacement of these wear parts will significantly increase service life. In this article, you will learn what you need to consider in general.

2. The right choice of wear parts

Service life – a topic that is extremely important in robot welding, because every minute that a welding robot cannot weld costs time and money. For this reason, no savings should be made on quality and thus on the costs of wearing parts such as the contact tip, gas nozzle, contact tip holder and liner. Every unplanned stop of the robot welding system costs more in relation to a high-quality wear part.

Contact tip

The contact tip – also commonly called the contact tube – has the task of optimally transferring the welding current to the continuously fed wire electrode. The feed speed of the wire is usually between 8-18 m/min. Depending on the process, it can also be higher.

If you want to ensure a long service life for your robot systems, you should also pay attention to the quality of the mass-produced contact tips – in terms of material and workmanship. Of course, there are also low-cost contact tips or contact tubes on the market, but on the one hand these have to be changed more often, which in turn interrupts the welding process. On the other hand, contact tips of poor material quality and workmanship are counterproductive for a good weld seam quality.

The main causes of contact tip wear are:

• Mechanical wear due to washing out of the bore – mainly influenced by the wire electrode material and the temperature of the contact tip
  → Consequence: The wire no longer runs centred and impairs the TCP

• Soiling of the bore – caused by wire abrasion and drawing agent residues
  → Consequence: The abrasion settles at the inlet of the contact tip or is conveyed through the contact tip and produces unstable arcs up to wire sticking.

• Electro-erosive wear – caused by micro arcs between wire and contact tip removing material inside the bore
  → Consequence: The contact tip is "washed out".

The requirements for the material of a contact tip or a contact tube are as follows:

• High wear resistance
• Good thermal conductivity
• High heat resistance
• Good electrical conductivity

Every material scientist thus immediately recognises: there is no material that combines all these properties. This means when selecting a contact tip, the welding wire, the welding parameters and the welding torch cable assembly used (air cooled or liquid cooled) must be taken into account. But the shielding gas used must also be included in the decision, because depending on its composition, it produces a more or less intense arc.

However, two materials are very close to fulfilling these requirements for a contact tip: electrolytic copper (E-Cu) and copper-chrome-zirconium (CuCrZr).

Material properties of E-Cu and CuCrZr

Compared to copper-chrome-zirconium, electrolytic copper is a better electrical conductor, but less wear-resistant. The zirconium content makes the contact tips or contact tubes more resistant to wear and thus the recommended material for robot welding. Contact tips made of electrolytic copper should be used for currents up to a maximum of 350 amperes. Although CuCrZr is the more expensive material, it guarantees a longer service life. The following shows the direct comparison:

Quality of workmanship

A high-quality contact tip can be recognised by its coherent overall geometry: a precise, centred bore with a smooth surface and well-made threaded connections. Inferior contact tips have a rough surface in the area of the bore. ABICOR BINZEL, for example, manufactures contact tips using a high-speed deep drilling process and guarantees consistently high quality in terms of bore tolerances and surface roughness in the bore.

Tip

→ When purchasing contact tips, be sure to look for quality.

Gas nozzles

As with manual welding, the gas nozzles for robotic welding are selected to guarantee optimum shielding gas coverage and allow good accessibility to the component. Since they are used directly at the welding process, they are permanently exposed to flying weld spatter. Excessive spatter adhesion is the main cause of gas nozzle wear and leads to gas coverage problems as well as bridging and subsequently a short circuit. 

Sometimes it is necessary to choose a larger so-called tip stick-out - the contact tip protrudes further from the gas nozzle, for example, in order to be able to reach closer to the joint to be welded. If, on the other hand, a gas nozzle is further away from the process, more gas must be supplied to ensure sufficient shielding gas coverage. In return, less weld spatter is deposited at the gas nozzle. Special narrow-gap gas nozzles, which are used in conjunction with narrow-gap contact tips, are a possible solution. Thanks to their geometry, even deep gaps can be reached and welding can be carried out from the root to the top layer.

In addition to the basic copper body, most gas nozzles for robotic welding have surface coatings to reduce spatter adhesion through reflection and a harder surface.

Welding spatter is a major problem for gas nozzles. Although spatter can be reduced with a well-adjusted welding process, it cannot be completely prevented in MIG/MAG welding.

Tips

→ In robot welding, cleaning intervals must be firmly programmed in order to remove adherent weld spatter and prevent spatter formation.

→ Equip your welding robots with screwed instead of pushed-on gas nozzles. These generally retain a firm, secure fit on the welding torch during the welding process as well as during the cleaning intervals at cleaning stations.

Spatter protection & gas diffusor

If not specified by the robot torch system, the choice between spatter protection and gas diffusor is a situation-based decision. A gas diffusor acts like a shower and thus enables very good shielding gas coverage. This wear part is mainly used when welding aluminum or stainless steel, where the gas coverage must be as laminar as possible. The spatter protection allows the cleaning mill optimal access deep into the welding torch head, often up to above the gas boreholes in the inner tube, and thus ensures consistently good gas coverage.

Tip

→Both the gas diffusor and the spatter protection should be made of high-temperature resistant material, as welding is very often carried out at high loads during robot welding. This keeps downtimes low and the service life of the robot welding torch system high.

Liners & wire cores

There are many different liners on the market and mistakes can already be made when choosing the right liner for welding robots. A liner always depends on the wire to be conveyed. Plastic liners are used for soft wires and steel liners are used for steel wires. The basic rule is: a liner must compensate for the frictional forces caused by the wire electrode.

For this purpose, the wire of the liner needs to:

• have a high initial strength
• be wound with a high preload
• have a clean and score-free surface – even after winding

Always select the liner that matches the wire

If, for example, a hard liner is selected for a very soft aluminum wire, metal powder is rubbed off and can collect and stick at a narrow point of the contact tip. The wire feed is blocked.

Follow the installation instructions of the liner

When mounting a liner, it is important to always put it slightly under pressure. This means: A liner should be approx. 3-5 mm longer than the cable assembly, so that it is positioned with pressure against the conical inlet of the contact tip. This prevents the liner from having slack inside the cable assembly or from moving. Especially with a soft wire, there is a danger that it can break out sideways and become entangled - the so-called "bird nesting" occurs. This can occur generally wherever the wire finds a space on its way to the process.

Replacing the liner on time

Even with a liner that is optimally suited for a wire, wire particles are rubbed off or abrasion is caused by the wire, which settles at the joints. Among other things, this can be recognised by the fact that the arc becomes unstable, the wire does not run smoothly through the cable assembly or can no longer be conveyed at all. Before the wire fuses with the contact tip - the so-called burn-back - or the process has to be interrupted to rework the component by hand, the liner should be replaced as preventive maintenance. Unplanned things that can be avoided are annoying and expensive - and should be prevented if possible.

Tips

→When installing liners, make sure that there is sufficient fixation in the form of a clamping nipple or retaining washer on the cable assembly connection.

→Liners must not be able to move freely back and forth in the cable assembly.

→All interfaces to the inside and outside must be absolutely free of burrs.

→ The liners must be inserted under pressure.

→ Always choose the liner that matches the wire.

→ Replace the liner according to a fixed rhythm that suits the process to increase the service life.

If you have any comments or questions about contact tips, gas nozzles & co. with regard to downtimes for welding robots, please feel free to leave them in the comments field.

In the next blog on this topic, we will take a closer look at the cable assembly, the cable assembly guide, the torch mount and the switch-off protection as well as the cleaning of the welding torch with a cleaning station.

See you and …

Happy welding!