While only a small part of a larger system, contact tips play a critical part in achieving high quality welding performance. The primary function of this consumable is to transfer the welding current to the welding wire as it passes through the center bore, effectively creating the arc. To achieve the most consistent results, the wire needs to feed through the contact tip with the least amount of resistance possible, while still maintaining electrical contact.
|Contact tip failure not only hinders productivity by creating downtime for change over, but it can also negatively affect weld quality and create rework. Understanding the typical types of contact tip failures and their causes is the best approach to preventing them. |
Because of the constant friction from the wire and the exposure to the heat of the arc (and, in some cases, the reflective heat from the base material), contact tips take a tremendous amount of abuse during the course of welding. This abuse can easily turn into contact tip failure without the proper precautions.
Not surprisingly, contact tip failure not only hinders productivity by creating downtime for change over, but it can also negatively affect weld quality and create rework. Adding to that, the cost of replacing contact tips on a frequent basis can add up over time.
Understanding the typical types of contact tip failures and their causes is the best approach to preventing them.
Types of contact tip failure
There are two main types of contact tip failure.
1. Failure that leads to a burnback and its associated problems
2. Failure that produces contact tip wear
Burnbacks occur when a weld forms within the contact tip and can occur at any point along the weld. They are not necessarily the result of poor contact tip performance, but rather burnbacks can result from too slow of wire feed speeds and/or incorrect contact-tip-to-work distance (also referred to as CTWD). The CTWD is the distance between the end of the contact tip and the base material; if the distance is too short (i.e. the contact tip is too close to the workpiece), a burnback can occur. The quality of the wire, incorrect parameter settings and micro-spatter buildup, as well as incorrect wire feeder and liner adjustments can all contribute to burnbacks. When they occur, burnbacks reveal themselves by way of poor arc starts, arc instability, inconsistent wire feeding and, ultimately, stoppages in wire feeding altogether.
Contact tip wear can be both mechanical and electrical. It occurs from the friction of the wire feeding through the bore of the contact tip and is especially prevalent in higher amperage semi-automatic and robotic applications. In the latter, contact tip wear can produce issues with tool center point (TCP), resulting in offset welds and potentially rework, especially in robotic welding systems that do not employ seam tracking. The design of the contact tip and the material it is composed of are two factors that affect a contact tip’s tendency toward wear. Typically, manufacturers use copper for contact tips because it is readily available and offers good electrical and thermal conductivity. Copper, however, has a relatively low resistance to wear, making it more prone to failures. For higher amperage applications, companies often turn to chrome zirconium contact tips due to their strength and their ability to resist wear by heat.
Rectifying contact tip failure
All contact tips, regardless of the material used to manufacture them, will eventually fail if used or abused for a long enough periods of time and/or at a high enough temperature. They are, after all, consumables with a finite lifespan. The goal, nonetheless, is to prolong the life of the consumables in order to avoid unnecessary downtime, as well as cost for additional inventory. A good step in achieving those goals is to understand the ways to help prevent contact tip failure.
Burnbacks: There is no one solution to minimize contact tip failure due to burnbacks; each situation is unique and may require a series of corrective actions. The goal is to address the associated errors or issues that are leading to the burnback in the first place.
|Burnbacks, as shown here, occur when a weld forms within the contact tip and can occur at any point along the weld.|
The two key solutions for minimizing burnbacks include increasing the wire feed speed and/or lengthening the distance of the MIG gun from the workpiece. The nozzle should be no further than one-half inch from the base metal.
Matching a welding wire with the appropriate cast for the contact tip bore tolerance can also reduce the risk for burnbacks, as it helps improve electrical contact and reduce CTWD variability. The wire’s cast is affected by three main factors: the supply reel (spool or drum); drive roll tension; and MIG gun neck angle. A tight wire cast may allow for a looser bore tolerances and still be able to make the appropriate electrical contact with the contact tip to create a stable arc. A straighter cast may require a contact tip with a tighter bore to exert pressure on the wire and create consistent conductivity. It is important to note that with a smaller contact tip bore, there is a risk of the spatter build up, so cleanliness is key.
Selecting contact tips with a smooth surface and bore can also help prevent the wire from snagging on the consumable and causing a burnback. Using a contact tip/gas diffuser design that maximizes the surface area between these consumables is another option to reduce the potential for this problem — the tight connection creates less heat and can reduce micro-spatter that could hinder the wire from feeding and becoming blocked in the contact tip bore.
Additional preventive measures include:
• Adjusting the drive rolls to ensure smooth wire feeding
• Properly selecting and installing the MIG gun liner
• Using shorter power cables when possible
• Eliminating loops or kinks in the power cable
• Using dust covers to protect the wire from contaminants that could clog the contact tip
Contact tip wear: The degree of wear on a contact tip depends on multiple factors, including operating temperatures; the wire cast; and the surface condition, material properties and bore tolerances of the contact tip.
Lowering operating temperatures, when feasible, is among the best defenses against contact tip wear. These lower temperatures can be achieved in a number of ways, for example, using a water-cooled MIG gun. These types of guns are especially well suited for higher amperage applications (usually between 300- and 600-amps). They do, however, introduce some additional complexities to the welding operation that companies need to consider.
|These images show the characteristics of contact tip wear. The image on the left (A) shows the bore conditions of a new contact tip. The image on the right (B) shows a contact tip that has experienced wear after many hours of welding.|
Namely, water-cooled guns have a weaker neck than air-cooled models, so in robotic applications specifically, they can be more prone to bending in the event of a crash. They also tend to be more expensive to maintain. When deciding whether to use a water-cooled MIG gun to help combat the excessive heat that could lead to contact tip
wear, users will have to weigh out the advantages and disadvantages of this equipment in terms of costs and productivity to determine if the product is the best choice.
An alternative to reduce contact tip wear via lower temperatures would be to use a thermally-effective air-cooled torch in combination with front end consumables designed to dissipate heat. Typically, high quality consumables have been designed to seat firmly together to minimize electrical resistance, thereby generating less heat and reducing the opportunity for contact tip wear and failure. Remember that cheaper isn’t always better. When it comes to purchasing consumables, it may be worth the extra cost upfront for such a design in order to minimize long-term costs and downtime associated with contact tip changeover.
The value of extending contact tip life
In any welding operation, there is no single solution to instill efficiencies — it can be a matter of technique, equipment and more. However, minimizing contact tip failure is an important way to reduce downtime and costs, while also ensuring higher weld quality. Be sure to train new welding operators as to the value of taking preventive measures to combat burnbacks and contact tip wear, emphasizing the impact of these occurrences on the overall welding operation. As with any process, education can go a long way in helping companies create a more productive and profitable business.
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