By Joe Hoffman,
Senior Welding Engineer,
FANUC Robotics America, Inc.

From the perspective of the robot, the welding process can be broken down into defined steps, beginning with striking an arc.

Starting an arc can be one of the most difficult steps in aluminum gas metal arc welding process. Mechanical properties of aluminum work against the welding process, and key factors, such as oxidization and thermal conductivity, and variables, such as soft or ductile filler wire, have to be taken into consideration.

Base metal oxidization is a natural enemy to the welding process and should be minimized. Oxides act as insulators and require greater arc energy to burn through. Because arc starting routines are predefined at set energy levels in robotic applications, there may not be enough energy to burn through a part that is contaminated by excessive oxidization. That can result in a failure at the start of the process, making it critically important to control that oxidization and to implement measures into the arc starting routine to overcome this natural occurrence.

Touch-Retract arc starting is one method that can be used to overcome the natural oxidization process and assist the starting of the arc.

Touch-Retract starting is a controlled process in which the robot sequences the weld power supply and the wire drive through a defined starting routine to draw the arc to ignition.

The process of drawing the arc eliminates the harsh, dead short, explosive start routines that are conventionally used. This method of drawing the arc provides the reliability required for robotic applications without impacting cycle time. It also dramatically improves contact tip life and the mean time between failures.

After the start of the arc, weld formation is the next step in robotic welding. The objective is to transition from the starting sequence to the formation of a weld, and common techniques include run-in, ramping or direct entry editing. Choosing the appropriate technique is directly related to the part, and material thickness, the need for multiple welds on a part, weld sequencing, and fixture design all play a role in the decision.

♦ Run-in typically is treated as a global condition in which the robot uses a defined weld condition to start all welds for a given process. This is a good tool to use when the base metal temperature is consistent and does not fluctuate during processing. If the base metal heats up due to weld processing, the run-in conditions used during the beginning phase of the process may not be appropriate throughout. Run-in can be set as the default condition for welding, and it can be disabled if an alternative starting condition is called for on a specific job.
♦ Ramping is a common technique that is used when welding thicker material. The theory behind ramping is to change gradually from the starting parameter to the welding parameter over a defined time. During the specified time, the weld output is ramped from the starting parameter to the welding parameter to provide the smooth transition into the welding mode. Ramping parameters are specified for each weld.
♦ Direct entry is a common technique used on thin materials in which the base metal temperature changes as welds are applied. The inconsistent temperatures of the base metal make it necessary to have specific control at each weld. This technique is different from ramping in that the change between parameters is immediate. Often on thinner material, the time between the start and weld is so short there is no advantage to use ramping.

Each of these techniques operates on the same principles. Touch-retract initiates an arc, the defined set of weld values are used to stabilize the arc, the weld values are changed and the weld is made. When welding aluminum, it is common to use higher weld values to start, stabilize and penetrate, then switch to a cooler parameter to complete the weld. Starting slightly hotter helps to initiate the arc and assists in overcoming the thermal efficiency of aluminum.

Weld deposition is the reward of successful starting and weld stabilization. The robot continues to play an important role and should not be overlooked.

The stability of the welding process is related directly to the ability of the robot to control the welding process. Programming techniques such as weaving may have to be applied to overcome part variations, and changes in the welding process may have to be made on the fly without interrupting the arc.

Advanced process techniques such as "Heat Wave" may have to be used to overcome large gaps or to weld varying metal thicknesses, or provide the desired cosmetic appearance, such as applying a gas tungsten arc welding (TIG weld) finish on the weld. The limitations of the robot should never affect the welding process. Understanding the common aluminum welding modes and how to apply them to robotic welding equipment will assist in achieving success. Here are several of the more standard welding modes:

♦ Pulse Welding is a common deposition mode used in robotic aluminum welding applications. The deposition of this mode is stable, the penetration is consistent and the cosmetic appearance is good. Because of the good stability of the arc, this mode is often used on fillet welds for which good travel speeds can be maintained.
♦ Variable Pulse Welding is a special deposition mode that is supported by only a handful of power supply manufactures. The deposition of this mode is stable, the penetration is slightly greater then conventional pulse and, due to the nature of the deposition, it tends to tolerate wider variations than conventional pulse. The cosmetic appearance is exceptional and, when properly tuned, resembles the "stacked dime" look that TIG welds are known for.
♦ Power Mode is special for aluminum. It provides a very clean, fast, spatter-free mode of deposition and is ideally suited for applications with good material fit-up that have little or no limitations on material thickness. When used on thicker material and combined with a circular weave, this mode can provide outstanding results. On thinner material, it can be cranked up, and let rip!
♦ Heat Wave is a proprietary weld process control special to Fanuc Robotics. The robot controls the welding deposition by changing the process parameters based on wire location. The sought after TIG appearance can be achieved, gaps can be bridged without problems and precise control of penetration simplifies the welding of dissimilar metal thicknesses.

Arc ending on aluminum requires special techniques to close the weld crater.

The weld crater is the void that remains at the end of all welds, and the current used in making the weld has a direct effect on the crater size.

Failure to fill this void leaves a stress point in the weld that will promote the formation of a defect called a crater crack that can propagate through the rest of the weld and cause weld failure.

There are a couple of welding techniques used to fill and close aluminum gas metal arc welding craters. The techniques operate with similar principles: The weld current is reduced and time is added to allow the weld puddle to close and the crater to fill. Personal preference and joint design play a role in determining the appropriate method, but the end result is most important – that the crater gets filled. Here are the standard techniques used to close and fill craters:

♦ Ramping to a cooler parameter. This technique provides a gradual transition from the "hot" welding parameter to the "cooler" crater parameter. However, ramping of the weld schedule typically will not fill the crater; some additional time – dwell – must be added to hold the weld process at the cooler setting until the crater is filled.

♦ Process switching between two modes of deposition to close and fill craters. This technique is used when welding with a "variable pulse" process. Variable pulse welding modes do not fill craters as consistently as conventional "pulse" welding modes. It is a good technique to switch from a variable pulse process to a straight pulse process to fill and close the crater. Straight pulse is predictable and can be programmed to achieve consistent crater results.

♦ Weld parameter change with an included dwell to close and fill craters. This technique is similar to ramping but does not have a gradual change between the weld and the crater parameter, and as with ramping, a dwell must be added to allow the cooler parameter to close and fill the crater. This technique is used when welding thinner material.

Burn back is the final step in making a weld, during this phase the filler wire is separated from the weld puddle and the arc is extinguished. A system with properly controlled burn back will terminate the wire crisply, leaving no ball on the end of the wire. Systems with poor burn back control end the wire erratically and often leave a large ball on the end of the wire or the wire is consumed into the contact tip. A clean ending prepares the wire for the next start.

The weld power supply dictates where the burn back control resides. Control can be locally within the weld power source or remotely from the robot. It is important to understand where burn back control resides to avoid communication conflicts.

If the control resides at the weld power source, the robot needs to be configured accordingly. The burn back feature on the robot should be disabled, and it may be necessary to add additional communications to synchronize the motion of the robot with the shut down routines of the power source. Failure to synchronize the motion of the robot with the power supply shut down routines often will result in poor ending conditions and failed arc starts.

Burnback and the influence it has on the weld wire is important and often misunderstood. Improper settings typically will result in a failed arc start for the next weld, and this adds confusion to the troubleshooting process. When trouble shooting a failed arc start, the ending conditions of the previous weld should be examined. Understanding where – and how – to adjust the burn back to get the desired weld ending results minimizes process problems.

Burn Back Rules The weld wire should be separated from the weld puddle. The weld wire should extend past the contact tip half the distance of the taught tool center point upon weld termination. Example: If the taught tool center point (TCP) is 12 mm, then the wire stick-out after burn back should be 6 mm or greater. The weld wire should not have a large ball formation on the end of the wire. The wire should have the appearance as if it was cut. Understanding the welding process and the capabilities of your equipment are the keys to success. If you have inadequate equipment, an upgrade to the latest technology may be necessary. If you do not thoroughly understand the welding process, or how to program the robot to give you the desired results, pursue training.