Monday, April 16, 2007
 
The Challenges of Robotic Aluminum MIG Welding
 
Understanding the welding process and the capabilities of your equipment are the keys to success

Robotic Aluminum MIG Welding presents many challenges. The aluminum MIG process is not as forgiving as steel and requires unique control to achieve successful results.  A good understanding of the welding process and how to control it with a robot is critical.  From a robotic perspective, the welding process can be broken down into defined steps.

 

Starting of the Arc can be a one of the most difficult steps of the aluminum MIG process. Mechanical properties of aluminum are working against the welding process.  Some of the key factors to consider are oxidization, thermal conductivity, and soft or ductile filler wire.

 

Base Metal Oxidization is a natural enemy to the welding process and measures should be taken to minimize this contamination.  Oxides act as an insulator and require greater arc energy to burn through. Since the arc starting routines in robotic applications are predefined at set energy levels, there may not be enough energy to burn through a part with excessive oxidization, resulting in a failure at the start. This is why it is important to control base metal oxidization and implement measures into the arc starting routine to overcome this natural occurrence.

 

Touch Retract Arc Starting is one method used to overcome the natural oxidization process and assist the starting of the arc.  Touch Retract Starting is a controlled process where the robot sequences the weld power supply and the wire drive through a defined starting routine, drawing the arc to ignition.  The process of drawing the arc eliminates the harsh, dead short, explosive start routines conventionally used. This method of drawing the arc provides the reliability required for robotic applications without affecting cycle time.  Contact tip life is dramatically improved and mean time between failures is improved.

 

Weld Formation is the next step following the start of the arc.  The objective is to transition from the starting sequence to weld formation.  Common techniques include Run-In, Ramping or Direct Entry editing. Choosing the appropriate technique is directly related to the part.  Material thickness, multiple welds on a part, weld sequencing, and fixture design all play a role in the decision.

 

·         Run-In is typically treated as a global condition.  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 dynamically disabled and an alternative starting condition may be used for the appropriate welds.

 

·         Ramping is a common technique 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 this duration, the weld output is ramped from parameter A to parameter B, providing a smooth transition into the welding mode.  Ramping is not global and can be specific for each weld.

 

·         Direct Entry is a common technique used on thinner material where the base metal temperature changes as welds are applied, making it necessary to have specific control at each weld.  This technique is different from ramping in that the change between parameter “A” to parameter “B” 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 the techniques described operate on the same principles.  Touch Retract initiates an arc, a 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, penetrate and then switch to a cooler parameter to make the weld.  Starting slightly hotter helps arc initiation as well as assists in overcoming the thermal efficiency of aluminum.  

 

Weld Deposition is the reward of successful starting and weld stabilization.  The robot still plays an important role, and should not be overlooked.  The stability of the weld process is directly related to the ability of the robot to control the welding process.  Programming techniques such as weaving may be needed to overcome part variations.  Weld process changes may need to be made on the fly without interruption of the arc.  Advanced, proprietary weld process techniques, offered exclusively by a manufacturer, may be needed to overcome large gaps, weld variations in metal thickness, or to provide the cosmetic “TIG” appearance.  The limitations of the robot should never have an affect on the welding process.  Understanding common aluminum welding modes, and how to apply them to robotic applications will assist in achieving success.

 

·         Pulse Welding is a common deposition mode used in conventional 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 maintaining good travel speeds.   

 

·         Variable Pulse Welding is a unique deposition mode only supported by 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 a wider degree of variation over conventional pulse.  The cosmetic appearance is exceptional and when properly tuned resembles that of the TIG stack dime analogy.   

 

·         Power Mode is unique for aluminum, providing a very clean, fast, spatter free mode of deposition.  It is ideally suited for applications with good material fit-up with little to no limitations to material thickness. On thicker material combine this deposition mode with a circular weave and the results are outstanding.  On thinner material crank it up and let it rip!

 

·         Proprietary Weld Process Controls are unique to a specific manufacturer.  FANUC Robotics, for instance, offers a process where the robot controls the welding deposition by changing the process parameters based wire location. This advanced process control has been instrumental in the evolution of robotic aluminum welding.  The sought-after TIG appearance can be easily achieved, gaps can be bridged without problems, and precise control of penetration simplifies the welding of dissimilar metal thicknesses.   

 

Arc ending on aluminum requires some special techniques to close the weld crater.  The weld crater is the void that remains at the end of all welds.  The amount of current used in making the weld has 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.  A crater crack will typically propagate through the rest of the weld causing weld failure.  There are a couple of welding techniques used to fill and close aluminum MIG craters. The techniques operate on the same principle, weld current is reduced and time is added to allow the weld puddle to close and the crater to fill. Personal preference as well as joint design will play a role in determining the appropriate method to fill the crater.   

 

·         Ramping to a cooler parameter is one techniques used to close and fill craters.  This technique provides a gradual transition from the “Hot” welding parameter to the “Cooler” crater parameter.  The ramping of the weld schedule alone typically will not fill the crater; some additional time or what is called dwell must be added to hold the weld process at the cooler settings until the crater is filled.

 

·         Process Switching between two modes of deposition to close and fill craters 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, without the gradual change between the weld parameter and the crater parameter. This technique is typical when welding thinner material. When using this technique the transition from the “Hot” welding mode to the “Cooler” crater mode is instantaneous; there is no down ramping.  As with ramping, a dwell must be added to allow the cooler parameter to close and fill the crater.

 

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 crisp, leaving no ball on the end of the wire. Systems with poor burn back control often leave a large melted ball of wire on the end of the wire or the wire is consumed into the contact tip. A clean ending properly prepares the wire for the next start.

 

The manufacture of 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 point of 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 will often result in poor ending conditions and failed arc starts.

 

Burnback and the influence it has on the weld wire is very important and often misunderstood. Improper settings will typically result in a failed arc start of the next weld. This adds confusion to the troubleshooting process. The poor ending condition of the previous weld creates a failure condition for the next weld. This is where the confusion happens.  When troubleshooting a failed arc start condition always look at the ending conditions of the previous weld. Understanding where and how to adjust the burn back to get the desired ending results will minimize 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 12mm then the wire stick-out after burn back  should be 6mm 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.

 

With proper equipment and an understanding of the welding process, robotic welding of aluminum is successful.  As the process gains acceptance, unique and more difficult challenges are presented. Understanding the challenges and the ability to develop the necessary tools to succeed should be the goal of your robotics supplier.

 

This article was written by Joe Hoffman, senior welding engineer, FANUC Robotics America, Inc.