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GAS/ENERGY: Miller Axcess - a Pipe Welding Solution from Afrox

 





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RMD and Accu-Pulse Provides Superior Weldability and Improved Deposition Rates in GMAW


Johan Pieterse, Business Manager Application Development at Afrox explains how Software-Driven RMD (Regulated Metal Deposition) and Accu-Pulse technology in multi-process GMAW equipment, improves deposition rates, and reduces heat input, labour time, spatter and fumes in the GMAW welding process.

Multi-process GMAW equipment is manufactured by Miller Electric Mfg. Co. of the United States and distributed in South Africa by Afrox.

When a new welding process lowers overall heat input, spatter and fumes - and can lower welding costs while improving weld quality - people tend to sit up and take notice. That’s the case with Miller Electric Mfg. Co.’s RMDTM process.


RMD is a unique, patented, advanced software application for modified short circuit transfer GMAW (MIG welding) that precisely controls the electrode current during all phases of the short (see Fig. 1 below). It is best suited for welding thin metal (especially stainless steels), overcoming less-than-ideal fit-up (gaps) and root passes on pipe.



Though RMD is a software-driven welding process, it is important to note that it is very user-friendly. In fact, RMD improves the performance of less experienced welders as it is very tolerant of gaps, putting about 5 to 20 percent less heat into the work, and compensates for changes in contact tip to work distance. RMD maintains optimum arc characteristics because the electrode current is closely monitored and controlled during each phase of the welding process.


Using RMD requires only familiarity with everyday welding terms. In fact, operators simply enter the wire type, wire diameter, gas combination and the desired wire feed speed on the welding system’s simple interface. The RMD software then automatically maintains optimum arc conditions regardless of wire feed speed and contact tip to work distance.

Unlike speciality welding systems from other manufacturers that offer limited process options, the multi-MIG inverter-based welding system for RMD provides unparalleled flexibility. It can create any GMAW output type imaginable; hence, RMD can be combined with other software generated processes or arc transfer modes from the same system. This allows selection of the best process for the application at hand.


Example: Pipe Welding: RMD could be used for the root pass in pipe and Accu-Pulse™ used for hot and cover passes without changing machines or secondary leads. In automated applications, RMD could be used to weld thin sections or fill gaps, and then switched using remote programme select to Accu-Pulse™ for higher travel speeds.


Short Circuit Limits

As a welding process designed to operate at lower heat levels (most commonly 30 to 200 amps/16 to 22 volts), standard short circuit transfer MIG is preferred for thinner metals. However, it has limitations, and Miller engineers designed RMD specifically to overcome them. To understand RMD benefits, a brief explanation of the standard short circuit process is necessary.

In a standard short circuit transfer, the welding wire initially contacts the workpiece, creating a dead short (voltage = 0). At that point, the power source rapidly reacts to the short, attempting to maintain the preset voltage value by dumping current into the wire to clear the short. As the wire heats from resistance to high current flow, it forms a ball at the end and almost simultaneously an arc is created in the space created. The ball is then pushed into the weld puddle at the end of the wire. This happens repetitively 90 to 150 times per second. Observation of these events using slow motion video emphasises these reactions and they can be viewed as violent chaos that creates spatter and affects the next short circuit.
Further, the excess uncontrolled current adds more heat. This increases the likelihood of warping on heat-sensitive metals such as stainless steel and, where the metal cannot support much heat (such as the edge of a thin plate or bevelled pipe); it can also produce burn-through. The combination of current spikes and spatter may also lead to engineers’ lack of confidence in fusion and/or consistent penetration. For this reason, some codes and specifications do not permit using short circuit MIG for certain applications.


Cool and in Control


Miller points out that RMD operates similar to the short circuit transfer, but with some important differences.


The RMD software programme, working with an inverter-based welding system and closed-loop feedback, closely monitors and controls the electrode current at speeds up to 50 microseconds (50 millionths of a second). By closely monitoring the change in energy and understanding exactly how much is required for a specific wire diameter, speed and gas combination, it is possible to predict future arc conditions and control the metal transfer accordingly. Basically, a precisely controlled waveform is applied at critical points in a short circuit cycle. The desired result is that when the ball separates from the wire, it happens at a lower current level. This minimises total heat input burn-through, warping, spatter, and smoke while greatly improving weld puddle control and gap filling.

As for predicting future welding conditions, Miller explains that the arc end of the weld wire has a heating history. How much heat is in the wire at the point of contact with the workpiece depends on how long it was in transit and the behaviour of the current during the transit time. Understanding these phenomena is what allows a great deal of latitude in fluctuating contact tip to work distance.


Miller has used the data from the heat history to create a method of controlling the arc. In the RMD process, a fine adjustment can be made based on this knowledge, giving the operator a range of adjustment to suit a technique or application. RMD is ideally suited for filling gaps in open-root piping applications for industries as diverse as transmission and process piping, fabrication, dairy, hospital and food industries; and where cosmetic, lower heat input welds are needed.

The fact that the weld puddle behaves much more calmly and ’freezes‘ faster with the RMD process is a critical factor in successfully welding open-root pipe in any position. Miller notes that excessive positive (convex) and negative (concave) reinforcement inside the pipe has historically been an issue using conventional GMAW processes for pipe root passes. As RMD removes that critical chunk of energy while the ball collapses into the weld puddle, the puddle freezes faster in the desired position. Initial field tests with major engineering firms have been very positive, Miller reports.


Accu-Pulse has excellent Out of Position-control


Accu-Pulse enables holding a shorter arc length with more focused arc cone, regardless of electrode stickout. With Accu-Pulse, the welder is no longer required to hold a longer arc to prevent short circuits. Whilst welding, Accu-Pulse allows the weld puddle to fill in at the toes of the weld much better than older technology. This results in an increase in deposition and travel speed. Accu-Pulse has very good out-of-position control.




Technology and Reliability


Miller equipment is renowned for its reliability. All Miller equipment undergoes the following reliability testing and has a three-year warranty:


40º C life testing - 60,000 hrs MTBF at rated load
Vibration testing
Low frequency and high displacement
High acceleration
Dust chamber and drop testing
Monitoring and inducing electrical noise

ESD

HF chamber
Power up / power down
Missing ground
Random primary line loss - 2,000 and 5,000 volt input surge

Certain models in the Miller range feature the following patented technology:

• Auto-Line

With Auto-Line, equipped with this feature, equipment can be connected to any global power supply, between 120 volt to 575 volt (50 or 60 Hz, single- or three-phase), depending on the model, without change the physical supply power cable connections within the unit.

• Wind Tunnel Technology
Wind tunnel technology ensures that only the components that need direct airflow for cooling are in the air stream.
All other components are not exposed to airflow, which reduces the exposure to dirt and moisture.

• Aluminium Casing/Housing
Certain models have casing manufactured from aluminium. This provides two benefits: firstly, it reduces the weight of the unit, and secondly, it is suitable for working in corrosive environments.

• Fan on Demand
With Fan on Demand, the fan used to keep the unit’s internal components cool only operates when the unit reaches a pre-determined temperature. With this feature, the unit is quieter when idling, uses less power and does not draw in any unnecessary dirt into the unit whilst operating, thus keeping the inside of the unit clean and protecting the internal components.

 
 
 
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