Switch to Modified Short Circuit MIG (RMD) to Significantly Improve Welding Productivity | MillerWelds

Switch to Modified Short-Circuit MIG (RMD®) to Significantly Improve Welding Productivity

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Modified short-circuit GMAW provides improved puddle control and significant productivity gains in many shop and field applications.
RMD welding in the field
Better control of the weld puddle

Help welding operations stay competitive

As organizations look for ways to improve profitability and maintain a competitive edge, there are solutions and technologies available that can help drive productivity and quality in field and shop welding applications.

Welding plays a significant role in the timely completion of construction projects — both pre-fabrication in the shop and installation on the jobsite. Making the switch from traditional welding processes to a modified short-circuit MIG process provides improved puddle control and significant productivity gains in many shop and field applications, while maintaining and sometimes improving weld quality.

Much of the return on investment in wire processes comes from increased productivity. Most of the costs associated with welding in the field are related to labor — approximately 80%. So, giving welders advanced technology and equipment helps improve their productivity, saving significant time and money in the long run.

This can help welding operations stay competitive and maintain the highest levels of quality and productivity. At the same time, they can address numerous challenges, including a growing welding operator shortage, changing materials and shortening project timelines.

How does modified short-circuit GMAW work?

A modified short-circuit MIG process offers numerous benefits that can help improve productivity, efficiency and quality. In a modified short-circuit MIG process, such as Regulated Metal Deposition (RMD) from Miller Electric Mfg. LLC, the welding system anticipates and controls the short circuit, then reduces the welding current to create a consistent metal transfer. This precisely controlled metal transfer provides uniform droplet deposition, making it easier for the welding operator to control the weld puddle. These processes are also more forgiving to variations in fit-up or changes in the wire stickout.

A key differentiator of a modified short-circuit GMAW process is the greater control of the current during the transfer process. This results in a smoother transfer and a calm weld puddle that’s easier for the operator to read. This can help a less-experienced welding operator complete quality welds more efficiently.

Think about dropping a pebble into a lake versus dropping a brick into the lake. The ripples from the pebble may be almost unnoticeable, while the brick produces a harder-to-control splash. The finer, more consistent droplets produced in RMD are like the pebble — where it’s easier to control the impact. Conversely, a traditional short-circuit MIG process produces a transfer more like the brick, creating sidewall splashing of the weld puddle.

In a traditional MIG process, this “splash” can lead to issues of cold lap in the finished weld if the welding operator does not thoroughly grind the material that splashed onto the sidewall. Cold lap is a lack of fusion that happens when the weld metal fails to completely fuse with the base metal or with the preceding weld bead in multipass applications. The more consistent molten pool of the RMD process helps prevent cold lap, resulting in better mechanical properties in the completed weld.

Improving productivity and quality

What do these differences mean for the end user? With RMD, welding operators can achieve the same code-quality welds they have been achieving with traditional welding processes. However, now they can do it with greatly improved deposition rates and travel speeds for significant productivity gains.

The smoother droplet transfer and more stable weld puddle can reduce rework, helping operations decrease costs, meet quality requirements and improve productivity to meet tighter deadlines. The minimized sidewall splashing of this process also results in less grinding — saving time and money.

In addition, because the weld puddle is easier to read and manipulate, new welders can be trained faster to produce quality welds.

Also, with RMD, the thicker root deposit can eliminate the need for a hot pass by reducing the risk of burn-through on the subsequent fill passes.

The lower heat input offers several benefits, including the ease of welding out of position with both RMD for the root weld and pulsed MIG for the fill and cap passes. The weld puddle freezes faster than in traditional MIG processes. This allows the welding operator to run a higher wire speed, therefore improving the deposition of each pass. In addition, the lower heat input typically helps improve the mechanical properties of the finished weld.

Eliminating the back purge

Another significant benefit of the RMD process is the ability to eliminate the back purge in some applications, thanks to the consumables used and the lower heat input of the process. In applications with P91 and some stainless steels, many contractors have had success using a modified short-circuit GMAW process with no back purge. Typically, when these materials are TIG welded, they require an argon back purge during the root pass and for some time afterward. Eliminating the back purge saves in wasted gas and the time it takes for the purge. This can be even more important in cases of a weld failure after the purge dams are removed, requiring the operator to purge the entire system to do the repair.

Tips and techniques for success

When using a process such as RMD, there are a few considerations that can help in achieving optimal results.

It’s important to maintain a minimum 1/8-inch gap while welding. This allows sufficient space for the weld metal since the wire is continuously fed from the wire feeder. Some materials, such as stainless steel, tend to expand when hot so a wider gap — like 5/16-inch — may be needed when welding these types of materials to maintain a 1/8-inch gap while welding.

Also, remember that RMD guides the puddle rather than how a traditional MIG process leads the puddle. When leading the puddle, defects can occur from overshooting the wire. Because RMD guides the puddle, it helps eliminate that problem.

In addition, the welding operator should sit in the puddle a bit deeper when using a modified short-circuit MIG process. It’s recommended to use a 5- to 10-degree drag angle. Place the wire about one wire diameter from the leading edge of the puddle.

It’s also recommended to use mixed gas such as 75% argon/25% CO2 or 90% argon/10% CO2, depending on the fill and cap weld process used. This helps reduce spatter and improve the quality of the finished weld. In many applications, the gas operations use for the cap pass may drive the choice of gas for the root pass. This simplifies the process and eliminates the need to change out the gas between passes.

Lastly, joint bevel preparation angles in a modified short-circuit GMAW process are standard and very similar to what an operation would use with a traditional MIG process. This means a switch to RMD requires minimal changes in joint preparation, resulting in additional time savings and productivity gains.

Training is critical

Training is critical when making the switch to RMD, especially for operations that have not previously run wire processes. Having a go-to welder who understands the functions of the equipment and the process is also a good idea.

While a process switch may require an investment in operator training, converting to a process that offers ease of use and greater productivity can help operations address the labor shortage facing the industry. An easier-to-learn MIG process can shorten training time, so welding operators can be on the job making quality welds sooner. An investment in these processes and training can pay off quickly — without negatively impacting weld quality.

When implemented properly, a modified short-circuit MIG process offers high weld quality along with increased productivity and efficiency by way of much greater travel speeds.

These more productive and forgiving welding processes can help operations save money, while also allowing them to complete jobs faster with a broader spectrum of welding operator skill levels.