Cause of weld spatter: GMAW welding and electric arc

Posted by Prof. Emil Schubert on Jul 6, 2021 9:21:13 AM
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Gas-shielded metal arc welding – also known as GMAW welding or MIG/MAG welding – is by far the most applied and most common welding process in welding technology. When searching for the cause of weld spatter, this process literally hits the bull's eye. Whereas with TIG welding and plasma welding almost no weld spatter occurs, spatter formation in GMAW welding can never be completely prevented. The question may arise why not use TIG or plasma welding in general if welding spatter should be avoided as much as possible. To get the answer it is necessary to take a closer look at the process of MIG/MAG welding and the cause of weld spatter.

Why MIG/MAG welding?

There are primarily economic reasons why the GMAW process is most frequently used in welding technology for joining sheet metal: With the gas-shielded metal arc welding process, a better melting performance of the filler metal and a higher welding speed are achieved compared to TIG welding and plasma welding. This is also due to the three types of electric arc used in MIG/MAG welding. More on this later.

However, due to the formation of spatter, the GMAW welding process is not suitable if the focus is on achieving a perfect appearance. Subsequent removal of weld spatter is time-consuming and expensive. With some metals, such as stainless steel, reworking should be avoided in any case, as any mechanical intervention can destroy the surface appearance. If minimal reworking is required and time is not a decisive factor, the TIG or plasma welding process is the first choice.

What happens during MIG welding? Why are different arcs used in MIG/MAG welding? In addition, what influence do they have on the cause of weld spatter?

The GMAW welding process – briefly explained

In gas-shielded metal arc welding a filler material in the form of an endless welding wire melts at high heat under a shielding gas cover. The energy required to melt the filler material is generated through a so-called electric arc. This arc is an ionised shielding gas, i. e. when the arc is ignited, high temperatures are generated so that the neutral shielding gas decomposes into positively charged atomic cores and negatively charged electrons. Since the workpiece is usually at the negative pole of the welding circuit, an arc column forms between the wire tip and the workpiece. The wire melts in this arc column and is then transported into the weld pool.

There are different types of arcs depending on the protective gases used and the current ranges.

The protective gas shield formed by the gas prevents welding defects such as oxygen, hydrogen and other particles from the ambient air contaminating the weld and causing undesirable reactions such as pore formation, cracks or the inclusion of particles.

The molten wire fills the gap to be joined between two sheets or components under the protective gas shield. The welded area cools down and creates a permanent joint between the two workpieces. This is the intended process.Verfahrensübersicht_MSG_Prozessprinzip_EN

In welding technology different types of arcs are used for MIG/MAG welding, depending on the thickness of the sheets to be welded as well as on the speed to be achieved during welding. These are the three most common types of arc:

  • Short arc

  • Spray arc

  • Pulsed arc

Electric arc types and the cause of weld spatter

In more than 90 percent of all applications, short arc, spray arc or pulsed arc welding is used.

The short arc

A characteristic feature of MSG welding with a short arc is the short circuit that always occurs when a piece of molten wire passes into the weld pool. This is an uncontrollable process and therefore a process with a high level of spatter. With the short arc it regularly happens that one of the wire droplets bursts open and lands as a weld spatter next to the weld pool. This process can be significantly improved by using a so-called controlled short arc. Examples of a controlled short arc in welding technology are brands* such as Cold Steel (CMT = Cold Metal Transfer), Cold Arc and Steel Root, each with special properties, but all limit the short-circuit current and suppress the spatter effect very strongly. This means that without a controlled short arc, the short arc process in general has a very high level of spatter.MSG_Lichtbogen-1The short arc is mostly applied for welding thinner sheets up to approx. 2 mm and for lower welding currents up to a maximum of 200 amperes.

The spray arc

If thicker sheets of approx. 2 mm or more have to be welded, the current intensities also increase to ensure a deep penetration. For this upper performance range at 200 to 500 amperes, the spray arc is used, which runs without interrupting the short circuit. The welding wire melts under high power and is distributed into very fine droplets. If these droplets reach the weld pool, the process is called low-spatter. However, some of them fly very finely and regularly distributed next to the weld pool and this can be a considerable amount of weld spatter.Spruehlichtbogen_auf_flachem_Bauteil-1Examples of a spray arc include the brands Power Steel and Fast Steel, which also allow a higher welding speed and in addition produce comparatively little spatter. Due to the higher welding speed, there is also less heat input into the workpiece, which in turn reduces deformation.

The pulsed arc

Among the GMAW processes, the pulsed arc provides the best control of the welding process. The process is best suited for sheets with a thickness of 0.5 to 4 mm. With the pulsed arc, basic current and pulsed current alternate after a predefined frequency and pulse time. The current intensity is increased accordingly until the wire melts and exactly one drop passes over, but without causing a short circuit. With each pulse, a liquid drop merges into the weld pool. Theoretically, every drop falls into the weld pool and normally no weld spatter is produced during this process. In practice, however, short circuits occur unintentionally between individual pulses. Sometimes such a welding droplet gets so long that it is still attached to the end of the wire, but already touches the weld pool at the bottom. This leads to a conductive connection between the wire and the weld pool and finally to a short circuit. As a result, this drop bursts, which in turn leads to increased spatter adhesion, similar to a single short arc.

Impuls

When searching for the cause of weld spatter, the three arc processes of short arc, spray arc and pulsed arc must be considered. They provide information on how spatter formation takes place and how often it occurs.

Optics or speed – when to use which arc type?

The pulsed arc is the first choice for a visually attractive and very high-quality weld seam on thin sheets. However, the deposition rate is not very high with this arc, because fewer kilograms of wire are brought into the weld joint per hour compared to the spray arc. For this reason, thick sheets are generally welded with the spray arc. The welding power sources or welding machines, as they are often colloquially called, are more expensive due to the additional pulse control required, because many other parameters have to be set. A favorable variant for thin sheets is a short arc, which does not require any complex control – but produces a correspondingly high level of spatter.

To control and limit spatter formation with a short arc, controlled short arcs are used. However, the welding power sources for this process are also more expensive, because they have to be equipped with an additional electronic control system.

Advantage: know the cause of weld spatter

Ultimately, the choice of welding arc always depends on the user's requirements. If the quality requirements are less stringent and reworking is not necessary, spatter formation is not a problem. If a workpiece is further processed after welding, for example by painting, the subsequent removal of spatter is time-consuming and cost-intensive.

The most frequently welded material in welding technology is structural steel, which is known for its high spatter formation. With stainless steels, the melt is a little more viscous, which already reduces spatter formation. Aluminum welding also produces a lot of spatter, but alternating current or pulse welding is used, which in turn reduces spatter.

Deciding which type of arc to use is a technical and economic decision. If the causes of weld spatter are known, it is possible to take targeted countermeasures with the right technology – or to protect the workpiece and the welding torch from spatter adhesion with special anti-spatter agents.

Prevent spatter adhesion with anti-spatter agents

Even under the best technical conditions, weld spatter cannot be completely prevented during MIG/MAG welding. In order to avoid spatter adhesion as far as possible, anti-spatter agents are highly recommended as a pre-treatment for the workpiece, but also for the wear parts on the welding torch or torch neck and the clamping elements. The protective film provided by the anti-spatter agent allows flying particles from the welding process to bounce off the workpiece or to adhere only slightly to it. Sprayed onto clamping devices, safe mounting and removal of the workpiece is also ensured throughout. Anti-spatter agents can also be sprayed onto the front end of the welding torch in order to optimally protect the wear parts of the gas nozzle, contact tip and gas diffuser from spatter adhesion.

If particles adhere to the inner wall of the gas nozzle and to the gas diffuser, there will be problems with the gas cover. A spatter formation at the contact tip leads to wire burn backs or wire sticking. In case a so-called spatter bridge forms between the gas nozzle and the contact tip, this leads to a short circuit and the functions of the welding torch are restricted or the welding torch is even destroyed.

The following anti-spatter agents from ABICOR BINZEL are recommended as preparation for the welding process:

For the workpiece

➡️ ABIBLUE NF

For the wear parts of the torch

➡️ Super Pistolenspray NF, Anti-spatter paste Düsofix, Ceramic spray

For the workpiece + wear parts + clamping elements

➡️ Super Pistolenspray NF

Since weld spatter formation can have many causes, it is necessary to understand how weld spatter is produced and how to prevent spatter adhesion by using effective anti-spatter agents. Welding equipment must be checked regularly in any case and in this context especially the wear parts. If you pay attention to these things when welding and take appropriate preventive measures, the pleasure of using the torch and the quality of the welding result will last for a long time. Because in the end, it is always about a high-quality weld seam.

Happy welding!

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Topics: Welding Chemicals