New Advances in CO2 Gas Metal Arc Welding

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In the United States, the standard shielding gas used in gas metal arc welding (GMAW) is generally a 75/25 percent argon (Ar)/carbon dioxide (CO2) gas mixture. Argon is a relatively expensive gas. A less costly weld grade CO2 gas as a replacement for Ar/CO2 mixtures is used for welding applications in Asian countries including Japan, China, and Korea. In Japan, the use of a CO2 shielding gas has all but replaced the typical Ar/CO2 mixture.

In fact, statistics show that in Japan,GMAW with a CO2 shielding gas accounts for 94 percent of the total gas usage. GMAW using an Ar/CO2 mixture accounts for only six percent. The usage rate of CO2 for GMAW in other Asian countries is reported to be even higher than that of Japan.

In Japan, the automotive and heavy construction industries were the first to shift from an Ar/CO2 mixture to a CO2 shielding gas as a means of drastically reducing their welding costs. This type of cost reduction has become even more critical as companies cope with the current worldwide economic downturn. The use of weld grade CO2 is spreading rapidly as its economic benefits are recognized and as advanced technology enables the more productive and efficient use of it.

KEY FACTORS FOR CHANGE
There are two key factors that have resulted in the high rate of GMAW using CO2 in Japan. First, CO2 grade welding gas is currently approximately one-third less expensive than argon. Therefore, the operational cost per weld can be immediately reduced while positively impacting profitability. Second, a welder can achieve the same quality of weld with CO2 as with the Ar/CO2 mixture. With a CO2 weld, penetration is deep during the weld process (Figure 1), therefore welding defects due to a lack of penetration rarely occur. When dealing with heavy-sectioned welded structures, deep penetration is easily maintained. CO2 gas shielded welding of large structures is currently used in the fabrication of steel building columns in Japan. As Japan is located in an area where earthquakes occur frequently, weld quality and strength are critical attributes. This method delivers a high quality and very robust weld comparable to that of a typical GMAW with an Ar/CO2 mixture.

CO2 QUALITY REQUIREMENTS

FOR GMAW
CO2 shielding gas used in arc welding applications are held to Japan Industrial Standards, JIS Z 3253, which require regulation of gas purity of 99.5 percent by volume or more.
Under these standards, the moisture content must be 150 ppm or less. In the US, AWS D1.1 regulates that the dew point must be 40F (-40C) or lower. In other words, the moisture amount must be at 120 ppm or less for CO2 gas when added into Ar/CO2 mixtures to be used in arc welding applications.

OVERCOMING SPATTER ISSUES
A frequently heard concern of welders when discussing CO2 GMAW is that the amount of spatter generated will be greater than that of GMAW using a typical Ar/CO2 mix. In the past, when using CO2 as a gas shield, excessive spatter was created. This required additional manpower for spatter removal, which increased weld costs and made welding with an Ar/CO2 shielding gas mix the better option despite the cost of argon.
In the early 1980s, Inverter Control Technology was introduced and incorporated into OTC DAIHEN arc welding power sources. Inverter Control Technology allowed a reduction in size and weight of arc welding power sources, improved electrical efficiency, and reduced spatter generation due to higher operating speed when compared to conventional thyristor controlled machines. OTC DAIHEN’s Digital Inverter Control Technology allows the welder to further control the “spatter phenomenon” discussed above, thereby eliminating the additional costs incurred for spatter removal. OTC DAIHEN’s solution to the “spatter phenomenon” was the creation of its waveform control methods. By measuring waveforms of the weld current and adjusting the current every 100 milliseconds based on digital feedback, a radical reduction of weld spatter can be achieved. This advanced CO2 welding technology was developed for GMAW at DAIHEN’s facility in Japan, where it is continuously perfected.

To better understand this technology, Figure 2 highlights the control capability of OTC DAIHEN’s newest welding power source. This was developed specifically to avoid the generation of weld spatter when using CO2. During detection of the “squeezed” characteristic of the molten metal droplet via our waveform feedback, the welding power source immediately reacts. Throughout the molten metal droplet stage, the current between both the wire tip and molten pool is measured. At the point just before re-igniting the arc, the welding current will decrease instantaneously with detection of a low level current. With this advanced CO2 technology, the welding equipment can control the energy at the moment of the arc re-ignition. This dramatically reduces spatter by avoiding the eruption of both the molten pool and the molten metal droplet.

WELDING MATERIALS
When using CO2, the solid wire for GMAW contains multiple deoxidizing agents such as silicon (Si) and manganese (Mn). This is very similar to the solid wire used with Ar/CO2 mixtures in GMAW. In Japan, solid wire is subdivided into several categories depending on welding current under JIS Z 3312. CO2 type welding wires, which optimize the molten pool viscosity to control the generation of weld spatter, and is available from most local suppliers. Recently, there has been an increased usage of flux cored wire, primarily among members of the shipbuilding industry. Flux cored wire generates less spatter and provides excellent welding performance in addition to being an acceptable wire for use in outdoor climates. When flux cored wire is combined with the OTC DAIHEN inverter controlled welding power source, arc starts are instantaneous and welding performance can be superior.

THE POSSIBILITIES
GMAW using CO2 and solid wire for CO2 gas shielded welding is a cost-effective method of welding structures of medium/thick plates that require the deep penetration of welding applications around the world. Welders in the US are now recognizing that GMAW using a weld grade CO2 delivers a quality of weld as high as the GMAW using an Ar/CO2 mixture. As the price of argon continues to rise in the US, interest in advanced CO2 welding technology grows as an attractive lower cost alternative to Ar/CO2 shielding gas mixtures. As CO2 welding is just beginning to take hold in the US, and much of CO2 in the US is used for food and beverage industries, it can be difficult to obtain welding grade CO2.
The US supply chain for CO2 is beginning to accommodate the growing demand of the expanding CO2 market for arc welding. The ongoing increases in cost for argon continue to make the case for switching to CO2 for GMAW more and more attractive. We estimate that a welding shop that changes over to a weld grade CO2 shielding gas, can reduce its gas costs by one-third compared to the price of Ar/CO2 mixtures. As welders begin to experiment with GMAW using welding grade CO2 in conjunction with OTC DAIHEN’s advanced CO2 welding technology, we believe CO2 gas shielded welding will become more widely recognized in the US.

For more information on OTC DAIHEN please contact Modern Machinery Company at 1-800-589-1444.

Article reprinted from www.cryogas.com

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