In much the same way that the automatic transmission has simplified the process of driving, Gas Metal Arc Welding (GMAW) has simplified the process of welding. Of all welding methods, GMAW is said to be one of the easiest to learn and perform. The main reason is because the power source does virtually all the work as it adjusts welding parameters to handle differing conditions; much like the sophisticated electronics of an automatic transmission.Because less skill is required, many operators are able to GMA weld at an acceptable level with limited training. These same operators run into trouble, however, when they begin creating inferior welds and are unable to diagnose and correct their own problems.
The guidelines listed below will help even inexperienced operators create high quality welds as well as offering tips for those who have been using the GMAW process for a number of years. Most common welding problems fall into four categories:
I. Weld porosity, II. Improper weld bead profile, III. Lack of fusion, and IV. Faulty wire delivery related to equipment set-up and maintenance.
I. Weld Metal PorosityPorosity
Problem #1: Improper Surface ConditionsThe most common cause of weld porosity is an improper surface condition of the metal. For example, oil, rust, paint or grease on the base metal may prevent proper weld penetration and hence lead to porosity. Welding processes that generate a slag such as Shielded Metal Arc Welding (SMAW) or Flux-Cored Arc Welding (FCAW) tend to tolerate surface contaminates better than GMAW since components found within the slag help to clean the metal’s surface. In GMAW, the only contamination protection is provided by the elements which are alloyed into the wire.
RemediesTo control porosity, use a deoxidizer within the wire such as silicon, manganese or trace amounts of aluminum, zirconium or titanium. Wire chemistry can be determined by referring to the American Welding Society (AWS) wire classification system. Test the various types of wire available to find the right chemistry for a given application. To start, try the most common wire type, ER70S-3 (Lincoln L50) which contains 0.9-1.4 percent manganese and 0.45-0.75 percent silicon. If porosity is still present in the finished weld, increase the amount of silicon and manganese found in the wire by switching to an ER70S-4 (Lincoln L54) or an ER70S-6 which has the highest levels of silicon (0.8 -1.15 percent) and manganese (1.4-1.8 percent). Some operators prefer to use a triple deoxidizer such as ER70S-2 (Lincoln L52) which contains aluminum, zirconium or titanium in addition to the silicon and manganese.In addition to changing the wire, further prevent porosity by cleaning the surface of the metal with a grinder or chemical solvents (such as a degreaser.) A word of caution though if using solvents, be certain not to use a chlorinated degreaser such as trichlorethylene near the welding arc -- the fume may react with the arc and produce toxic gases.
Porosity Problem #2: Gas Coverage The second leading cause of porosity in welds is a problem with the shielding gas coverage. The GMAW process relies on the shielding gas to physically protect the weld puddle from the air and to act as an arc stabilizer. If the shielding gas is disturbed, there is a potential that air could contaminate the weld puddle and lead to porosity.Remedies Shielding gas flow varies depending on wire size, amperage, transfer mode and wind speed. Typical gas flow should be approximately 30-40 cubic feet per hour. Using a flow meter, check that the shielding gas flow is set properly. There are a variety of flow meters on the market today ranging from simple dial gauges to ball flows all the way up to sophisticated, computerized models. Some operators mistakenly think that a pressure regulator is all that is needed, but the pressure meter will not set flow. A pure carbon dioxide shielding gas requires the use of special flow meters designed specifically for carbon dioxide. These special flow meters are not affected by the frosting that may occur as the carbon dioxide changes from liquid form to a gas.If high winds are blowing the shielding gas away from the puddle, it may be necessary to erect wind screens. According to the AWS Structural Welding Code, it is advisable not to GMA weld when wind speeds are greater than 5 mph. Indoors, ventilation systems may hamper gas coverage. In this case, redirect air flow away from the puddle. If fume extraction is necessary, use equipment designed specifically for this purpose such as MAGNUM™ Extraction Guns from Lincoln Electric -- they will remove the fume, but not disturb the shielding gas.A turbulent flow of gas as it exits the gun may also lead to porosity problems. Ideally, the gas will lay over the weld puddle much like a blanket. Turbulent gas flow can be caused by too high a flow, an excessive amount of spatter inside the gun nozzle, or spatter build-up in the gas diffuser. Other possible causes of insufficient gas flow may be damaged guns, cables, gas lines, hoses or loose gas fittings. These damaged accessories may create what is referred to as a “venturi effect” where air is sucked in through these openings and flow is reduced.Lastly, welding with a drag or backhand technique can lead to gas coverage problems. Try to weld with a push or forehand technique which lays the gas blanket out ahead of the arc and lets the gas settle into the joint. Porosity Problem#3: Base Metal PropertiesAnother cause of weld porosity may be attributed simply to the chemistry of the base metal. For instance, the base metal may be extremely high in sulfur content. Remedy Unfortunately, if the problem with porosity lies within the base metal properties, there is not much that can be done. The best solution is to use a different grade of steel or switch to a slag-generating welding process.II. Improper Weld Bead ProfileIf operators are experiencing a convex-shaped or concave-shaped bead, this may indicate a problem with heat input or technique. Improper Bead Problem #1: Insufficient Heat InputA convex or “ropy” bead indicates that the settings being used are too cold for the thickness of the material being welded. In other words, there is insufficient heat in the weld to enable it to penetrate into the base metal.
Remedies
To correct a problem with running “too cold,” an operator must first determine if the amperage is proper for the thickness of the material. Charts are available from the major manufacturers, including Lincoln Electric, that provide guidelines on amperage use under varying conditions. If the amperage is determined to be high enough, check the voltage. Voltage that is too low usually is accompanied by another telltale sign of a problem: a high amount of spatter. On the other hand, if voltage is too high, the operator will have problems controlling the process and the weld will have a tendency to undercut. One way to check if the voltage is set properly is to test it by listening. A properly running arc will have a certain sound. For instance, in short arc transfer at low amperages, an arc should have a steady buzz. At high amperages using spray arc transfer, the arc will make a crackling sound. The arc sound can also indicate problems -- a steady hiss will indicate that voltage is too high and the operator is prone to undercut; while a loud, raspy sound may indicate voltage that is too low. (continued)
(Source : Lincoln Electric Company)
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