 |
Mig-welding-tips:do you master them all?SOLUTIONS with Effective, Powerful AdviceMig-welding-tips presented hereafter review important aspects and point to special features of this popular process, officially designated as GMAW - The still popular name MIG (Metal Inert Gas) was initially adopted when the inert gases used to weld aluminum were only argon or helium. Later on, the introduction of mixes of active gases like oxygen, hydrogen and carbon dioxide to weld other metals too, suggested the need to drop the word "inert" from the description, with the adoption of the accepted new name. Although Mig-welding-tips should be universally known, being the process quite diffused, widely popular and successful in numerous applications, it is contended by no less authority than Ed Craig, a universally acclaimed world expert, (author of the fundamental book "A Management and Engineers Guide to Mig Welding, Quality-Costs-Training", available through www.weldreality.com) that unfortunately many misconceptions still limit its use. UPDATED PAGE
One of the most damaging errors is letting welders do it their way, even if they, despite the considerable skill they may have in stick (Shielded Metal Arc) welding, ignore basic Mig-welding-tips. In particular it seems that no coordinated effort is dedicated by those in charge, mainly managers and welding engineers, to educate designers to make the best use of the process, and to train welders by providing them with Mig-welding-tips on how to select their parameters to improve quality and reduce costs. We hope that these Mig-welding-tips will spur renewed interest in the principles of this process bringing about a review of practical applications, to check if they are performed in an effective and economic way. GMAW uses a continuous, consumable electrode wire, fed from a spool, to provide the needed filler metal. A welding arc is established between electrode and work with energy drawn from a suitable power supply. Shielding of hot and molten metal is provided by a protective gas flowing from proper containers. The Gas Metal Arc Welding (GMAW) process provides the following main Advantages:
Among all important Mig-welding-tips one should remember that the Limitations are:
The first of our Mig-welding-tips reminds that most of Gas Metal Arc Welding (GMAW) applications are performed with direct current reverse polarity (electrode positive) from constant voltage (CV) power supplies. Only if shallow penetration is required (i.e. for weld surfacing and cladding) straight polarity (electrode negative) can be used. Once started with correct parameters the process is self regulating. A built in feature is designed to keep the arc length constant despite minor changes in the distance between the hand held torch and the work, as explained hereafter. With constant current (CC) power supplies, the wire feed speed is regulated by the voltage measured by a sensor across the arc. A rise in voltage caused by an occasional longer arc due to increased torch distance causes the wire feeder to increase the wire speed, thus reducing length and voltage to the previous condition. With constant voltage (CV) power supplies, the current is determined by the wire preselected constant feed speed. Arc length and wire melt-off rate are in equilibrium. An occasional longer arc due to increased torch distance causes a current drop, due to increased electrode extension (see hereafter), that limits the wire melt-off. More wire is thus available to shorten the arc length because of the constant wire feed. Current is then increased and a new equilibrium is reached at the previous arc length. The total current supplied in the welding operation is partitioned between two competing processes. The current flowing in the wire between the contact and the arc, contributes to heat that length of wire by resistance heating (at least with steel, not with aluminum) and prepares its tip to melt more easily. The remaining amount of current is used up to feed the arc. It is the arc current that determines weld penetration. Therefore an increased torch to work distance, although not changing the arc length because of the self regulating processes explained above, has the consequence of increasing electrode extension and of decreasing arc current, therefore reducing weld penetration. Gas Metal Arc Welding (GMAW) provides the unique capability to establish the metal transfer mode adapted to the application, by selecting properly the most controlling factors following proper Mig-welding-tips:
The first and the last of the above Mig-welding-tips are those most often considered. An Article on Shielding Gases (7) was published in Issue 48 of Practical Welding Letter for August 2007. Click on PWL#048 to see it.
Short Circuit Arc Transfer Mode It consists in a rapid succession of short circuits generated when the wire tip contacts the work. Metal is transferred in the form of small drops. Although current is lower than with other modes, a great influence is played by the gas mix used. Excessive voltage will cause objectionable spatter. Globular Transfer Mode Spray Transfer Mode Pulsed Arc Transfer Mode This low heat input mode, called GMAW-P for pulsed, permits to provide the advantages of spray transfer mode to thin sections, by reducing the average current level to an acceptable level that does not cause burn through. Different versions exist of this basic principle, contributing to over choice confusion, and to its selection even for application that could be dealt with by using a standard approach. This one may be one of the most important Mig-welding-tips. Given the substantial equipment cost difference, it would always pay to check first if the given job can be welded with regular equipment. Note the following Mig-welding-tips: two additional features available sometimes in equipment should need some attention. Slope Control When using small size wires and low currents, however, that is for Short Circuit Transfer Mode, there is a need to limit the amount of short-circuit current that the power supply can deliver. A high short-circuit current generates a powerful "pinch-effect" or magnetic squeezing force that separates violently the molten drop from the wire. If the short-circuit current is limited to a moderate value, by selecting a suitable slope, the detachment will be smooth with no or little spatter. Note that if the slope is too steep, the current will be too low, and the unmelted electrode will stick to the work. Variable Inductance One of the Mig-welding-tips that can have practical influence is the following. In Short Circuit Transfer Mode inductance limits the pinch effect, which will be applied more gradually, by controlling the rate of current rise in time. Higher inductance will decrease the number of short-circuits per second. The weld pool will become more fluid, resulting in smoother, flatter weld bead. Too much inductance however will negatively affect arc initiation. In Spray Transfer Mode, more inductance will only affect arc starts, which will result softer. No influence will be detected once the arc is running. Additional information and Mig-welding-tips on the principles governing parameter selection for GMAW has been presented in four consecutive issues of our Practical Welding Letter. You can see them by clicking, that opens new windows, on the following links: The following documents include information essential to the subjects dealt with in this page on Mig-welding-tips. Recommended Practices for Gas Metal Arc Welding
Recommended Practices for Shielding Gases for Welding and Plasma Arc Cutting
Specification for Carbon Steel Electrodes and Rods for Gas Shielded Arc Welding
Standard Welding Procedure Specification (Swps) for Gas Metal Arc Welding (Short Circuiting Transfer Mode) of Galvanized Steel (M-1), 18 through 10 Gauge, in the As-Welded Condition, with or without Backing
Standard Welding Procedure Specification (SWPS) for Gas Metal Arc Welding (Short Circuiting Transfer Mode) of Carbon Steel to Austenitic Stainless Steel (M-1 to M-8, P-8, or S-8), 18 through 10 Gauge, in the As-Welded Condition, with or without Backing An Article on Process Extensions of GMAW was published in section 2 of the issue 30 of Practical Welding Letter. To see the article click on PWL#030. A new page on Robotic Arc Welding (Opens a new page) was recently added to this website. It deals with applications of GMAW for increased productivity.
NEW RESOURCES
Do you need more Resources?
Any specific questions or comments? Write them down and send them to us by e-mail, by a click on Your Questions and Feedback. We plan to update this Mig-welding-tips page following readers' input
To reach a Guide to the collection of the most important Articles from Past Issues of Practical Welding Letter, click on Welding Topics. Back HOME
Tig Welding Tips
POWERED BY:
Click on this Logo NOW! Copyright (c) 2006, 2007, 2008 by Elia E. Levi
Mig-welding-tips on fundamental properties and characteristics of GMAW can establish your reputation as an expert, and contribute largely to your successful applications. -
|
