Modeling and Optimization of Gas Metal Arc Welding (GMAW) Process

Introduction

The gas metal arc welding (GMAW) process is a welding process that yields coalescence of metals by heating with a welding arc between continuous filler metal wire electrode and the work piece. The continuous wire electrode, which is drawn from a reel by an automatic wire feeder, and then fed through the contact tip inside the welding torch, is melted by the internal resistive power and heat transferred from the welding arc. Heat is concentrated by the welding arc from the end of the melting electrode to weld pool and by the molten metal that is being transferred to weld pool. Molten weld pool and electrode wire are protected from contaminants in the atmosphere by a shielding gas obtained from various combinations. Figure 1 shows the basic circuit diagram of GMAW process.

Figure 1.

GMAW circuit diagram

• The basic circuit consists of the following:

  • 1.

    Welding gun: It has a number of key parts – a control switch, a contact tip, a power cable, a gas nozzle, an electrode conduit and liner, and a gas hose. The control switch, when pressed by the operator, initiates the wire feed, electric power, and the shielding gas flow, causing an electric arc to be struck. The contact tip, normally made of copper, is connected to the welding power source through the power cable and transmits the electrical energy to the electrode while directing it to the weld area. Before arriving at the contact tip, the wire is protected and guided by the electrode conduit and liner, which help prevent buckling and maintain an uninterrupted wire feed. The gas nozzle is used to evenly direct the shielding gas into the welding zone.

  • 2.

    Work piece: It can be a metal or alloy

  • 3.

    Power supply: Most applications of GMAW use a constant voltage power supply. As a result, any change in arc length results in a large change in heat input and current. This helps operators keep the arc length consistent even when manually welding with hand-held welding guns. Alternating current is rarely used with GMAW, instead, direct current is employed and the electrode is positively charged. This results in faster melting of the feed wire, which increases weld penetration and welding speed.

  • 4.

    Wire feed unit: It supplies the wire electrode to the work, driving it through the conduit and onto the contact tip. Most models provide the wire at a constant feed rate but more advanced machines can vary feed rate in response to the arc length and voltage as high as 30 m/min.

  • 5.

    Electrode source: It contains the electrode and the selection of electrode is based primarily on the composition of the metal being welded, but also on the process variation, joint design, and the material surface conditions. All commercially available electrodes contain deoxidizing metals such as silicon, manganese, titanium, and aluminum in small percentages to help prevent porosity.

  • 6.

    Shielding gas: the shielding gases are necessary to protect the welding area from atmospheric gases such as oxygen and nitrogen, which can cause fusion defects, porosity, weld metal embrittlement, etc. the choice of a shielding gas depends on several factors, most importantly the type of material being welded, and the process variation being used. Pure inert gases such as argon and helium are only used for nonferrous welding. Pure carbon dioxide, on the other hand, allows for deep penetration welds but encourages oxide formation, which adversely affect the mechanical properties of the weld. Low cost makes it an attractive choice, but because of the violence of the arc, spatter is unavoidable and welding thin materials is difficult. As a result, argon and carbon dioxide are frequently mixed in a 75%/25% to 90%/10% mixture. Shielding gas mixtures of three or more gases are also available to improve weld quality (e.g. mixtures of argon, carbon dioxide, and oxygen)