16-02-2013, 09:28 AM
Study Gas Metal Arc Welding Process and Microstructural Analysis of Mild Steel
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Introduction
Gas Metal Arc Welding (GMAW) is an arc welding process that joins metals together by heating them with an electric arc that is established between a consumable electrode (wire) and the work-piece. Shielding of the arc and the molten weld pool is often obtained by using inert gases such as argon and helium, and this is why GMAW is also called the metal–inert gas (MIG) welding process. Since non-inert gases, particularly CO2, are also used, GMAW seems a more appropriate name. [1][3]
Although the basic GMAW concept was introduced in the 1920s, it was not commercially available until 1948. At first, it was considered to be fundamentally a high-current-density, small-diameter, bare-metal electrode process using an inert gas for arc shielding. Its primary application was aluminum welding. As a result, it became known as metal-inert gas (MIG) welding, which is still common nomenclature. The GMAW process can be operated in semi-automatic and automatic modes. All commercially important metals, such as carbon steel, high-strength low-alloy steel, stainless steel, aluminum, copper, and nickel alloys can be welded in all positions by this process if appropriate shielding gases, electrodes, and welding parameters are chosen. [1]
Modes of Metal Transfer in Gas metal arc welding
The molten metal at the electrode tip can be transferred to the weld pool by three basic transfer modes: globular, spray, and short-circuiting.
A. Globular Transfer
Discrete metal drops close to or larger than the electrode diameter travel across the arc gap under the influence of gravity. Figure 2 shows globular transfer during GMAW of steel at 180A and with Ar–2% O2 shielding. Globular transfer often is not smooth and produces spatter. At relatively low welding current globular transfer occurs regardless of the type of the shielding gas. With CO2 and He, however, it occurs at all usable welding currents. As already mentioned, a short buried arc is used in CO2-shielded GMAW of carbon and low-alloy steels to minimize spatter. [2] [4]
B. Spray Transfer
Above a critical current level, small discrete metal drops travel across the arc gap under the influence of the electromagnetic force at much higher frequency and speed than in the globular mode. Figure 3 shows spray transfer during GMAW of steel at 320A and with Ar–2% O2 shielding. Metal transfer is much more stable and spatters free. The critical current level depends on the material and size of the electrode and the composition of the shielding gas. [2] [4]
C. Short-Circuiting Transfer
The molten metal at the electrode tip is transferred from the electrode to the weld pool when it touches the pool surface, that is, when short circuiting occurs. Short-circuiting transfer encompasses the lowest range of welding currents and electrode diameters. It produces a small and fast-freezing weld pool that is desirable for welding thin sections, out-of position welding (such as overhead-position welding), and bridging large root openings.[2] [4]
Heat flow in welding:
Heat flow during welding strongly affect phase transformations during welding and thus the resultant microstructure and properties of the weld. It is also responsible for weld residual stresses and distortion.
SOLIDIFICATION MODES AND CONSTITUTIONAL SUPERCOOLING
During the solidification of a pure metal the S/L interface is usually planar, unless severe thermal
undercooling is imposed. During the solidification of an alloy, however, the S/L interface and
hence the mode of solidification can be planar, cellular, or dendritic depending on the
solidification condition and the material system involved. In order to directly observe the S/L
interface during solidification, transparent organic materials that solidify like metals have been
used. Shown in Figure 6.8 are the four basic types of the S/L interface morphology observed
during the solidification of such transparent materials: planar, cellular, columnar dendritic, and
equiaxed dendritic [8,9]
NONNON EPITAXIAL GROWTH AT FUSION BOUNDARY
When welding with a filler metal (or joining two different materials), the weld metal composition is different from the base metal composition, and the weld metal crystal structure can differ from the base metal crystal structure. When this occurs, epitaxial growth is no longer possible and new grains will have to nucleate at the fusion boundary. [12]
COMPETITIVE GROWTH IN BULK FUSION ZONE
The grain structure near the fusion line of a weld is dominated either by epitaxial growth when the base metal and the weld metal have the same crystal structure or by nucleation of new grains when they have different crystal structures. Away from the fusion line, however, the grain structure is dominated by a different mechanism known as competitive growth. During weld metal solidification grains tend to grow in the direction perpendicular to pool boundary because this is the direction of the maximum temperature gradient and hence maximum heat extraction. However, columnar dendrites or cells within each grain tend to grow in the easy-growth direction. As we know the easy-growth directions in several materials and, as shown, it is <100> for both fcc and bcc materials. Therefore, during solidification grains with their easy-growth direction essentially perpendicular to the pool boundary will grow more easily and crowd out those less favorably oriented grains, as shown schematically in Figure 10. This mechanism of competitive growth dominates the grain structure of the bulk weld metal. [13]
Weld Metal Solidification II: Microstructure within Grains
The microstructure within the grains in the fusion zone, focusing on the solidification mode, dendrite spacing and cell spacing, how they vary across the weld metal, and how they are affected by welding parameters.
SOLIDIFICATION MODES
As constitutional super-cooling increases, the solidification mode changes from planar to cellular and from cellular to dendritic. Figure 11 shows schematically the effect of constitutional super-cooling on the microstructure within the grains in the weld metal. The solidification mode changes from planar to cellular, columnar dendritic, and equiaxed dendritic as the degree of constitutional super-cooling at the pool boundary increases. Heterogeneous nucleation aided by constitutional super-cooling promotes the formation of equiaxed grains in the weld metal [14,15,16].
Formation of the Partially Melted Zone
Severe liquation can occur in the partially melted zone during welding. Several fundamental liquation-related phenomena are discussed in this chapter, including liquation mechanisms, solidification of the grain boundary (GB) liquid, and the resultant GB segregation. The partially melted zone (PMZ) is the area immediately outside the weld metal where liquation can occur during welding. [17]
The Heat-Affected Zone
CARBON STEELS
According to the American Iron and Steel Institute (AISI), carbon steels may contain up to 1.65wt% Mn, 0.60wt% Si, and 0.60wt% Cu in addition to much smaller amounts of other elements. This definition includes the Fe–C steels of the 10XX grades (up to about 0.9% Mn) and the Fe–C–Mn steels of the 15XX grades (up to about 1.7% Mn). The last two digits in the alloy designation number denote the nominal carbon content in weight percent, for instance, about 0.20% C in a 1020 and about 0.41% C in a 1541 steel. Manganese is an inexpensive alloying element that can be added to carbon steels to help increase hardenability.[17]
Low-Carbon Steels and mild steel
These steels, in fact, include both carbon steels with up to 0.15% carbon, called low-carbon steels, and those with 0.15–0.30% carbon, called mild steels. For the purpose of discussion 1018 steel, which has a nominal carbon content of 0.18%, is used as an example. Figure 13 shows the micrographs of a gas–tungsten arc weld of 1018 steel. The base metal consists of a light-etching ferrite and a dark-etching pearlite (position A). The HAZ microstructure can be divided into essentially three regions: partial grain-refining, grain-refining, and grain-coarsening regions (positions B–D).The peak temperatures at these positions are indicated in the phase diagram. The partial grain-refining region (position B) is subjected to a peak temperature just above the
effective lower critical temperature Ac1. The prior pearlite (P) colonies transform to austenite and expand slightly into the prior ferrite (F) colonies upon heating to above Ac1 and then decompose into extremely fine grains of pearlite and ferrite during cooling. The prior ferrite colonies are essentially unaffected. The grain-refining region (position C) is subjected to a peak temperature just above the effective upper critical temperature Ac3, thus allowing austenite grains to nucleate. Such austenite grains decompose into small pearlite and ferrite grains during subsequent cooling. The distribution of pearlite and ferrite is not exactly uniform because the diffusion time for carbon is limited under the high heating rate during welding and the resultant austenite is not homogeneous. The grain coarsening region (position D) is subjected to a peak temperature well above Ac3, thus allowing austenite grains to grow. The high cooling rate and large grain size encourage the ferrite to form side plates from the grain boundaries, called the Widmanstatten ferrite.