9411 Gas Shielded Arc Welding

Before World War II, shielded metal arc welding (SMAW) using a flux-coated electrode was one of the few ways aluminum could be welded. This process, however, was inefficient and often produced poor welds. In the 1940s, inert gas-shielded arc welding processes were developed that used argon and helium instead of flux to remove the oxide and quickly became more popular. Other methods of welding aluminum are used (and will be discussed below), but today most aluminum welding is by the gas-shielded arc processes.

There are two gas-shielded arc methods: gas metal arc welding (GMAW), also called metal inert gas welding, or MIG, and gas tungsten metal arc welding (GTAW), also called tungsten inert gas welding, or TIG. MIG welding uses an electric arc between the base metal being welded and an electrode filler wire. The electrode wire is pulled from a spool by a wire-feed mechanism and delivered to the arc through a gun. In TIG welding, the base metal and, if used, the filler metal are melted by an arc between the base metal and a nonconsumable tungsten electrode in a holder.

Tungsten is used because it has the highest melting point of any metal [6170°F (3410°C)] and reasonably good conductivity, about one-third that of copper. In each case, the inert gas removes the oxide from the aluminum surface and protects the molten metal from oxidation, allowing coalescence of the base and filler metals.

Tungsten inert gas welding was developed before MIG welding and was originally used for all metal thicknesses. Today, however, TIG is usually limited to material \ in. (6 mm) thick or less. TIG welding is slower and does not penetrate as well as MIG welding. In MIG welding, the electrode wire speed is controlled by the welding machine and once adjusted to a particular welding procedure does not require readjustment, so even manual MIG welding is considered to be semiautomatic. MIG welding is suitable for all aluminum material thicknesses.

The weldability of wrought alloys depends primarily on the alloying elements, discussed below for the various alloy series:

1xxx Pure aluminum has a narrower melting range than alloyed aluminum. This can cause a lack of fusion when welding, but generally the 1xxx alloys are very weldable. The strength of pure aluminum is low, and welding decreases the strength effect of any strain hardening, so welded applications of the 1xxx series are used mostly for their corrosion resistance. 2xxx The 2xxx alloys are usually considered poor for arc welding, being sensitive to hot cracking, and their use in the aircraft typically has not required welding. However, alloy 2219 is readily weldable, and 2014 is welded in certain applications.

3xxx The 3xxx alloys are readily weldable, but have low strength and so are not used in structural applications unless their corrosion resistance is needed. 5xxx The 5xxx alloys retain high strengths even when welded and are free from hot cracking and are very popular in welded plate structures such as ship hulls and storage vessels.

6xxx The 6xxx alloys can be prone to hot cracking if improperly designed and lose a significant amount of strength due to the heat of welding, but are successfully welded in many applications. Postweld heat treatments can be applied to increase the strength of 6xxx weldments. The 6xxx series alloys (like 6061 and 6063) are often extruded and combined with the sheet and plate products of the 5xxx series in weldments. 7xxx The low copper content alloys (such as 7004, 7005, and 7039) of this series are weldable; the others are not, losing considerable strength and suffering hot cracking when welded.

Some cast alloys are readily welded and some are postweld heat-treated because they are usually small enough to be easily placed in a furnace. The condition of the cast surface is key to the weldability of castings; grinding and machining are often needed to remove contaminants prior to welding. The weldability of the 355.0, 356.0, 357.0, 443.0, and A444.0 alloys is considered excellent.

Filler alloys can be selected based on different criteria, including: resistance to hot cracking, strength, ductility, corrosion resistance, elevated temperature performance, MIG electrode wire feedability, and color match for anodizing. Recommended selections are given in Table 9.28, and a discussion of some fillers is given below. Material specifications for these fillers are given in AWS A5.10, Specification for Bare Aluminum and Aluminum Alloy Welding Electrodes and Rods [19]. There is no ASTM specification for aluminum weld filler.

Filler alloys 5356, 5183, and 5556 were developed to weld the 5xxx series alloys, but they have also become useful for welding 6xxx and 7xxx alloys. Alloy 5356 is the most commonly used filler due to its good strength, compatibility with many base metals, and good MIG electrode wire feedability. Alloy 5356 also is used to weld 6xxx series alloys because it provides a better color match with the base metal than 4043 when anodized. Alloy 5183 has slightly higher strength than 5356, and 5556 higher still. Because these alloys contain more than 3% magnesium and are not heat treatable, however, they are not suitable for elevated temperature service or postweld heat-treating. Alloy 5554 was developed to weld alloy 5454, which contains less than 3% magnesium so as to be suitable for service over 150°F(66°C).

Alloy 5654 was developed as a high-purity, corrosion-resistant alloy for welding 5652, 5154, and 5254 components used for hydrogen peroxide service. Its magnesium content exceeds 3% so it is not used at elevated temperatures.

Alloy 4043 was developed for welding the heat-treatable alloys, especially those of the 6xxx series. Its has a lower melting point than the 5xxx fillers and so flows better and is less sensitive to cracking. Alloy 4643 is for welding 6xxx base metal parts over 0.375 in. (10 mm) to 0.5 in. (13 mm) thick that will be heat treated after welding. Alloys 4047 and 4145 have low melting points and were developed for brazing but are also used for some welds; 4145 is used for welding 2xxx alloys and 4047 is used instead of 4043 in some instances to minimize hot cracking and increase fillet weld strengths.

Alloy 2319 is used for welding 2219; it's heat treatable and has higher strength and ductility than 4043 when used to weld 2xxx alloys that are postweld heat treated.

Pure aluminum alloy fillers are often needed in electrical or chemical industry applications for conductivity or corrosion resistance. Alloy 1100 is usually satisfactory, but for even better corrosion resistance (due to its lower copper level), 1188 may be used. These alloys are soft and sometimes have difficulty when fed through MIG conduit.

The filler alloys used to weld castings are castings themselves (C355.0, A356.0, and A357.0), usually a |-in. (6-mm) rod used for TIG welding. They are mainly used to repair casting defects. More recently, wrought versions of C355.0 (4009), A356.0 (4010), and A357.0 (4011) have been produced so that they can be produced as MIG electrode wire. (Alloy 4011 is only available as rod for GTAW, however, since its beryllium content produces fumes too dangerous for MIG welding.) Like 4643,4010 can be used for postweld heat-treated 6xxx weldments.

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