Often in the supply industry we are asked what type of wire should be used for certain jobs. In general, terms the only requirement is the chemical make-up, which results in an equal to or higher tensile strength than the weaker of the base materials and the correct deposition materials.
Some of us have used a Stainless steel electrode to weld mild steels before. This is not a recommended case since the chemical compositions do not mesh. The chromium from the stainless readily bonds with the carbon of the mild steel. This reaction is referred to as innergrannular corrosion. Innergrannular corrosion is where the boundaries of crystallites of the material are more susceptible to corrosion than their insides.
Innergrannualar corrosion can cause cracking, which will affect heat-affected zone (where the chromium meets the carbon for an extended period of time). The chromium in a stainless steel usually forms with an oxide; this is what keeps the iron-oxide or rust from forming. This is exactly why austenitic stainless steels (most 3xx series) are welded with an 3xxL electrode. The “L” stands for less than .08% carbon, there are also stabilized grades that include alloys such as titanium, columbium, or tantalum.
Image Source: Met Tech
When welding stainless steels the electrode selection process is simple, the most common electrodes are 308L, 308LSi, 309L, 316L, and 409L. 309L is specifically designed to weld dissimilar materials such as an AISI 304L to an AISI 1018. When welding dissimilar stainless steels you should reference a code, manufacturer, or supplier. It gets very complex with the ferrite number of stainless steels to get the correct weld deposit composition.
When welding aluminum alloys the most important thing to keep in mind is the heat treatable alloys. Heat treatable alloys are 2xxx, 6xxx, 7xxx, and a few 4xxx series. If the alloy is heat treatable then the filler metal must be heat treatable to the same level that produces a sound base material, heat affected zone, and weld deposit.
For example, after welding a 6xxx series aluminum alloy such as a 6061 regardless of the filler material 4xxx, 5xxx,etc., the base material loses 40-50% of its strength. This material must be heat-treated and either age hardened or precipitation hardened to regain its strength. Since a 5xxx series is not heat treatable it would most likely not be the suitable electrode choice for a 6xxx series. Also, keep in mind quenching an aluminum alloy does not cause it to harden instantly like carbon steels, actually in most cases it will soften. Heat treatments such as precipitation hardening, age hardening, and artificial age hardening are where aluminum gains its strength.
Image Source: Fabricating and Metal Working
Most welding codes and all specifications (WPS) include the electrode specification. These are the best guidelines to go by when selecting a filler material. Since someone has already done all the testing to prove the reliability.
Image Source: Civil Engineering
When it comes to preference, in most cases it comes down to welding position. When welding in the flat or horizontal position, a free flowing electrode, such as an s-6 or a metal core will produce the most visually appealing welds. Most metalcore electrodes can’t be used out of the flat or horizontal position. When welding out of position, vertical up or overhead, a stiffer electrode such as an s-2 or s-3 will give you more control over the weld pool. The “s” stands for silicon content the higher the number the more silicon there is in the electrode. Silicon does mainly two things, it stabilizes the arc and wets out. This results in less spatter and the ability to weld through thicker mill scales.
Image Source: Wordpress
The last variable and usually the determining variable is the shielding gas you have available. When welding mild steels a 90/10 (90%Argon/10%CO2) can be used for any GMAW electrode. If you are limited to 100% CO2 then solid wires will be your only option. A 75/25 (75% Argon/ 25% CO2) can only be used with some metalcores, and all solid wires. If you have any questions you can always contact your local supply house or the manufacturer of the electrode.
If you are a beginner welder, or just looking to brush up on some of the best electrodes for GMAW/Arc Welding, feel free to check out our website Baker's Gas for more tips on The Best Electrode for the Beginner Arc Welder.