Welding Thin Sheet metal- Why it is important?
Welding of sheet gauge looks simple but it require lot of skills and perfection to produce good looking welds. The welding problems of warpage due to heat, burn through, high melting of base metal, control of heat input are always a concern. To produce a strong weld, atleast stronger than the sheet is very important when welding thin sheet gauge materials. Gauge sheets are available in mild steel, carbon steel, stainless steel and aluminum. They are mostly zinc galvanized when made from mild steel or carbon steel to prevent corrosion.
People use common term ”Gauge” for sheet metal worldwide and many people are not aware what is Gauge meaning. For example, what is mean by 20 gauge sheet steel? So, before we dig deep into welding guidelines, first let us understand the gauging system used for sheet metal.
What is mean by Sheet metal Gauge?
Sheet metal are identified by gauge for example 10 gauge, 12 gauge, 15 gauge or so on. Gauge number are unique number given to tell the thickness of the sheet metal. Gauge chart, as shown in the below picture for carbon steel are used to find out the sheet thickness for a given gauge number. For example here, Gauge Number 7 has sheet thickness of 0.1793 inch or 4.5 mm. So, Gauge Number 7 in realty has no direct relationship with the sheet thickness rather than a unique numbering system used around the world.
Welding processes for Sheet Metal Welding
Various welding processes are used for sheet metal welding, both fusion welding processes as well as resistance welding processes. Fusion welding processes offers portability, can weld out of shape parts, big sizes components while resistance welding processes are limited to smaller parts and to shop welding only as the resistance welding equipment’s are bulky and require quit a lot of equipment setup.
The following are the main welding processes used for Sheet Metal welding:
- Stick Welding or SMAW
- Gas Tungsten Arc Welding (GTAW or TIG)
- GMAW (MIG-MAG Welding)
- Spot Welding
- Seam welding
- Resistance welding
MIG-MAG welding of thin sheets (22 gauge to 7 gauge)
MIG Welding or GMAW are the most common welding technique used for welding of thin sheet metals. MIG/ MAG process is widely used in automobiles as well as structural welding of gauge sheets. MIG welding offers good usability to weld thin sheets as well as thick sheets with same welding machine by varying the welding parameters.
The 0.023, 0.030, 0.035, and 0.045 diameter GMAW Filler wire are the suggested for the short-circuiting GMAW-S mode. For carbon steel use carbon dioxide (CO2) or a combination of 25% CO2 and 75% argon (Ar) for Shielding gas. The amperage range may be as low as 35 for materials of 24 gauge or as high as 225 for materials up to 1/8 in. (3 mm)
in thickness on square groove weld joints.
GMAW-S gives low heat and make it easy to use in all positions on sheet metal, low carbon steel, low alloy steel, and stainless steel sheet metal from 25 gauge (0.02 inch or 0.5 mm) to 12 gauge (0.1 inch or 2.6 mm).
The most important thing when welding thin sheet with MIG is to use the smallest diameter wire as this will give the minimum heat input for welding. Also, small diameters runs quite well with short-circuit mode, which is very good for thin sheets welding. It is good to use 0.023- or 0.025-inch diameter filler wire with MIG welding. As a rule of thumb, use a wire diameter that is thinner than base metal. Typical welding parameters for MIG welding using 0.025 inch (0.6 mm) and 0.030 inch (0.8 mm) wire are given in below table. When welding 20 gauge sheet, use welding current around 30-35 ampere with a gas flow rate of 15-20 CFh.
The shielding gas mixture of Ar+Co2 (80% argon to 20% carbon dioxide) shall be used. Pure argon will give deep penetration and is good for 7 gauge to 12 gauge sheets although mixture of argon + CO2 is also good.
Another option using MIG welding for thin sheet is to use Pulsed welding power source or pulse welding machines. The pulse welding offer welding with minimal heat input, less spatters, good control on welding arc. Pulse welding can be very beneficial for thin sheet welding. Pulse transfer mode gives very less distortion with very low heat input which is required for sheet metal welding. But the pulse welding machines are costlier than regular mig welding machine, so you need to put more money for buying the pulse power source.
When welding stainless steel sheet metal (Such as 304L or 316L), you will need different shielding gases either usually pure Argon or Helium or a mixture of both. The welding wire used is ER308L or ER308LSi for welding of 304L sheet metals. Always use the smallest diameter available for the filler wires.
Stick welding of SMAW welding for sheet metal welding
Material as thin as 16 gauge, approximately 1/16 in. (2 mm) thick up to several feet thick, can be Stick welded. Stick welding is one of the option to perform sheet metal welding especially in field welding applications. Stick welding of sheet metal shall be carried out using AWS A5.1/A5.1M E6013 (CWB/CSA W48-06 E4313) type electrode of diameter 5/64 inch or 2.0 mm. E6013 types electrode offer very good arc stability, easy to start arc, shallow penetration and less power in arc compare to E6010 or E7018 type rods. E6013 can be used with AC, DCEP and DCEN polarity.
Use 50 ampere current when welding with E6013 dia 5/64 inch. For smaller gauge sheets such as 20 or 22 lower the ampere before weld. Welding shall not be carried out continuously but rather by giving frequent stops to minimize the heat input and allow the material to cool. Recommended welding parameters are show in the below table using E6013 for sheet metal welding.
TIG welding for sheet metal welding
Welding sheet metal with TIG welding is very good when welding without filler wire, also called autogenous TIG Welding. TIG without filler requires less welding current compare to welding with filler and offers easy to join thin sheet metals easily. Pure argon for shielding is required when welding thin sheets with TIG, doesn’t matter either carbon steel or stainless steel or aluminum.
When welding stainless steel, gas purging is mandatory to avoid oxidation of the root side of the weld. Use pure argon gas for purging gas.
Use ceriated tungsten electrode instead of thoriated one. Tungsten electrode diameters up to 0.020, 0.040, and 1/16 inch are most commonly used for thin sheet welding. The smaller the electrodes, it will be easier to start the arc and also lower heat input with improved weld appearance. This prevents burnout and allows the arc to be concentrated in a smaller area which is good for fusion as well as low heat input.
FCAW Welding of Sheet metal
Flux cored arc welding is not much popular for sheet metal welding as it will gives high heat input and results in more distortion. If still FCAW is used for thin sheet welding, use a smaller diameter wire with low welding current. Use argon + Co2 gas mixture for gas shielding or use self shielded cored wire.
Carry the welding with short circuit mode with regular stop and not using long weld runs. Clean the material before use to avoid welding porosity issue with thin sheet welding along with properly stored welding wire. Both FCAW-S and FCAW-G use DCEN when welding on thin-gauge materials to keep the heat in the base metal and the small diameter electrode at a controllable burn-off rate.
Sheet metal welding technology
Use shorter arc
Because shorter arcs produce less heat and penetration, they are best suited for use on thin metal or thin-to-thick metal joints. Using this technique, metal as thin as 16 gauge can be arc welded easily. Shorter arc gives better control on weld puddle as well as produce narrow welds compare to broader welds produced by long arc.
Apply skip welding technique
Skip welding technique involves welding using welds apart from each other at a defined distance as shown in the below picture. Skip weld help to minimize the distortion and prevent accumulation of weld heat during welding. Skip welding is similar to the intermittent welds or stitch welds that help to distribute the welding heat evenly.
Use Backing bars
Use of backing or chill bar allow the faster heat dissipation during welding thus allowing distortion control. Materials having high thermal conductivity are used as backing bar such as copper which dissipate the heat from the welding area to the backing bar. Backing bars are placed under the weld joint and as copper do not fuse with carbon steel, copper backing bar are very famous in welding applications.
Use restrains and clamps
Using clamps or restrains during thin sheet welding help to prevent distortion. Clamps holds the part until it cool down to room temperature thus prevent ay warpage. Various types of clamps are easily available in the market for these applications.
Tips for welding thin sheet metals
- Find the lowest diameter welding electrode or filler wire.
- Use the minimum possible welding current and voltage.
- Use clamps and restrains to hold the parts.
- Never weld continuous welds in sheet metals.
- Always use low heat input during welding.
- Use short circuit transfer mode when welding GMAW or FCAW.