Done right, welding creates long-lasting joints that are as strong or stronger than the parent metals. Pick the wrong welding process though, and the joint will be brittle and prone to cracking, and will most likely fail soon after the part is put into service.

This blog discusses what to consider when choosing a welding process. Before going through the selection criteria, it provides a brief description of the most widely used processes. But to provide some context, let’s start by discussing why the choice of welding process matters.

The Challenges of Welding

Almost all welding processes involve heating pieces of metal until they melt and flow together. Usually, a third metal – the filler – is added to increase the volume of metal available for fusion. How much heat, how it’s applied, and whether the process allows any contamination, determine what happens as the metal subsequently cools and solidifies.

The two biggest issues to address are:

  • Metal properties: Different types of metal have different melting points, thermal conductivity, and expansion coefficients. Heat a metal with low conductivity and a weld pool might form quickly, while if the conductivity is high the heat flows away from where it’s needed and perhaps causes a lot of expansion.
  • Preventing contamination: The biggest problems are porosity and embrittlement, caused by oxygen and hydrogen in the weld. These are present in the air and moisture, so joints must be dry before welding, and ideally, all air excluded from the weld region. Some welding processes take care of this, others don’t.

The consequence of these variables is that the weld process must be compatible with the material being welded. If it isn’t, the pieces being joined will distort and the joint may crack as it cools. It’s also possible to burn through the pieces, especially if they are thin, so there will be nothing to weld.

Common Welding Processes

Here’s a list of the welding processes you can expect to see in most metal fabrication shops:

  • Gas Metal Arc Welding (GMAW): This splits into Metal inert Gas (MiG) and Tungsten inert Gas (TiG), plus some less widely used variants. Both MiG and TiG work by creating an electric arc between an electrode in the welding torch and the workpiece. The arc is shielded by an inert gas, or a blend of inert gases, to exclude contaminants from the weld pool and keep the arc stable. In MiG, the electrode is consumed to provide filler metal while TiG requires an additional filler wire.
  • Resistance/spot welding: Passing an electric current through a metal and resistance makes it heat up. Resistance welding uses this effect to create small weld nuggets between overlapping pieces of sheet metal. (Note that this process does not form continuous, watertight welds.)
  • Laser welding: Lasers create heat with light. For a long time CO2 lasers were used for welding, but today they’ve been largely superseded by fiber lasers. These are much smaller and deliver their energy to where it’s needed through a fiber optic system. One consideration with lasers is to match the wavelength of the light to the absorption characteristics of the metal being welded, (or at least avoid high reflectivity.)
  • Oxy-acetylene/gas welding: In this welding process oxygen and acetylene gas are mixed and burnt, with the gas velocity producing a high-temperature jet. The temperature is lower than in an electric arc though, (6,000°F versus 10,000°F), so it tends to heat more slowly.

Other welding processes used in specialized applications include friction welding, friction stir welding, and submerged arc welding. You might see these used with particular materials or in exotic applications, like spacecraft, but they are not mainstream metal fabrication welding processes.

Welding Process Selection

Choosing a compatible welding process starts by analyzing the joint being welded. The factors to assess are:

  • Material type: Most metals routinely used in metal fabrication can be welded. However, cast iron, aluminum, and magnesium pose challenges that require careful preparation and process optimization to join. Some alloys weld better with some processes than others. For example, welds in stainless steel and titanium are more successful with TiG. Dissimilar metals can be welded, but this needs a filler metal compatible with the parent pieces.
  • Material thickness: It can be hard to achieve full penetration in a thick material and careful preparation is essential. Conversely, with thin sheet material, there is a risk of applying too much heat and burning through. TiG is preferred for the precision it provides, although oxy-acetylene is often an option too. Welding is easier when the pieces being joined are of similar thickness as the heat flows are easier to manage.
  • Access: Does the design allow space for a welding torch to be brought in close? Are the materials being overlapped or butted together? (Overlapped sheet might be the best resistance welded.)
  • Orientation: Will the weld be made with the joint below the torch and horizontal? If not, it may be difficult to maintain the weld pool. A positioning table may be needed to put the workpiece in a convenient orientation.
  • Appearance: If the weld will be on show, as is the case for some architectural or structural fabrications, TiG might be preferred because it produces cleaner, more attractive welds.
  • Speed: Time is money in fabrication, so it may be better to accept a less attractive finish but get the welds made faster. This would mean using MiG rather than TiG.
  • Welder skill: All welding requires skill, but some forms are harder to master than others. Making quality TiG welds needs more practice than doing the same with MiG. In addition, to meet design or service standards, some types of metal fabrication may need a certified welder.

Welder Skill and Certification

One of the biggest challenges in welding is to verify the integrity of the completed weld. Visual inspection can only find surface defects and sectioning is usually needed to establish penetration depth and degree of fusion achieved.

Sectioning is of course, a destructive inspection useful for showing whether weld process parameters were set correctly, but is impractical on large or high-value fabrications. Consequently, the best form of quality assurance in welding is to have it done by welders whose training and expertise have been independently verified. This is the purpose of certification programs like those operated by the American Welding Society (AWS).

In addition, some types of fabrication are governed by regulations addressing welder certification. Pressure vessels, aerospace, and marine applications all have specific requirements.

Partner With Fabrication Experts

Many metal fabrication projects require welding. Done right, it joins metal pieces in a durable manner that ideally improves aesthetics too. Choose the wrong welding process though and the result will probably be a weld that’s ugly and unsafe.

The information given here should help with choosing an appropriate welding process for your fabrication. It may also help with design decisions relating to the welding needed. However, for best results, we recommend discussing your project with an experienced metal fabrication specialist. At Wiley, some of our team members have decades of fabrication experience and will be happy to help you understand your options. Contact us to set that in motion.