Laser Welding Metals: Troubleshooting 4 Common Issues

Every year, our application team fields hundreds of questions from customers who are experiencing challenges with their laser welding processes. We polled the team about the most commonly reported issues and the winners are:

• Gaps that cause welding defects
• Porosity caused by trapped gasses in the weld pool
• Cracking during and after welding
• Overheating during welding

In this post, we’ll discuss each of these issues and offer solutions that consider materials, equipment, and processes. They fall into two general categories: “gaps” usually result from an error in set-up, while the other three result in poor weld quality.

Set up issues: gaps

A gap between parts is the source of numerous welding issues including underfill, low strength, and porosity. To make a successful weld, the laser must heat both materials and fuse them together. Because the spot size can be very small – for fiber lasers it can be <100 microns – the two parts need to be really, really close to each other. In an ideal scenario, they are in intimate contact with zero gap. In reality, there is almost always some space between the parts but it must be smaller than the diameter of the laser beam (for butt and fillet welds) or less than the thickness of the thinnest part (for lap welds).

Ultimately, it comes down to this: the more gap that is present, the larger the void that needs to be filled by the welding process. What causes gaps? Poor part design, manufacturing tolerances, and fixturing.

When designing a complex part, it can be easy to overlook welding requirements in favor of design for ease of assembly. Or perhaps the part was designed for another technology where the diameter of the heating zone was much larger. Companies transitioning from TIG welding where the arc is much broader and filler material is used, often run into this problem because the designed spacing between parts may not be tight enough for laser welding.

Manufacturing tolerances can also be a major issue in the micro-welding world. Parts are usually designed for easy fit-up and assembly, but fiber lasers have a typical spot size of < 100 microns (0.004”). For successful laser welding (without further welding techniques), the gap between the parts needs to be smaller than this – approximately the width of a human hair.

Finally, consider the tooling. Does it hold the materials firmly in place? Or do parts shift during welding? Because laser welding is a non-contact process, the tooling must be precise and hold the two parts together in intimate contact.

So we’ve learned that there are several ways to address gaps:

• Ensure tight tolerances during the design and procurement phase
• Design the parts to use the best welding configuration for the parts purpose
• Tack welding the parts before welding by using a lighter energy to hold in place and prevent the contraction forces after welding to induce a gap

 Welding issues: porosity

Porosity is the appearance of voids (air bubbles) in the melt pool which are created by the laser welding process. This reduces weld strength, leading to increased risk of breakage or cracking under stress. It also reduces hermeticity and can affect appearance.

There are three main causes of porosity:

1. Improper gas coverage -an incorrect type, quantity, of flow of cover gas allows nitrogen, oxygen, and hydrogen to enter the molten weld pool
2. The presence of contaminants and/or material metallurgy: some materials are simply more likely to result in porous weld. Also, impure materials may cool unevenly or prematurely
3. Inadequate pre-cleaning: dirt and oil may block access to the pure material, introducing gas into the weld, and causing gaps. Contaminants can react with the weld metal and form oxides or other compounds.

Proper gas coverage is high on the list of solutions for porosity issues. Choose a gas mixture that matches your material and provides adequate protection from atmospheric contaminants. Check your gas flow rate, pressure, and hose connections for any leaks or blockages; avoid welding in drafty conditions that could disrupt your shielding gas coverage and make sure the gas flow is laminar rather than turbulent.

The material weldability chart below provides some guidance on the common welding materials and which are known good combinations. Remember that plating can also mix into the melt pool and create porosity, so be sure to use known good combinations that limit that effect. Also, be sure to contact your vendors and test new incoming materials versus a known good reference. It can occur that a new vendor may provide a slightly different mixture and create issues!

Finally, be sure to pre-clean your parts before welding to remove any contamination. Use a wire brush, grinder, solvent or degreaser to remove any oil, grease, rust, moisture or dirt from the surfaces to be welded. The wire brush can also be used to remove the aluminum oxide layer – which is a particular challenge for laser welding. In some cases, it may be advantageous to use a laser to clean your parts. Find more information on laser cleaning here.

Welding issues: cracking

There are a couple of sources for cracking, i.e. where the two materials separate during or after the welding process.

Cracking can happen when the melt pool is insufficient to fill the space between the solidifying materials creating a stress fracture. This can occur for the entire weld nugget or manifest itself in smaller micro-cracks. Consequences include reduced strength, hermeticity and conductivity as the cracks break the electrical flow path.

Cracking can also be caused by the metallurgy itself, since some materials are more susceptible to cracking, including those high in carbon or hydrogen. High carbon steels, like 440C, for example, are susceptible to cracking with pulsed welding. CW fiber lasers can help control heating/cooling to avoid cracking.

Another example of material dependent cracking is aluminum from the 6000 series. This has been a traditionally difficult alloy to weld, due to the lower silicon content. It is often paired with another aluminum alloy to provide the right balance of materials to make a successful, hermetic weld.

Finally, cracking can be the result of incorrect weld energy/input, which can cause excess stress in the joint, leading to weaker and less stable welds. Careful development of the weld and definition of the weld window will help ensure successful welds.

Welding issues: overheating

Overheating happens when the part absorbs too much laser energy, creating a too wide or too deep weld. It can create a larger than normal heat affected zone, which can cause burns on the surface of the part and/or over penetration and damage surrounding components. Overheating reduces weld strength, leading to unstable welds and the risk of breakage. It also results in welds that are not aesthetically appealing, and the weld shape may be different than intended.

Overheating is typically caused by improper gas coverage; unbalanced excessive heat; or the use of a weld process that generates too much energy for the material in the first weld.

One solution  is ensuring proper gas coverage. Check that cover gas is consistently applied in a laminar, rather than turbulent flow, and be certain that the type of gas matches the material. A second solution is heat sinking or laser beam biasing. If the parts are close in size, biasing the laser beam will help balance the heat for each. Finally, consider controlling the weld process by using pulsing. Upslope slowly introduces heat into the material, while downslope controls the cooling at the end of the weld.

Summing it all up

Here is a quick overview of the common laser defect issues causes and solutions.

For more information on these issues and solutions, watch the webinar.