Nickel alloys, known for their exceptional corrosion resistance and high-temperature stability, play a vital role in various applications. The complexity and sensitivity of these alloys demand a careful approach to welding to avoid costly mistakes. In this comprehensive guide, we’ll delve into the challenges of welding nickel alloys, techniques to prevent issues like porosity and cracking, an overview of different nickel alloys, pre-weld cleaning procedures, filler metal selection, and insights into the best welding processes.

Challenges of Welding Nickel Alloys:

Nickel alloys are prone to porosity, cracking, contamination, and oxide inclusions. If the weld fails, it’s almost always necessary to mechanically remove all weld metal and re-do everything again. So, mistakes when welding nickel alloys can cause setbacks for any welding business. 

Porosity:

Like mild steel, nickel alloys can develop porosity if the oxygen and nitrogen from the atmosphere are trapped in the weld. However, you must also worry about the surface hydrogen presence with nickel alloys. Therefore, it’s not a bad idea to slightly preheat the welded part if you suspect any moisture has condensed on the metal’s surface. 

But you can actually use up to 10% hydrogen in your shielding gas to reduce the chances of hydrogen porosity. The bubbles of the hydrogen will gather up the diffused hydrogen from the metal. However, if you decide to experiment with a hydrogen shielding gas, don’t exceed the 10% content.

You must use adequate shielding when MIG or TIG welding the nickel alloys. In addition it’s helpful to use a larger shielding cup size when TIG welding to improve shielding gas coverage. As little as 0.025% nitrogen in the weld pool will cause porosity. So, maximum shielding gas coverage is vital. Ensure that the work environment doesn’t have any wind drafts. In addition, gas purging is crucial when making critical welds.

Surface Oxide Inclusions:

Nickel alloys often contain aluminium, titanium, chromium, and other elements in small quantities but large enough to produce surface oxides. These oxides melt at a higher temperature than the base metal underneath. So, unless you remove these oxides before welding, they’ll get stuck in the weld puddle and make the joint quite weak. Think of them as hard pieces of metal that were just kind of laid inside the weld pool without actually melting. Such inclusions represent the hard spots that can easily lead to weld cracks.

Nickel alloy surface oxides should be removed with abrasive grinding or machining. Using a stainless steel brush will likely not suffice — you’ll just polish up the oxides on the surface instead of removing them.

If you are making a multipass weld, don’t forget to remove the surface oxides between each pass. Surface oxides produced at high temperatures are even more detrimental, so each welding pass should be cleaned with an angle grinder. This is even more important to keep in mind if repairing a nickel alloy part that was exposed to high heat during its service life. For example, jet turbine engine parts can have oxides that must be removed before welding.

Be careful of using compressed air when grinding between passes to clean the beads. The compressed air can contain moisture, which brings us back to the issue of hydrogen presence. It’s best to avoid using any compressed air altogether.

Weld Puddle Sluggishness:

Recognizing the unique characteristics of nickel alloy weld puddles is crucial.Unlike stainless steel or mild steel, nickel alloys’ puddle doesn’t flow freely. It feels like a sluggish goo that’s more challenging to navigate into the joint. As a result, you can easily create an overly convex weld bead with a poor toe blend into the surrounding metal. The sluggish nature requires a V-butt configuration to guide the puddle efficiently into the joint, ensuring a smooth blend with the surrounding metal.

Cracking:

Nickel alloys are prone to various types of weld and heat affected zone (HAZ) cracking, but the most prominent cracking issue results from weld pool contamination. The worst element to introduce to nickel alloys is sulphur, but phosphorus, lead, zinc, cadmium, tin, boron, silver, bismuth, and other elements with a low melting point can cause hot cracking. 

Many oils used for cutting and machining contain sulphur, and other shop processes and tools can introduce metals like lead or zinc. So, be extra careful of cross-contamination because nickel alloys will become brittle if the weld pool is contaminated.

Types of Nickel Alloys:

There is a huge number of nickel alloys, and each has some unique characteristics, but many share similar properties. 

The most widely welded nickel alloy is the pure nickel 200 and nickel 201. They are both corrosion-resistant, but nickel 201 has better resistance to carbon precipitation when used at temperatures higher than 315°C (600°F)

Most of what we are about to discuss below can be applied to various nickel alloys. But, you should always consult industry codes when welding anything critical.

Pre-cleaning Nickel Alloys Before Welding:

Cleaning nickel alloys from contaminants such as sulphur and lead is crucial before welding. Removal of oils, paints, cutting fluids, crayon markings, polishing substances, grime, dirt, and cross-contamination from other metals in the shop. It’s imperative to avoid cross-contaminating via grinding belts, hammers, cutting tools, and abrasives. Even a dirty rag can contaminate the weld and cause post-weld cracking to occur. Clean the oils and paints by using degreasing solvents and alkaline cleaners. Then, remove the surface oxides we discussed earlier using the carbide deburring tools because they are most likely to completely remove the oxides.

Filler Metal Selection:

Choosing filler metals close to the chemical composition of the nickel alloy is recommended. However, overmatching for improved corrosion resistance is also an option. 

The table below describes the most commonly used bare electrode filler metals for welding nickel alloys:

Welding Process Selection:

While all arc welding processes can be used, but we highly recommend sticking to TIG and MIG welding to achieve the best results. Furthermore, we specifically recommend using a TIG welding process because it provides the best puddle control and weld aesthetics. However, MIG welding is more efficient for thicker sections, with options like short-circuit arc transfer, spray transfer, and pulsed MIG welding.

MIG Welding Nickel Alloys

MIG welding nickel alloys is more efficient than TIG welding because it’s easier, faster, and allows you to weld thicker sections. However, MIG welds are less likely to look aesthetically pleasing, unlike TIG welds. 

Short-Circuit Arc Transfer

It’s most efficient to use a standard short-circuit arc transfer when MIG welding thin sheets of nickel alloys. Short-circuit arc won’t input too much heat, and you may avoid excessive distortion or burn-through. However, there will be more spatter, and your weld beads may be more convex. This  requires post-weld grinding and can prolong your work hours. In addition, welding thick nickel alloys will likely suffer from a lack of fusion when using short-circuit transfer.

Spray Transfer

The spray transfer provides a much higher deposition rate and heat input, so you shouldn’t set your MIG welder for spray transfer when welding anything under 0.25 inches thick. You’ll get a more stable arc and less spatter, but the spray transfer works best for thicker joints.

Pulsed MIG Welding

The pulsed MIG welding offers the best of both worlds — it provides more power, but its not likely to burn-through the thin materials. A pulsed MIG welder, like our MAC AFRICA Digital IGBT 250A MMA/PULSE MIG Welding Machine, alternates between the high and low amperage output to provide a high deposition rate and penetration but with a less fluid puddle so that you have more control over it. Therefore, we recommend pulsed MIG welding for nickel alloys, but you can also use a standard short-circuit transfer. Your welds may not be as pretty, but short-circuit works well most of the time. However, if you need to clad metal with nickel alloys, we recommend sticking with either spray transfer or pulsed MIG welding, and avoiding short-circuit altogether.

Shielding Gas Selection:

For MIG welding, we recommend using 100% argon, while TIG welding benefits from 100% argon or a mix with helium for specific applicationsAvoid pure helium because you’ll get excessive spatter and erratic arc. DO NOT use standard MIG welding gas like 75/25 Ar/CO2 mixture because nickel alloys are far more likely to oxidise and contaminate the weld.

TIG Welding Nickel Alloys

TIG welding is a widely applied welding process for joining nickel alloys. While more challenging from the technique standpoint, TIG offers better precision and weld quality.

You should use TIG welding for nickel alloys whenever you weld thin materials, need a high-quality finish, and when making a root pass of a joint with an inaccessible back of the weld.

The direct current electrode negative (DCEN) setup is used to TIG weld nickel alloys. You can use standard DC TIG or a pulsed TIG welder when welding very thin sheets to prevent burn-through. Pulsed TIG welding is also beneficial for pipe root joints. Use high-frequency start when TIG welding nickel alloys to avoid contamination.

Arc Starting

Using a scratch or lift TIG start may leave tungsten residue particles on the nickel alloy’s surface and cause weld contamination. If your machine doesn’t support HF start, you can just initiate the arc on a scrap piece of metal adjacent to the joint and move the arc to the nickel alloy joint. That way, you’ll prevent joint contamination. Mind you, this is not a perfect way to start the arc, so we recommend getting an HF capable TIG machine. 

Tungsten Selection

You can use standard thorium tungsten electrodes to weld nickel alloys. Pure tungsten rods also work well, but cerium or lanthanum alloyed tungsten provide better results and offer a longer life. The thoriated tungsten is radioactive, so you must be careful when grinding it not to inhale its dust. But, lanthanated or ceriated tungsten are non-radioactive and safe to use. Cerium tungsten works best at lower amperages, so we recommend it for welding thin joints. 

Shielding Gas Selection

We recommend using a 100% argon shielding gas for TIG welding nickel alloys. You can add some helium to the mix, but again, you’ll get a more erratic arc and a wider bead profile. 

Using a large enough TIG torch cup is vital to provide efficient gas shielding over the joint. As we discussed earlier, you must provide maximum protection over the molten nickel alloy metal. Otherwise, you are risking oxidation and porosity. 

Final Thoughts

Mastering the art of welding nickel alloys requires great attention to cleanliness, proper shielding, and using specific welding techniques. While not inherently challenging, the diversity of nickel alloys requires a specialised approach for the best results. With that said, talking with engineers is advised for critical projects involving specialised nickel alloys.