Corrosion testing in additive manufacturing: Laser Cladding Venture explains their approach

Laser cladding coatings offer numerous ironclad advantages that are in many cases unmatched. In contrast, there are new challenges concerning corrosion. On this subject, we would like to give the floor to expert in the field, technology manager (CTO) at LCV: Stijn Clijsters. 

He explains a bit about how LCV works, corrosion concerns within additive manufacturing, and how this can be remedied by having the proper tests performed.

Good morning Stijn, I hear LCV is doing well! Tell me, how did it all start?

"Laser Cladding Venture was launched in 2015 as a spin-off of the Flemish Institute for Technological Development (VITO). In the beginning there were 3 of us but in the meantime our tightly-knit team counts 9 employees."

How is laser cladding employed in LCV?

"Laser cladding (or laser metal deposition) is a welding process where a laser creates a molten pool on the target surface. A metal-containing filler material (powder or wire) is then melted into the substrate. Traditionally, this process was used to repair turbine shafts and similar items. What makes laser cladding appealing is the minimal heat input.

Today LCV aims to evolve laser cladding into industrial series production. Here we focus mainly on applying coatings to new pieces and sometimes on refurbishing in series production. Laser cladding coatings extend the life of equipment, reducing the need for ad hoc repairs. The philosophy behind this is to cost-competitively apply a coating at an industrial level that provides a longevity extension.

LCV provides coatings via laser cladding for, among others:

• Hydraulics (large piston rods)
• Process industry
• Steel industry
• Compacting and earthmoving

LCV operates a technique that is slightly different from the classic powder bed process. How does this work exactly? 

"The powder bed method involves applying layers of powder to a plate and, layer by layer, you proceed to melt where you want the powder to be. At LCV, we blow the powder into the laser light very focused through a nozzle.

This technique is interesting because you can work on larger areas and more hybrid which also allows you to treat more complex surfaces. That is why we use it as a coating technique because of its flexibility."

In laser cladding you can use either wire or powder. Why does LCV prefer to use powder?

"Powder comes in many more varieties so we have more liberty in the development of alloys, compositions and gradients. In addition, we can also switch powder during the process and powder is also in fact still easier to melt with the laser. Wire requires more heat input which can be a disadvantage to the resulting properties of the material."

What makes your welding process so unique?

"We apply a laser source so there is less heat input. This results in less distortion and a harder microstructure: typically 20% harder and 4x more wear resistant. Also, it requires less material because not as much material from the substrate is mixed in. There are typical criteria for the highest permissible level of mixing from a corrosion perspective.

Thermal spray coatings (such as ceramic coatings for piston tanger) are only mechanically bonded therefore they are not impact resistant. Consequently, when the coating is damaged, the underlying material is exposed with an increased risk of corrosion.

Our welding process results in a metallurgical, and therefore better, bond while we can also guarantee good corrosion properties thanks to the low mixing."

As the name suggests, laser cladding is the focus but you are also working on 3D printing in addition to surface coatings. Will this assume an equally important position as cladding coatings for LCV?

“Coatings cladding still remains our core business. At VITO we were already doing a bit of 3D printing with laser cladding. While it was then primarily about characterization of material properties, now the focus is more on developing the 3D printing process in order to be able to use it at an industrial level.

As mentioned earlier, LCV uses the powder bed method. To integrate powder bed into 3D printing is more complex because there is a real risk of going out of focus, for example with deformations, which requires constant supervision. LCV is therefore working on developments to better keep this process under control."

How do you see your place within the 3D printing landscape?

"Traditionally, 3D printing has the advantage of greater design versatility. The additional design capabilities make it easy to eliminate material in the design where it is not needed. Think of the aerospace sector where one can make more complex structures that are much lighter.

However, we do believe that we have our own unique place within this landscape. Our resolution, unlike that of other companies, is more in the range of cast pieces with simple geometry. For example, for one customer, we were able to reduce the production lead time from 3 months to 1 month by shifting from a casting process to 3D printing by laser cladding. As icing on the cake, we obtained better material properties (harder and more wear resistant), less material loss during the process and a lower cost.

Right now, we are focused on relatively simpler rotationally symmetric geometries within our 3D printing R&D roadmap. This requires a less complex tool path which in addition makes the process easier to control."

How have the applications evolved over time?

"3D printing used to primarily focus on rapid prototyping. Customers often need something urgently but following the traditional process this has to be cast. The process of mold making, validation of the casting process can easily take 3 months. This is where rapid prototyping offers an interesting alternative because something of equal quality can be produced in 1-3 weeks. This process has been stepped up and up. In the process, rapid prototyping has moved on to a serial production technique."

What is the main challenge regarding material properties in this process?

"Thermal tensions from the welding process can cause the pieces to warp and crack.  Heat treatment helps to relax these stresses. When certain pieces have dimensional tolerances, they are typically re-milled but as a result of internal stresses, they can start to move during the milling process so that the dimensional tolerances no longer quite match.

Obtaining the right microstructure is certainly an important point in terms of corrosion resistance. For example, we work a lot with super duplex stainless steel for repairs in the petrochemical industry. The microstructures of these materials are not straightforward which makes it a challenge to guarantee with a welding process that you maintain the right microstructure during the welding process."

Are there any specific tests that LCV performs or has performed on laser cladding coatings or samples?

"For additive manufacturing, we get samples verified to identify microdefects in the material. These have an influence on the mechanical properties of the products. In the future, we want to rely on the expertise of METALogic for this matter because we can count on a convenient lead time that we have already experienced in previous projects. 

There are also several standard tests that we have performed on overlay welds and hard facings. We typically apply these layers as a coating to create more corrosion resistance or wear resistance. We do this according to certain standards.  There are a number of tests mentioned in ISO and ASME standards (ISO 15614 & ASME_IX) such as measuring hardness, composition, measuring admixture,...

Concerning corrosion resistance, there is ASTM G28 to test the resistance to intergranular corrosion. If the results of this are not satisfactory, the reason is typically that the microstructure is not adequate. Possible causes can be, for example, too high temperatures during welding or too rapid solidification of the material."

Stijn, thank you for your time and elaborate answers!

Be sure to take a look at the LCV website for more information! METALogic is of course also at your service as an expert in testing the corrosion resistance of welds, the characterization of (3D printed) materials and offers you a solution for all kinds of corrosion related issues.