Article 1: Introduction to Laser Cladding Technology

Preface

Some of you are senior technical experts in laser cladding, some are just starting to gain their expertise and some are even not familiar with the process. Considering that, I have decided to write my version of the Laser Cladding Technology review. I want to publish a blog of short articles, related to Laser Cladding. The idea behind this is to give you my subjective overview of the process and the latest developments in the world of Laser Cladding.

By that, my intention is not to bother you with deep technical details, but rather to give a summary with simple explanations. I wish, that everyone, who is not familiar with Laser Cladding will have a chance to understand the technology and its advantages. If you are a more experienced reader, I would like to invite you to join me in the discussions, ask your questions, and leave your personal opinion. If there is a related topic you want me to write an overview of, I will be happy to consider that. I hope it will be interesting to read and will be happy to get your feedback.

What is laser cladding about?

I think it is important to start with the roots, and I am sure that most of you are familiar with the term - welding. Welding technology is used to join two pieces of metal together and is quite commonly used in the industry already for more than 100 years (according to the literature the first arc process was patented already in 1881). You can find results of the welding process anywhere – almost every metal construction has a welding joint – public transport, cars, ships, bridges, etc. I am sure, even at your home place you can find examples of welding. On Figure 1 you can see some examples from my flat:

Figure 1: Examples of welding joint

a)    A welding joint on my bar chair leg

b)    A welding joint from my balcony metallic construction

How does welding work? To start a welding process you need an energy source. With the help of an energy source, you produce heat on the metal surface, heating it to a liquid phase. In those conditions, two parts can be fused together. That helps not only to connect two metallic parts but also to give freedom on design for metallic constructions.

You can, however, use welding not only for bringing two metal parts together but also for covering the surface of one metal with another material. In that case, you build a layer of filler material on the surface of the substrate. That material usually has better properties and can provide protection against corrosion or/and wear. This process is known as hardfacing or overlaying. In other words, we place a protective coating on the surface of the base material, applying the same principle as in joint welding. Those types of coatings are nowadays widely used in many markets, for example, oil and gas, agriculture, mining, chemical, and steel industries.

The second direction, where overlay coatings are commonly used is related to 2D/3D dimensional parts recovery. Quite commonly parts, suspected of wear can be repaired, instead of replaced. This helps to save the costs and also decreases long standby times (as new components require a sufficiently long time for delivery). For the parts, recovery materials of similar properties to the worn base material are commonly used, however also here a better grade material can be selected, which will result in an increase of the lifetime of repaired components.

What does hardfacing have to do with Laser Cladding? In my opinion everything! Laser cladding can be even named as a “Queen” among all welding deposition methods. I personally define Laser cladding as a coating process by welding, where we use the laser beam as an energy source to heat the surface of the substrate. The laser as an energy source offers many benefits in terms of heat control and precision. This allows us to speak about controlled heat input into the base material and offers quality advantages compared to conventional hardfacing processes. The process scheme is shown in Figure 2.

Figure 2: Laser Cladding Process

In Laser Cladding with the help of the laser source, a laser beam will be produced and transferred to the surface of the substrate. The laser beam builds a spot on the surface, where the heat will be introduced. The main benefit is that with the help of optical components (fibers, lenses, collimator) the geometry and dimensions of a produced spot can be defined and controlled. This helps to achieve the following advantages:

-       low HAZ [1] (heat affected zone).

-       low dilution [2] with the substrate (around 5%), resulting in better coating properties.

-       metallurgical bonding, which leads to the highest impact resistance.

-       low heat input, resulting in negligible distortion.

-       high process reliability and repeatability.

-       high deposition efficiency [3] by working with powder materials (80-95%).

-       the high variety of deposition options.

In the area of the spot, we do produce a melting bath on the surface of the substrate.

In parallel, filler material in the form of powder or wire is transferred into the melting pool area, melts, and builds a coating on the surface of the base material. As the process happens in the motion of a part or working head we do build so-called welding seams of defined size on the surface (the width is typically equal to the spot size of the laser beam). If we need to cover a bigger surface, those seams have to be overlapped with the next passes as shown in Figure 2.

I have tried to summarize the benefits of Laser Cladding and also five important take-aways in Figure 3.

Figure 3: Key facts about Laser Cladding

Variety of process possibilities

Laser Cladding has existed in the market already for more than 40 years, but still can be called as one of the youngest coating methods. It was not so commonly used in the past, due to the expensive costs of lasers and related components, compared to other deposition methods. Also, the main applications were in the high-costs market. However, in the last decade, the situation has significantly changed, and the technology is now one of the most promising overlaying methods, combining price and quality benefits! Imagine, the prices of laser sources have dropped by a factor of ten compared to the beginning of the 21st century.

Figure 4: Laser Cladding sub-groups

Another driving factor for the success of technology is related to the multi-functionality of Laser Cladding. Below, I would like to give some examples on the variety of deposition options, as with Laser Cladding you have different direction on how to apply the process:

Laser Cladding of internal surfaces with special heads (see Figure 4a). Nowadays with so-called ID heads, you can do parts as small as 25mm internal diameter. The length of ID heads is limited to around 1500mm due to possible vibration at the end of the head during deposition. I have seen 2000mm long heads, but the process stability is not always on the highest level by longer tools. Nevertheless, with 1500mm you can cover internal surfaces of parts up to 3000mm depth, simply by turning the components.

Wire cladding (see Figure 4b). The main benefit here is 100% of DE, resulting in full consumption of the material. In some industries with restrictions on operating with toxic powders, wire cladding can be a promising alternative. The latest developments by wire cladding have also opened new horizons for additive applications. With the help of the coaxial wire-feeding principle, 3D geometries with high precision and excellent process control can be achieved.

Additive Manufacturing (AM). AM of metals became very popular in the last couple of years. However, not everyone knows that Laser Cladding can also produce 3D structures by the process called LMD (laser metal deposition) or DMD (direct metal deposition). In this process we can build structures layer by layer directly on the surface of components, changing their design. For that process wires and powders can be used. Figure 4c shows a typical image of this type of process. On that image, you can see a multi-layer restoration of the turbine blade surface.

High-speed laser cladding (Figure 4e). This is a newly developed method also known as EHLA (Extrem Hochgeschwindigkeit Laserauftragsschweissen). Translated from German it means extreme high-speed laser cladding. The unique idea of the process is that you melt the powder before it interacts with the surface, where over 80% of laser energy goes into the powder melting. This allows for the production of thin coatings with low surface roughness and excellent properties. Due to high deposition velocities for rotation symmetrical bodies and deposition rates already going into direction over 2m2/h EHLA treats in the competition with processes like thermal spraying and hard chrome plating. Another advantage of high-speed cladding is the deposition efficiency of 90%.

High power laser cladding (Figure 4d). I know already many companies who do operate with laser power of 10-20KW. In most cases, they do use a rectangular spot to cover the big surface in one path and to produce thick coatings. The main benefit here is a high surface coverage rate for massive components. Imagine you can work with spot dimensions over 10mm in diameter or with a rectangular spot of 20mm width. For massive parts this offers extremely high productivity.

Laser Surface Engineering is not directly related to coating technology, but still can be realized using the same components as for Laser Cladding. I would like to mention one process here – Laser Surface Hardening (Figure 4f). Laser Hardening allows the selective increase of the hardness of base material, simply by focusing the laser beam on the required surface. As in Laser cladding, you do create a spot on the surface. By controlling the spot temperature it is possible to heat selective areas of the part. Control of temperature helps to stay below the meting point of the base material, avoiding any changes in surface structure and roughness. Selective heat input, combined with rapid cooling causes the martensitic transformation of the steel surface and increase the hardness of the material in the depth up to 2mm. Another advantage of this process is – no requirements on post-treatment (mechanical of heat-treatment).

Acknowledgment

I want to thank my team for their great work, motivation, and a high level of professionalism by making our Laser Center a great place to work. Thank you J?rg Spatzier, Kemal Coskun, and Pether ?hninger.

Special thank you goes to my manager Dr. Alexander Schwenk for his trust and support.

Laser Center of Competence by Oerlikon Metco

In July 2019 we have started restructuring our Laser Cladding R&D capacities and decided to open a Laser Center of Competence. Here we work as a team on providing new solutions for the protection of parts by laser surface engineering.

Our main goals are:

• to provide engineering, industrialization and service support for our customers
• to support all ongoing projects in materials development
• to be involved in the latest developments in the laser cladding technology
• to provide high-level technical expertise by having a senior technical team on board

Now we are in the final stages of the shop-floor restructuring process and we will re-open our department with full capacity in September 2020.

[1] HAZ is resulting changes in the properties of base material (microstructure, hardness), due to heat introduced during cladding process.

[2] Dilution is a term, which describes how much of base material is mixed with deposited coating, causing changes in the composition of the overlay. Higher dilution leads to decreased properties of the coating. For example, a classical welding overlaying can have up to 50% of dilution in the first layer, which require multi-layer deposition to achieve better properties of coating and compensate dilution.

[3] Deposition efficiency (DE) indicates how much percentage of used powder will land in the melting pool, by building a coating. Lower DE means less efficient material consumption. For example, thy typical DE for processes like thermal spray is below 60%.

Dr. Arkadi Zikin - Head of Laser Center of Competence by Oerlikon Metco

Email: [email protected]; LinkedIn: https://www.dhirubhai.net/in/arkadi-zikin/