Trends in the Laser Cladding Technology - part 1

Preface

In my previous articles, we have discussed the basics of Laser Cladding, and also had a look at acronyms related to the technology. Today I would like to address the topic of the technology itself by giving an overview of trends, developments, and possibilities. I try to focus on facts, which I know, and which are based on my personal practical experiences. I will give my overview of spot geometries and sizes variations, and deposition velocities. Together we will also look at how laser power and velocity influence the process and what are the latest trends in the technology.

My intention is to help you to decide for yourself whether EHLA, high-power laser cladding, or any other type of process is the right choice for you.

I will be happy to receive your feedback if the information you read doesn’t give a full picture and want to learn more.

Note: All the images I use are agreed with content providers or are taken from open access internet sources, and are linked to the reference. My intention is not to advertise any of the companies, it is more about to show the trends in the world of laser cladding.

Where will technology development lead us?

Nowadays more and more companies show interest in Laser Cladding. Some of them want to purchase a system for their applications, some might be interested in service or cooperation, and some just want to understand how competitive this technology is compared to existing solutions like thermal spraying, hardfacing, or, for example, hard chrome plating. With this article, I want to give you a short overview of the latest developments within the technology, which might help you get an idea of the possibilities and answers to questions like:

• What are the latest trends in the Laser Cladding world?
• What are the main developments in terms of productivity?
• What possibilities do we have, and what might be important to consider if you decide to work with Laser Cladding?
• Should I work with a circular or rectangular spot?
• Is EHLA the right choice for me?

Let’s try to answer those questions together. Probably the best would be to start from the roots. Laser Cladding was already established in certain high-cost market segments like Aero and Energy since twenty years ago. Then Lasers were quite expensive, and an energy source of 2-3KW power was considered high-tech – enough to repair turbine blades, but way too expensive for other applications. In those twenty years, many things have changed – prices for lasers have dropped almost by a factor of ten, and also the high power lasers (let’s say 10+ kW) became convenient under industrial conditions. I think that was one of the main factors explaining why the technology is finally of high industrial interest – it became price competitive to alternative surface treatment solutions on a wider scale of applications. Furthermore, technology has also developed and offers nowadays a variety of deposition options, which I have tried to summarize in Figure 1:

Figure 1: Classification of Laser Cladding

For starters, let us focus on three main groups: High-speed, High-power, and 3D.

High-speed Laser Cladding. Let’s keep the name of EHLA, as it is mostly related to the principle of melting most of the powder before it interacts with the surface. This process has been grabbing attention in the market in the last couple of years. It is phenomenal how rapidly it has developed. Imagine, that the first R&D system was installed by Hornet at Fraunhofer ILT back in 2013, and now we have over 50 systems operating worldwide under research and production activities. My personal view on the EHLA development curve is presented in Figure 2. Please let me know if I have missed some of the facts.

Figure 2: EHLA development curve

What makes EHLA so exciting? Imagine you can use it for rotational symmetrical bodies and deposition velocities, which are at the range of 100-200+ m/min. With that, we can produce thin, dense metallurgically bonded to substrate coatings of excellent properties. On average per path, it is possible to get the thickness between 100-250μm. The surface roughness after deposition can be achieved by values of Rz <50μm. Additionally, it is possible to remelt the surface by the interaction of the laser spot with the deposited coating. It does remove some of the asperities and smooth the coating, dropping Rz values below 20μm. Some examples of EHLA surface quality and cross-sectional images are shown in Figure 3. You can see, that EHLA can be applied directly on the surface of the cast-iron, or the propagation of cracks on hard materials like Metco 15E (Hardness over 800HV), can be significantly reduced or avoided (for example by pre-heating of components).

Figure 3: Surface quality after EHLA and re-melting; Cross-section of microstructure for Metco 15E (on the left side) and new generation Metco Fe-based alloy, deposited directly on cast-iron.

Above mentioned benefits make EHLA an attractive technology and push further developments. In 2020 everyone speaks already about high-power EHLA and deposition rates exceeding 2m2/h. Those values put EHLA in competition with thermal spraying, spray and fuse or even hard chrome plating. High-speed laser cladding is getting more and more attention also from the application side – most common discussions and advertising flow in directions of hydraulic rods and brake discs – a sneak peek to the iceberg of potential technology directions.

If to look at technological challenges, I see two main directions for the high-speed laser cladding:

• Materials. I strongly believe, that due to the rapid cooling and application of fine-grade powder particles (-53+20μm) the developments of new generation materials are of high importance and interest. Also at Oerlikon, we put a lot of attention on that topic. One of the examples is shown in Figure 3, as a modern coating solution by working on complex substrates. I will go more in detail here in one of the next publications.
• Industrialization, and by that I primarily mean the reliability of the process and new generation nozzles, which can ensure stable operation under series production. There is a lot of work done in that direction and in 2020 we have a couple of new generation nozzles already available on the market. I don’t want to compare the nozzle suppliers, as I intend to show, what is available, and it is up to you to decide, which of the ways you would prefer to take. The examples of the latest nozzles are shown in Figure 4. There are mainly two working principles: coaxial powder supply and multi-jet powder supply. Both new generation nozzles can already work with powder volumes over 100 g/min and with energy power over 10KW. 

Figure 4: Highno EHLA nozzle by HD; Multi-jet EHLA nozzle by Trumpf

High Power Laser Cladding (and by that I mean the laser power exceeding 10kW) can be divided into two main fields (Figure 5):

1. Working with circular spot geometry. Here the latest development is the spot size which can be cladded up to 12mm in diameter, with depositions velocities over 1.5 m/min. The logic behind this is very simple – a bigger spot and higher velocity are compensated by the power of the energy source.
2. Working with rectangular spot geometry. Nowadays there are flat-jet powder nozzles available with rectangular geometries up to 45mm in width (Figure 6). The powder consumption, however, also increases accordingly and can go upwards of over 300g/min. Also here due to the large area of the interaction between the laser spot and the substrate surface, the high power laser is required.

Figure 5: Examples of high power nozzles with circular and rectangular spots (Fraunhofer IWS)

Historically this direction was always in the focus of Fraunhofer IWS in Dresden. No wonder, that their products like COAX powerline and COAX11 are well established in the market. Both of these principles have their advantages and disadvantages, (which I will discuss in part 2). Working with high power laser of over 10KW, in general, requires more care for optical components due to back-reflections and heating effects. Also, the life-time of processing heads under industrial conditions might strongly depend on the hours of operation.

Figure 6: 45mm spot with 20kW Laser power (image courtesy Fraunhofer IWS)

3D Laser Cladding or LMD (or any other convenient acronym). In this process, we primarily speak about 3D structures, the building of components, or multi-layer repair of complex geometries. This part of the technology became popular parallel to the developments happening right now in the additive manufacturing world. It is a very promising direction, which has proved to be very productive and is already widely used in many industrial segments. The main questions experts commonly ask related to this technology are as follow:

• How to minimize and prevent the overheating the parts, due to multi-layer deposition? Over-heating can have a strong influence on the microstructural features of the produced component and as a result, lead to decreased mechanical and/or wear behavior properties.
• How to increase productivity and get faster? The production time is always a question customers ask us first.
• How to increase deposition efficiency and ensure almost no overspray? The topic of material consumption and toxicity of overspray particles for example.

In Figure 7 the main trends of the technology are presented and should give an idea about possible solutions to the above-mentioned questions. One of the promising trends is related to wire-feed laser cladding. Wire as a filler material has one significant benefit versus powder – 100% consumption of the deposited material. With the development of coaxial wire feeding principles, the issues of constant wire feeding were mostly solved. Additional improvement was also achieved by changing the cladding trajectory at constant deposition velocities – in the past that could lead to the defects in “corners”. This short video, taken from Laserline webpage gives a good idea about coax-wire cladding features. 

Figure 7: 3D laser cladding development directions

Another promising direction of technology development is related to a combination of two processes in one system – cladding and milling. This helps to address multiply topics and produce parts of very complex geometries in the same system. In my opinion, that is a very interesting approach, which is nicely described by company DMG Mori, which is one of the pioneers in providing such systems to the market. I think this solution is great for the production of prototypes, repair/production of single individual components, or small series of parts. On another hand, there are also a couple of challenges for such a complex, and for sure, an expensive hybrid system under medium and high volume productions. It is a matter of simple calculations to find out what is more effective – one hybrid system or two machines (one cladding, another milling), working in parallel on the same product. In general, the main benefits here are related to the development of CAD/CAM chains for the technology itself. It has given a push to the digitalization of Laser Cladding. Offline programming combined with process monitoring options, during the deposition process, ensures excellent reliability and high quality of products.

3D EHLA is one of the latest trends, recently presented to the market. It combines high-speed laser cladding and LMD. The latest developments are still ongoing and the progress can be monitored in the project – futureAM – Next Generation Additive Manufacturing, which is lead by my colleagues at Fraunhofer ILT. It is really impressive to see this method taken to new heights and the incredible benefits it can offer in the future. For now, it is still an R&D process and it sure is exciting to see how this will develop.

Acknowledgment

I want to express my sincere gratitude to Dr. Maria Barbosa from Fraunhofer IWS and Marco G?bel from Trumpf for supporting me with graphic materials and technical discussions we have.

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 thanks goes to my manager Dr. Alexander Schwenk for his trust and support.

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

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