A cure in 3D

© Photo | Laserzentrum Hannover e.V.

Whether 3D printers will some day be found in every home shop remains to be seen. But one thing is certain: generative processes are well on their way to maintaining health and saving lives.

The general public is dreaming about the 3D laser printer. This multifunctional device that can take some dust and light and conjure up whatever the heart desires, from a coffee cup to a compact car. And yet hardly anyone realizes that industry has taken this dream and started achieving real results. Its goals are just very different from those of the ordinary person.

There is more

The LZH

The Laserzentrum Hannover finds interdisciplinary solutions for all fields of laser applications through close cooperation between production engineers, material scientists, and physicists.

The author

Christian_Noelke_LZH

Christian Nölke is head of the Surface Treatment Group at the Laserzentrum Hannover e.V. He is active in a number of committees, among them the FA13 “Generative Processes” technical committee of the German Welding Association (DVS). He also works on this topic for the standards committee of the German Institute of Standardization (DIN), where he is co-drafting the DIN ISO/TC 261 standard. Contact: c.noelke@lhz.de

The research project

The work on laser melting of magnesium alloys is funded by the German Research Foundation DFG under the project code HA 1213/77-1.

The work on laser melting of NiTi shape memory alloys is funded by the German Federal Ministry of Education and Research BMBF as part of the GentleCI project under the project code 16SV3944.

The work on SLµM in the REMEDIS joint research project is funded by the German Federal Ministry of Education and Research BMBF as part of the framework program “Leading-edge Research and Innovation in the New German Länder” (project code: 03IS2081).

Industry does not want a device that can do a little bit of everything. It works instead on developing processes, materials, and systems that can perform specific tasks in a highly precise manner — tasks that can be programmed and reproduced at industrial scale.

From rapid prototyping to rapid manufacturing

In recent years, the major progress made in the realm of laser additive manufacturing (LAM) has gone largely unnoticed. Based on rapid prototyping, additive manufacturing has become firmly established in a broad range of areas thanks to the geometric freedom it allows. It is also common now for small-scale production and in tool manufacturing.

Rapid manufacturing and rapid tooling are key concepts here. Especially in the aerospace and automotive industries, experts are hard at work to develop industrial processes based on innovative process designs. Taking a look at the other end of the scale, it is medical technology research that seeks to extend the boundaries of the feasible.

Miracle cure for medical technology?

One of the aims of medical technology is to make components compatible with the natural biological surroundings in which they are used. Today, computer tomography provides high-resolution 3D images prior to complicated surgery.

These precise scans of the patient’s bones, organs, and extremities make it possible to produce made-to-fit surgical implants and accessories. Surgical tools such as drilling templates for the operating room are already commonplace, as are tools that are designed to support the planning of surgical interventions and the fitting of implants.

Tailored implants on an industrial scale

One of the first implants to be made in large numbers using LAM is the acetabular prosthesis made by Lima Corporate s.p.a. Its porous surface structure is designed to facilitate tissue integration, albeit with the additional help provided by a system that exposes the implant to electron beam radiation. Literature on LAM regularly reports on customized implants also being used in oral, jaw, and facial surgery. Dental implants are already being produced on a large scale using this technology.

Despite the progress that has already been made, further work is needed on the complex software infrastructure required for the widespread use of LAM implants. Moreover, production processes still need to be certified to meet EU Directive 93 /42 / EEC. At the moment, industry and research are working at full speed to close gaps with regard to production technology, software, and eligibility for certification.

Fundamental research into welding materials

In so doing, medical technology is not only conducting research close to the applications. The needs in this sphere are also initiating a great deal of basic research. Here, experimentation focuses on processing hard-to-weld, bioresorbable or functional materials.

Magnesium Selective Laser Melting Samples

Deposition welding with magnesium. Processing this medically highly attractive material is far more effective under excess pressure.

For instance, the Surface Technology Group at Laserzentrum Hannover e.V. is currently researching laser additive processing for magnesium powders. These investigations are part of a project carried out jointly by the German Research Society, the Department of Oral and Maxillofacial Surgery at Hannover Medical School, and the Hannover School of Veterinary Medicine. The aim of the project is to enable the production of hybrid implants with controlled bioresorption for nearly natural reconstruction of craniofacial defects.

An implant that makes itself disappear

Such an implant could initially replace the injured parts of bones and support reconstructed tissue where the bone itself is missing. Over the course of the healing process, this implant disintegrates and makes way for the regenerating bone cells. This also means that the forces encountered by the bone are increasingly transferred to the new bone as healing progresses.

The aim is to manufacture the resorbable magnesium base body using laser additive methods. Once produced, it is coated with a polymer. Where necessary, the compound is reinforced with a titanium component. To speed up and improve tissue integration, the hybrid implant is populated with bone cells in the laboratory at the very start.

Welding with magnesium

The biggest challenge of laser additive manufacturing for three-dimensional components made from magnesium is the small difference between the melting and vaporization points. Under normal ambient conditions, selective laser melting (SLM) will only be able to produce simple fusion track and exhibit a strong tendency toward “balling”.

Magnesium Phase Diagram

The strategy employed in the project aims to carry out the process under conditions of elevated pressure, increasing the spread between the melting and vaporization points. With support provided by the SLM Solutions company, a commercial laser melting system was modified and equipped with a process chamber developing 2 bar of gauge pressure. The first three-dimensional test structures have already been produced, and this is a major step toward achieving the project’s aim.

Microimplants and drug delivery systems

The Surface Technology Group also focuses on laser-based additive microprocessing, including selective laser micro-melting (SLμM) and laser micro-deposition welding (μLMD). Here, it is possible to build up medical micro-implants or to lend a functional structure to surfaces.

Drug delivery systems made of surgical steel for the direct administration of medication into target tissue is one area of focus, as is the processing of the shape-memory alloy Nitinol — used in micro-actuators, for instance.

The application determines the process:

As a result, the application determines the process, involving either a two-stage SLμM powderbed process or the single-stage μLMD. Both process approaches create objects without pores or imperfections, incorporating structures within the range of 30 to 40 μm in size.

In contrast to SLM, μLMD does not require a powder bed, as it introduces the additive in powder form directly into the processing zone. μLMD is better suited to creating surface structures on complex freeform surfaces or on relatively large components. When it comes to producing a high number of components and complex undercut 3D structures, the SLμM process shows significant benefits.

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On the way to industrial application

Recent events and publications indicate that laser additive manufacturing is well on its way to being used for industrial applications, either as an alternative to conventional process approaches, or because it opens up new opportunities that can only be realized with this technology.

While the maturity and penetration of various processes are clearly divergent, it is important to note that SLμM und μLMD are in the early stages of development. Ongoing developments have encouraged interaction between users and research, contributing to a clearer picture of the requirements of industry and of the direction that new research must take. In the years to come, significant progress can thus be expected in the realm of laser additive manufacturing.

 

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