Selective laser melting

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Scheme of the manufacturing process

The selective laser melting (English selective laser melting , abbr. SLM ), and laser powder fusion Bed ( LPBF or LPBF called), is an additive manufacturing process to the group of beam melting process belongs. Similar processes are electron beam melting and selective laser sintering .

Procedure

With selective laser melting, the material to be processed is applied in powder form in a thin layer on a base plate. The powdery material is locally completely remelted by means of laser radiation and forms a solid material layer after solidification. Then the base plate is lowered by the amount of a layer thickness and powder is applied again. This cycle is repeated until all layers have melted. The finished component is cleaned of excess powder, processed as required or used immediately.

The typical layer thicknesses for the structure of the component range between 15 and 500 µm for all materials.

The data for guiding the laser beam are generated from a 3D CAD body using software. In the first calculation step, the component is divided into individual layers. In the second calculation step, the paths (vectors) that the laser beam travels are generated for each layer. In order to avoid the contamination of the material with oxygen , the process takes place in a protective gas atmosphere with argon or nitrogen .

Components manufactured by selective laser melting are characterized by high specific densities (> 99%). This ensures that the mechanical properties of the additively manufactured component largely correspond to those of the base material.

However , a component with selective densities can also be specifically manufactured according to bionic principles or to ensure a partial modulus of elasticity. In lightweight construction in aerospace and body implants, such selective elasticities are often desired within a component and cannot be produced in this way using conventional processes.

Compared to conventional processes ( casting process ) is characterized the laser melting of the fact that tools or molds omitted (formless production) and thereby the time to market can be reduced. Another advantage is the great freedom of geometry, which enables the production of component shapes that cannot be produced with form-related processes or only with great effort. Furthermore, storage costs can be reduced, since specific components do not have to be stored, but can be produced generatively if required.

Exposure strategy

The tendency is that the higher the laser power, the higher the roughness of the component. Modern system technology can control density and surface quality according to the "shell-core principle". The segmented exposure specifically influences the outer areas of the component, overhangs and high-density component areas. An optimized exposure strategy improves the quality level and at the same time the build-up speed. The performance profile of a component can be significantly increased with the help of segmented exposure.

Quality aspects and topology

The plant manufacturers pursue different quality assurance approaches, i. d. Usually on the one hand off-axis (or ex situ) or on the other hand on-axis (or in situ).

Classic off-axis inspections have a lower resolution and a lower acquisition rate. For example, an infrared-sensitive camera is used, which is positioned outside the process chamber - i.e. ex situ. The advantage of an ex-situ solution is the simple system integration of the system and camera system. An off-axis structure enables statements to be made about the overall melting and cooling behavior. However, a detailed statement about the weld pool cannot be derived.

The on-axis / in-situ structure (e.g. Concept Laser structure ) is based on a coaxial arrangement of the detectors. A camera and a photodiode, which use the same optics as the laser, are used as detectors. This coaxial integration enables a high coordinate-related 3D resolution. The recognition rate results from the scan speed. If this is 1,000 mm / s, the result is 100 µm, i.e. the distance for which a recording is made. At 2,000 mm / s the value is 200 µm. A coaxial arrangement has the advantage that the molten bath emissions are always focused on one point of the detectors and the image section can be reduced and thus the sampling rate can also be increased. A detailed analysis of the melt pool characteristics (melt pool area and melt pool intensity) is possible.

Designations and naming

The process was largely developed at the Fraunhofer Institute for Laser Technology (ILT) in Aachen in cooperation with F&S (Dr. Matthias Fockele and Dr. Dieter Schwarze). In the course of further process and system development, different machine manufacturers have coined different names for the process principle described:

Characteristics and special aspects of the technology

  • Freedom of geometry

The freedom of geometry enables the production of complex structures that cannot be technically or economically realized using conventional processes. These include undercuts that can occur in jewelry or technical components .

  • Lightweight construction and bionics

Open-pore structures can also be produced, which means that lightweight components can be produced while maintaining strength. The potential of lightweight construction is considered a very important advantage of the process. The porous structure of bones can be cited as a bionic model from nature. In general, approaches to bionics play an increasingly important role on the constructive side.

  • Redesign and one shot approach

Compared to classic cast or milled parts, which are often assembled together to form an assembly, it is possible to assemble a complete assembly or at least many individual parts in one shot (one-shot technique). The number of components in an assembly tends to decrease. One then speaks of a redesign of the previous construction. The generative component can thus be installed more easily and the assembly effort is generally reduced.

  • Reverse engineering

The construction elements can be extracted from a finished product and mapped in data sets. Based on this data, the component can be copied and optimized in reengineering (redesign),

  • Mixed construction / hybrid construction

The mixed construction / hybrid construction method in the SLM process means the manufacture of a component that is only partially additive. In the subsequent SLM process, a second, additively manufactured component area is built up on a flat surface of a first, conventionally manufactured component area. The advantage of the hybrid construction method is that the building volume to be produced using the SLM process can be greatly reduced and simple geometries can be constructed conventionally, and geometrically more demanding areas using the SLM process. This reduces the construction time and the costs for the metallic powder material due to the lower volume for the component area manufactured using the SLM process.

  • Prototypes and unique items

Mold-based processes require certain series sizes in order to allocate the costs for the molds to the unit costs. These restrictions do not apply to the SLM process: it is possible to produce samples or prototypes in a timely manner. In addition, very individual parts can be created as unique items , such as those required for dentures , implants , watch elements or jewelry . A special feature is the parallel production of unique items in one installation space (e.g. dental implants, hip implants or spinal support elements). It becomes possible to design and manufacture components that are specially tailored to the patient.

  • Selective densities

With a classic milled or turned part, the density of the part is always evenly distributed. With a laser melted part you can vary. Certain areas of a component can be rigid and others can be applied elastically, e.g. B. with a honeycomb structure (bionic principles) component requirements can be designed much more creatively compared to conventional techniques.

  • Function integration

The higher the complexity, the better a generative process comes into play. Functions can be integrated (e.g. with temperature control channels or air injectors or the part is given a hinge function or sensory instruments are integrated into the component). The components that are thus enhanced in value are more efficient than conventionally manufactured components.

  • "Green Technology"

Environmental aspects, such as low energy consumption when operating a system and resource conservation (exactly the material used is used / no waste) are elementary features of laser melting. There are also no oil or coolant emissions, as is often found in machine technology today. Even the residual heat can be used. A 1,000 W laser emits approx. 4 kW of heat, which can be used by the building services in a water cooling circuit. Conventional technologies and their disadvantages are increasingly being viewed from a sustainability perspective. Laser melting also means a contribution to reducing CO₂ emissions under the four special aspects of lightweight construction, tool-free production, decentralized production and “on demand”. It is the combination of resource conservation combined with high economic efficiency and quality standards. Additive manufacturing can serve these trends.

  • Production-on-demand

An essential aspect of laser melting is the temporal and local production if required. This can change the logistics concepts (e.g. at aircraft manufacturers) very significantly, because spare parts no longer have to be stored, but can be printed out when required. In addition, with production-on-demand, aircraft overhaul times can be reduced.

  • reduced use of materials

The lower material consumption is particularly noticeable compared to milling from a solid part. It is assumed that on average the pure component weight and around 10% material are used for the support structures (these are the support structures required for assembly).

Materials

The materials used for selective laser melting are usually series materials that do not contain any binding agents. The machine manufacturers and their material partners certify the series materials for the users (e.g. for dental or medical applications in accordance with EU directives and product liability law ).

Series materials are converted into powder form by spraying. This creates spherical particles . The minimum and maximum diameter of the particles used is selected depending on the layer thickness used and the component quality to be achieved. All powder materials are 100% reusable for subsequent construction processes. A refresher with unused material is not necessary.

The material consumption is i. d. Usually calculated as follows: component weight + 10% (the 10% surcharge is caused by the support structure, which must be separated from the component after the manufacturing process).

The materials used are, for example:

Applications and industries

The process can be used in numerous industries. These include:

Assembly speeds and lot size consideration

The factors of assembly speed and lot sizes define the consideration of the profitability of additive manufacturing. These factors change continuously with the state of the art.

The following rules of thumb apply to lot sizes:

  • Number of items from 1 to approx. 1000 per year: additive manufacturing processes are typically the most economical option.
  • Number of pieces 1000 to 100,000 per year: The production in a mold made of metal should be taken into account as a possible variant in the profitability analysis using additive processes.
  • Number of items greater than 100,000 per year: a particularly durable geometry made of solid material in the classic way is likely to make the most sense.

The building speeds z. B. selective laser melting are steadily increasing.

The reasons are: Higher laser powers (such as 1 kW laser sources or the use of multiple laser sources - keyword: multi-laser technology).

To illustrate this, a comparison of the build-up rates as expected by the management consultancy Roland Berger:

See also

Individual evidence

  1. ↑ SLM is acc. Trademark entry 30094322 , German Patent and Trademark Office a word mark of the SLM Solutions Group . In the specialist literature, SLM is used as an abbreviation for the process of selective laser melting, see for example Joachim Tinschert, Gerd Natt: Oxidkeramiken und CAD / CAM-Technologie , Deutscher Ärzteverlag, 2007, p. 39, ISBN 9783769133424 ; Andrzej Grzesiak et al .: Generative manufacturing with plastics , Springer-Verlag, 2013, p. 31. ISBN 9783642243257
  2. ↑ a b Laser Powder Bed Fusion - Fraunhofer ILT. Retrieved December 27, 2018 .
  3. ↑ Jean-Pierre Kruth et al. a .: Binding Mechanisms in Selective Laser Sintering and Selective Laser Melting , In: Rapid prototyping journal , 11 (1), 2005, pp. 26-36. ISSN  1355-2546
  4. ↑ Wilhelm Meiners: Direct Selective Laser Sintering of One-Component Metallic Materials , Dissertation, RWTH Aachen 1999.
  5. ↑ DMG Mori company homepage / Additive Manufacturing  ( page no longer accessible , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. , accessed on February 21, 2018@1@ 2Template: Dead Link / journal.dmgmori.com  
  6. ↑ Homepage: 3D Systems, ProX-DMP-320 , accessed on February 21, 2018
  7. ↑ Special Tooling 1/02: Generating tool cores with laser energy , 2002, pp. 12–15.
  8. ^ Additive manufacturing - A game changer for the manufacturing industry? - Lecture Munich (November 2013)