Selective Laser Melting

 

Direct, Mold-less Production Systems

Selective Laser Melting

Solving the dichotomy between mass-production and individualized single piece production is the aim of ICD A2. Additive manufacturing methods, like Selective Laser Melting (SLM), can be a main driver for producing individualized products at nearly the same costs as mass produced products. The basic principle of Additive Manufacturing is to manufacture a part layer by layer, utilizing e.g. metal powder. In contrast to sintering, this metal powder is fully molten during the SLM process and therefore 100% dense parts with properties comparable to conventional manufactured ones can be produced. Neither tools nor molds are required in contrast to conventional manufacturing (e.g. die-cast). Comparing additive manufacturing with conventional manufacturing methods, cost curves differ significantly: Due to the lower start-up costs, small lot sizes can be produced cost efficiently with additive manufacturing. This is a key-enabler for new business models, centering on highly individualized products ("Individualization for free"). Manufacturing costs of additively manufactured parts are almost independent of part complexity ("Complexity for free"), since time and effort needed for manufacturing is only depending on component volume. Innovative products with integrated functionalities can be realized with additive manufacturing methods like SLM.

 

Practical Issues

Functional integrated rim-support

Off-the-shelf SLM machines manufactured by considerable OEMs like EOS, SLM Solutions, Realizer and Concept Laser are limited in regards to their productivity. By increasing laser power from 200 W to 1 kW and developing
a double-beam concept to realize the skin-core principle, the process efficiency was increased by a factor of
10 in the last five years. The double-beam concept utilizes a beam switch to shape two different beam sizes either of
200 μm or 1 mm beam diameter. This ability to switch beam diameter, enables the machine to build
the contour of the additively manufactured part with a laser spot size of 200 μm and small layer thicknesses
(approx. 50 μm) to achieve high accuracy, surface quality and resolution. In contract, core sections only resemble
bulk material and therefore a beam diameter of 1 mm and layer thicknesses of >200 μm are used to increase
the build-up rate without scarifying part quality. However, today there is not much experience yet in industrial applications of the skin-core principle with laser powers exceeding 1 kW. Additionally, pre- and post-process work
is needed to prepare the machine results in a rather high amount of nonproductive time and therefore resulting in
a lower efficiency of the SLM process. Also, utilization of the laser source itself is limited, since powder dispersion
and exposure to laser radiation are distinctive, nonparallelized process steps. A 100% laser utilization cannot
be achieved with today’s SLM machines. Mechanical properties of additively manufactured lattice structures
under load-bearing conditions are only sparsely available. The huge potential of functionally optimized parts, by
integration of lattice structures (see figure right), today is far from being utilizable.

 

Approach

Overall concept of the SLM-machine (CAD model)

A possible solution needs to be based on a holistic analysis of the SLM process and the development of a reference architecture for “mould-less” production systems. This holistic approach is necessary to harness SLM for serial production. To improve the SLM machine technology, multiple concepts of each SLM machine module are developed and evaluated in a systematic approach. Achieving nearly 100% utilization of the laser, and minimal non-productive time is the major guideline during evaluation and creation of a complete nextgeneration SLM machine (see fig. left). The new concept for the laser utilization calls for using the laser while the powder is distributed (“on-the-fly”). Also, using multiple laser sources in parallel will significantly reduce primary processing time. An innovative process chamber design should increase the handling of the SLM machine. Multiple designs were created and evaluated, with the “sliding system” with two scanners systems and two separate lasers having the most positive impact on handling and process efficiency.

 

Multi Spot Machine Center

Multi-Spot Lehrstuhl für Lasertechnik LLT

A second SLM-machine based on a multi-diode array, which is placed on moveable axis, is developed. The official presentation of this SLM-machine was performed at Euromold 2014 in Frankfurt. The overall feedback was very positive and currently ways for commercialization of the SLM-machine concept are investigated. Moreover a SLM-specific cost model is developed to allow the estimation of part costs depending on the SLM-machine configuration that is used for the build-up. The SLM-specific cost model strongly contributes to the development of a production theory and allows the systematic development of SLM-machines regarding concrete requirements.

 

Technical Challenges

Major technical challenges are developing and shaping innovative concepts for the different machine modules,
in order to achieve a nearly 100% utilization of the laser source, and minimize non-productive time. The degree
of novelty of the chosen concepts, e.g. the multi-beam concept, also requires adapted process management in
the overlapping areas and is managed on innovative control system software, as well as innovative machine components.