Selective laser melting

Selective laser melting (SLM) or direct metal laser sintering (DMLS) is a rapid prototyping, 3D printing, or additive manufacturing (AM) technique designed to use a high power-density laser to melt and fuse metallic powders together. In many SLM is considered to be a subcategory of selective laser sintering (SLS). The SLM process has the ability to fully melt the metal material into a solid three-dimensional part unlike SLS.

History
Selective laser melting, one of the several 3D printing technologies, started in 1995 at the Fraunhofer Institute ILT in Aachen, Germany, with a German research project, resulting in the so-called basic ILT SLM patent DE 19649865. Already during its pioneering phase Dr. Dieter Schwarze and Dr. Matthias Fockele from F&S Stereolithographietechnik GmbH located in Paderborn collaborated with the ILT researchers Dr. Wilhelm Meiners and Dr. Konrad Wissenbach. In the early 2000s F&S entered into a commercial partnership with MCP HEK GmbH (later on named MTT Technology GmbH and then SLM Solutions GmbH) located in Luebeck in northern Germany. Recently Dr. Dieter Schwarze is with SLM Solutions GmbH and Dr. Matthias Fockele founded Realizer GmbH.

The ASTM International F42 standards committee has grouped selective laser melting into the category of “laser sintering”, although this is an acknowledged misnomer because the process fully melts the metal into a solid homogeneous mass, unlike selective laser sintering (SLS) which is a true sintering process. Another name for Selective Laser Melting is Direct Metal Laser Sintering (DMLS), a name deposited by the EOS brand, however misleading on the real process because the part is being melted during the production, not sintered, which mean the part is fully dense. This process is in all points very similar to other SLM processes, and is often considered as a SLM process.

A similar process is electron beam melting (EBM), which uses an electron beam as energy source.

Procedure
In 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 completely remelted locally by means of laser radiation and forms a solid layer of material after solidification. Subsequently, 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 been remelted. The finished component is cleaned of excess powder, processed as needed or used immediately.

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

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

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

But it can also be targeted, manufactured according to bionic principles or to ensure a partial modulus of elasticity, a component with selective densities. In lightweight aerospace and body implants such selective elasticities are often desired within a component and can not be produced using conventional methods.

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 allows the production of component shapes that can not be produced with molded processes or only with great effort. Furthermore, storage costs can be reduced because specific components do not need to be stored, but are produced generatively when needed.

Exposure Strategy
The tendency is that the higher the laser power, the higher the roughness of the component. Modern plant engineering can control the density and surface quality according to the “shell-core principle”. The segmented exposure has a specific influence on 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 speeds. The performance profile of a component can be significantly increased with the help of the segmented exposure.

Quality Aspects and Topology
The plant manufacturers pursue different quality assurance approaches that i. d. R. On the one hand off-axis (or ex situ) done or on the other hand on-axis (or in situ).

Classic off-axis inspections have a lower resolution and a lower detection rate. For example, an infrared-sensitive camera is used, which is positioned outside the process chamber – ie ex situ. The advantage of an ex-situ solution is the simple system integration of system and camera system. An off-axis design allows statements about the overall melting and cooling behavior. However, a detailed statement about the molten bath is not derivable.

On-axis / in-situ setup (eg build concept laser) is based on a coaxial arrangement of the detectors. The detectors used are a camera and a photodiode, which use the same optics as the laser. This coaxial integration enables 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, ie the distance for which a picture is ever taken. At 2,000 mm / s, the value is 200 μm. A coaxial arrangement has the advantage that the Schmelzbademissionen are always focused on a point of the detectors and the image detail is reduced and thus the sampling rate can be increased. A detailed analysis of the melt pool characteristics (melt pool area and melt pool intensity) becomes possible.

Process
DMLS uses a variety of alloys, allowing prototypes to be functional hardware made out of the same material as production components. Since the components are built layer by layer, it is possible to design organic geometries, internal features and challenging passages that could not be cast or otherwise machined. DMLS produces strong, durable metal parts that work well as both functional prototypes or end-use production parts.

The process starts by slicing the 3D CAD file data into layers, usually from 20 to 100 micrometres thick, creating a 2D image of each layer; this file format is the industry standard.stl file used on most layer-based 3D printing or stereolithography technologies. This file is then loaded into a file preparation software package that assigns parameters, values and physical supports that allow the file to be interpreted and built by different types of additive manufacturing machines.

With selective laser melting, thin layers of atomized fine metal powder are evenly distributed using a coating mechanism onto a substrate plate, usually metal, that is fastened to an indexing table that moves in the vertical (Z) axis. This takes place inside a chamber containing a tightly controlled atmosphere of inert gas, either argon or nitrogen at oxygen levels below 500 parts per million. Once each layer has been distributed, each 2D slice of the part geometry is fused by selectively melting the powder. This is accomplished with a high-power laser beam, usually an ytterbium fiber laser with hundreds of watts. The laser beam is directed in the X and Y directions with two high frequency scanning mirrors. The laser energy is intense enough to permit full melting (welding) of the particles to form solid metal. The process is repeated layer after layer until the part is complete.

The DMLS machine uses a high-powered 200 watt Yb-fiber optic laser. Inside the build chamber area, there is a material dispensing platform and a build platform along with a recoater blade used to move new powder over the build platform. The technology fuses metal powder into a solid part by melting it locally using the focused laser beam. Parts are built up additively layer by layer, typically using layers 20 micrometers thick.

Selective heat sintering
Selective heat sintering (SHS) is a type of additive manufacturing process. It works by using a thermal printhead to apply heat to layers of powdered thermoplastic. When a layer is finished, the powder bed moves down, and an automated roller adds a new layer of material which is sintered to form the next cross-section of the model. SHS is best for manufacturing inexpensive prototypes for concept evaluation, fit/form and functional testing. SHS is a Plastics additive manufacturing technique similar to selective laser sintering (SLS), the main difference being that SHS employs a less intense thermal printhead instead of a laser, thereby making it a cheaper solution, and able to be scaled down to desktop sizes.

Selective laser melting Characteristics

Geometric freedom
The freedom of geometry enables the production of complex structures that can not be realized technically or economically with conventional methods. These include undercuts, as they can occur in jewelry or technical components.

Lightweight construction and bionics
It is also possible to produce open-porous structures, whereby lightweight components can be produced while maintaining strength. The potential of lightweight construction is considered a very important advantage of the process. A bionic template from nature is the porous structure of bones. In general, bionics approaches 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 build 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 speaks then of a redesign of the previous construction. The generative component can be installed more easily and the assembly effort is thus generally reduced.

Mixed construction / hybrid construction
Under the mixed construction / hybrid construction in SLM process refers to the production of a partially generatively manufactured component. Here, on a flat surface of a first, conventionally manufactured component area in the subsequent SLM process, a second, generatively manufactured component area is constructed. The advantage of the hybrid construction is that the construction volume to be produced by the SLM process can be greatly reduced and simple geometries can be built conventionally, geometrically more demanding areas by means of the SLM process. Thus, the construction time and the costs for the metallic powder material are reduced due to the smaller volume for the manufactured by the SLM process component area.

Prototypes and unique items
Molded processes require some batch size to transfer the cost of the molds to the unit cost. The SLM process eliminates these limitations: It becomes possible to produce samples or prototypes in a timely manner. In addition, very individual parts can arise as unique, as they are required for dentures, implants, clock elements or jewelry. A special feature is the simultaneous production of unique items in a space (eg dental implants, hip implants or spinal column support elements). It becomes possible to design and manufacture individualized components especially for the patient.

Selective densities
In a conventional milling or turning part, the density of the part is always evenly distributed. With a laser-melted part one can vary. Certain areas of a component may be rigid and others may be elastically applied, e.g. For example, with a honeycomb structure (bionic principles) component requirements can be much more creative compared to conventional techniques.

function integration
The higher the complexity, the better a generative process comes into play. Functions can be integrated (eg with temperature control channels or air injectors or the part receives a hinge function or sensory instruments are integrated into the component). The thus increased value components are more efficient than conventionally manufactured components.

“Green Technology”
Environmental aspects, such as low energy consumption in the operation of a plant and conservation of resources (it is used exactly the material used / no waste) are elementary features of laser melting. There are also no oil or coolant emissions, as is still 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 techniques are increasingly being considered with their disadvantages in terms of sustainability. Laser melting also means a contribution to the reduction of CO₂ emissions in 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 efficiency and quality standards. Generative manufacturing can serve these trends.

Production on demand
An essential aspect of laser melting is the temporal and local production as needed. This can change the logistics concepts (eg at aircraft manufacturers) very much, because spare parts no longer need to be stored but can be printed out if necessary. In addition, one can reduce the inspection times of aircraft in a production-on-demand.

reduced material usage
Especially compared to milling from a full part of the lower material use is striking. It is assumed that on average the pure component weight and about 10% material for the support structures (these are the support structures necessary for the construction) is consumed.

Materials
Many Selective Laser Melting (SLM) machines operate with a work space up to 400 mm (15.748 in) in X & Y and they can go up to 400 mm (15.748 in) Z. Some of the materials being used in this process can include copper, aluminium, stainless steel, tool steel, cobalt chrome, titanium and tungsten. In order for the material to be used in the process it must exist in atomized form (powder form). Currently available alloys used in the process include 17-4 and 15-5 stainless steel, maraging steel, cobalt chromium, inconel 625 and 718, aluminum AlSi10Mg, and titanium Ti6Al4V.

The materials used for the selective laser melting are usually standard materials that contain no binders. The machine manufacturers and their material partners certify the series materials for the users (eg for dental technology or medical applications in accordance with EU Directives and the Product Liability Act).

Series materials are converted by atomization in powder form. This creates spherical particles. The minimum and maximum diameter of the particles used is selected as a function of the layer thickness used and the component quality to be achieved. All powder materials are 100% reusable for subsequent construction processes. Refreshing with unused material is not necessary.

The material consumption is i. d. R. 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).

Used materials are for example:
Stainless steel
Tool steel
Aluminum and aluminum alloys
Titanium and titanium alloys
Chromium-cobalt-molybdenum alloys
Bronze alloys
Precious metal alloys
Nickel-based alloys
Copper alloys
Ceramics

Applications
The types of applications most suited to the selective laser melting process are complex geometries & structures with thin walls and hidden voids or channels on the one hand or low lot sizes on the other hand. Advantage can be gained when producing hybrid forms where solid and partially formed or lattice type geometries can be produced together to create a single object, such as a hip stem or acetabular cup or other orthopedic implant where oseointegration is enhanced by the surface geometry. Much of the pioneering work with selective laser melting technologies is on lightweight parts for aerospace where traditional manufacturing constraints, such as tooling and physical access to surfaces for machining, restrict the design of components. SLM allows parts to be built additively to form near net shape components rather than by removing waste material.

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

Aerospace
Automotive engineering
Dental technology (dentures, implants)
Medical technology (medical devices, endoscopy, implants or orthopedics)
mechanical engineering
Machine tool construction (eg fine and precision drills)
Tool construction (eg inserts for contour-near temperature control)
Lifestyle products, such as jewelry, fashion, shoes or watches
Prototype construction, such as: rapid prototyping
bionically designed lightweight components (technical components that mimic the bone structure, for example)
Small series for racing (automobile sport and motorcycle sport)
technical components of metal

Traditional manufacturing techniques have a relatively high set-up cost (e.g. for creating a mold). While SLM has a high cost per part (mostly because it is time-intensive), it is advisable if only very few parts are to be produced. This is the case e.g. for spare parts of old machines (like vintage cars) or individual products like implants.

Tests by NASA’s Marshall Space Flight Center, which is experimenting with the technique to make some difficult-to-fabricate parts from nickel alloys for the J-2X and RS-25 rocket engines, show that difficult to make parts made with the technique are somewhat weaker than forged and milled parts but often avoid the need for welds which are weak points.

This technology is used to manufacture direct parts for a variety of industries including aerospace, dental, medical and other industries that have small to medium size, highly complex parts and the tooling industry to make direct tooling inserts. DMLS is a very cost and time effective technology. The technology is used both for rapid prototyping, as it decreases development time for new products, and production manufacturing as a cost saving method to simplify assemblies and complex geometries. With a typical build envelope (e.g., for EOS’s EOSINT M280) of 250 x 250 x 325 mm, and the ability to ‘grow’ multiple parts at one time,

The Northwestern Polytechnical University of China is using a similar system to build structural titanium parts for aircraft. An EADS study shows that use of the process would reduce materials and waste in aerospace applications.

On September 5, 2013 Elon Musk tweeted an image of SpaceX’s regeneratively-cooled SuperDraco rocket engine chamber emerging from an EOS 3D metal printer, noting that it was composed of the Inconel superalloy. In a surprise move, SpaceX announced in May 2014 that the flight-qualified version of the SuperDraco engine is fully printed, and is the first fully printed rocket engine. Using Inconel, an alloy of nickel and iron, additively-manufactured by direct metal laser sintering, the engine operates at a chamber pressure of 6,900 kilopascals (1,000 psi) at a very high temperature. The engines are contained in a printed protective nacelle, also DMLS-printed, to prevent fault propagation in the event of an engine failure. The engine completed a full qualification test in May 2014, and is slated to make its first orbital spaceflight in April 2018.

The ability to 3D print the complex parts was key to achieving the low-mass objective of the engine. According to Elon Musk, “It’s a very complex engine, and it was very difficult to form all the cooling channels, the injector head, and the throttling mechanism. Being able to print very high strength advanced alloys… was crucial to being able to create the SuperDraco engine as it is.” The 3D printing process for the SuperDraco engine dramatically reduces lead-time compared to the traditional cast parts, and “has superior strength, ductility, and fracture resistance, with a lower variability in materials properties.”

Industry applications
Aerospace – Air ducts, fixtures or mountings holding specific aeronautic instruments, laser-sintering fits both the needs of commercial and military aerospace
Manufacturing – Laser-sintering can serve niche markets with low volumes at competitive costs. Laser-sintering is independent of economies of scale, this liberates you from focusing on batch size optimization.
Medical – Medical devices are complex, high value products. They have to meet customer requirements exactly. These requirements do not only stem from the operator’s personal preferences: legal requirements or norms that differ widely between regions also have to be complied with. This leads to a multitude of varieties and thus small volumes of the variants offered.
Prototyping – Laser-sintering can help by making design and functional prototypes available. As a result, functional testing can be initiated quickly and flexibly. At the same time, these prototypes can be used to gauge potential customer acceptance.
Tooling – The direct process eliminates tool-path generation and multiple machining processes such as EDM. Tool inserts are built overnight or even in just a few hours. Also the freedom of design can be used to optimize tool performance, for example by integrating conformal cooling channels into the tool.

Other applications
Parts with cavities, undercuts, draft angles
Fit, form, and function models
Tooling, fixtures, and jigs
Conformal cooling channels
Rotors and impellers
Complex bracketing

Potential
Selective laser melting or additive manufacturing, sometimes referred to as rapid manufacturing or rapid prototyping, is in its infancy with relatively few users in comparison to conventional methods such as machining, casting or forging metals, although those that are using the technology have become highly proficient. Like any process or method selective laser melting must be suited to the task at hand. Markets such as aerospace or medical orthopedics have been evaluating the technology as a manufacturing process. Barriers to acceptance are high and compliance issues result in long periods of certification and qualification. This is demonstrated by the lack of fully formed international standards by which to measure the performance of competing systems. The standard in question is ASTM F2792-10 Standard Terminology for Additive Manufacturing Technologies.

Difference from selective laser sintering (SLS)
The use of SLS refers to the process as applied to a variety of materials such as plastics, glass, and ceramics, as well as metals. What sets SLS apart from other 3D printing process is the lacked ability to fully melt the powder, rather heating it up to a specific point where the powder grains can fuse together, allowing the porosity of the material to be controlled. On the other hand, SLM can go one step further than SLS, by using the laser to fully melt the metal, meaning the powder is not being fused together but actually liquified long enough to melt the powder grains into a homogeneous part. Therefore, SLM can produce stronger parts because of reduced porosity and greater control over crystal structure, which helps prevent part failure. However, SLM is only feasible when using a single metal powder.

Benefits
DMLS has many benefits over traditional manufacturing techniques. The ability to quickly produce a unique part is the most obvious because no special tooling is required and parts can be built in a matter of hours. Additionally, DMLS allows for more rigorous testing of prototypes. Since DMLS can use most alloys, prototypes can now be functional hardware made out of the same material as production components.

DMLS is also one of the few additive manufacturing technologies being used in production. Since the components are built layer by layer, it is possible to design internal features and passages that could not be cast or otherwise machined. Complex geometries and assemblies with multiple components can be simplified to fewer parts with a more cost effective assembly. DMLS does not require special tooling like castings, so it is convenient for short production runs.

Constraints
The aspects of size, feature details and surface finish, as well as print through error in the Z axis may be factors that should be considered prior to the use of the technology. However, by planning the build in the machine where most features are built in the x and y axis as the material is laid down, the feature tolerances can be managed well. Surfaces usually have to be polished to achieve mirror or extremely smooth finishes.

For production tooling, material density of a finished part or insert should be addressed prior to use. For example, in injection molding inserts, any surface imperfections will cause imperfections in the plastic part, and the inserts will have to mate with the base of the mold with temperature and surfaces to prevent problems.

Independent of the material system used, the DMLS process leaves a grainy surface finish due to “powder particle size, layer-wise building sequence and [the spreading of the metal powder prior to sintering by the powder distribution mechanism.”

Metallic support structure removal and post processing of the part generated may be a time consuming process and require the use of machining, EDM and/or grinding machines having the same level of accuracy provided by the RP machine.

Laser polishing by means of shallow surface melting of DMLS-produced parts is able to reduce surface roughness by use of a fast-moving laser beam providing “just enough heat energy to cause melting of the surface peaks. The molten mass then flows into the surface valleys by surface tension, gravity and laser pressure, thus diminishing the roughness.”

When using rapid prototyping machines,.stl files, which do not include anything but raw mesh data in binary (generated from Solid Works, CATIA, or other major CAD programs) need further conversion to.cli &.sli files (the format required for non stereolithography machines). Software converts.stl file to.sli files, as with the rest of the process, there can be costs associated with this step.

Machine components
The typical components of a DMLS machine include: a laser, roller, sintering piston, removable build plate, supply powder, supply piston, and optics and mirrors.

Source from Wikipedia