Introduction

Selective laser melting (SLM) is an additive manufacturing technique that uses a high-powered laser to fuse small particles of metal powder into a solid 3D object. SLM is similar to other powder bed fusion technologies like direct metal laser sintering (DMLS), but SLM fully melts the powder to create solid parts with high densities and mechanical properties comparable to traditionally manufactured metal parts.

SLM enables the production of complex geometries and customizable metal parts efficiently and without the geometric restrictions of traditional subtractive manufacturing methods. In SLM, a 3D model is digitally sliced into thin layers, and a laser selectively fuses powdered metal material layer-by-layer based on the cross-section of each slice. This article provides an in-depth overview of the SLM process, the technologies involved, materials used, applications, advantages and limitations.

How Selective Laser Melting Process Works

Overview

The key steps involved in producing a part using selective laser melting include:

1. Creating a Computer Aided Design (CAD) model of the desired part geometry
2. Converting the CAD model into an STL file format
3. Slicing the STL file into thin layers
4. Selectively melting the metal powder layers based on the sliced file
5. Post-processing to remove the part from the build plate and to improve surface finish

Overview of the SLM process (Image source: EOS)

CAD Model Creation

The first step is to design the 3D model geometry of the part to be manufactured using CAD software. Complex shapes with internal features like lattices and cavities can be modeled in CAD, which would be impossible to manufacture with conventional techniques. Standard CAD model formats like STEP and IGES can be imported into the SLM machine software.

Conversion to STL Format

The CAD model is then converted into an STL (stereolithography) file format, which approximates the surfaces of the 3D model with a mesh of triangles. The STL file describes only the surface geometry of the 3D model without any representation of color, texture, or other common model attributes.

The size of the triangles in the STL mesh determines the resolution of the printed part. A finer mesh with smaller triangles enables higher resolution but also increases the file size. The optimal STL settings are chosen based on the part geometry, desired resolution, build time, and cost.

Slicing the STL file

The STL file is virtually sliced into thin cross-sectional layers by the SLM machine software. The thickness of each layer corresponds to the resolution of the Z-axis and can range from 20 microns to 100 microns.

Slicing software calculates the cross-sectional geometry of the 3D model for each layer and generates instructions for the SLM machine to selectively melt the powder layers. Slicing optimizes the scan path of the laser to minimize build time.

Selective Laser Melting of Powder Layers

The SLM machine spreads a thin layer of metal powder across the build platform using a recoater arm mechanism. The typical layer thickness is 20 to 100 microns. The laser then selectively scans the powder bed to melt the areas that correspond to the cross-section of the part for that layer, as defined by the sliced STL file.

The areas scanned by the laser fully melt to form a solid. The surrounding unmelted powder serves as support for subsequent layers. The build platform lowers after each layer is formed, and a new layer of powder is spread across it. This process repeats layer-by-layer until the complete 3D object is formed.

Key components in SLM machine:

• Laser: A high-powered fiber laser is used as the heat source to selectively melt the metal powder layers. Lasers with power ranging from 100 W to 500 W are typically used.
• Scanners: Galvanometer scanning mirrors or scanning heads direct the laser beam across the powder bed surface based on the predefined scan path. This controls the melting of the cross-sectional geometry for each layer.
• Powder bed: The powder bed chamber contains the build platform and powder dispensing system. The atmosphere is filled with inert gas like argon or nitrogen to prevent oxidation.
• Recoater: A recoater arm spreads new powder layers across the build platform after each layer is melted.
• Build platform: The platform lowers down after each layer is completed so that new powder can be deposited on top. The final part is anchored to the platform.
• Powder dispensers: Supply fresh powder material to the recoater arm for deposition. Excess powder is collected in overflow containers and reused.
• Computer and software: Software slices the CAD model and controls operating parameters like laser power, scan speed, hatch spacing, and layer thickness.

Post-Processing

After the SLM fabrication process is complete, the build platform is raised, and the part is removed. Support structures are detached from the part either manually or using wire EDM. Additional post-processing may be required to improve surface finish and properties:

• Removal of part from platform: The parts are separated from the build platform using wire EDM or band saws. For delicate parts, the entire platform can be placed in a furnace to heat and loosen the supports.
• Support removal: Support structures are manually broken away from the part and removed. They can also be detached using wire EDM.
• Surface finishing: The stair-stepping effect inherent in the layer-by-layer SLM process results in a rough surface finish. Various finishing operations like grinding, polishing, shot peening are used to improve the surface texture.
• Stress relieving: Heat treatment stress relief cycling is done to remove residual stresses developed during the rapid melting and solidification.
• Hot isostatic pressing: HIPing helps eliminate internal voids and improves density, strength, and surface finish.

Materials Used in SLM Process

SLM can process various alloys ranging from steels, titanium, aluminum, nickel, and cobalt-chrome. The most commonly used materials include:

• Stainless steel: Austenitic steels like 316L and 17-4PH are popular for their strength, corrosion resistance and weldability.
• Tool steel: Maraging steels, H13 tool steel have high hardness and wear resistance ideal for tooling applications.
• Titanium alloys: Strong and biocompatible titanium alloys like Ti6Al4V and Ti64 are extensively used in aerospace and medical industries.
• Aluminum alloys: AlSi10Mg, AlSi12, and Scalmalloy® are examples of aluminum alloys processed by SLM known for their low density.
• Nickel alloys: Inconel 625 and 718 are nickel-based superalloys with properties needed for high-temperature environments.
• Cobalt-Chrome: CoCrMo alloy is widely used for biomedical implants due to its corrosion resistance and biocompatibility.

The powder material properties like particle size distribution, flowability, and minimal internal porosity are critical for high-density SLM parts. Most pre-alloyed powders suitable for SLM range 20-65 microns in size.

Applications of Selective Laser Melting

The key advantages of SLM like the ability to fabricate complex geometries with customized properties has led to rapid adoption across industries like:

• Aerospace: Lightweight aerospace components like turbine blades, fuel nozzles manufactured in nickel and titanium alloys.
• Medical: Customized implants, surgical instruments in titanium, cobalt-chrome and stainless steel alloys.
• Automotive: Lightweight customized parts for racing cars and high-performance vehicles.
• Industrial: Lightweight structural components, custom tooling for injection molding in maraging steels and aluminum alloys.
• Defense: DMLS produces light armor and vehicle parts from high-strength titanium alloys.
• Jewelry: Intricate jewelry designs in precious metals like gold, silver and platinum.

Advantages of SLM Additive Manufacturing

Key benefits of SLM:

• Design freedom: Complex geometries with internal features like lattices can be produced without the design constraints of traditional machining.
• Customization: Patient-specific implants, tooling for one-off production runs can be easily customized.
• Weight reduction: Lightweighting by incorporating lattices and complex shapes not feasible with subtractive methods.
• Rapid prototyping: Concept models, prototypes can be quickly fabricated without tooling to compress development time.
• Minimal material waste: High buy-to-fly ratio as only material required for the final part geometry needs to be melted. Minimal machining waste.
• High strength: Fully dense parts with mechanical properties equaling traditionally manufactured parts.
• Microstructure control: Rapid melting and solidification kinetics results in finer microstructures and unique material properties.

Limitations and Challenges of SLM

Despite the benefits, there are some challenges:

• Slow build rates: Low deposition rates of 5-20g/hour makes SLM time-consuming for large parts compared to casting or CNC machining.
• Anisotropic behavior: Layer-by-layer melting results in anisotropic properties depending on build orientation.
• Rough surface finish: Stair-stepping effect leads to vertical ridges and surface roughness typically Ra 10-15 microns.
• Limited scalability: Maximum build envelope is typically 500 x 280 x 365 mm restricting part dimensions.
• High thermal stresses: High heating and cooling rates result in residual stresses up to the material’s yield strength. Cracking and distortion are concerns.
• Process parameter optimization: SLM requires optimization of parameters like laser power, scan speed, hatch spacing for each material.
• Post-processing: Additional finishing operations needed to improve surface finish and correct defects.
• Costs: High equipment investment of over $500,000 as well as material costs are key limitations.

The Future of Selective Laser Melting

SLM technology continues to advance rapidly with innovations in hardware, software, materials, quality control, and applications:

• Larger build volumes: New machines allow parts up to 750 mm x 550 mm x 500 mm for complete assemblies.
• Multi-laser and multi-material systems: Multiple lasers and powder hoppers enable fabrication of complex multi-material components and composites.
• In-situ monitoring: Vision systems and sensors allow real-time monitoring of defects. Closed-loop adaptive control minimizes errors.
• Simulation and modeling: Thermal and mechanical modeling guides support structure design and parameter optimization specific to part geometry.
• New materials: Light alloys, composites, and functionally graded materials will expand applications. Biomaterials and polymers are being explored.
• Quality standards: Consistent parameters and best practices will be established as experience is gained with novel alloys. Certification and documentation will increase adoption in regulated industries.
• Automation: Automated powder handling, unpacking stations, and integration with CNC machining centers will enable scaled-up production.
• Cost reduction: As the technology matures, higher build rates and competition will lower equipment and operating costs, making SLM more economically viable.

In summary, advancements in SLM systems and materials will drive increased part quality, customizable properties “on-demand”, and expanded applications across aerospace, medical, automotive, and industrial sectors.

Frequently Asked Questions

What is the difference between SLM and SLS?

The main difference is that Selective Laser Melting (SLM) fully melts the powder to create a fully dense part, while Selective Laser Sintering (SLS) only partially melts the powder together into a porous part that requires infiltration with epoxy or bronze to become solid. SLM produces metal parts, whereas SLS is mainly used for plastics and polymers.

What types of metals can be processed with SLM?

SLM can process various metals and alloys including stainless steels, tool steels, titanium alloys, aluminum alloys, nickel superalloys, precious metals, and cobalt-chrome. The most common are 316L, 17-4PH, H13 tool steel, Ti6Al4V, Inconel, and CoCrMo.

What is the typical accuracy and surface finish of SLM parts?

SLM can achieve accuracy around ±100 microns with smoother surfaces compared to other AM processes. However, stair-stepping effects lead to a rough finish of around 10-15 microns Ra surface roughness. Additional machining and polishing is often required to achieve finer finishes.

What is the key benefit of SLM for the aerospace industry?

For aerospace, SLM enables lightweight optimized designs by incorporating internal lattices and complex geometries not feasible with subtractive machining. Parts consolidation also reduces assembly components. The buy-to-fly ratio when machining aerospace parts from solid billets can be up to 20:1 whereas SLM only uses the required material.

How does the cost of SLM compare to CNC machining or other manufacturing methods?

Currently, SLM has a higher part cost compared to conventional techniques for medium to high production volumes due to the high equipment costs and slow build rates. However, for single parts or small batches, SLM has a low incremental cost and avoids high tooling expenses. The breakeven point compared to CNC machining is dependent on factors like build time, material scrap rates, surface finishing needs, etc.