1. Metal AM Processes Overview
Metal Additive Manufacturing (AM) can be broadly categorized into two major categories: Powder Bed Fusion (PBF) and Directed Energy Deposition (DED). These two technologies can be further classified based on the type of energy source used. In PBF technology, thermal energy selectively melts regions of the powder bed. Major PBF technologies include Selective Laser Sintering (SLS), Selective Laser Melting (SLM), Direct Metal Laser Sintering (DMLS), and Electron Beam Melting (EBM). In DED technology, focused thermal energy is used to melt materials (powder or wire) for deposition. Common DED techniques include Laser Engineered Net Shaping (LENS), Direct Metal Deposition (DMD), Electron Beam Free Form Fabrication (EBFFF), and Arc Additive Manufacturing. This paper primarily discusses the fundamental principles, features, and applications of SLS, SLM, DMLS, EBM, and LENS metal 3D printing technologies.
1.1 Selective Laser Sintering (SLS)
Selective Laser Sintering (SLS) is a type of additive manufacturing technology that uses a liquid phase sintering mechanism. During the forming process, a laser partially melts the powder material, and the powder particles retain their solid-phase form, bonding through subsequent liquid phase solidification and solid particle rearrangement. The SLS system consists of a laser, scanning system, powder spreader, powder bed, and powder delivery system, as shown in Figure 2. A 3D CAD model is first drawn on a computer, converted into an STL file format, and sliced into a series of ordered layers using slicing software. These sliced layers are sent to the SLS system, where the powder is preheated before sintering begins. The powder bed is then gradually lowered, and new powder is applied after each scan, with the laser sintering the metal powder layer by layer until the 3D part is complete.
SLS technology offers the advantages of directly manufacturing complex metal parts, fast production time, a wide range of materials, high material utilization, low cost, and simple manufacturing processes. It also allows for integration of design and manufacturing. The process does not require support structures since unsintered powder supports the suspended parts. The accuracy is typically between 0.05 to 2.5 mm, making it suitable for small batches of customized parts. However, the process has some drawbacks, including high material and equipment costs, porous and rough surface finish, and limitations in part size.
1.2 Selective Laser Melting (SLM)
Selective Laser Melting (SLM) is an advanced rapid prototyping technology developed from SLS. The basic principle of SLM is to use computer-aided design software (such as UG or Pro/E) to design the part, then slice the 3D model into layers. Laser beams selectively melt metal powder layer by layer based on a computer-controlled path. The entire process occurs in an inert gas environment to prevent oxidation at high temperatures. The main difference between SLS and SLM is that while SLS does not completely melt the metal powder, SLM fully melts it, resulting in denser and stronger parts.
SLM offers high part density (>99%) and excellent mechanical properties similar to forged parts. It achieves high dimensional accuracy (up to ±0.1 mm) and better surface finishes (Ra 20-50 μm). However, it comes with the drawbacks of high equipment costs, slower production speed, and complex processing parameters.
1.3 Direct Metal Laser Sintering (DMLS)
Direct Metal Laser Sintering (DMLS) uses a high-energy laser beam (200W) to sinter thin layers of metal powder (20-60 μm) to form dense, solid parts. The process is similar to SLS, with the main difference being the powder type. The system components include the build platform, dispenser, recoater, laser system, optical components (e.g., F-θ lens), and high-speed scanners. Parts created with DMLS have varying material structures and mechanical properties, depending on the material used. DMLS offers high accuracy (±0.05 mm) and a part density of over 90%, making it suitable for small-batch production. However, it requires support structures and the use of wire-cutting machines for post-processing.
1.4 Electron Beam Melting (EBM)
Electron Beam Melting (EBM) is another PBF-based additive manufacturing process that uses a high-energy electron beam in a vacuum environment to selectively melt metal powder layers or metal wire for deposition. EBM has several advantages, including high energy efficiency, high formability, and the ability to process materials that are difficult to work with using traditional methods. EBM's disadvantages include the high cost of specialized equipment, limited part sizes, and the generation of strong X-rays during the process, which requires protective measures.
1.5 Laser Engineered Net Shaping (LENS)
Laser Engineered Net Shaping (LENS) is an additive manufacturing technology developed from laser cladding, which combines the principles of SLS with laser sintering. A high-power laser beam forms a melt pool on a metal substrate, and metal powder is injected into the pool where it solidifies rapidly, layer by layer, until the part is formed. LENS offers high design flexibility and can produce parts with complex internal geometries and functionally graded materials.
Summary and Future Outlook
Metal 3D printing technologies have numerous advantages and are becoming widely used in aerospace, oil and gas, marine, automotive, manufacturing tools, and medical fields. The technology can significantly reduce material wastage, lower the material-to-part ratio, and produce highly complex and customized parts. As the technology advances, we expect the costs of metal 3D printing to drop, part quality to improve, and part sizes to increase. Future developments will focus on improving manufacturing speed, surface finish, and the ability to predict material properties to optimize performance in different industrial applications.
