We bring to you the details of one of the most important of all additive manufacturing processes, Metal 3D printing
The adage ‘change is the only constant’ holds true across all segments of life, including manufacturing. Traditionally, manufacturing has been about bringing raw materials and making a product through various processes by removing all that is not necessary, while keeping the required design. Such manufacturing requires use of excess raw materials, which are then finished off to form the required shape and design. It leads to use of excess amount of raw materials that could potentially be wasted during the process.
An alternative form of manufacturing that has been in continuous development and have found multiple use cases is additive manufacturing or 3D printing. Instead of machining or ‘subtracting’ material to form an object, the idea of 3D printing is to add each layer of material on another to create a product. Specialised 3D printers begin printing a solid object once designers and engineers upload a digital computer-aided-design (CAD) file.
METAL AS RAW MATERIAL
Thermoplastics are the most frequently used materials for additive manufacturing. However, machines are capable of 3D printing products from photopolymers, epoxy resins, metals, and others. Metal 3D printing is gaining popularity due to the possible advantages that this material brings when compared to others. For example, additive manufacturing is extensively used to make tools and dyes for special purpose machines, and the fact that these dyes can be made directly in metal is a boon, both in terms of cost-efficiency and quick turnaround.
There are a few main technologies concerning metal-based additive manufacturing – Powder Bed Fusion (PBF), Metal Binder Jetting, Laser Cladding, Direct Energy Deposition (DED) and Material Extrusion. Metal 3D printing brings together the design flexibility of 3D printing with mechanical properties of metal. It finds applications in the products of tooling inserts with cooling channels and lightweight structures for aerospace as well as any application requiring complex metal parts. Some metals used for metal 3D printing include aluminium, titanium, stainless steel and Inconel. Raw materials can be used in the form of metal powder or wire, while the energy source used could be laser/electron beam or arc.
METAL 3D PRINTING METHODS
PBF technology is the most common type of metal 3D printing and has three main forms of additive manufacturing – Direct Metal Laser Sintering (DMLS), Selective Laser Melting (SLM) and Electron Beam Melting (EBM). Components produced using the PBF melting technology are said to be clear of residual stresses and internal imperfections, making them ideal for applications in the areas of aerospace, automotive, medical equipment, tooling and turbo machinery. Under the PBF technologies, thermal energy selectively fuses regions of powder bed. The DMLS process uses metal powder and a high-power laser to sinter together a useable part, and is capable of producing dense parts, for which post-treatment is usually needed.
Meanwhile, under the EBM technology, a heated powder bed of metal is used in a vacuum that is melted and formed layer upon layer, using an electron beam energy source. This technology results in the creation of a part that is not as precise as SLM, but has an advantage in being a faster process for larger parts.
In the case of Direct Energy Deposition (DED) technologies, focussed thermal energy is used to fuse materials in either powder or wire form by melting, as they are being deposited. Some of the DED-based metal additive manufacturing processes are Laser Engineered Net Shaping, Direct Metal Deposition, Electron Beam Free Form Fabrication and Arc-based 3D printing. The raw material (powder or wire) and the laser both sit on a single print head in the case of DED technology, which dispenses and fuses material simultaneously. This results in the production of parts that are similar to PBF.
The method of Metal Binder Jetting is large in scale and highly accurate, and is considered to be replacing SLM as the premier loose powder-based method of 3D printing. This technology has grown in the additive manufacturing arena over the past few years. Due to its speed and scalability, this technology propels metal additive manufacturing capabilities into production volumes.
The technology behind metal binder jetting reflects what a conventional (2D) printer uses to quickly jet ink onto paper. First, a binder jetting machine evenly distributes metal powder over its print bed, forming an unbound layer. Then, a jetting head much like one in a 2D printer distributes binding polymer in the shape of the part cross section, loosely adhering the powder. The process repeats until the machine yields a finished build of completed parts.
METAL 3D PRINTING PROCESS
The basic fabrication processes for most forms of 3D metal printing remain the same, with the first step consisting of the build chamber being filled with inert gas. This step is carried out in order to reduce the oxidation of the metal powder. The build chamber is subsequently heated to the optimal build temperature. In the second process, a thin layer of metal powder is spread across the build platform, following which a laser of high power scans the cross-section of the component. This leads to the melting (or fusing) of metal particles together and creating the next layer. Additionally, the part is built fully solid, for which the entire area of the model is scanned. The third process begins when the scanning process is complete. Here, the build platform moves down by one-layer thickness and the coating element spreads another thin layer of metal powder. This process is repeated until the entire part is completed.
Once the build process is finished, the parts are fully covered in the metal powder. In case of SLM and DMLS, the parts are attached to the build platform through support structures, which are built using the same material as the part itself. These support structures are required to counteract the warping and distortion that could affect the part due to the high processing temperatures. When the product bin cools to room temperature, the excess powder is manually removed and most parts are typically heat treated, while being still attached to the build platform so that all residual stresses are relieved. Finally, the parts are detached from the build plate using various methods including cutting, machining or wire EDM, and are either ready for use or further post-processing.
BENEFITS & LIMITATIONS
Metal 3D printing comes with its own set of advantages and disadvantages, just as any other form of manufacturing. This process can be used to manufacture complex, bespoke parts with geometries that traditional manufacturing methods are unable to produce. It can make parts that can be topologically-optimised to maximise their performance, while minimising their weight and total number of components in an assembly. Another benefit of this technology is that metal 3D printed parts have a higher level of physical properties and the available material range includes metals that are difficult to process otherwise, such as metal super alloys.
In terms of banes, the materials and manufacturing costs connected with metal 3D printing are high; so these technologies are not suitable for parts that can be easily manufactured through traditional methods. In addition, the production capacity of metal 3D printing systems is limited since precise manufacturing conditions and process controls are required. Finally, the existing designs may not be suitable for metal 3D printing, which may require complete reworking of product designs.
The overall requirement for lean & efficient development and manufacturing across industries highlights the role that additive manufacturing can play. Within this non-traditional form of manufacturing, metal 3D printing is bringing in more capabilities to create one-off and custom parts as well as enable the production of final parts that were earlier required to use other forms of production.
Metal 3D printing has enabled reduction in the amount of time required to make certain parts, while also permitting components to be made out of different metals using additive manufacturing. This is a form of manufacturing that is expected to witness higher user adoption in times to come, especially where requirements include metal as a material, lean manufacturing and short prototyping timelines.
(Inputs from Stratasys, EOS, ScienceDirect, 3D Hubs, 3DPrinting.com, Markforged)
TEXT: Naveen Arul