Learning Objectives
By the end of this section, students will be able to:
- Describe the basic categorization of AM technologies originally developed by ASTM.
- Describe the limitations of ASTM definitions.
- Apply the AM subfunctions.
During the early 2000s, engineers were continually inventing new AM machines and technologies. With each new development, they brought forth a new process with specific advantages and disadvantages in relation to others that already existed. These new AM technologies required different types of energy sources, feedstocks, and material types, and it soon became confusing for people – even those with deep expertise and experience – to understand exactly what type of AM was being discussed.
To solve this confusion, in 2012, the American Society for Testing and Materials (ASTM) F42 committee defined seven categories of AM processes, as follows:
- VAT Photopolymerization - process in which liquid photopolymer in a vat is selectively cured by light-activated polymerization
- Material Jetting - process in which droplets of build material are selectively deposited
- Binder Jetting - process in which a liquid bonding agent is selectively deposited to join powder materials
- Material Extrusion - process in which material is selectively dispensed through a nozzle or orifice
- Powder Bed Fusion - process in which thermal energy selectively fuses regions of a powder bed
- Sheet Lamination - process in which sheets of material are bonded to form a part
- Direct Energy Deposition - process in which focused thermal energy is used to fuse materials by melting as they are being deposited
By grouping AM technologies into seven different types, it became easier for people to understand exactly what type of AM technology was being described. However, the development of process variants and hybrid AM processes began to accelerate at an unprecedented pace. By 2019, processes were being created that did not fit neatly into the seven categories.
To maintain the classification method and develop a more logical way to differentiate between the technologies, experts began looking at the AM process itself in combination with the subfunctions are used to create a part. Specifically, how each Layer is created, where or how the Material feedstock is added to the layer and what Energy source is used during the process.
Additional post-processing and, sometime, Densification steps, described below, may also be required for some processes. Table 2.1 defines the options for each of these technologies and how often they are required.
| Remove Supports or Substrate | Subtractive process for Dimensions | Process for Surface Finish | Process for Material Density | Heat Treat/Cure for Material Performance | |
|---|---|---|---|---|---|
| Vat Photopolymerization | Very often | Rare | Sometimes | Rare | Sometimes |
| Material Extrusion | Very often | Sometimes | Sometimes | Rare | Rare |
| Power Bed Fusion | Very often | Sometimes | Sometimes | Rare | Very often |
| Directed Energy Deposition | Sometimes | Sometimes | Sometimes | Rare | Very often |
| Material Jetting | Very often | Rare | Sometimes | Sometimes | Rare |
| Binder Jetting | Rare | Sometimes | Sometimes | Sometimes | Very often |
| Sheet Lamination | Very often | Sometimes | Rare | Rare | Rare |
For example, the Material Extrusion process is characterized by: a) wire material feedstock commonly known as filament, b) mechanical fusion from a nozzle, using c) liquification as the means to push the feedstock through the nozzle, resulting in a d) fully dense part. By deconstructing AM processes into three distinct subfunctions of material, layers, and energy sources, and an optional fourth, densification when required, an unlimited number of AM processes can be mapped that stretch beyond the limits of the limited ASTM process definitions.
Energy Source
The energy source subfunction describes how the AM process initiates to create a chain of events that produces AM hardware. Using thermal energy and/or pressure overcomes the material flow stress to change the physical characteristics of the material feedstock. This can be accomplished using a number of different technologies, these include:
- Laser, which is a device that emits a special type of light that can be focused in a tight spot.
- Nd:YVO4 (aka UV): Used in commercial SLA machines to generate ultraviolet laser energy
- Nd:YAG: Used in some metal powder bed fusion machines. Typically, older machines CO2: Used in SLS polymer machines
- Yb-fiber: Used in most metal powder bed fusion machines and DED blown powder or wire fed machines
- Plasma: Similar to conventionally manufacturing process known as plasma arc welding (PAW), AM machines have been outfitted with PAW deposition heads to build up freeform structure
- Fluid Flow: Spouts a liquid that is made up of nanoparticles of ceramic or metallic material to freeform build a part. This category may include low pressure and temperature deposition process that deposits a variety of materials through an inkjet printing head system. Materials could be conductive for circuitry or polymer based.
- High Pressure Gas: A gas is compressed at a high pressure to act as a catalyst to the remaining process.
- Electrical Resistance: An energy source that opposes the flow of electrical current in a circuit, thereby converting electrical energy to thermal energy
- Compression: The phase transformation of a solid material to a liquid typically through a compression and/or material sheering in a heated process.
- Electron Beam: A discharging of a stream of electrons generated by magnetic and electric fields
Material Feedstock
Material feedstock is defined as the raw material form factor that is used at the beginning of the AM process, that is ultimately, physically transformed by the energy source. These may be in the forms of the following:
- Liquid
- Epoxy based photopolymer
- Nano particle slurry
- Powder Either metallic or polymeric
- Stationary before the energy source is applied
- Dynamically moving when the energy source is applied
- Wire Typically, in the form of conventional welding wire that is ordered in spools
- Polymer filament A polymer monofilament feedstock that comes in spools
- Pellets of polymer Similar to injection molding feedstock. Resin pellets created specifically for engineering plastic applications
- Sheets Sheets or foil made from metal, plastic, paper or other fibrous material
Layer Creation
Once the raw material is physically transformed into the AM processing material form factor, new layers are created during the AM process fabrication. As such, the layer is created with the energy and material coming together at a focused point. The following are ways that AM layers are formed:
- Curing: A change in physical state of transformation within a material. The material is hardened by a cross linking of polymer chains typically from a liquid solution to a solid.
- Welding: Joining of additive layers using high thermal energy to melt the feedstock together thereby creating a fusion of metallic or polymer layers into a homogenous structure
- Mechanical fusion from nozzle: Joining liquified polymer material together by welding a heated bead of material to an already cooled region of polymer material
- Kinetic energy: The successive buildup of layers by impacting new material on top of previously impacted material to form a thick coating or structure
- Ultrasonic energy: The bonding of materials using an ultra-high frequency sound waves that is vibrates material thereby merging feedstock together to form a mechanical layered bond
- Friction: The use of a tool or rotating body to locally increase temperature of a mater to allow solid state bonding.
For some of the layer creation method, a material other than the feedstock can be applied. For example, a low-melting polymer powder used to fuse sheet materials together, an agent that increases the feedstock’s ability to absorb energy and fuse, or that prevents fusion for control of features.
Full Densification
Though not a strict definition, full densification is an aggregate of each layer creating hardware that results in a structure which is greater than 99.5% dense directly from the machine without post-processing steps being necessary. These post-processing steps would include other non-AM related processes such as sintering, ultraviolet curing, etc. In the densification sub function, it is a binary decision. Will the part that is removed from the AM machine exhibit 99.5% microstructural density or not?