Solid Freeform Fabrication is a general term used for different types of rapid or layered manufacturing techniques. In this three dimensional parts are manufactured as per part geometry layer by layer, with the aid of Computer Aided Design (CAD) software. While SFF processes were originally developed for rapid prototyping recently its use has gradually moved towards the direct layered manufacturing for fully functional end use parts.
SFF technologies seek to improve upon the conventional processing technologies through a reduction in processing steps, reduction in use of materials and reduction in fabrication time and cost. Geometric freedom is one of the main advantage advantages SFF is widely used in Aerospace, Automobile, Medical industries etc. some of the Solid Freeform Fabrication technologies are listed below
Selective laser sintering is one of layered manufacturing technology. According to the CAD model components/parts can be prepared from powder-based materials layer by layer (2). To prepare the selective laser sintering build, the powdered material is added to the feed chamber. The feed chamber is brought to the operating condition which is slightly below the melting point of the material. Upon launching the build, the build piston lowers the build chamber to one layer thickness i.e. 0.1 - 0.15mm (3). A feed piston rises to present powder and a powder delivery roller traverses the build chamber, depositing the powder into the gap between the build piston and the working surface. A stationary CO2 laser is directed to scanning mirrors that redirect the laser for travel in the X-Y plane. As the laser scans across the powder surface it imparts the thermal energy into the powder bed causing the material to sinter. The depth to which the powder sinters is a function of the laser power and travel velocity. The longer the laser stays the dipper the sintering depth. In this process the support structure is not required hence unsintered powder acts as a support for the object. Therefore the first layer constructed is actual part geometry (the bottom of the part). After completing a layer the build piston lowers and the roller spread a fresh layer of powder across the build chamber. The process is repeats for the subsequent layers. After completing the part cool-down process is done to lower the operating temperature. It has reasonable dimensional accuracy having rough and porous surface finish.
Stereolithography
Stereolithography (SL) is a rapid prototyping process in which parts of a plastic monomer are directly built by photopolymerization process with the model constructed using CAD software. The process of SL involves modelling of part with CAD software to generate 3D solid model conversion of 3D solid model into standardized triangular language (STL) file format to create volumetric mesh and creation of support structure (4). In this process the build is made with the addition of the photopolymer resin to bring the level of operating condition. The build platform is lowered leaving a gap of one layer thickness i.e. 0.03 - 0.25mm (3) between the platform and the top surface of the resin. A stationary UV laser, formerly gas charged and now solid state, is directed to X-Y scanning mirrors. The mirrors redirect the laser beam for travel in the X-Y plane. As the laser scan across the top surface of the resin, it imparts UV energy into the photopolymer, which causes the material to solidify. The depth of curing is a function of the laser power and the travel velocity. Longer laser dwells result in deeper cure depths. In this process support structure is required, hence after constructing approximately 6.4 mm (3) of support structure the first layer of part geometry- the bottom of the part is solidified. To create the part geometry the laser traces the outline borders of the profile and then solidifies the internal area with the overlapping of the X-Y axes. After the layer is complete the build platform lowers by one layer thickness. To level the resin, a blade sweeps over the surface. In preparation for curing of the next layer, sensors check the resin level and the laser power. The process is repeated until the part is complete. In stereolithography dimensional accuracy and surface finish is best of all the rapid prototyping method.
Fused deposition modelling
Process does not require material replenishment prior to the build. If the material supply is insufficient to complete a build, or if the supply is exhausted prior to the end of the build, a new filament cartridge must be inserted into the device. The build chamber temperature is raised to operating level which is just below the melting point of the material. Parts are constructed in open space. The parts are not surrounded with liquid or powdered materials. Therefore upon launching a build the platform is not lowered by one layer thickness. Instead its position provides a gap between it and the extrusion tip. The building process uses an extrusion head called the liquefier which travels in the X and Y direction. In semi molten state the filament is extruded through the tip. Each pass of the extrusion tip deposits a road. The thickness and the depth of the road depend upon the diameter of the extrusion tip and the velocity of the extrusion head. Typically layer thickness is of 0.13- 0.30mm (3). This process is also continuous. In this process support structure are needed. Supports are fixed to the disposable mat placed on the build platform. Starting with the bottom of the part the first layer of the part geometry is deposited, and the process is repeated for the subsequent layer. Dimensional accuracy for this process is better than SLS and shrinkage percentage is similar to the STL process. It gives rough surface finish.
3D printing
3D printing is a unique process that manufactured some complex three dimensional structures. In 3D printing process materials such as ceramics, metals and polymers are used for manufacturing the parts (5). At the beginning of the process, the binder supply is replenished and fresh powder is added to the feed box. When a build is launched, the build platform is lowers into the build chamber to a depth of one layer thickness usually 0.089-0.178mm (3). A piston rises in the feed chamber to present fresh powder, and roller spreads the material across the build chamber. In this process there is no need of support structures. The process uses an inkjet print-head assembly that deposits liquid binder. Traversing the build chamber on a gantry, the print head passes onto the powder bed. After each print head pass, the gantry moves across the build chamber for the next pass.
Laser Engineered Net Shaping (LENS)
LENS process is a laser-assisted, direct metal manufacturing process under development at Sandia National Laboratories. This process uses the features from stereolithography and laser surfacing using computer-aided design file cross section to control the forming process. Powder metal particles are delivered in a gas stream into the focus of a laser to form a molten pool. The part is driven on an x/y stage to generate a 3 dimensional part by layer-wise, additive processing (6).
Electron Beam Melting
A thin layer of metal powder is spread across the build platform. Using an electron beam to melt compositions of metal powder from a pre-laid powder bed. The high heat intensity generated by the electron beam is capable of completely melting the powder together creating high density metal parts. Due to the high heat intensity EBM process needs surface finishing operation (7).
Ultrasonic Consolidation
Ultrasonic Consolidation is a additive manufacturing process where 3 dimensional parts are manufactured layer by layer from thin metal foils (8). Ultrasonic Consolidation process consists of transducer which is piezoelectric that convert's high frequency electrical energy into high frequency mechanical oscillation. Booster which is having specific geometry component to increase or decrease amplitude gain at required frequency. Sonotrode is a tool which transfers the mechanical energy into the foil material. Dummy booster acts as a support for the sonotrode so that force can be applied equally on either side of the sonotrode. Ultrasonic welded foil material that are layered and overlapped to create a solid metal blank. This foil is mounted on the base plate which provides a solid flat expendable surface for the foil to be ultrasonically deposited onto prior to CNC machining, whereas base plat is mounted on the anvil which provides a solid platform for holding the base plat in place and providing a reaction force for the compression from the sonotrode. The process applies ultrasonic oscillations to metal foil under an applied load. The oscillation bonds the thin metal foil together with a very low heat.
Chapter - 2 Selective Laser Melting
SLM Process
Selective Laser Melting (SLM) process is layerwise process. SLM is a SFF process whereby a three-dimensional part is built layer wise by laser scanning over a pre-deposited powder bed (9).SLM uses a laser beam to melt the small metal powder particles deposited onto a powder bed to build a 3D-object. On the surface of the powder bed, a laser selectively fuses powdered material by scanning the cross-section generated from a 3D CAD model of the part.. After scanning each cross-section, the powdered bed is lowered by one layer thickness, a new layer of powder material is applied on the top, and the process is repeated until the part is completed. Figure 1 shows the schematic and working principle of SLM process (10).
The processing chamber of the SLM machine is inerted using argon gas. A stainless steel or any different powder according to the need was installed on the forming substrate and levelled with the hopper. The fresh powder is delivered through a powder feeding equipment, after the powders are rolled by a moving a flat edge, scraper roller, thus a smooth powder layer can be obtained. By using a scanning strategy the powder is melted using different scan speeds and laser powers. When building multi-layers samples, the powders could be melted via the fibre laser scanning a sliced layer according to the objective's CAD model, the substrate/platform is moved down a layer thickness (11).
Figure Selective Laser MeltingBy the use of SLM process the parts are manufactured with greater geometric freedom and great accuracy.
SLM Key Parameters
Within the SLM process there are many parameters that affect the properties of a fabricated part. Laser power, spot size, scanning velocity and application of protective gas atmosphere are the parameters which should be considered while manufacturing a prototype. Scanning strategy, orientation of the prototype, distance in between the prototypes (10).
Laser Power
The amount of laser beam penetrates the material is a function of power. If there is too much power the material will simply vaporise (12).
Spot size
Spot size is the actual size of the pulse laser beam as it strikes the surface of the material it is processing. Spot size does not have any impact on the depth of the melting.
Wavelength
Laser with shorter wavelengths have more energetic photons that can be absorbed by a greater number of bound electrons.
Pulse Energy
Pulse energy is the amount of energy contained within each laser pulse.
Pulse Duration
Pulse duration is the time for which a single laser pulse is emitted.
Scan Speed
Scan speed is the rate at which the laser spot moves relative to the powder bed. As scan speed goes up, energy delivered goes down.
Scanning strategies
There are six different laser scanning strategies can be used while using SLM and SLS process for SFF. The following are the six scanning strategies
Line X
Line Y
Sector 5 - successive
Sector 5 - Least Heat Influence (LHI)
Sector 2.5 - successive
Sector 2.5 - Least Heat Influence (LHI)
When the laser scans the surface or powder bed horizontally it is termed as Line X scanning strategy, whereas when laser scans the powder bed or surface vertically then it is termed as Line Y scanning strategy. For the 3,4,5,6 scanning strategy the part is divided into small division which is known as 'vector' (5 x 5) or (2.5 x 2.5). Initially laser scans the left bottom sector of the part horizontally i.e. in 'X' direction. The next sector at right is scanned with a line Y pattern, the next again with line 'X' starting point of the corner also varied with the subsequent layer when this type of scanning is done then it is Sector 5 - successive. Starts scanning randomly at selected sector, and next sector is scanned according to the least heated vector, which is farthest vector away from the part is scanned and so on between the subsequent sector the orientation of the line scan is altered between X and Y, when this type of scanning is done then it is termed as sector 5 - LHI (13). These types of scanning strategies are used to build up the part completely.
Types of Scanning Strategies
For building the cubes and cylindrical parts SLM 100 can be used. Following are the scanning strategies which we can achieve on SLM 100:
Standard XY (Alternate each layer)
XY within one layer (Remelt)
Stripes (2.5 x 4mm stripe)
Hatching first & then Boundary
Checker board
1. Standard XY (Alternate each layer)
In this scanning strategy laser scans the first layer of part in horizontal i.e. in X direction, and for subsequent another layer of part scans in vertical direction i.e. in Y direction. It continues until complete part is made.
2. XY within one layer (Remelt)
In this scanning strategy laser scan the first layer in horizontal i.e. in X direction and in Y direction alternately for same layer. In this process, laser scans one layer twice that is known as remelting process.
3. Stripes (2.5 x 4mm stripe)
In this scanning strategy laser scans in shorter distance i.e. stripes.
4. Hatching first & then Boundary
In this scanning strategy laser scan the core portion of the part first and then to the boundary of the part.
Preheating
Materials
There are variety of materials can be used in SLM process.
Steels
The advancement has developed new biomedical metal devices which were impractical to be produced. Medical grade metals such as stainless steel are considered to be biocompatible and commonly used in orthopaedic, dental, and maxillofacial implants (Lawrence et al.,2005)
Among medical grade metals, 316L Stainless Steel (SS) is a well-known alloy to manufacture internal fixation devices (Hao et al., 2005). It is noted that although pure SS is an acceptable for these applications, but it does not promote new tissue growth and, hence, is inferior to a material which possesses the required mechanical properties and encourages tissue re-growth (14).
Ceramics
On the other hand, usage of hydroxyapatite (HA), as a bioactive ceramic based on calcium phosphate (Wang, 2003), actively encourages bonds with natural bone (Hing et al., 1997). This property, if be combined with the porous structure originated from would result in an implant that exhibits similar properties to those of the original bone and encourages bone to re-grow within the implant (Hao et al., 2009). According to the individual properties of 316L SS and HA, combination of high strength 316L SS with HA bioceramic leads to a load-bearing and bioactive composite. In other words, a SS/HA composite could provide improved properties for medical applications such as internal fixation implants, hip implants, and maxillofacial implants by combining the strength of the SS and the ability of the HA to stimulate bone and tissue re-growth at the interface of the implant and bone tissue in the human body (14).
Aluminium
Aluminium alloys are mostly used in all variety of industries, i.e. car industry, mechanical engineering and aerospace industry. For some applications the additive manufacturing technology represents an economical alternative to the various types of mould casting (15).
Applications
The most complex geometries ever to be generated in dense non-infiltrated metals direct from individual data (16). Also manufacturing time of the part is less as compared to the traditional manufacturing process with greater accuracy and precision. Because of these advantages SLS process is used in aerospace, automobile and medical industry.
Microstructure analysis
Microstructure is defined as the structure or internal structure of built material part as revealed by the microscope. Optical microscopes can be used for determining the porosity of prepared cross-sectioned part. Electron microscopes can be used for checking the microstructure of a cross-sectioned part. The microstructure of the material strongly influences the physical properties such as strength, toughness, hardness, etc.
Cracks, porosity, crystalline structure can be observed by microstructure analysis. Depending upon different scanning strategies internal microstructure is varied which ultimately changes the mechanical properties of the build part or prototypes.
Mechanical testing
Ultimate Testing Strength (UTS)
Maximum stress a material can withstand the tension, compression or shearing.
Yield
Change in the material from its elastic deformation to permanent plastic deformation of a structure.
Elongation at Break
Change in increase in length to its original length is known as elongation of length.
Hardness
Hardness refers to various property of material or matter which has resistance to various shapes.
Young's Modulus
When stress is directly proportional to strain in the limit of Hook's law then it is termed as a young's modulus.
