Additive Manufacturing (AM) is a group of manufacturing technologies which are capable of producing 3D solid parts by adding successive layers of material. Parts are fabricated in an additive manner, layer by layer; and the geometric data can be taken from a CAD model directly [3, 4, and 9]

. The main revolutionary aspect of AM is the ability of quickly producing complex geometries without the need of tooling, allowing for greater design freedom [5]. The other advantages include the reduction in manufacturing steps and use of materials; therefore causing reduction in fabricating cost. Additive Manufacturing is also referred to as Solid Freeform Fabrication (SFF) [10], Layered Manufacturing (LM) [11], and e-Manufacturing [12].

Additive Manufacturing can go back to the late 1980’s, early 1990’s [13, 14]. In 1991, CIRP’s STC-E devoted a first keynote paper on a survey of additive manufacturing, which surveys one decade of innovation in AM [13]. Although most processes were already known in 1991, most of them were still in a pre-commercial stage, with some of them reaching the commercialisation stage painfully [15]. The first successful process, Stereolithography from 3D systems, came out in 1991. Followed by other companies, there is a clear breakthrough in 1994 in AM at which time machine sales took off exponentially [15]. Many AM techniques existing today can process materials such as polymers, metals, ceramics and composites. The bonding of material can be achieved by different physical and chemical methods.

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2.1.1 Classification

The classification of AM technologies can be based on the raw material used in the process [16], which divides these technologies into three different categories: Liquid-based processes, Powder-based processes and Solid-based processes. Figure 2-1 shows a family tree of AM technologies. It may not cover all current technologies, but it shows the categorisation of major AM techniques.

Figure 2-1 Family tree of Additive Manufacturing Technologies [16]

Additive

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2.1.2 Liquid-based processes

Most Liquid-based AM systems form solid parts by selectively curing regions of photosensitive polymers using a particular wavelength of light. The light source can be either a scanning laser beam or a wide area light source.

Photocurable resins which can achieve stable properties over time and in different environments are widely used in Liquid-based additive manufacturing technologies [16].

Stereolithography (SLA) system, released by 3D systems in 1987, is widely considered to be the founding process within the field of AM [17]. The stereolithography process uses an ultraviolet (UV) laser to cure a photocurable resin. Parts can be built from a CAD model and the whole process can be controlled by the machine’s software, including automatically generating the supports.

2.1.3 Solid-based processes

AM processes which use solid raw materials in non-powder form have been an integral part of the AM industry since early 1990s [16]. They are still developed and improved by both the suppliers and academic institutions today.

Fused Deposition Modelling (FDM) was commercialised by Stratasys Inc.

in 1991 [8, 18]. In this process, solid wire-shaped materials are heated to a semi-molten consistency before depositing using single or multi nozzle systems. The nozzle systems traverse in X and Y direction to create a two-dimensional layer. FDM can process materials such as polycarbonate, polyphenylsulfone and acrylonitrile butadiene styrene (ABS). This process can build solid parts with little waste, but the size of the extrusion limits the size that any features smaller than double the track width cannot be produced [18].

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Ultrasonic Consolidation (UC) is also a solid-based technology developed by Solidica Inc. in 2002 [19]. It combines the ultrasonic welding of metal foils and additive manufacturing techniques to produce solid parts. The process applies sonic oscillations to metal foil under an applied load. The oscillation bonds the thin metal foil together with a very low heat. This process is capable to process a range of metals such as Iron, Copper, Nickel and dissimilar combinations like Al/Stainless steel and Al/Ni [20].

2.1.4 Powder-based processes

Powder-based additive manufacturing technologies offer a wide range of material possibilities such as polymers, metals and ceramics. Parts can be built with similar material properties and stability compared with solid material.

These technologies can be divided into two main types: powder feed beam. The powder in the melt pool created by the laser can form a cladding line and then cool to form a solid structure when the laser moves away. It is important to melt the powders and homogenise the melt pool for successful building [4]. Fully dense parts can be achieved by this technique [22-24].

Three-Dimensional Printing (3DP) developed at MIT is the basis of a number of technologies that use the application of a binder to a powder layer to construct parts [16-18, 25]

. In the process, a thin powder layer is selectively bonded by ink-jet droplets of adhesive binder. A range of materials can be used in this technique, including metals. But the parts fabricated by this technique usually have high surface roughness and need further post processing operations to obtain final properties [26, 27].

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Selective Laser Sintering (SLS) is an important additive manufacturing technique widely used today, which is referred to as powder bed deposition.

It was first invented in 1979 by Ross Householder, and commercialised in the late 1980s by the University of Texas at Austin, when the first machine came out in 1992 developed by DTM Corporation [28-30]. The process is in many ways similar to Stereolithography; but is capable of processing a variety of materials including polymers, metals and ceramics. The powdered raw material is sintered or partially melted by a laser which selectively scans the surface of the powder bed to create a two-dimensional solid shape, and then a fresh layer of powder is added to the top of the bed to form another solid during the process, it usually operates in an inert protect gas environment [18].

Electron Beam Melting (EBM) is a process very similar to SLS but replaces the laser with an electron beam. It was developed by Arcam in Gothenburg Sweden in 1997 [33].The electron beam is stationary and there is no need for scanning mirrors as the beam can be directed by changing an electromagnetic field, which allows for high scanning speed and fast build rates [34]. The technique offers the ability to fully melt a wide range of metal powders due to the high power developed by the electron beam. However the process is limited to conductive materials and surfaces [16].

Selective Laser Melting (SLM) is also a process very similar to SLS, but it uses a higher energy density to enable full melting of the powder. This technique is capable of building fine details such as thin vertical walls, complex lattice structures and fine cylindrical struts [35, 36]. SLM is capable of processing many standard metal materials like Stainless Steel, Inconel, Titanium alloys and Aluminium alloys [16, 25]. Due to the high temperature involved in processing metals, the use of a protecting gas is important to avoid oxidation. It can also enhance the wet-ability of the molten material and

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reduce the porosity caused by oxidation [37]. However, due to full melting process and high temperature, there can be big thermal stresses and large shrinkage after solidification, which need to be improved.

In document Further process understanding and prediction on selective laser melting of stainless steel 316L (Page 31-36)