Additive Manufacturing (AM)

Additive manufacturing (AM) is defined by ASTM as the “process of joining materials to make objects directly from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies, such as traditional machining” (ASTM Standard F2792 – 12a 1994). Additive manufacturing technologies can be used anywhere throughout the product life cycle from preproduction (i.e., rapid prototyping) to full scale production (also known as rapid manufacturing) and even for tooling applications or postproduction customization. Since its emergence 25 years ago, additive manufacturing has found applications in industries ranging from architecture, aerospace, automation, defense, precision engineering, oil and gas, dentistry, orthopaedics, and consumer products. Across all industries, additive manufacturing accounted for $1.3B in worldwide sales of materials, equipment, and services in 2010 and is poised to exceed $3B by 2016 (Scott et al. 2012).
The fastest growing application for AM parts is end use products or functional part production. Unlike rapid prototyping, AM is used as a step in the design or production process. In direct part production, AM creates a final product for sale or use. This application category has grown from 4 % of total AM revenues in 2003 to nearly 20 % in 2010 (Scott et al. 2012). This rise in direct part production has happened by increasing material quality from AM processes, reducing cost, and growing awareness of the potential of additive processes (Gu et al. 2012).
From the processing point of view, all AM technologies can be divided into three categories: powder-based, liquid-based, and solid-based rapid manufacturing (Chua et al. 2003). However, using powders as starting materials for making 3D functional objects is the most common in the AM industry. Since the scope of this section is on powder processing of materials, some specific manifestations of powder AM will be examined in more detail.

Powder Bed Additive Manufacturing

Powder bed additive manufacturing is the most common and successful technology for manufacturing of metallic, ceramic, and polymeric components. In this technique, the powder is spread by a counter-rotating roller onto a build platform inside a build box. By means of a binding technology which can be laser, electron beam, or even a binder dispenser the cross section of the object is selectively fused or bonded on the powder bed. Subsequently, the built platform advances downward by one layer thickness, and a new layer of powder is spread and the process is continued layer by layer until full object is complete. A schematic of the process is shown in Fig. 71. One of the advantages of powder bed 3D printing is the supportive function of the powder bed for the complex object. Therefore, the need for support material is greatly reduced and the achievable complexity of the parts is improved.
Common technologies based on powder bed additive manufacturing include inkjet 3D printing, selective laser melting, selective laser sintering, direct metal laser sintering, and electron beam melting.

Powder Jet Additive Manufacturing

As schematically shown in Fig. 72, in powder jet additive manufacturing, a high-powered laser is used to melt metal powder supplied coaxially to the focus of the laser beam through a deposition head. The laser beam typically travels through the center of the head and is focused to a small spot by one or more lenses. The head itself, or the table, will be moving to make one layer from the cross-sectional data of the object. Metal powders are delivered and distributed around the circumference of the head either by gravity or by using a pressurized carrier gas. An inert gas is often used to shield the melt pool from atmospheric oxygen for better control of properties and to promote layer-to-layer adhesion by providing better surface wetting. Laser engineered net shaping or laser aided additive manufacturing are some common names for this technology.

The advantage of this method is the ability to make functionally graded materials through changing composition or rate of powder feeding via the process. Since no support material is used, this process is very limited in the complexity of the object. However, this method has been very successful for repair and remanufacturing purposes.

Colloidal Additive Manufacturing

Other powder-based additive manufacturing can be categorized under colloidal methods, in which ceramic or metallic powder are incorporated into a polymer or solvent and ink jetted or extruded through a nozzle and immobilized using polymerization or solidification mechanisms. Depending on the material and use, the solvent or carrier might need to be burnt off or removed afterwards, and parts must be fired for densification. Since particle friction is greatly reduced in liquid form solutions, the packing and density of the achievable parts can be enhanced and controlled. Methods such as 3D stereolithography, robocasting, and paste dispensing apparatus are some of the common methods for making 3D objects based on colloidal paste or slurries. Most of this technology is in the research stage.

Functionally Graded Materials by Powder Processing

Functionally gradient materials (FGMs) are a category of composite materials characterized by a compositionally graded interface between two component phases, most commonly ceramic/metal, although ceramic/ceramic, metal/metal, and metal/polymer FGMs are also fabricated. The concept originated in Japan in 1984 as a hypothetical spaceplane skin for Mach 10 + with the capacity to withstand a surface temperature of 2,000o C and a temperature gradient of 1,000o C across a cross section of 10 mm or less, an application for which FGMs are the only practical solution (Niino 1990). While the thermal barrier role of FGMs is predominantly a high-technology application, improvements in ceramic–metal and ceramic–ceramic bonding through theFGM concept have applications, from nuclear fusion reactors to biomaterials to wear-resistant tiles. This is because FGMs are an ideal solution to the problem of thermomechanical property mismatch at the interface in a ceramic/metal bond and also in a ceramic/ceramic bond. The main use of FGM films is in bonding dissimilar materials. They are also useful for minimizing thermomechanical mismatch for coatings, for example, a hydroxyapatite-coated alumina femoral component for a knee prosthesis. For hypersonic spaceplanes, an FGM skin is the only viable thermal barrier material.
Most of the published FGM research has involved modelling the properties of hypothetical FGMs, not fabrication techniques. Reported fabrication methods can be split into two main categories: thin film/interfacial FGMs (gradient thickness in the micron range) and bulk FGMs (gradient thickness millimeters to centimeter thick). Bulk FGMs can be further split into two broad categories, namely, stepwise/layered bulk FGMs and continuous bulk FGMs. FGM thin films are primarily made by plasma spraying or CVD (Kerdic et al. 1996). Reported fabrication methods for layered bulk FGMs include laser cladding, slipcasting, sedimentation, slurry dipping, centrifugal casting, co-sedimentation, and electrophoretic deposition (Kerdic et al. 1996; Chavara and Ruys 2007; Chavara et al. 2009a).
Continuous bulk FGMs are the most advanced manifestation of FGMs. They can be made by reconstructive methods or constructive methods. Reported reconstructive methods include sedimentation forming, centrifugal forming, slipcasting, and thixotropic casting. “Reconstructive methods” means that gradient formation is achieved by gravitational segregation of an originally homogenous two-component powder blend as a slurry. It is almost impossible to achieve a linear gradient via a reconstructive method, usually it is irregular, or sigoidal at best. The only reported constructive method for continuous bulk FGMs (constructively engineering the gradient) is impellor dry blending (IDB) (Chavara and Ruys 2007; Chavara et al. 2009a). IDB involves the following stages:

• Preparing ceramic powder feedstock, comprising submicron powders granulated and sieved for appropriate flow rate.
• Sourcing metal spheroidal powder feedstock for appropriate flow rate.
• Flow rate optimization studies of the feedstock and, based on these, programming the computer-controlled feedgate to produce a linear gradient for the chosen feedstock pair.
• Powders are loaded into their respective two feed hoppers of the computer-controlled feedgate.
• Hoppers are vibrated as the computer-controlled feedgate switches on powder flow and feeds the powders into the blending impellor from initially 100 % metal to ultimately 100 % ceramic.
• The powder blend exits the blending impellor and, guided by the exit port, settles like snowflakes onto the cylindrical mold with its rotating screed.
• FGM powder blend is pressed and sintered, usually by hot-pressing. The submicron ceramic powders enable optimal sintering.

Hydrostatic Shock Forming

High-rate forming is a long-established method for forming metals (Rinehart and Pearson 1963). Commonly, it involves placing metal sheet or plate against a mold, immersed in a liquid medium. An explosive charge is detonated in the medium, and the shock waves passing through the liquid press the sheet metal against the mold with high intensity, thereby producing a perfect shape replica in microseconds. It is not a widely used method as it is costly, hazardous, unsuitable for mass production, and therefore rarely commercially justifiable. Its main niche application is for one-off situations requiring high forming pressures, with no high-capacity press available, or press-tooling unviable.

In the case of ceramic powders, however, there are powders which cannot be densified by conventional means. An even more extreme case is a ceramic–metal functionally graded material for which the melting point of the ceramic powder is 1,000o C or more higher than the metal powder. For example:

• Aluminum (660o C)–alumina (2,050o C)
• Copper (1,080o C)–magnesia (2,800o C)
• Stainless steel (1,400o C)–silicon carbide (2,700o C)

Densification by combustion synthesis was proposed as one possible solution to this problem (Ruys et al. 2001). However, this limits FGM densification to the rare situation in which both the metal and ceramic components are combustible, which is not the case for most commercially useful metal–ceramic and ceramic–ceramic combinations. Spark plasma sintering (SPS) is also an option, but even SPS cannot succeed with a 1,000o C sintering temperature difference of metal and ceramic. Hot-pressing enables sintering to take place a few hundred degrees below that required for pressureless sintering. However, with the melting point differentials of typical metal–ceramic combinations commonly in excess of 500o C, this is often not viable either.

Hydrostatic shock forming (HSF) involves the use of a focussed explosive charge which enables localized pressures in excess of 30 GPa, in the order of a thousand times larger than the pressures for hot-pressing. Moreover, 30 GPa is higher than the microhardness of all ceramics (typically 5–15 GPa) except diamond and its polymorphs. Indeed, at pressures in the 30 GPa magnitude range, ceramics can flow hydrodynamically. In addition, there is the instantaneous adiabatic heating from the detonation, which further enhances the potential for densification of metal–ceramic or ceramic–ceramic FGMs with a large difference in melting points between the two components.
The velocity of a detonating high-explosive charge can be anywhere from 3 to 9 km/s and is typically in the order of 6 km/s, and this immense velocity generates an intense pressure wave. A focussed explosive charge transmits a shockwave through a conical water column into the FGM powder blend. Densification of metal–ceramic FGMs by hydrostatic shock forming (HSF) has been demonstrated for Cu–SiC and stainless steel–SiC FGMs (Ruys et al. 2001). Metal–ceramic powder blends were prepared as pellets using impeller-dry-blending (Chavara and Ruys 2007; Chavara et al. 2009b) and placed into pre-machined mild steel dies with a water-filled conical shock-focussing chamber linking the explosive charge with the powder preform. The ceramic powder side of the FGM was on the side in contact with the conical shock-focussing chamber since the ceramic powder needed the shock wave much more than the metal powder.
It is unlikely that HSF will see widespread adoption. However, for specialty applications, such as metal–ceramic FGMs for hypersonic spaceplane skins, it is sometimes the only option and therefore is of significance for the future of powder forming.