Advantages and disadvantages

The main advantage of electroforming is that it reproduces the external shape of the mandrel within one micrometer. Generally, forming an internal cavity accurately is more difficult than forming an external shape, however the opposite holds true for electroforming because the mandrel's exterior can be accurately machined.

Compared to other basic metal forming processes (casting, forging, stamping, deep drawing, machining and fabricating) electroforming is very effective when requirements call for extreme tolerances, complexity or light weight. The precision and resolution inherent in the photographically produced conductive patterned substrate, allows finer geometries to be produced to tighter tolerances while maintaining superior edge definition with a near optical finish. Electroformed metal is extremely pure, with superior properties over wrought metal due to its refined crystal structure. Multiple layers of electroformed metal can be molecularly bonded together, or to different substrate materials to produce complex structures with "grown-on" flanges and bosses.

Tolerances of 1.5 to 3 nanometers have been reported

A wide variety of shapes and sizes can be made by electroforming, the principal limitation being the need to strip the product from the mandrel. Since the fabrication of a product requires only a single pattern or mandrel, low production quantities can be made economically.

Electron beam melting (EBM) is a type of additive manufacturing for metal parts. It is often classified as a rapid manufacturing method. The technology manufactures parts by melting metal powder layer per layer with an electron beam in a high vacuum. Unlike some metal sintering techniques, the parts are fully dense, void-free, and extremely strong.

This solid freeform fabrication method produces fully dense metal parts directly from metal powder with characteristics of the target material. The EBM machine reads data from a 3D CAD model and lays down successive layers of powdered material. These layers are melted together utilizing a computer controlled electron beam. In this way it builds up the parts. The process takes place under vacuum, which makes it suited to manufacture parts in reactive materials with a high affinity for oxygen, e.g. titanium.

The melted material is from a pure alloy in powder form of the final material to be fabricated (no filler). For that reason the electron beam technology doesn't require additional thermal treatment to obtain the full mechanical properties of the parts. That aspect allows classification of EBM with selective laser melting (SLM) where competing technologies like SLS and DMLS require thermal treatment after fabrication. Comparatively to SLM and DMLS, EBM has a generally superior build rate because of its higher energy density and scanning method.

The EBM process operates at an elevated temperature, typically between 700 and 1.000°C, producing parts that are virtually free from residual stress, and eliminating the need for heat treatment after the build.

Melt rate: up to 80 cm³/h. Minimum layer thickness: 0.05 millimetres (0.0020 in). Tolerance capability: +/- 0.2 mm.

This technology was developed by Arcam AB in Sweden.

Titanium alloys are widely used with this technology which makes it a suitable choice for the medical implant market.

CE-certified acetabular cups are in series production with EBM since 2007 by two European orthopedic implant manufacturers, Adler Ortho and Lima-Lto. The acetabular cups are manufactured with integrated, engineered trabecular structures for enhanced osseointegration, and more than 10.000 cups have been implanted to date.

Aerospace and other highly demanding mechanical applications are also targeted.

The EBM process was recently developed for manufacturing parts in gamma titanium aluminide, and is currently used by Avio S.p.A. for the production of turbine blades in γ-TiAl for aero engines.

An electron beam furnace (EB furnace) is a type of vacuum furnace employing high-energy electron beam in vacuum as the mean for delivery of heat to the material being melted. It is one of the electron beam technologies.

Electron beam furnaces are used for production and refining of high-purity metals (especially titanium, vanadium, tantalum, niobium, hafnium, etc.) and some exotic alloys. The EB furnaces use a hot cathode for production of electrons and high voltage for accelerating them towards the target to be melted.

An alternative for an electron beam furnace can be an electric arc furnace in vacuum. Somewhat similar technologies are electron beam melting and electron beam welding.

ELECTRON BEAM PROCESSING

Electron beam processing involves irradiation (treatment) of products using a high-energy electron beam accelerator. Electron beam accelerators utilize an on-off technology, with a common design being similar to that of a cathode ray television.

Electron beam processing is used in industry primarily for three product modifications:

Crosslinking. The cross-linking of polymers through electron beam processing changes a thermoplastic material into a thermoset. When polymers are crosslinked, the molecular movement is severely impeded, making the polymer stable against heat. This locking together of molecules is the origin of all of the benefits of crosslinking, including the improvement of the following properties:

Cross-Linking is the interconnection of adjacent long molecules with networks of bonds induced by chemical treatment or Electron Beam treatment. Electron Beam processing of thermoplastic material results in an array of enhancements, such as an increase in tensile strength, and resistance to abrasions, stress cracking and solvents. Joint replacements such as knees and hips are being manufactured from Cross-Linked Polyethylene because of the excellent wear characteristics.

Polymers which are commonly crosslinked using the electron beam irradiation process include polyvinyl chloride (PVC), thermoplastic polyurethanes and elastomers (TPUs), polybutylene terephthalate (PBT), polyamides / nylon (PA66, PA6, PA11, PA12), polyvinylidene fluoride (PVDF), polymethylpentene (PMP), polyethylenes (LLDPE, LDPE, MDPE, HDPE, UHMWPE), and ethylene copolymers such as ethylene-vinyl acetate (EVA) and ethylene tetrafluoroethylene (ETFE). Some of the polymers utilize additives to make the polymer more readily irradiation crosslinkable.

Cross-Linked Polyethylene piping called PEX is commonly used as an alternative to copper piping for water lines in newer home construction. PEX piping will outlast copper and has performance characteristics that are superior to copper in many ways.

Chain-scissioning. Chain scissioning or polymer degradation can also be achieved through electron beam processing. The effect of the electron beam can cause the degradation of polymers, breaking chains and therefore reducing the molecular weight. The chain scissioning effects observed in polytetrafluoroethylene (PTFE) have been used to created fine micropowders from scrap or off-grade materials.

Chain Scission is the breaking apart of molecular chains to produce required molecular sub-units from the chain. Electron Beam processing provides Chain Scission without the use of harsh chemicals usually utilized to initiate Chain Scission.

An example of this process is the breaking down of cellulose fibers extracted from wood in order to shorten the molecules, thereby producing a raw material that can then be used to produce biodegradable detergents and diet-food substitutes.

Teflon (PTFE) is also Electron Beam processed, allowing it to be ground to a fine powder for use in inks and as coatings for the automotive industry.

Sterilization (microbiology). Electron beam processing has the ability to break the chains of DNA in living organisms, such as bacteria, resulting in microbial death and rendering the space they inhabit sterile. E-beam processing has been used for the sterilization of medical products and aseptic packaging materials for foods as well as disinfestation, the elimination of live insects from grain, tobacco, and other unprocessed bulk crops.

Electron Beam processing has the ability to kill bacteria, thereby rendering a product sterile. Sterilization with electrons has significant advantages over other methods of sterilization currently in use. The process is quick, reliable, and compatible with most materials. Does not require any quarantine following the processing. For some materials and products that are sensitive to oxidative effects, radiation tolerance levels for electron beam irradiation may be slightly higher than for gamma exposure. This is due to the higher dose rates and shorter exposure times of e-beam irradiation which have been shown to reduce the degradative effects of oxygen.

Pest & Pathogen Control. Electron Beam processing as a disinfestation method replaces antiquated environmentally unfriendly methods such as fumigation and chemical dipping. A significant area for this technology is the herb and spice industry. These commodities are valued for their distinctive flavors, aromas and colors. They can be processed by this technology to reduce bacterial contamination without compromise to their sensory properties.

Fruits, vegetables, grains and other food items can be processed by Electron Beam to control fruit flies and other insects that use these commodities as a host for propagation. Suitable as a quarantine measure, several countries rely on this technology to treat food commodities prior to exporting.

In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow - the natural science of fluids (liquids and gases) in motion. It has several subdisciplines itself, including aerodynamics (the study of air and other gases in motion) and hydrodynamics (the study of liquids in motion). Fluid dynamics has a wide range of applications, including calculating forces and moments on aircraft, determining the mass flow rate of petroleum through pipelines, predicting weather patterns, understanding nebulae in interstellar space and reportedly modeling fission weapon detonation. Some of its principles are even used in traffic engineering, where traffic is treated as a continuous fluid.

Fluid dynamics offers a systematic structure that underlies these practical disciplines, that embraces empirical and semi-empirical laws derived from flow measurement and used to solve practical problems. The solution to a fluid dynamics problem typically involves calculating various properties of the fluid, such as velocity, pressure, density, and temperature, as functions of space and time.

Historically, hydrodynamics meant something different than it does today. Before the twentieth century, hydrodynamics was synonymous with fluid dynamics. This is still reflected in names of some fluid dynamics topics, like magnetohydrodynamics and hydrodynamic stability - both also applicable in, as well as being applied to, gases.

Plasma processing is a plasma-based material processing technology that aims at modifying the chemical and physical properties of a surface.

Plasma processing techniques include:

Plasma activation (or Plasma functionalization). It is done with the intent to alter or improve adhesion properties of surfaces prior to coating, painting, etc. In most cases, the surface in question is surface of a polymer material and weakly ionised oxygen plasma is used.

Surface activation is a result of following processes:

Removal of weak boundary layers. Plasma removes surface layers with the lowest molecular weight, at the same time it oxidises the uppermost atomic layer of the polymer.

Cross-linking of surface molecules. Oxygen radicals (and UV radiation, if present) help break up bonds and promote the three dimensional cross bonding of molecules.

Generation of polar groups. Oxidation of the polymer is responsible for the increase in polar groups which is directly related to the adhesion properties of the polymer surface.

Plasma activation (or Plasma functionalization). It is done with the intent to alter or improve adhesion properties of surfaces prior to coating, painting, etc. In most cases, the surface in question is surface of a polymer material and weakly ionised oxygen plasma is used.

Surface activation is a result of following processes:

Removal of weak boundary layers. Plasma removes surface layers with the lowest molecular weight, at the same time it oxidises the uppermost atomic layer of the polymer.

Cross-linking of surface molecules. Oxygen radicals (and UV radiation, if present) help break up bonds and promote the three dimensional cross bonding of molecules

Generation of polar groups. Oxidation of the polymer is responsible for the increase in polar groups which is directly related to the adhesion properties of the polymer surface.

Plasma polymerization uses plasma sources to generate a gas discharge that provides energy to activate or fragment gaseous or liquid monomer, often containing a vinyl group, in order to initiate polymerization.

Plasma polymerization can be used to deposit polymer thin films. By selecting the monomer type and the energy density per monomer, known as Yasuda parameter, the chemical composition and structure of the resulting thin film can be varied in a wide range.

Plasma cleaning involves the removal of impurities and contaminants from surfaces through the use of an energetic plasma created from gaseous species. Gases such as argon and oxygen, as well as mixtures such as air and hydrogen/nitrogen are used. The plasma is created by using high frequency voltages (typically kHz to >MHz) to ionise the low pressure gas (typically around 1/1000 atmospheric pressure), although atmospheric pressure plasmas are now also common.

In a plasma gas atoms are excited to higher energy states and also ionised. As the atoms and molecules 'relax' to their normal, lower energy states they release a photon of light, this results in the characteristic “glow” or light associated with plasma. Different gases give different colours. For example, oxygen plasma emits a light blue color.

A plasma’s activated species include atoms, molecules, ions, electrons, free radicals, metastables, and photons in the short wave ultraviolet (vacuum UV, or VUV for short) range. This 'soup', which incidentally is around room temperature, then interacts with any surface placed in the plasma.

If the gas used is oxygen, the plasma is an effective, economical, environmentally safe method for critical cleaning. The VUV energy is very effective in the breaking of most organic bonds (i.e., C-H, C-C, C=C, C-O, and C-N) of surface contaminants. This helps to break apart high molecular weight contaminants. A second cleaning action is carried out by the oxygen species created in the plasma (O₂⁺, O₂^-, O₃, O, O⁺, O^-, ionised ozone, metastably-excited oxygen, and free electrons). These species react with organic contaminants to form H₂O, CO, CO₂, and lower molecular weight hydrocarbons. These compounds have relatively high vapour pressures and are evacuated from the chamber during processing. The resulting surface is ultra-clean.

If the part to be treated consists of easily oxidised materials such as silver or copper, inert gases such as argon or helium are used instead. The plasma activated atoms and ions behave like a molecular sandblast and can break down organic contaminants. These contaminants are again vapourised and evacuated from the chamber during processing.

Most of these by-products are small quantities of harmless gasses such as carbon dioxide, and water vapor with trace amounts of carbon monoxide and other hydrocarbons. To put this in perspective, 10 minutes of automobile exhaust is approximately equivalent to one year of plasma cleaning exhaust.

Whether or not organic removal is complete can be assessed with contact angle measurements. When an organic contaminant is present, the contact angle of water with the device will be high. After the removal of the contaminant, the contact angle will be reduced to that characteristic of contact with the pure substrate.

Plasma electrolytic oxidation (PEO), also known as microarc oxidation (MAO), is an electrochemical surface treatment process for generating oxide coatings on metals. It is similar to anodizing, but it employs higher potentials, so that discharges occur and the resulting plasma modifies the structure of the oxide layer. This process can be used to grow thick (tens or hundreds of micrometers), largely crystalline, oxide coatings on metals such as aluminium, magnesium and titanium. Because they can present high hardness and a continuous barrier, these coatings can offer protection against wear, corrosion or heat as well as electrical insulation.

The coating is a chemical conversion of the substrate metal into its oxide, and grows both inwards and outwards from the original metal surface. Because it is a conversion coating, rather than a deposited coating (such as a coating formed by plasma spraying), it has excellent adhesion to the substrate metal. A wide range of substrate alloys can be coated, including all wrought aluminium alloys and most cast alloys, although high levels of silicon can reduce coating quality.

Metals such as aluminium naturally form a passivating oxide layer which provides moderate protection against corrosion. The layer is strongly adherent to the metal surface, and it will regrow quickly if scratched off. In conventional anodizing, this layer of oxide is grown on the surface of the metal by the application of electrical potential, while the part is immersed in an acidic electrolyte.

In plasma electrolytic oxidation, higher potentials are applied. For example, in the plasma electrolytic oxidation of aluminium, at least 200 V must be applied. This locally exceeds the dielectric breakdown potential of the growing oxide film, and discharges occur. These discharges result in localised plasma reactions, with conditions of high temperature and pressure which modify the growing oxide. Processes include melting, melt-flow, re-solidification, sintering and densification of the growing oxide. One of the most significant effects, is that the oxide is partially converted from amorphous alumina into crystalline forms such as corundum (α-Al₂O₃) which is much harder. As a result, mechanical properties such as wear resistance and toughness are enhanced.

The part to be coated is immersed in a bath of electrolyte which usually consists of a dilute alkaline solution such as KOH. It is electrically connected, so as to become one of the electrodes in the electrochemical cell, with the other, being a stainless steel counter-electrode, often the wall of the bath itself.

Potentials of over 200 V are applied between these two electrodes. These may be continuous or pulsed direct current (DC) (in which case the part is simply an anode in DC operation), or alternating pulses (alternating current or "pulsed bi-polar" operation) where the stainless steel counter electrode might just be earthed.

Plasma electrolytic oxide coatings are generally recognized for high hardness, wear resistance, and corrosion resistance. However, the coating properties are highly dependent on the substrate used, as well as on the composition of the electrolyte and the electrical regime used (see 'Equipment used' section, above).

Even on aluminium, the coating properties can vary strongly according to the exact alloy composition. For instance, the hardest coatings can be achieved on 2XXX series aluminium alloys, where the highest proportion of crystalline phase corundum (α-Al₂O₃) is formed, resulting in hardness of ~2000 HV, whereas coatings on the 5XXX series have less of this important constituent and are hence softer. Extensive work is being pursued by Prof. T. W. Clyne at the University of Cambridge to investigate the fundamental electrical and plasma physical processes involved in this process, having previously elucidated some of the micromechanical (& pore architectural), mechanical and thermal characteristics of PEO coatings.

A shape memory alloy (SMA, smart metal, memory metal, memory alloy, muscle wire, smart alloy) is an alloy that "remembers" its original, cold-forged shape: returning the pre-deformed shape by heating. This material is a lightweight, solid-state alternative to conventional actuators such as hydraulic, pneumatic, and motor-based systems. Shape memory alloys have applications in industries including medical and aerospace.

The three main types of shape memory alloys are the copper-zinc-aluminium-nickel, copper-aluminium-nickel, and nickel-titanium (NiTi) alloys but SMAs can also be created by alloying zinc, copper, gold and iron. NiTi alloys are generally more expensive and change from austenite to martensite upon cooling; M_f is the temperature at which the transition to martensite completes upon cooling. Accordingly, during heating A_s and A_f are the temperatures at which the transformation from martensite to austenite starts and finishes. Repeated use of the shape memory effect may lead to a shift of the characteristic transformation temperatures (this effect is known as functional fatigue, as it is closely related with a change of microstructural and functional properties of the material).

The transition from the martensite phase to the austenite phase is only dependent on temperature and stress, not time, as most phase changes are, as there is no diffusion involved. Similarly, the austenite structure receives its name from steel alloys of a similar structure. It is the reversible diffusionless transition between these two phases that results in special properties. While martensite can be formed from austenite by rapidly cooling carbon-steel, this process is not reversible, so steel does not have shape memory properties.

In this figure, ξ(T) represents the martensite fraction. The difference between the heating transition and the cooling transition gives rise to hysteresis where some of the mechanical energy is lost in the process. The shape of the curve depends on the material properties of the shape memory alloy, such as the alloying and work hardening.

One-way vs. two-way shape memory. Shape memory alloys have different shape memory effects. Two common effects are one-way and two-way shape memory. A schematic of the effects is shown below.

In the figure above, the procedures are very similar: starting from martensite (a), adding a reversible deformation for the one-way effect or severe deformation with an irreversible amount for the two-way (b), heating the sample (c) and cooling it again (d).

One-way memory effect. When a shape memory alloy is in its cold state (below As), the metal can be bent or stretched and will hold those shapes until heated above the transition temperature. Upon heating, the shape changes to its original. When the metal cools again it will remain in the hot shape, until deformed again.

With the one-way effect, cooling from high temperatures does not cause a macroscopic shape change. A deformation is necessary to create the low-temperature shape. On heating, transformation starts at A_s and is completed at A_f (typically 2 to 20 °C or hotter, depending on the alloy or the loading conditions). A_s is determined by the alloy type and composition and can vary between −150 °C and 200 °C.

Two-way memory effect. The two-way shape memory effect is the effect that the material remembers two different shapes: one at low temperatures, and one at the high-temperature shape. A material that shows a shape memory effect during both heating and cooling is called two-way shape memory. This can also be obtained without the application of an external force (intrinsic two-way effect). The reason the material behaves so differently in these situations lies in training. Training implies that a shape memory can "learn" to behave in a certain way. Under normal circumstances, a shape memory alloy "remembers" its high-temperature shape, but upon heating to recover the high-temperature shape, immediately "forgets" the low-temperature shape. However, it can be "trained" to "remember" to leave some reminders of the deformed low-temperature condition in the high-temperature phases. There are several ways of doing this. A shaped, trained object heated beyond a certain point will lose the two-way memory effect, this is known as "amnesia".

Pseudo-elasticity. One of the commercial uses of shape memory alloy exploits the pseudo-elastic properties of the metal during the high-temperature (austenitic) phase. The frames of reading glasses have been made of shape memory alloy as they can undergo large deformations in their high-temperature state and then instantly revert back to their original shape when the stress is removed. This is the result of pseudoelasticity; the martensitic phase is generated by stressing the metal in the austenitic state and this martensite phase is capable of large strains. With the removal of the load, the martensite transforms back into the austenite phase and resumes its original shape.

This allows the metal to be bent, twisted and pulled, before reforming its shape when released. This means the frames of shape memory alloy glasses are claimed to be "nearly indestructible" because it appears no amount of bending results in permanent plastic deformation.

The martensite temperature of shape memory alloys is dependent on a number of factors including alloy chemistry. Shape memory alloys with transformation temperatures in the range of 60–1450 K have been made.

The first reported steps towards the discovery of the shape memory effect were taken in the 1930s. According to Otsuka and Wayman, A. Ölander discovered the pseudoelastic behavior of the Au-Cd alloy in 1932. Greninger and Mooradian (1938) observed the formation and disappearance of a martensitic phase by decreasing and increasing the temperature of a Cu-Zn alloy. The basic phenomenon of the memory effect governed by the thermoelastic behavior of the martensite phase was widely reported a decade later by Kurdjumov and Khandros (1949) and also by Chang and Read (1951).

The nickel-titanium alloys were first developed in 1962–1963 by the United States Naval Ordnance Laboratory and commercialized under the trade name Nitinol (an acronym for Nickel Titanium Naval Ordnance Laboratories). Their remarkable properties were discovered by accident. A sample that was bent out of shape many times was presented at a laboratory management meeting. One of the associate technical directors, Dr. David S. Muzzey, decided to see what would happen if the sample was subjected to heat and held his pipe lighter underneath it. To everyone's amazement the sample stretched back to its original shape.

There is another type of SMA, called a ferromagnetic shape memory alloy (FSMA), that changes shape under strong magnetic fields. These materials are of particular interest as the magnetic response tends to be faster and more efficient than temperature-induced responses.

Metal alloys are not the only thermally-responsive materials; shape memory polymers have also been developed, and became commercially available in the late 1990s.

Crystal structures. Many metals have several different crystal structures at the same composition, but most metals do not show this shape memory effect. The special property that allows shape memory alloys to revert to their original shape after heating is that their crystal transformation is fully reversible. In most crystal transformations, the atoms in the structure will travel through the metal by diffusion, changing the composition locally, even though the metal as a whole is made of the same atoms. A reversible transformation does not involve this diffusion of atoms, instead all the atoms shift at the same time to form a new structure, much in the way a parallelogram can be made out of a square by pushing on two opposing sides. At different temperatures, different structures are preferred and when the structure is cooled through the transition temperature, the martensitic structure forms from the austenitic phase.

Manufacture. Shape memory alloys are typically made by casting, using vacuum arc melting or induction melting. These are specialist techniques used to keep impurities in the alloy to a minimum and ensure the metals are well mixed. The ingot is then hot rolled into longer sections and then drawn to turn it into wire.

The way in which the alloys are "trained" depends on the properties wanted. The "training" dictates the shape that the alloy will remember when it is heated. This occurs by heating the alloy so that the dislocations re-order into stable positions, but not so hot that the material recrystallizes. They are heated to between 400 °C and 500 °C for 30 minutes. Typical variables for some alloys are 500 °C and for more than 5 minutes.

They are then shaped while hot and are cooled rapidly by quenching in water or by cooling with air.

Date: 2016-04-22; view: 712