Comparison of basic methods of thermal spraying with powders [3].
Abstract
This chapter of the book presents the basis of classical powder metallurgy technologies and discusses powder fabrication, preparation, preliminary moulding, sintering and finish treatment operations. A general description of the materials and products manufactured with the classical powder metallurgy methods is presented. New variants are characterised along with special and hybrid technologies finding their applications in powder metallurgy. Special attention was drawn to microporous titanium and to TiAl6V4 alloy fabricated using hybrid rapid manufacturing technologies with selective laser sintering/selective laser melting (SLS/SLM) used for innovative implant scaffolds in medicine and regenerative dentistry. Laser deposition, thermal spraying and detonation spraying of powders are also discussed as special methods in which powders of metals and other materials are used as raw materials.
Keywords
- powder metallurgy
- moulding
- sintering
- special power metallurgy methods
- additive manufacturing
- near net shape
- implant scaffolds
- laser deposition of powders
- thermal spraying and detonation spraying of powders
1. General description of the classical technological process of powder metallurgy for product manufacturing, especially products used in medicine and dentistry
Powder metallurgy (PM) is an important area of the technology of material processes which is still being intensively developed. In general, powder fabrication, preparation, preliminary moulding, sintering and finish treatment operations can be distinguished in a classical technological process of products fabricated by the powder metallurgy method [1–8]. Deviations from a typical technological process often occur in industrial or research practice. For example—preliminary moulding and sintering can often be combined into a single operation or, in so-called sinter-hardening, fast controlled cooling is used from a sintering temperature to harden or appropriately supersaturate the sintered element. Sometimes, the sinter achieved, having high porosity, is next saturated with melted metal with its melting point lower than this of the main component by infiltration. Other deviations can also exist from the said typical technological process, but it is distinctive that an input material in the form of powder is always produced and its sintering takes place at a temperature lower than the melting point of the main component [1, 3].
Metal powders are produced as a result of mechanical or physiochemical disintegration of the input solid material or as a result of chemical or physiochemical reactions—from other materials or chemical compounds, by reducing oxides, decomposition in high temperature, electrolytically, in hydrometallurgy processes or by sputtering a metallic liquid [1, 9]. Powder fabrication methods are split into five basic groups preconditioning the powder size, thus the properties in the subsequent pressing and sintering processes:
mechanical from the solid phase (stripping, milling, grinding, breaking, grinding, crushing, shattering);
physiochemical from the liquid phase (sputtering, granulation, sputtering and mechanical granulation);
physical (evaporation and condensation);
physicochemical (solidification, decomposition of carbonyls, reduction of oxides and other compounds, dissociation of oxides and other compounds, self-decomposition)
chemical (sol-gel, electrolysis of melted salts or aqueous salt solutions, thermally from chemical compounds).
Disc-shaped, multiwalled-shaped or fraction-shaped powders are obtained by mechanical methods by crushing in ball, vibration or centrifugal-impact mills (Hametag powder in Figure 1a) [3, 10]. Mechanical methods are those having small efficiency and can be used basically for crushing metals and non-metals cleaned with the mill lining and ball materials, which then requires mechanical cleaning. A Hametag centrifugal and impact mill is a device most often used for powder crushing. Two steel propellers rotating in opposite directions at high speed in the mill drum create air whirls which capture the particles of the metallic charge such as cut wires, chippings and other residues. The particles are crushed as a result of being hit by propellers and drum walls and by colliding against each other. The gas blown into the drum by a fan is lifting the powder and directs the powder via a segregator into a settler. Powder is periodically collected to air-tight magazines.
Sputtering is carried out by breaking up a stream of liquid (Figure 2a) into fine droplets by a sputtering agent working under high pressure. The agent is usually water, water vapour, air or inert gases. The liquid droplets solidify (Figure 1b) before falling onto the tank bottom. In addition, in a method known as DPG, mechanical disintegration takes place of a stream of liquid metal with blades—wedges mounted on a rotating disc (Figure 2b). In a method known as RZ (Figure 1c), a sputtering process is combined with chemical reactions of oxidisation, carbon burning or reduction taking place during the process or used later [3, 9, 10].
The electrolytic method consists of metal precipitation on a cathode, most often in the form of sponge, which is crushed into powder after drying (Figure 1e). Metal powders with a low boiling point, for example Zn, can be produced with the metal evaporation method and condensation of its vapours in a tank called condenser [1, 3, 9].
The aim of powder preparation is to produce an appropriate charge for further technological operations. Charge preparation processes include powder segregation into different grain fractions, mixing in the right proportions, adding slipping and pore-forming agents, as well as powder granulation [3, 4, 10].
In cold moulding, the powder is pressed in a closed space as a result of which compression occurs. A relevant cold-moulding method is selected depending on the die shape and powder properties, especially on powder plasticity, compactibility and mouldability, [3, 4, 10] namely:
cold pressing in different types of presses in closed dies (Figure 3a),
isostatic pressing in high-pressure chambers,
vibration compaction of powders,
circumferential pressing (Figure 3b),
rolling pressing (Figure 3c),
rolling of powders (Figure 4a),
cold extrusion of powders (Figure 4b),
impact moulding,
cold forging
casting and sputtering of slip, that is, a strongly compressed suspension of the powder of basic material in liquid with addition of agents preventing agglomeration of grains.
Matrices are created as a result of moulding, for example mouldings, forgings, wire rods.
Powder sintering refers to a technological operation (applied to a moulded element or loosely poured powder grains) whereby particular powder grains are bonded under the influence of heat and form a composite with specific mechanical and physiochemical properties [1, 3, 4, 6, 10–12]. Metal sinters or ceramic-metallic sinters called cermetals are produced as a result of sintering. Sintering can be carried out:
freely,
under the influence of force, that is, combined with moulding ensuring a specific shape, for example, by hot pressing of powders, hot rolling or hot forging.
A compact material is achieved due to sintering, which is usually however porous to a certain degree, with a single- or multi-phase structure. A homogenous or heterogeneous structure can be achieved by sintering with the solid phase and liquid phase.
Sinters have the following characteristics:
particular grain powders are bonded,
new grain boundaries are created,
properties different than properties of matrices,
volume usually smaller than matrices
density higher than matrices.
Volume may be increase sometimes, however, due to sintering. Volume change caused by sintering should be taken into account in designing by using appropriate additives for matrices [1, 3].
The finish treatment of sinters includes:
cold re-pressing or hot re-pressing, including hot isostatic pressing;
plastic working, including forging;
calibrating;
machining;
infiltration with metals with lower melting point;
impregnation with organic materials, for example, oil;
heat treatment and thermochemical treatment;
plating;
coatings deposition
de-burring and burnishing.
In order to improve mechanical and physical properties, metal sinters can be subjected to re-pressing with various methods (cold re-pressing, hot re-pressing, hot isostatic pressing) as well as normal heat treatment which—depending on the chemical composition of the sinter—consists of hardening and tempering, supersaturation and aging, surface hardening, as well as thermochemical treatment, mainly carburising, carbonitriding or nitriding and passivation (steam treatment and blueing) [13]. Due to lower thermal conductivity, the heating and cooling rates of sinters are smaller than for conventional materials and their annealing time is longer [3, 4, 6, 10].
Finished products are calibrated under loads much lower than during a cold-moulding operation to achieve high dimensional accuracy and to achieve the required geometrical characteristics and properties, semi-products made of sintered metals shaped as blocks undergo plastic working, for example, forging or rolling.
Sinters with high porosity can be saturated with metals with a melting point lower than that of the ready sinter in the process of infiltration [1–3]. This can take place by immerging the sintered and porous skeleton in the melted saturating metal or by heating the skeleton with the powder of saturating metal in a furnace with a controlled atmosphere. Various liquid metal infiltration methods are utilised for a porous matrix of a product, among others a vacuum method and low- and high-pressure method, ensuring the thorough filling of the moulding pores with a metal matrix. Porous sinters can be soaked with organic materials, for example, oils in impregnation processes.
Machining, for example, grinding, allows to achieve the final shape and the required surface smoothness. Various surface treatment operations are also applied, including the deposition of coatings, for example, by physical and chemical vapour deposition PVD/CVD, plating with, for example, copper or stainless steel to enhance surface resistance to corrosion or for decoration purposes and also de-burring and burnishing [3, 4, 6, 10, 13].
Some of the mentioned technologies are considered special powder metallurgy methods and are presented in detail in one of the following sub-chapters.
2. Special powder metallurgy methods
Apart from classical powder metallurgy methods, many new special and hybrid variants and technologies have been developed and implemented, where the fabrication and application of metal powders and their alloys are of basic importance. Further works are taking place to improve the methods established so far and new ones are also being developed, for example, shock consolidation which uses high-pressure shock waves [4, 14–16]. The selected unconventional methods applied in powder metallurgy are characterised further in the sub-chapter.
hot upsetting in which a preform is subject to considerable lateral flow of material;
hot re-pressing, also called re-striking or hot coining, where the flow of material takes place mainly towards deformation.
This technology, known already before, was developed about 20 years ago mainly in connection with production of connecting rods in diesel engines in the car industry due to positive influence on the level of noise, reduced vibrations, improved surface roughness and lower production costs. This technology allows to reduce the number of technological operations from 17 to 8, to lower investment and operational costs of technological machines and decreases production costs by approx. 10% compared to a connecting rod forged from a solid material. It also became possible to reduce an active section and the mass of connecting rods made by P/F using an Fe-1.8Cu-0.4C powder blend as compared to those forged from a solid material made of microalloy steel despite decreasing fatigue strength by nearly 15% in relation to forged products [20, 21]. Pistons were also manufactured by this method, made of an aluminium alloy produced from the mixture of Al-4.5Cu-0.5Mg-0.7Si powders.
For conventional hot isostatic heating, heat is created inside the mould and a charge is subjected to the activity of a high-frequency electromagnetic field generated with an induction coil coupled with an electronic generator and with pressure applied to the punch by one or two cylinders. The mould is positioned inside the induction coil. Powders undergo this process even in the liquid phase and low pressure is possible, as well. An advantage of this method and the equipment used is that pressure and induction supply act independently. In case of indirect resistance heating, the mould is placed in a heating chamber (Figure 6a). The chamber is heated electrically with graphite heating elements and heat is transferred by convection. Regardless of mould conductivity, temperature and pressure, a high treatment temperature can be achieved which is an advantage of this method, while long heating time is the method’s obvious disadvantage. The method is mainly used for producing hard and brittle materials, in particular for consolidating composite diamond-metal cutting tools and technical ceramics [1, 2].
Powder materials are placed between a punch and a die between electrodes, which ensures rapid temperature growth to 1000–2500°C. As compared to hot press sintering (HP) and hot isostatic pressing (HIP) methods or atmospheric furnaces, the SPS method ensures easy operation and high reliability and accurate control of sintering energy at a high heating rate of 1000°C/min to the sintering temperature within several minutes, depending on the material type, dimensions and shape of the treated element, as well as device power and type (Figure 7). This distinguishes this method positively from conventional methods where sintering lasts more than ten and even several dozens of hours; hence, the SPS is finding more and more applications [30–32]. A temperature gradient inside the element being produced must be reduced to achieve sintering homogeneity, the decomposition of which is dependent upon electrical conductivity of powder, thickness of the die wall and presence of graphite insulation preventing direct contact with the element being treated and also ensuring electric contact between all the elements. The SPS is more and more widely used [2, 4, 7, 30–33] with the reservation that it enables to solid sinters only. The use of metal powders and die conductivity ensures fast heating of the treated part, especially when moulds have large diameter and relatively small height. This process is especially suitable for applications requiring high heating rates. This refers to materials which can stay shortly at high temperature or in processes requiring high heating rates to achieve high temperature. Such materials can be sintered to obtain final density with near-net-shape accuracy; thus, it is not necessary to perform their final mechanical treatment, especially that they cause major difficulties when mechanical treatment is applied. The SPS technology has been recently employed for, among others, fabrication of sputtering discs and high-performance ceramic parts such as boron carbide, titanium diboron and sialon. This technology is becoming more and more important in the sector of friction materials, for production of sintered brake shoes used in fast trains and different cars and also in wind energy devices and even in quads and mountain bikes. Sintered clutch plates manufactured by this technology are used mainly for heavy trucks, ships, tractors and other farming machinery.
Several powder moulding techniques which employ binders exist, which can be generally classified into one of the pressure-free moulding techniques [1]:
immersion method in which binder content is about 50%;
casting of a polymer-powder slip into rubber moulds, in which a paraffin-based binder is used;
pouring where the surface is covered with a binder and then powder is poured, which binds to the plasticised binder;
electrophoresis;
strip casting;
surface lamination with thin coatings produced by strip casting;
stream printing on part surface;
stereolithography using a laser;
pressing a slip through capillary nozzles arranged as x–y
surface spraying.
Regardless of the methods of pressure-free moulding, extrusion and injection moulding, the entire process consists of mixing the powder and a binder, moulding, removal of binder and sintering [1, 4, 6, 7]. Classical injection moulding in moulders does not differ from the moulding of thermoplastic polymers, while the injected preforms should be subjected to binder removal and sintering to achieve the expected functional properties (Figure 8).
The use of thermoplastic polymers as a binder which is binding metallic or ceramic powder also enables to transport it and mould it in an injector socket. Two types of binders based on, respectively, paraffin or polymers and an aqueous methylcellulose solution are used most often. Owing to the key advantage of this method where ready parts are produced without additional treatment necessary, it is used more and more extensively for producing hard materials, including tool materials which are exceptionally difficult and costly to machine. The formability of metal and ceramic powders and their mixtures allows to fabricate, in particular, metal tools with relatively high ductility, ceramic tools with high hardness or composites with a metal matrix composites (MMC) and ceramic matrix composites (CMC) matrix combining high properties characteristic for metals and ceramics [1, 40]. A powder-to-binder ratio is closely linked to the shape, the size of powder particles, powder wettability by a binder and the properties of the binder itself as well as is linked to mixture production conditions. Despite numerous advantages, an injection moulding process is not suitable for fabrication of large parts with dimensions exceeding 100 mm.
Binder removal is costly, which affects the final price of the materials so produced and removal can be either [1]:
thermal,
hydrolytic,
mechanical,
environmental,
by biodegradation or
by photodegradation.
Thermal and solvent degradation is utilised predominantly for removing polymers serving as binders in the PIM method. The mixing method is linked to combined degradation techniques, for example, solvent and thermal technique.
At present, for exceptionally small preforms, binder degradation is associated with heating to a sintering temperature [1]. At least two binder components are required for fast binder removal so that one of them, that is, skeleton polymer, maintains the composite shape to high temperature in which sintering occurs. A temperature of this polymer’s thermal degradation should be as high as possible. The second binder component should be removed in low temperature or during solvent or catalytic degradation. Paraffin can be an example of this. The component which is degraded first should account for 30–98% of the binder content. Oils and wax have low melting points. Oil or wax can be removed by filtering off, that is, can be sucked away by porous pads.
A binder is often removed with a solvent and thermally [1]. A solvent removes one of binder components by opening pores in the whole volume of the preform, which allows for the quick thermal removal of the next binder component. In all binder degradation types, higher velocity is possible using a higher temperature, which, however, increases the probability of preform damages or deformations. An atmosphere of flowing gas, which evacuates degradation products and is constantly replenished, also assists in higher degradation velocity.
Solvent degradation is relatively fast, but necessitates solvents which are frequently aggressive and unfriendly for the environment [1, 34–36]. Water-thinnable binders are recommended for this purpose. Another solution is to use water as a binding substance, together with starch, salt or sugar. Once a preform is formed, it is dried or frozen and water is removed by freeze drying. Catalytic degradation combines thermal and solvent degradation, where the rate is determined by the temperature and a catalyst concentration. Skeleton polymer, on which a catalyst has no influence, maintains the element’s shape until sintering temperature. As degradation takes place at the interface between polymer and a catalytic atmosphere, a nearly flat degradation front moves through the whole moulding. The final decomposition of the binding substance occurs due to thermal degradation and requires slow heating which is preventing damages.
Solvent and catalytic methods cause a smaller deformation as compared to thermal degradation made at the same time, but they require two operations. Sintering is initiated by growth in the concentration of carbon resulting from binder degradation; however, in case of some materials such as stainless or high-speed steels, a carbon concentration must be closely monitored due to their properties or influence on heat treatment [35, 36, 38]. This type of the binder used influences the final carbon concentration.
Preform density depends on injection pressure, powder particle size and binder content and it is about 60% of theoretical density when the binder is completely removed. Binder content is 30–55% depending on the powder shape and wettability. Irrespective of the preform density, it is subjected to densification and shrinkage due to sintering, which is higher for high porosity. Sintering is usually the last stage of the technological process decisive for density and properties of the ready product and is irreversible. If the ready element should have high mechanical properties, final heat treatment and often machining are used to ensure accurate dimensions of the produced sinters. The sintering of injection moulded or pressure-free moulded powders does not differ largely from the sintering of powders formed by other methods. High-speed steels are subject to supersolidus sintering. Sintering activators in the form of boron, copper, phosphorous copper, carbon, molybdenum, tantalum, titanium, vanadium and tungsten powder are added to iron powders and iron alloys, while the process itself is called activated sintering with the forming liquid phase. In case of graphite, if it has not been used as a reducer of the oxides situated on the surface of powder particles, it leads to a reduction in solidus temperature, as in the sintered high-speed steels. In general, the growing content of carbon is, however, reducing sintering temperature, broadening the range of sintering temperature and lowering the content of pores and allows to produce a homogenous structure with small carbide precipitates [1, 35, 36, 38]. Fine-grained powders are remelted faster and the size of a powder particle is also decisive for surface roughness.
An atmosphere inside the furnace chamber is an important factor conditioning sintering [1]. It is not suitable to choose inert gas, for example, argon, for high-speed steels, as this gas is not soluble in steel and gaseous bubbles may form. An atmosphere during sintering should also be selected in terms of costs generated by the selected gases. Vacuum sintering is a costly alternative, but nevertheless, vacuum is often used for sintering high-speed steels. The sintering of injection moulded high-speed steels in high vacuum is quite difficult due to gas products being released, coming from the thermal degradation of skeleton polymer residues; hence, a better solution is an atmosphere of flowing gas or a gas mixture, most often N2-5%H2 or N2-10%H2 with the right dew point temperature.
In
Several types of mills are used, in particular vibration and planetary ones, atritors and others [4, 9]. In high-power milling, welding takes place repeatedly along with crushing (breaking) and re-welding of powder grains. It is accompanied by the crushing of the powder structure and many defects are formed. The structure becomes metastable and can be either a solid solution, an intermetallic phase and a mixture of components or have the amorphous form. The mechanical alloying method produces a nanocrystalline structure in metals. This is a result of dislocations being generated successively in powder grains. Their density is constantly increasing and annihilation takes place during recovery and polygonisation and then static recrystallisation occurs. Recovery in metals with a low melting point takes place easier and the substantial refinement of structure grains usually is not taking place. The crushed powder is next consolidated with different methods to achieve the desired shape. In some cases, it is required to apply a hot isostatic pressing (HIP) process for uniform powder compaction and sintering [1]. Ready products are heat treated to ensure the required structure and properties.
The advantages of the mechanical alloying method include crossing the solubility limit in the solid state, the synthesis of new crystalline and quasi-crystalline phases. Amorphous phases as well as ordered and unordered intermetallic phases are produced. Certain elements can be used in an unrivalled way as alloy additives and chemical reactions can be triggered, which are usually occurring at a low temperature. Process scalability is also achievable; besides, it is possible to dispergate, as described below, the particles of the second phase (usually oxides) in a matrix of the material produced. The strength and physiochemical properties of products of mechanical synthesis depend on the type of mill, temperature and milling atmosphere. High-melting nitrides, carbides, borides and oxides can also be achieved with mechanical alloying methods [1, 43].
The mechanical alloying (MA) method, describing a process in which a mixture of powders is milled at the same time—usually different metals or alloys and phases—thus leading to the fabrication of homogenous alloys, can also be otherwise called in literature mechanical milling (MM). In this method, the powder with the uniform composition of, for example, pristine metal, intermetallic phase or metal alloy is subjected to the process. Even the name of mechanical disordering (MD) can be met when, as a result of long-lasting milling of intermetallic phase powder, its disorder takes place or an amorphous phase is created. Moreover, the term of mechanical grinding (MG) can be found in some works [44–46], which is normally used, however, for abrasive machining processes, inherent to the formation of chippings, which is not appropriate for the process discussed.
Each layer of powder is levelled each time with a scraper. A computer-controlled laser beam is guided across the powder surface with a CAD programme in successive layers (corresponding to the cross section of a virtual special model of an item recorded using CAD 3-D digital recording), causing the sintering of the powder particles in a strictly defined manner and in the selectively chosen places on the powder surface. A table with another layer of powder is lowered to the set height corresponding to one thickness of a layer resulting from the automatic virtual division of the spatial item model into layers with the set thickness, after each cycle corresponding to one scanning section of the powder bed and another, new layer of material is applied on the top [51, 52, 54, 56, 57]. The powder particle size range is between 20 and 70 μm, while the thickness of a single layer corresponds to approx. 0.1 mm. The process of powder distribution and laser sintering cycle is repeated until a fully bonded item is achieved in line with the model designed, which can be put into use after cooling down and cleaning off the excess powder. This is critical for increasing or decreasing dimensional accuracy and surface roughness of the parts produced. If the unsintered powder surrounding such an element during the process cannot be removed mechanically with a scraper, it can be removed through small windows or drain openings specially designed in a given element, which prevents any small volume of powder from being seized in its closed sections. This distinguishes such technologies positively from other technologies, by ensuring design freedom unavailable in other traditional technologies. The system ensures temperature monitoring of the item produced and laser sintering conditions of the item with mechanical properties reproducible in the whole volume. Depending on the powder material, after laser sintering, it is possible to achieve up to 100% of the density and properties of the materials with the identical chemical composition, but which were produced with conventional methods.
The quality of elements fabricated by SLS may be influenced by powder quality and laser power having an effect on the density of the item produced. As density depends on the peak laser power, hence the impact time of a laser is not very important, a pulsed laser is used most often. An SLS device is heating a loose powder material in a bed just below its melting point to facilitate the local growth of temperature with a laser beam. The quality of the fabricated elements depends on the maintenance level of the device, an operator’s design and operational experience and even the manufacturer’s business practices. It is important to align the element being produced relative to the working table, for example, the best accuracy of element diameter in different directions can be achieved by aligning a wheel in the XY plane level and then approx. 0.5 mm thick layers can be best produced by aligning them in the Z axis, while some parts can be achieved most advantageously in a plane oriented at the angle of 45° [54, 58].
Additive manufacturing methods are used for sintering a broad array of metallic powders, including, among others, light metal alloys, titanium alloys, steel, cobalt and chromium alloys and superalloys and their mixtures and also polymer materials
SLS technologies are commonly used across the world as it is easy to fabricate parts with highly complex shapes directly from digital data in CAD systems, without having to produce any tools. The applications of SLS technologies in arts and industrial design and even in jewellery and for decoration, were initially less expected but are witnessing sharp growth. SLS technologies enable to fabricate single elements or elements in small series, according to individual market demands, ensuring hollows, cuts and internal channels which are difficult or even impossible to achieve conventionally [51–60]. As several or multiple, even small, elements can be manufactured at the same time on a single working table and after loading once to a device’s working chamber, very high process performance can be attained. All the quantified types of SLS/SLM belong to advanced additive manufacturing methods, among others to
Innovative implant scaffolds used for implantation to replace the bone pieces removed surgically due to cancerous disorders or inflammatory conditions were developed (Figure 11). An implant scaffold is comprised of a solid zone and a porous zone with pores sized 100–600 µm, fulfilling the functions of scaffolds. It is possible to cover the internal surface of micropores with a thin layer of a bioactive material up to 500 µm thick and to integrate the implant scaffold with joint implants. The porous zone ensures appropriate osteosynthesis with bone elements remaining after the removal of joints and enables the living tissue to outgrow across the porous zone after implantation. For this method of manufacturing a bone implant scaffold, individual patient data have to be acquired with medical imaging methods, for example, computer tomography. A virtual model prepared in, for example, STL format, created by means of software, for example, AutoFab, enables to design a virtual technological model of an implant scaffold with a method of repeatable unit cells. Such a model enables to manufacture a ready real bone implant scaffold by selective laser sintering with the shape adjusted to the patient’s bone loss. A manufacturing method of personalised dental implant scaffolds used in the treatment of teeth and bone losses situated in the dental system and in a craniofacial bone is identical. Individually fabricated tooth implant scaffolds with geometrical features reflecting the shape of a post-extraction alveolar bone with integrated abutments will ensure a single-operation surgical procedure of tooth extraction and implantation without having to bore the periodontal tissue bone of the upper or lower jaw, guaranteeing a substantial improvement of osseointegration compared to classical implants [55, 56, 61–63].
A beam of accelerated electrons emitted by an electron tungsten gun after passing through a beam concentrating electrode and obtaining a beam size with the required diameter is specially deflected and directed as designed in a CAD system in the right place on the powder bed surface and as a result of impacting the metal powder, its kinetic energy is converted into thermal energy [65]. If the temperature rises above the powder melting temperature, it can be melted and sintered with the previously constituted layer of the fabricated element. A scanning rate of the beam reaches 1000 m/s, which allows to constitute a newly created element with the efficiency rate of up to 60 cm3/h and ranks this technology among the most efficient additive manufacturing technologies.
3. Laser deposition, thermal spraying and detonation spraying of powders
Apart from the classical powder metallurgy methods and new special and hybrid variants and technologies already discussed in this chapter, numerous derivative methods have been established requiring the use of powders, manufactured with classical methods, serving to improve the properties of different products produced with other technologies. Selected laser deposition, thermal spraying and detonation spraying of powders methods are characterised further in the sub-chapter.
alloying,
cladding
melting (hardfacing).
Laser alloying may also be used to introduce non-metals, carbon, nitride, silicon and boron into the surface layer. Laser remelting usually consists of enriching a surface layer of materials with such metals as Co, Cr, Sn, Mn, Nb, Ni, Mo, W, Ta, V or with some of their alloys, for example, Cr-Mo-W, Ni-Nb. Machine steels are employed most often by remelting with powdered carbides WC, TiC or mixtures of WC-Co and tool steels—with different mixtures of carbides, tungsten, tungsten carbide and titanium carbide, chromium or vanadium borides, vanadium carbides, boride carbides or their mixtures with chromium and mixtures of Mo-Cr-B-Si-Ni [68–71].
In
Laser alloying, cladding and hardfacing ensure the highest quality of surface layers with the thickness of 0.1–1.5 mm although the main difficulty is a cracking tendency in the area of surface layers and in the heat-affected zone of the substrate [13, 66, 67, 71, 73–80]. A cracking tendency can be limited or totally eliminated by applying a graded fraction of hard ceramic phases or a gradient concentration of an alloying component in the surface layer, as well as by applying an indirect layer between a top layer and a substrate [68–70, 81].
Powders of different materials can also be used in traditional hardfacing technologies, which were not described in this chapter. The binder material may then be fed through a magazine or powder feeder. In plasma cladding, an additional material, in particular in the form of powder with the grain size of 0.06–0.3 mm with a slightly remelted substrate, is melted in a plasma arc with the temperature of 18,000–24,000°C. The powder is entered into a plasma burner from a feeder by means of transporting gas, normally argon, ensuring thorough protection against the access of air, so that after melting in a plasma arc exiting the nozzle, it is transported by gases under pressure to the substrate, creating a padding weld with the minimum fraction of the substrate material and thickness of 0.25–6 mm in one pass. An additional material is powder on a matrix of cobalt, nickel, iron, chromium, copper and zinc. Plasma cladding is applied to parts of combustion engines, cutting tools, cutting blades of tools for earthworks, valves, valve sockets, steel roll necks, connectors of drilling lines made of non-alloy steels, stainless steels, cast steel and some type of cast iron [13, 82].
The following thermal spraying technologies are distinguished:
arc spraying,
plasma spraying
classical or ultrasound flaming.
The cover material powder is situated in an electric or plasma arc or in a gas flame where it is melted and scattered by a stream of gas such as argon, nitride or compressed air or in a gas flame. The stream of gas is covering liquid particles with the diameter of 0.01–0.05 mm directs them onto the surface of the covered element with the velocity presented for each method in Table 1. Those particles have an impact on the surface, are cooled on and joined with it. Little heat is delivered onto the substrate, as a result of which its temperature is rising only to 100–250°C. Thermal spraying is therefore not causing any structural changes or plastic deformation of the substrate and this type of coverings can be used, among others, in thin and precise parts and for materials susceptible to the activity of heat. The substrate being sprayed has a layered structure with a varied concentration of pores, which can reach up to 20%. Coatings are usually between 0.05 and 2.5 mm thick although may be even 12 mm thick. The coatings attained can also be thinner than for other methods [3, 13, 82, 89–91]. A spraying operation can be carried out by a structure fabricator, on an assembly area or in the place where the structure is to be operated. This allows to reduce transport and assembly costs.
The application of thermally sprayed coatings has been growing year by year and thermally sprayed coatings have gained numerous practical applications, including those ensuring [13]:
anticorrosion resistance—Zn or Al on cast irons or steels, notably on structural parts of steel bridges, in petroleum industry, car industry, shipbuilding industry, road sector, railway, electrical engineering, construction and infrastructure components;
heightened heat resistance and corrosion resistance—high-chromium alloys on internal surfaces of boilers, Al or modified on the Al matrix, such as Al-Si, Al-Cr, Al-Pt, Pt-Al-Cr, Ni alloys crystallised directionally and monocrystals on parts of gas turbines;
heightened heat resistance and fatigue strength—coatings as MeCrAlY, where Me is Co, Ni, NiCo and also CoNiCrAlYHfSi and CoCrAlYSi and thermal barrier coatings TBC providing thermal insulation against high temperature, ZrO2-Y2O3 or Al2O3, Al2O3+5% Ni coatings and Me-CrAl coatings on a nickel alloy substrate as interlayers, sometimes with other coatings;
surface hardening—when smaller thickness is required, this is achievable by cladding, for example, in car engine cylinders, piston rings, parts of textile machines, pump and bearing parts;
electrical conductivity—Cu, Al or Ag on surfaces of weak conductors or materials not conducting electric current;
insulating performance—for example, Al2O3 on glass or polymer materials;
surface porosity—Co or Ti and ceramic materials on medical implants to ensure adhesion and growth of living cells;
light reflection—Al on glass forming mirrors;
decorative effects—precious metals or such with high costs in return for conventional plating and decorative materials on different products and parts of architectural structures;
repair of surface recesses—technological or service surface defects of, for example, parts of aviation engines
regeneration of worn parts—porous coatings enabling the penetration of lubricating substances into pores and improving bearing operation, on rollers in the bearing location of usually steel or made of copper alloys.
The negative characteristics of a thermal spraying process include losses of the material used for producing coatings, which are largely influencing costs, connected with the use of different coating materials and guns with a different construction, including an open or closed system for sputtering the melted material ensuring small particle size and lower porosity.
Acknowledgments
The works have been implemented within the framework of the BIOLASIN project entitled ‘Investigations of structure and properties of newly created porous biomimetic materials fabricated by selective laser sintering’ headed by Prof. L.A. Dobrzański, funded by the Polish National Science Centre in the framework of the ‘Harmony 4’ competitions. The project was awarded a subsidy under the decision DEC-2013/08/M/ST8/00818.
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