Feasibility study of chemical liquid deposition based solid freeform fabrication

Materials and Design 21 Ž2000. 83᎐92

Feasibility study of chemical liquid deposition based solid freeform fabrication Zongyan He a,U , Jack G. Zhoua , Ampere A. Tseng b a

Department of Mechanical Engineering and Mechanics, Drexel Uni¨ ersity, Philadelphia, PA 19104, USA b Manufacturing Institute, Arizona State Uni¨ ersity, 85287-5106, Tempe, AZ 85287-5106, USA

Abstract The Chemical Vapor Deposition ŽCVD. process has been used in Solid Freeform Fabrication ŽSFF. for several years, and it has faced some critical problems in deposition rate, product accuracy and facility cost. To overcome these shortcomings and explore new chemical deposition methods and materials, a new rapid tooling and SFF technique named Chemical Liquid Deposition based Solid Freeform Fabrication ŽCLD-SFF. is proposed. CLD-SFF can further be sub-divided as Thermochemical Liquid Deposition Based SFF ŽTCLD-SFF. and Electrochemical Liquid Deposition Based SFF ŽECLD-SFF.. TCLD-SFF is based on the following experimental fact: when cold Žroom temperature. liquid reactants are sprayed from a nozzle and come in contact with a hot substrate, the reactants can decompose or react with one another, and then the solid products are deposited on the substrate. By controlling the motion of the nozzle and the spray time, a desired three-dimensional shape of deposited material can be formed through layer-by-layer scanning. In ECLD-SFF a special anode was designed to deposit metallic materials among powder particles on a plate cathode. By controlling the motion of the anode and the current and voltage of the electrical field, a desired three-dimensional part made of connected powder particles can be formed through layer-by-layer scanning. According to our initial experimental results, the feasibility of CLD-SFF is discussed and demonstrated. 䊚 2000 Elsevier Science Ltd. All rights reserved. Keywords: Solid freeform fabrication ŽSFF.; Chemical vapor deposition; Organometallic compounds; Electroplating; Fractal

1. Introduction Rapid Prototyping ŽRP. has brought in a new revolution in manufacturing processes of materials by using additive and layer-by-layer material processing technique, however, its focus has gradually shifted to Rapid Tooling ŽRT. and Solid Freeform Fabrication ŽSFF. ŽJacobs, 1996; Zhou and He, 1998a,b, 1999.. When the same Computer Aid Design ŽCAD. database is used not only to generate rapid prototypes for design optimization and verification but also to create production tools or directly fabricate products, rapid prototyping becomes rapid tooling and manufacturing. The development list of Rapid Tooling technology based on

U

Corresponding author.

rapid prototyping and manufacturing has been growing during recent years. Over 10 RT methods have been proposed, and examples are three-dimensional Systems’ Keltool, DTMs RapidSteel, CEMCOMs Nickel Ceramic Composite ŽNCC. Tooling, Dynamic Tooling’s PolySteel, ExpressTool’s Electroforming, and Extrudehone’s PROMETAL ŽAshley, 1998.. Each approach comes with a unique set of limitations, yet each promises to reduce the time it takes to produce metal tooling. None of the new RT approaches offer the choice of materials, work volume, or accuracy of Computer Numerical Control ŽCNC. machining. However, if the new technologies can be successfully put into use, rapid tooling and solid freeform fabrication promise to be a great boon to the multibillion-dollar worldwide tooling industry. Solids can be free-formed by direct gas-phase chemi-

0261-3069r00r$ - see front matter 䊚 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 1 - 3 0 6 9 Ž 9 9 . 0 0 0 6 2 - X

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cal reactions or electrochemical reactions. The main research work in this field can be briefly introduced as follows.

2. Gas-phase reaction There is a number of continuing efforts to use gasphase laser-induced reaction to form parts. A selective laser sintering system has been used to form alumina shapes by oxidation of aluminum powder in air and to form silicon nitride by laser sintering of silicon in nitrogen or ammonia ŽBirmingham and Marcus, 1995.. Other efforts are focused on converting laser-induced Chemical Vapor Deposition ŽCVD. from a filmforming to a shape-making technique ŽLehmann and Stuke, 1994; Maxwell et al., 1995.. A technique named Selective Area Laser Deposition ŽSALD. was proposed by Jakubenas et al. Ž1998.. Their proposed process of depositing material from one or more organometallic gases on a substrate selectively heated by a scanning laser beam offers an opportunity of forming shapes under a lower temperature with a wide range of materials. However, the researchers may meet some severe obstructs when using this technique to real production. First, the initial gaseous reactant needs to have a very low absorptivity to laser energy ŽSudarshan, 1989., and the deposited material also needs to have a very low reflectivity to laser beam, otherwise the substrate temperature and the deposition rate will reduce greatly ŽAllen, 1984.. Second, the deposited material should have a low thermal conductivity, or else the high temperature region on the surface of the material will become wider, which will cause two defects: low deposition rate and low accuracy of the part ŽSudarshan, 1989.. Since metallic materials usually have a very high thermal conductivity and a higher reflectivity to laser beam, SALD may be limited in fabricating metal or alloy parts. This technique needs a high cost CO 2 , Ar or Kr ion laser device. Some shortcomings of conventional CVD, such as the thick boundary layer and the fringe crystal structure of the deposited materials ŽSudarshan, 1989., still exist in SALD. The primary limitation of SALD and other gas-phase reaction methods is its very low deposition rate. Based on the common deposition rate of organometallics Ž0.1᎐1.0 ␮ mrmin. ŽSudarshan, 1989., it may take several months to make a solid part of a cubic inch when using CVD methods.

3. Electrochemical reaction Solids can also be free-formed by electrochemical methods. There are two SFF techniques related to electrochemical reactions.

3.1. Tooling with NCC (Ashley, 1998) The NCC tooling method uses plastic RP models as a master pattern, it is first coated with a conductive silver-based material, then placed in an electroforming bath of nickel sulfamate where a thin nickel layer is plated over it. The typical nickel plating thickness varies from 0.04 to 0.2 inches ŽAshley, 1998.. After electroforming, Chemical Bonded Ceramic ŽCBC. is cast to support the nickel shell. Once the CBC is cured, the ejector pins are drilled and installed. This technique needs a RP part as a pattern to be used prior to do coating and electroforming, hence it is not a direct free-forming method. Furthermore, the filled-in ceramic material usually has large shrinkage that will cause shear stress on the interface and tooling distortion. 3.2. Express tool (Ashley, 1998) In this technique, a graphite mandrel is first made with a CNC machine, then the mandrel is put in a bath of nickel sulfamate for electroforming. The formed nickel shell with a thickness of 1᎐2 mm needs to be backed with aluminum-filled epoxy. The aluminum fraction helps heat conduction. Two main advantages of this process are the ability to produce large parts and the high accuracy of the products due to a very small shrinkage. A key disadvantage is that deep holes do not elctroform well, this is also the shortcoming of conventional electroforming techniques caused by the heterogeneity of electric current density. This technique is also not a direct freeform fabrication.

4. Principle of CLD-SFF A new SFF technique, called Chemical Liquid Deposition Based Solid Freeform Fabrication ŽCLD-SFF., is proposed by the authors to overcome the main limitations of the above chemical or electrochemical reaction deposition techniques. The proposed CLD-SFF can further be sub-divided as two different methods: one is to use heat to accelerate a chemical reaction called Thermochemical Liquid Deposition based SFF ŽTCLDSFF.; the other is to use electric current to conduct a chemical reaction called Electrochemical Liquid Deposition based SFF ŽECLD-SFF.. Although the two methods have different chemical reaction principles, their reactants are all liquid, their products are all solid and the corresponding system structures are very similar. The two methods will be studied and compared in the following. In TCLD-SFF, when cold Žroom temperature. liquid reactants are sprayed from a nozzle and come into contact with a hot substrate, the reactants can decom-

Z. He et al. r Materials and Design 21 (2000) 83᎐92

85

Fig. 1. The conceptual diagram of the TCLD-SFF system.

pose or react with each other, and the solid products will be deposited on the substrate. By controlling the movement of the nozzle and spray time, a desired three-dimensional shape of the deposited material can be formed through layer-by-layer scanning. Fig. 1 is a conceptual diagram of our TCLD-SFF system. It consists of six sub-systems: a substrate heating system, an X᎐Y᎐Z scanning and elevating system, a reactants providing and controlling system, a pressure generating system, a gaseous product treating and recovery system, and a central computer control unit. The main technical processes of TCLD-SFF can be described as follows: to design a part by computer using a three-dimensional CAD software and translate the designed part file into an STL format file; to install the substrate and heat it to a certain temperature which is maintained; to close the chamber and then run the pressure generating system until it reaches a certain pressure which is maintained; to run the reactant providing and controlling system and adjust the flow and pressure of the liquid reactants; to run the scanning system and let the nozzle spray the reactants towards the hot substrate at prescribed positions and time according to the STL file to form the part by layering; and then to remove and examine the final product. In ECLD-SFF, the substrate is made of or coated with conductive materials Žmetals or graphite., and is connected to a DC power supply as a negative electrode Žcathode.. Then the substrate is put into a plating bath that is filled with electroplating liquid. A very thin pin that is made of deposition metal is connected to the DC power as a positive electrode Žanode.. Between the substrate and the tip of the pin there is a thin layer of metal powderrparticles. Between cathode and anode there is an electric field named Z direction field. Two assistant electric fields are arranged perpendicular to

each other to form an X᎐Y surface electric field. A magnetic field is applied in the Z direction of the substrate to form a tight connection of powders for ferrite materials Žsee Fig. 2.. Under the effects of the electric and magnetic fields, metal ions from the electrode moving to chemical liquid will deposit onto the powder particle and grow Žthe spread powder layer providing mechanism is not shown in Fig. 2.. The metal particles will be bound by the deposited materials to form freeform solid. By controlling the pins movement and electrified time, a desired three-dimensional shape can be formed through layer-by-layer scanning. The formed product will be further treated, such as sintering and infiltration. The motion and control units of the ECLD-SFF system are very similar to the TCLDSFF system shown in Fig. 1. If the new technique can be realized, the following advantages can be expected: 1. Unlike other RT techniques, CLD-SFF is a direct Žone-step. SFF technique, in which many intermediate steps can be omitted, so it has a shorter processing time, and no pattern duplication error. 2. In CLD-SFF, since the nozzle or pin has very small bore diameter or pin diameter Ževen less than 1 ␮ m., and the substrate always has a constant and uniform temperature for deposition on the contacted region, the products made by CLD-SFF can be very detailed and have a high accuracy. 3. The solid product is formed by a large ensemble of atoms during the chemical reaction process, so its micro-structure will be much denser than that made by physical process methods, such as laser sintering, cold gas dynamic spray, and other SFF techniques ŽAshley, 1998.. Compared with SLS, SALD and other RT techniques that must use polymer

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Fig. 2. The conceptual diagram on ECLD-SFF technique.

4.

5.

6. 7.

binders, CLD-SFF does not require any binder, so the density of the product will be higher and the shrinkage will be smaller. Compared with other gas-phase deposition SFF, CLD-SFF has a much higher deposition rate and much lower deposition temperature. Compared with those SFF techniques using phase transformation of deposition material or binder, in CLD-SFF, only the substrate needs to be heated, thus the system is much simpler and easier to control. Compared with laser assisted SFF, CLD-SFF does not require a laser and is therefore much cheaper. Most importantly, by using CLD-SFF, a wide range of materials such as metals, alloys, ceramics and carbon materials can be deposited to form desired parts, and their chemical compositions, density and purity can be controlled by designing suitable technical parameters.

There are two main limitations and also challenges in the CLD-SFF: Ž1. due to the limited deposition rate, in its current stage of development the CLD-SFF is not ready to make big size parts; and Ž2. the final deposited parts need further treatment to decrease their residual stress and increase their hardness. CLD-SFF can be applied to many fields such as rapid tooling, semiconductor epitaxial deposition, and micro-electromechanical system problems that need to be solved, such as the design of CLD-SFF system, the modeling of the CLDSFF process and the design of electrodes.

5. Feasibility study on CLD-SFF 5.1. Modeling and experiments on TCLD-SFF The TCLD-SFF process is very different from con-

ventional CVD. First, the reactants are forced to contact the hot substrate, the chemical reactions occur only in a very small region of the substrate, and only the substrate is heated, not the whole reaction system. Second, the gaseous products and the rest of the nonreacted reactants are discharged from the reaction system very quickly, and will not react in the system. Thirdly, the shape, size and arrangement of the reaction chamber will be no longer important to the flow type of the reactants and the heat transfer model. Lastly, near the surface of the deposited material there is a large temperature gradient due to contact with cold Žroom temperature. reactants, and the reactants have a much higher concentration than that in conventional CVD. Therefore, many existing models and characteristic parameters, such as Fr , Sh, Nu and N kn ŽOxley, 1966. used in conventional CVD will not be effective in the model on the TCLD-SFF process. When building a model of CLD-SFF we must consider three major factors: 1. Technical parameters and flow characteristics: temperature, pressure, reactant concentration and composition, and flow type, rate and viscosity. 2. Chemical system: reaction principles, possible reaction products, reaction dynamics and thermodynamics. 3. Mass transfer characteristics: diffusion, forced convection and phase transformation heat transfer. It is impossible to build a complete model of TCLDSFF by considering all factors, since up to now we do not have enough data on chemical reaction dynamics at high temperatures, and we even do not know how many kinds of intermediate products can be formed. We have to consider some simplified models and build upon the work of other researchers. Fig. 3 shows a conceptual diagram of an initial depo-

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Fig. 3. Three deposition models of the TCLD-SFF process.

sition model on TCLD-SFF. First we assume that the liquid reactant has a smaller viscosity and a better wettability to most metals or ceramic materials in a large temperature scope Žfrom room temperature to 1000 F.. When a drop of liquid is extruded from the nozzle, if the substrate temperature T is too high, the molecules on the bottom of the drop will first contact the hot substrate, and then vaporize very quickly. Their volume will expand greatly in a very short time, and the liquid molecules in the upper part of the drop will be blown out, and then the deposition cannot be carried out ŽFig. 3a.. If T is too low, most liquid molecules will flow on the substrate and only the molecules in contact with the substrate will vaporize. Thus a gaseous layer containing high-density gaseous reactant will be formed above the substrate, this is called gas film boiling mode. The solid resultant will deposit on the substrate quickly but it will not have the desired shape due to the flow of liquid reactants ŽFig. 3b.. When T is in a proper range, there is not enough time for the liquid drop to flow before vaporization, and the gaseous layer on the bottom of the drop can be formed. In this way, the solid resultant can deposit quickly at the desired position on the substrate ŽFig. 3c.. Using the model introduced above it is possible to calculate the deposition rate that depends upon the kinetics of the decomposition of the reactant and the heat transfer of the gas film boiling on the bottom of the drop. We found that there are many kinds of liquid reac-

tants that can decompose or react very quickly under a suitable temperature, and in 1 or less than 1 atm of pressure. The solid deposits can be inorganic salts, oxide ceramics, metals, alloys or Diamond-Like Carbon ŽDLC.. In fact, most chemical solutions can vaporize quickly when contacting hot plate and deposit solid reactants on the plate, and the most useful deposits in production should be ceramics or metallic materials. In order to deposit metallic materials, we chose some organometallic compounds, such as metal carbonyls, as initial reactants. At room temperature pentacarbonyliron ŽFeŽCO.5 . is an air-stable liquid that can be prepared by direct reaction of finely ground iron with carbon monoxide, and it is commercially available and inexpensive. The decomposition process of the liquid reactant FeŽCO.5 can be described as follows. When liquid FeŽCO.5 contacts the hot substrate it is first decomposed into FeŽCO.4 and CO Fe Ž CO. 5 ª Fe Ž CO. 4 q CO

Ž1.

Therefore, the gaseous layer on the bottom of the drop will be full of gaseous FeŽCO.4 and CO. The further decomposition reactions will occur in the layer. Fe Ž CO. 4 ª Fe Ž CO. 3 q CO

Ž2.

Fe Ž CO. 3 s Fe q3 CO

Ž3.

For the homogeneous reactions Ž2. and Ž3., the de-

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88

composition rate can be given as

ž

k Ž py x . dx s 2 dt homo 1 q b2 x

/

Ž4.

where x is the decomposed amount Žexpressed in units of pressure., t is time, p is the initial pressure of FeŽCO.4 , k 2 is the reaction coefficient of Ž2. and b 2 s 4 k 2rk 3 , in which k 3 is the reaction coefficient of Ž3.. For a heterogeneous reaction, the decomposition rate is assumed to depend on the surface area of the adsorption sites:

ž

k 2 Ab Ž py x . dx s dt hetero 1 q b Ž p y x . q b1 x

/

Ž5.

where A is the surface area covered by the drop, b is the Langmuir constant for adsorption of Fe and b1 is the Langmuir constant for adsorption of CO ŽChan and McIntosh, 1962.. It is often assumed that b1 s 0. Based on the above model, we have performed some initial experiments and obtained pure iron deposited on substrates. Other experiments on TCLD-SFF have also been done. Table 1 shows our findings on deposits and the corresponding technical conditions used in the TCLD-SFF process. From this table one can see that for the listed materials the deposition temperature is not high Žfrom 45 to 450⬚C., and in this temperature range the deposited metals or ceramic materials can maintain their original solid state. One can also see that the deposition rates are much higher Ž2᎐10, even 30 times. than that in conventional CVD, which is usually 0.1᎐5.0 ␮ mrmin ŽHolzl, 1968; Hough, 1972.. The chemical liquid deposition rate is good enough to make a metal or ceramic part with a moderate thickness. According to our experiments Žsee Figs. 4 and 5., when FeŽCO.5 Žroom temperature. is directly sprayed on a 316 stainless steel plate Ž500⬚C., the deposition rate of the Fe film can reach 15᎐20 ␮ mrmin, which is good enough for an SFF process. Fig. 4 shows an X-ray electronic scanning ŽXES. spectrum of a nickel coating from nickelocene on a 316 stainless steel plate. Fig. 5 is a photomicrograph of the DLC film deposited on a quartz plate. The deposition experiment of DLC film was carried out in a vacuum chamber, the liquid mixture of

water and benzene sprays to a hot ceramic or metal substrate from a nozzle. The pressure in the chamber is kept from 10 to 20 kPa Žif the pressure reaches 100 kPa, it is necessary to use nitrogen for protection.. The temperature of the substrate is maintained from 150 to 250⬚C. The deposition rate can reach 10 ␮ mrmin. The film has a hardness of 10 000᎐120 000 MPa, a resistivity of 10 12 ᎐10 16 ⍀rcm and a density of 1.1᎐1.4 grcm3. From Fig. 5 one can see the film is not very uniform. By reducing the bore diameter of the nozzle the uniformity can be improved, in this process the more important factors should be pressure, temperature and the ratio of reactant in the mixture, which needs further researches and experiments to find suitable parameters. Another possible way to make ceramic parts by TCLD-SFF is to use metal alkoxide. It can react rapidly with water to form hydroxide or oxide particles. Water can be introduced by using a second nozzle or through reaction with atmospheric moisture during forming ŽCalvert and Crockett, 1997.. The main limitation of this method lies in the very large shrinkage during the reaction. However, in TCLD-SFF the part is formed layer-by-layer; each layer is formed line-by-line, and the next layer spraying through a layer thickness milling and calibration mechanism can compensate for the shrinkage of the previous layer. Although shrinkage is evident for each layer during conversion from alkoxide with water to oxide, the shrinkage will not affect the final accuracy of the part.

6. Experiments on ECLD-SFF Some existing research and our initial experiments on ECLD-SFF revealed the following interesting phenomena. 1. Research and experiments have shown that metal particles in a suitable electroplating medium can be connected by directed electrochemical growth ŽBradley et al., 1997, 1998.. Our experiments further show that if some metal powders are laid on the cathode plate Žsubstrate . and sunk into the liquid medium ŽFig. 2., the powder particles can be bound under the action of electric fields and form a

Table 1 Deposits and their corresponding technical conditions of salt, metal, alloy and DLC Technique

Deposit

Reactant

Composition

Temperature Ž⬚C.

Pressure ŽkPa.

Deposition rate

TCLD-SFF TCLD-SFF TCLD-SFF TCLD-SFF

NaCl Al2 O3 Fe DLC

Solution of NaCl Solution of C15 H21O6 Al FeŽCO.5 C6 H6 q H2 O

357 grl ŽH2 O. 120 grl Žtoluene. FeŽCO.5 only 25 mlrl ŽH2 O.

130᎐150 125᎐300 150᎐450 150᎐250

100 100 15᎐100 10᎐150

1᎐5 mmrmin 150 ␮ mrmin 2.5᎐50 ␮ mrmin 0.8᎐10 ␮ mrmin

Z. He et al. r Materials and Design 21 (2000) 83᎐92

Fig. 4. XES spectrum Žnickel coating on steel..

2.

3.

4.

5.

solid with certain strength. In Fig. 6a, an SEM photo shows a rope-like deposit connecting to a particle at the initial growth stage. Our research result, in Fig. 6b, shows the formed powder layer bound by the deposits. Research and experiments have shown that the electrochemical deposition is a fractal growth process where the distance between two particles is very short, and the fractal growth always occurs in the direction of the electric field ŽBrady and Ball, 1984.. The fractal dimensions of the growing branches and the width of the deposited metal band all decrease with the increasing electric field. Fig. 6c shows several stages of the fractal growth, from which one can see when some new fractal branches are forming, some formed fractal branches may disappear at the same time. After a longer time only a few main branches can survive. Research and experiments have shown that by using assistant electric fields in surrounding directions ŽFig. 2., a web-like deposit among particles can be formed ŽFig. 6d.. Our initial research has shown that the surface topography of the bound powder particles has fractal structures, and the fractal dimension depends mainly on the composition of the medium and the strength of the electric and assistant magnetic fields ŽFig. 6e.. An assistant magnetic field can increase the particle density and affect the dimensions of the fractal surfaces. Research and experiments have shown that during the electrochemical process the voltage and current are unstable. There is a critical point for each particle growing process Žsee Fig. 7.. At this point the constant voltage will drop to a very low voltage sharply, and after this the electrochemical deposition process stops basically, until new powders are

89

Fig. 5. Photo of DLC film on quartz plate.

added on the top surface and then the voltage recovers to the initial value. This phenomenon can be used to detect and control the fractal growth process.

7. Anode design and experiments on ECLD-SFF In conventional electrolytic deposition, the coatings are not distributed uniformly over the part serving as the cathode. The local thickness differences depend on a large number of factors, and are more pronounced when the profile of the part is irregular. The ECLD-SFF can eliminate this shortcoming, if its deposition is always concentrated in a very small plane even though the part has a very complex shape. It has been realized that the deposition area size is very important not only to the accuracy of the part but also to the thickness distribution. The geometrical characteristics of the electrode will be an important factor affecting the area size. The primary current distribution is determined by the electrical field strength. The electrical field strength ␧ s Erd at projecting points of the cathode is inversely proportional to the diameter d of the point and proportional to the voltage E. The projecting cathode area therefore always has a higher current density than the receding area. On the other hand, the smaller the distance between the electrode and the substrate, and the smaller the ratio of the electrode diameter to the bath size, the greater the current density difference between electrode edge and center. In order to get a smaller deposition area and higher current density in this area, the requirements of the design should be: smaller diameter of the pin elec-

90

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Fig. 6. The SEM photos showing particle binding and growth phenomena.

trode, shorter distance between the electrode and substrate and smaller ratio of the electrode diameter to the bath size. The initial calculation and analysis on the current distribution corresponding to different electrode designs have been carried out. Fig. 8 shows a schematic representation of the primary current distribution for different electrode designs, and the pin anode is our selected design. In the ECLD-SFF process, most conventional electroplating liquids can be used to deposit various metal alloy or composite parts on the cathode substrate. The amount of deposited material and required current density can be calculated based on the following formulas. According to Faraday’s first law and considering the current efficiency, we have E iFt␩ Gs el 6000

Ž6.

where G is the weight of the deposited metal Ž g ., Eel denotes the electrochemical equivalent Ž grah., i is the current density Žamprdm2 ., F denotes the area of the local depositing region, t is the exposure time Žminutes. and ␩ is the current efficiency Ž%.. The depositing thickness d Ž ␮ m. among particles will be ds

Eel it␩ 60␥

Ž7.

where ␥ is the density Žgrcm3 . of the deposited metal. The required current density to obtain the desired thickness, in a given time is is

60␥ d Eel t␩

Ž8.

These formulas have been used in our experiments on ECLD-SFF. Table 2 shows some experimental parameters, composition and deposition rates.

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8. Conclusions

Fig. 7. The relationship between the voltage and time in the ECLDSFF process

According to the above study, it has been realized that CLD-SFF can be used to directly deposit solid materials with a desired shape. Based on the results of the conducted feasibility study we have moved forward to develop a prototype machine of the CLD-SFF system and to develop more practical research on CLDSFF.

From this feasibility study we made several findings. There are many kinds of liquid reactants that can be used to deposit solid materials by TCLD-SFF. The solid deposits can be inorganic salts, oxide ceramics, metals, alloys or DLC. A desired shape formed by TCLD-SFF can be obtained as long as processing parameters, especially the temperature of the substrate, are controlled in a defined range. In ECLD-SFF, most conventional electroplating liquids can be used to deposit materials among metal powder to form desired parts by connecting the particles of the powder. The key parameters in ECLD-SFF are the shape and size of the anode, electrical current density and the distance between the powder surface and the pin-tip. If these parameters are well controlled a desired shape can be consistently obtained. The deposition rate in CLD-SFF is much higher than that in the conventional CVD method. References Allen SD. Laser chemical vapor deposition ŽLCVD., emergent process methods for high temperature ceramics. Materials Science Research, vol. 17. New York: Plenum Press, 1984:397᎐413.

Fig. 8. Conceptual diagram of current density distribution in different electrodedesigns Žthe dash or curved lines are electric field lines..

Table 2 The technical parameters and deposition rates in the experiments on ECLD-SFF Technique

Deposit

Electrolyte

Composition

Temperature

pH

Deposition rate

ECLD-SFF

Ni

Solution of Ni salt

60᎐70⬚C

3.8᎐4.2

20᎐50 ␮ mrmin

ECLD-SFF

Ni᎐Co

Solution of Ni᎐Co salt

Aminosulfonic acid Ni 600 grl Nickel chloride 10 grl, boracic acid 40 grl Aminosulfonic acid Ni 200 grl Aminosulfonic acid Co 24 grl, boracic acid 35 gr1

45⬚C

3.8᎐4.2

20᎐50 ␮ mrmin

92

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