STUDIES REGARDING THE PREPROCESSING THROUGH POWDER METALLURGY OF PARTS MADE OF TI-TIB AND TI-TIB 2 METALLIC MATRICS COMPOSITES

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1 STUDIES REGARDING THE PREPROCESSING THROUGH POWDER METALLURGY OF PARTS MADE OF TI-TIB AND TI-TIB 2 METALLIC MATRICS COMPOSITES Prof.Dr. Eng. Marius BIBU, Prof. Dr.Eng. Valentin PETRESCU, Prof.Dr.Eng. Toderita NEMES, Assoc.Prof.Dr.Eng. Claudiu ISARIE Lucian Blaga University of Sibiu, Romania, Faculty of Engineering, Materials Science and Technology Dept. Emil Cioran str., 4, Sibiu, marius.bibu@yahoo.com Abstract The current state of the art in the domain of preprocessing through powder metallurgy of parts made of Ti-TiB and Ti-TiB 2 Metallic Matrix Composites includes procedures starting from the liquid and solid phase, as well as from the gaseous phase. This paper analyses an optimal technological flow for the realizing of a sintered part made of titanium alloys, comprising stages of mixing, compacting, sintering, heat treating and processing. 1. TITANIUM AND TITANIUM ALLOYS Titanium is a metallic materials characterized by a silver-white metallic luster. It is situated at the fine line which delimitates the heavy metals and the soft metals. Its properties recommends it for heavily use in the industrial processes: low density, high resistance ratio R m /ρ (Rm break resistance, ρ - density), refractivity (up to 500 C) and an excellent corrosion resistance. The most important mechanical properties of the titanium alloys are presented in table 1, [1]. Table 1. The most important mechanical properties of the titanium alloys Titanium alloys Melting point [N/mm 2 ] Break resistance [N/mm 2 ] Break elongation [%] Alloy with α mono-phase structure: Ti-5%Al-2.5%Sn Alloy with β mono-phase structure: Ti-13%V-11%Cr-3%Al Alloys with α+β structure: Ti-8%Mn Ti-6%Al-4%V Ti-7%Al-4%Mo Ti-6%Al-6%V-2%Sn The most common titanium-based alloys are: TiAl 6 V 4 and TiAl 6 Sn 2 Zr 4 Mo 2. Titanium and its alloys are heavily used in the following industries: aeronautics, space transportation, automotive, metallurgy, chemical, food, optics, medical, sportswear, energetic installation, medicine, clock manufacturing, security equipment, and measurement and control instruments

2 2. METAL MATRICS COMPOSITE MATERIALS BASED ON TITANIUM AND/OR WITH TITANIUM WHISKERS Many of the recent researches in the presented fields have been oriented towards Metal Matrix Composites based on titanium. The term of metal matrix composite (MMC) refer to a wide range of systems of materials, sizes and microstructures. The hardening component is generally a ceramic material, so in present MMCs can include hardened materials too, with relatively soft phases and/or easy deformable (graphite particles, lead particles, even gases). Also, it can be used refractory materials, inter-metallic compounds or semiconductors. Generally, composite materials have the advantage of combining the properties of two or more types of materials (metallic materials and ceramic), improving the useful properties and eliminating the weak parts of each type. MMCs are divided based on hardening phase: particles, short fibers or long fibers. Regarding the MMCs reinforced with particles, even in the present time, the most common types are with alumina matrix, there have been various researches regarding the use of titanium as metal matrix material. Also, even the ranforced particles used continues to be based on silica carbide, SiC and alumina oxide, Al 2 O 3. It is taken into consideration materials like titanium boride, TiB 2, or titanium carbide, TiC. These MMCs hardened by fibers are obtained usually by a melting and casting procedure, or by agglomerating and consolidating some powders. Other possibilities include the reactive processing or co-deposition by spraying. Regarding the MMCs reinforced with short fibers, usually the diameter of these fibers is of some microns, their initial lengths being of hundreds of microns. The superior properties can be achieved when the fine structural grain is replaced by a singular grain. The result is called whiskers with a 0.1 µm and a dimensional ratio of hundreds. Traction stresses are usually very high. The cheaper methods have been developed, but initially the technology was expensive. Still, the industrial applications continue to be limited. In the field of MMCs based on alumina, the Al-SiC composites ranforced with particles or SiC whiskers are already common. The highly reactivity of titanium is a continuous breaking factor in development of some metal matrix composites based on titanium and classic SiC, Al 2 O 3, Si 3 N 4 or B 4 C whiskers, different reaction products being formed at the interface. Therefore, the ideal solution is to use combinations like Ti-TiC or Ti-TiB, Ti-TiB 2. Table 2 shows a comparative analysis of the mechanical properties of some titanium compounds, which can be used in the composite materials. Table 2 The mechanical properties of some titanium compounds Properties Ti TiB TiB 2 TiC TiN Density [kg/m 3 ] Modulus of elasticity [GPa] Coefficient of thermal 8,6*10-6 7,15*10-6 6,2*10-6 7,95*10-6 9,35*10-6 expansion Hardness Vickers (kg/mm 2 ) Melting point ( C) MMCS FROM Ti-TiB AND Ti-TiB 2 SYSTEMS In the Ti-B system of alloys (fig. 1) there are three main compounds: TiB, Ti 3 B 4, and TiB 2, with 18%, 22% respectively 30% mass concentration of boron. TiB 2 can be considered as optimal solution of titanium hardening, because of its high melting point (3054 C), its modulus of elasticity (540 GPa) and its harness (Hardness Vickers 2200 kg/mm 2 ), also the

3 possibility to form Ti 3 B 4 and TiB at the interface Ti-TiB 2 have decreased the development of this type of composites. From this point of view, using TiB as hardener is more advantageous, because between titanium and TiB there are no intermediate phases, and TiB formation requires a lower quantity of boron than TiB 2. The processing temperatures in the solid state of these composites are relatively reduced ( C), which ease their fabrication. Figure 1 Equilibrium phase diagram of Ti-B In figure 2 is presented the free energy of forming some compounds of titanium, [7]. Figure 2 Free forming energy of titanium compounds TiB forms mono-crystals with long whiskers inside of titanium matrix, therefore, with a small quantity of ranforcing fibers it can be achieved important increases of modulus of elasticity and the composite material s resistance. Other combinations of titanium, including TiB 2 as well as TiC, TiN, Ti 3 Si 5, don t form whisker fibers in the thermodynamic equilibrium state. Forming energy of TiB 2 is negative, while the titanium and TiB 2 can combine to form TiB due to small negative value of free energy at this reaction. Therefore, the forming reaction of TiB by combining titanium with TiB 2 is possible as long as the boron quantity is lower than the necessary quantity to form TiB 2. The commonly used method to obtain TiB or TiB 2 is powder metallurgy, which uses commercial powders, atomized pre-alloyed powders in gas or mixed powders or mechanically alloyed. After the initial alloying process and/or cold pressing, the composites are sintered or hot isostatic pressed. K.S. Ravi Chandran and K.B. Panda, [4], have demonstrated that Ti-TiB composites can be fabricated with any percentage of TiB by a singular sintering process, obtaining a dense structure, with a uniform distribution of fibers. Same authors have verified the possibility to alloy a titanium matrix with three modals powder mixes, in order to maintain in the structure the β phase. Table 3, [4], shows different types of composites mentioned in field literature, indicating their properties and their recommended fabrication methods.

4 Table 3 Mechanical properties, at room temperature, of MMC with matrix of titanium alloys and different percentages of TiB as ranforced fibers Matrix composition (mass %) % vol. TiB E (GPa) R p0,2 (MPa) R m (MPa) A 5 (%) Manufacturing process Ti (ASTM Grade-4) ,0 Forging Ti6Al4V , ,25 metal alloy + hot isostatic pressing Ti24Al10Nb 10 * sintering process + hot isostatic pressing Ti6Al4V ,470 3,1 argon atomizing + hot isostatic pressing / extrusion Ti ,4 vacuum melt + die forging Ti5Al2,5Fe * ,0 sintering process + hot isostatic pressing Ti * sintering process Ti6Al4V ,5 metal alloy + hot isostatic pressing Ti4,3Fe7Mo1,4Al1,4V ,5 mechanic merge + cold isostatic pressing + sintering process + die forging Ti4,3Fe7Mo1,4Al1,4V ~3 idem Ti4,3Fe7Mo1,4Al1,4V ~1 idem Ti6,4Fe10.3Mo * sintering process + extrusion Ti24,3Mo * ,85 sintering process Ti43Nb ,65 sintering process Ti6Al4V sintering process + extrusion Sample cracked before the plastic deformation. Data from this table shows that, by inserting 10% of TiB fibers in the structure, can be achieved an increase up to 20-25% of modulus of elasticity, at higher proportions, even greater values can be achieved (for example, the composite Ti24,3 Mo + 34% TiB have an elasticity modulus of 171 GPa). Figures 3.a ad 3.b shows the TiB whiskers fiber distribution inside the titanium matrix in a Ti - 30%TiB composite. TiB have an orthorhombic crystalline structure and the formed fibers usually show a hexagonal section. The crystal growth on transversal direction is much slower than on axial direction. Figure 3 The structure of TiB whiskers fibers Figures 4.a... 4.d show the microstructures, obtained by Scanning Electronic Microscopy, of some composites with different titanium alloys matrix, ranforced with TiB fibers. Regarding the thermal properties, for a TiB-16TiB 2 mixture [5] it was achieved a thermal dilatation coefficient of 7.15*10-6 K -1 at room temperature, respectively of 11.32*10-6 K -1 at 1625 K. The corresponding values for TiB 2 polycrystalline are 5.6*10-6 K -1, respectively 10*10-6 K -1 at 1625 K.

5 Figure 4 Micrographics of TiB fibers distribution in different titanium matrix: a) and b) Ti6,4Fe10,3Mo + 34%TiB; c) Ti24,3Mo + 34%TiB; d) Ti53Nb + 34%TiB According to matrix composition and ranforcing proportion, composite s break resistance may vary between 673 MPa and 1820 MPa, as indicated in table 3. All these values are higher than commercial pure titanium, which have a break resistance of 550 MPa. Most of the Ti-TiB and Ti-TiB 2 composites have a very low plasticity. That is related to the fragility of Ti-TiB and Ti-TiB 2 and ranforcing fibers, which appear at higher proportions than 30% in the structure. Data from table 3 shows that by alloying titanium with β-stabilizing elements, such as iron, molybdenum and niobium, the ductility is increasing, even when the TiB ratio overcome 30%. At Ti24,3Mo composites with 34% TiB, the breaking elongation is about 0.9% in breaking resistance conditions of 1100 MPa. At Ti53Nb composite with matrix, the elongation is even higher, probably because of the presence of a large quantity of β phase in the matrix, but the resistance and the elasticity modulus is even lower than Ti24,3Mo. It can be concluded that by choosing the right combination of composition and processing technologies, better materials with higher plasticity can be obtained. 4. PREPROCESSING OF TI-TIB AND TI-TIB 2 COMPOSITES The research in the field of preprocessing, by P/M techniques some MMCs in Ti-TiB 2 and Ti- TiB systems and by processing by plastic deformation followed by heat treatment, can be done according to figure 5, which is proposed in the glossary of field notions for composites materials with metal matrix, [2]. The technological flow for manufacturing the sintered parts by Ti-TiB or Ti-TiB 2 systems is represented in figure 6.

6 Preprocessing and processing methods of MMCs Figure 5 Liquid state processing Solid state processing Vapour state processing (P, SF) (P, SF, CF) (P) Preform preparation Metal Powder (P, SF) Metal Foil (CF, MF) Milling, mixing Preparation foil-fiber-foil (CF, MF) (P, SF) Secondary processing Primary processing Preprocessing Preprocessing Fiber displacement Composite fabrication Homogen forming Infiltration Sputter deposition Liquid phase Sintering Hot isostatic pressing Sintering process Extrusion, rolling Joining by difusion Deposition Forming Casting Insertion casting Extrusion forging rolling Extrusion forging rolling End product PROCESSING OR JOINING P MMC hardened by particles, SF MMC hardened by short fibers, CF - MMC hardened by continuous fiber, MF MMC hardened by whiskers

7 Pure titanium Powder of the main alloy Mixing Compacting Sintering Heat treatment Processing Superficial treatment Product Figure 6 The technological flow for manufacturing sintered parts by Ti-TiB or Ti-TiB 2 systems Mixing or homogenizing the compounds in order to obtain mixes with a specific chemical composition is realized by mechanical or chemical processing. The mechanical homogenization is executed in installation called homogenizers by inserting into the mix of powders of some bonding agents or pro-formers for the porous parts. The chemical homogenization consists in deposition of some addition substances on the surface of the base metal grains. The addition substances are mineral salts of the base metal, which evaporates, process which intensify the homogenizing process. The quality of the homogenizing process depends on the density, size and shape of grains, blending grain size composition, particles structure, blending component ration and homogenizing type. Compacting powders by rolling is a productive method of continuous forming of parts, especially sheet-metal and bands. Rolling process consists in compacting powders between rolling cylinders (fig. 7, [3]). The thickness of rolled bands depends on grain size, particles shape, rolling pressure, rolling surface of the cylinders and rotation speed. Rolled bands from metal powders usually don t have a specific resistance, therefore, during the production; the sintering process is part of a continuous fabrication flow with a major complexity. The rolling process allows the manufacture of multilayered sheet-metals with different structural characteristics for each layer, based on used grain size fractions. Extruding process assure a continuous manufacturing process in order to obtain tubular parts with high densities. Free casting of powders is a simple process of forming parts without pressing. For a complete fill of the form and for a uniform porosity the form is usually vibrated. The final porosity depends on grain s size and vibration process parameters, like amplitude and frequency. The sintering process is realized with the corresponding form.

8 a) b) c) d) e) f) Figure 7 Methods of powder rolling process [11]: a); d); e) - vertical rolling; b); c) horizontal rolling; f) interior rolling of the tubular parts Annealing heat treatment is applied for stabilizing the grain s surface in an inert atmosphere or in vacuum, at a temperature of of melting temperature of metal powder. As a reducing element hydrogen or dissociated ammonia is used. The annealing is usually applied to the powders obtained by mechanical grinding, electrolysis, carbonyl dissociation or pulverization inside of sintering furnace. TiB 2 powders are manufactured [6] by reaction in solid state between titanium nitride, TiN, and amorphous boron. The two components have been pre-heated at 600 C, in the vacuum of 5*10-5 torr, then combined in an molar ration B/TiN = 2.2 and heated at 1,400 C during 360 minutes. The obtained particles of TiB 2 have diameters of approximately 1 µm. The powders can be sintered by heated rolling, at 20 MPa and 1,800 C, for 60 minutes. LITERATURE REFERENCES 1. V. Deac and others, Titanium casting in the dental prosthetics, University Publishing House, Sibiu, I. Mitelea, E. Lugscheider, W. Tillmann, Materials Science in machine construction, Welding Magazine Publishing House, Timişoara, G. Vermeşan and others, Introduction in metal powders, Dacia Publishing House, Cluj-Napoca, K.S. Ravi Chandran, K.B. Panda, S.S. Sahay, TiBw-Reinforced Ti Composites: Processing, Properties, Application Prospects, and Research Needs. Journal of Materials, nr. 5/2004, p R. Boyer, G. Welsch and E.W. Collings, ed., Materials Properties Handbook: Titanium Alloys, Materials Park, Ohio, SUA, ASM International, H. Itoh and others, Preparation of TiB2 sintered compacts by hot pressing, Journal of Materials Science, vol. 25, 1990, p C.M. Ward-Close, P.G. Partridge, A Fiber Coating Process for Advanced Metal Matrix Composites. J. Mat. Sci., vol. 25, 1990.