'99: SCIENCE AND TECHNOLOGY

USE ·OF ELECTRON BEAM SKULL MELTING WITH ELECTROMAGNETIC STIRRING OF MELT FOR THE TITANIUM CASTINGS PRODUCTION

Dr. S. LADOKHIN, eng. N. LEVITSKY, p.g. A. KOSHELAP

Physico-Technological Institute of and Alloys of National Academy of Sciences of Ukraine, 34/1 Vemadsky Ave.,Kiev-142, 252680, Ukraine

ANNOTATION

A technology of the electron beam skull melting of titanium alloys is discussed. The efficiency of the electromagnetic stirring of melt in the skull crucible at the electron beam melting of titanium alloys (EBSM with EMS) is demonstrated. In particular, the possibility of the reduction of the specific electric energy consumption at the use of electromagnetic stirring systems is shown. ·

Results of the investigation of highly alloyed titanium alloys melting by means of the'EBSM with the EMS, including alloys based on intermetallic compounds TiAl and TiNi and alloys containing highly volatile component Al, are presented. The possibility of an efficient alloying of melt in the course of melting is shown. Mechanical (including high temperature) properties of alloys produced at the EBSM with the EMS, as well as their chemical content are described. Comparative analysis of the EBMS with the EMS and the vacuum arc skull melting is performed. key words: titanium, electron, beam, skull, melting '

1. INTRODUCTION

Titanium and its alloys are the one of the most long-range among materials used in contemporary machine building. Now, titanium alloys are widely used in aircraft and ship building industries, chemical machine building, aerospace complex and oil and gas industry. During the last 10 ... 20 years titanium stopped to be an exotic material that used only in war industry and so-called "special technologies" area and now has wider use in production of medicine tools, sport implements and many other widespread goods for everyday use. But despite all that was told before the Ti-based materials still remain enough expensive that essentially brakes an increase of their consmnption. ' One of basic reason for high cost of titanium and its alloys is very low product yield (PY) at semifinished items production (cast parts, ingots, roll, forging and die products). It can be possible to increase essentially technical-and-economic indexes of semifinished items production from titanium and its alloys at the expense of wider use of technology of mold casting that permits to obtain an item with a maximum proximity to final product in shape. But use of this technology has some difficulties and the basic of them is the extremely high chemical activity of Ti at high temperatures and especially in liquid ~tate. So, in particular, titanium melt intensely interacts with air oxygen and nitrogen, and reaction products dissolve in liquid; moreover, it is not any refractory material that could be inert to titanium melt. All above mentioned directly determines vacuum skull melting as the only technology suitable for titanium alloys melting for production of cast parts. Now, the vacuum skullmelting (V ASM) is most widely used for production of cast parts from titanium alloys. This technology having obvious advantages, such as relatively low specific energy consumption, simplicity of vacuum and electric equipment used, low loss of alloying elements and base, at the same time is being characterized with essential disadvantages, in particular, a dependent heat source, difficulties with remelting of substandard charge, low refining ability, problems with such technological operations as additional alloying at process of melting and modification, and also a low overheat of molten metal. Therefore one can make a conclusion that it would be prospective to develop a melting technology free from above mentioned disadvantages. In our opinion it could be the electron beam skull melting with electromagnetic stirring of molten metal (EBSM-EMS) technology developed in Physico-Technological Institute for Metals and Alloys of the National Academy of Science of Ukraine.

1399 TITANIUM'99: SCIENCE AND TECHNOLOGY

2.. USE OF ELECTROMAGNETIC STIRRING OF MOLTEN METAL AT ELECTRON BEAM MELTING OF TITANIUM

·. An essential disadvantage of classic EBSM is impossibility to obtain a large volwne of molten metal without big evaporation losses of it. It is connected with the fact that depth of molten metal bath obtained at heat of its surface with EB is not more than 100 ... 150 mm and an increase of heat power starting from certain limit does not increase the volwne.of molten metal but intensifies the evaporation process. Use of shields increases a volwne of poured metal ortly a little (15 ... 20% more) but finally cannot be believed to be an effective solution of problem stated. The experience we col~ected showed that a. substantial increase of molten metal volwne in skull crucible is possible oiily at the expense of considerable intensification of convective movement of molten metal in running electromagnetic field. At the same time it is necessary to mark that the development of effective EMS system able to work in EB melting conditions is a far away from trivial problem. The basic difficulty lies in the fact that at interaction of electron beam with electromagnetic field created by stirring system the electron beam becomes to be non­ controlled and is being redirected from crucible to vacuwn chamber walls. To solve the problem the following measures were taken:. - electron beam gun locates coaxially with crucible. So the. electron beam heater cathode is located in direct visibility of molten metal; in such a severe conditions axial EB guns with ~ultistage beam beam guiding system evacuation showed an enough workability; the external surface of electromagnetic coils of stirring system is covered with the magnetic core of electrical-sheet steel that permits both to reduce the leakage field intensity and increase the efficiency of EMS system (EMSS); . . . it is desirable to keep the molten metal level in crucible under the level of upper coil of EMSS, because it permits a guided movement of beam spot across ihe molten metal bath surface, ~~erwise an electron beam is strictly located ~t the center of the bath. · · The . water-cooled crucible with. so-called magnetically transparent walls is used as a melting capacity. The crucible with EMSS used in this work have a capacity'of30 kg of Ti. The vacuwn in melting chamber is created by oil-vapor pwnps of booster type. Numerous investigations we fulfilled showed that the melting of titaniwn at use of above mentioned EB heater can be stably held in vacuum of 1 ... 3 Pa (the vacuwn in block cathode· is about 0,01 Pa). Results of inve.stigations of melting of titaniwn alloy BT20L with (meltings No. I - 5) and without use (melting No:6) ofEMSS are given in table 1. . . . l)tble l Parameters and technical-and-economic in~exes of EBSM of BT2ff alloy. . ' . .

.. Melting Time of Time of heating at Mass of Mass of 'specific energy No. EMSSwork, maximwn power, alwninium poured metal, consumption, min . min alloying, kg kg kW·hlkg 1 14 8 0 13,2 2,87 2 15 9 0 13,7 2,74 3 15 . 9 Q. . =13,5 . 2,80 4 14 10 0,3· 13,8 2,82 5 15 11 0,3 13,2 2,88 6 0 IO 0,3 6,8 4,57

Analyzing table 1 data it is necessary to take into account that the niass of poured metal at melting with EMSS application is maximally possible for this crucible. At the same time maximwn·mass we obtained in the crucible of given design is 32 kg. So, application of EMSS permits a more than 4 times increase of the mass of poured metal. Moreover, as it is seen from fig. I, a, specific energy conswnption (SEC) essentially reduces at increase of mass of poured metal and at industrial application of EB SM-EMS technology (when mass of molten metal is about 200 ... 300 kg) one can expect the SEC to·be about 0,8 ... 1,0 kWh/kg (fig. l ,b ). Application of EMSS also permits to make operations of additional alloying of molten metal at melting. In table 2 the chemical composition of BT22L alloy obtained in our experiments is given. As seen from data ·given, an injection of highly volatile component is not effective without EMSS application and results in large loss of itself (melting No.6). Alloys melted with stirring of molten metal and.additionally alloyed at melting have composition complied with standard requirements. As it will be shown later, the

1400 TITANIUM'99: SCIENCE AND TECHNOLOGY

10 I .I<: 9 ~ c: \ a0 8 E 7 \ ::J I Cll \ '6 ,, ~ 8 a) >- 5 I. f:> . Cl> ,. c 4 Cl> CJ "'-..;.. I;:: 3 'll ' .... -:---.:.... Cl> ~ enQ. 2 • 0 5 10 15 20 25 30 Mills d ixum rrEta ~. kg

7

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0 0,05 0,1 0,15 0,2 1 / M, kg-1

Figure 1. Dependence of specific energy consumotion on mass of poured metal

1401 TITANIUM'99: SCIENCE AND TECHNOLOGY

opportwtities of EBSM-EMS technology are the clearest in production of highly alloyed titanium alloys at melting of 13-titanium and intermetallic Ti-based alloys. Table 2 Chemical composition of EB-melted BT20 alloy

Melting Melting conditions Al Mo v Zr No. Standard 5,5 ... 7,5 0,5 ... 2,0 0,8 ... 1,8 1,5 ... 2,5 Primarv 6,20 1,60 1,50 2,00 ·,. 1 Without alloying 4,80 1,62 1,40 1,90 2 Without alloying 4,63 1,64 1,32 1,90 3 Without alloying 4,65 1,67 1,36 1,80 4 With alloying -- 6,17 1,62 1,41 2,09 5 With alloying 6,08 1,71 1,62 1,97 6 With alloying 4,95 2,09 2,37 2,12

The investigations fulfilled showed that application ofEMSS at EBSM had high economic effectiveness and essentially increased the technological flexibility. It is necessary to mark that in all meltings a titanium scrap and chip were applied as a charge.

3. MELTING OF NONTRADITIONAL Ti-BASED MATERIALS BY MEANS OF EBSM-EMS

The objective of our further investigations was to show the possibility of obtaining of principally new materials, which production using traditional technology is now impossible, difficult or the technology used for this purpose is expensive and unproductive, by means of EBSM-EMS. These materials include corrosion­ resistant alloy of Ti-Mo system with about 30% content of Mo, Ti+50% Nb alloy and high-temperature alloys based on monoaluminide of Ti, intermetaliic compound TiNi-based materials with shape memory effoct and superelasticity.

3.1 titanium-based alloys with high refractory P-stabilizers

The alloys 4201 (Ti+32%Mo) and HT-50 (Ti+50%Nb) were chosen as an object of investigations of alloys of this group. Mo and Nb form a continuous series of solid solutions in 13-Ti and have a limited solubility in a-Ti; they belong to 13-stabilizing elements group. Ti-alloys containing these elements in more than critical concentration (it is the one when 13-phase of Ti becomes stable at room temperature) ·are related to corrosion­ resistant alloys and now it is a good base to apply them in chemical machine building. Moreover, Ti-Nb system alloys are of interest because at 25 ... 40% content of Nb (close to Nb3Ti) they have a combination of high superconduction transition temperature, upper critical magnetic field and critical current density and can be used as a superconductive material. The problem that appears at obtaining of these materials lies in necessity of alloying of elements essentially different in both melting point and density (see table 3). Currently, to obtain them, multiple VAR with the homogenization or powder metallurgy technology are used. We have been faced a problem of obtaining of alloy of designed chemical composition at one remelting. The test meltings were held in electron beam installation ELLU-4 constructed on base of vacuum induction furnace ISV-0,04 in copper water-cooled crucible with two-phase bottom and wall EMSS. We tested some means of alloying of components that permitted to obtain a sought-for result to a certain extent. The most simple and convenient means is when charge materials are being loaded in crucible at the same time so that the first to be melt is more refractory component and only after its complete melting the molten metal is progressively being enriched with the less refractory one as a result of stirring of molten metal. As experiments showed, this means was available for HT-50 alloy melting providing use of mix of small pieces of Nb ofHE-1(0,1%Ta,0,01%Mo, 0,01 %Fe, 0,01 %02), the same material chip and BTl titanium alloy of tube crop ends as a charge. The substantial volume of such a charge and its developed surface permitted to hold the melting at small power which provided small evaporation and progressive melting of niobium charge right up to the level of titanium one. Results of chemical analysis showed that at such a melting process titanium loss was not more than 3 .. .4% and it

1402 Table 3 Results of experiments oo melting of highly alloyed alloys based oo titanium and its compounds

Allov Properties of alloying element Melting technology Results 3 Tm• °C . T boih °C P, g/cm 2400 4900 8.5 Small pieces of Nb rods and its chip were loaded above the An alloy of sought-for composition 1s Ti charge obtained. Loss of Ti is 3-4% Mixture of Nb beads and Ti Technology is not workable. Ti loss is more than 60%. SEC = 19 Klh.q I KI-

Nb beads are collected in a cone with a hole for EB An alloy of sought-for composition is passing and installed above the Ti charge .obtained. The heat efficiency of process is high. Loss of Ti is 2%. SEC = 5.2 kW.h I kg l----+~~~~~~-1-~~~~-1-~~~~~+-~~~~+-~~~~~~~~~~~~~~~~~~~~~-+-~...;,_~~~~~-'"'-~~~~~~~~--1~ 2 Ti+32%Mo 2650 4600 10.2 Mo beads are hung up on the manipulator. Mo is being The technology is not effective. Mo is sinking ~ melted with EB in the liquid titanium bath. on the bottom of crucible. The bath is being z strongly freezed. Mo content in alloy is < ~ 6%. . ~ l---+~~~~~~-1-~~~~-+-~~~~~+-~~~~+-~~~~~~~~~~~~~~~~~~~~~-+-~~~~~~~~~~~~~~~~--1~ Ti charge is melted primarily. The bath solidifies An alloy of sought-for composition 1s ~ forming a monolyth. Mo alloying is injected on its obtained. Loss of titanium is < 5%. ~ surface and melted on low power of EB through the Q surface. Then power increases to nominal value. ~ l---+~~~~...;,_~-1-~~~~-+-~~~~~+-~~~~+-~~~~~~~~~~~~~~~~~~~~~-+-~~~~~~~~~~~~~~~~--10 3 1450 2900 8.9 Mixture of Ti and Ni Charge is igniting. A substantial deviation ;l l---l-~~~~~~~--+-~~~-1-~~~~~~~~~~~~~~~~~~-+..::..:::::.....::.:=z::::_:::;:_=:=c=::=-~~~~~from sou1:1ht-for composition. n Ni is injected in liquid titanium bath. An alloy of sought-for composition and ~ l-----+-~~~~~--1~~---"'~--l-~~~~-+~~~~+-~~~~~~~~~~~~~~~~~~~-4-=~.w...... ;"--"'"-'--':....:....;_:.::...:..;:.L.,_.:.=.;:....:....;_..:..;:_..:..:..c~;.:_;_~--19having shape memory effect is obtained 5 4 Ti+50%.,AI 660 2500 2. 7 Al is injected in liquid titanium bath. An alloy of sought-for composition is ..... obtained. loss is less than 2% Ti 1667 3500 4.5 TITANIUM'99: SCIENCE AND TECHNOLOGY was possible to obtain the alloy having sought-for chemical composition with uniform distribution of alloying ·elements. To regret, this means did not appear to be availabie at use of niobium beads. In this case niobium when being melted (at power that is 2 ... 2,5 times more than for rip charge) leaked on the bottom of crucible through holes between titanium rods or froze on these rods. The repeated melting lead to leak of Nb again, then titanium started to be melt. As a result, the obtained alloy did not contain more than 10% of Ti and SEC reached to 19 kWh/kg. . At use of the similar charge the following means appeared to be effective: titanium crop ends are being loaded in skull crucible, then niobium beads primarily constructed in a cone having a hole specially designed for electron beam are put on the previous loading. The inelting in that case is held in such a way: firstly the bath of liquid titanium is being made by means of EB pass.ing through the hole; then EMSS of molten metal is being turned on, and as a result the liquid titanium is starting to wash niobium beads out. The presence of shield over the molten metal prevents titanium loss because its vapor condenses on niobium b~ads and finally fetches to molten metal up again. At the same time niobium starts to heat up at the expense of radiation from bath surface and condensation heat of titanium vapor that increases a heat efficiency of process. So it is becoming possible to obtain an alloy of sought-for composition at insignificant titanium loss and available SEC (results are given in table 3). · · But despite the high technical and economic effectiveness of the proposed means of alloying of components it is not believed to be a universal process because to hold it, an availability of charge materials of certain standard size is necessary. Therefore, we tested other technological means for melting of 4201 alloy. Originally, we used the following means: beads of different length were being tied together and hung up on the special device; after the titanium bath build-up these beads were being fed under the beam and Mo progressively drained off to molten metal at EMSS turned on. The necessity to maintain the liquid bath caused the by-tum direction of EB to both on molten metal and molybdenum fed; this procedure prolonged an alloying process; moreover, as the results of chemical analysis of obtained metal showed, molybdenum drops were badly accumulated by titanium melt. Molybdenum is sinking on the bottom of crucible and is accumulated by skull, as a result cast part contained only 5 ... 6% of Mo. Obtaining of alloy of sought-for composition became possible after the application of two-stage process: titanium charge is being melted and a liquid-metal bath is being formed across the diameter of crucible; the heating is being turned off and bath solidifies forming a titanium monolith; the EB power is being increased until the necessary volume of molten metal will. be accumulated, then pouring occurs; Titanium loss is not more than 5% at such a melting means. Investigations we held have clearly demonstrated the possibility of effective EBSM-EMS technology application for alloying of titanium with more refractory and dense elements.

3.2 obtaining of intermetallic titanium compounds (/'iAI, TiNi)-based alloys

Recently, the. problem of wide industrial application of alloys based on titanium intermetallic compounds is becoming more and more widespread. The most prospective in this case are titanium monoaluminide and nickelide. TiAl compound could be a base for production of high-temperature material with 3 work temperature of up to 1000 °C at density of about 3,5 g/cm , and compete with Ni-based superalloys. Titanium nickelide also has a series of unique properties. One can call following of them: • shape memory effect; • superelasticity; • ductility at cryogenic temperatures. TiNi-based materials now are applied in medicine and are prospective for industrial applications in branches where their unique properties.can be used. The objective of investigations we held was to show the possibility of effective application of EBSM­ EMS technology for titanium intermetallic compounds obtaining. The traditional means of obtaining is powder metallurgy. A development of cheaper casting technology could substantially reduce the production cost of these materials that would contribute to their more widespread inculcation, at the same time an application of powder metallurgy for production of certain types of parts is not excepted. At melting of alloys of TiNi, TiAl systems we obtained a little unexpected results. Taking into account close values of melting and boiling points of titanium and we originally chose a technology of alloying when titanium (iodide Ti rods) and nickel (chopped sheets of Hlll technical nickel) charge in proportion of 50 at.% to 50 at.% were loaded in a mixed up way and heated with a defocused EB. At heating of charge materials

1404 TITANIUM'99: SCIENCE AND TECHNOLOGY below melting points of components a self-ignition process was observed, and the further melting of charge occurred without a heat absorbtion. The obtained alloy had perfect fluidity (the spiral of complex sample, more than 500 mm in length, was filled completely) but appeared to be so brittle that at kriock-out fell to pieces. The reason for such brittleness lied in substantial deviation from chemical composition. As the results of chemical analysis showed the alloy obtained contained 32,4% of Ni, the rest was Ti and it corresponded to Ti2Ni compound on constitution diagram. This compound has melting point of 980 °C and is very brittle. In our opinion it is connected with that a joint heating of titanium and nickel up to the temperature which corresponds to the first eutectic point on constitution diagram leads to exothennic process of synthesis of Ti2Ni compound that is stable in this temperature range. The rest of nickel staying in mol~n metal is being evaporated because its vapor elasticity is assumably higher than one of the fomied compound. The stated ·problem of obtaining of mononickelide of Ti was managed to be solved by means that was mentioned before for meltmg of alloys of Ti­ Nb and Ti-Mo systems when the second component (in this case it is Ni) was injected in molten titanium bath. The fonnation of TiNi compound in such conditions is more probable because the temperature of molten metal is far closer to one of fonnation of this compound than to Ti2Ni. Results of test melting confmned our assumption. We obtained the alloy with high ductility and, what is more important, with shape memory. The chemical analysis showed that a Ni content equaled to 56,2% that corresponds to TiNi compound. The mentioned method appeared to be effective at melting of TiAl-based alloy (Ti+36%Al) also, though, taking into account a substantial difference in properties of these elements, it was expected that this means would require improvements if not be effective at all. Therefore, at the first test melting. we charged a furnace with an excess of aluminium (AOOO). So, providing 5% presence of Al in titanium charge (BT5-1 alloy), to obtain a necessary composition 43% of Al were injected. As chemical analysis resulted, the alloy contained 42,08% of Al that was an evidence of almost complete adoption of Al by molten metal. In our opinion, it could be explained by a thennodynamic benefit of compound fonnation in molten metal and, as a result, the dissolution of aluminium in titanium with a substantial deviation from Raul's law fonnation. In whole, the fulfilled investigations showed the possibility of effective EBSM-EMS technology application aimed at intennetallic titanium compounds synthesis.

CONCLUSIONS Results ofexperiments permit to assert that EBSM-EMS is very flexible technological process giving an opportunity to realize · a plenty of melting schemes that in tum gives the possibility of development of economically effective casting technology. An application of systems of electromagnetic stirring of molten.metal permits to increase technical-and-economic indexes of process and widens a range of obtained materials substantially. The technology considered here permits to obtain both highly alloyed industrial alloys contained highly volatile elements and nontraditional Ti-based materials complied with sought-off requirements. Although it is necessary to mark that melting schemes proposed in our report·require further finishing and improvement. Results we obtained can be a ponderable argument in' favor of industrial application of EBSM-EMS technology for production of cast parts from both "ordinary" Ti-based alloys and intennetallic Ti based compounds.

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