The Albany Research Center of the U.S. Department of Energy has been investigating a means to form useful wrought products by direct and continuous casting of titanium bars using cold-wall induction melting rather than current batch practices such as vacuum arc remelting. Continuous ingots produced by cold-wall induction melting, utilizing a bottomless water-cooled copper crucible, without slag (CaF2) additions had minor defects in the surface such as ''hot tears''. Slag additions as low as 0.5 weight percent were used to improve the surface finish. Therefore, a slag melted experimental Ti-6Al-4V alloy ingot was compared to a commercial Ti-6Al-4V alloy ingot in the areas of physical, chemical, mechanical, and corrosion attributes to address the question, ''Are any detrimental effects caused by slag addition''

Argetsinger, Edward R.

Gerdemann, Stephen J.

Hansen, Jeffrey S.

Hartman, Alan D.

Holcomb, Gordon R.

Paige, Jack I.

Schrems, Karol K.

Turner, Paul C.

UNT Digital Library

ADVANCED TITANIUM PROCESSINGA. D. Hartman, K. K. Schrems, G. R. Holcomb, E. R. Argetsinger,J. S. Hansen, J. I. Paige, and P. C. TurnerAlbany Research CenterDepartment of Energy1450 Queen Ave. SWAlbany, OR 97321ABSTRACTThe Albany Research Center of the U.S. Department of Energy has been investigating a means to formuseful wrought products by direct and continuous casting of titanium bars using cold-wall inductionmelting rather than current batch practices such as vacuum arc remelting. Continuous ingots produced bycold-wall induction melting, utilizing a bottomless water-cooled copper crucible, without slag (CaF2)additions had minor defects in the surface such as “hot tears”. Slag additions as low as 0.5 weight percentwere used to improve the surface finish. Therefore, a slag melted experimental Ti-6Al-4V alloy ingot wascompared to a commercial Ti-6Al-4V alloy ingot in the areas of physical, chemical, mechanical, andcorrosion attributes to address the question, “Are any detrimental effects caused by slag addition?”.INTRODUCTIONDirect and continuous casting of titanium bars using cold-wall induction melting, rather than currentbatch practices such as vacuum arc remelting (VAR), has been investigated at the Albany ResearchCenter. The induction melting technology allows for the use of various forms of charge materials such asloose titanium sponge, powder, scrap, and turning alloys to be consolidated into continuous ingots. Thistechnology is envisioned to produce starting ingots from inexpensive charge material for furtherrefinement to non-aerospace applications. Ingots produced by cold-wall induction melting at the AlbanyResearch Center had minor defects in the surface (Fig. 1) therefore, calcium fluoride slag additions wereused to facilitate melting and improve the surface finish (Fig. 2). Results are presented that show nodetrimental effects were discernible from slag additions. Also, a transition gas seal was developed toallow continuously melted ingots to pass from an argon atmosphere into air. A patent application has beenfiled for the transition gas seal.Fig. 1 Ingot surface tearwithout slag additions.Fig. 2 Smooth ingot surface meltedwith 0.5 weight percent slag.EQUIPMENT AND PROCESSINGCold-wall induction melted ingots were produced with the equipment shown in Fig. 3. This equipmenthas slight modifications from the original equipment described in U.S. Patent 3,775,091 (ref. 1). Thecrucible was a 24 segment, approximate 10 cm (4 in.) round, bottomless water-cooled copper crucible.The crucible was surrounded by an induction coil within a vacuum chamber. A solid titanium bar (startingstub) of approximately 17.8 cm (7 in.) length and a slightly undersized diameter was drilled and tapped toallow attachment of a water-cooled copper shaft. This starting stub was positioned within the center of thecrucible while the attached shaft extended through the bottom withdraw chamber via a vacuum gland.Fig. 3 Cold-wall induction furnaceschematic.The vacuum-tight aluminum furnace chamber was approximately 76 cm (30 in.) diameter by 76 cm longwith a 76 cm diameter hinged door. The crucible was mounted in the bottom of the chamber with anadditional approximate 18 cm (7 in.) diameter by 76 cm long extension chamber for ingot withdrawal.Two approximate 15 cm (6 in.) diameter view ports were located in the chamber, one directly above thecrucible and one in the door. All flanges were sealed with o-rings and water-cooled. The furnace wasinstalled on an elevated deck to facilitate ingot withdrawal. The power supply was a 100 kW, 9600 Hzmotor generator rated at 440 VAC and 228 AAC. The vacuum system consisted of a rotary blower backedby a mechanical pump.Charge material preparation consisted of hand screening Ti-6Al-4V chips to eliminate foreign materialand fines. Next, the chips were washed with acetic acid and rinsed with hot water to remove any residuesuch as cutting fluids. Then the chips were dried at 177°C. Charge material mixed with 0.5 weight percentCaF2 slag was placed in an approximate 50 cm (20 in.) diameter by 50 cm high chamber mounted on a 25 cm (10 in.) diameter extension to the right side of the furnace chamber. The furnace chamber wasevacuated to less than 50 millitorr and then backfilled with 1/5 atmosphere of an inert gas, typicallyargon. The charge composition was gravity fed through the bottom of the chamber via a funnel-shapedopening onto an electric vibratory feeder. The vibratory feeder delivered the charge into the crucible by achute extended over the edge of the crucible.The CaF2 slag has a lower melting point (1330°C) than the titanium charge (~1660°C). This allowed theslag to melt and migrate to the outer edge of the melt to solidify on the surface and outer diameter of theingot. This slag boundary layer facilitated withdrawal of the semi-solid ingot from the crucible.As the charge material melted, an equivalent amount of solidified bar was withdrawn continuously fromthe coil area and into an unheated water-cooled portion of the crucible for further cooling before enteringthe withdraw chamber. The withdraw mechanism consisted of an initial starting stub of ingot material ofsimilar composition as the charge material, and was positioned inside the crucible within the coil. Thebottom of the stub was drilled and tapped to allow attachment of a water-cooled copper shaft. This shaftextended through the bottom of the withdraw chamber via a vacuum gland. Withdraw was accomplishedby an electric motor and worm drive screw mechanisms.The CaF2 slag layer on the outer diameter of the resulting experimental alloy bar was easily chipped offand removed by a hammer. The resulting experimental alloy bar surface was machined before x-ray andultrasonic determination of defects. Ultrasonic defects were detected at a length of 8.89 to 12.70 cm(3.5 to 5 in.) and a depth of 2.54 cm (1.0 in.) and 27.94 to 31.12 cm (11.0 to 12.25 in.) length, 1.27 cm(0.5 in.) deep in the ingot. Cutting and machining the ingot at the defect locations revealed a CaF2 (slag)inclusion in one location and a WC (tungsten carbide) defect location in the other. The probable WCsource was a cutting tool chip in the scrap charge.Further processing of the experimental alloy included step forging two pieces at 1066°C from 8.73 cm(3 7/16 in.) diameter to 3.81 cm (1.5 in.) plate. The pieces were then reheated at 954°C for 30 minutes andcrossed and straight-away rolled to a final thickness of 1.59 cm (0.625 in.) The final step was to annealfor 30 minutes at 788°C. The final dimensions of the plates were approximately 14.6 ´ 38.10 cm (5 3/4 ´15 in.) and 13.34 ´ 26.21 cm (5 1/4 ´ 11 1/2 in.). Samples were cut from these for corrosion andmechanical testing. A commercial alloy Ti-6Al-4V ingot was similarly step forged and rolled inpreparation for comparison samples.RESULTSCHEMISTRYTable 1 shows the chemical analysis of typical reported literature values, the slag melted experimentalalloy, and the commercial alloy composition. The experimental alloy, which was produced from Ti-6Al-4V scrap chips, had elevated oxygen levels.Table 1. Chemistry percent comparison of Ti-6Al-4V alloy titanium castingsSource MIL T-9046Jliterature values(2)Slag meltedexperimental alloyCommercial alloyAl 5.5–6.75 6.42 6.81C 0.08 0.05 0.03Fe 0.30 0.22 0.20H 0.0125 Not analyzed Not analyzedN 0.05 0.06 0.06O 0.20 0.24 0.18V 3.5–4.5 4.17 4.46Others Total 0.40 0.16 0.07Balance Ti Ti TiMECHANICAL TESTINGTest blanks were cut from the forged and rolled plates for determination of mechanical properties. Theroom temperature mechanical properties of the two Ti-6Al-4V alloys were determined from specimenssectioned parallel to the rolling direction, so that the load axis of the tensile samples was aligned in thelongitudinal direction. The Charpy specimens were notched transverse to the rolling direction. Impactspecimens were standard Charpy V-notch specimens conforming to ASTM specification E23-91 (ref. 3).Tensile specimens were sub-size specimens proportional to the standard in ASTM E8-91 (ref. 4). Becauseof the notch sensitivity of titanium, the end of the tensile bar was designed to fit into a 12.7 mm colletgrip. The dimensions of the tensile test specimen are shown in Fig. 4.Fig. 4 Tensile test specimen dimensionsTensile specimens were tested on a servo hydraulic machine with a load capacity of 220 kN. Thespecimens were loaded into hydraulic collet grips. Strain was collected by an extensometer until 5%extension, after which the extensometer was removed. The tensile test was performed by loading at astrain rate of 0.02 mm/mm*min–1.Charpy V-notch specimens were tested on a Dynatup Model 8250 drop weight impact tester with dataanalysis by Dynatup 830-I software. The drop weight fixture was designed in accordance with ASTME23-91. The velocity of the calibrated tup was 3.05 m/sec with an impact energy of 210 J. Thetemperature of the test was 20°C.A total of five impact samples were tested for the experimental alloy that was melted with slag additionsand the commercial Ti-6Al-4V alloy. A total of four samples were tensile tested for each alloy. The meanresults are shown in Table 2.Table 2. Mechanical testing resultsMaterial Ultimate tensilestrength (MPa)Yield strength(0.2% offset)(MPa)Elongation(percent)Reduction inarea (percent)Charpy(J)Slag melted Ti-6Al-4Vexperimental alloy1047 1004 15 38 15.7Ti-6Al-4V commercialalloy1010 957 17 42 18.2Minimum SAEspecifications(5)896 827 10 25 N/ATypical properties ofannealed wrought bar(6)1000 925 16 34 22In comparison to the commercial values, the experimental alloy had slightly higher strengths and lowerductilities. Both the experimental and commercial alloys exceeded the SAE specifications for titaniumalloy bars and forgings and are similar to typical properties for an annealed wrought bar of Ti-6Al-4V,with the exception of low values for the impact energy. Prior to sectioning, the forgings received a millanneal, so the slightly lower impact values may be due to the differences in thermomechanical processing,testing direction, or impurity levels. Overall, slag additions do not appear to detrimentally affect themechanical properties of Ti-6Al-4V ingots prepared in a cold-wall induction furnace.CORROSIONTitanium and its alloys exhibit excellent corrosion resistance in a wide variety of environments. Thecorrosion resistance results from the formation of a very stable, continuous, and highly adherent oxidefilm. This film can break down under reducing and acidic conditions when TiO2 reduces to Ti2+ or Ti3+.Increasing temperature favors this reduction.Titanium castings can result in alloys with higher oxygen (O), nitrogen (N), and iron (Fe) contents.Table 1 shows that the slag melted Ti-6Al-4V experimental alloy ingot had more O and about the same Nand Fe contents as compared to the commercial alloy. However, the nature of the oxide film is basicallyunaltered by the presence of most minor alloying (<2 to 3%) additions.(7) For example, despite smalldifferences in O, N, and Fe contents in the various grades of unalloyed titanium, they all have about thesame corrosion resistance under low corrosion rate conditions (below 0.13 mm/y) (ref. 8,9). It is undermore extreme conditions that O, N, and Fe may increase corrosion rates.(8,9) Minor variations in alloychemistry are only a concern under conditions where passivity of titanium is borderline or non-existent.(7)Boiling HCl, an aggressive reducing acid, was chosen for initial corrosion testing at HCl concentrationsof 2%, 0.2%, and 0.02%. The corrosion rate of Ti-6Al-4V in boiling 2% HCl is reported to be 260 mils/y(6.6 mm/y) (ref. 10), which is too aggressive for useful service, but well into the active range where minorvariations in alloy chemistry may result in corrosion rate differences.Corrosion samples were cut from the slag melted experimental alloy (nominal dimensions of 2.8 by1.5 by 0.2 cm with a 0.24 cm diameter hole) and ground to 400 grit, pickled to remove approximately2 mils, cleaned, dried, and weighed. Pickling was done in a bath containing 35 volume percent stocknitric acid (HNO3, 70%), 5 volume percent stock hydrofluoric acid (HF, 60%), and the balance deionizedwater for 38 minutes at 40°C (ref. 11). The samples, in groups of two slag melted Ti-6Al-4V samples andtwo commercial samples, were individually suspended by teflon thread into deaerated HCl solutions andplaced in autoclaves. There was an additional group of four slag melted samples exposed to 2% HCl. Thesamples were heated to 103°C (the boiling temperature) after three cycles of gas evacuation with avacuum pump and backfilling with argon. The samples were removed after five days. After five days, thesamples were cleaned, dried, and weighed.The samples exposed in 0.02% HCl were scale-free and not etched. The samples exposed in 2% werescale-free and lightly etched. However in 0.2% HCl, a gray surface layer appeared on three of the fourslag melted experimental samples (Fig. 5). No such layer appeared on the commercial alloys. Both alloyswere lightly etched. Based on other results in hot, strong reducing acids,(11) the gray layer is probably athin layer of hydrided titanium that deposited onto the sample from Ti3+ ions in solution. Typically thisgray hydrided surface does not have significant effects on weight-loss measurements.(11)The corrosion rates were found to be strongly dependent upon the HCl concentration. The individualsample results and linear fit with HCl concentration and are shown in Fig. 6, while average values arelisted in Table 3. The average value of 6.6 mm/y for the slag melted experimental alloy in the 2% HClFig. 5. Ti-6Al-4V alloy samplesafter 5 days in boiling 0.2% HClsolution.Table 3. Averaged corrosion rates in mm/y for boiling HCl solutionsHCl concentration Slag melted Ti-6Al-4Vexperimental alloyTi-6Al-4V commercialalloyTi-6Al-4V literaturevalue(10)0.02 % <0.1 <0.1 –0.2 % 0.4 1.5 –2.0 % 6.6 8.0 6.6exposure agrees with the literature value of 6.6 mm/y (ref. 10). However, as Fig. 6 shows, there was oneslag melted sample with a corrosion rate close to 10 mm/y, while the rest were close to 6 mm/y. Initialcorrosion results in aggressive reducing acids (where active corrosion occurs) indicates no increasedcorrosion rates for the slag melted experimental alloy as compared with either the literature value, or thecommercial alloy.FATIGUEFatigue test specimens were tested in the elastic regime (<1000 MPa) for comparison of the slag meltedalloy with the commercial alloy. The white circles in Fig. 7 are the slag melted alloy and it appears thatthe slag melted alloy has lower fatigue life and more scatter in the data than the commercial alloy.02468100.0% 0.5% 1.0% 1.5% 2.0% 2.5%HCl Concentration, wt%Corrosion Rate, mm/yr Commercial alloyInduction slag meltedFig. 6. Individual samples and linearfit (dashed lines) corrosion results after5 days in boiling HCl solutions.Fig. 7 Fatigue comparison of slag melted (white circles)and commercial (black circles) Ti-6Al-4V alloys.CONCLUSIONSThe bottomless cold-wall copper crucible induction furnace provides the capability to consolidate manyforms of charge materials such as titanium sponge, scrap and turning alloys, and powder into a continuousingot with and without slag additions. The produced ingot could be used as stock material for castingfurnaces or be processed further for other products such as bar and wire. Although a slag defect wasidentified in the resulting ingot the following specific results can be drawn from this research:· Slag additions as low as 0.5 weight percent enhance melting by providing a “lubricated” surfacebetween the ingot and the copper crucible. This slag layer facilitates ingot withdrawal resulting in theelimination of surface tears and providing better surface finishes.· The experimental alloy chemistry produced with slag additions is in agreement with chemicalspecifications cited in literature for Ti-6Al-4V alloys.· Slag additions do not appear to detrimentally affect the mechanical properties of Ti-6Al-4V ingotsprepared in a cold-wall induction slag furnace.· No increased corrosion rates for the slag melted experimental alloy were noted from corrosion testingin an aggressive reducing acid.REFERENCES1. P.G. Clites and R.A. Beall. “Induction Melting of Metals in Cold, Self-Lined Crucibles”, U.S. Patent3,775,091, Nov. 27, 1973.2. ASM. Materials Properties Handbook, Titanium Alloys. Materials Park, OH, 1994, p. 487.3. ASTM. Standard Test Methods for Notched Bar Impact Testing of Metallic Materials. E23-91 in1991 Annual Book of ASTM Standards: Volume 03.01: Metals—Mechanical Testing; Elevated andLow-Temperature Tests; Metallography. Philadelphia, PA, 1991, pp. 197–212.4. ASTM. Standard Test Methods of Tension Testing of Metallic Materials. E8-90a in 1991 AnnualBook of ASTM Standards: Volume 03.01: Metals—Mechanical Testing; Elevated and Low-Temperature Tests; Metallography. Philadelphia, PA, 1991, pp. 130–145.5. Society of Automotive Engineers. Titanium Alloy Bars and Forgings. Aerospace MaterialsSpecification AMS4928H. Warrendale, PA, Jan. 15, 1977, 9 pp.6. ASM. Metals Handbook, Ninth Edition. Volume 3, Properties and Selection: Stainless Steels, ToolMaterials and Special-Purpose Metals. Metals Park, OH, 1980, p. 370.7. R. W. Schultz and D. E. Thomas, “Corrosion of Titanium and Titanium Alloys”, Metals Handbook,Volume 13–Corrosion, ASM, 1987, pp. 670–673.8. R. W. Schultz, J. S. Grauman, and J. A. Hall, “Effect of Solid Solution Iron on the CorrosionBehavior of Titanium”, Titanium–Science and Technology, Proceedings of the Fifth InternationalConference on Titanium, Deutsche Gesellschaft fur Metallkunde E. V., 1985, pp. 2617–2624.9. L. C. Covington and R. W. Schultz, “Effects of Iron on the Corrosion Resistance of Titanium”,Industrial Applications of Titanium and Zirconium, ed. E. W. Kleefisch, ASTM STP 728(Philadelphia PA, 1979: American Society of Testing and Materials) pp. 163–180.10. Corrosion Resistance of Titanium (1997: Titanium Metals Corporation) p. 11.11. R. W. Schutz and L. C. Covington, “Guidelines for Corrosion Testing of Titanium”, IndustrialApplications of Titanium and Zirconium, ed. E. W. Kleefisch, ASTM STP 728 (Philadelphia PA,1979: American Society of Testing and Materials) pp. 59–70.

Advanced titanium processing

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Advanced titanium processing

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