Titanium Alpha Case Prevention

Titanium Alpha Case PreventionA Major Qualifying Project report to be submitted to the faculty ofWORCESTER POLYTECHNIC INSTITUTEin partial fulfillment of the requirements for the Degree of Bachelor of ScienceSubmitted byJustin ChretienMatthew KingWilliam ProiaStacy RudolfSubmitted onApril 27, 2010In Cooperation WithWyman-Gordon CompanyApproved:Professor David DiBiasio, Advisor Professor Richard Sisson, Co-Advisori

AbstractIn an effort to more accurately predict diffusion kinetics of oxygen into titanium duringheat-treatment, the Wyman-Gordon Company commissioned this study to obtain an accurate rateof alpha case formation within titanium and to investigate the use of coatings to reduce alphacase formation. This project consisted of heat-treating Ti-6Al-4V and Ti-6Al-4V ELI samplesthat were uncoated, coated with an SJ, and SJ advanced coating over a period of time consistentwith their heat-treatment cycle. These samples were analyzed through optical microscopy andmicrohardness to determine alpha case depth. The project concluded with a cost analysis, whichaimed to find the most economic and optimal solution to reducing alpha case formation.ii

Executive SummaryThe Wyman-Gordon Company (WGC) is an industry leader in forging titaniumaerospace components. Before the forging process can take place, these titanium pieces are heattreated in large gas furnaces. During the heat treatment process, oxygen diffuses into the titaniumcreates a stabilization of the alpha phase. This causes the formation of an alpha case layer. Thisalpha case layer is a hard, brittle shell. Its fragility makes it undesirable for aerospaceapplications. Titanium’s high strength to density ratio makes it ideal for the aerospace industrybut formation of alpha case compromises this strength. Even a small fracture in the alpha casemay cause a part to fail. Therefore, all alpha case formed during the heat treatment process mustbe removed through a chemical milling process involving corrosive acids: hydrofluoric andnitric. The objective of this project was to understand diffusion kinetics of oxygen into titaniumduring the heat treatment process in order to obtain an accurate rate of alpha case formationwithin titanium over time and to investigate coatings in an attempt to reduce alpha caseformation.To determine the rate of alpha case formation and compare a coating’s resistance tooxygen diffusion 12-piece sample sets of Ti-6Al-4V (Ti-6-4) and Ti-6Al-4V ELI (ELI) wereheat treated under three coating conditions for a maximum of 6 hours and at 1750 F. The coatingconditions were as follows:1. Uncoated2. Coated with SJ, current WG production procedure3. Coated with SJ advancedAfter heat treating the samples and performing the necessary preparation techniques ofcutting, mounting, grinding, polishing, and etching each sample they were analyzed throughoptical microscopy and microhardness profiles. Using a microscope synchronized with acomputer program we were able to photograph and measure the visible alpha case region on eachsample under 20 times magnification. Microhardness testing was then performed to determine atwhat percentage range beyond the visible alpha case region the hardness profile of the sampleiii

stabilized. This point indicated the minimum depth necessary to remove through chemicalmilling.To visually represent the amount of alpha case formed and to compare the effect of eachcondition on alpha case formation, the optically measured depth was plotted against the squareroot of time each sample was heated. This representation showed a linear, positive correlation inthe data thus indicating a parabolic growth of alpha case over time. The maximum average depthof alpha case observed was about 67 microns. Samples coated with SJ advanced showed a 37%reduction in alpha case on Ti-6-4 samples and a 54% reduction on ELI samples.Microhardness profiles revealed that sample hardness did not stabilize until an average of65% beyond the optically viewed depth of alpha case, with a maximum increase of 120%. Themaximum depth of stabilization for Ti-6-4 samples was 110 microns and similarly 100 micronsfor ELI. With SJ advanced coated samples, however, the maximum depth before hardnessstabilization was only 50 microns for Ti-6-4 and 30 microns for ELI.After completing analysis upon our test samples and collecting data from WGC from the2009 year a raw material cost comparison was conducted. It was found that by chemicallymilling only as much alpha case as was found through our analysis and by transitioning to the SJadvanced coating that a significant amount of money could be saved annually. Table 1 representsthe potential savings associated with both these proposed changes.Table 1: Cost FindingsCurrent ProcessOptimized MillingSJ Advanced CoatingCoating Cost ( /yr) 42,500 42,500 47,300Acid Bath Cost ( /yr) 688,000 267,000 172,000Total Coating and AcidCost ( /yr) 731,000 310,000 219,000 421,000 512,000Savings ( /yr)iv

Our results provide an accurate profile of alpha case accumulation on titanium for theWGC to reference when heating and milling. Optimizing the chemical milling process so thatonly the alpha case layer is removed would show immediate savings for WGC as their acid bathswould begin lasting much longer. This would also benefit the company environmentally as lessacid would need to be treated and disposed of, therefore lessening the risk of a spill.Furthermore, while the SJ advanced coating did not eliminate alpha case altogether, it didsignificantly reduce the amount of alpha case formed by protecting against oxygen diffusion. It isrecommended that the WGC begin to use the SJ advanced coating in place of SJ. The projectgroup recommends that our results be used to aid future research outside the scope of thisproject. In particular we recommend the following projects:1. Determine coating durability and practicality within the forging process bysimulating forging, cooling, and reheating cycles.2. Investigate alternate methods of coating removal to further lengthen life of acidbaths.3. Continue experimenting with different coatings to completely prevent alpha caseformation.v

AcknowledgementsWe would like to begin by thanking the WGC for allowing us access to their facility andequipment as we investigated the formation of alpha case over time and possible coatings toreduce the depth of contamination. In particular, we would like to thank Brian Postale and BriantCormier for the creation of our project and for their continual discussion, interest, and assistancewith becoming acclimated with the facility, equipment, and important people of interest that weutilized during at time at WGC.We also extend thanks to David Markey, WGC’s principle metallurgist, for providingreference laboratory reports and discussing gas versus electric furnace effects on alpha caseformation. We extend a very special thanks is also extended to Ernie Brackett, samplepreparations supervisor, for his assistance in acquiring titanium test samples as well as theindustrial staff for cutting our tested samples to a usable size.We appreciate the assistance of Advanced Technical Products in supplying our projectgroup with a five gallon sample of the SJ advanced coating. Also, we would like to thank RogerFabian of Bodycote Thermal Processing for being able to prepare a test sample under vacuumconditions for our group.In addition, we would like to thank Professor Boquan Li for his continual assistance inour sample preparations in the Washburn Laboratories. Professor Li was responsible for trainingour group on numerous procedures such as mounting, polishing, and etching our samples.Without his assistance we could not have gathered any data. Similarly, we thank Rita Shilansky,Mechanical Engineering Administrative Assistant, for allowing our group access to theappropriate laboratories during the sample preparation and analysis stages of our project.Finally, we would like to thank both our advisors, David DiBiasio and Richard Sisson,for their continual assistance, guidance, and feedback throughout the entirety of the MQPprocess.vi

Authorship PageJustin Chretien, Matthew King, William Proia, and Stacy Rudolf all contributed to theoutline and final edit of this research report. Each person’s contributions in terms of authorshipand editing are recorded on this page. Authors were responsible for writing an initial draft ofeach section. Editors revised, updated, and checked sections for appropriate voice, grammar, andmechanics.Abstract:Author – Bill ProiaEditor – Justin ChretienAcknowledgments:Author – Justin ChretienExecutive Summary:Authors – Justin Chretien, Bill ProiaEditor – Justin ChretienIntroduction:Authors – Matthew King, William Proia, Stacy RudolfEditor – Stacy RudolfLiterature Review:Authors – All membersEditor – Justin Chretien, Stacy RudolfMethodology:Authors – Justin Chretien, Matthew King, Stacy RudolfEditor – Stacy RudolfFindings and Discussion:Authors – Justin Chretien, Matthew KingEditor – Justin Chretien, Stacy RudolfConclusions:Author – Stacy RudolfEditor – Matthew KingRecommendations:Author – Stacy RudolfEditor – Justin Chretien, Matthew Kingvii

Table of ContentsAbstract . iiExecutive Summary . iiiAcknowledgements . viAuthorship Page . viiTable of Contents . viiiList of Figures . xiList of Tables . xii1Introduction . 12Literature Review. 42.1Titanium . 42.1.1 Titanium Phases . 52.1.2 Titanium Alloys . 72.2Heat-treatment of Titanium . 82.3Alpha Case Overview. 92.4Dominating Factors . 92.4.1 Theoretical Calculations . 112.5Alpha Case Prevention . 112.5.1 Molds . 112.5.2 Coatings . 143Methodology . 183.1Create alpha case samples and prepare for analysis . 183.1.1 Creating Alpha Case . 183.1.2 Cutting Samples . 183.1.3 Mounting, polishing, and etching . 193.2Determine parameters to be tested and data to be collected . 203.2.1 Parameters varied . 20viii

3.2.2 Data recorded . 203.3Analyze alpha case depth on samples . 203.3.1 Optical Microscopy . 203.3.2 Micro-hardness analysis . 2043.4Cost Analysis. 213.5Vacuum Testing . 21Results and Discussion . 224.1Optical assessment of heat-treated specimens . 224.1.1 Phase contrast . 224.1.2 Phase Structure . 234.2Alpha case depth trends. 234.2.1 Microscopic analysis . 244.2.2 Microhardness analysis. 264.3Cost analysis . 284.3.1 Acid Bath . 284.3.1 Coatings . 294.3.2 Overall . 304.4Environmental Benefits . 314.5Coating comparison. 314.6Vacuum Testing . 314.7Limitations to findings . 324.7.1 Application method . 324.7.2 Furnace trays. 324.7.3 Coating durability . 324.7.4 Coating viability and removal . 335Conclusions . 346Recommendations . 36ix

References . 38Appendix A – Equipment used in testing and analysis . 40A.1 Electric Oven (5217) and P.P.E. . 40A.2 Saw in Washburn . 40A.3 Vibro-peen . 40A.4 Coater. 40A.5 Sample Mounter . 40A.6 Grinding . 41A.7 Optical Microscope. 41A.8 Micro-hardness Tester . 41Appendix B – Traceability and labeling technique. 42Appendix C – Heating Procedure . 43Appendix D – Cutting Procedure . 44D.1Procedure in the Test Prep Department of WGC . 44D.2Procedure in Worcester Polytechnic Institute’s Materials Lab . 44Appendix E – Sample Preparation Procedure . 46E.1Mounting . 46E.2Polishing . 46E.3Etching . 47Appendix F- Optical Microscopy Analysis Procedure . 48Appendix G – Microhardness analysis Procedure . 49Appendix H – Alpha Case Optical Depth Photos . 50Appendix I – Alpha Case Optical Depth Data . 55Appendix J – Microhardness Profiles . 58Appendix K – Microhardness Data. 60x

List of FiguresFigure 1: Titanium Phase Diagram (Gale & Totemeier, 2003) . 5Figure 2: Hexagonal Close Packed Structure (Callister, 2006) . 6Figure 3: Body Centered Cubic Structure (Crystal structure, 2009) . 6Figure 4: Predicted depth of α-case for titanium alloy IMI 834 at different exposed temperatures(Gurappa, 2003). . 10Figure 5: Measured microhardness profiles

Titanium’s high strength to density ratio makes it ideal for the aerospace industry but formation of alpha case compromises this strength. Even a small fracture in the alpha case may cause a part to fail. Therefore, all alpha case formed during the heat treatment process must be removed through a chemical milling process involving corrosive acids: hydrofluoric and nitric. The objective of .