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IMIII i f~lflllll= j4i AD-A I I1" WL-TR ELECTF. Materials Directorate USE OF TITANIUM CASTINGS WITHOUT A CASTING FACTOR

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AD-A IMIII i f~lflllll= WL-TR USE OF TITANIUM CASTINGS WITHOUT A CASTING FACTOR Dr. Dianne Chong McDonnell Douglas Missile Systems Company PO Box 516 St Louis MO September 1992
AD-A IMIII i f~lflllll= WL-TR USE OF TITANIUM CASTINGS WITHOUT A CASTING FACTOR Dr. Dianne Chong McDonnell Douglas Missile Systems Company PO Box 516 St Louis MO September 1992 Final Report for Sept Sept Approved for Public Release; Distribution is Unlimited =-.-.. SDTI C ELECTF S MWAY 1q. i= Materials Directorate Wright Laboratory Air Force Materiel Command Wright-Patterson Air Force Base, Ohio j4i I I1 NOTICE When Government drawings, specifications, or other data are used for any purpose other than in connection with a definitely Government-related procurement, the United States Government thereby incurs no responsibility nor any obligation whatsoever. The fact that the government may have formulated, or in any way supplied the said drawings, specifications, or other data, is not to be regarded by implication or otherwise as in any manner construed, as licensing the holder or nay other person or corporation, or as conveying any rights or permission to manufacture use, or sell any patented invention that may in any way be related thereto. This report is releasable to the National Technical Information Service (NTIS). At NTIS, it will be available to the general public, including foreign nations. This technical report has been reviewed and is approved for publication. TEW.T C)0N THEODORE I- F' AINHART, Chief Engineering & De n Data Materials Engineering Branch Materials Engineering Branch Systems Support Division FOR THE COMMANDER THOMAS D. COOPER, Systems Support Divisia Materials Directorate Wright Laboratory hf If your address has changes, if you wish to be removed from our mailing list, or if the addressee is no longer employed by your organization please notify WL/MLSE BLDG 652, 2179 TWELFTH ST STE 1, WPAFB OH to help us maintain a current mailing list. Copies of this report should not be returned unless return is required by security considerations, contractual obligations, or notice on a specific document. 1. A0CV U10 GNLMT P .,,,Wm. U m ,e SPO ET E AND DATES COVERED September 1992 Final Report 26 Sept Sep TTL AMS VIBTLI! L PFUNDNONUMBI Use of Titanium Castings Without a Casting Factor C-F C-5621 PE-62102F An... PR-2418 fl Inonp TA-04 Dianne Chong WU-73.PIRPORAOG OVAOANATMAI0N N() AMS AD011ongEl L. UPERORSOG GWAI NIZ 1EPORT NUMM McDonnell Douglas Missile Systcuis Company P.O. Box 516 St. Louis MO I. 8I SOOeONG /MOMTOIUNO AwCY Nm ANS ADODSS(E 10. SPONsOI / MONTOAIN Maerials Directorate AGENCY REPORT NUMBER Wright Laborato y (WI.MLSE) WL-TR Air Force Materiel Command Wright-Patterson Air Force Base OH (Steven R. Thompson; ) 11. SUPPLEMENTARV NOTES 12L. DISTIBUTION / AVAIABLITY STATEMENT a6. DISTRIOUTON0 CODE Approved for Public Rcleasc: Distribution is Unlimited 1i. ABSTRACT (Maidmum 2W0 wo) The Use of Titanium Castings Without a Casting Factor program was conducted to establish A and B allowables for Ti.6A1-4V. Taguchi methods were used to develop a more restrictive chemistry and annealing condition to provide parts with less variability in properties. A and ' design allowablcs that were determined for full scale Ti-6A1-4V missile fins and step plates produced using these new pmmasneers showcd very low variability: the standard deviations of these data were less than 2 ksi. The A -basis allowables are Ftu=125 ski and Fry 120 ksi. A nondestructive inspection technique was dcvcloped to correlate m csurcment of microstructurm features to mechanical properties. This was found go be of limited value because of the narrow property band that was established. A new AMS specification that included the new allowables, the microstructural inspection criteria, and the more refined chemistry and post-casting treatments was established for investment cast Ti-6AI-4V. Limited fracture mechanics evaluation was also performed on the cast fins. 14. GdECT TF*4 It. NUM ER OF PAGES Titanium Castings, casting factor. Ti-6AI-4V. mcchanical properties, design allowables. 122 specification Is. C 17. SECURITY CLASSPICATION I&, SECUITYrCLASSI1PATION It. SECURITY CLAAIICATION 20. UMITA ION OF ABSTRACT OF REPORT OF THIS PAE OP ANSTRACT UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED UL NSN COMPUTEA GENErFATED Standdwd Form 290 (Rey 240) 2*1e0V1 due*31 FOREWORD This program was conducted by the McDonnell Douglas Missile Systems Company (MDMSC) in cooperation with Scldosser Casting Company under Contract Number F C Under this effort A and B design allowables were determined fdr Ti-6AI-4V castings. A new microstructural inspection techniques and a new AMS specification were established for investment cast Ti-6Al-4V. Mr. Steven R. Thompson managed the program for Wright Laboratory. His guidance on the program is greatly appreciated. Funding for the program was provided by Wright Laboratories Materials Directorate. We are also grateful for the support provided by the members of the MIL- HDBK-5 Titanium Casting Task Group, for their inputs and support of this program. Their guidance was invaluable to the program. DTIC Q!ALMT r1-ki'ected 3715 GRA&1 DTIC TAB 0 Una,usno ed 0 JuJt l ct erion- By- Distr 'Ibu ti/ Avail bllt C0od4l IAVSI 1 d/ot Dist 0pealI ViIi CONTENTS SPAOE FOREW ORD... iii LIST CF T'iGUL R... vii LIST OF TABLES... ix I INTRODUCTION AND SUMMARY BACKGROUND PROGRAM PHASES CONTROL OF VARIABILITY COMPOSITIONAL VARIABILITY POST-CASTING TREATMENT PREPRODUCTION PART ANALYSIS MECHANICAL PROPERTY TESTING NONDESTRUCTIVE INSPECTION ESTABLISHMENT OF SPECIFICATION A' AND B ALLOWABLES ALLOWABLES DETERMINATION REDUCED RATIOS NONDESTRUCTIVE INSPECTION DAMAGE TOLERANCE FRACTURE TOUGHNESS FATIGUE CRACK GROWTH CONCLUSIONS AND RECOMMENDATIONS REFERENCES V CONTENTS (CONT'i)) APPENDICES APPENDIX A: CONTROL OF VARIABILITY TAGUCHI ANALYSIS APPENDIX B: PREPRODUCTION PART ANALYSIS APPENDIX C: PROPOSED AMS SPECIFICATrION FOR TITANIUM ALLOY CASTINGS, INVESTMENT 6AL-4V APPENDIX D: CAST TI-6AL-4V PRODUCTION FIN ANALYSIS, MECHANICAL PROPERTY DATA, MICROSTRUCTURAL NDI DATA, A AND 'B ALLOWABLE ANALYSIS APPENDIX E: FRACTURE MECHANICS DATA vi LIST OF FIGURES 1 VARIATION IN MECHANICAL PROPERTIES OF TI.6AL-4V CASTINGS FROM DIFFERENT SUPPLIERS PROGRAM 'LOW DIFFEREN1 CHEMISTRIES PRODUCE DIFFERENI STRENGTH LEVELS AND DISTRIBUTON OF POPULATION EFFECTS OF INTERSTITIAL ALLOYING ELEMENTS ON UNALLOYED TITANIUM THE TI-6AL-4V PHASE DIAGRAM CAST TI-6A),-4V STEP PLATES CAST TI-6A,-4V MISSILE FINS LOCATION knd ORIENTATION OF TENSILE SPECIMEN'; EXCISED FROM PREPRODUCTION FINS AND ST'EP PLATES RELATIONS HIP OF CHEMISTRY TO STRENGTH RELATION'.HIP OF THICKNESS TO TENSILE S)'RENGTH DATA GENERATED FROM PARTS CAST TO MORE RESTRICTIVE CHEMISTRY AND POST-CASTING TREATMENT SHOW LESS VARIABILITY THAN DATA GENERATED FROM PARTS CAST TO CURRENT PUBLIC SPECIFICATIONS TYPICAL PHOTOMICROGRAPHS OF REPLICAS TAKEN FROM TI-6AL.-4V CASTINGS VARIATION OF MICROSTRUCTURAL FEATURE WITH PART THICKNESS vii LIST OF FIGURES (CONT'D) - PAG 14 SIZES OF THE MICROSTRUCTURAL FEATURES VARY LITTLE OVER THE NARROW PROPERTY RANGE SEEN IN THESE CASTINGS PRODUCTION FINS FROM SUPPLIER COMPRESSION, BEARING, AND SHEAR SPECIMENS FROM TI-6AL-4V CAST[NGS SUMMARY OF ALLOWABLES FOR CAST TI-6AL-4V COMPACT TENSION SPECIMENS AND BEND SPECIMENS USED FOR FRACTURE TOUGHNESS ASSESSMENT FRACTURE TOUGHNESS DATA FROM THE COMPACT TENSION SPECIMENS ARE COMPARABLE TO LITERATURE VALUES (REFERENCE 32) FATIGUE CRACK GROWTH SPECIMEN FATIGUE CRACK GROWTH RATE AS A FUNCTION OF STRESS INTENSITY FACTOR FOR ALL THREE SPECIMENS TESTED COMPARISON OF FATIGUE CRACK GROWTH DATA WITH LITERATURE CITATIONS (REFERENCE 32) CRACK GROWTH VERSUS CONSTANT-AMPLITUDE STRESS CYCLES FOR THREE TI-6AL-4V CAST FINS viii LIST OF TABLES TARL.E PAGE 1 A AND B ALLOWABLES CORRESPONDING TO CURVES IN FIGURE PROPOSED CHEMISTRIES FOR TI-6AL-4V CASTINGS CONTRIBUTION OF ALLOYING ELEMENTS TO MECHANICAL PROPERTY VARIABILITY PROGRAM CHEMISTRY ACTUAL CI{EMISTRIES OF PREPRODUCTION PARTS MICROSTRUCTURAL FEATURES - MAXIMUM LIMITS.23 7 COMPARISON OF FEATURES OF SPECIFICATIONS FOR CAST TI-6AL-4V A AND B; ALLOWABLES REDUCED RATIOS FOR TI-6AL-4V CASTINGS FRACTURE TOUGHNESS TESTS R-CURVE MEASUREMENTS FATIGUE CRACK GROWTH CONDITIONS ix SECTION 1 INTRODUCTION AND SUMMARY Casting has been demonmtrated to be a cost-effective means of manufacturing aerospace parts compared to other fabrication processes such as machining or forging. The casting process produces net or near net shape parts that require little or no machining. For titanium alloys castings are particularly cost-effective for several reasons. Since the raw material cost of titanium is high, efficient use of the raw material as in castings results in little waste. Using traditional methods to machine titanium is expensive. Elimination of machining would further reduce costs. Although castings have been found to be cost-effective, their usage in critical aircraft structures is limited due to the imposition of a margin of safety (i.e., casting) factor. In early casting technology, poor controls over the material composition resulted in parts with entrapped gas or inclusions. Lack of process control produced castings with shrinkage, cold-shuts, and hot tears. Many parts had coarse, nonuniform microstructure and chemical segregation. These defects caused variabilities in the mechanical properties of castings. This led to the institution of an added margin of safety for castings, or a casting factor, that is still used in the design of cast components despite the advancements that have been made in casting technology that have increased the reliability and quality of parts. Foundries have focussed on several parameters in order to improve the quality of castings. Refinement of chemistries has been performed to increase consistency in processing as well as in the final product. Analysis of casting design has provided information for the optimization of gating and mold fill to prevent the formation of flaws during casting and to improve producibility. Heat treatment of castings has been developed to modify microstructures to improve properties as verified by tests of separately cast bars or prolongations. Extensive nondestructive inspection techniques have been developed to verify quality in castings. These techniques and inspection criteria have been tailored to the criticality of castings in use. While the better inspection methods increase confidence in the quality of the parts being used, they also add to the cost of using castings. Despite all these improvements in foundry practice, the process controls are not well enough established to permit the establishment of design allowables. I 1 1.1 BACKGROUND Aircraft companies have been reluctant to use castings (primarily aluminum) due to their inconsistent mechanical properties and quality. To compensate for the scatter in properties, a margin of safety (i.e., a casting factor) of 1.33 was defined for missiles (Reference 1) and aircraft (Reference 2). During the 1960s, aluminum foundries demonstrated that the property scatter could be reduced by providing better control of the process. To eliminate the uncertainty that properties of separately cast test bars did not reflect those of castings, strength was verified using specimens excised from parts. While the use of separately cast bars provides a good means of checking chemistry and heat treatment response, it is not representative of the properties of the part since the solidification environment is different. In 1970, MIL-A (Reference 3) was issued and addressed the problem of variability in properties by requiring more detailed inspection criteria. Even with improvements in foundry practice, variability in mechanical properties was still considered excessive. In 1985 acceptance criteria based upon measurement of dendrite arm spacing (DAS) of aluminum castings was established (Reference 4). Subsequently, the Society of Aerospace Engineers (SAE) issued an Aerospace Recommended Practice, ARP 1947, (Reference 5) describing the procedure for determining DAS and relating it to tensile strength and also issued a material specification, AMS 4241 (Reference 6), that specified a more restrictive chemistry for aluminum alloy 357. Despite the advances that have been made in titanium foundry technology, there is a reluctance to eliminate the casting factor because of the history of property variability in aluminum castings. In titanium alloys, hot isostatic pressing and appropriate heat treatment have been shown to offer the potential of near-wrought properties, including fatigueresistance and ductility. For these reasons and because of the cost effectiveness of using these castings, there has been an increased interest in using and establishing design allowables for these parts. In response to this need, in 1986, the Military Handbook 5 Coordination Committee established an ad hoc committee to compile data from investment cast Ti- 6AI-4V for the purpose of determining A and B design allowables. Data from suppliers and users supplied to the Titanium Casting Task Group showed that investment cast Ti-6AI-4V parts supplied to the aerospace industry could not be represented by a single set of A and B design allowables (Reference 7). Figure 1 demonstrates this point. This figure shows the mechanical property distribution by supplier for an investment cast Ti-6AI-4V elevon housing supplied to the Boeing Corporation (Reference 8). Data from each supplier can be represented by its own population distribution. The implication is that foundry practices significantly affect the variability of mechanical properties in castings. However, the differences in properties in no way compromise the quality of the parts since mechanical properties 2 of the parts met the minimum values specified in the Boeing specification (BMS 7-181). Attempts to determine A and B basis design allowables from data with such a large variation in properties would result in conservative values. The Task Group concluded that the casting and processing of Ti- 6A1-4V needed to be reduced to a standard practice that was tightly controlled by a specification in order to reduce the variability in mechanical properties. Only when the variability was reduced and meaningful A and B allowables established, could reduction or elimination of the casting factor be considered. 30 Frequency 20- C percent A FTU. kh GOP34,OO4.16-v , FIGURE 1. VARIATION IN MECHANICAL PROPERTIES OF TI-6AL-4V CASTINGS FROM DIFFERENT SUPPLIERS The primary objective of our program was to establish meaningful A and B design allowables for Ti-6A1-4V castings. It is important to emphasize that this did not necessarily result in obtaining castings with the highest properties, but rather the most consistent. We employed the strategy of first reducing the variability in mechanical properties by imposing tighter restrictions on chemistry and post-casting treatment. We also utilized a microstructural nondestructive technique to verify properties of castings. Castings produced to these tightened parameters would then be controlled by a new specification and a microstructural nondestructive inspection technique. The technical program consisted of the following phases: control of variability, preproduction analysis, nondestructive inspection, specification establishment, establishment of 3 A and w allowables. and damage to isenae. Th program flow is shown in PFgure 2. CONTROL OF VARIABILITY IFICAT IO PARTS PPRODUCTION A AND B ALLOWABLES DAJ'IAGE1 TOLERANCE SPECIFIC L-,,ATO FIGURE 2. PROGRAM FLOW 4 1.2 PROGRAM PHASES In Phase I, we used Taguchi methods to determine the sources of variability in Ti-6AI-4V castings. The primary factors that were investigated in this study were the chemical composition and post-casting treatment. These factors were defined with the intent of producing castings with small variability in mechanical properties. In Phase II, we analyzed mechanical properties of preproduction missile fins and step plates produced using the composition and postcasting treatment defined by the results of the Taguchi study. We also utilized a nondestructive inspection (NDI) technique developed by MDMSC to correlate physical and mechanical properties of castings with features such as prior beta grain size, alpha colony size, and grain boundary alpha. In Phase III, a new AMS specification was written to incorporate the refined chemistry and post-casting treatment. Mechanical property testing of specimens from of step plates and preproduction fins was used to provide S basis allowables. In Phase IV, specimens from production lots of parts were tested to determine A and B allowables for these castings. Compression, bearing, and shear properties were also determined for the establishment of reduced ratios. These properties were used to revise the AMS specification. Finally, in Phase V, fracture mechanics testing of specimens from the castings was performed. 5 SECTION 2 CONTROL OF VARIABILITY In Phase I, we utilized Taguchi methods to identify the causes of and minimise the variation in the tensile strength of titanium castings. We applied Taguchi analysis of means and variance methods to the test data provided by the Boeing Corporation as well as other available data. As a result of this analysis, we were able to discern the individual effects of chemistry, HIPing, and heat treatment on the average and variance of the mechanical properties for Ti-WAI-4V castings. It was considered beyond the scope of this program to include analysis of other factors such as cooling rates (due to differences in mold temperature prior to casting), weld repair conditions, and heat treatments above the beta transus. 2.1 COMPOSITIONAL VARIABILITY In this task we used Taguchi methods to define compositional limits for Ti-6AI-4V castings to provide more consistent mechanical properties. The relative strengthening effect of each alloying element was taken into account in our analysis. A detailed description of this analysis can be found in Appendix A. Based on our findings, we felt that a tightening of allowable chemistry variations was feasible for the alloying elements in Ti-6AW-4V. Because of extensive experience obtained in the production of titanium alloys over the last 30 years, control of alloy chemistry is fairly routine. Of the interstitials, carbon and nitrogen are usually not adjusted by the primary metal supplier and tyically do not exceed 0.01 weight percent (w/o). Oxygen levels are usaly higher than those for carbon and nitrogen primarily because the starting titanium sponge can contain oxygen levels as high as 0.08 w/o. Melting operations conducted by titanium foundries typically raise the oxygen content of the melt by approximately 0.02 w/o. With current commercial practice, therefore, it is possible to obtain a titanium alloy casting with well-controlled oxygen levels in the range w/o. As stated in Section 1, the intent of the program was to establish parameters to produce the most consistent properties and not necessarily the highest average properties. An example of this is shown below. Differences in chemical composition that were still within the limits of the current public specifications can produce variations in population 6 distributions (Figure 3). If we target a tighter chemical composition, we obtain the population labelled minimum variance. The average strength of the parts is approximately 134 kei. On the other hand, if our target were to be a chemistry that would produce mazirnum average strength, the resultant mechanical properties would show a much larger spread in values. The A'- and B -basis allowables (Table 1) for each of these groups verify the influence that population distribution has upon allowable values. so 40 - Minimum Variance (Run 5) Frequoncy 30- percent Maximum Average (Run 4) Uttimate Strength -kul OnhM4.toW. v, FIGURE 3. DIFFERENT CHEMISTRIES PRODUCE DIFFERENT STRENGTH LEVELS AND DISTRIBUTION OF POPULATION TABLE 1. A AND B ALLOWABLES CORRESPONDING T
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