Brown, H.J.

Fahrenholtz, W.G.

Hilmas, G.E.

English

Engineering Conferences International

Engineering Conferences InternationalECI Digital ArchivesUltra-High Temperature Ceramics: Materials ForExtreme Environmental Applications II ProceedingsSpring 5-15-2012Fusion Welding of ZrB2-Based CeramicsH.J. BrownMissouri University of Science and TechnologyW.G. FahrenholtzMissouri University of Science and TechnologyG.E. HilmasMissouri University of Science and TechnologyFollow this and additional works at: http://dc.engconfintl.org/uhtcPart of the Materials Science and Engineering CommonsThis Conference Proceeding is brought to you for free and open access by the Proceedings at ECI Digital Archives. It has been accepted for inclusion inUltra-High Temperature Ceramics: Materials For Extreme Environmental Applications II by an authorized administrator of ECI Digital Archives. Formore information, please contact franco@bepress.com.Recommended CitationH.J. Brown, W.G. Fahrenholtz, and G.E. Hilmas, "Fusion Welding of ZrB2-Based Ceramics" in "Ultra-High Temperature Ceramics:Materials For Extreme Environmental Applications II", W. Fahrenholtz, Missouri Univ. of Science & Technology; W. Lee, ImperialCollege London; E.J. Wuchina, Naval Service Warfare Center; Y. Zhou, Aerospace Research Institute Eds, ECI Symposium Series,(2013). http://dc.engconfintl.org/uhtc/13Fusion Welding of ZrB2-Based CeramicsH.J. Brown-Shaklee, W.G. Fahrenholtz, and G.E. HilmasMissouri University of Science and TechnologyRolla, MO United StatesUltra-High Temperature CeramicsMaterials for Extreme Environment Applications IIHernstein, AustriaThis work was supported by a Phase I STTR sponsored by AFOSR Grant # FA 9550-10-C-0143Goals and Outline• Goals– Characterize microstructures of weldments– Assess the mechanical behavior of welded ceramics• Outline– Welding methods and needs– Thermal shock considerations– Welding methods employed in the present study– Characterization of fusion welds– ConclusionsJoining Methods and Needs• Common joining methods– Polymers and cements – Glassy interlayersOxide and bulk metallic glass– BrazesIncreasing TemperatureNon reactive alloysReactive– SHS joining– Mechanical fixtures– Diffusion Bonding – Plasma deposition– Fusion weldingE-beam, GTAW, plasma arcIncreasing Temperature• Considerations for TPS joining methods– Oxidation of joint and structure– Elevated temperature properties– CTE of parts– ManufacturabilityHeat Flux and DistributionHeat flux distribution for a 5.5 mm 100A W-arc discharged from a 75° sharpened tip• Thermal stresses are generated during welding– Temperature gradient– Heat flux• Must mitigate thermal stresses to prevent thermal shock and successfully join brittle materialsRef. Wu and Gao., “Analysis of the heat flux distribution at the anode of TIG welding arc.” Comp. Mat. Sci. 24 (2002).Weld Specimens• Compositions– ZrB2 + 20 vol% SiC (Z20S)– ZrB2-SiC-B4C (ZSB)– Hot pressed (HP) or sintered (S)• Bars were cut and ground to size and then butt weldedButt Welded Bars45 mm3 mm4 mm• Disks were sectioned– Diamond saw– Cleaned and chamfered• Welding methods– Gas tungsten arc welding (GTAW)– Plasma arc welding (PAW)Butt Welded DiscsØ~45 mm3 mmDiamond CutInitial GTAW Attempts• Incomplete penetration due to low power input• Extensive warping due to welding stressesCaused failure of larger specimens such as disks >25 mm diameter• Very promising!Vickers Fusion EpoxySolidification VoidIndentWeld InterfaceBottom of Weld PoolZonePreheating and Post HeatingI. A B C D II.GTAW Weld Pool Microstructure 3 mm Heat Affected Zone Fusion Zone Parent ZrB2-SiC Joint Chamfers Parent ZrB2-SiC Top SurfaceBottom Surface2.2 mmGTAW Weld Pool MicrostructureTop SurfaceBottom Surface2.2 mmMechanical Properties of Welds5.2 mm wide test bars were cut from billet and tested in 4-point flexure without finish grinding5.2 mmWelded JointBillet # w (mm) t (mm)Load (N)Stress (MPa)Bead In Tension?2 5.14 2.87 494 161 Y3 5.15 2.83 267 97 Y1 5.35 2.96 274 88 N1 5.11 2.95 214 72 N1 5.11 2.95 352 119 NCritical flaw of the weld section from Billet #2 was estimated to be ~200 µm based on Griffith analysisFailure Analysis of WeldsADB CEFEstimated joint strengths were calculated by assuming a joint KIC of ~4.5 MPa-m½– σf,estimated compares well with measured strengths.FlawFlaw Size(µm)σf, estimated(MPa)A 440 108B 450 107C 220 153D 290 133E 80 254F 60 2931 mm 200µmFull Penetration Welds in ZSBWeld Direction 1 mm 100 µmPorosity in Welds• Some porosity at boundary of parent material and fusion zonePossibly due to volume change upon solidification• Void formation in heat affected zone (HAZ)Active oxidation of SiC?• Porosity and void formation could be reduced by optimizing the starting composition, atmosphere, or heat inputHAZParent MaterialFusionZoneFusionZoneThermal Properties of Welds• Thermal diffusivity increases after welding– 0.13 cm2/s at 200°C for Z20S– 0.155 cm2/s at 200°C after welding• Welding alters the ceramic– Larger grain size– SiC polytype changes from 6H to 3C– May remove oxygen impurities through active oxidation• Thermal conductivity at 200°C~43 W/m•K for Z20S~51 W/m•K after weldingConclusions• ZrB2-based ceramics were joined by fusion welding– Parts up to 3 mm thick were joined by GTAW• Joint microstructures varied through the part– ZrB2-rich near the top of the weld– SiC rich near the bottom of the weld• Joint strengths were lower than the parent materials– <200 MPa for joined specimens compared to ~800 MPa for Z20S– Void formation at the HAZ-parent material interface• Thermal conductivity of joints was higher than parent material– Could be due to grain size, reduced impurities, or SiC polytype

Fusion Welding of ZrB2-Based Ceramics

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Fusion Welding of ZrB2-Based Ceramics

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