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Titanium

Titanium. Resistance Welding. Learning Activities View Slides; Read Notes, Listen to lecture Do on-line workbook. Lesson Objectives When you finish this lesson you will understand:. Keywords. Donachie, Titanium, ASM, 1988. Donachie, Titanium, ASM, 1988. Unique Properties

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Titanium

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  1. Titanium

  2. Resistance Welding • Learning Activities • View Slides; • Read Notes, • Listen to lecture • Do on-line workbook • Lesson Objectives • When you finish this lesson you will understand: Keywords

  3. Donachie, Titanium, ASM, 1988

  4. Donachie, Titanium, ASM, 1988

  5. Unique Properties • Light Weight • Excellent Corrosion Resistance • Strong tightly adherent oxide • High strength-to-weight ratio • Applications • Chemical Industry • Aerospace Industry • Marine Fields • Medical Applications • Phases in Pure Titanium • Alpha {hcp} < 1620F (880C) • Beta {bcc} > 1620F

  6. Classification of Titanium Alloys • Alpha & near Alpha Alloys • Alpha-Beta Alloys • Metastable Beta Alloys AWS Welding Handbook

  7. Can Lead to Brittleness Especially in welds (see below) AWS Welding Handbook

  8. AWS Welding Handbook

  9. Commercially Pure Titanium • Yield Strengths 35-80 ksi (interstitial O, N, C) • Alpha Titanium Alloys • Not heat treated to increase strength • Used for moderate elevated-temp strength and creep resistance • Near-alpha Alloys • Contain small amount of Beta stabilizers • Marginally heat treatable • Most contain Si for improved high temp properties

  10. Alpha-Beta Alloys (e.g. Ti-6Al-4V) • Mixture of Alpha & Beta • Annealed or Solution heat treated and aged • Outstanding strength to density ratio in heat treated condition • Excellent fracture toughness when annealed • Metastable Titanium Alloys • Contain high percentage of Beta stabilizing elements • Slow transformation - air cooling gives 100% Beta • Aging precipitates of fine Alpha can occur • Aging give strength but ductility and fracture toughness sacrificed • Frequently used for fasteners and springs

  11. Applications Using Titanium Alloys For more information about applications see: http://www.titanium.org/GIsec2a.htm

  12. General Weld Problems • Oxygen Contamination Cracking • Oxygen > 3000ppm = WM & HAZ Cracking • Alpha (hcp) particularly susceptible • Removal of surface oxide film recommended • Gas shielding generally recommended (especially for seam welds) • Degree of Oxidation • Silver (mild oxidation) • Straw • Blue • White (severe oxidation - weld problems likely)

  13. General Weld Problems • Iron Contamination Cracking • Iron particle dissolve into surface causing loss of : • Corrosion • Embrittlement (when sufficient iron) • Prevention • Avoid steel fabrication near titanium fabrication • Avoid airborne dust • Avoid tools used for both Fe and Ti • Scratch Brush just prior to welding • Use clean gloves for handling Http://www.twi.co.uk/bestprac/jobknol/jk24.html

  14. General Weld Problems • Hydrogen Embrittlement • Precipitation of titanium hydrides • Hydrogen > 200ppm can cause problems • Use low moisture shielding gas AWS Welding Handbook

  15. Example of Hydride Formation in Resistance Seam Weld 25 ppm Hydrogen 2056 ppm hydrogen H 450 days-Full around 2 days– no hydrides 227 days-1/2 around Toosky, R. “Evaluation of Titanium Hydride Formation in Resistance Seam Welded Dissimilar Titanium Joints”, Boeing Co., 1998 43 days-at toe

  16. Seam Weld Start, Nugget Pull-Out Toosky, R. “Evaluation of Titanium Hydride Formation in Resistance Seam Welded Dissimilar Titanium Joints”, Boeing Co., 1998

  17. General Weld Problems • Porosity • H & O partitioned between solidifying dendrites • Micropores form when diatomic gas overcomes head pressure • Mircopores coalesce to form macropores • To keep O & H out of weld pool, clean & degrease AWS Welding Handbook

  18. General Weld Problems • Ductility Dip • Alpha-beta alloys containing Nb rather than V more susceptible • Alpha films at prior beta boundaries more susceptible Niobium Containing • Material in HAZ with these peak Temp can experience Dip • Cracks are intergranular • Believed to be caused by volumetric difference during transformation AWS Welding Handbook

  19. Resistance Spot Welding • Surface Cleaning - Mechanical or Chemical {HF-HNO3} • To lower surface resistance to below 50 microhms • To keep Titanium Oxide out of weld metal - embrittlement • Handle with gloves • Store in low-humidity less than 48 hours • Higher resistivity for Alloys than steel = lower current Same Schedule as SS for Pure Titanium, But Not For alloys • Pure Ti can use SS Schedule • Class 2 Electrodes with 3 in radius spherical face • Gas shielding not required for spot welds AWS Welding Handbook

  20. Schedule for Two Typical Alloys AWS Welding Handbook

  21. Equiaxed Grains Columnar Grains due to Effect of Alloys AWS Welding Handbook

  22. Resistance Seam Welding • Same Cleaning procedures as spot welding • Class 2 Electrode Wheels with radius face = wheel radius • Inert shielding gas recommended • Electrode Force and Welding Current slightly higher than for spot welds in the same material AWS Welding Handbook

  23. Flash Welding Titanium • Use machine capacities similar to that for steel • Gas Shielding not absolutely necessary but recommended (Titanium Oxidizes rapidly) • Use fast flashing rates & shorter flashing times than steel (minimize weld contamination) • Lower upset force than for steel • Flashing and upset distances slightly less than for steel Flash Welding Has Been Done on Following: • Commercially Pure Ti • Ti-6Al-4V • Ti-8Al-1Mo-1V • Ti-5Al-2.5Sn • Ti-6Al-2Sn-4Zr-2Mo • Ti-6Al-2Sn-4Zr-6Mo Info from AWS and Titanium Industries Inc.

  24. Some Titanium Welding Applications

  25. Froes, FH, et al, “Non-Aerospace Applications of Titanium” Feb 1998, TMS

  26. SAAB 340 Aero Engine Shroud constructed from commercially pure titanium using resistance, spot and TIG welding. http://www.swantec.com/industry.htm

  27. Welding Battery Connectors for Implantable Medical Device Shield Gas (Ar) Tube Weld Conditions 1000 amps 40 msec 3.5 kgrams Electrodes Battery Case Top Feed Through Contacting Pin (glass sealed in case top) Weld Combinations (inch) 0.018 Ti tab to 0.035 Nb pin 0.018 Ti tab to 0.020 Mo pin 0.018 Ti tab to 0.020 Ta pin 0.004 Ni tab to 0.020 Mo pin Battery Case Battery Tab Berkowitz, F, et al, “Implantable Medical Device With High Reliability Electrical Connection using Reactive Metals” US Patent 5,712,462 Jan 27, 1998

  28. Resistance Weld Ti Alloy Honeycomb Structures for High Speed Civil Transport (Requirement 72,000 hours @ 350F) (Al-Li max use temp = 225F) Weld Weld between Ti 6242S face sheet and Beta 21S honeycomb low heat treatment – Low mechanical Boundary Present Heat Treat 1675-1825 F 15 min – 4 hours El-Soudani, S, “Process for Enhancing the Bond Strength of Resistance Welded Joints Between Titanium Alloy Articles” US Patent 5,830,289 Nov 3, 1998 Weld with heat treatment – good properties

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