Titanium

Resistance Welding Lesson Objectives When you finish this lesson you will understand: Learning Activities 1.View Slides; 2.Read Notes, 3.Listen to lecture 4.Do on-line workbook Keywords

Donachie, Titanium, ASM, 1988

Donachie, Titanium, ASM, 1988

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

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

Can Lead to Brittleness Especially in welds (see below)

AWS Welding Handbook

Commercially Pure Titanium Yield Strengths 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

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

Applications Using Titanium Alloys For more information about applications see:

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)

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

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

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

Seam Weld Start, Nugget Pull-Out

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

AWS Welding Handbook 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

Resistance Spot Welding Surface Cleaning - Mechanical or Chemical {HF-HNO 3 } 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 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 Same Schedule as SS for Pure Titanium, But Not For alloys

AWS Welding Handbook Schedule for Two Typical Alloys

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

Resistance Seam Welding AWS Welding Handbook 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

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.

Some Titanium Welding Applications

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

SAAB 340 Aero Engine Shroud constructed from commercially pure titanium using resistance, spot and TIG welding.

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

Resistance Weld Ti Alloy Honeycomb Structures for High Speed Civil Transport (Requirement 72, F) (Al-Li max use temp = 225F) Weld Heat Treat F 15 min – 4 hours Weld between Ti 6242S face sheet and Beta 21S honeycomb low heat treatment – Low mechanical Boundary Present Weld with heat treatment – good properties 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